INTRODUCTION

Two large epidemiologic studies, one from the Mayo Clinic and the second from the Central Brain Tumor Registry of the United States (CBTRUS) gave a remarkably similar incidence of symptomatic brain tumors with a rate of about 12 per 100,000 populations per year. Given the current United States population, this comes to about 35,000 new patients with symptomatic brain tumors who are diagnosed each year (De Angelis, 2005).

Brain tumors are the fourth most common solid malignancy in the under 45 age group, and the eighth most common in the under 65 age group. Overall, brain tumors are the second most common cause of death from neurological disease, after stroke (Hart and Grant, 2007).

Brain Metastasis is the most common intracranial tumor, with an estimated annual incidence of more than 100,000 cases. In 20 to 40% of patients with cancer, metastatic lesions travel to the brain (Sawaya

INDEX Master degree thesis submitted by Dr Hisham Ibrahim | www.yassermetwally.com

INTRODUCTION

MOLECULAR PATHOGENESIS OF BRAIN TUMORS

TARGETING CRITICAL POINTS

IMMUNOTHERAPY

ANTIANGIOGENIC THERAPY

STEREOTACTIC RADIOSURGERY

CHEMOTHERAPY

ENDOCRINAL THERAPY

GENE AND VIRAL THERAPY

DISCUSSION

et al., 1994). Gliomas are the most common primary brain tumors (Belda-Iniesta et al., 2006).

In children, brain tumors are the most common solid tumor of childhood and are second only to leukemia in their overall incidence of malignancies in the pediatric population. In children, low grade astrocytomas and medulloblastomas predominate. In adults, malignant astrocytoma and meningioma are the most common tumors (De Angelis, 2005).

Collected data on symptoms before and after surgical resection report that 32% had an improvement in their symptoms, 58-76% were not different, and 9-26% had a worsening. Neurosurgery can improve some symptoms, but it can also create new ones. Neurological complications following surgery include focal haematoma, abscess and seizures. Systemic complications include pneumonia and thromboembolic disease. Morbidity rates range from 11% to 32%, with a mortality rate of 0-20% (Hart and Grant, 2007).

High-grade primary and metastatic brain tumors are common, deadly, and refractory to conventional therapy and have median survival duration of less than one year (Heimberger and Priebe, 2008).

The diffuse infiltrative nature of gliomas makes resection rarely complete and recurrence inevitable, so resection is rarely curative (Hart and Grant, 2007). Despite advances in radiation and chemotherapy along with surgical resection, the prognosis of patients with malignant glioma is poor. Therefore, the development of a new treatment modality is extremely important. Biotherapy for brain tumors will open a novel reality now (Yamanaka and Itoh, 2007).

Medulloblastoma is the most common malignant brain tumor of childhood. Surgery, radiation therapy, and chemotherapy cure many patients, but survivors can suffer long-term toxicities affecting their neurocognitive and growth potential; furthermore, there is no curative therapy in up to 30% of cases, mainly because of our incomplete understanding of many of the underlying molecular and cellular processes (Grizzi et al., 2008).

There has been substantial progress in understanding the molecular pathogenesis of these tumors, especially in the critical role of tumor stem cells. In addition, there have been important technologic advances in surgery and radiation therapy that have significantly improved the safety of these therapies, and these advances have allowed the widespread application of techniques, such as stereotactic radiosurgery, to treat brain metastases and some primary brain tumors that cannot be removed surgically. Most excitingly, improved understanding of the biology of brain tumors finally is being translated into novel therapies using targeted molecular agents, inhibitors of angiogenesis, and immunotherapies. The preliminary results with these therapies are encouraging (Wen and Schiff, 2007).

Immunotherapy is an appealing therapeutic modality for brain tumors because of its potential to selectively target residual tumor cells that have invaded the normal brain. Most immunotherapeutic studies are designed to exploit the capacity of dendritic cells for inducing cell-mediated effects as well as immune memory responses for destroying residual tumor cells and preventing recurrence (Parajuli et al., 2007). Recent reports demonstrate that systemic immunotherapy using dendritic cells or peptide vaccines successfully induce an antitumor immune response and prolong survival in those patients without major side effects (Yamanaka, 2008).

Malignant tumors of the central nervous system are difficult to cure, despite advances made in the fields of neurosurgery and radiation oncology. These tumors are generally resistant to standard chemotherapeutic agents, and new strategies are needed to overcome these tumors. Tumors of the brain demonstrate various features of angiogenesis. Correlation has been made between the degree of increased vascularity and outcome, making these tumors enticing targets for anti-angiogenic agents. Important advances in utilizing anti-angiogenic agents as an additional therapeutic modality for patients with central nervous system tumors have been identified. Rapid developments in understanding the molecular basis of angiogenesis and brain tumors have been made over the past decade, and application

of this knowledge is currently being brought to the clinic (Warren and Fine, 2008).

Overactivation of epidermal growth factor receptor signaling has been recognized as an important step in the pathogenesis and progression of multiple forms of cancer of epithelial origin. The important role of aberrant epidermal growth factor receptor signaling in the progression of malignant gliomas makes epidermal growth factor receptor-targeted therapies of particular interest in this form of cancer. The use of anti-epidermal growth factor receptor therapies against malignant brain tumors, although in its infancy, promises to yield exciting results as these new drugs probably will enhance the usefulness of existing therapies (Nathoo et al., 2004).

Combined inhibition of vascular endothelial growth factor and platelet derived growth factor signaling resulted in enhanced apoptosis, reduced cell proliferation, and clonogenic survival as well as reduced endothelial cell migration and tube formation compared with single pathway inhibition. So, dual inhibition of vascular endothelial growth factor and platelet derived growth factor signaling significantly increased tumor growth delay versus each monotherapy (Timke et al., 2008).

Combination of radiotherapy with vascular endothelial growth factor and platelet derived growth factor anti-angiogenic agents has the potential to improve the clinical outcome in cancer patients (Timke et al., 2008). Recent studies incorporating cytotoxic therapy plus anti-vascular endothelial growth factor agents among recurrent glioma patients have achieved unprecedented improvements in radiographic response, time to progression and survival (Reardon et al., 2008).

In prolactin secreting pituitary adenomas, dopamine agonists are considered the first-line of treatment. Dopamine agonists (bromocriptine and cabergoline) effectively normalize prolactin levels in as many as 89% of patients and decrease tumor volume by at least 50% in more than two thirds of patients within the first several months of therapy. Recent reports highlight impressive advancements in the pharmacologic treatment of growth hormone-secreting adenomas. Traditionally, the two options for medical therapy for these tumors have been dopamine agonists and somatostatin analogues. Dopamine agonists provide symptomatic relief in the majority of patients but normalize insulin like growth factor I levels only in approximately 20% to 40% of cases. Somatostatin analogues (octreotide and lanreotide) can normalize insulin like growth factor I levels in up to 60% of patients and have a more favorable side-effect profile compared with dopamine agonists agonists. The recently introduced growth hormone receptor antagonist (pegvisomant) has normalized insulin like growth factor I levels in 90% to 100% of patients who have refractory disease (Jagannathan et al., 2007).

Somatostatin and its analogues have the potential to be effective in a wider group of pituitary and other tumors. More potent and broader-spectrum somatostatins are likely to play an increasing role in the treatment of tumors (Hubina et al., 2006). Somatostatin and its analogues have direct anti-proliferative effect and anti-angiogenic effect (Grozinsky-Glasberg et al., 2008). A frequent finding in meningiomas is expression of progesterone receptors. So, anti-progesterone treatment can be considered in recurrent unresectable benign meningiomas (Sioka and Kyritsis, 2009).

Until recently, the use of systemic chemotherapy was restricted and ineffective, due to the fact that the blood brain barrier inhibits the adequate therapeutic concentrations of most chemotherapeutic agents into the tumor and peritumoral area (Argyriou et al., 2009). Objective benefit from the temozolomide treatment (stabilization or remission) was observed in 49% of patients irrespective of histological diagnosis. Tolerability of treatment with temozolomide in patients with high-grade gliomas is good (Ziobro et al., 2008). Temozolomide can also enhance tumor cell radiosensitivity in vitro and in vivo and this effect involves an inhibition of DNA repair (Kil et al., 2008).

Radiosurgery is a procedure in which spatially accurate and highly conformal doses of radiation are targeted at well-defined structures with an ablative intent. It has been used increasingly as a primary or adjuvant treatment for various brain diseases, including primary brain tumors, secondary metastatic tumors, and arterio-venous malformations. Radiosurgery is used to treat much smaller volumes of tissue

and has traditionally used a high dose of precisely focused radiation delivered in a single session (Oh et al., 2007).

Gamma Knife Radiosurgery provides safe and effective alternative treatment that is less invasive and has fewer side effects (Nesbitt, 2007). Gamma Knife Radiosurgery for brain metastases from conventionally radioresistant primary cancers provides better local control of the brain disease and improves survival time (Sin et al., 2009). Gamma Knife Radiosurgery for brain metastases without prophylactic whole brain radiation therapy prevents neurological death and allows a patient to maintain good brain condition (Serizawa et al., 2008). Gamma Knife Radiosurgery also provides excellent control of pineal region brain tumors when it is used in conjunction with surgery, conventional radiation therapy, or both (Lekovic et al., 2007). Gamma Knife Radiosurgery can control hemangioblastomas for as many as 10 years (Tago et al., 2005).

One of the novel strategies for treatment of brain tumors is gene therapy that includes the use of diverse viral and non-viral vectors and different genes to treat these tumors (Benítez et al., 2008).

The development of new approaches for the treatment of these tumors has led to the emergence of oncolytic virotherapy, with the use of conditionally replicating viruses, as a potential new intervention. Herpes simplex virus type 1 has emerged as the leading candidate oncolytic virus (Markert et al., 2006). These oncolytic viruses can specifically replicate and lyse in cancers, without spreading to normal tissues (Cutter et al., 2008).

MOLECULAR PATHOGENESIS OF BRAIN TUMORS

Primary brain tumors are a genetically and phenotypically heterogenous group of neoplasms that vary prognostically as a function of location, pathologic features, and molecular genetics (Levin et al., 2001). They are comprised of tumors that originate from within the brain and from structures associated intimately with brain as meninges and ependymal tissues. Understanding of molecular and genetic changes causing tumorigenesis is critical for development of new therapeutic strategies for brain tumors (Sauvageot et al., 2007).

Gliomas

Gliomas are the most common type of primary brain tumor and are grouped into three major categories astrocytomas, oligodendrogliomas, and mixed oligoastrocytomas based on their histologic similarity to normal astrocytes or oligodendrocytes. Gliomas are subgrouped by WHO into grades based on histologic factors, such as nuclear atypia, mitotic activity, vascular proliferation, and necrosis. Tumor grade is predictive of patient survival (Sauvageot et al., 2007).

Astrocytomas

Astrocytomas account for more than 60% of all primary brain tumors (Cavenee et al., 2000). They are classified into four grades. Grade I pilocytic astrocytomas are essentially benign. Grade II diffuse astrocytomas are characterized by nuclear atypia. Grade III anaplastic astrocytomas show high mitotic activity and nuclear atypia. Grade IV glioblastomas are highly infiltrative, proliferative, and necrotic. Glioblastomas are the most common form of gliomas, accounting for more than half of all primary gliomas and representing more than 75% of astrocytomas (CBTRUS, 2006).

Glioblastoma subtypes

Malignant transformation results from the accumulation of genetic abnormalities, including chromosomal loss, mutations, and gene amplifications and rearrangements. Two subtypes of glioblastomas are identified that are indistinguishable clinically but that have different genetic profiles. Primary glioblastomas typically appear in older patients who have no previous history of the disease,

whereas secondary glioblastomas usually manifest themselves in younger patients as low-grade astrocytomas that transform within 5 to 10 years into glioblastomas (Sauvageot et al., 2007).

Defects in the p53 Apoptotic and Cell Cycle Pathway

P53 is a tumor suppressor gene that plays a critical role in apoptosis and cell cycle arrest. Loss of function-mutations in the p53 protein are found in more than 65% of low-grade astrocytomas, anaplastic astrocytomas, and secondary glioblastomas, suggesting that this is an early event in formation of these tumors. Unlike secondary glioblastomas, primary glioblastomas infrequently display mutations in p53 (less than 10%), although this pathway is deregulated at other levels (Sauvageot et al., 2007). Approximately 75% of glioblastomas exhibit functional inactivation of the p53 pathway, which results in defects in apoptosis and increased cell cycle progression (Fig. 1) (Ichimura et al., 2000).

Defects in the Retinoblastoma Cell Cycle Pathway:

Retinoblastoma is a critical gatekeeper of cell cycle progression by its ability to restrict or allow a cell to progress through the G1 phase of the cell cycle pathway. Hypophosphorylation of retinoblastoma maintains the cell in a quiescent state by preventing the transcription of genes important in mitosis. Retinoblastoma can be phosphorylated by cyclin-dependent kinases 4 and 6, which are regulated negatively by cyclin-dependent kinase inhibitors, including p16Ink4a and p21Cip1. The transition from low-grade to high-grade astrocytoma is associated with allelic losses on chromosomes 9p and 13q and with amplification of 12q, which are chromosomal loci that house genes associated with the retinoblastoma pathway (Fig. 1). More specifically, approximately 25% of high-grade gliomas have mutations in retinoblastoma and 15% exhibit gene amplification of cyclin-dependent kinase 4, whereas inactivation of the p16Ink4a cyclin-dependent kinase inhibitor occurs in 50% to 70% of these tumors (Sauvageot et al., 2007).

Olig2 is a transcriptional repressor that is involved in lineage specification in the nervous system (Novitch et al., 2001). It is expressed in astrocytomas regardless of grade, suggesting it is an early oncogenic event (Ligon et al., 2004).

An article has revealed that Olig2 is required for glioma formation and that it is a direct repressor of the cyclin-dependent kinase inhibitor gene, p21Cip1 (Ligon et al., 2007).

 Defects in Growth Factor Signaling:

Excessive signaling resulting from the overexpression of either the growth factor ligand or the receptor gives a proliferative and survival advantage to the cells leading to their transformation and progression. The most common defects in growth factor signaling involve the platelet-derived growth factor and epidermal growth factor signaling pathways (Sauvageot et al., 2007).

Figure 1. The p53 and retinoblastoma cell cycle and cell death signaling pathways: double arrows indicate multistep stimulation. Rb: retinoblastoma. p14ARF: p14 alternate reading from, MDM2: murine double minute-2, Bax: BCL2-associated X protein, cdc2: cell division cycle 2 (Sauvageot et al., 2007).

Overexpression of both platelet-derived growth factor receptors and ligands within the same cells lead to autocrine loops, which can drive cell proliferation (Sauvageot et al., 2007). Platelet-derived growth factor activation promotes the upregulation of vascular endothelial growth factor, which enhances angiogenesis (Guo et al., 2003). Platelet-derived growth factor ligands and receptors are found in low-grade and high-grade astrocytomas, suggesting that this is an early oncogenic event in secondary glioblastomas. Platelet-derived growth factor receptors gene amplification is associated with loss of function of p53, placing these two genetic lesions as hallmarks of secondary glioblastomas (Sauvageot et al., 2007).

Defects in epidermal growth factor receptors occur almost exclusively in primary glioblastomas, where approximately 40% of these tumors show amplification of the region in chromosome 7 that encodes the epidermal growth factor receptor gene (Sauvageot et al., 2007). Overactivity of this receptor results in cellular proliferation, tumor invasiveness, increased angiogenesis and motility, and inhibition of apoptosis (Shinojima et al., 2003). Within tumors exhibiting epidermal growth factor receptors amplification, 40% also express a constitutively autophosphorylated variant of the epidermal growth factor receptors that lacks the extracellular ligand-binding domain, known as epidermal growth factor receptor mutant version III. This truncated receptor enhances tumorigenicity by increasing tumor proliferation and invasiveness and decreasing apoptosis (Lal et al., 2002).

Phosphatase and tensin homolog deleted on chromosome 10 is a tumor suppressor gene that is mutated or deleted in gliomas. Inactivating mutations in phosphatase and tensin homolog deleted on chromosome 10 results in increased survival, proliferation, and tumor invasion. Approximately 30% to 40% of glioblastomas exhibit mutations within the phosphatase and tensin homolog deleted on chromosome 10,

which occur almost exclusively in primary glioblastomas (Sauvageot et al., 2007).

Oligodendrogliomas:

Oligodendrogliomas are diffusely infiltrating tumors whose cells resemble immature oligodendrocytes. They account for more than 4% of all primary brain tumors and represent approximately 20% of all glial tumors (DeAngelis, 2001). Oligodendrocytomas are classified into two grades, grade II oligodendroglioma and grade III anaplastic oligodendroglioma (Sauvageot et al., 2007).

Genetic alterations:

The main genetic abnormalities associated with oligodendrogliomas are loss of chromosomes 1p and 19q, which occur in approximately 50% to 90% of grade II and grade III tumors, suggesting that this is an early oncogenic event in the formation of oligodendrogliomas (Jenkins et al., 2006). In addition, Olig2 is expressed in grade II and grade III tumors (Ligon et al., 2004). The retinoblastoma cell cycle pathway is altered in 65% of oligodendrogliomas at the level of cyclin-dependent kinases 4 amplification, mutations in retinoblastoma, or deletions of p16Ink4a (Watanabe et al., 2001). p53 gene mutations are seen in approximately 10% to 20% of all oligodendrogliomas (Sauvageot et al., 2007).

Finally, similar to high-grade astrocytomas, approximately 9% of anaplastic oligodendrogliomas exhibit mutations in phosphatase and tensin homolog deleted on chromosome 10, and more than 20% exhibit loss of the 10q locus which houses phosphatase and tensin homolog deleted on chromosome 10 (Sasaki et al., 2001). Such mutations, combined with growth factor abnormalities, would result in increased proliferation of cells (Sauvageot et al., 2007).

Defects in Growth Factor Signaling

Oligodendrogliomas also exhibit abnormalities within the epidermal growth factor receptors and platelet derived growth factor receptors signal transduction pathways. Unlike astrocytomas, which exhibit epidermal growth factor receptors amplification and overexpression only in high-grade tumors, overexpression of this protein occurs in approximately 50% of low-grade and high-grade oligodendrogliomas. Coexpression of platelet derived growth factor ligands and receptors occur in almost all oligodendrogliomas (Sauvageot et al., 2007). Abnormalities within epidermal growth factor receptors and platelet derived growth factor receptors signaling indicate that this is an early oncogenic event in the formation of these tumors (Weiss et al., 2003).

Oligoastrocytomas

As their name implies, oligoastrocytomas are composed of a heterogeneous population of cells that have characteristics of astrocytomas and oligodendrogliomas. Oligoastrocytomas are divided into two grades, grade II oligoastrocytomas and grade III anaplastic oligoastrocytomas which are clinically aggressive and characterized by high mitotic activity, nuclear atypia, and necrosis (Sauvageot et al., 2007).

The cellular heterogeneity seen in oligoastrocytomas is reflected in the genetic heterogeneity of these tumors. Approximately 30% to 50% of oligoastrocytomas exhibit 1p and 19q deletions reminiscent of oligodendrogliomas, whereas a separate 30% of these tumors house genetic anomalies similar to astrocytomas, including mutations in p53. Microdissection of the astrocytoma versus oligodendroglioma components of these tumors have revealed common genetic changes in all components of a given tumor, suggesting that these histologically mixed tumors may arise from a single transforming event (Sauvageot et al., 2007).

Meningiomas

Meningiomas originate from the transformation of meningothelial cells and account for approximately

13% to 26% of all primary brain tumors (Louis et al., 2000). The large majorities of meningiomas are benign and classified as WHO grade I tumors, although 5% to 7% of all meningiomas are grade II atypical meningiomas, and 1% to 3% are grade III anaplastic meningiomas (Sauvageot et al., 2007). Atypical meningiomas are characterized by an increased mitotic index and sheeting architecture, macronucleoli, hypercellularity, or necrosis, whereas anaplastic meningiomas exhibit high mitotic index and frank anaplasia (Perry et al., 2004).

Genetic alterations

The most common change in meningiomas is loss of heterozygosity of chromosome 22, which houses the neurofibromatosis type 2 gene (Sauvageot et al., 2007). Loss of function of neurofibromatosis type 2 is an early event in approximately 60% of meningiomas and occurs in almost all neurofibromatosis type 2 associated meningiomas. The neurofibromatosis type 2 gene encodes a protein known as merlin which is part of the protein 4.1 family (Perry et al., 2004). Loss of merlin in vivo is associated with increased cell growth and formation of highly motile and metastatic tumors (Giovannini et al., 2000). Another protein 4.1 family member known as DAL-1 has a growth suppressive effect. It is lost in up to 76% of benign and atypical meningiomas and 87% of malignant meningiomas (Gutmann et al., 2000). Loss of protein 4.1R occurs in 40% of meningiomas (Robb et al., 2003). These data suggest that the protein 4.1 family of proteins plays a critical role in meningioma formation (Sauvageot et al., 2007).

Telomerase enzyme is associated with tumor progression, with infrequent expression in benign meningiomas (approximately 8%) and higher levels in atypical and anaplastic meningiomas (approximately 60% and approximately 90%, respectively) (Falchetti et al., 2002). Progesterone receptors are present in more than 80% of meningiomas. Progesterone receptors expression is inversely proportional to tumor proliferation and histologic grade, with expression occurring in 96% of benign tumors compared with 40% in malignant tumors. Progesterone receptors expression is a favorable prognostic factor for meningiomas (Sauvageot et al., 2007).

Defects in Growth Factor Signaling

Growth factors may play a role in meningioma initiation and progression. Coexpression of Platelet derived growth factor ligands and receptors occurs in the majority of meningiomas (Sauvageot et al., 2007), with higher expression correlating with increasing tumor grade (Yang et al., 2001). Epidermal growth factor receptors and their ligands; epidermal growth factor and transforming growth factor ?, are expressed in almost all meningiomas but not in normal meninges. Increased transforming growth factor ? expression is associated with aggressive meningioma growth. Insulin-like growth factor II, its receptor, and insulin-like growth factor binding protein are expressed in meningiomas, where a high ligand/receptor ratio is associated with malignant progression (Sauvageot et al., 2007).

Medulloblastomas

Medulloblastomas are an invasive embryonal tumor of the cerebellum that show tendency to metastasize. Although medulloblastomas occur most frequently during childhood, approximately 30% of cases arise in adults (Sauvageot et al., 2007). Medulloblastomas are considered WHO grade IV tumors because of their invasive and malignant phenotypes and are grouped into four subtypes consisting of: classic, desmoplastic, extensive nodularity and large-cell medulloblastomas. The histologic subtype is associated with the clinical outcome (Eberhart et al., 2002).

Adult medulloblastomas differ from pediatric medulloblastomas in terms of histology, localization, and relapse frequencies (Carrie et al., 1994). Unlike pediatric medulloblastomas, which are localized in the midline cerebellar vermis and extend into the fourth ventricle, approximately 50% of adult medulloblastomas are located laterally within the cerebellar hemispheres (Prados et al., 1995).

Genetic alterations

The most common genetic abnormality in medulloblastoma consists of an isochromosome of chromosome 17, which arises in approximately 50% of cases, and which occurs with a higher frequency in classic than in desmoplastic medulloblastomas. Deletions of the short arm of chromosome 17p also occur in 30% to 45% of cases. Although the tumor suppressor gene, p53, is located on 17p and is mutated in many cancer types, only rare mutations in this gene are found in medulloblastomas. Also deletions on chromosomes 1q and 10q are found in 20% to 40% of medulloblastomas (Sauvageot et al., 2007).

Defects in Growth factor Signaling

The main growth factor pathways that are defective in medulloblastomas are (Fuccillo et al., 2006):

1) Sonic hedgehog pathway.

2) ErbB pathway.

3) Wingless pathway.

Signaling within Sonic hedgehog pathway is initiated by binding of the ligand Sonic hedgehog to its receptor PTC that normally inhibits the activity of a protein known as Smoothened. This binding removes inhibition of Smoothened leading to release of glioma-associated oncogene homolog-1 that enters the nucleus to activate gene transcription (Fig. 2). Hence, PTC can play the role of a tumor suppressor, whereas Sonic hedgehog and Smoothened are putative oncogenes (McMahon et al., 2003). Inactivating mutations of PTC was identified in approximately 8% of medulloblastomas whereas activating mutations within Sonic hedgehog and Smoothened was identified in rare cases (Sauvageot et al., 2007).

ErbB2 is a receptor tyrosine kinase. Overexpression of ErbB2 or a single point mutation in the transmembrane domain of this receptor can induce tumor formation (Yarden and Sliwkowski, 2001). 80% of medulloblastomas show overexpression of ErbB2 which is associated with a worse prognosis (Gajjar et al., 2004). The tendency of these tumors to metastisize may be mediated in part by aberrant ErbB2 activation, as ErbB2 overexpression in medulloblastomas increases cell migration and promotes the upregulation of prometastatic genes (Hernan et al., 2003).

Figure 2. Sonic hedgehog signaling pathway: Shh: Sonic hedgehog, SMO: Smoothened, GLI1: glioma-associated oncogene homolog-1, SuF: suppressor of fused (Sauvageot et al., 2007).

Wingless activation of the frizzled transmembrane receptor inhibits ?-catenin degradation by the GSK-3/APC/axin complex (Baeza et al., 2003). The resulting increases in ?-catenin levels enable it to enter the nucleus and activate proliferation genes such as MYC and cyclin D1 (Fig. 3) (Henderson and Fagotto, 2002). Approximately 15% of sporadic medulloblastomas exhibit mutations within Wingless pathway (Baeza et al., 2003).

Figure 3. Wingless signaling pathway: Wnt: Wingless, Dsh: disheveled, LEF: lymphoid enhancer bindin factor, TCF: transcription factor protein, UUUU: ubiquitination (Sauvageot et al., 2007).

