Antimicrobial Agents

II- Antifungal Drugs

Antifungal drugs

•Polyenes•Azoles•Allyamines•Echinocandins•Flourocytosine and Grisofulvin

Antifungal drugso Antifungal drug is a medication used to treat fungal infections.

o They work by exploiting differences between mammalian and fungal cells to kill fungi without dangerous effects on the host.

o Fungal infections Mycoses

o Fungi and humans are eukaryotes and similar at molecular level.

o It is difficult to find or design drugs that target fungi without affecting human cells.

o Many cause side-effects due to similarity of fungi & host.

o Side-effects are life-threatening if drugs are not used properly

o No effect of antibacterial on fungi due to differences in targets

Fungio Fungi of medical importance are;o True yeast (Unicellular Eucaryotic) o e.g. Candida albicans, Cryptococcus neoformans

o Dimorphic fungio Grow as yeasts or filamentous fungi depending on cultural condition o e.g. Histoplasma capsulatum, Blastomyces dermatitidis, Dermatophytes

o Mould (Multicellular Eucaryotic): o Fungal infectionso Superficial mycosiso The skin, nail or hair (Dermatophytosis, Ringworms or Tinea)o Mucous membrane (Candidasis orThrush)

o Systemic mycosiso Cryptococcal meningitis and invasive candidasiso Histoplasmosis, blastomycosiso Aspergillosis

Dermatophytes

Dermatophytes is group of dimorphic fungi that cause skin

diseases which include 3 genera

Trichophyton, Epidrmophyton, and Microsporum

They cause infections of the skin, hair and nails

They able to obtain nutrients from keratinized material.

Dermatophycosis such as

Ringworm or tinea.

Onychomycosis (Toe- and fingernail infection)

Dermatophycosis Athlete's foot or tinea pedis

Jock itch or tinea cruris (infect foot, groin and perianal)

Ringworm of the body or tinea corpora, tinea versicolor

Facial ringworm or tinea faciei

Blackdot ringworm or tinea capitis (infect hair shaft)

Scalp ringworm or tinea capitis

Ringworm of the hands or tinea manuum

Ringworm of the nail, Onychomycosis, or tinea unguium

Structure of fungal cell wall Multilayers contains mannan, chitin, α and β glucan.

β glucan and mannoprotein composed 80% of the cell mass

Outerlayer (mannan, mannoprotein, β (1,6) glucan & chitin).

Innerlayer (β (1,3) glucan & chitin with mannoprotein).

β-Glucans are polymers of D-glucose

They occur as cellulose in plants, the bran of cereals, the cell

wall of bakers' yeast, certain fungi, mushrooms & bacteria.

Structure of cell membrane Ergosterol

Bioregulator of membrane fluidity

Membrane integrity of fungal cells

Integrity of the cell membrane requires that inserted sterols lack

C-4 methyl groups (ergosterol).

Human cell doesn't contain ergosterol.

Human cell contains cholesterol

Biosynthesis of ergosterol Squalene is converted into sequelene-2,3-epioxoid by enzyme

squalene epoxidase Allyamines (e.g. Terbinafine) inhibit sequalene epioxidase.

sequelene-2,3-epioxoid is converted to lanosterol Lanosterol is converted to erogesterol by 2 pathways

Lansterol 4,14-Dimethylzymosterol Zymosterol Ergosterol

Lansterol 24-Methylenedihydrolanosterol 14-Mmethylfectosterol Ergosterol

Azoles inhibit 14-α-demthylase leads to depletion of ergosterol and accumulation of sterol precursors including 14-α-demthylated sterols (lanosterol, 4,14-Dimethylzymosterol, 24-Methylenedihydrolanosterol).

