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CHEM E-120Harvard University Extension School
Spring 2011
Disorders of Mood and BehaviorAnxiety
Classical Benzodiazepines – GABAA Allosteric Modulators
Partial AgonistsSubtype Selective Benzodiazepines – α2/α3
5-HT1A agonist
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Anxiety
A normal response to threatening situations
Key feature is increased fearfulness accompanied by subjective (heightened sense of awareness to a deep fear of impending disaster and death) and physiological manifestations.
Panic Disorder (DSM-IV 300.01)Post-traumatic stress disorder (PTSD, DSM-IV 309.81)Generalized Anxiety Disorder (GAD, DSM-IV 300.02)Social phobia (DSM-IV 300.23)Obsessive-compulsive disorder (OCD, DSM-IV 300.3)
Anxiety and depression often occur together (comorbidity)e.g. 50% of patients with panic attacks have depression
Most common of psychiatric disorders, 10-30% of population
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Neurobiology of Anxiety
Animal models of fear and avoidance1
Fear network has been proposed centered on the amyglada2
1. Animal Models of Anxiety… Trends in Pharmacological Sciences 2008, 29, 4932. Neuroanatomical Hypothesis of Panic Disorder, American J. Psychiatry 2000, 157, 493
Sensory thalamus cortex
amyglada
HypothalamusHPA axis Locus cerules
NE, CRH
periaqueductal gray region axisautonomic response (fight or flight)
hippocampusmemory, learning
dopaminergicnoradrenergicserotonergiccholinergic
Main Neurotransmitters are GABA (inhibitory) and Glutamic Acid (excitory)
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Therapeutic Targets – Approved AnxiolyticsBenzodiazepines
Allosteric modulators of GABAA ligand-gated Cl- ion channel where they increase the influx of Cl-, leading to hyperpolarization – reduction in neuronal activity
Allosteric modulation – a drug binds to a different site on a protein than where GABA binds, changing the conformation of the ion channel, increasing the affinity of GABA. Positive Cooperativity.
Alprazolam (Xanax) Panic Disorder, Social PhobiaChlordiazepoxide (Librium) AnxietyClonazepam (Klonopin) GAD, Panic Disorder, Social PhobiaDiazepam (Valium) Anxiety
Side effects
Sedation, dose-related transition anxiolytic sedativeAddiction – mildMemory impairment - Rohypnol (flunitrazepam) date rape
Lyrica (Pfizer) not approved for GAD
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Benzodiazepines
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GABAA Ion Channels
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Ion channel is pentameric structure where each pentamer subunit is composed of four polypeptides, 19 subtypes.
Known the benzodiazepines (BZ) bind to sites on the interface of and subunits. GABA binds to 2 sites on subunits.
Currently thought that the BZ binding site involves a combination of 1, 2, 3 or 5 + 2 or 3.
1 thought to play a role in amnesia and sedative effects of BZ21 and 32 thought to be most important for anxiolytic effects of BZ5 memory component?
Barbiturates and alcohol also bind to GABAA
Zolpidem (Ambien) binds to GABAA
1 Science 2000, 290(5489)131 (transgenic mice)2 Journal of Neuroscience 2005, 25(460 10682
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Approved Nonbenzodiazepine Anxiolytics
Sertonergic System
SSRI’sSertraline (Zoloft) Panic disorder, OCD, PTSD, social phobiaFluoxetine (Prozac) OCD
5-HT1A agonistBuspirone (BuSpar) GAD
Noradrenergic System
Yohimbine blocks α2 adrenergic autoreceptors (activation inhibits noradrenergic neuronal activity) activating noradrenergic activity – induces panic attacks
NSRIVenlafaxine (Effexor) GAD
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GABAA – Benzodiazepines (BZ)
Benzodiazepines first discovered in 1957, found to have potent anxiolytic activity in animal models of sedation. No binding assays, went directly from flask into animal models
inclined screen – muscle relaxation & sedationfoot shock induced aggression in mice and rats – taming effectcat – muscle relaxationpentylenetetrazole – sedation & anticonvulsant
Later found to bind to the GABAA Cl- ion channelallosteric binding increases binding affinity of GABA
Librium
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BZ potentiate Cl- influx
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GABA invokes 0.9 nA current response
Addition of BZ triples amplitude
Chemicals delivered to cultured mouse spinal cord neuronvoltage-clamped at -70 mV with amicroelectrode.