Pathogenesis of Brain Tumors Spread

In a mathematical model it has been calculated that the velocity of expansion is linear with time and varies from about 4 mm/year for low-grade gliomas and 3 mm/month for high grade gliomas (Swanson et al., 2003). Motility and invasion of glioma cells are due to events involving extracellular matrix, adhesion molecules and properties of the cells themselves (Schiffer, 2006).

Several genes involved in glioma invasiveness have been identified and include members of the family of metalloproteases. Expression of metalloprotease 2 and, to a lesser extent, metalloprotease 9 correlates with invasiveness, proliferation and prognosis in astrocytomas (Wang et al. 2003).

Other non-metalloprotease proteases, including urokinase-type plasminogen activator and cysteine proteases (e.g. cathepsin B), are elevated in high-grade gliomas. Invasion inhibitory protein 45, a potential tumor suppressor gene on chromosome 1p36, its product inhibits invasion and it is frequently down-regulated in glioblastomas. Other invasion- and migration-associated genes have been identified including P311, death-associated protein 3, and FN14 (Furnari et al., 2007). Other proteins are overexpressed in invasive areas of glioblastoma, such as angiopoietin-2, which in addition to its involvement in angiogenesis also plays a role in inducing tumor cell infiltration by activating metalloprotease 2 (Hu et al. 2003).

Many factors intervene in regulating cell migration and invasion, first of all epidermal growth factor. Highly amplified cells for epidermal growth factor receptors are found at the invading edge of the tumor. Another very important factor in regulation of glioma cell motility is phosphatase and tensin homolog deleted on chromosome 10; its phosphatase-independent domains reduce the invasive potential of glioma cells (Schiffer, 2006). Phosphatase and tensin homolog deleted on chromosome 10 mutation

has been implicated in an invasive phenotype due to its ability to stabilize E-cadherin and modulate cell matrix adhesion complexes (Furnari et al., 2007).

Integrins variously occur on glioma cells and mediate cell adhesion to extracellular matrix (Tysnes and Mahesparan, 2001). Integrins, especially ?V?3 complexes, are elevated in glioblastomas and appear to be relevant to processes of glioma invasion and angiogenesis (Kanamori et al. 2004). Integrins also play a role in glioma cell growth and proliferation. Other factors involved in controlling cell motility are scatter factor/hepatocyte growth factor and transforming growth factor ?1 (Schiffer, 2006).

Cell-of-Origin of Brain Tumors

Primary malignant brain tumors have a high rate of recurrence after treatment. Studies have led to the hypothesis that the root of the recurrence may be brain tumor stem cells, stem-like subpopulation of cells that are responsible for propagating the tumor. Current treatments combining surgery and chemoradiotherapy could not eliminate brain tumor stem cells because these cells are highly infiltrative and have several properties that can reduce the damages caused by radiation or anti-cancer drugs. Brain tumor stem cells are similar to the neural stem cells. Furthermore, data from various models of malignant brain tumors have provided evidence that multipotent neural stem cells and neural progenitor cells could be the cell origin of brain tumors. Thus, the first event of brain tumorigenesis might be the occurrence of oncogenic mutations in the neural stem cells or neural progenitor cells. These mutations convert a neural stem cell or neural progenitor cell to brain tumor stem cells, which then initiates and sustains the growth of the tumor (Xie, 2009).

What are Stem Cells?

Stem cells are defined as cells capable of self-renewal and multipotency. Self-renewal refers to the ability of a cell to generate an identical copy of it, whereas multipotency refers to the ability to give rise to all the differentiated cell types within a given tissue. Stem cells are able to divide asymmetrically to produce mother and daughter cells. The mother cell has equivalent self-renewal and multipotency capabilities as the original stem cell, whereas the daughter cell (a progenitor cell) has a more limited set of developmental options and are able to transiently divide before giving terminally differentiated cells (Sauvageot et al., 2007).

Stem Cells in the Nervous System and in Brain Tumors

Historically, it has been believed that the cell-of-origin of gliomas must be a mature glial cell. More recently, neural stem cells have been identified within the adult brain, providing an alternate source of cycling cells capable of transformation. Neural stem cells capable of self-renewal and multipotency were identified in germinal areas of the adult nervous system within the subventricular zone and the dentate gyrus of the hippocampus (Sanai et al., 2004).

Evidence suggests that tumorigenic stem cells also exist within brain tumors. Initial reports showed that neural stem cells could be isolated from glioblastomas and medulloblastomas (Singh et al., 2003). Subsequent experiments revealed that xenotransplantion of neurospheres derived from human brain tumors could form neoplasms that have the morphology and lineage characteristics of the original tumors (Singh et al., 2004).

Implantation of as few as 100 of these cells that were shown to express the stem cell surface marker, cluster destination 133 (CD133), suffices to generate a tumor compared with implantation of 10,000 cluster destination 133 negative cells, which are not able to form a tumor. Several studies have indicated the presence of cluster destination 133 positive brain tumor stem-like cells within the human glioblastomas (Anhua et al., 2008). These studies do not exclude the possibility that cycling progenitor cells, or even more differentiated cells, also may play a role in contributing the growth and malignancy of these tumors (Sauvageot et al., 2007). The neural progenitor cells are one source of gliomas as the

marker phenotypes, morphologies, and migratory properties of cells in gliomas strongly resemble neural progenitor cells in many ways (Canoll and Goldman, 2008).

Mouse Models

Mouse models of gliomas, in which different transforming events are targeted to specific subpopulations of cells within the nervous system, have been created to help clarify the cell-of-origin of gliomas. Although these experiments have shown that tumors can arise from progenitor cells and from more differentiated cells, they reveal that progenitor cells are more permissive to oncogenic transformation (Holland et al., 2000). This idea is supported by data showing that the transforming epidermal growth factor receptors mutant version III gene induces glial tumors more readily when introduced into progenitor cells than in differentiated ones (Bachoo et al., 2002).

These data reveal that malignant transformation occurs more readily in progenitor cells, but that multiple oncogenic pressures can drive more differentiated cells to reacquire progenitor cell phenotypes, which subsequently also can drive tumorigenesis (Sauvageot et al., 2007).

Functional Similarities between Stem Cells and Tumor Cells:

Several similarities exist between stem cells and tumor cells, including 1) extensive proliferation, 2) heterogeneity of progeny, and 3) high motility (Sauvageot et al., 2007).

Neural stem cells and gliomas are highly proliferative, and the epidermal growth factor receptors are involved in this response in both cell types. It plays a prominent role in promoting proliferation of gliomas, both as a result of epidermal growth factor receptors amplification, as well as expression of the constitutively active epidermal growth factor receptor mutant version III (Sauvageot et al., 2007).

Neural stem cells and gliomas show a diversity of progeny. Neural stem cells can give rise to heterogeneous population of cells. Likewise, gliomas are a heterogeneous population of cells that arise from a much localized transformation event, most likely occurring in a multipotent tumor stem cell. In cases where clonality cannot be established, this may suggest that independent transformation events in different cells of origin. Recent evidence reveals that some of the heterogeneity may arise from progenitors that are recruited to the tumor mass after the original transformation event occurred (Assanah et al., 2006).

A cardinal feature of malignant gliomas is their ability to migrate and infiltrate into adjacent regions. Similarly, neural stem cells and neural progenitors exhibit a migratory capacity in the normal adult brain (Lois and Alvarez-Buylla, 1994). After injury, the progenitor cells migrate from the subventricular zone to the site of injury (Hayashi et al., 2005). Once again, this shared phenotype may result from epidermal growth factor activity and epidermal growth factor receptor mutant version III which upregulates factors of tumor invasion (Lal et al., 2002).

Molecular Similarities between Stem Cells and Tumor Cells

Several proteins that are normally involved in neural stem cells development also are present in gliomas, including platelet derived growth factor, phosphatase and tensin homolog deleted on chromosome 10, Oolig2, and Sonic hedgehog, providing further support for the notion that the origin of gliomas may be neural stem cells and their progenitors (Sauvageot et al., 2007).

Coexpression of platelet derived growth factor ligands and receptors are a common occurrence in brain tumors. Platelet derived growth factor also plays an important role in regulating glial progenitor proliferation (Sauvageot et al., 2007). In the adult brain, platelet derived growth factor receptor expression is restricted to the germinal zone of the brain and, more specifically, is expressed in a subset of subventricular neural stem cells capable of differentiating into neurons and oligodendrocytes.

Infusion of platelet derived growth factor into the lateral ventricles induces the formation of atypical hyperplastic cells that resemble gliomas (Jackson et al., 2006).

Phosphatase and tensin homolog deleted on chromosome 10 also is involved in the growth of neural stem cells and gliomas. It is inactivated in a high percentage of brain tumors, and this inactivation results in increased cell migration, invasion, survival, and proliferation. Correspondingly, phosphatase and tensin homolog deleted on chromosome 10 plays a critical role in neural stem cells maintenance and function (Li et al., 2002).

Expression of Olig2 further extends the connection between normal neural stem cells and gliomas. Olig2 expression found in all malignant gliomas is required for glioma formation (ligon et al., 2007). During normal development, Olig2 is expressed in precursor cells within germinal areas of the brain (Menn et al., 2006), and is necessary for normal and tumorigenic progenitor cell growth (ligon et al., 2007).

Finally, the Sonic hedgehog and glioma-associated oncogene homolog-1 signaling pathway controls the proliferation of precursor cells within the adult germinal zones (Sauvageot et al., 2007). A variety of primary brain tumors express glioma-associated oncogene homolog-1, and inhibition of this pathway inhibits tumor growth (Dahmane et al., 2001).

In addition, targeted disruption of the Sonic hedgehog receptor results in medulloblastoma formation. The parallels between these developmental programs and tumorigenesis lend further evidence that tumor formation results from aberrations of developmental programs normally active in normal neural stem cells and their progenitors (Sauvageot et al., 2007).

Localization of Stem Cell-Derived Tumors

If normal neural stem cells are the cell of origin of brain tumors, then it is expected that these neoplasms be localized preferentially in the germinal areas of the brain. Although this is not the case, much evidence suggests that they arise from these areas and then migrate out to their final locations (Zhu et al., 2005). This dissociation of transformed cells from germinal areas is consistent with current understanding of stem cell niches (Li and Neaves, 2006). The germinal areas in which stem cells reside are known as niches, and they maintain a homeostatic balance of factors that control the self-renewal and differentiation states of normal neural stem cells revealing these cells' dependence on the niche for survival (Xie and Spradling, 2000). Tumorigenesis of normal neural stem cells reflect mutations that make the normal neural stem cells independent of the signals necessary for self-renewal and survival in the niche, thereby enabling it to grow at any location within the brain. The ability of niche-independent normal neural stem cells to migrate and grow in aberrant locations of the brain is suggestive of the highly infiltrative nature of brain tumors (Sauvageot et al., 2007).

TARGETING CRITICAL POINTS

Knowledge of the relevant pathogenesis pathways aids selection of targets and agents. It is probable that, for any given molecular target, multiple pathways and multiple cell types can be affected. Different effects may reinforce or contradict each other (Wieduwilt and Moasser, 2008).

Before choosing a molecular target or pathway, it is necessary to select the cell, or cells, that will be the primary focus, and how directly they will be tied to tumor attack. Targeted therapy could be directed also at cells that participate in the antitumor immune response. Once a molecular target has been chosen, what agent should be selected to attack it, a small molecule inhibitor, an antibody, does one prefer to stimulate an immune response against the target (Lampson, 2009).

Despite the molecular heterogeneity of brain tumors, there exist common signal transduction pathways that are altered in many of these tumors. In malignancies, mutation or over-expression can occur in growth factor ligands and their receptors, as well as in intracellular effector molecules. Overexpression

or mutations causing activation of these growth factors and their downstream effector molecules result in uncontrolled cell proliferation, survival, and invasion (Mercer et al., 2009). Targeting of these signaling pathways provides new molecularly targeted treatment options (Argyriou and Kalofonos, 2009).

Targeting the Growth Factors and their Receptors

Growth factors bind to their receptors extracellularly and initiate an intracellular signal transduction cascade. Recent research has discovered many different monoclonal antibodies and small-molecule inhibitors that specifically target these ligands and receptors (Mercer et al., 2009).

Targeting the Epidermal Growth Factor Receptors

Epidermal Growth Factor Receptor Inhibitors

Epidermal growth factor receptor is amplified in 40-60% of glioblastomas, and approximately 30-50% of these tumors also possess the epidermal growth factor receptor mutant variant III, in which coding regions in the extracellular ligand domain are absent (Pelloski et al., 2007). Epidermal growth factor receptors play an integral role in the malignant phenotype of brain tumors, and it is a common hope among researchers that functional inhibition of epidermal growth factor receptor-mediated signaling will slow tumor growth (Sarkaria et al., 2007).

Treatment options that target epidermal growth factor receptors include erlotinib and gefitinib (Fig. 4). However, efficacy of these agents is modest (Argyriou and Kalofonos, 2009). Erlotinib was evaluated in a randomized phase II trial used in patients with recurrent glioblastoma multiforme. The 6-month progression-free survival was 12% (Mercer et al., 2009). Gefitinib was evaluated in a phase II clinical trial in patients with the first recurrence of a glioblastoma. The study showed a PFS-6 of 13% but no radiographic response was observed (Argyriou and Kalofonos, 2009). Overall survival rates in several phase I/II clinical trials for erlotinib and gefitinib treatment were similar, but erlotinib was more effective than gefitinib treatment in terms of objective radiographic responses (Argyriou and Kalofonos, 2009).

Epidermal Growth Factor Receptor Peptide Vaccines

As mentioned previously, epidermal growth factor mutant variant III is an attractive target because it is not expressed in normal tissues but is expressed in a wide number of malignancies, including 31-50% of malignant gliomas (Heimberger et al., 2005). Initial preclinical studies targeting epidermal growth factor mutant variant III in rodents were promising. A peptide vaccine targeting epidermal growth factor mutant variant III was successful in extending the survival rate in vaccinated rats by 72%. The success of these preclinical studies led to several human trials that also produced promising results (Ciesielski et al., 2005).

In a phase I human trial, patients were treated with up to four peptide vaccines. The vaccine was well tolerated and the majority showed increased cellular and humoral responses. Clinical results were moderately favorable; 5 of 21 patients showed a partial radiographic response and 8 of 21 had stable disease. The median survival among patients with glioblastoma was 622 days (Yajima et al., 2005).

Phase I human trial involving epidermal growth factor mutant variant III the keyhole limpet vaccine showed that the vaccine was well tolerated and the clinical results were also promising in patients with malignant glioma. The vaccine was successful in stimulating cellular immunity. Among patients with grade III tumors, 2 of 3 patients had stable disease. Among patients with glioblastoma, the mean time to disease progression was 46.9 weeks. For the phase II trial, preliminary results have indicated that the vaccine is well tolerated and studies have demonstrated humoral and cellular immunity; the final results have not yet been reported (Sampson et al., 2008).

Targeting the Vascular Endothelial Growth Factor Receptors

Vascular proliferation is a hallmark of tumor survival and growth. It was suggested that patients with highly vascularized tumors would have a poorer prognosis than patients with less neovascularization; however, this supposition requires further validation (DeGroot and Gilbert, 2007).

Glioma cells produce many different proangiogenic factors, including vascular endothelial growth factor. For these reasons, vascular endothelial growth factor ligands and receptors have been targeted for treatment of gliomas. Strategies for targeting this ligand and its receptor include using: 1) vascular endothelial growth factor antibodies, 2) vascular endothelial growth factor receptor tyrosine kinase inhibitors, and 3) protein kinase C inhibitors (Mercer et al., 2009).

Bevacizumab is, to date, the most promising vascular endothelial growth factor monoclonal antibody. Bevacizumab targets the vascular endothelial growth factor ligand with the goal of interfering with ligand-receptor signaling (Fig. 4). Although at first there was hesitation about using bevacizumab for the treatment of patients with brain tumors, because of its known history of intra-tumoral hemorrhage in patients with colorectal carcinoma (Mercer et al., 2009).

A phase II trial evaluated bevacizumab in combination with chemotherapy (irinotecan). The radiographic response rate was positive and 14 of 23 patients (61%) responded to therapy. The 6-month progression-free survival was 30% and the overall median survival time was 40 weeks (Vredenburgh et al., 2007).

It was suggested that bevacizumab as monotherapy may be the best way to maximize the efficacy of treatment while minimizing systemic toxicity. In a phase II trial of bevacizumab alone, the results showed that approximately 60% of patients who were treated with bevacizumab alone had objective radiographic responses, as well as a 6-month progression-free survival of 30% and a median progression-free survival of about 110 days (Fine, 2007).

Targeting the Platelet-Derived Growth Factor Receptors

Platelet derived growth factor and its receptors play an important role in tumor interstitial pressure, tumor growth, and angiogenesis. Over-expression of platelet derived growth factor and its receptors in gliomas made them to be targets for anti-tumor treatments (Ostman, 2004).

The platelet derived growth factor receptor inhibitor for which most data have been obtained is imatinib mesylate. Imatinib is a small-molecule inhibitor of platelet derived growth factor receptors ? and ? (Fig. 4). However imatinib showed some anti-tumor effects in preclinical studies, minimal clinical benefit was seen using imatinib as monotherapy, with a radiographic response rate of <6% and a 6-month progression-free survival of <16% (Wen, 2006).

Imatinib with chemotherapy (hydroxyurea) showed promising results (Dresemann, 2005). These initial results of imatinib with hydroxyurea were supported in a phase II trial in patients with recurrent glioblastoma, which reported a radiologic response rate of 42% and a 6-month progression-free survival of 27% (Reardon et al., 2005). The mechanism of action of this combination is still unknown, but it has been shown that imatinib decreases the interstitial tumor pressure and may increase delivery of the chemotherapeutic agent hydroxyurea. This gives a possible explanation for the initial success associated with this combination. Unfortunately, a phase III trial did not provide further evidence of efficacy (Mercer et al., 2009).

There are many other therapeutic agents that are beginning to be explored further such as sunitinib which inhibits platelet derived growth factor receptors and vascular endothelial growth factor receptors. Sunitinib was shown in vivo to increase survival by 36% in mice with glioblastoma. Also, treatment with sunitinib increased tumor necrosis, reduced angiogenesis and caused a 74% reduction in microvessel

density (DeBouard et al., 2007).

Another example is epidermal growth factor receptor and vascular endothelial growth factor receptor inhibitor vandetanib that have been shown in vitro to have anti-tumor effects (Sandstrom et al., 2008).

Targeting Downstream Intracellular Effector Molecules

Activation of the growth factor receptors leads to recruitment of intracellular effector molecules (downstream molecules) to the cell membrane. Gliomas are associated with either the activation of these effector molecules or the inactivation of their suppressive regulators. This stimulates cell proliferation and invasion. Crucial pathways in gliomas are (Mercer et al., 2009):

1) Ras/ Raf/MAPK,

2) PI3K/AKT/mTOR, and

3) Protein kinase C.

Ras/Raf/MAPK Pathway

The Ras superfamily of genes regulates many important cellular functions including cell proliferation, differentiation, protein trafficking, and cytoskeletal organization. The Ras pathway is an important downstream of the epidermal growth factor receptors and platelet derived growth factor receptors. Many glioblastomas have hyperactive Ras pathway due to mutation or amplification of their upstream growth factor receptors (Knobbe et al., 2004). Activation of Ras is followed by overactivation of downstream Ras and Raf molecules, which triggers Mitogen-Activated Protein Kinase (MAPK), causing cell proliferation and release of proangiogenic growth factors (Fig. 4) (Mercer et al., 2009).

Tipifarnib and lonafarnib are the two most prominent drugs used in an attempt to indirectly inhibit Ras. A phase II study of tipifarnib reported a 6-month progression-free survival of 12% in patients with glioblastoma and 9% in patients with anaplastic glioma (Cloughesy et al., 2006). A phase I study of lonafarnib with the chemotherapeutic agent, temozolomide, was performed in patients with recurrent glioblastoma in an attempt to overcome tumor resistance to temozolomide. Using this therapy, 27% of patients who were previously resistant to temozolomide treatment showed a partial response, and the 6-month progression-free survival was 33% (Mercer et al., 2009).

PI3K/AKT/mTOR Pathway

PI3K (Phosphatidylinositol 3-Kinase) is a serine/threonine kinase that controls several malignant characteristics as avoidance of apoptosis, and cell growth and proliferation. Overactivation of Phosphatidylinositol 3-Kinase is a result of activation of receptor tyrosine kinases such as epidermal growth factor receptors. This pathway is down-regulated by phosphatase and tensin homolog deleted on chromosome 10. When phosphatase and tensin homolog deleted on chromosome 10 function is lost, activation of the Phosphatidylinositol 3-Kinase pathway occurs. Activation of this pathway activates many downstream molecules including AKT (Fig. 4) (Chakravarti et al., 2004).

AKT is also a serine/threonine kinase that decreases apoptosis and therefore increases cell growth and proliferation. AKT regulates many central biological processes; therefore it is difficult to target it directly, so inhibition of its upstream (preceding steps as Phosphatidylinositol 3-Kinase) and downstream (following steps as mammalian target of rapamycin) has been suggested (Fig. 4) (Mercer et al., 2009).

mTOR (mammalian target of rapamycin) is a downstream target of AKT (Vivanco and Sawyers, 2002).

Mammalian target of rapamycin controls important cellular functions, including modulation of cell growth (Fig. 4). Mammalian target of rapamycin inhibitors that have been used in clinical trials are synthetic analogs of rapamycin (e.g. temsirolimus, everolimus and sirolimus). Phase II trials of temsirolimus in patients with glioblastoma have not shown much efficacy. Although a radiographic improvement was seen in 36% of patients, no real survival benefit was recorded (Galanis et al., 2005).

Protein Kinase C Pathways

Protein Kinase C is a family of 14 protein kinases that regulate cell growth, proliferation, and angiogenesis (Couldwell et al., 1991). Protein Kinase C is a downstream of growth factor receptors such as epidermal growth factor receptors and platelet derived growth factor receptors (Fig. 4) (DaRocha et al., 2002).

Tamoxifen and enzastaurin are the two protein Kinase C inhibitors, for which significant data have been recorded (Hui et al., 2004). Tamoxifen in glioma xenografts showed some anti-tumor activity, but in the clinical practice it showed only moderate efficacy with no improvement in the median survival time (Spence et al., 2004).

Enzastaurin has shown promising efficacy in the clinical practice, with a 29% radiographic response rate in patients with recurrent high-grade gliomas (Fine et al., 2005). Progress with enzastaurin was stopped when the phase III trial was stopped prematurely because the significant survival benefits were not seen (Mercer et al., 2009).

Figure 4. Targeting growth factors & their receptors, and downstream intracellular effector molecules: &#9;EGFR: epidermal growth factor receptors, &#9;PDGFR: platelet derived growth factor receptors, VEGFR: vascular endothelial growth factor receptors, MAPK: mitogen-activated protein kinase, PKC: Protein Kinase C, PI3K: Phosphatidylinositol 3-Kinase, mTOR: mammalian target of rapamycin,

(Argyriou and Kalofonos, 2009).

Targeting Cancer Stem Cells

The evidence strongly suggests that the cells-of-origin of brain tumors are cancer stem cells or transformed progenitor cells that arise from these stem cells. Traditional therapies for brain tumors have focused on shrinking the bulk of the tumor. As cancer stem cells represent only a tiny minority of the tumor bulk, therapies that focus on tumor bulk regression may not eliminate all of the tumor-initiating cells completely, thereby enabling regrowth and recurrence. Furthermore, the nature of stem cells makes them more resistant to current therapies. Hence, therapies targeting tumor stem sells are important in preventing recurrence of these tumors (Fig. 5) (Sauvageot et al., 2007).

Figure 5. Therapies directed against the cancer stem cells would eliminate the cells capable of repopulating the tumor, resulting in a tumor that cannot grow (Sauvageot et al., 2007).

Selective eradication of cancer stem cells may be achieved by targeting the tumor microenvironment rather than the cancer stem cells directly. Anticancer therapies combining anti-angiogenic and cytotoxic effects reduce the cancer stem cell fraction in glioma xenograft tumors (Folkins et al., 2007).

Targeting Tumor Spread

Integrins are cell surface receptors that play important roles in tumor invasion. Integrins, especially ?V?3 complexes, are elevated in glioblastoma (Kanamori et al. 2004). Cilengitide is a drug that targets integrins (?V?3) and demonstrated some efficacy against malignant gliomas in a preclinical study and in a phase I clinical trial. Considering these preliminary results, cilengitide appears to be a promising treatment. A larger phase II trial of cilengitide is currently ongoing in patients with newly diagnosed glioblastomas concurrent with radiation therapy (Argyriou and Kalofonos, 2009).

Matrix metalloproteinases are implicated in tumor growth, invasion, extravasation into distant sites, and angiogenesis. The most well studied matrix metalloproteinases inhibitor to date is marimastat. In vitro studies have shown cytostatic effects. Animal models have shown efficacy in preventing growth and vascularization. Marimastat in combination with chemotherapy (temozolomide) showed a 6-month progression-free survival of 39%, compared with 18 to 21% with temozolamide alone. Prinomastat is a lipophilic matrix metalloproteinases inhibitor, which crosses the blood-brain barrier, making it promising for the treatment of glioma (Vogelbaum and Thomas, 2007).

Src kinase facilitates cellular invasion. Targeted deletion of Src in mice reduced glioma cell invasion. Dasatinib, an inhibitor of Src kinase, is under clinical development in recurrent glioblastoma patients (Sathornsumetee et al., 2007).

Miscellaneous

Proteasome Inhibitors

The ubiquitin-proteasome system is important in regulating cell proliferation and apoptosis. Proteasome inhibitors can induce cell-growth arrest and apoptosis. Bortezomib is a proteasome inhibitor that can induce cell-cycle arrest and apoptosis in glioma cells. A phase I/II study of bortezomib in gliomas is ongoing (Sathornsumetee et al., 2007).