This leads to formation of plasma membrane with altered structure and function

Classes of Antifungal drugso A. Antifungals affecting fungal sterolso 1. Ergosterol inhibitorso a. Polyene antibiotics (ergosterol binding change cell wall

permiability)o e.g. Nystatin and Amphotricin B

o b. Azoles (lanosterol 14 α-demethylase inhibitors)o Imidazoles e.g. Clotrimazole, Miconazole, Econazole, Ketokenazoleo Triazoles e.g. Fluconazole, Itraconazoleo Thiazoles e.g. Abafungin

o c. Allylamines (squalene monooxygenase inhibitors) e.g. Terbinafineo B. Antifungal active against fungal cell wall o β-glucan synthase inhibitorso Echinocandins (e.g. Caspofungin, Micafungin)

o C. Antifungals acting intracellularo Pyrimidine analogues/Thymidylate synthase inhibitors (Flucytosine)o Mitotic inhibitors (Grisofulvin)

A. Antifungal Affecting Fungal Sterols 1. Ergosterol inhibitors

a. Ergosterol binding (Polyene antibiotics)

NystatinAmphotericin B

Mechanism of action of polyene antibiotics A polyene antifungals are a macrocyclic polyene contain a

multiple conjugated double bonds and contain a heavily hydroxylated region on the ring opposite the conjugated system.

This makes polyene antifungals amphiphilic. The polyene antimycotics bind with ergosterols in the fungal cell

membrane. The hydroxylated hydrophilic portion of the polyene antibiotic is

pulled into the membrane interior When present in sufficient concentrations, it forms aqueous pores

in the membrane This leads to alter permeability, leakage and death.

Ergosterol is fairly unique to fungi, so the drug does not have such catastrophic effects on animals.

Nystatin (Mycostatin®) Nystatin is polyene antifungal

It was isolated from Streptomyces noursei in 1950

It is sensitive to yeast (Candida and Cryptococcus) and many

moulds infections

Used to treat cutaneous, vaginal mucosal & esophageal Candida infections.

Nystatin has some toxicity when given intravenously, but it is not

absorbed across intact skin or mucous membranes.

It is a relatively safe drug for treating Thrush.

Amphotricin B It was originally extracted from Streptomyces nodosus in 1955 Amphotericin B (Fungizone ®) is the most antifungal available. Its narrow therapeutic index limit its use. Liposomal preparation reduce toxic effects Uses

Oral preparations are used to treat thrush; these are virtually nontoxic, in contrast to typical IV doses.

Intravenous uses used in treating various systemic fungal infections, including cryptococcal meningitis

Side effects Nephrotoxicity Electrolyte imbalance (e.g. hypokalemia) Hepatotoxicity

1. Ergosterol inhibitors b. Lanosterol 14 α-demethylase inhibitors

(Azole based antifungal)

Clotrimazole

a. Imidazole antifungal drugsMiconazole

Ketoconazole

b. Triazole antifungal drugFluconazole

c. Thiazole antifungal drugAbafungin

Mechanism of action and resistance of azolesLanosterol 14 α-demethylase inhibitors

Mechanism of action (see slide of ergosterol biosynthesis) Azoles are synthetic antifungal drugs cause alterations in fungal cell

membranes by blocking the biosynthesis of ergosterol. Azoles inhibit demethylation of 14--methyl group from lanosterol by

inhibit cytochrome P450 lanosterol 14 α-demethylase (CYP51A1). This enzyme converts lanosterol to ergosterol. This leads to a toxic accumulation of lanosterol. Azole drugs also block steroid synthesis in humans.

Mechanism of resistance Over-expression of CYP51A1 can lead to resistance to azoles. Drug active efflux.

Azoles: Imidazoles They have broad antimycotic spectrum They have some antibacterial activity Clotrimazole (Cansten®) is used mainly for superficial fungal

infection and used for treatment of; Vaginal and oral yeast infections (Thrush) Ringworm (Tinea) such as athlete's foot and jock itch.

Miconazole (Daktarin ®) It is applied topically to cure fungal infections. Oral or vaginal thrush

It has an advantage over nystatin in treatment of neonatal oral thrush. Ringworm (Athlete's foot, and jock itch). Intravenous administration (infusion) used for treatment of;

Chronic and invasive candidiasis Dermatophytes Histoplasmosis

Ketoconazole (Nizoral ®):

Ketoconazole is used for topical infections such as

Candidiasis (yeast infection or thrush)

Ringworm (athlete's foot and jock itch).

It used as a body wash for the treatment of tinea versicolor

It is for treatment of systemic fungal infections: candidiasis,

blastomycosis, histoplasmosis, coccidioidomycosis.