Deacetylase Inhibitors

Histone deacetylase inhibitors target gliomas by modifying gene transcription to cause cell-cycle arrest and induction of apoptosis. Vorinostat, one of the histone deacetylase inhibitors, was used in the treatment of glioma and showed potential benefit. Pretreatment with vorinostat can make glioma cells more susceptible to chemotherapy and radiation (Chinnaiyan et al., 2005).

In a phase II trial of vorinostat in patients with recurrent glioblastoma, the drug was well tolerated and anti-tumor efficacy was noted; 6-month progression-free survival was 23% (Galanis et al., 2007). Other deacetylase inhibitors (e.g. romidepsin, LBH589, and valproic acid) are being developed clinically for treatment of glioblastoma (Mercer et al., 2009).

Ligand-Toxin Conjugates

Glioma cells commonly express cell surface receptors at greater density than normal brain cells. The ligands for these receptors can be engineered to deliver toxins or radioisotopes to produce tumor cytotoxicity (Sathornsumetee et al., 2007).

Examples of these ligands include interleukin-13, epidermal growth factor, and antibodies against tenascin-C (extracellular matrix glycoprotein expressed by glioma cells). Several ligand-toxin conjugates was developed for glioma therapy including cintredekin besudotox, which consists of IL-13 ligand conjugated to a pseudomonas exotoxin. Cintredekin besudotox targets interleukin-13 receptors that are over-expressed in gliomas (Argyriou and Kalofonos, 2009).

Small Molecular inhibitors of Signal Transducer and Activator of Transcription

Signal transducer and activator of transcription is a key transcriptional factor that drives the fundamental components of tumor malignancy in CNS. Signal transducer and activator of transcription promote tumorigenesis by enhancing proliferation, angiogenesis, invasion, metastasis, and immunosuppression. A group of potent, small molecule inhibitors of signal transducer and activator of transcription display marked efficacy with minimal toxicity against brain tumors in murine models. The mechanism of this in vivo efficacy of Signal transducer and activator of transcription inhibitors is a combination of direct tumor cytotoxicity and immune cytotoxic clearance. Given their ability to achieve good CNS penetration, these drugs will be taken forward into clinical trials for patients with CNS malignancies (Heimberger and Priebe, 2008).

Heat Shock Protein 90 Inhibitors

Heat shock protein 90 assists in protein folding and cell signaling (Richter and Buchner, 2001). Heat shock protein 90 is involved in folding, stabilization, activation, and assembly of a wide range of proteins, thus playing a central role in many biological processes. Especially, several oncoproteins act as heat shock protein 90 client proteins and tumor cells require higher heat shock protein 90 activity than normal cells to maintain their malignancy. For this reason, heat shock protein 90 has emerged as a promising target for anti-cancer drug therapy (Hahn, 2009).

Targeting of heat shock protein 90 may act either as a direct anti-tumor agent, or as an enhancer of efficacy of chemotherapy and radiotherapy. Heat shock protein 90 inhibition preclinically using geldanamycin against glioma cells showed anti-tumor effects, and 17-AAG, a less toxic derivative of geldanamycin, was shown to inhibit tumor growth in glioma xenografts in mice (Yang et al., 2001). 17-AAG exhibits promising antitumor activity in clinical studies, but is limited by poor solubility and hepatotoxicity (Zhang et al., 2009).

Challenges Associated with Targeted Therapies

There are many controversies surrounding molecularly targeted therapies. Drug delivery across the blood-brain barrier is one of the vital problems, so the dilemma now is what is the most efficient way to deliver drugs to the brain (Sawyers, 2006)? Also, it is difficult for the drugs to reach the intraparenchymal regions in the brain beyond the resection cavity. This becomes a burden with micrometastases and infiltrative satellites that are commonly seen in glioma (Mercer et al., 2009).

Therefore, researchers developed convection-enhanced delivery as a novel way to deliver drugs across the blood-brain barrier as well as over a large volume of tissue. With convection-enhanced delivery, a catheter(s) is placed in a predetermined area of the brain using imaging techniques and the drug is infused over a period of hours to days. A constant pressure gradient created by an infusion pump permits the drug to be delivered homogeneously to circumscribed areas of brain tissue without causing significant tissue damage. This method has become especially relevant in brain tumors as it bypasses the blood-brain barrier and reduces systemic toxicities (Mercer et al., 2009).

Lastly, higher efficiency of targeted therapies will require technological advances that will improve response evaluations. The progress of technology will allow investigators to see whether treatment has true antitumor effects or not (Mercer et al., 2009).

The Future of Targeted Therapies

Targeting of a single signaling pathway in cancer may not be sufficient to combat tumor progression, and it has been theorized that targeting multiple growth factors may be beneficial (Reardon et al., 2006). Combination treatment regimen induced greater tumor cell apoptosis and cell cycle arrest, and reduced proliferation compared with single-agent therapy (Goudar et al., 2005).

The effects of these agents as a combination were greater than their effects as monotherapies. There will be a continuing effort to develop novel combinations of targeted therapies in the future using the estimated 214 possible drug combinations available at present. With this many combinations, there is infinite potential in targeted therapies that remain to be explored (Mercer et al., 2009).

IMMUNOTHERAPY

Immunotherapy gives the promise of targeting tumor cells for destruction with an excellent specificity and efficiency, while at the same time almost completely sparing normal cells from harm. The sensitivity and specificity of the immune system is refined to the point at which the body is capable of recognizing foreign pathogens within minutes of infection, responding with innate, humoral, and cellular effector mechanisms that can control a rapidly expansive infection, and eliminating almost all infected cell from the body. For more than 100 years, tumor immunologists have hoped to use this amazing cytotoxic power for use against malignant cancer cells (Mitchell et al., 2008).

Immunologic Treatments for Brain Tumors can take the form of (Mitchell et al., 2008):

"Passive immunotherapy: involving administration of antibodies or toxins to the patients without specifically inducing antitumor immune response in vivo (inside the patients).

"Active immunotherapy: involving immunization of the patients to induce specific antitumor immune response in vivo.

"Adoptive immunotherapy: involving expansion of effector cells (sensitized immune cells) ex vivo (outside the patients) and introducing of these effector cells to the patients (not commonly used nowadays).

Passive Immunotherapy

Passive immunotherapy may be defined as the transfer of immunity to a patient in the form of either antibodies (not developed by the patient) or targeted toxins (Stragliotto et al., 1996).

Potential tumor antigens fall into two categories consisting of 'tumor-specific' antigens, which are composed of antigens that are only found within the tumor and 'tumor-associated' antigens, which are expressed in other tissues but overexpressed within the tumor (Mercer et al., 2009).

Antibodies against tumor-specific antigens or tumor-associated antigens have been used as: [1] biologic response modifiers (Bigner et al., 1998) and [2] delivery system for other agents such as (a) radionucleotides, (b) plant or bacterial toxins, or (c) chemotherapeutic agents (Sampson et al., 2000).

Monoclonal Antibodies as Biologic Response Modifiers

Studies targeting epidermal growth factor receptor in brain tumors have generally met with disappointment, but a still promising use of antibodies as response modifiers involves targeting epidermal growth factor receptor mutant variant III (Sauter et al., 1996). In preclinical studies, targeting of this mutant variant with a single intratumoral injection of Y10 (anti-epidermal growth factor receptor mutant variant III monoclonal antibodies) increased the median survival of mice bearing epidermal growth factor receptor mutant variant III expressing tumors by an average of 286% (Sampson et al., 2000).

The most exciting and promising advance in the use of monoclonal antibodies as biologic response modifiers has been the use of monoclonal antibodies against vascular endothelial growth factor. Bevacizumab (Avastins®) is anti-vascular endothelial growth factor monoclonal antibodies. Recent studies examining the capacity to inhibit the growth of tumors by combination of bevacizumab and chemotherapy have shown promising response rates and evidence of prolongation of survival in recurrent glioblastoma (Vredenburgh et al., 2007).

Monoclonal Antibodies for Delivery of Radionucleotides

The specificity of an antibody for a tumor antigen allows its use as a delivery system for a variety of effectors that may be conjugated artificially to the antibody. These effector molecules are guided specifically to their tumor targets by the antibody specificity. Radionucleotides have been the most commonly utilized conjugate (Brady, 1998).

The radiolabeled antibody 81C6 was used in a number of clinical studies. Immunoreactivity and tumor-localizing capacity of 81C6 have been reported as superior to other anti-glioma monoclonal antibodies. It has demonstrated both safety and ability to increase survival in patients with leptomeningeal neoplasms, recurrent glioma, and newly diagnosed glioma. This latter application, in particular, has demonstrated promising results (Mitchell et al., 2008).

In 33 patients with newly diagnosed malignant glioma treated with 81C6 monoclonal antibodies into the surgically created resection cavity, there was a median survival of 86.7 weeks for all patients and 79.4 weeks for glioblastoma patients. These results were favorable in comparison with prognostic factor-matched historical controls and acceptable toxicity profile was observed during this trial. A phase III

study of the clinical efficacy of 81C6 monoclonal antibodies treatment is planned based on these encouraging data (Mitchell et al., 2008).

Immunotoxins

Plant and bacterial toxins represent alternative biologic effectors that may be conjugated to either antibodies or peptide ligands to produce either immunotoxins or fusion toxins, respectively (Frankel et al., 1995). These are designed to selectively deliver toxins into tumors. Once arrived at their target, toxins exhibit cytotoxicity. Such mechanisms give important advantages for targeted toxins over radiotherapy or chemotherapy (Hall and Fodstad, 1992). Therefore, cancer cells exhibiting resistance to both radiotherapy and chemotherapy may be susceptible to the cytotoxic effects of a targeted toxin (Mitchell et al., 2008).

The truncated form of pseudomonas exotoxin has also been used as a targeted toxin, which was conjugated to a circularly interleukin-4 (Rand et al., 2000). The use of interleukin-4 as the delivery system made benefit of a proposed enrichment of interleukin-4 receptor on the surface of glioma cells (Puri et al., 1994). The interleukin-4 toxin conjugate is capable of arresting tumor cell protein synthesis (Pastan et al., 1992). One patient remained disease-free more than 18 months after the procedure, and no systemic toxicities were observed. Seven of nine patients treated required craniotomy during protocol to relief increased intracranial pressure (Rand et al., 2000).

A similar line of preclinical work has been translated into clinical trials and includes targeting of the interleukin-13 receptors that are known to be expressed on a significant proportion of gliomas. A wide array of human glioblastoma cell lines expressing the interleukin-13 receptors were killed by a chimeric protein composed of human interleukin-13 and a mutated form of pseudomonas exotoxin (Debinski et al., 1995).

Recently published results of clinical trials using intracerebral convection-enhanced delivery of this chimeric protein showed capacity to induce tumor necrosis but no clinical benefit in over 50 treated patients with glioma (Kunwar et al., 2007). Recently completed phase III study of treatment of first recurrence of glioblastoma with convection-enhanced delivery of this chimeric protein compared with chemotherapy by carmustine wafers showed that the chimeric protein was comparable but not statistically superior to carmustine wafers (Mitchell et al., 2008).

Advantages and Disadvantages of Passive Immunotherapy

Results of passive immunotherapy trials have left reason for optimism. Indeed, they offer much improvement over current modalities in the field of specificity. However, a number of limitations still exist on the clinical efficacy of targeted treatment with antibodies. The main unresolved problem stems from the question whether treatments administered systemically can achieve clinically significant levels at the site of intracranial tumors. Sufficient levels may not be achieved with the dosage and systemic route of administration. It was demonstrated that 0.0006-0.0043% of the total injected dose localized to the intracerebral tumor following systemic intravenous administration; thus, sufficient targeting of intracranial tumors may require higher or more sustained doses of antibody in the peripheral blood. Clinical attempts to 'open up' the blood-brain barrier to drug delivery have met with both failure and toxicity (Mitchell et al., 2008).

Other problems that remain for systemic delivery are the high interstitial pressures in the tumor and surrounding tissue which prevent perfusion. Furthermore, systemic administration would require clearance by the liver and/or kidney. This would be problematic during delivery of conjugated radioisotopes or toxins, as it may concentrate the toxic substances in these organs (Jain, 1990).

As a result, antibodies are administered in almost exclusively loco-regional fashion, providing them specificity and enhanced targeting. Effectiveness of this regional mode of delivery has been enhanced by

the emergence of convection-enhanced delivery (Choucair et al., 1997).

Other challenges exist for the passive immunotherapy of gliomas. Even with a tumor specific antigen, the heterogeneity of these tumors makes it unlikely that targeting a single antigen would produce a curative therapeutic effect. In addition, the use of antibodies introduces a problem that is the antibodies themselves are antigenic. The development of human anti-human antibodies against the therapeutic antibodies is not infrequent event. This outcome may represent strict limitations regarding repeated use of the therapy (Mitchell et al., 2008).

Active Immunotherapy

Active immunotherapy defines a strategy of initiation of antitumor immune response in vivo following immunization with tumor antigen (Yu et al., 2001).

The ongoing development of vaccine-based immunotherapies offers a new hope for more effective low-toxicity targeted treatments. A successful vaccine for CNS cancers has been a 'holy grail' in neuro-oncology. Recent clinical trials involving vaccines against glioblastoma offer promising results. Other trials have inspired additional hope and optimism in the field of cancer immunotherapy (Ebben et al., 2009).

Peptide-Based Vaccines

Peptide-based vaccines represent a major focus of cancer vaccine research that offers exciting clinical possibilities. The underlying assumption of peptide-based cancer vaccines is similar to other conventional vaccinations. These vaccines are produced by introducing small peptides that are immunogenic and expressed by the targeted cancer cells (Fig. 6). These peptides are processed by host antigen presenting cells, which travel to the lymph nodes and sensitize cytotoxic T-lymphocytes to the targeted cancer cells (Yamanaka and Itoh, 2007).

The development of a peptide-based vaccine requires successful isolation of an effective immunogenic peptide that can be administered to induce an efficient in vivo patient immune response against the cancer. Furthermore, this peptide must be universally expressed by cancer cells, and must be in itself, crucial for tumor survival. This could be particularly challenging, given the heterogeneous nature of cancers, as well as the difficulty in isolating and identifying a particular peptide that demonstrates both specific restriction to cancer cells and efficiently stimulates an appropriate antitumor immune response (Ebben et al., 2009).

Figure 6. Peptide-Based Vaccines. CTLs: cytotoxic T-lymphocytes (Ebben et al., 2009).

Targeting Epidermal Growth Factor Receptors mutant variant III

An epidermal growth factor receptors mutant variant III-specific peptide has shown promise as an immunogenic antigen. This peptide provides a novel target for vaccine based therapies with a high degree of specificity to glioblastoma tumor cells. In a phase I clinical trial, glioblastoma patients were vaccinated with the epidermal growth factor receptors mutant variant III-peptide after surgical resection. This approach resulted in median time to progression of 46.9 weeks and overall median survival of 110.8 weeks giving similar results to current chemotherapy. The concern remains about the universality of this approach to effectively treat glioblastoma because not all glioblastomas express the epidermal growth factor receptors mutant variant III (Sampson et al., 2008). Although additional testing is needed in other models, vaccination against epidermal growth factor receptors mutant variant III could be of therapeutic value (Ebben et al., 2009).

Wilms' Tumor Peptide

Other tumor-specific antigens have been identified as Wilms' tumor peptide produced by Wilms' tumor gene. This peptide was first identified in renal cancers as Wilms' tumor, but has been also isolated in a number of haematopoietic and solid tumors. This gene is frequently overexpressed in glioblastoma, and its protein product has also demonstrated immunogenic properties, making it an attractive target for potential vaccines (Ebben et al., 2009).

A phase II clinical trial has been performed using the Wilms' tumor peptide for vaccination of patients with recurrent glioblastoma. Twenty-one patients with failed standard therapy were treated with intradermal injections of Wilms' tumor peptide weekly for at least 12 weeks. Two patients showed partial response, ten patients had stable disease and disease progressed in nine patients. No adverse effects of vaccination were noted (Izumoto et al., 2008).

Identifying Novel Antigens

The continual refinement and development of new tumor antigens is an essential component of the peptide-based vaccine development process because of the inherent molecular heterogeneity of tumors that has been one of the major challenges that hinder effective implementation of peptide-based therapeutic strategies. With the development of a more expansive library of target antigens, there is

hope that individualized, or personalized, peptide-based vaccines can be developed. The development of these vaccines would involve exposing the immune system to multiple tumor antigens, to evaluate their immunogenicity to identify a particular immunogenic cocktail that will be most effective for the given patient's tumor (Itoh and Yamada, 2006).

In the case of pediatric glioma, three promising tumor antigens were recently identified; ephrin receptor EphA2, interleukin-13 receptor ?2, and survivin (Okada et al., 2008).

Another set of potentially fruitful targets is tumor angiogenesis-associated antigens. In a phase I trial, six patients with recurrent malignant brain tumors was vaccinated with human umbilical vein endothelial cells. MRI showed partial or complete resolution of enhancing cancer masses in three patients. This may represent a promising strategy. More research is needed to determine whether this strategy promotes improved patient survival (Okaji et al., 2008).

Dendritic Cell-Based Vaccines

Another strategy used to induce an immune response to CNS malignancy involves vaccination with patient dendritic cells that have been treated with various tumor components. Dendritic cells are specialized antigen presenting cells. In their immature state, they show high ability to capture and process antigens of all types. After antigen capture, they mature and become excellent antigen presenting cells (Banchereau and Steinman, 1998).

Several methods have been reported for stimulating or charging dendritic cells with tumor antigens, including: (1) pulsing of dendritic cells with tumor peptides, (2) fusion of dendritic cells with tumor cells, and (3) loading of dendritic cells with tumor lysate, apoptotic tumor cells or tumor messenger RNA (Steinman and Dhodapkar, 2001).

Fusion of dendritic cells with tumor cells (Fig. 7) showed no adverse effects with this approach (Kikuchi et al., 2001), and a second trial with co-administration of interleukin 12 showed partial radiographic response in 3 of 12 patients (Kikuchi et al., 2004).

Figure 7. Fusion of dendritic cells with tumor cells: DCs: dendritic cells, CTLs: cytotoxic T-lymphocytes (Ebben et al., 2009).

Loading of dendritic cells with tumor antigen followed by vaccination of the patient with the loaded cells has also shown potential efficacy as a method of overcoming chemotherapy resistance, a serious problem that is frequently associated with higher grade gliomas and glioblastoma (Liu et al., 2006).

Viral Vaccination Strategies

In an effort to expand the library of target antigens, a novel technique has been developed. Intratumoural vaccination with herpes simplex virus has been demonstrated to generate an immune response by cytotoxic T-lymphocytes. After intratumoral injection of herpes simplex virus into a mouse glioma model, a new glioma-specific immunogenic antigen was identified and isolated for study in future therapeutic strategies (Iizuka et al., 2006).

Tumor was obtained from 23 patients and infected with Newcastle disease virus. The tumor cells were then irradiated and returned to the patient via subcutaneous injection on a set schedule. Treated patients had a longer time to progression and longer median survival than those not receiving this therapy. Observation of resected tumors from patients with recurrent disease revealed that the vaccine induced an antitumour immune response (Steiner et al., 2004).

Heat-Shock Protein Vaccination

It is theorized that while each patient may have unique tumor antigens, targeting of autologous heat-shock proteins that are coupled with these unique tumor antigens offers a method of targeting cancer cells without the need to identify these unique tumor antigens (Przepiorka and Srivastava, 1998). This technique relieves clinicians from relying on a library of targets that may not be complete or may not contain relevant antigens (Amato, 2007). Studies of heat-shock protein vaccination were found to effectively increase number of active natural killer cells. This carries important implications due to the vital role of natural killer cells in the antitumor immune response (Qiao et al., 2008).

Current Vaccines

There are several promising agents under investigation in different phases:

CDX-110 TM:

This is a peptide-based vaccine that targets epidermal growth factor receptors mutant variant III. It is given intradermally. The therapy was well tolerated and time to progression from surgery was an impressive 12 months. The median survival exceeded 18 months (Heimberger et al., 2006).

DCVax TM:

This vaccine utilizes autologous dendritic cells generated by leukopheresis (separation of white blood cells) with antigenic peptides derived from patient's own tumor. In phase I study, treatment was well tolerated. There were hematologic and local evidences of antitumor response but no objective clinical response or survival benefit were found (Liau et al., 2005). Currently, a Phase II randomized trial targeting 141 patients with newly diagnosed glioblastoma is ongoing (Das et al., 2008).

Oncophage TM:

This vaccine is based on isolation of the heat-shock proteins from the patient's surgical specimen. This is then delivered intradermally starting 2-8 weeks after surgery. This therapy was well tolerated and laboratory studies found evidence of a tumor-specific immune response that correlates with favorable clinical response to therapy (Das et al., 2008).

Poly-ICLC:

This vaccine is a double-stranded ribonucleic acid and stimulates the immune system broadly. In phase I trial, therapy was well tolerated and 66% of patients had an objective response and median survival for glioblastoma patients was 19 months (Das et al., 2008).

Adoptive Immunotherapy

Adoptive immunotherapy involves treatments that include transfer of immune cells manipulated in some fashion ex vivo (outside patients) to patients. Treatment approaches have differed in the types of cells administered, the route of administration, and the activation status of the cells. Cell types that have been used in adoptive immunotherapy include (Mitchell et al., 2008):

(i)&#9;Peripheral blood mononuclear cells.

(ii)&#9;Lymphokine-activated killer cells.

(iii)&#9;Mitogen-activated killer cells.

(iv)&#9;Tumor infiltrating lymphocytes.

(v)&#9;Antigen-specific and unselected.

Routes of administration are generally either systemic or into the tumor cavity. Infusing T-cell populations previously activated against specific tumor antigens is one strategy of adoptive T-cell therapy (Mitchell et al., 2008).

Immunotherapy and Brain Tumor Blood Vessels

A stimulating new approach to immunotherapy for tumors involves the immunologic targeting of tumor endothelial cells for destruction instead of the targeting of the tumor itself. Tumor endothelial cells express distinct antigenic markers that can be exploited in vaccine strategies (Wei et al., 2002).

It is currently being investigated whether strategies involving transfer of tumor endothelium-specific T cells can effectively inhibit tumor growth through cutting off of blood supply. There are several advantages to this approach: (1) the potential for killing many tumor cells per effector lymphocyte, (2) the ability to avoid escape mutations, (3) the lack of concern of T-cell movement across the blood-brain barrier because the endothelial targets are in contact with the blood, and (4) such approach may overcome the limitations of cytostatic angiogenic therapy, in which escape mechanisms frequently occur (Mitchell et al., 2008).

Immunotherapy and Brain Tumor Stem Cells

The brain tumor stem cells are widely believed to be responsible for tumor propagation and resistance to conventional therapies. Some anti-tumor approaches may spare brain tumor stem cells. Therapies targeting molecular properties that define these cells have a greater biologic impact and reduce the need for intensive conventional therapy (Liu et al., 2006).

Although brain tumor stem cells appear resistant to radiation and chemotherapy, their susceptibility to immunotherapy has not been formally tested. A recent study in a glioma model has shown that the immunologic targeting of brain tumors using dendritic cells pulsed with lysates from tumor cells grown in stem-cell culture media was more efficacious than dendritic cells pulsed with tumor lysates from cells grown in standard culture, suggesting that targeting of brain tumor stem cells may be of therapeutic

benefit. Thus, identification of antigens expressed in brain tumor stem cells and development of immunotherapy that effectively targets these brain tumor stem cells is a promising approach for treatment of brain tumors (Mitchell et al., 2008).

New Hope in Immunotherapy

The recent advances carry great promise for development of immunotherapies that will lead to significant and reliable antitumor responses. They also hold significant promise for having a favorable impact on patient survival. Although immunotherapy did not emerge as a single-modality treatment for brain tumors, it will take its place as a very effective adjuvant therapy (Mitchell et al., 2008). Dendritic cells and peptide-based therapies appear promising as an approach to successfully induce antitumor immune response and prolong survival. Dendritic cells and peptide-based therapies seem to be safe and without major side effects. Its efficacy should be determined in randomized and controlled clinical trials (Yamanaka, 2009).

An effective vaccine will be a potent addition to the current therapeutic approaches against brain tumors. Significant progress has been made that offers new hope and novel therapeutic opportunities. It is important to realize that much additional work lies ahead before an effective brain tumor vaccine is validated for clinical use (Ebben et al., 2009).

ANTIANGIOGENIC THERAPY

Angiogenesis, the growth of new blood vessels from pre-existing vessels, is considered to be the key regulating factor of vascular development of brain tumors and especially for glioblastoma multiforme. The development and growth of malignant glioma seems to be dependent on angiogenesis, since the microvascular proliferation can only be observed in high grade gliomas. Additionally, the aggressiveness and clinical recurrence of malignant gliomas are closely related to the degree of neovascularization (Argyriou et al., 2009).

The discovery that tumor progression depends on angiogenesis led to the development of anti-angiogenic therapy for cancer (Kerbel, 2008). A large body of preclinical evidence suggests that anti-angiogenic therapy may be effective in treating gliomas, and preliminary clinical data suggest that anti-angiogenic agents have a beneficial effect in patients with gliomas (Norden et al., 2008).

Currently, anti-angiogenic molecularly targeted therapies including administration of monoclonal antibodies or tyrosine kinase inhibitors are being increasingly adopted for treating glioblastoma (Argyriou et al., 2009).

Furthermore, antiangiogenic agents have potent anti-edema (Fig. 8 and 9) and steroid-sparing effects in patients. Emerging data suggests that these drugs may modestly improve progression-free survival (Chi et al., 2009).

Vascular Endothelial Growth Factor Pathway Inhibitors

Because of vascular endothelial growth factor prominent role in glioma angiogenesis, its pathway was rapidly identified as an attractive therapeutic target. Thus; agents that target the vascular endothelial growth factor pathway have become the most clinically advanced anti-angiogenic drugs. Vascular endothelial growth factor-targeting approaches include strategies that target vascular endothelial growth factor ligand and receptors (Chi et al., 2009).

Vascular Endothelial Growth Factor Ligand Inhibitors

Bevacizumab (Avastin®) is a humanized monoclonal antibodies against vascular endothelial growth factor ligand. Aflibercept is a soluble vascular endothelial growth factor decoy receptor (vascular

endothelial growth factor Trap). Both of them are vascular endothelial growth factor sequestering molecules (hide and isolate the factor) and characterized by high specificity (Chi et al., 2009).

In the first reported prospective study of an anti-vascular endothelial growth factor therapy in malignant glioma, high radiographic response and 6-month progression-free survival proportions were observed with the combination of bevacizumab and conventional chemotherapy (Vredenburgh et al., 2007).

In a phase II clinical trial, 68 patients (33 recurrent anaplastic glioma and 35 recurrent glioblastoma) were treated with bevacizumab and chemotherapy (irinotecan). Radiographic response (Fig. 8) proportions of 57% for recurrent glioblastoma and 61% for recurrent anaplastic glioma patients were observed. Responses were also associated with neurological improvement and reduction or discontinuation of corticosteroid requirements. Although treatment was generally well tolerated, toxicity was observed. There were thromboembolic complications in 8 of the 68 patients (12%), and 2 patients (3%) had CNS hemorrhages (Wagner et al., 2008).

Additionally, several retrospective studies have reported similar findings with bevacizumab in recurrent malignant glioma. Radiographic response proportions between 35% and 50% have been observed with the combination of bevacizumab and conventional chemotherapy in several case series with responses often occurring rapidly after treatment initiation (Guiu et al., 2008). These studies also reported delayed tumor progression, suggesting that clinical benefits were derived. One study reported that 6-month progression-free survival was 42% for recurrent glioblastoma patients, and apparent antiedema effect of bevacizumab was evident, in that 33% of patients reduced their corticosteroid requirements. Treatment was generally well tolerated in these retrospective series, although thromboembolic and hemorrhagic complications were reported (Norden et al., 2008).

All of these studies were of bevacizumab in combination with cytotoxic chemotherapy. Bevacizumab was most often combined with irinotecan in these studies, and irinotecan is known to have minimal efficacy in recurrent malignant glioma patients (Batchelor et al., 2004). How much chemotherapy contributed to the effect of bevacizumab and whether bevacizumab had single-agent activity remained unanswered. To address these questions, a phase II trial randomized 167 recurrent glioblastoma patients to receive either bevacizumab alone (10 mg/kg every 2 weeks) or in combination with irinotecan. The radiographic response and 6-month progression-free survival proportions were 37.8% and 50.3% respectively, with combination therapy. These were similar to the proportions seen with bevacizumab alone (28.2% and 42.6%, respectively). In addition, the median overall survival for the combination therapy (8.7 months) was similar to that observed with bevacizumab alone (9.2 months). In this study, most patients were able to reduce their corticosteroid dose by at least 50%, and treatment was well tolerated (Cloughesy et al., 2008).

Figure 8. A 44-year-old man with recurrent glioblastoma (A) treated with bevacizumab and irinotecan: showing marked reduction in enhancement after 4 weeks of therapy (B) (Cavaliere et al., 2007).

Another recently reported phase II study evaluated the benefit of bevacizumab monotherapy in recurrent glioblastoma patients. In 48 patients, the authors observed a 35% radiographic response and a 6-month progression-free survival of 29%. Furthermore, when irinotecan was added to bevacizumab at progression in 19 patients, there were no objective radiographic responses. These two studies suggest that irinotecan adds little, if any, benefit to bevacizumab (Kreisl et al., 2009).

Aflibercept is a soluble vascular endothelial growth factor decoy receptor that consists of a vascular endothelial growth factor receptor fused to an immunoglobulin constant region. It has vascular endothelial growth factor receptors binding affinity several hundred times greater than bevacizumab (Holash et al., 2002). An ongoing phase II clinical trial of 48 recurrent malignant glioma patients treated with aflibercept monotherapy reported response proportions of 50% for anaplastic glioma and 30% for glioblastoma patients, values similar to the response proportions observed in bevacizumab trials. There was moderate toxicity; however, treatment was discontinued in 12 patients (25%), on average less than 2 months into therapy (De Groot et al., 2008).

 Vascular Endothelial Growth Factor Receptor Inhibitors

Currently many vascular endothelial growth factor receptors-targeted tyrosine kinase inhibitors are in clinical trials for malignant gliomas. These small molecule inhibitors were initially developed as specific inhibitors of the vascular endothelial growth factor receptors, but they have the capacity to inhibit many other receptors such as the platelet-derived growth factor receptor. Although this lack of specificity may result in more off-target effects, the potential for simultaneous inhibition of several proangiogenic pathways may be a favorable feature (Karaman et al., 2008).

Cediranib is potent pan-vascular endothelial growth factor receptors tyrosine kinase inhibitors with modest activity against platelet-derived growth factor receptors. In a recent phase II trial of cediranib monotherapy in recurrent glioblastoma patients, radiographic response (Fig. 9) was observed in 56% of patients and the 6-month progression-free survival was 26%. There was also a modest improvement in median overall survival in cediranib-treated patients relative to a historical database. Furthermore, an anti-edema effect (Fig. 9) was detected with cediranib monotherapy and corticosteroid requirements

were discontinued or decreased in 15 of 16 patients. Toxicity was moderate; only 2 of 31 patients were removed from the study because of toxicity and no treatment-related deaths or intracranial hemorrhages occurred, a high frequency of hypertension was observed. Fatigue and diarrhea were also frequent toxicities (Batchelor et al., 2008).

As a result of this study, a phase II study of cediranib with chemotherapy and radiation in newly diagnosed glioblastoma has been launched, and a multicenter, randomized phase III trial in recurrent glioblastoma is underway, comparing cediranib versus cediranib plus chemotherapy (Chi et al., 2009).

Vatalanib, an inhibitor of the vascular endothelial growth factor receptors and platelet derived growth factor receptors tyrosine kinases, has been clinically investigated in several advanced solid tumors (Thomas et al., 2005). Several phase I/II clinical trials have studied vatalanib in recurrent glioblastoma patients, either as monotherapy (Conrad et al., 2004) or in combination with chemotherapy (Reardon et al., 2004). Clinical benefits were limited in these studies. Response proportions ranged between 4% and 8%, and progression-free survival was not significantly higher than historical benchmarks. These results may have been affected by suboptimal dosing. More recently, the combination of vatalanib, the platelet derived growth factor receptors inhibitor imatinib, and chemotherapy resulted in a modest response proportion (22%) in a phase I trial of 37 recurrent malignant glioma patients (Kirkpatrick et al., 2008).

Figure 9. A 47-year-old woman with recurrent glioblastoma treated with the pan-vascular endothelial growth factor receptors inhibitor (cediranib): showing marked reduction of T1-contrast enhancement (upper row) and peri-tumoral edema on FLAIR images (lower row) over 56 days (Cavaliere et al., 2007).

The broad-spectrum tyrosine kinase inhibitors sorafenib and sunitinib have the capacity to inhibit a number of tyrosine kinases including vascular endothelial growth factor receptors and platelet derived growth factor receptors (Morabito et al., 2006). Both agents are being studied in early phase clinical trials for recurrent malignant glioma. Sorafenib is being evaluated as monotherapy and in combinations with the mammalian target of rapamycin inhibitor temsirolimus and epidermal growth factor receptor inhibitor erlotinib. Sunitinib is being studied as monotherapy and in combination with chemotherapy (irinotecan). Mature data have yet to be reported, although to date these agents appear to be only moderately well tolerated. In addition, several other vascular endothelial growth factor receptors inhibitors are in clinical trials for recurrent malignant glioma as single agents or in combination regimens (Chi et al., 2009).

Non-Vascular Endothelial Growth Factor Pathway Inhibitors

Epidermal Growth Factor Receptor Inhibitors

Cetuximab targets the extra cellular domain of the epidermal growth factor receptors. The clinical relevance of this effect is currently under investigation in patients with recurrent glioblastoma (Sadones et al., 2006). A phase I/II study investigating the efficacy and safety of cetuximab plus concomitant chemoradiotherapy is currently ongoing (Argyriou et al., 2009).

Treatment options which target epidermal growth factor receptors mainly include the administration of gefitinib and erlotinib. Gefitinib (500-1,000 mg orally) has been tested in a phase II trial assessing 57 patients with first recurrence of a glioblastoma. Gefitinib yielded a modest degree of efficacy, while no neuroimaging response was observed. The 6-month progression-free survival was 13%, and the median overall survival time was 39.4 weeks (Rich et al., 2004). Likewise, limited activity was observed in another phase II trial of gefitinib assessing patients with malignant glioma recurrence following surgery plus radiotherapy and chemotherapy. The 6-month progression-free survival was 14.3% and the median overall survival was 24.6 weeks (Franceschi et al., 2007). Considering these data, monotherapy with gefitinib is not superior in terms of efficacy over conventional chemotherapy (Argyriou et al., 2009).

Erlotinib seems to influence the survival rates of patients with malignant glioma to a similar degree. In a multicentre phase I trial, 19 patients with glioblastoma were treated with erlotinib plus radiation therapy, median overall survival was 55 weeks (Krishnan et al., 2006). In another study, 83 patients with stable or progressive malignant glioma received monotherapy with erlotinib or in combination with chemotherapy (temozolomide). Eight of 57 finally assessed patients had a partial response and 6 had a progression-free survival of longer than 6 months (Prados et al., 2006).

Overall, therapy with erlotinib or gefitinib is associated with modest therapeutic efficacy in only few patients with malignant gliomas. Therefore, biomarkers reliably predicting the response of gliomas to these agents need to be identified (Argyriou et al., 2009).

Platelet Derived Growth Factor Receptor Inhibitors

The platelet derived growth factor pathway plays an important role in angiogenesis and thus represents a particularly attractive therapeutic target (Jain et al., 2007). Clinical trials evaluating the platelet derived growth factor receptors inhibitor imatinib as a single agent in recurrent malignant glioma patients have been disappointing. The 6-month progression-free survival rate was 16% for glioblastoma (Raymond et al., 2008). Combination of imatinib with chemotherapy (hydroxyurea) in several studies reported promising 6-month progression-free survival proportions in recurrent malignant glioma patients (24%-32%) (Desjardins et al., 2007). However, a recent large multicenter study failed to confirm these findings (Dresemann et al., 2008).

The newer platelet derived growth factor receptors inhibitors, tandutinib and dasatanib, have potentially greater efficacy in malignant glioma patients, due to improved CNS penetration, and they are in clinical trials for recurrent malignant glioma (Chi et al., 2009).

Fibroblast Growth Factor Inhibitors

The fibroblast growth factor pathway is an important proangiogenic pathway. Recently, the fibroblast growth factor pathway has been implicated in resistance to vascular endothelial growth factor receptors-targeted therapy (Bergers and Hanahan, 2008). Strategies targeting fibroblast growth factor had limited efficacy in malignant glioma patients in earlier trials as the inhibitors used in these studies were relatively nonspecific (Chi et al., 2009).

Thalidomide, an inhibitor of both fibroblast growth factor and vascular endothelial growth factor, was

shown to have minimal activity as monotherapy or in combination with chemotherapy (carmustine or temozolomide) (Baumann et al., 2004). The more potent thalidomide analog lenalidomide has better tolerability; however, there was little suggestion of efficacy, either as a single agent or in combination with radiotherapy. There were no radiographic responses with lenalidomide monotherapy in 24 recurrent glioblastoma patients (Fine et al., 2007) and one response in 20 newly diagnosed glioblastoma patients treated with lenalidomide in combination with radiotherapy (Drappatz et al., 2009). Toxicities associated with this class of agents, which include overlapping hematologic toxicity between lenalidomide and chemotherapy, may limit further clinical development (Chi et al., 2009).

Several other inhibitors of the fibroblast growth factor pathway had limited efficacy in clinical trials of malignant glioma patients, including interferon-?, interferon-? and suramin. Fibroblast growth factor receptor tyrosine kinase inhibitors with greater specificity for the fibroblast growth factor pathway were developed. These drugs, which include tyrosine kinase inhibitors 258, XL-999, brivanib and BIBF1120, have potential utility in malignant glioma patients (Chi et al., 2009).

Protein Kinase C Inhibitors

Signaling through the protein kinase C pathway plays an important role in angiogenesis. The protein kinase C inhibitor, enzastaurin, was evaluated as monotherapy in a phase III randomized trial, compared against chemotherapy (lomustine), in recurrent glioblastoma, but the study was stopped because efficacy milestones were not met (Fine et al., 2008).

 Cyclooxygenase-2 (COX-2) Inhibitor

Cyclooxygenase-2, which promotes the expression of proangiogenic factors (Iniguez et al., 2003) has been targeted in recurrent malignant glioma patients with the selective cyclooxygenase inhibitor celecoxib. However, only modest 6-month progression-free survival of 19% and 25% was observed when celecoxib was combined with chemotherapy (irinotecan) (Reardon et al., 2005) and 13-cis-retinoic acid, respectively (Levin et al., 2006).

Hepatocyte Growth Factor/Scatter Factor Inhibitors

The hepatocyte growth factor/scatter factor pathway is another important mediator of glioma angiogenesis (Abounader and Laterra, 2005). There has been increased interest in targeting hepatocyte growth factor/scatter factor and its tyrosine kinase receptor in malignant glioma. Drugs in clinical development that target this pathway include XL184 and AMG 102 (a human monoclonal antibody against hepatocyte growth factor/scatter factor) (Chi et al., 2009).

Endothelial Cell Migration Inhibitors

The ???3 and ???5 integrins (cell surface receptors) promote endothelial cell migration and survival during angiogenesis and represent attractive therapeutic targets (Avraamides et al., 2008).

Cilengitide, the competitive ???3 and ???5 integrin inhibitor, has demonstrated modest activity in recent clinical trials in malignant glioma patients. In a phase II trial, 81 patients with recurrent glioblastoma were treated with cilengitide in two dose groups. Although the drug was well tolerated, cilengitide exhibited modest antitumor activity. A radiographic response of 13% and a 6-month progression-free survival of 15% were observed in the higher dose group, with 2000 mg twice weekly (Reardon et al., 2008).

Another phase II trial reported promising results using cilengitide in newly diagnosed glioblastoma in which 81 patients received either cilengitide in addition to standard therapy (radiotherapy and chemotherapy) or standard therapy alone. A 6-month progression-free survival of 65.4% in patients treated with cilengitide was significantly higher than 6-month progression-free survival with standard

therapy alone (53.6%). Cilengitide toxicity was minimal. Toxicity was similar in both arms of the study (Stupp et al., 2007). In addition, a clinical study observed good tumor penetration after intravenous drug administration (Gilbert et al., 2007). Based on these encouraging results, a multicenter randomized phase III trial is underway using cilengitide in newly diagnosed glioblastoma (Chi et al., 2009).

Metronomic Chemotherapy

Metronomic chemotherapy is a conventional chemotherapy administered at low doses. It targets mainly tumor vasculature and prevents tumor growth (Kerbel and Kamen, 2004). Preclinical studies suggest that certain conventional chemotherapeutic agents can function as anti-angiogenic drugs when administered at low doses on a continuous or frequent schedule (Gille et al., 2005). In glioma animal models, metronomic chemotherapy is an effective anti-angiogenic strategy (Kim et al., 2006). Studies evaluating metronomic chemotherapy reported favorable toxicity profiles, but no significant survival benefits were observed (Chi et al., 2009). Recently, two reports evaluating metronomic chemotherapy in recurrent glioblastoma documented modest gains in 6-month progression-free survival (Perry et al., 2008). Clinical trials evaluating the addition of bevacizumab to metronomic chemotherapy are ongoing (Chi et al., 2009).

Current Challenges

A number of issues remain unresolved concerning the clinical use of anti-angiogenic drugs, including: (1) toxicity profiles, (2) radiographic assessment, (3) biomarkers, and (4) resistance (Chi et al., 2009).

Toxicity profiles

The risks of hemorrhage and thrombosis have been ongoing concerns with the use of antiangiogenic therapy. Cumulative toxicity data indicate that major systemic bleeding is rare, but risk of epistaxis is increased. The intracranial hemorrhage risk appears to be low, and events are often asymptomatic (Norden et al., 2008). Although some reports suggest an increased risk of thromboembolic events in patients treated with bevacizumab, this risk is difficult to assess, as patients with glioblastoma have an intrinsically higher risk of thrombosis (Chi et al., 2009).

It appears that some toxicities are shared by all inhibitors of angiogenesis, but certain toxicities are associated with specific classes of anti-angiogenic agents. Toxicities common to both vascular endothelial growth factor antibodies and vascular endothelial growth factor receptors tyrosine kinase inhibitors include fatigue and hypertension. Impaired wound healing is observed, and may be problematic in treating glioblastoma patients immediately after surgery (Lai et al., 2008). Hemorrhage and gastro-intestinal perforation are less frequent. With tyrosine kinase inhibitors, skin toxicity, hypertension, diarrhea, and mucositis are more common, whereas proteinuria is more frequent with bevacizumab. Other rare but serious complications include myocardial infarction, stroke, posterior leukoencephalopathy, and thrombotic thrombocytopenic purpura (Chi et al., 2009).

Radiographic assessment

Many studies reported high radiographic response proportions in patients treated with anti-angiogenic agents. Obtaining accurate assessment of response to anti-angiogenic therapy is problematic in brain tumor patients (Sorensen et al., 2008). Standard response criteria currently in use for assessing treatment response are dependent on contrast-enhancement on CT or MRI (Macdonald et al., 1990). However, contrast-enhancement reflects vascular endothelial growth factor mediated blood-brain barrier dysfunction and may not be a reliable representation of the underlying tumor. Moreover, anti-vascular endothelial growth factor agents decrease permeability of cerebral vessels and diminish the contrast enhancement making it difficult to distinguish the anti-tumor effects of treatment from the effects of these drugs on blood vessel permeability (Batchelor et al., 2007).

Biomarkers

There is a need for validated circulating or tissue biomarkers that can accurately predict anti-angiogenic efficacy and indicate progression. Several blood and tissue components have been identified as candidate biomarkers (Duda et al., 2007). A recent study suggests that vascular endothelial growth factor protein expression in tumor tissue may be predictive for radiographic response. This study reported tumor expression of hypoxia-induced carbonic anhydrase 9 associated with shorter survival (Sathornsumetee et al., 2008). In recurrent glioblastoma patients, radiographic tumor progression during cediranib treatment was associated with elevated levels of fibroblast growth factor and circulating endothelial cells. That study and others have shown that serum vascular endothelial growth factor is significantly elevated in patients being treated with anti- vascular endothelial growth factor therapy. These serum markers have potential utility as pharmacodynamic or pharmacokinetic biomarkers (Chi et al., 2009).

Resistance

Clinical data suggest that benefits gained from anti-angiogenic agents will be short-lived. Almost all patients treated with anti-angiogenic therapy progress during treatment, as tumors finally acquire resistance. After failure of treatment with bevacizumab, tumor progression is usually rapid (Quant et al., 2009).

Several mechanisms of resistance to angiogenesis inhibitors have been proposed in glioblastoma. Tumors may induce revascularization by activating alternate proangiogenic pathways (Bergers and Hanahan, 2008). In recurrent glioblastoma, patients who progressed during cediranib treatment had elevated levels of circulating mediators of alternative proangiogenic pathways (Batchelor et al., 2007). A second mechanism of resistance involves the recruitment of vascular progenitor cells and proangiogenic monocytes from the bone marrow (Murdoch et al., 2008). Hypoxia, by inducing hypoxia-inducible factor attracts to the tumor a heterogeneous population of bone marrow-derived cells that facilitate neovascularization (Du et al., 2008).

Other studies raise the possibility that tumors may not develop resistance per se. Treatment failure may be due to inability of anti-vascular endothelial growth factor drugs to control the continued growth of the tumor (Chi et al., 2009).

Future Directions

Current evidence suggests that anti-angiogenic therapy has clinical benefits including steroid-sparing effects and increased progression-free survival. A definite survival advantage has yet to be established. Several issues and obstacles remain in the clinical development of anti-angiogenic agents. Prospective randomized controlled trials that use survival as an endpoint will be required to determine whether anti-angiogenic strategies increase survival of patients. Furthermore, clinical trials with correlative imaging and molecular studies will be critical to overcome challenges (Chi et al., 2009).

STEREOTACTIC RADIOSURGERY

There are three fundamental strategies of delivering the ionizing radiation to any intracranial neoplasm: (1) Stereotactic radiosurgery (2) Stereotactic radiotherapy (3) Fractionated radiation therapy. The essential difference between these strategies is in number of fractions or doses that the physician uses to deliver the total dose to the patient. In stereotactic radiosurgery, the total dose is typically delivered in one fraction, although the current definition permits a maximum of 5 fractions. In stereotactic radiotherapy and in fractionated radiation therapy, tens of fractions may be required, depending on the total dose to be delivered, and they are differentiated only by the fact that stereotactic radiotherapy is delivered stereotactically (Barnet et al., 2007).

Stereotactic (stereotaxis) refers to the technology of precisely localizing a target within the 3 dimensional space (Barnet et al., 2007).

Radiosurgery signifies any kind of application of ionizing radiation energy aiming at precise and complete destruction of chosen target structures containing healthy and/or pathological cells, without significant concomitant or late radiation damage to adjacent tissues (Larsson, 1992).

Stereotactic radiosurgery is performed by Cobalt-60 gamma source unit (Gamma Knife) or by Linear accelerators system. Masks or cranial pin frames provide immobilization. Preoperative imaging is paramount (Thng et al., 1999).

Gamma knife and Linear accelerator systems are radiosurgical equipments to treat predetermined intracranial targets through the intact skull without damaging the surrounding normal brain tissue. The technical developments allow more precise control of intracranial lesions and offer more conformal dose plans for irregular lesions (Vesper et al., 2009).

Current Radiosurgical Techniques

Gamma Knife (Models A, B, C, 4-C, and PFX®)

Gamma Knife uses multiple fixed cobalt sources inside a helmet, which are arranged to intersect at a given point. A head frame is attached to the patient's skull and the patient is positioned within the helmet so that the target corresponds to the focal point of the device (Fig. 10) (Mayberg and Vermeulen, 2007). The latest model, Leksell Gamma Knife® PERFEXION® (PFX®), was released in 2006 (Fig. 11) (Niranjan et al., 2007).

The disadvantages of Gamma Knife may be (1) the need to attach the frame to the skull, (2) limitation of treatment to intracranial lesions above the foramen magnum, and (3) the inability to fractionate or divide the dose (Mayberg and Vermeulen, 2007).

Figure 10. Gamma-Knife principle (Mayberg and Vermeulen, 2007).

Figure 11. Photograph of the world's first PerfeXion Gamma Knife (Elekta Instrument AB, Stockholm, Sweden) installed at Timone University Hospital (Régis et al., 2009).

Linear accelerator-Based Radiosurgery

There are many Linear accelerator based systems such as XKnife®, Linear Accelerator Scalpel®, Novalis®, the Peacock System®, and CyberKnife® (Niranjan et al., 2007).

CyberKnife uses a linear accelerator attached to an industrial robot to precisely define the target tissues and adjacent critical structures, and create a highly conformal dose (Fig. 12). CyberKnife radiosurgery can be utilized for lesions anywhere in the body. The patient does not need rigid frame fixation and the doses can be fractionated to provide additional safety. On the other hand, CyberKnife has not been in widespread use as many years as Gamma Knife and follow-up data is not as well developed for this modality (Mayberg and Vermeulen, 2007).

Table. 1. A comparison of Gamma Knife and CyberKnife Radiosurgery (Niranjan et al., 2007).

Radiosurgery for Brain Metastasis

Figure 12. The CyberKnife: consists of a linear accelerator mounted on a six-axis robotic manipulator that permits a wide range of beam orientations (Niranjan et al., 2007).

Stereotactic radiosurgery with a Gamma Knife or a linear accelerators system is now being used as primary management or booster treatment with whole brain radiation therapy in an increasing number of patients with brain metastasis, both radiosensitive and radioresistant, single and multiple. Radiosurgery became relatively routine, rather than special, 'high-tech' treatment (Yamamoto, 2007).

Although the size limitation on treatable lesions is crucial in radiosurgery, tumor control rates of 90%, or even better, can be expected if 1-4 lesions are irradiated with a peripheral dose of 20 gray (Gy) or more. In such cases, true recurrence is extremely rare. Postradiosurgical MRI reveals disappearance of the tumors and complete cure, as confirmed by autopsy. The incidence of radiosurgery-related complications, i.e. symptomatic radionecrosis of the normal brain, is no more than 3.0%. Prolonged survival cannot be expected because survival duration depends primarily on nonbrain lesions (including the primary tumor). 80-90% of patients die of causes other than brain tumor progression (Pollock and Brown, 2002).

Controversy persists as to whether stereotactic radiosurgery should be combined with whole brain radiation therapy. Although some reports have supported combining treatment with whole brain radiation therapy to achieve optimal tumor control and survival periods (Chidel et al., 2000), others have questioned usefulness of whole brain radiation therapy (Sneed et al., 2002). Neither the survival rate nor the local tumor recurrence rate differs significantly between groups with vs. without whole brain radiation therapy (Yamamoto, 2007).