It is used to treat eumycetoma, the fungal form of mycetoma

Azoles:Triazoles Fluconazole (Diflucan ®):

Fluconazole is a triazole antifungal drug used in treatment and prevention of superficial and systemic fungal infections.

Fluconazole is active against the following microorganisms: Yeast {Candida spp. (except C. krusei and C. glabrata)

and Cryptococcus neoformans} Dermatophytes {Epidermophyton spp., Microsporum

spp., Trichophyton spp.} Dimorphic fungi {Blastomyces dermatitidis, Histoplasma

capsulatum, Coccidioides immitis} It is indicated for the treatment and prophylaxis of;

Candidiasis caused by susceptible strains of Candida, Tinea corporis, tinea cruris or tinea pedis, Onychomycosis, Cryptococcal meningitis Blastomycosis, histoplasmosis.

Azoles:Triazoles Itraconazole (Sporanox ®):

It has a broader spectrum of activity than fluconazole. It is active against Aspergillus, which fluconazole is not. Itraconazole is active against;

Yeasts (Candida spp., Cryptococcus neoformans). Dimorphic fungi (Histoplasma capsulatum, Blastomyces

dermatitidis, Coccidioides immitis, Paracoccidioides brasiliensis, and dermatophytes).

Moulds (Aspergillus spp.) It is indicated for the treatment and prophylaxis of;

Cryptococcal and coccidioidal meningitis Aspergillosis, blastomycosis, and histoplasmosis Onychomycosis of the toenail and fingernail.

Azoles:Thiazoles It is a new synthetic broad spectrum antifungal agents belong to

a new class of antifungal compounds called arylguanidines. It is active against a number of different pathogens including

yeasts, Dermatophytes, and moulds. It also has antibacterial properties.

Abafungin has a novel mechanism of action for the treatment of dermatomycoses.

In addition to the inhibition of the fungal cytochrome P450 enzyme 14α-demethylase, abafungin inhibits the enzyme sterol 24-C-methyltransferase, modifying the composition of the fungal membrane.

1. Ergosterol inhibitors c. Squalene monooxygenase inhibitors (Allylamines)

Terbinafine (Lamisil ®)

Squalene

Squalene epioxide

Squalene epioxidase

Allylamines inhibit this

step

Allylamines (terbinafine) Mechanism of action (see slide of ergosterol biosynthesis) Allyamines inhibit squalene epioxidase Squalene monooxygenase (squalene epoxidase) is an

enzyme that uses NADPH and molecular oxygen to oxidize squalene to squalene epoxide.

Squalene epoxidase catalyzes first oxygenation step in sterol biosynthesis.

Uses Terbinafine is mainly effective on the dermatophytes. Terbinafine has good activity against azole-resistant

Candida albicans and Cryptococcus neoformans

B. Antifungals Active Against Fungal Cell Wall β-glucan synthase inhibitors

Echinocandins (e.g. caspofungin) Echinocandins (see slide of structure of fungal cell wall and cell

membrane) are antifungals that inhibit the synthesis of β-glucan in the cell wall via noncompetetive inhibition of 1,3-β glucan synthetase.

1,3-β-glucan synthetase is a glucosyltransferase enzyme involved in the generation of β-glucan in fungi. This enzyme is a pharmacological target for antifungals such as caspofungin.

Uses Fungicidal against yeast e.g. most species of Candida

Not active against Cryptococcus, Trichosporon & Rhodotorula Fungistatic against mould e.g. Aspergillus

Not active against Fusarium & Rhizopus Modest or minimally active against dimorphic fungi

e.g. Blastomyces and Histoplasma.

C. Antifungal acting intracellular: Griseofulvin It is isolated from Penicillium griseofulvum It was the first antifungal agent to be described It is administered orally and it is a relatively non-toxic drug Spectrum

It is active against dermatophytes Trichophyton that causing ringworm It is ineffective against C. albicans and filamentous fungi

Mechanism of action Griseofulvin binds to polymerized microtubules and inhibits

fungal mitosis It binds to keratin in keratin precursor cells and makes them resistant to

fungal infections. Griseofulvin will enter dermatophyte through energy dependent transport

processes and bind to fungal microtubules. This alters the processing for mitosis.