Retrospective analysis of 619 patients underwent linear accelerators-based stereotactic radiosurgery for 1569 brain metastases between 1989 and 2006 revealed encouraging results. The median survival was 7.9 months. 1- and 2-year survival probabilities were 0.36 and 0.14, respectively. Factors that were associated with improved survival are: 1) female sex, 2) younger age, 3) higher Karnofsky performance status, 4) controlled primary tumor, 5) absence of systemic metastases, 6) asynchronous presentation of brain metastases, 7) fewer brain metastases, 8) smaller total volume of brain metastases, 9) surgery prior to radiosurgery, and 10) multiple radiosurgical treatments. Local control was achieved in 84.3% of all lesions treated. Linear accelerator-based stereotactic radiosurgery was a safe and effective treatment for patients with brain metastases. Selection of patients who are likely to benefit most from stereotactic radiosurgery is complex and treatment decisions should be based on the entire clinical picture (Swinson and Friedman, 2008).

Frameless stereotactic radiosurgery (linear accelerators system) has been used for treatment of 53 patients with 158 brain metastases. The radiation doses were delivered in a single fraction (dose range, 12-22 Gy). The overall survival rate was 70% at 6 months, 44% at 1 year, 29% at 18 months, and 16% at 24 months. Local control was achieved in 90% at 6 months, 80% at 12 months, 78% at 18 months, and 78% at 24 months. Local control tended to be improved in lesions treated with >or=18 Gy and for lesions <0.2 cm. Adverse events occurred in 5 patients (9.6%). No evidence of mistargeting of the radiation isocenter (Breneman et al., 2009).

Number of Lesions That Can Be Treated

'How many tumors can and should be treated?' has long been a central question in the field of stereotactic radiosurgery for brain metastases. In a linear accelerators system, the upper tumor number limit is generally considered to be 4-5 lesions in a single session due to mainly the prolonged procedure time. In Gamma Knife radiosurgery, the maximum number of lesions that can be treated in a 1-day session is 30 or even slightly more. If a patient has > 40 lesions, it is recommended that the procedure be divided into two sessions, 1 day apart, with the patient keeping the stereotactic head frame on overnight (Yamamoto, 2007).

A comprehensive review found no correlation between lesion number, even in patients with eight or more lesions, and survival (Boyd and Mehta, 1998). Gamma Knife radiosurgery for multiple metastases produces survival rates similar to those achieved using Gamma Knife for single metastasis (Suzuki et al.,

2000). Presently, most patients can be treated using a Gamma Knife in one frame position, without replacing the frame, as was previously required (Yamamoto et al., 2003). Stereotactic radiosurgery is beneficial for carefully selected end-stage patients with 10 or more brain metastases. Even those patients can maintain good performance status for a major portion of their remaining life (Yamamoto, 2007).

Advantages of Gamma Knife Radiosurgery over Whole Brain Radiation Therapy

Gamma Knife radiosurgery is superior to whole brain radiation therapy for several reasons: (1) Only a brief hospital stay is necessary, (2) Higher control rates and earlier symptom palliation can be achieved, (3) All observable lesions on MRI can be treated, even if such lesions are radioresistant, (4) Other treatments, such as radiation therapy for other parts of the body, surgery and chemotherapy, need not be interrupted, (5) The availability of an alternative treatment for brain metastases allows whole brain radiation therapy to be reserved for subsequent treatment attempts. Whole brain radiation therapy should be postponed until it is absolutely necessary, (6) Gamma Knife radiosurgery can be repeated, even after whole brain radiation therapy. Whole brain radiation therapy usually cannot be repeated, (7) Patients never lose all of their hair, (8) A very low incidence of dementia can be expected (Yamamoto, 2007).

Analysis of the outcomes and cost-effectiveness of gamma Knife radiosurgery and whole brain radiation therapy was done over a period of 5 years in 156 patients with multiple brain metastases The mortality rate was lower for Gamma Knife than for whole brain radiation therapy. The median survival time for Gamma Knife and whole brain radiation therapy was 9.5 months and 8.3 months, respectively. Gamma Knife resulted in higher cost-effectiveness than whole brain radiation therapy for treating multiple brain metastases (Lee et al., 2009).

Safety of Gamma Knife Treatment for Multiple Metastases

The cumulative irradiation doses to the normal brain were calculated based on the treatment protocols for 92 patients who underwent Gamma Knife radiosurgery for 10 or more metastases. The doses to the whole normal brain were estimated to be 4.0-6.0 J in most patients (range 2.2-11.9 J). These estimates confirm that cumulative irradiation doses for patients with numerous radiosurgical targets do not exceed the threshold level associated with necrosis. In fact, the symptomatic complication rate is very low (1%). Two thirds of patients died less than 12 months after treatment, i.e. one third of patients lived long enough to experience radiation-induced complications. The incidence of such complications was 3% in the latter special patient group (Yamamoto et al., 2002).

Radiosurgery as Post-operative Irradiation

Very little information is available on Gamma Knife radiosurgery as an alternative to post-operative irradiation. Surgical removal followed by Gamma Knife radiosurgery was used in 23 patients. Gamma Knife irradiation was delivered to the cavity remaining after tumor removal. Local tumor control rate of 78.3% was comparable to the rates of local control achieved with surgery followed by whole brain radiation therapy. Therefore, radiosurgery can be used as post-operative irradiation instead of whole brain radiation therapy, which has several disadvantages (Yamamoto, 2007).

Radiosurgery for Brain Meningiomas

Surgeons have traditionally preferred to use Gamma Knife radiosurgery as an adjunctive treatment modality after surgical resection or for patients who are at high risk for operative morbidity or mortality. Multiple studies demonstrated the efficacy and safety of Gamma Knife with tumor control rates ranging from 60 to 100% depending on the proportion of atypical or malignant meningiomas (Lee et al., 2002). Given the benefit of radiosurgery, Gamma Knife has been increasingly used to treat patients as a primary therapy based on imaging criteria only (Flickinger et al., 2003).

Radiosurgery of meningiomas at the skull base represents particular concern to the vascular and neural structures that are often embedded within or around the meningioma. It was found that the motor cranial nerves within the cavernous sinus are particularly less sensitive to radiation doses used in radiosurgery while the sensory nerves appear to be more sensitive (Lee et al., 2002).

Radiosurgery was performed in almost 1,000 patients with meningiomas at the University of Pittsburgh. This treatment modality can be performed either as an adjunct to surgical resection or as a primary procedure. Gamma Knife provides 5- and 10-year tumor control rates of 93% for benign meningiomas. The 5-year control rate for atypical and malignant meningiomas was 83 ± 7% and 72 ± 10%, respectively. The complication rate remains low. The incidence of adverse radiation effect ranged from 5.7 to 16% and was gradually reduced with the advent of MRI and lower dosing (Lee et al., 2007).

Stereotactic radiosurgery was used in patient group consisted of 972 patients with 1045 intracranial meningiomas. The overall control rate for patients with benign meningiomas was 93%. In those without previous histological confirmation tumor control was 97%. However, for patients with atypical and malignant meningiomas, the tumor control was 50% and 17%, respectively. Delayed resection after radiosurgery was necessary in 5% of patients at a mean of 35 months. After 10 years, benign meningiomas were controlled in 91% and in those without histology 95% were controlled. None of the patients developed a radiation-induced tumor. The overall morbidity rate was 7.7%. Radiosurgery provided high rates of tumor control or regression in patients with benign meningiomas. This study confirms the role of radiosurgery as an effective management choice for patients with small to medium-sized symptomatic, newly diagnosed or recurrent meningiomas of the brain (Kondziolka et al., 2008).

CyberKnife was used in a series of 199 benign intracranial meningiomas. The tumor volume decreased in 36 patients, was unchanged in 148 patients, and increased in 7 patients. Three patients underwent repeated radiosurgery, and 4 underwent operations. One hundred fifty-four patients were clinically stable. In 30 patients, a significant improvement of clinical symptoms was obtained. In 7 patients, neurological deterioration was observed. The procedure proved to be safe. Clinical improvement seems to be more frequently observed with the CyberKnife than in other previous linear accelerator experience (Colombo et al., 2009).

Radiosurgery for Brain Gliomas

There are several papers summarizing the experience of radiosurgery in gliomas. In 16 cases of low-grade astrocytomas treated with Gamma Knife, it was found disappearance of tumor in 50% and decrease or static tumor size in 31% of cases (Barcia et al., 1994). Gamma Knife was used in 8 cases of pilocytic astrocytoma and 5 cases of low-grade astrocytoma in children. In pilocytic astrocytoma, the tumor either disappeared or decreased in 7 cases. In low-grade astrocytoma, the tumor either disappeared or decreased in 2 cases (Grabb et al., 1996).

Stereotactic radiosurgery with a linear accelerator system and standard radiation therapy were used in 32 patients with glioblastoma multiforme underwent tumor debulking or biopsy. The 12-month survival was 37% (Masciopinto et al., 1995). The impact of stereotactic radiosurgery on the survival of patients with malignant gliomas was evaluated and found that the 2-year and median survival was 45% and 96 weeks, respectively. Patients treated with stereotactic radiosurgery had a significantly improved 2-year and median survival (Sarkaria et al., 1995).

Stereotactic radiosurgery represents an alternative or supplementary treatment modality to conventional surgery in small-volume low-grade astrocytomas especially in deep-seated critical locations. There is also evidence for beneficial effect of stereotactic radiosurgery on the survival of patients with high-grade glioma (Szeifert et al., 2007).

Out of a series of brain tumor patients treated at the Lars Leksell Center for Gamma Knife Surgery, 74 astrocytomas were selected. In astrocytoma grade I; the tumor either disappeared or decreased in 60%

and increased in 40% of cases following Gamma Knife. Best results occurred when the pretreatment volume was < 3 cm3. The mean volume of tumors with response to the treatment was 2.7 cm3. In astrocytoma Grade II; the tumor either disappeared or decreased in 58.8%, remained unchanged in 12.5% and increased in 31.3% cases. In astrocytoma Grades III and IV; a decrease in the tumor size was observed in 22% of cases with complete disappearance in 2 cases. There was no change in the tumor volume of 28.5% and increase in size was noticed in 45% of cases. The mean survival time interval was 26 months. At 5 years, 25% of patients were live (Szeifert et al., 2007).

In another recent study, stereotactic radiosurgery was well tolerated in glioblastoma treatment. There was no difference in survival whether stereotactic radiosurgery is delivered upfront or at recurrence, so the treatment should be individualized for each patient. Future studies are needed to identify patients most likely to respond to stereotactic radiosurgery (Biswas et al., 2009).

Radiosurgery for Pituitary Adenomas

Stereotactic radiosurgery is a safe and highly effective treatment for patients with pituitary adenomas. Radiosurgery provides control of tumor growth in nearly all cases and hormone normalization in the majority of secretory tumors (Mayberg and Vermeulen, 2007).

Numerous studies documented that more than 95% of pituitary adenoma show either shrinkage or stabilization after stereotactic radiosurgery. Biochemical remission is possible in approximately 80% of properly selected patients. The time to endocrinal normalization typically ranges from 1 to 5 years. Since the effects of radiosurgery are gradual, it is effective for persistent or recurrent tumors after surgery or for patients with high risk for open surgical procedures (Pollock, 2007).

Patients with persistent or recurrent hormonal overproduction now typically undergo stereotactic radiosurgery. Patients stop all pituitary suppressive medications for 4-6 weeks before radiosurgery. At radiosurgery, every attempt is made to provide a maximum tumor margin dose that exceeds 20 Gy while still limiting the radiation dose to the optic apparatus to less than 12 Gy. In this manner, biochemical remission is possible for approximately 80% of patients. The majority of patients achieve hormonal normalization 12-24 months after stereotactic radiosurgery. Also, radiosurgery has replaced radiation therapy for patients with recurrent or progressive non functioning pituitary adenomas (Pollock, 2007).

Between 1997 and 2007, 347 patients with secretory pituitary adenomas were treated with Gamma Knife radiosurgery. In 47 of these patients some form of prior treatment such as transsphenoidal resection or craniotomy and resection was done. In the others, Gamma Knife served as the primary treatment modality. Of the 68 patients with adrenocorticotropic hormone-secreting adenomas, 89.7% showed tumor volume decrease or remain unchanged and 27.9% experienced normalization of hormone level. Of the 176 patients with prolactinomas, 23.3% had normalization of hormone level and 90.3% showed tumor volume decrease or remain unchanged. Of the 103 patients with growth hormone-secreting adenomas, 95.1% experienced tumor volume decrease or remain unchanged and 36.9% showed normalization of hormone level. None of the patients in the study experienced injury to the optic apparatus or had other neuropathies related to Gamma Knife (Wan et al., 2009).

Complications of Radiosurgery

Treatment complications have been reported from the early experience of 2 Canadian radiosurgery programs. Patients were accumulated until August, 2007, and a total of 973 patients were included in this report. During the radiosurgical procedure, 19 patients (2%) suffered anxiety or syncopal episodes, and 2 patients suffered acute coronary events. Treatments were incompletely administered in 12 patients (1.2%). Severe pain was a delayed complication; 8 patients suffered unexpected headaches, and 9 patients developed severe facial pain. New motor deficits developed in 11 patients. Four patients required shunt placement for symptomatic hydrocephalus, and 16 patients suffered delayed seizures. Gamma Knife radiosurgery is a minimally invasive treatment modality for many intracranial diseases.

Treatment is not risk free, and some patients will develop complications that will decrease as institutional experience matures. Expanding availability and indications necessitate discussion of these risks with patients (Vachhrajani et al., 2008).

CHEMOTHERAPY

Chemotherapy has played primarily an adjuvant role in the treatment of brain tumors because of efficacy limitations related to drug-delivery issues and inherent tumor chemoresistance (Cairncross et al., 1998).

The blood-brain barrier is mediated by tight junctions between endothelial cells of brain vasculature, preventing the penetration of large molecules with poor lipid solubility or tight protein binding. Even in areas in which there is a breakdown of the blood-brain barrier (areas of contrast enhancement on MRI), variable intra- and peri-tumoral hydrostatic pressures from cerebral edema make drug delivery at useful concentrations challenging (Calatozzolo et al., 2005).

Recent developments in chemotherapy of brain tumors include the combination of cytotoxic, cytostatic and targeted therapies. Multitargeted compounds that simultaneously affect multiple signaling pathways are increasingly employed (Soffietti et al., 2007).

Cytotoxic Chemotherapy

During the past three decades, thousands of patients with malignant gliomas have been entered on adjuvant chemotherapy trials. The most commonly used chemotherapeutic drugs have been nitrosoureas, either alone as carmustine or in combination with other agents as in PCV that include procarbazine, lomustine, and vincristine. Nitrosureas were the mainstay of adjuvant therapy. Carmustine (BCNU) and orally available lomustine (CCNU) are liposoluble drugs. Studies of carmustine documented a modest 10-15% benefit in survival at 18 months, but no difference in median survival or survival at 12 or 24 months. Major toxicities include fatigue, nausea, myelosuppression, and dose-related pulmonary fibrosis (Walker et al., 1980).

Two meta-analyses of clinical trials for high-grade gliomas have demonstrated therapeutic benefits from addition of nitrosourea-based chemotherapy to radiotherapy. Glioma meta-analysis was performed on individual patient data from 12 randomized trials and showed a modest, but significant prolongation of survival associated with the addition of chemotherapy. An absolute increase in 1-year survival of 6% (from 40 to 46%) and in 2-year survival of 5% (from 15 to 20%) was found (Soffietti et al., 2007).

Temozolomide is an oral alkylating agent with ability to cross an intact blood-brain barrier and excellent toxicity profile (Newlands et al., 1997). Temozolomide's favorable toxicity profile makes it the preferred therapy. Temozolomide has demonstrated antitumor activity as a single chemotherapeutic agent in recurrent glioma after conventional therapies with response rates ranging from 35% for anaplastic astrocytomas to 5% for glioblastomas (Brada et al., 2001).

The Food and Drug Administration (FDA) approved temozolomide for the treatment of recurrent anaplastic astrocytomas only, whereas the European authorities approved the drug for both anaplastic astrocytomas and glioblastoma. The approved schedule ('standard regimen') was a dose of 150- 200 mg/m2/day for 5 days of every 28-day cycle. On the basis of experimental data showing a synergism between temozolomide and radiation (Van Rijn et al., 2000), and the availability of an extended continuous schedule of the drug for up to 7 weeks (Brock et al., 1998), Stupp et al. conducted a pilot phase II trial for newly diagnosed GBMs using conventional fractionated radiotherapy with concomitant temozolomide (75 mg/m2/day), followed by up to 6 cycles of adjuvant temozolomide using the standard regimen. This regimen showed promising clinical activity including median survival of 16 months and 2-year survival rate of 31% (Stupp et al., 2002).

Why did this trial show a greater benefit with temozolomide than all the previous trials with nitrosoureas? Several explanations can be put forward. First, most patients had favorable prognostic factors (i.e. age under 70 years, good performance status, total or partial resection). Second, temozolomide is better tolerated and may be more active than nitrosoureas. Third, the concurrent administration of the drug during radiotherapy, by utilizing the radiosensitizing effect of temozolomide, may have played an important role in improving the outcome (DeAngelis, 2005).

Multi-institutional trial enrolled 120 patients with recurrent glioblastoma, anaplastic astrocytoma, or anaplastic oligodendroglioma. An initial oral dose of 200 mg/m2 of temozolomide was followed by 9 consecutive doses of 90 mg/m2 every 12 hours. Treatment cycles were repeated every 28 days. Doses were escalated to 100 mg/m2 twice daily in the absence of unacceptable toxicity or were reduced if unacceptable toxicity occurred. For glioblastoma, anaplastic astrocytoma, and anaplastic oligodendroglioma patients, respectively, the median progression-free survival was 4.2 months, 5.8 months, and 7.7 months, whereas the median overall survival was 8.8 months, 14.6 months, and 18 months. The overall response rate for glioblastoma, anaplastic astrocytoma, and anaplastic oligodendroglioma patients was 31%, 46%, and 46%, respectively. Twice-daily dosing may enhance the efficacy of temozolomide in treatment of recurrent gliomas without increasing toxicity. This regimen compares favorably with other dosing schedules of temozolomide reported in the literature (Balmaceda et al., 2008).

Other alkylating agents that penetrate the blood-brain barrier include procarbazine, thiotepa, and busulfan, but they have little activity in gliomas as single agents. Platinum compounds are used with modest results for recurrent disease, but they also have poor blood-brain barrier penetration (Warnick et al., 1994). Plant alkaloids have poor brain entry and little activity in gliomas. Vincristine is used most often in combination with procarbazine and lomustine for oligodendroglioma. Etoposide was studied in combination with carboplatin and as monotherapy with modest results (Franceschi et al., 2004).

Efforts to maximize efficacy of cytotoxic chemotherapy by overcoming blood-brain barrier limitations include intra-arterial delivery, blood-brain barrier disruption and implantation of polymer-based drug delivery vehicles directly into the tumor bed. Dose intensification intended to increase drug gradients across the blood-brain barrier for improved penetration is often faced by the added systemic toxicity. Intra-arterial administration allows higher concentrations to be delivered to brain parenchyma without first-pass metabolism (Ashby et al., 2001). Combining Intra-arterial administration with osmotic blood-brain barrier disruption improves treatment responses, especially in chemosensitive brain tumors, such as germ cell tumors and CNS lymphoma. However, increased neurotoxicity is prohibitive (Fortin et al., 2005).

The chemosensitivity of primary CNS lymphoma is exceptional. Addition of chemotherapy to standard radiotherapy had an appreciable impact on tumor response and patient survival. Combined-modality regimens prolong survival compared with radiotherapy alone and are the mainstay of treatment of primary CNS lymphoma. They consist of high-dose methotrexate-based combination chemotherapy and radiotherapy. These regimens result in impressive survival benefit with median overall survival ranging from 25 to 60 months. Methotrexate is typically infused over several hours to optimize CSF penetration and is given weekly or every 2 weeks. High-dose methotrexate-based regimens are given in multiple cycles with the goal of inducing a radiographic complete response. This regimen is followed by whole brain radiation therapy (Mohile and Abrey, 2007).

Molecularly Targeted Therapy

Trials of molecularly targeted drugs as monotherapy for gliomas were disappointing, with some potential benefit when used in combination with hydroxyurea or temozolomide. Combinations of multitargeted therapy are increasingly the focus of clinical trials (Kreisl, 2009).

Everolimus, sirolimus, and temsirolimus are mammalian target of rapamycin inhibitors that have been

studied as single agents and combination therapy with epidermal growth factor receptor inhibitors (Doherty et al., 2006). Novel agents like enzastaurin and tamoxifen inhibit protein kinase C (Fine et al., 2008). Tamoxifen showed only moderate efficacy in clinical practice with no improvement in the median survival time. Enzastaurin has shown promising efficacy in the clinical practice, with a 29% radiographic response rate in patients with recurrent high-grade gliomas. Progress with enzastaurin was stopped when a phase III trial was stopped prematurely as significant survival benefits were not seen (Mercer et al., 2009).

Vorinostat, one of the histone deacetylases inhibitors, was used in the treatment of glioma and showed potential benefit. Pretreatment with vorinostat can make glioma cells more susceptible to chemotherapy and radiation (Chinnaiyan et al., 2005). In a phase II trial of vorinostat in recurrent glioblastoma, the drug was well tolerated and anti-tumor efficacy was noted; progression-free survival for 6 months was 23% (Galanis et al., 2007). Other deacetylase inhibitors (e.g. romidepsin, LBH589, and valproic acid) are being developed clinically for the treatment of glioblastoma (Mercer et al., 2009).

The interpretation of "molecularly targeted therapy" can be widened to include therapies that are conjugates of a cytotoxic factor (radioisotope, toxin or standard drug) and a tumor-specific vector (an antibody or other tumor-specific ligand). Most of this work is done in the setting of convection-enhanced delivery. This allows delivery of large molecular weight drugs that are unable to penetrate the blood-brain barrier. Two randomized phase III trials have investigated convection-enhanced delivery of toxin conjugates; transferrin-conjugated diphtheriae toxin (Weaver and Laske, 2003) and interleukin 13-conjugated pseudomonas exotoxin (Kunwar et al., 2007).

The invasive nature of the treatment and associated neurotoxicity make this modality appropriate for only selected patients with tumors of size and location physically amenable to convection-enhanced delivery (Kreisl, 2009).

Combination of Cytotoxic Agents 'Cytotoxic Synergy

One can combine drugs with either similar or different mechanism of action. Nitrosoureas (carmustine, lomustine, fotemustine) and temozolomide are both alkylating agents with independent activity in gliomas (Gerson, 2002).

Preclinical models observed a synergistic activity of carmustine and temozolomide, but did not identify the optimal sequence and schedule to maximize antitumor activity and minimize toxicity. Different schedules have been employed clinically, including administration of both drugs on day 1 (Plowman et al., 1994). This regimen resulted in significant myelotoxicity, without any additive antitumor activity in phase I and II studies (Prados et al., 2004).

The best tolerated sequence seems represented by carmustine (on day 1) followed by temozolomide (days 1-5) (Hammond et al., 2004). As neoadjuvant therapy in inoperable glioblastomas, this combination has exhibited promising activity (Barrie et al., 2005). To avoid overlapping toxicities, the combination occurs between locally administered carmustine ('Gliadel Wafers') and temozolomide, and seems attractive as serum carmustine levels are undetectable after gliadel implant (Olivi et al., 2003) and the results of a phase I study are encouraging (Gururangan et al., 2001). Moreover, there seems to be a rationale for sequencing Gliadel Wafers and temozolomide + radiotherapy in resected patients with newly diagnosed glioblastomas: first, to kill with carmustine the resistant tumor cells in the postoperative period; second, to exploit a synergism between carmustine and radiation during the first weeks of radiotherapy. A phase II study of radiation with concomitant and adjuvant temozolomide in patients with newly diagnosed supratentorial malignant glioma who have undergone surgery with carmustine wafers insertion is ongoing; preliminary results have been reported, and include a 1-year survival rate of 63%. If the final results confirm the efficacy and safety of this combination, a phase III study comparing carmustine wafers and temozolomide + radiotherapy versus temozolomide + radiotherapy would be warranted (Soffietti et al., 2007).

The combination of lomustine, temozolomide and radiotherapy has yielded promising survival data (2-year survival rate of 44.7%), and acceptable toxicity in patients with newly diagnosed glioblastomas (Herrlinger et al., 2006).

Carmustine and cisplatin have been suggested to be synergistic, and moreover cisplatin has a well-known radiation sensitizer effect that can persist long after administration, thus being attractive to be incorporated in chemotherapy regimens in the pre-radiation setting. Despite these premises and impressive response rates in phase II trials (Grossman et al., 1997), continuous infusion of carmustine and cisplatin before radiotherapy in newly diagnosed malignant gliomas has not led to improvements in survival and having relatively high toxicity (Grossman et al., 2003).

Alkylating agents are ideal candidates for combination therapy with irinotecan, as they have different mechanisms of action and different systemic toxicities. Irinotecan is a topoisomerase I inhibitor with substantial activity in glioma cell cultures and human glioblastoma xenografts. After intravenous injection, it is metabolized in the liver into the active metabolite SN-38 that has some ability to cross the blood-brain barrier (Hare et al., 1997). Clinical trials using irinotecan in patients with recurrent malignant gliomas reported response rates ranging from 2 to 15%, and extended progression-free survival at 6 months as high as 56% (Batchelor et al., 2004).

Carmustine and irinotecan display a schedule-dependent synergistic activity against glioma cells, with maximal activity achieved when administering carmustine before irinotecan (Castellino et al., 2000). To date, carmustine and irinotecan for patients with malignant gliomas have not been proven to be superior to irinotecan alone, and may be associated with increased pulmonary toxicity (Reardon et al., 2004). To enhance the blood-brain barrier penetration and reduce the systemic toxicity, a locally delivered irinotecan (by biodegradable wafers) could be coupled with systemic carmustine (Storm et al., 2004).

Another alternative option is represented by temozolomide combined with irinotecan that has demonstrated encouraging preclinical activity (Patel et al., 2000). A phase I trial with escalating doses of irinotecan and a standard dose of temozolomide in recurrent glioblastomas has shown a response rate of 14%, with progression-free survival for 6 months of 27%. A number of phase II studies are ongoing (Soffietti et al., 2007).

Combination of cytotoxic, cytostatic and targeted molecular agents

The rationale for combining cytotoxic and cytostatic agents lies in the different mechanism of antitumor activity and non-overlapping side effect profiles. Cytostatic agents may have anti-angiogenic, anti-invasion/migration and differentiating properties (Drappatz and Wen, 2004).

Thalidomide inhibits endothelial cell proliferation and basic fibroblast growth factor-induced angiogenesis, and has immunomodulatory effects. To enhance its modest activity as single agent in gliomas, thalidomide has been combined with cytotoxic agents such as carmustine and temozolomide. The combination of carmustine and thalidomide has been evaluated in patients with predominantly recurrent glioblastomas. The response rate was 24% and the progression-free survival for 6 months was 27%. These results compared favorably with carmustine alone (Fine et al., 2003). The combination of temozolomide and thalidomide (both oral agents) has yielded interesting results. The median survival of newly diagnosed glioblastoma patients (73 weeks) was better than that after radiation therapy alone, but similar to that after radiotherapy and adjuvant nitrosoureas (Chang et al., 2004). Lenalidomide is an analog of thalidomide, which has increased anti-angiogenic activity and fewer side effects and is under investigation (Soffietti et al., 2007).

Temozolomide and cilengitide, an integrin antagonist, could be synergistic in inhibiting the increased expression of ?v?3 integrin after radiotherapy, thus interfering with the radiation-induced upregulation of angiogenesis (Wick et al., 2002).

Cyclooxygenase-2 is often upregulated in gliomas and there is increasing evidence that cyclooxygenase-2 inhibitors may block angiogenesis and growth. Clinical trials combining cyclooxygenase-2 inhibitors, such as celecoxib or rofecoxib, with irinotecan or temozolomide have been performed with some activity (Soffietti et al., 2007).

High-dose tamoxifen, a protein kinase C inhibitor, has shown some activity both alone and in combination with carboplatin or procarbazine. Protein kinase C inhibitors as tamoxifen, staurosporine, hypericin and calphositin C act as chemo-sensitizers, but association of high-dose tamoxifen and continuous temozolomide in recurrent gliomas is not effective and relatively toxic (Soffietti et al., 2007).

Enzastaurin is an inhibitor of protein kinase-?2 with potent anti-angiogenic activity in preclinical models. A phase II trial in patients with recurrent gliomas has shown promising activity (14 objective radiographic responses out of 79 evaluable patients, i.e. 18%) (Fine et al., 2005). A phase III trial of enzastaurin compared with standard therapy with lomustine in patients with glioblastoma at first relapse, failed to meet its efficacy endpoints, but significant radiographic responses and excellent toxicity profile were observed (Fine et al., 2008).

Marimastat is a low-molecular weight peptide that inhibits matrix metalloproteinases which degrade the extracellular matrix, and thereby enable tumor invasion and migration. In-vitro studies of marimastat showed a significant inhibition of invasion of glioma cell lines (Tonn et al., 1999). Single-agent marimastat failed to improve survival in patients with glioblastoma or gliosarcoma following surgery and radiotherapy (Levin et al., 2006).

Two phase II trials tested marimastat in combination with temozolomide in patients with recurrent glioblastoma and anaplastic gliomas, respectively. In the first study, the combination resulted in a progression-free survival for 6 months that exceeded the historical control group by 29%. Joint and tendon pain were the major side effects, occurring in 47% of patients (Groves et al., 2002). In the second study, the regimen was roughly equivalent to single-agent temozolomide and was associated with additional toxicity (Groves et al., 2006).

Differentiating agents, such as retinoids, induce the differentiation of malignant cells into mature cells, and can also suppress cell proliferation and induce apoptosis. Retinoids have been studied in clinical trials with good tolerance but modest success (Drappatz and Wen, 2004). The combination of 13-cis-retinoic acid with temozolomide in gliomas (Jaeckle et al., 2003) and both of them with concurrent radiotherapy, has yielded results not superior to that obtained with conventional therapy (Butowski et al., 2005).

Many targeted molecular agents are currently being evaluated in the treatment of gliomas (Reardon et al., 2006). Studies in colorectal cancer showed that targeted molecular agents can enhance the activity of cytotoxic chemotherapy, being the combinations far more active than single agents alone (Hurwitz et al., 2004).

Farnesyltransferase inhibitors inhibit the activity of enzyme implicated in the Ras/MAPK signaling pathway. Such inhibitors as tipifarnib and lonafarnib are being tested in glioblastoma clinical trials, either alone or with radiation and/or temozolomide (Soffietti et al., 2007).

Upfront Chemotherapy for Newly Diagnosed Malignant Gliomas

A phase III trial compared radiotherapy + temozolomide with radiotherapy alone in 573 patients with newly diagnosed gliomas from 85 institutions in 15 countries. Patients were randomized to receive radiation alone (60 Gy) or with concurrent temozolomide, 75 mg/m2 daily, followed by 6 months of adjuvant treatment at 150 to 200 mg/m2 on 5- out of 28-day schedule. Survival benefit was observed for chemotherapy added to radiation in the initial management of patients with glioblastoma. The median survival was 14.6 months after radiotherapy and temozolomide compared with 12.1 months after

radiotherapy alone, with a 37% reduction in the risk of death. The 2-year survival rate was 26.5% with the combined treatment while it was 10.4% with radiotherapy alone. The combination of radiotherapy and temozolomide was associated with a significant improvement in median survival and also it was well tolerated in all subgroups of patients. Many if not most neuro-oncologists extend the adjuvant therapy to at least 1 year, anticipating inevitable recurrence for most patients (Stupp et al., 2005). More recent evidence supports temozolomide's effect as a radiosensitizer (Van Nifterik et al., 2007).

Efforts to improve on initial therapy for malignant glioma patients include ongoing and anticipated studies that depend on concurrent chemo-radiation with temozolomide. Novel radiosensitizers, neoadjuvant regimens, and temozolomide combination trials are all being investigated (Kreisl, 2009).

Mechanisms of chemoresistance

A number of mechanisms of resistance to chemotherapeutic drugs have been reported in brain tumors, but to date only a few have known clinical relevance (Bredel et al., 2006).

Mechanisms of intrinsic chemoresistance are poorly understood. The most important mechanism of resistance in malignant gliomas is based on the activity of the enzyme O6-methylguanine DNA methyltransferase which causes resistance to some alkylating chemotherapeutic agents. This enzyme repairs the DNA damage induced by alkylating agents. A correlation of tumor O6-methylguanine DNA methyltransferase levels with the outcome of glioma patients has been reported. Patients with low O6-methylguanine DNA methyltransferase levels (i.e. those with a reduced capacity to repair DNA damage) displayed significantly longer survivals and/or higher response rates during treatment with carmustine or temozolomide (Kreisl, 2009).

Some data suggest that epidermal growth factor receptor overexpression/amplification could be associated with resistance to alkylating agents (Leuraud et al., 2004).

Fortunately, advances in cancer therapeutics have afforded novel agents directed at tumor-specific oncogenic targets (Kreisl, 2009).

Future Directions

Concomitant chemo-radiotherapy followed by single-agent adjuvant treatment with the alkylating agent temozolomide is the current standard of care for the patients with glioblastomas. The concomitant administration of chemotherapy with radiotherapy, as well as the early introduction of chemotherapy, appears to be the key to the improving outcome. Better understanding of the molecular pathways of carcinogenesis, particularly gliomagenesis, will allow development of specifically targeted therapies. Correlative molecular studies, now included in most ongoing trials, will enhance our understanding of this disease and allow for rapid further improvement in outcome (Stupp et al., 2007).

Although conventional chemotherapeutic agents continue to be developed to reduce toxicity and/or improve efficacy, the remarkable advances made in knowledge of tumor biology in the past decade have shifted the focus onto novel agents that target molecular changes crucial for tumor proliferation or survival. These selective agents are predicted to be less toxic to normal cells and it is anticipated that they will be more effective than currently used nonspecific chemotherapeutic agents. The toxicity and efficacy of these novel agents is currently being assessed with brain tumors. Ultimately, if these novel therapies prove effective, their role in combination with established chemotherapeutic agents will need to be assessed (Gottardo and Gajjar, 2008).

ENDOCRINAL THERAPY

Pituitary Tumors

Pituitary tumors represent about 15% of the primary intracranial neoplasms and the hormone-secreting tumors account for about 30% of all pituitary tumors (Patil et al., 2009).

The medical approach to pituitary adenomas has been greatly improved since the availability of dopamine agonists such as: bromocriptine, cabergoline and quinagolide, and availability of somatostatin analogues such as: lanreotide slow release and autogel, and octreotide long-acting repeatable (Colao et al., 2009).

The efficacy of dopamine agonists in prolactin secreting adenomas and efficacy of somatostatin analogues in growth hormone and thyroid stimulating hormone secreting adenomas is well established. More recently, data are accumulating suggesting a potential therapeutic role of dopamine agonists also in patients with adrenocorticotrophic hormone secreting and clinically non-functioning pituitary adenomas (Colao et al., 2009).

Available Dopamine Agonists and Somatostatin Analogues

The class of dopamine agonists includes ergot derivatives (bromocriptine, pergolide, metergoline, lisuride, terguride, and cabergoline) and non ergot derivatives (Quinagolide). The ergot alkaloids have a wide spectrum of pharmacological effects. These effects are mainly responses mediated by noradrenaline, serotonin and dopamine receptors. Within the class of non ergot derivatives, quinagolide is the most active and when compared to bromocriptine, it was about 35 times more potent (Colao et al., 2002).

Octreotide was the first commercially available somatostatin analogue and was formulated for subcutaneous administration. It has a high affinity for somatostatin receptors. Octreotide is administered with multiple daily injections (2-4) and its growth hormone suppressive effect lasts at most 5 h after injection (Lamberts et al., 1996). Octreotide is also available in a long-acting repeatable formulation. The active compound is encapsulated in microspheres of a biodegradable polymer. After intramuscular injection, the drug levels begin to rise over 7-14 days and remains plateau for 20- 30 days (Lancranjan et al., 1996). Dosing every 4 weeks is typical (McKeage et al., 2003). The pharmacological effect of the drug can be manipulated by varying the dose from 10 to 30 mg with a fixed dosing interval of 28 days. It can be used up to 40 mg every 28 days (Colao et al., 2007).

Lanreotide slow release is a medium-acting somatostatin analogue. The drug is encapsulated in microspheres that provide prolonged release over 10-14 days after intramuscular administration (Colao et al., 1999). Lanreotide slow release is provided in a 30 and 60 mg dosage (the latter only in Italy) and the pharmacological effect is manipulated by changing the dosing interval among every 7, 10, or 14 days (Freda, 2002). More recently another long-acting formulation, lanreotide autogel, has become available in many European countries and in United States (Caron et al., 2006). Autogel forms a slow-release aqueous gel that can be administered subcutaneously and its pharmacological effect can be manipulated by varying the dose from 60, 90, or 120 mg with a fixed 28 days administration schedule or by varying the interval between injections between 28-56 days maintaining the dose of 120 mg (Lombardi et al., 2008).

Pasireotide (SOM 230) is another somatostatin analogue that underwent experimental investigation. It has high affinity for somatostatin receptors (Bruns et al., 2002). Available data about pasireotide is only related to its effect on growth hormone level (Van der Hoek et al., 2004). However, in a study in mice bearing growth hormone/prolactin secreting adenomas, pasireotide induced greater tumor shrinkage than octreotide (Fedele et al., 2007).

Dopamine Agonists and Somatostatin Analogues in Pituitary Adenomas

1. Prolactin Secreting Adenomas:

Bromocriptine was introduced into clinical practice as the first medical treatment for prolactinomas (Gillam et al., 2006). It has a relatively short elimination half-life, so that it is usually taken 2 or 3 times daily, although once daily may be effective in some patients. Generally, the therapeutic doses are in the range of 2.5-15 mg/day and most patients are successfully treated with 7.5 mg or less. However, doses as high as 20-30 mg/day may be necessary for patients who demonstrate resistance. For microprolactinomas bromocriptine is successful in 80-90% of patients in normalizing serum prolactin level, restoring gonadal function and shrinking tumor mass (Colao et al., 2002).

For macroprolactinomas normalization of serum prolactin level and tumor mass shrinkage occur in about 70% of patients treated with bromocriptine. In most patients, headache and visual field defects improve dramatically within days after the first administration of bromocriptine with gonadal and sexual function improving even before complete normalization of serum prolactin level (Colao et al., 2009).

Cabergoline is widely used to treat prolactinomas. Cabergoline treatment normalized prolactin level in 86% of 425 patients (Verhelst et al., 1999). Generally, the dose of cabergoline at the start of therapy was 1 mg/week in patients with macroprolactinomas and 0.5 mg/week in those with idiopathic hyperprolactinaemia or microprolactinomas. Remarkable tumor shrinkage was observed in patients with microprolactinomas; 12-24 months treatment with cabergoline induced more than 20% decrease of baseline tumor size in more than 80% of cases with complete disappearance of the tumor mass in 26-36% of cases. The tumor shrinking effect is very rapid and improvement of visual field defects can be detected even after the administration of the first tablet (Colao et al., 2000).

The superiority of cabergoline over bromocriptine was supported by a comparative retrospective study by Di Sarno et al. and by the evidence of further tumor shrinkage in patients previously intolerant or resistant to bromocriptine and in those already responsive to it (Di Sarno et al., 2001).

A recommendation in patients with large macroadenomas, is to begin cabergoline treatment (as well as any other dopamine agonists) with very low doses as in some very responsive tumors treatment can be associated with remarkable early tumor shrinkage leading to rhinorrhea that need surgical intervention (Cappabianca et al., 2001). In patients chronically treated with cabergoline, withdrawal of treatment for as long as 5 years was not associated with tumor re-growth. In this study, patients with macroadenomas extending into critical areas such as cavernous sinuses or cerebral areas were excluded (Colao et al., 2003).

Quinagolide is one of the dopamine-agonists used to treat prolactinomas. Several studies demonstrated that once-daily quinagolide in women with hyperprolactinaemia reduced prolactin level and tumor size and relieved gonadal dysfunction thereby restoring fertility (Gillam et al., 2006). Tumor mass was reduced by at least 30% in 21 of 26 patients with macroprolactinomas. Rare dissociation between prolactin control and tumor shrinkage can be encountered (Colao et al., 2009).

It is expected that pasireotide could have some role in treating patients with prolactinomas resistant to dopamine agonists, but there are no clinical data at present. Experimental data recently published showed that pasireotide produced a dose-dependent inhibition of prolactin release similar to that of cabergoline in dopamine agonists-sensitive prolactinomas (Fusco et al., 2008).

2. Growth Hormone Secreting Adenomas:

Dopamine agonists are primarily effective only in those growth hormone secreting tumors that co-secrete prolactin or that exhibit immunostaining for prolactin (Melmed et al., 2002). In a collection of 28 series including over 500 patients with acromegaly, bromocriptine produced a symptomatic improvement in up to 70% of the patients but lowered growth hormone levels below 5 ?g and induced tumor shrinkage in only 10-15% of patients (Jaffe and Barkan, 1994). In contrast, it was reported tumor shrinkage in 13 of 21 patients with a tumor reduction of 50% in 5 growth hormone/prolactin secreting

adenomas (Abs et al., 1998).

In the last decade, several studies have indicated that some tumor shrinkage is achieved using somatostatin analogues as first-line treatment (Colao et al., 2009).

Many studies have analyzed tumor shrinkage with somatostatin analogues therapy. The definition of tumor shrinkage differed across studies but overall, 217 of 478 patients (45%) had a reduction in tumor size. In patients treated first-line with somatostatin analogues, 51% had tumor shrinkage while in patients treated after unsuccessful surgery and/or radiation, 27% had tumor shrinkage (Bevan, 2005). Similarly, it was reported that, an approximately 50% decrease in pituitary mass is achieved when a somatostatin analogues is used exclusively or before surgery or radiotherapy. In 14 studies including 424 patients, the results showed that 36.6% of patients receiving primary somatostatin analogue therapy for acromegaly experienced a significant reduction in tumor size (Melmed et al., 2005).

In 99 patients treated with lanreotide slow release and octreotide long-acting repeatable for 12 months, the tumor shrinkage was absent (<25% of baseline size) only in 22.2% of patients, mild (25-50% of baseline size) in 31.1%, moderate (50- 75% of baseline size) in 30.3% and notable (>75% of baseline size) in 14.1% of patients. There was no difference in the amount of tumor shrinkage with the two analogues (Colao et al., 2006).

 In  a  study  including  patients  treated  only  with  somatostatin  analogues  up  to  18  years,  the  mean reduction in tumor volume was 43% (range 13-97%) and shrinkage >20% was obtained in 72% of the patients (Maiza et al., 2007). A very recent study has confirmed that, results obtained with lanreotide slow release and autogel and with octreotide long-acting repeatable are similar (Murray and Melmed, 2008).

In a study for 2 years of continuous treatment with octreotide long-acting repeatable, increased doses up to 40 mg every 28 days increased the chance to control growth hormone level and increased the rate of tumor shrinkage. A significant shrinkage (>25%) was found in 52.9% of patients after 12 months when they received 30 mg every 28 days and in all patients after 24 months when treated with 40 mg every 28 days. No data are currently available with lanreotide autogel. A preliminary experience in 26 patients treated first-line with this somatostatin analogue formulation had tumor shrinkage after 6 months of treatment (Colao et al., 2009).

It has been demonstrated that both somatostatin receptors and dopamine receptors are frequently coexpressed in adenomas from acromegalic patients and thus immunohistochemistry may determine receptor expression in pituitary adenomas to select patients responsive to different treatments (Ferone et al., 2008).

The combination treatment with somatostatin analogues and dopamine agonists could be beneficial to better suppress growth hormone level and also on tumor shrinkage but results on tumor shrinkage are lacking. The effect of somatostatin analogues on tumor shrinkage is reversible as demonstrated by tumor re-growth after stopping somatostatin analogues (Colao et al., 2009).

3. Thyroid Stimulating Hormone Secreting Adenomas:

This adenoma histotype is rare and frequently at diagnosis tumors are macros presenting with mass effect symptoms with variable symptoms of hyperthyroidism (Beck-Peccoz et al., 1996). The medical treatment of thyroid stimulating hormone secreting adenomas depends mainly on administration of somatostatin analogues, as dopamine agonists failed to persistently block hormone secretion in almost all patients and caused tumor shrinkage only in those with co-secretion of thyroid stimulating hormone and prolactin (Kienitz et al., 2007).

Somatostatin inhibits thyroid stimulating hormone secretion both in physiological conditions and in

thyroid stimulating hormone secreting adenomas as well as thyroid stimulating hormone secreting adenomas express somatostatin receptors. So, somatostatin analogues are a very efficient treatment in patients with thyroid stimulating hormone secreting pituitary tumors to improve the clinical signs, the hormone levels and to produce tumor shrinkage (Fischler and Reinhart, 1999).

Octreotide given subcutaneously induced an acute thyroid stimulating suppression. In more than 90% of thyroid stimulating hormone secreting adenomas, octreotide subcutaneous suppressed thyroid stimulating hormone secretion and in about 50% of cases adenoma size was reduced (Chanson et al., 1993). Octreotide treatment was also considered useful preoperatively as it allowed an easier tumor removal as it produces tumor shrinkage (Beck-Peccoz et al., 1996).

Another multicenter study reported treatment with octreotide long-acting repeatable in thyroid stimulating hormone secreting adenomas. Eleven patients received octreotide subcutaneous (at dosages of 200-900 ?g/daily) first and then octreotide long-acting repeatable (at 20 mg, every 28 days) after 4 weeks, both formulations significantly reduced thyroid stimulating hormone and thyroid hormone levels without causing significant side effects (Caron et al., 2001). The dose of octreotide needed to achieve thyroid stimulating hormone normalization was reported to be lower than that needed to suppress growth hormone in growth hormone secreting adenomas (Colao et al., 2003).

Lanreotide slow release treatment was similarly shown to suppress plasma thyroid stimulating hormone level and to normalize thyroid hormones with no significant change in adenoma size (Kuhn et al., 2000).

In five cases with thyroid stimulating hormone secreting pituitary adenomas, the hormone levels were successfully normalized and remained normal throughout the treatment period. The dosage of octreotide long-acting repeatable, or lanreotide slow release, required to normalize thyroid stimulating and thyroid hormone levels was indeed low (300 ?g/day, 10 mg every 28 days, and 30 mg every 14 days, respectively) compared with dosages generally required in patients with growth hormone secreting adenomas, however, dosage should be titrated on the basis of individual patients' responsiveness and tolerance (Colao et al., 2003).

4. Adrenocorticotrophic Hormone Secreting Adenomas:

The medical treatment of adrenocorticotrophic hormone secreting adenoma is reserved for patients with unsuccessful surgery (Pivonello et al., 2008) even after repeated pituitary surgery that is efficacious in approximately two-thirds (50-70%) of patients (Biller et al., 2008).

In long-term studies with bromocriptine, disease remission was confirmed in only a small minority of patients (Miller and Crapo, 1993).

A small, short-term study suggests that cabergoline at dosages of 2-3.5 mg/week may be effective (Pivonello et al., 2004). An extension study of this latter enrolling 20 patients after unsuccessful surgery, demonstrated that after 3 months of treatment, 75% of patients had normalization of free urinary cortisol that in 50% of them was maintained for 12 months (Colao et al., 2009). Tumor shrinkage after 6 months of cabergoline treatment was observed in 4 of 10 patients responsive to the treatment (Pivonello et al., 1999).

The use of cabergoline is still experimental and more data are required before proposing this treatment as an official therapy for adrenocorticotrophic hormone secreting tumors (Colao et al., 2009).

The possible role of somatostatin analogues has also been re-evaluated; pasireotide has been demonstrated to reduce adrenocorticotrophic hormone secretion in cell culture of corticotroph tumor (Hofland et al., 2005).

No definitive data are available on clinical trials, just a preliminary experience on very short-term

treatment that looks encouraging (Boscaro et al., 2005). These data on the effectiveness of specific dopamine agonists and somatostatin analogues suggest that their combination may also be a possible therapeutic approach (Colao et al., 2009).

5. Non Functioning Adenomas:

Bromocriptine was used in non-functioning adenomas with disappointing results (Bevan et al., 1986). Quinagolide efficacy in the treatment of these tumors was tested in a few studies and significant tumor shrinkage was documented only in 4 out of 12 reported patients (Ferone et al., 1998).

Cabergoline was administered to 10 patients with non-functioning adenomas and significant adenoma shrinkage was found only in 2 out of 10 patients with non-functioning adenomas (Colao et al., 2000). In 18 patients with remnant non-functioning adenomas, 12 months of cabergoline treatment induced tumor shrinkage in 56%. Tumor shrinkage was associated with dopamine D2 receptor expression (Pivonello et al., 2004).

The results of chronic octreotide treatment were controversial; it was reported to induce a rapid improvement of headache and visual disturbances, without any change in tumor volume (Warnet et al., 1997). Decrease of tumor volume by 30±4% was reported in some patients with non-functioning adenomas treated with a combined octreotide + cabergoline treatment. There are no data on the effects of somatostatin analogues, octreotide or lanreotide slow release or autogel in non-functioning adenomas (Andersen et al., 2001).

Although first-line therapy of non-functioning adenomas is surgery, only 35.5% are considered cured after surgery. Conventional radiotherapy is reported to delay tumor regrowth and causes hypopituitarism in all patients after 10 years. Therefore, there is a need for an additional treatment in the majority of non-functioning adenomas patients. Both somatostatin analogues and dopamine agonists were found to be of some efficacy in selected patients with non-functioning adenomas, with the latter drugs being significantly more effective in reducing tumor volumes. However, no placebo-controlled long-term studies are available to suggest the use of somatostatin analogues or dopamine agonists or a combination of them (Colao et al., 2008).

Meningiomas

It has been demonstrated that most of meningiomas express hormone receptors on their cell membranes (Marosi et al., 2008). Actually, up to 90% of meningiomas express progesterone receptors, while 30 to 48% express estrogen receptors (Korhonen et al., 2006).

It has been reported that the expression of the progesterone receptors alone in meningiomas indicates a favourable clinical and biological outcome. A lack of progesterone receptors or the presence of estrogen receptors correlates with an increasing potential for aggressive clinical behaviour, progression and recurrence of meningiomas (Pravdenkowa et al., 2006).

Epidemiological and immunohistochemical data have led to some attempts of treatment with anti-hormonal therapies for patients with progesterone receptors positive meningioma (Strik et al., 2002).

Mifepristone is oral anti-progesterone commonly used in treatment of estrogen receptors positive breast cancer with a significant benefit in global survival. Mifepristone inhibits the activity of progesterone receptors by complex mechanisms (Edwards et al. 2000). The use of progesterone antagonists in the palliation of meningioma has been discussed for more than 10 years (Strik et al., 2002).

The only prospective study on this topic was conducted on 160 patients in whom 80 patients were treated with mifepristone 200 mg and 80 patients were treated with placebo for a median duration of 10 months. The trial was prematurely closed. No difference was observed between the two arms in terms of time to

progression, but 55% of patients had stable disease, 1% of patients had partial response and no patient achieved complete response (Newfield et al. 2001).

Feasibility of hormonal therapy was assessed in 28 patients with unresectable meningioma treated with oral mifepristone 200 mg/day and oral dexamethasone 1 mg/day for the first 14 days. With a median duration of therapy of 35 months, minor responses (improved visual field examination or improved CT or MRI scan) were noted in 8 patients, 7 of whom were male or premenopausal female, which can result in significant clinical benefit in this subgroup of patients (Grunberg et al., 2006).