Flucytosine (5-FC) 5-FC is a fluorinated pyrimidine with inhibiting activity against yeast 5-FC must not be used as a sole agent in life-threatening fungal

infections due to relatively weak antifungal effects and fast development of resistance, but rather in combination with amphotericin B and/or azole antifungals.

Spectrum It is active against some strains of Candida and Cryptococcus It may be value against infections with Aspergillus & other fungi.

Mechanism of Actions 5-FC (antimetabolite) enter inside fungal cell by permease 5-FC is converted by cytosine deaminase into 5-fluorouracil (5-FU,

cytostatic)

Flucytosine (5-FC) 5-FU Mechanism of action:

Converted to 5 FU by FUNGAL CYTOSINE DEAMINASE 5FU 5FUTP(5-fluorouridine triphosphate ) RNA DEFECTIVE=

disruption of protein synthesis 5FU 5dUMP (5-fluorodeoxyuridine monophosphate ) inhibitor of thymidylate

synthetase, an enzyme involved in fungal DNA synthesis and nuclear division.

Human cells cant convert it to 5FUMechanism of resistance:

Decreased uptake (loss of permease activity) Loss of enzymatic activity responsible for conversion to FUMP

Loss of cytosine deaminase

Antimicrobial Agents

III- Antiviral Drugs

Antiviral Agents

Structure of Viruses The basic viral particle is a virion. Virion consists of core surrounded by capsid (or lipid envelope). Core contains nucleic acid genome either DNA or RNA, not both. DNA and RNA are required for cell to reproduce, viruses invade host

cells and take over their genetic equipment. Capsid contains protein spikes (help penetrate host cells).

Viral Life Cycle Attachment:

A virus contacts with a host cell & attaches specific receptors on cell. Entry:

A virus secretes chemicals that weaken host cell wall. DNA or RNA is injected through.

Replication: Virus instructs the host cell to make more viral DNA and proteins

Assembly: Cellular activity of the host cell helps assemble new viruses until the

host cell is filled with new viruses. Release:

Virons release enzymes that digest the host cell wall. New virons released go to infect new cells where cycle is repeated.

Classification of Viruses Viruses can be placed in one of the seven following groups DNA viruses

dsDNA viruses (e.g. Adenoviruses, Herpesviruses, Poxviruses) ssDNA viruses (+)sense DNA (e.g. Parvoviruses)

RNA viruses dsRNA viruses (e.g. Reoviruses) (+)ssRNA viruses (+)sense RNA (e.g. Picornaviruses,

Togaviruses) (−)ssRNA viruses (−)sense RNA (e.g. Orthomyxoviruses,

Paramyxoviruses, Respiratory Syncytial Virus, Rhabdoviruses) ssRNA-RT viruses (+)sense RNA with DNA intermediate in life-

cycle (e.g. Retroviruses)

Classification of Viruses

Reverse transcribing (RETRO) viruses Group VI:

Viruses possess ssRNA genomes and replicate using reverse transcriptase (RT).

Retroviruses are example, of which HIV is a member. Group VII:

dsDNA-RT viruses (e.g. Hepadnaviruses) Viruses possess ds DNA genomes and replicate using RT. The hepatitis B virus can be found in this group.

Antiviral Drugs Antivirals are used specifically for treating viral infections. They do not destroy virus, but inhibit their development. Virucides are not medication but destroy virus outside body. Most of the antivirals are designed to help deal with HSV,

HZV, HIV, HBV and HCV and Influenza A and B viruses. Extension of range of antivirals to cover other virus families.

Designing safe and effective antiviral drugs is difficult,

because viruses use the host's cells to replicate. This makes it difficult to find targets for the drug that would interfere

with the virus without also harming the host organism's cells.

Steps of virus replication, enzymes involved and Targets for drug therapy

Anti-viral targeting I. Before cell entry

1- Entry inhibitor A very early stage of viral infection is viral entry. Entry- or entry-blocking drugs are developed to fight HIV. HIV target is "helper T cells“ and identifies this target through T-

cell surface receptors "CD4“. Attempts to interfere with the binding of HIV with CD4 receptor

have some succeed to stop HIV from infecting helper T cells. 2- Uncoating inhibitor

Amantadine & rimantadine introduced to combat influenza. These agents act on penetration/uncoating.