In a phase II evaluation of hormonal therapy by tamoxifen (an orally active selective estrogen receptors modulator that competitively binds to estrogen receptors and inhibits oestrogen effects) in unresectable or refractory meningiomas, only 5% of patients achieved partial response, 10% had minor response that was of short duration, 32% remained stable for a median duration of more than 31 months while 53% demonstrated progression. A definite recommendation for the use of tamoxifen in refractory meningiomas could not be made (Goodwin et al. 1993).

Brain Tumors and Steroids

A major cause of death in glioblastoma patients (more than 60% of cases) is brain herniation due to cerebral edema and increased intracranial pressure (Altaha, 2009).

Corticosteroids are usually indicated in any patient with brain tumor with symptomatic peri-tumoral edema. Dexamethasone is used most commonly as it has little mineralocorticoid activity and, possibly, a lower risk for infection and cognitive impairment compared with other corticosteroids (Batchelor and DeAngelis, 1996).

Vasogenic edema associated with brain tumors results from increased capillary permeability and causes neurologic dysfunction by exerting pressure on brain structures. This leads to headache with nausea, emesis and exacerbation of seizures; if severe, edema can result in herniation and death (Papadopoulos et al., 2001).

The mechanism of action of corticosteroids is not well understood. It has been argued that the anti-edema effect is due to reduction of the permeability of tumor capillaries by causing dephosphorylation of the tight junction component proteins (Drappatz et al., 2007).

Dexamethasone reduces the tumor-associated edema in patients with brain metastases or primary brain tumor as illustrated by CT studies (Sturdza et al., 2008). Dexamethasone produces symptomatic improvement within 24 to 72 hours. Generalized symptoms, such as headache and lethargy, tend to respond better than focal ones. Improvement on CT and MRI studies often lags behind clinical improvement (Drappatz et al., 2007).

Side effects of corticosteroids were dose-dependent, while the degree of neurologic improvement was independent of the dose. It is therefore generally accepted that corticosteroid dose should be titrated to the minimal dose needed to relieve symptoms. The usual starting dose is a 10 mg load, followed by 16 mg per day in patients who have significant symptomatic edema. Lower doses may be effective, especially for less severe edema. The dose may be increased up to 100 mg per day if necessary (Drappatz et al., 2007). The time to peak concentration for an oral tablet is 1-2 hours. The plasma half-life is about 2-4 hours but its biologic half-life is 36 to 54 hours (Sturdza et al., 2008).

Special care should be given to the tapering of the corticosteroid dose. Decreasing the dose by 25% every 4-5 days is a rational approach. In patients who have required corticosteroids for more than 2 weeks, the "steroid withdrawal syndrome" may occur (Rosenfeld and Pruitt, 2007). There is no standard recommendation regarding steroid dose used in patients with cerebral metastases, prior, during or following palliative radiotherapy (Sturdza et al., 2008).

A number of important aspects should be considered when prescribing steroids to maximize quality of life: (I) the lowest effective dose possible of corticosteroids should be used; (II) regarding peri-tumoral edema, treat the patient and not the MRI image (clinically asymptomatic edema does not require steroid treatment); (III) to reduce insomnia linked to steroids, avoid prescribing it in the evening; (IV) because of the risk of adrenal insufficiency following abrupt discontinuation, corticosteroids should be tapered progressively unless steroids have been administered for two weeks or less (Stupp et al., 2008).

Corticosteroid intake leads to a wide range of side effects. Side effects of corticosteroids include endocrine disorders such as Cushing's syndrome and disturbances of the blood sugar level ranging up to diabetes mellitus, muscular weakness, increasing risk of infectious diseases, gastrointestinal complications such as ulcers or bleeding, atrophic changes of skin, and hematological and psychiatric disorders (Sturdza et al., 2008). Longer duration of treatment (more than 3 weeks), higher doses, and hypoalbuminemia are associated with greater toxicity (Drappatz et al., 2007).

The side effects associated with corticosteroids are the driving force to search for alternative therapies for cerebral edema. Corticotropin-releasing factor reduces peritumoral edema by a direct effect on blood vessels through Corticotropin-releasing factor 1 and 2 receptors. Phase I/II trials of this agent suggest that it is relatively well tolerated. Several phase III trials are in progress examining the efficacy of this drug in the treatment of acute and chronic peritumoral edema. Preliminary studies suggest that cyclooxygenase-2 (COX-2) inhibitors may be effective in treating cerebral edema but further clinical studies using cyclooxygenase-2 inhibitors have been delayed due to the cardiovascular complications of this class of drugs. Inhibitors of vascular endothelial growth factor or inhibitors of its receptors reduce tumor-related edema. These classes of drugs eventually may prove more effective and less toxic alternatives to corticosteroids (Drappatz et al., 2007).

Future Endocrinal Therapy

Thyroid hormone plays a major role in normal brain maturation and affects the development of astrocytes. Increasing evidence has suggested that aberrant expression of thyroid hormone receptors isoforms could be associated with tumorigenesis. Thyroid hormone receptors expression was compared between low grade and high grade astrocytomas. A study demonstrated for the first time that thyroid hormone receptors isoforms are indeed expressed in human astrocytomas. The expression of thyroid hormone receptors isoforms is correlated to the malignancy grading of astrocytomas. The frequency of thyroid hormone receptors ?1 or ?2 expression significantly decreased with the grade of malignancy. However, the frequency of thyroid hormone receptors ?1 expression significantly increased with the grade of malignancy. This result provides insight into the potential use of hormonal therapy for brain tumors that overexpress or underexpress thyroid hormone receptors (Hwang et al., 2008).

GENE AND VIRAL THERAPY

Advances in understanding and controlling genes and their expression have set the stage to alter genetic material to prevent or fight disease with brain tumors being among one of the first human malignancies to be targeted by gene therapy. All proteins are coded for by DNA and most neoplastic diseases ultimately result from the expression or lack of one or more proteins (e.g., coded by oncogenes or tumor suppressor genes, respectively). Therefore, diseases could be treated by expression of the appropriate protein in the affected cells (Asadi-Moghaddam and Chiocca, 2009).

Currently available treatments for brain tumors necessitate the development of more effective tumor-selective therapies. The use of gene therapy for brain tumors is promising as it can be delivered in situ and selectively targets tumor cells while sparing the adjacent normal brain tissue (Germano and Binello, 2009).

Gene therapy is an experimental treatment that involves introducing genetic material (DNA or RNA) into cells, and it has made important advances in the past decade. Within this short time span, it has

moved from the conceptual laboratory research stage to clinical translational trials for brain tumors (Asadi-Moghaddam and Chiocca, 2009).

Two crucial considerations in gene therapy relate to: (1) what gene should be delivered for expression, and (2) how to deliver it. In its simplest form, gene therapy is the process by which either defective or missing genes are replaced, or new genes for new functions are introduced. For this purpose, the genetic material is coupled to additional regulatory sequences (e.g., promoters and enhancers) and is packaged inside a gene delivery vehicle to enable transfer and expression of the gene product inside the cell (Fig. 13) (Asadi-Moghaddam and Chiocca, 2009).

The first step of gene therapy involves gene delivery to facilitate the expression of the therapeutic gene in the interior of a cell. The simplest method is the direct introduction of therapeutic DNA into target cells by physical (i.e., electroporation) or chemical techniques (i.e., lipofection). This approach still remains limited in its application, as it is relatively inefficient, it can be used only with certain tissues and requires large amounts of DNA (Anwer, 2008).

The next difficulty for the foreign genetic material is that once within the cell, it must escape intracellular degradation to enter the nucleus to be expressed (Lechardeur and Lukacs, 2006). Therefore, gene delivery systems (vectors) were designed to protect the genetic material (Fig. 13). An ideal vector needs to meet three criteria: (1) it should protect the transgene (the transferred gene) against degradation by nucleases in the extracellular matrix; (2) it should bring the transgene across the plasma membrane and then into the nucleus of the target cells; and (3) it should have no harmful effects (Bergen et al., 2008).

Vectors for Gene Therapy

Gene delivery can be accomplished using different vectors. Historically, viral vectors were the first used. Cell-based transfer and synthetic vectors have subsequently been developed and seem to be promising gene delivery methods (Germano and Binello, 2009).

(1) Viral Vectors:

Viruses used in brain tumor therapy can be divided into two categories:

A) Replication-incompetent viruses (viral vectors) that can not replicate and they act as mere vectors for gene transfer. In this case, the vector is derived from a virus from which all or most of the viral genes have been removed to minimize the virus-mediated toxicity. Two replication-incompetent viruses, retrovirus and adenovirus, were studied in clinical trials (Asadi-Moghaddam and Chiocca, 2009).

B) Replication-competent viruses (oncolytic viruses) that infect then replicate and lyse tumor cells with or without gene transfer and these viruses are used in the oncolytic virotherapy. In this case, selected viral genes are deleted or mutated so that viral targeting and/or replication can occur selectively in the tumor cells. Four replication-competent viruses; herpes simplex virus, replicating adenovirus, reovirus and Newcastle disease virus, were studied in clinical trials (Asadi-Moghaddam and Chiocca, 2009).

The most effective way to transfer DNA into somatic cells remains the use of viral vectors. Gene transfer using viral vectors has been carried out in numerous clinical trials. Retrovirus and adenovirus are the most studied vectors for brain tumors. Retroviruses were the first used vectors. Adenoviral vectors offer several theoretical advantages over retroviruses. These include increased efficiency and ability to transcribe genes without insertion in the host genome, thereby eliminating the risk of insertional mutagenesis (Germano and Binello, 2009).

(2) Cell-based Vectors:

Neural stem cells, neural progenitor cells, embryonic stem cells derived astrocytes, bone marrow-derived stem cells, mesenchymal stem cells, endothelial progenitor cells, and fibroblasts were used for gene transfer (Germano and Binello, 2009).

(3) Synthetic Vectors:

Liposomes are the most studied nanoparticles for gene delivery. Liposomes are spherical vesicles with a membrane composed of a phospholipid and cholesterol bilayer, and can deliver and release DNA and drugs (Benítez et al., 2008). Liposomes become an attractive alternative to viral vectors. Their main advantage includes safety and lack of immunogenicity but they have low efficiency. Liposomal vectors have been used to deliver therapeutic genes in rodents and in clinical trials for brain tumors (Germano and Binello, 2009).

Figure 13. Mechanism of vector-mediated gene therapy: a vector binds to the cell membrane then enters the cell. The gene then travels to the nucleus, where it becomes active. In the nucleus, the gene becomes transcribed into mRNA then mRNA becomes protein, and the desired effect occurs. In this case, the desired effect was to cause apoptosis of tumor cells by introduction of an apoptotic gene (Asadi-Moghaddam and Chiocca, 2009).

Approaches of Gene Therapy

Five gene therapy approaches are currently being explored: (1) Suicide gene therapy (pro-drug activating gene therapy or chemotherapy-sensitizing gene therapy); (2) Tumor suppressor gene therapy; (3) Immunogene therapy (genetic immune modulation); (4) Anti-angiogenic gene therapy; and (5) Oncolytic virotherapy.

(1) Suicide gene therapy:

It is the most commonly used technique in clinical trials for brain tumors. It involves transducing tumor cells with a gene encoding an enzyme that can metabolize a nontoxic pro-drug to its toxic form (Asadi-Moghaddam and Chiocca, 2009).

Herpes simplex virus type-1 thymidine kinase/ ganciclovir: in this approach, the tumor cells are transduced to express herpes simplex virus thymidine kinase using retroviral or adenoviral vector and treated with the antiviral agent ganciclovir. Herpes simplex virus thymidine kinase is an enzyme that metabolizes nontoxic drugs such as ganciclovir, acyclovir, or valacyclovir, into a cytotoxic metabolite. The ganciclovir metabolite is incorporated into DNA, causing DNA elongation to terminate and subsequently cause cell death (Chen et al., 1994). Preclinical experiments demonstrated marked tumor

elimination, despite gene transfer into only a small fraction of tumor cells. This was due to the cytotoxic effect of the transduced cells on the adjacent non-transduced cells which is called the bystander effect. Furthermore, the tumor cells treated with this approach displayed enhanced sensitivity to radiation in culture and in vivo (Asadi-Moghaddam and Chiocca, 2009).

Cytosine deaminase/5-Fluorocytosine: in this approach, the tumor cells are transduced to express cytosine deaminase and treated with the antifungal agent 5-Fluorocytosine. 5-Fluorocytosine is a pro-drug converted into cytotoxic active agent 5-fluorouracil by cytosine deaminase. 5-Fluorocytosine is nontoxic to human cells because of the lack of cytosine deaminase. The toxic effects of 5-fluorouracil are mediated by its intracellular metabolites, which cause DNA strand breakage leading to cell death (Aghi et al., 1998). Two preclinical studies for glioblastoma using an adenoviral vector carrying the cytosine deaminase gene demonstrated promising results (Asadi-Moghaddam and Chiocca, 2009).

Cytochrome P450/cyclophosphamide: as in previous approaches, cyclophosphamide is a pro-drug that is activated by liver-specific enzymes of the cytochrome P450 family. The active form of cyclophosphamide (phosphoramide mustard) is an alkylating agent that generates DNA cross links and consecutive DNA strand breaks. The efficacy of cyclophosphamide in treating brain tumors has been limited by the fact that its active metabolites are poorly transported across the blood-brain barrier (Wei et al., 1994).

Replacement of parts of the herpes simplex virus type-1 genome with the cytochrome P450 gene has led to the design of a herpes simplex virus type-1 vector that can kill tumor cells through three modes: 1) using viral oncolysis, 2) making the infected cell sensitive to cyclophosphamide, and 3) making the infected cell sensitive to ganciclovir. Animal studies of glioma cell lines showed tumor regression when the animals were treated with this modification (Asadi-Moghaddam and Chiocca, 2009).

(2) Tumor suppressor gene therapy:

This includes transfer of tumor suppressor genes and cell-cycle modulators. The p53 tumor suppressor protein regulates cell-cycle progression and apoptosis in response to many external insults (e.g., DNA damage and oncogenic mutations) (Vousden K and Lane, 2007).

Mutations in the p53 gene resulting in loss of its function are common in astrocytomas, and are associated with tumor progression from low-grade astrocytoma to glioblastoma (Louis et al., 2001). Accordingly, the p53 gene became an attractive candidate for gene transfer in an attempt to restore cell cycle regulation in p53-mutated cells and induce apoptosis, even in tumors with intact functional genes, by causing enhanced expression of the gene product (Roth, 2006).

Another commonly affected cell-cycle pathway in gliomas is the retinoblastoma protein/cyclin-dependent kinases/cyclin-dependent kinase inhibitors circuit. Preliminary studies restoring the genomic region of these defects in glioblastoma cell lines demonstrated tumor growth arrest or apoptosis. Another candidate for a gene therapy approach is the epidermal growth factor receptor gene, which shows frequent amplification in primary glioblastomas (Asadi-Moghaddam and Chiocca, 2009).

Transfer of several pro-apoptotic genes, such as factor-related apoptosis-inducing ligand has shown promising pre-clinical results in vitro and in vivo studies. Transfer of caspase-8 was showed to augment apoptosis in vitro on human malignant glioma cells (Germano and Binello, 2009).

 (3) Immunogene therapy:

Gene therapy approaches aiming at genetic immune modulation enhance the immune response against tumors by expressing cytokines and lymphokines. The frequently used cytokines to achieve genetic immune modulation are interleukin-2, interleukin-4, interleukin-12, interferon ?, interferon ?, and granulocyte-macrophage colony stimulating factor. Several studies have been performed by infecting tumor cells ex vivo (outside the patient) with cytokine genes. Another model introduces immune

modulating genes into the tumor so infection occurs in situ. Several phase I trials are currently underway using this strategy. However, for interleukin-2 and interferon ? severe CNS toxicity has been reported when these cytokines were secreted by tumor cells intracranially (Asadi-Moghaddam and Chiocca, 2009).

In a recent clinical trial in humans with recurrent gliomas, an adenoviral vector was employed to deliver the gene for human interferon ? and this resulted in increased apoptosis in tumors (Chiocca et al., 2008). Gene therapy can also be used to generate tumor vaccines by inducing tumor antigen presentation by antigen presenting cells (Asadi-Moghaddam and Chiocca, 2009).

(4) Anti-angiogenic gene therapy:

Neovascularization is a feature of malignant gliomas and is dependent on several potent angiogenic factors secreted by tumor cells. Vascular endothelial growth factor is an important overexpressed angiogenic factor in gliomas. It has been shown that in vivo transfer of an adenoviral vector carrying gene for vascular endothelial growth factor in an antisense form inhibits glioma growth. Antisense form means single strands of DNA or RNA that can bind to a complementary RNA sequence preventing protein synthesis (i.e. preventing vascular endothelial growth factor synthesis) (Asadi-Moghaddam and Chiocca, 2009).

Other strategies are directed at the vascular endothelial growth factor receptors. A mutant vascular endothelial growth factor receptor was transferred into xenografted glioma using retroviral vector. Histological analysis revealed reduced vascular density, decreased tumor cell proliferation and increased apoptosis and necrosis (Benítez et al., 2008).

(5) Oncolytic Virotherapy:

Oncolytic viruses are engineered to selectively replicate in cancer cells killing these cells without affecting healthy cells. The virus replicates in the cancer cell then thousands of new viruses are released and infect neighboring cancer cells (Fig. 14). The cycle continues until tumor cells are completely eradicated (Benítez et al., 2008).

These viruses are capable of selectively lysing tumor or dividing cells, making their use potentially safe in the brain where most cells are not-dividing (Germano and Binello, 2009).

Oncolytic Adenovirus

Oncolytic adenoviruses are engineered or naturally occurring strains of virus that replicate better in tumor cells versus normal cells (Asadi-Moghaddam and Chiocca, 2009).

Figure 14. General mechanism of oncolytic virus selectivity (Asadi-Moghaddam and Chiocca, 2009).

Herpes Simplex Virus Type-1

Herpes simplex virus type-1 as an oncolytic virus offers several advantages: 1) the ability to incorporate large load of foreign DNA; 2) the ability to infect human cells with high efficiency; 3) the sensitivity to the anti-herpetic agents, such as ganciclovir providing a safety mechanism by which virus replication can be eliminated when needed; and 4) herpes simplex virus type-1 never integrates into the host genome so the risk of insertional mutagenesis is absent. The disadvantage of herpes simplex virus type-1 is that its genetic manipulation is more difficult than that of adenoviruses due to the large size of the viral genome (Asadi-Moghaddam and Chiocca, 2009).

Newcastle Disease Virus (NDV)

A phase I/II trial of intravenous infusion of an oncolytic Newcastle disease virus in glioblastoma patients reported that good tolerance to the virus was observed, but only one patient achieved a complete response (Freeman et al., 2006).

Reovirus

Reovirus can replicate to very high levels in an infected cell, and thus it could provide a good therapeutic effect because the large number of virus could infect a large number of neighboring tumor cells. One of the issues that limit use of this virus relates to its small size which makes genetic manipulation difficult (Asadi-Moghaddam and Chiocca, 2009).

Gene Therapy Trials

Six viruses; two replication-incompetent (retrovirus and adenovirus) and four replication-competent (adenovirus, herpes simplex virus type-1, Newcastle disease virus and reovirus) have been studied in clinical brain tumor trials in which the results have been published. Measles virus has been studied in glioma patients in a trial that is still ongoing and whose results have yet to be published (Asadi-Moghaddam and Chiocca, 2009).

Retrovirus and adenovirus have been genetically modified to express herpes simplex virus thymidine kinase with concurrent ganciclovir administration. The first study using gene therapy in glioma patients used stereotactic intratumoral inoculation of a retroviral vector carrying herpes simplex virus thymidine kinase gene in 15 patients with recurrent malignant brain tumor. Although this study showed

some promising results in terms of antitumor efficacy, it was not a controlled or randomized protocol (Ram et al., 1997).

Two subsequent phase I and II studies in patients with recurrent glioblastoma were performed using a herpes simplex virus thymidine kinase followed by ganciclovir administration. The first study involved 12 patients and no treatment-related adverse effects were noted. The overall median survival was 6.8 months, with three patients surviving > 12 months and one patient was recurrence-free 2.8 years after treatment (Klatzmann et al., 1998).

A comparable international, multicenter study used herpes simplex virus thymidine kinase with concurrent ganciclovir administration in 48 patients with recurrent gliomas. The median survival time was 8.6 months, with a 12-month survival rate of 27%. Tumor recurrence was absent on MRI in seven patients for at least 6 months and in two patients for 12 months, and one patient remained recurrence-free at 24 months (Shand et al., 1999).

A similar phase I study was performed in 12 children (aged 2 to 15 years) with recurrent malignant supratentorial tumors. This study also used the previous approach. Disease progression occurred at a median time of 3 months after treatment, and the longest time until progression was 24 months with no adverse effects were noted (Packer et al., 2000).

A large controlled phase III trial seemed necessary for confirmation of the efficacy of the retroviral herpes simplex virus thymidine kinase with ganciclovir administration approach. This study used an adjuvant gene therapy protocol to the standard therapy of maximum surgical resection and irradiation for newly diagnosed glioblastoma. After 4 years of follow-up of 248 patients, survival analysis showed no advantage of gene therapy in terms of tumor progression and overall survival (Rainov, 2000).

A phase I/II trial evaluated an adenoviral vector expressing the herpes simplex virus thymidine kinase with ganciclovir administration gene in patients with primary or recurrent high-grade gliomas. This study was performed in a controlled randomized fashion on 36 patients (i.e., 17 in the treatment arm and 19 in the control group). The treatment group underwent surgical resection and adenoviral injection, followed by intravenous ganciclovir on postoperative day 5 for 14 days. The control group underwent resection only. The median survival in the gene therapy group was significantly longer than the control group (62 vs. 37 weeks) (Immonen et al., 2004).

An adenoviral vector delivering a wild-type copy of the p53 transgene was also evaluated in humans with recurrent gliomas. The vector was initially introduced by an implanted catheter placed in the middle of the tumor bed, followed by resection of the tumor to allow for studies related to p53 gene delivery and distribution. Again, while the treatment was well tolerated, the distribution of p53 gene into tumor was relatively low (Lang et al., 2003).

One recent trial evaluated an adenoviral vector in 11 patients with recurrent high-grade glioma introduced by stereotactic injection, followed by surgical resection and an additional injection of the vector into the tumor bed. The vector was well-tolerated in all patients. One patient experienced confusion after the postoperative injection, which was believed to be caused by local brain toxicity. Tumor resection after the viral treatment showed a biologic effect from the transferred gene (Chiocca et al., 2008).

A phase I trial evaluated the safety of an oncolytic herpes simplex virus type-1 in 9 patients with recurrent glioblastoma. Direct intratumoral injection was performed with no adverse clinical symptoms or induction of encephalitis, nor reactivation of latent herpes simplex virus, thus appearing as if tumor progression was controlled with some efficacy (Rampling et al., 2000).

Another clinical trial using a different herpes simplex virus type-1 mutant included 21 patients with recurrent glioma with similar results as they relate to safety (Markert et al., 2000).

Strategies to decrease the immune response prior to administration of virus have also provided strong pre-clinical data. The therapeutic effects of the oncolytic herpes simplex virus can be enhanced by co-administration of cyclophosphamide, which decreases production of gamma-interferon and results in additional spreading of the virus (Fulci et al., 2006).

A large phase III trial in Europe using herpes simplex virus type-1 is being conducted (Asadi-Moghaddam and Chiocca, 2009).

Enhancing Gene Therapy

Currently, two delivery methods to enhance gene transfer have provided promising laboratory results: (1) convection-enhanced delivery and (2) ultrasound. The use of convection-enhanced delivery to homogenously cover larger areas of the brain has been described in laboratory studies and clinical trials for drug delivery. In convection-enhanced delivery, programmable pumps and specifically designed catheters are used. The hypothesis that ultrasound can be used to enhance efficacy of chemotherapeutics and intracellular gene delivery in glioma cells in vitro was recently tested in gliosarcoma model. The findings suggest that ultrasound may be useful to increase the efficacy of chemotherapy and gene therapy (Germano and Binello, 2009).

Finally, the problem of delivering genetic vectors into brain tumors and the efficient in situ gene transfer remains one of the most significant difficulties in gene therapy. The completed brain tumor gene therapy trials have offered some promising results. However, the results of most clinical studies have not lived up to the expectations created by experimental data. Although gene delivery to human patients seems to be safe, these studies have not yet translated into benefits in the clinic. At present, gene therapy is being studied in trials for brain tumors, and so far it is not available outside of a clinical trial (Asadi-Moghaddam and Chiocca, 2009).

DISCUSSION

It is thought that maximal surgical resection of gliomas, the commonest primary brain tumor, significantly improves survival but there is no good evidence from randomized trials that resection offers any survival advantage (Metcalfe and Grant, 2001).

Collected data on symptoms before and after surgical resection of brain tumors report that 32% had an improvement in their symptoms, 58-76% were not different, and 9-26% had a worsening. Neurosurgery can improve some symptoms, but it can also create new ones, it can be associated with local or systemic complications (Hart and Grant, 2007).

Despite advances in radiotherapy and chemotherapy along with surgical resection, the prognosis of patients with malignant brain tumors is poor. Therefore, the development of new treatment modalities is extremely important. Most brain tumor patients usually undergo multimodality treatments after histological diagnosis (Yamanaka and Itoh, 2007).

There are many controversies surrounding current non-surgical therapies. Drug delivery across the blood-brain barrier is one of the vital problems, so the dilemma now is what is the most efficient way to deliver drugs to the brain (Sawyers, 2006)? Also, it is difficult for the drugs to reach the intraparenchymal regions in the brain beyond the resection cavity. This becomes a burden with micrometastases and infiltrative satellites that are commonly seen in glioma. Therefore, researchers developed convection-enhanced delivery as a novel way to deliver drugs across blood-brain barrier as well as over a large volume of tissue. This method has become especially relevant in brain tumors as it bypasses blood-brain barrier and reduces systemic toxicities (Mercer et al., 2009).