II- During viral synthesis 1- Reverse transcription Nucleotide or nucleoside analogues (building blocks of RNA or DNA)

deactivate the enzymes that synthesize the RNA or DNA once the analogue is incorporated.

This approach is associated with inhibition of RT (RNA to DNA). Acyclovir and zidovudine are a nucleoside analogue.

An improved knowledge of the action of RT has led to better nucleoside analogues to treat HIV infections. lamivudine, has been approved to treat hepatitis B, which uses RT

as part of its replication process. 2- Transcription Production of mRNA is initiated by proteins ‘transcription factors (TF)’. Several antivirals are designed to block attachment of TF to viral DNA.

3-Translation / antisense Genomics help to find targets for many antivirals. It has provided entirely new type of drug, based on "antisense" molecules. Segments of DNA or RNA (antisense oligodeoxynucleotide) designed as

complementary molecule to critical sections of viral genomes. Binding of segments to the target sections blocks operation of the genomes. Phosphorothioate antisense drug used to treat CMV retinitis in AIDS Other antisense antivirals are in development.

4- Translation / ribozymes Another antiviral genomic technique is set of drugs based on ribozymes . Ribozymes are used as part of viral manufacturing sequence. Synthetic ribozymes designed to cut RNA & DNA at sites disable them. A ribozyme antivirals are being developed to deal with HIV and HCV.

5- Protease inhibitors Some viruses produce a protease that cuts viral protein chains apart

so they can be assembled into their final configuration. HIV includes a protease, and research has been performed to find

"protease inhibitors" to attack HIV at that phase of its life cycle. Protease inhibitors became available in 1990s & have proven effective Improved protease inhibitors are now in development. III- During release phase Final stage in life cycle is release of completed viruses from host cell. This step has been targeted by antiviral drug developers. Zanamivir and oseltamivir prevent the release of viral particles by

blocking a neuraminidase that is found on the surface of flu viruses.

Antiviral Drugs1. Anti-herpes virus

A. Nucleoside and Nucleotide Analoguesa. Acyclovir, Idoxuridine, Cytarabine, Ribavirin, Vidarabine

B. Pyrophosphate analogue non nucleoside compoundsb. Sodium Phosphonoformate, Phosphonoacetic acid

2. Anti-influnza virusA. M2 inhibitors (adamantanes) e.g. Amantadine and rimantadineB. Neuramindase inhibitors e.g. Zanamivir and oseltamivir

3. Anti-HIVa. Nucleoside/nucleotide RT inhibitors e.g. Zidovudine, Abacavir.b. Non-Nucleoside/ nucleotide RT inhibitors.e.g. Nevirapinec. Protease inhibitors e.g. Saquinavir, Indinavird. Viral entry inhibitors e.g. Enfuvirtide

a. Anti-HBV & HCV e.g. Interferons, lamivudine

I. Anti-herpesvirus compoundsA- Nucleoside and nucleotide analogues

They are phosphorylated by kinases

Phosphorylated analogues work as antimetabolites which act

as chain terminators and stop viral DNA Polymerase.

They are not specific to viral DNA & affect mitochondrial DNA.

Resistance can develop quickly with mutation(s) in kinases.

Nucleoside and nucleotide analogues

Idoxuridine(Tymidine)

Cytarabine(Cytosine)

ZidovudineAnti-HIV

(Thymidine)

Acyclovir (Guanosine)

Abacavir Anti-HIV

(Guanosine)

Ribavirin(Guanosine)

Vidarabine (Adenosine)

LamivudineAnti-HBV

I. Anti-herpesvirus compounds A- Nucleoside and nucleotide analogues

Acyclovir (Zovirax) is a guanosine analogue. It is extremely selective and low in cytotoxicity.

One of the most commonly-used antiviral drugs It is primarily used for treatment of HSV & HZV infections. It is converted into acyclo-GMP by viral thymidine kinase. Acyclo-GMP is further phosphorylated into the active acyclo-GTP. Acyclo-GTP is a very potent inhibitor of viral DNA polymerase Acyclo-GTP has 100 times greater affinity for viral DNA polymerase than

for host cell DNA polymerase. Acyclo-GTP is incorporated into viral DNA, resulting in chain termination.