There are several non-surgical treatment modalities for brain tumors starting with radiation therapy and chemotherapy that represent the backbone of the non-surgical treatment, till immunotherapy and

antiangiogenic therapy that showed promising clinical results, and finally gene and viral therapies that hold promise in the near future. Also, we can not forget the endocrinal therapy that showed great success in the field of management of pituitary adenomas.

Radiation therapy plays a primary role in the management of most malignant and many benign brain tumors. It is used frequently postoperatively as (Stieber and Mehta, 2007):

(a) Adjunctive therapy: to 1) decrease local failure, 2) delay tumor progression, and 3) prolong survival (as in malignant gliomas).

(b) Curative treatment for diseases such as primitive neuroectodermal tumors and germ cell tumors.

(c) Therapy that stops further tumor growth as in schwannoma, meningioma, pituitary tumors, and craniopharyngioma.

Survival benefit for postoperative whole brain radiation therapy for glioma was demonstrated in studies going back three decades ago. Median survival increased from 17 weeks in patients treated with conventional measures to 37.5 weeks in patients treated with postoperative whole brain radiation therapy (Norden and Wen, 2006).

Subsequent advances in radiotherapy techniques have used advanced imaging of the tumor and focused on radiotherapy techniques that maximize treatment to the tumor while minimizing radiation to normal brain tissue. Focal radiotherapy, termed involved field radiotherapy has replaced whole brain radiation therapy as the standard approach. Some types of involved field radiotherapy include 3 dimensional conformal radiotherapy and intensity modulated radiotherapy that provides particular advantages when the target is critically close to the radiation-sensitive structures, as the dose to these structures can be minimized without affecting the dose to the tumor that needs to be treated (Omay and Vogelbaum, 2009).

Radiotherapy has side effects that include local scalp irritation, hair loss, somnolence and fatigue in the short term. Late side effects include radiation leuco-encephalopathy (comprising of dementia, incontinence and gait disturbance). Its effectiveness, ease of administration and acceptable short-term side effects profile make it a standard treatment for the most of malignant brain tumors (Hart and Grant, 2007).

Because the majority of malignant gliomas recur in close proximity to the original tumor and multifocal or disseminated disease is uncommon, there is great interest in maximizing the radiation dose to the tumor bed without increasing radiation exposure to the surrounding brain tissue. Stereotactic radiosurgery offer this advantage. It can deliver a high accurate single dose of radiation (Omay and Vogelbaum, 2009).

Stereotactic radiosurgery with a Gamma Knife or CyperKnife is now being used as (A) primary management or (B) booster treatment with whole brain radiation therapy, in brain metastasis (Yamamoto, 2007). Controversy persists as to whether stereotactic radiosurgery should be combined with whole brain radiation therapy? Although some reports have supported combining stereotactic radiosurgery with whole brain radiation therapy to achieve optimal tumor control and survival periods (Chidel et al., 2000), others have questioned usefulness of whole brain radiation therapy (Sneed et al., 2002). Neither the survival rate nor the local tumor recurrence rate differs significantly between groups with vs without whole brain radiation therapy (Yamamoto, 2007).

Although the size limitation on treatable lesions (should be < 4 cm) is crucial in stereotactic radiosurgery, tumor control rates of 90% can be expected if 1-4 lesions are irradiated with a peripheral dose of 20 Gy or more. In such cases, true recurrence is extremely rare. Post-radiosurgical MRI reveals disappearance of the tumors and complete cure (Pollock and Brown, 2002).

Gamma Knife radiosurgery is superior to whole brain radiation therapy for several reasons: (1) a brief hospital stay, (2) Higher control rates and earlier symptom palliation, (3) All observable lesions on MRI can be treated, (4) Other treatments, such as surgery and chemotherapy, need not be interrupted, (5) Availability of an alternative treatment allows whole brain radiation therapy to be reserved for subsequent treatment attempts when it become absolutely necessary, (6) Radiosurgery can be repeated, even after whole brain radiation therapy which usually cannot be repeated, (7) Patients never lose all of their hair, (8) A very low incidence of dementia can be expected, (9) Radiosurgery can be used as postoperative irradiation instead of whole brain radiation therapy that has several disadvantages (Yamamoto, 2007).

Stereotactic radiosurgery may represent also an alternative or supplementary modality to conventional surgery in small-volume low-grade astrocytomas. There is also evidence for beneficial effect of stereotactic radiosurgery on the survival of patients with high-grade gliomas (Szeifert et al., 2007). Future studies are needed to identify patients most likely to respond to stereotactic radiosurgery (Biswas et al., 2009).

Multiple studies demonstrated the efficacy and safety of stereotactic radiosurgery in treatment of meningioma, with tumor control rates ranging from 60 to 100% depending on the proportion of atypical or malignant meningiomas (Lee et al., 2002). Given the benefit of radiosurgery, stereotactic radiosurgery is increasingly used to treat patients as a primary therapy based on imaging criteria only (Flickinger et al., 2003). Radiosurgery is considered as an effective management choice for patients with small to medium-sized symptomatic, newly diagnosed or recurrent meningiomas of the brain (Kondziolka et al., 2008).

Stereotactic radiosurgery is also safe and highly effective treatment for patients with pituitary adenomas. Radiosurgery provides control of tumor growth in nearly all cases and hormone normalization in the majority of secretory tumors (Mayberg and Vermeulen, 2007).

Although, stereotactic radiosurgery is not a risk-free treatment and some patients may develop complications. Complications of stereotactic radiosurgery that have been reported include acute coronary events, severe pain, headaches, facial pain, new motor deficits, symptomatic hydrocephalus and delayed seizures (Vachhrajani et al., 2008).

Chemotherapy has played primarily an adjuvant role because of efficacy limitations related to the non-specific nature of chemotherapeutic agents, drug delivery issues, and inherent tumor chemoresistance (Cairncross et al., 1998). There is now clear evidence that chemotherapy can improve survival. Debates have continued over whether this increase in survival is clinically significant. Further trials need to consider the quality of life as well as survival benefit (Hart and Grant, 2007). The most commonly used drugs were nitrosoureas, either alone or in combination with other agents (Walker et al., 1980).

Temozolomide is now the preferred agent for concurrent and adjuvant chemotherapy in patients with gliomas (Batchelor and Supko, 2008). More recent evidence supports temozolomide effect as a radiosensitizer also (Van Nifterik et al., 2007). Temozolomide was associated with significant improvements in median survival, progression-free survival, overall survival, and two-year survival. This improvement in survival was achieved with no harmful effect on the quality of life of the patients (Batchelor and Supko, 2008).

Concomitant chemo-radiotherapy followed by single-agent adjuvant treatment with the temozolomide was associated with a significant improvement in median survival and also it was well tolerated in all patients (Stupp et al., 2005). It is the current standard of care for glioma patients. Concomitant chemo-radiotherapy as well as the early introduction of chemotherapy, appears to be the key to improving outcome (Stupp et al., 2007).

Efforts to maximize efficacy of chemotherapy by overcoming the blood-brain barrier limitations

included: (1) intra-arterial delivery, (2) disruption of the blood-brain barrier and (3) implantation of the drug directly in the tumor bed. Dose intensification intended to increase drug gradients across the blood-brain barrier for improved penetration is often faced by added systemic toxicity (Ashby et al., 2001).

Trials of molecularly targeted drugs as monotherapy for gliomas were disappointing, with some potential benefit when used in combination with nitrosurea or temozolomide. Combinations of multitargeted therapies are currently the focus of clinical trials (Kreisl, 2009).

Two aspects are noteworthy in regard to chemotherapeutic molecular agents. Pretreatment molecular profiling of tumors will be increasingly needed to determine if the mechanism of a drug is appropriate to the genetic alterations found within individual tumors. In addition to traditional clinical endpoints, biological end-points (change in serum tumor markers, measures of target inhibition) seem to be appropriate, and in particular dosing schedules in phase I trials that focus on the determination of an optimal biological dose rather than the maximum tolerated dose must be explored. Finally, multitargeted agents are needed to target simultaneously multiple signaling pathways that concur at the same time or sequentially, as a compensatory activation, to tumor growth and resistance to treatments (Soffietti et al., 2007).

Better understanding of the molecular pathogenesis of brain tumors will allow development of specifically targeted therapies. Correlative molecular studies, now included in most ongoing trials, will enhance our understanding of this disease and allow for rapid further improvement in outcome (Stupp et al., 2007).

Immunotherapy gives the promise of targeting the tumor cells for destruction with an excellent specificity and efficiency, while at the same time completely sparing the normal cells (Mitchell et al., 2008). Immunotherapeutic approaches include: 1) passive immunotherapy, 2) active immunotherapy (tumor vaccines), and 3) adoptive immunotherapy (largely abandoned nowadays) (Omay and Vogelbaum, 2009).

Passive immunotherapy using the anti-vascular endothelial growth factor monoclonal antibodies bevacizumab (Avastins®) showed good results specially the combination of bevacizumab and chemotherapy that have shown promising response rates and evidence of prolongation of survival in recurrent glioblastoma patients (Vredenburgh et al., 2007).

Active immunotherapy or a successful vaccine for brain tumors has been a 'holy grail' in neuro-oncology (Ebben et al., 2009). Peptide-based vaccines represent a major focus of cancer vaccine research (Yamanaka and Itoh, 2007). Dendritic cell-based vaccines has also shown potential efficacy as a method of overcoming chemotherapy resistance (Liu et al., 2006). There are several promising vaccines under investigation in different phases including: (1) CDX-110, (2) DCVax, (3) Oncophage, (4) Poly-ICLC (Das et al., 2008).

Immunotherapy did not emerge as a single-modality treatment for brain tumors, but it may take its place as a very effective adjuvant therapy. Dendritic cell and peptide-based therapies appear promising as an approach to successfully induce antitumor immune response and prolong survival. They seem to be safe and without major side effects. Their efficacy should be studied in randomized controlled clinical trials (Yamanaka, 2009).

However, a number of limitations still exist on the clinical efficacy of immunotherapy with antibodies. The main unresolved problem stems from the question whether treatments administered systemically can achieve clinically significant levels at the site of intracranial tumors. Sufficient levels may not be achieved with the dosage and systemic route of administration. Clinical attempts to 'open up' the Blood-brain barrier to drug delivery have met with both failure and toxicity. Other problems that remain for systemic delivery are the high interstitial pressures in the tumor and surrounding tissue which prevent

perfusion. In addition, the use of antibodies introduces a problem that is the antibodies themselves are antigenic. The development of human anti-human antibodies against the therapeutic antibodies is not infrequent event (Mitchell et al., 2008).

Currently, anti-angiogenic therapy is being increasingly adopted for treating glioblastoma (Argyriou et al., 2009). Because of vascular endothelial growth factor prominent role in glioma angiogenesis, its pathway was rapidly identified as an attractive therapeutic target (Chi et al., 2009).

Bevacizumab (Avastin®), as mentioned before, is an anti-vascular endothelial growth factor monoclonal antibodies but also has strong anti-angiogenic effect. Bevacizumab showed promising radiographic response proportions in recurrent glioblastoma and in recurrent anaplastic glioma. Responses were also associated with neurological improvement and reduction or discontinuation of corticosteroid requirements (Wagner et al., 2008). Additionally, several retrospective studies have reported similar findings with bevacizumab. Radiographic response proportions between 35% and 50% were observed with combination of bevacizumab and conventional chemotherapy (Guiu et al., 2008).

Cediranib which is a potent pan-vascular endothelial growth factor receptor inhibitor with modest activity against platelet derived growth factor receptor showed radiographic response in 56% of patients and there was a modest improvement in median overall survival. Furthermore, an antiedema effect was detected (Batchelor et al., 2008).

Other anti-angiogenic drugs include epidermal growth factor receptor inhibitors such as cetuximab, gefitinib and erlotinib. A phase I/II study investigating the efficacy and safety of cetuximab plus concomitant chemoradiotherapy is currently ongoing. Monotherapy with gefitinib is not superior in terms of efficacy over conventional chemotherapy. Overall, therapy with erlotinib or gefitinib is associated with modest therapeutic efficacy (Argyriou et al., 2009). The newer platelet derived growth factor receptor inhibitors, tandutinib and dasatanib, have potentially greater efficacy in malignant gliomas due to improved CNS penetration, and they are in clinical trials for recurrent gliomas (Chi et al., 2009).

A number of issues remain unresolved concerning the anti-angiogenic therapy, including: 1) toxicity profiles, 2) radiographic assessment, 3) biomarkers, and 4) resistance. A definite survival advantage has yet to be established but current evidence suggests that anti-angiogenic therapy has clinical benefits including steroid-sparing effect and increased progression-free survival (Chi et al., 2009).

Toxicity profiles of anti-angiogenic therapy include risks of hemorrhage and thrombosis that have been ongoing concerns with the use of anti-angiogenic drugs. Epistaxis is not infrequent. The intracranial hemorrhage risk appears to be low, and events are often asymptomatic (Norden et al., 2008). Common toxicities include fatigue and hypertension. Impaired wound healing is also observed (Lai et al., 2008). Other toxicities include proteinuria, skin toxicity, diarrhea and mucositis. Other rare but serious complications include myocardial infarction, stroke, posterior leukoencephalopathy, and thrombotic thrombocytopenic purpura (Chi et al., 2009).

Standard criteria of response to anti-angiogenic therapy, currently in use, are dependent on contrast-enhancement on CT or MRI (Macdonald et al., 1990). Obtaining accurate assessment of response is problematic in brain tumor patients (Sorensen et al., 2008).

Clinical data suggest that benefits gained from anti-angiogenic agents will be short-lived. One of the major problems is that almost all patients treated with anti-angiogenic therapy progress during treatment, as tumors finally acquire resistance (Quant et al., 2009).

Currently available treatments for brain tumors necessitate the development of more effective tumor-selective therapies. The use of gene therapy for brain tumors is promising as it can be delivered in situ and selectively targets the tumor cells while sparing the adjacent normal tissue (Germano and Binello,

2009). Five gene therapy approaches are currently being explored: (1) Suicide gene therapy; (2) Tumor suppressor gene therapy; (3) Immunogene therapy; (4) Anti-angiogenic gene therapy; and (5) Oncolytic virotherapy (Asadi-Moghaddam and Chiocca, 2009).

Suicide gene therapy is the most commonly used technique. Preclinical studies showed marked tumor elimination. Furthermore, the tumor cells treated with herpes simplex virus thymidine kinase with ganciclovir administration displayed enhanced sensitivity to radiation. A phase I/II trial evaluated this approach in glioma patients. The median survival in the gene therapy group was significantly longer than the control group (62 vs. 37 weeks) (Asadi-Moghaddam and Chiocca, 2009).

At present, gene therapy is being studied in trials for brain tumors, and so far it is not available outside of these clinical trials (Asadi-Moghaddam and Chiocca, 2009). Finally, gene therapy has not yet been an effective tool in the treatment of gliomas. It offers new ways to attack these tumors and when used together with surgery, chemotherapy and radiation therapy, it will be a valuable adjuvant therapy to improve survival and the quality of life of the patients (Benítez et al., 2008).

Problem of the delivery of genetic vectors into solid brain tumors and efficient in situ gene transfer remains one of the most significant hurdles in gene therapy. The efficiency of transduction could be improved. The currently used manual injection of vectors might be improved by the use of three-dimensional neuronavigation techniques and convection-enhanced delivery methods (Asadi-Moghaddam and Chiocca, 2009).

The disadvantages of gene therapy include low tumoricidal effect and a limited distribution of the transgenes (transferred genes) and/or the vectors to tumor cells, making treatment localized peripherally from the main tumor mass. In addition virus-derived vectors may have the potential to create damaging immunological reactions by immune-mediated toxicity, especially in the presence of circulating antibodies to the virus vectors or by triggering immune reactions to self or transgene antigens (Omay and Vogelbaum, 2009).

The medical approach to pituitary adenomas greatly improved since availability of dopamine agonists such as: bromocriptine, cabergoline and quinagolide, and availability of somatostatin analogues such as: lanreotide and octreotide (Colao et al., 2009).

Bromocriptine is successful in 80-90% of patients with microprolactinomas in normalizing serum prolactin level, restoring gonadal function and shrinking tumor mass. For macroprolactinomas, normalization of serum prolactin level and tumor mass shrinkage occur in about 70% of patients. Cabergoline also is widely used to treat prolactinomas. The tumor shrinking effect of cabergoline is very rapid and improvement of visual field defects can be detected even after the administration of the first tablet. Cabergoline is superior over bromocriptine and can be given to patients previously intolerant or resistant to bromocriptine (Colao et al., 2009).

For growth hormone secreting adenomas, dopamine agonists are primarily effective only in tumors that co-secrete prolactin or that exhibit immunostaining for prolactin (Melmed et al., 2002). Somatostatin analogues appear to be more effective. They showed reduction in tumor size in 45% of patients (Bevan, 2005). Combination treatment with dopamine agonists and somatostatin analogues could be beneficial to better suppress growth hormone level and also on tumor shrinkage. The effect of somatostatin analogues on tumor shrinkage is reversible as demonstrated by tumor re-growth after stopping somatostatin analogues (Colao et al., 2009).

For thyroid stimulating hormone secreting adenomas, treatment depends mainly on the administration of somatostatin analogues, as dopamine agonists failed to persistently block thyroid stimulating hormone secretion in almost all patients and caused tumor shrinkage only in tumors that co-secrete prolactin (Kienitz et al., 2007).

A potential disadvantage of medical therapy for pituitary adenomas in general is that it requires life-long daily or weekly treatment. Hence, issues about compliance and cost become important, especially for younger patients. Somatostatin analogues are costly, and long-term use of these drugs makes this a significant issue. Common but often transient side effects of somatostatin analogues include nausea, abdominal discomfort and diarrhea. Furthermore, somatostatin analogues inhibit the secretion of insulin and glucagon and cause cholelithiasis in up to 20% of patients (Patil et al., 2009).

Side effects of bromocriptine include nausea, dizziness (orthostatic hypotension), nasal stuffiness, difficulty concentrating, depression, psychosis, and peripheral vasospasm. These side effects can be minimized by a slow increase in medication dosage and initiation of therapy at night. A rare side effect of bromocriptine is delayed reversible visual loss. Side effects with cabergoline are similar, but appear to be less common and less severe than bromocriptine. A recent study has suggested that cabergoline administration may be associated with cardiac valve insufficiency in patients with Parkinson's disease (Patil et al., 2009).

Corticosteroids are an established treatment for symptomatic relief from brain edema and usually indicated in any patient with brain tumor. Dexamethasone is used most commonly as it has little mineralocorticoid activity and, possibly, a lower risk for infection and cognitive impairment compared with other corticosteroids (Batchelor and De Angelis, 1996). Dexamethasone produces symptomatic improvement within 24 to 72 hours. The usual starting dose is a 10 mg load, followed by 16 mg per day in patients who have significant symptomatic edema (Drappatz et al., 2007). There is a lack of randomized controlled trials to prove their effectiveness. Care needs to be taken when withdrawing steroids. Side effects are common and can be significant (Hart and Grant, 2007).

The side effects of corticosteroids are the driving force to search for alternative therapies for cerebral edema. Corticotropin-releasing factor reduces peritumoral edema by a direct effect on blood vessels. Inhibitors of vascular endothelial growth factor or inhibitors of its receptors reduce the tumor-related edema and they are more effective and less toxic alternatives (Drappatz et al., 2007).

Although the progress in understanding of the biology of brain tumors has improved, survival rates are improving only marginally. The heterogeneity of genetic and biochemical alterations and the invasive nature of malignant brain tumors especially gliomas, make the future treatment strategies for these neoplasms are likely to consist of a combination of agents or strategies that will act synergistically at different levels to stop tumor growth. Surgery and radiotherapy have not lost their importance and a recent advance in the use of concurrent chemo-radiation has improved survival in a large fraction of patients. A variety of new treatment modalities are undergoing development and are studied in various clinical trials. These new treatment modalities hold promise for further survival gains and improvement in the quality of life of the patients in the near future. Trials in this area need to be larger and randomized, with greater attention to symptom profile and quality of life in their outcome analysis.

There is no magic bullet for malignant brain tumors in the foreseeable future, and clinical improvements will likely be because of the synergistic effects of a multitargeted attack. Although preclinical data are promising, clinical trials have been delayed by ethical concerns, and all of the treatment strategies are still searching for a significant survival benefit in phase II or III clinical trials. These strategies have their own distinct advantages and limitations.

SUMMARY

Despite advances in surgery, radiation, and chemotherapy, the prognosis for patients with malignant brain tumors has not markedly improved. Moreover, the non-specific nature of the conventional therapies for brain tumors often results in incapacitating damage to surrounding normal brain and systemic tissues. There is an urgent need for a search for new treatment modalities that precisely target the tumor cells while minimizing collateral damage to the neighboring brain tissue.

For more than 30 years, stereotactic radiosurgery has provided patients with brain tumors an alternative focal treatment to surgery. Initially, only benign tumors, such as meningioma and acoustic neuroma, were selected for treatment. In the last 15 years, radiosurgery as a therapeutic option was also considered for patients with brain metastases who had controlled systemic disease and/or a good prognosis. Also we can use radiosurgery to treat small tumor residua following surgery. Radiosurgery is a safe and effective therapy. It provides local tumor control and prolongs survival.

Chemotherapy may be of value in selected patients, increasing survival by 2-3 months, and the two regimes that have been most widely used in the past were lomustine, as a single agent, and procarbazine, lomustine and vincristine as a combination regimen, more recently temozolamide is being increasingly used. There is also evidence that giving temozolamide in combination with radiotherapy, and continuing the drug thereafter, improves the survival by about 6 months, when compared to radiation alone.

Concomitant chemo-radiotherapy followed by single-agent adjuvant treatment with the alkylating agent temozolamide is the current standard of care for the patients with gliomas. The concomitant chemo-radiotherapy, as well as the early introduction of chemotherapy, appears to be a key to the improving outcome.

Recent developments in chemotherapy of brain tumors include the combination of cytotoxic, cytostatic and targeted therapies. Multitargeted compounds that simultaneously affect multiple signaling pathways, such as those involving epidermal growth factor receptors, platelet derived growth factor receptors and vascular endothelial growth factor receptors, are needed and increasingly employed.

Immunotherapy has emerged as a promising tool in the management of brain tumors. Many novel immunotherapeutic strategies are currently being investigated in human trials, and have already been found safe and efficacious in preliminary studies. Dendritic cell and peptide-based strategies are promising approaches. Their efficacy should be determined in large randomized controlled clinical trials.

An effective vaccine would be a potent addition to current therapeutic arsenal against brain cancers. Significant progress has been made that offers new hope and novel therapeutic opportunities. However, it is important to realize that much additional work lies ahead before an effective brain tumor vaccine is validated for clinical use.

Current evidence suggests that angiogenesis inhibitors have clinical utility for brain tumor patients. Clinical benefits have manifested primarily as steroid sparing effects and increased progression-free survival but a definite survival advantage has yet to be established with these drugs. Several issues and obstacles remain in the clinical development of effective anti-angiogenic agents for brain tumor patients.

Anti-angiogenic drugs such as bevacizumab, cediranib, vandentanib, aflibercept, and cilengitide are being evaluated in combination with standard therapy. Other trials are evaluating combinations of anti-angiogenic therapy with different cytotoxic agents or targeted agents in attempts to improve upon the efficacy gains already observed.

Tremendous progress has been made in the field of gene therapy over the past decade and clinical trials are beginning to show some clinical efficacy. As we learn more about the biology of brain tumors and identify efficacious genes to stop their growth, the clinical utility of gene delivery strategies will become increasingly evident.

Gene therapy offers new approaches to treat brain tumors specially gliomas. However, the results from clinical trials have not yet fulfilled the expectations generated from very successful basic experiments. Gene therapy offers new ways to attack these tumors and when used together with surgical, chemotherapeutic and radiation treatments will be a valuable adjuvant therapy to improve survival and the quality of life of patients.

The management of pituitary tumors poses a unique challenge that also requires a multimodality treatment approach. This includes neurosurgical tumor resection, medical therapy, radiotherapy and radiosurgery. Since the late 1990s, several new medical therapies have emerged. The efficacy of dopamine agonists in prolactin secreting adenomas and efficacy of somatostatin analogues in growth hormone and thyroid stimulating hormone secreting adenomas is well established. More recently, data are accumulating suggesting a potential therapeutic role of dopamine agonists also in patients with adrenocorticotrophic hormone secreting adenomas and non-functioning pituitary adenomas.

A major cause of death in glioblastoma patients (more than 60% of cases) is brain herniation due to cerebral edema, so corticosteroids are usually indicated in any patient with brain tumor with symptomatic peri-tumoral edema. The mechanism of action of corticosteroids is not well understood. Corticosteroids reduce the tumor-associated edema in patients with brain metastases or primary brain tumor as illustrated by CT studies. It produces symptomatic improvement within 24 to 72 hours.

The prognosis for patients with malignant brain tumors is poor. Conventional treatments such as surgery, radiation therapy, and chemotherapy have done little to affect long-term survival, and new methods of treatment are urgently needed. The overall treatment of brain tumor patients remains a challenge. Although mortality remains a primary concern, morbidity and quality of life are important issues.

RECOMMENDATION

Brain tumors are a group of heterogeneous neoplasms that need multimodality treatments aiming at curing the patients or at least increasing survival and at the same time improving quality of life of those patients.

The available trials have provided results that have challenged previously held beliefs and breathed new air into treatment areas.

Based on the available data the following recommendations for further research are suggested:

(1) More experimental studies to clear the vague points in the genetics and molecular pathogenesis of brain tumors.

(2) More large randomized controlled double-blinded trials for the different treatment modalities to conclude specific treatment protocols.

(3) More trials interesting in improving quality of life of the patients. Although survival is an important endpoint, quality of life is as important.

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Master degree thesis submitted by Dr Hisham Ibrahim, Supervised by professor Yasser Metwally, Ain Shams university, Cairo, Egypt

www.yassermetwally.com