Mechanism of antiviral action of acyclovir (ACV) ACV targets viral DNA polymerases, such as the HSV

DNA polymerase

Ribavirin (Virazole) is guanosine analogue. It is a prodrug & its metabolite resembles purine RNA nucleotides. Mechanism of action: similar to acyclovir and interferes with early

events of viral transcription. It is active against numbers of DNA and RNA viruses

It is indicated in treatment of HCV, HSV, RSV, Influenza A & B.

Vidarabine (deoxyadenosine analogues) Vidarabine is nucleoside analogue & active against HSV & VZV. Vidarabine works by interfering with the synthesis of viral DNA. It has to be phosphorylated to be active by kinases to the ara-ATP. Ara-ATP is inhibitor and a substrate of viral DNA polymerase. Ara-ATP inhibits dATP leading to formation of ‘faulty’ DNA.

Mechanism of Action of Vidarabine

ARA needs to be phosphorylated, in three steps, to the triphospate form before it can interfere with the DNA polymerase reaction.

Idoxuridine (IDU) It is a thymidine analogue which inhibits the utilization of

thymidine in the rapid synthesis of DNA that normally occurs in herpes-infected cells.

It is restricted to topical treatment of herpes-infected eyes. Cytarabine (cytosine arabinoside ) It has anti neoplastic and antiviral properties. Rapid conversion into cytosine arabinoside triphosphate, which

damages DNA It inhibits DNA & RNA polymerases & nucleotide reductase It used topically to treat herpes keratitis resistant to IDU.

B. Pyrophosphate analogue non nucleoside compounds

These are organic analogue of inorganic pyrophosphateThey form complexes with & inhibit viral DNA polymerases.Sodium Phosphonoformate It Inhibits herpes DNA polymerase. It is non-toxic when applied to the skin It is used in treatment of herpes simplex labialis (cold sores). Phosphonoacetic acid It has a high specificity for herpes simplex DNA synthesis,

and has been shown to be non-mutagenic in experimental animals, but is highly toxic.

Anti-influnza virusA. M2 inhibitors (adamantanes)

These drugs are substitution of adamantane backbone. Amantadine & rimantadine were approved by FDA in 1966 &

1994 respectively. They have similar biological properties.

Antiviral and antiparkinsonian activity. They interfere with a viral protein, M2 (an ion channel). Prevents passage of H+ ions that are required for necessary

acidity to allow for the viral uncoating process "decapsidation".

Amantadine and rimantadine These drugs are effective against influenza A. They are ineffective against influenza B (lack M2 molecules). 91% (2005) of American isolates of H3N2 were resistant to them. 100 % (2009) of seasonal H3N2 have shown resistance to them. Now, these drugs are not longer recommended to treatment of flu. Amantadine is approved only for use in humans. In China, amantadine was used to protect birds against avian H5N1. H5N1 in China and southeast Asia are now resistant to amantadine. Amantadine-resistance is associated with amino acid mutations in the

residue of the transmembrane domain of the M2 proton channel

B. Neuraminidase inhibitors (NIs) N-acetylneuraminic acid analogue

NAIs were introduced in clinical practice since 1999 It is the 1st neuraminidase inhibitor followed by oseltamivir. They used in treatment & prophylaxis of Influenza virus A & B. They bind to active site on neuraminidase & rendering

influenza virus unable to escape its host cell and infect others. Neuraminidase is an enzyme on surface of influenza viruses

It plays an essential role in release of viruses from infected cells.

Zanamivir (Relenza) Oseltamivir (Tamiflu)

B. Neuraminidase inhibitor

Oseltamivir is not effective in treating Influenza viruses as

zanamivir, especially in H1N1 Seasonal Flu.

Oseltamivir-resistance H5N1 strain is susceptible to zanamivir.

Zanamivir has advantages over oseltamivir in binding to the active

pocket of H5N1 neuraminidase.

10-2009, H3N2 & influenza B have not resistance to oseltamivir.

Mutations conferring resistance are single amino acid residue

substitutions (His274Tyr) in the neuraminidase enzyme.

I- Nucleoside Reverse Transcriptase Inhibitors (NRTIs) Zidovudine (AZT) is nucleoside (thymidine) analogue RT inhibitor. It was the first approved treatment for HIV (types1 and 2). It works by inhibiting the action of RT.

Reverse transcription is necessary for production of viral ds DNA, which is integrated into genetic material of infected cell.

Resistance to AZT by mutation of its reverse transcriptase. Abacavir is guanosine analogue RT inhibitor. It is used to treatment of HIV (types1 and 2). Targeted HIV RT, act as chain terminator, following intracellular

phosphorylation and conversion (deamination) to gaunaosine analogue triphosphate (carbovir), and, after removal of diphosphate group, incorporation of carbovir 5’-monophospahte at the 3’-end of the viral DNA chain.

Mechanism of Action of Zidovudine (Azothymidine, AZT)

AZT needs to be phosphorylated, in three steps, to the triphospate form before it can interfere with the reverse transcriptase reaction (after De Clercq, E., 2002).

II- Non-NucleosideRT inhibitors (NNRTI) Nevirapine is NNRTI used to treat HIV-1 infection and AIDS.

Both nucleoside and non-nucleoside RTIs inhibit RT enzyme.

Unlike nucleoside RTIs, NNRTIs bind allosterically at a distinct

site away from the active site termed the NNRTI pocket.

Nevirapine is not effective against HIV-2, as the pocket of the

HIV-2 reverse transcriptase has a different structure.

HIV rapidly develops resistance if nevirapine is used alone.

Resistance develops rapidly due to mutations in RT.

III- Protease inhibitors (PIs).

Saquinavir was the first a PIs approved by FDA (1995).

Proteases are enzymes that cleave protein molecules into

smaller fragments.

HIV protease is vital for both viral replication within the cell

and release of mature viral particles from an infected cell.

Saquinavir inhibits both HIV-1 and HIV-2 proteases.

IV- Viral entry inhibitors Enfuvirtide was approved by FDA in 2003 as the first HIV

fusion inhibitor, a new class of antiretroviral drugs. Drugs that disrupt fusion of virus and target cell are termed

entry inhibitors or fusion inhibitors. It is used in combination therapy for treatment of HIV-1. HIV binds to CD4+ cell receptor via viral protein (gp160 gp41). Then undergoes a conformational change that assists in the

fusion of the viral membrane to the host cell membrane. Enfuvirtide is 36 amino acids peptide corresponding to viral

glycoproteins gp160 and gp41. It binds to gp41 preventing the creation of an entry pore for the

capsid of the virus, keeping it out of the cell. Resistance result from mutated 10 amino acid motif in viral gp41.

Anti HBV and HCV Interferons (IFNs) belong to glycoproteins (cytokines). INF is produced by host cell in response to the virus particle. INFs induces formation large amount of protein kinase R

inhibiting transcription of viral mRNA. Seven of INFs have been grouped among three IFN classes. Type I INFs

They comprise 3 classes IFN-α, IFN-β and IFN-ω They protect neighboring cell. IFN-α released by stimulated leukocytes IFN-α involved in innate immune response against viral infection. IFN-β released by stimulated fibroblasts.

IFN-β have antiviral activity which is mainly involved in

innate immune response.

Type II IFNs ( INF)

These are acid-labile and are also known as 'immune' .

They produced by stimulated T-lymphocyte

They prevent the spread of virus from cell to cell and are more

active in inhibiting growth of tumor cells.

Recently, recombinant interferon produced by recombinant DNA

technology in E. coli and Saccharomyces cerevisiae.

It is usually used for treatment of chronic HBV and HCV.

Lamivudine Lamivudine (3-TC) is an analogue of cytidine. It can inhibit both types (1 and 2) of HIV RT (Higher dose). Also inhibits the RT of hepatitis B (lower dose). It is often given in combination with zidovudine. It needs to be phosphorylated intracellularlly to its

triphosphate (3TC-triphosphate) form before it is active. 3TC-triphosphate also inhibits cellular DNA polymerase. Resistance is associated with the mutation in RT gene.

In HIV M184V/I. In HBV M204V/I/S.