CRC Press GABA and benzodiazepine Receptors Vol 1 1988
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Benzodiazepine PotenciesCorrelation between receptor affinity and behavioral potency among various benzodiazepines. Benzodiazepines were tested for their potency (Ki) in displacing [3H]diazepam from specific BDZ binding sites on rat cerebral cortical membranes. The resulting values were highly correlated (r=.90, p<.0001) with the behavioral potency of the same drugs (EDmin) in the cat muscle relaxant test.
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Benzodiazepine SAR
- stackingvan der waalsC7 electronegativegroup tends inc funcanxiolytic activity
HBA critical
C3 substitution decreasesantagonist activity but not agonist activity
tolerant of substituents
Not required for in vitro bindingbut is for in vivo efficacy4’ very sensitive to substitution
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Synthesis of Benzodiazepines
2-aminoaryl ketones
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PharmacophoreThose parts of the drug molecule that interact with the protein: key interactions that are responsible for the biological activity of the drug.
Medicinal Chemists commonly refer to the pharmacophore as consisting of
a scaffold (core)pendant functional groups bioactive conformation (i.e. the sterochemistry).
A pharmacophore does not necessarily represent a real molecule.
dab
dbc
dcd
dad
dac
a
b c
d
abcbcdetc
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Ligand-based Drug Design
In structure-based computerized drug design, the 3D structure of theTarget and drug should be known with a high degree of resolution.
The exact 3D structure of most receptors is not know.
1. Series of assayed ligands2. Generate energy-minimized structures – molecular mechanics3. Calculate various properties - descriptors4. Determine if a common set of shape or descriptors describe
the ensemble - pharmacophore
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Theoretical MethodsMolecular Mechanics
Treats atoms as balls and bonds as springs - classical mechanicsAssigns x,y,z coordinates to atoms in a molecule and calculates the potential energy at positionsForce Field: collection of values that define the change in energy with geometryForce fields can be transferred from one molecule to another to predict geometry.
calculate the potential energy of a conformation
Methods: MM1, MM2, AMBER, OPLS
Quantum Mechanics
Represents molecules in terms of their electron distributionThe shapes of orbitals (sp3, sp2, sp, π) and the electron distribution derive from QM.
partial atomic charges (areas of high and low electron density)electrostatic potentialdipole moments (polar bonds)
Methods: MINDO, ZINDO, PM3
QSAR (Quantitative Structure Activity Relationships) combine aspects of bothsize & charge
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Descriptors of Molecules
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Generating the Pharmacophore
Usually some type of iterative process where a compound(s) a modelIs developed. A training set of compounds is overlayed and the fitquantified by a scoring function. This is used to predict the biological activity (K i) for a set of compounds and compared to the known values.
The model is then adjusted as needed.
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Intermolecular Bonding
Hydrophobic Pocket
Salt bridge3/2/11 18CHEM E-120
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Benzodiazepine Binding Models
Relative locations of the descriptors and regions of the unified pharmacophore/ receptor model. The pyrazolo[3,4-c] quinolin-3-one CGS- 9896 (dotted line), a diazadiindole (thin line), and diazepam (thick line) aligned within the unified pharmacophore/receptor model for the Bz BS.
H1 and H2 represent hydrogen bond donor sites within the Bz BS
A2 represents a hydrogen bond acceptor site necessary for potent inverse agonist activity in vivo.
L1, L2, L3 and LDi are four lipophilic regions
S1, S2, and S3 are regions of negative steric repulsion.
LP = lone pair of electrons on the ligands
Current Medicinal Chemistry, 2007, 14, 2755-2775
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Benzodiazepine Binding Models
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Benzodiazepine Binding Models
Current Med Chem 2007, 14, 27553/2/11 CHEM E-120
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Receptors/Drugs - Partial Agonists
epartial agonist = [A]2
[A]1
Emax of partial agonist
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Alprazolam (Xanax)
This type of modification is leading intonew nonbenzodiazepines that bind selectivelyto the 2 site and/or are partial agonists
BZ use is limited by side effects, e.g., sedation and amnesia, the development of tolerance, and concerns about dependence and withdrawal. These side effects are a natural extension of their mechanism of action. Considerable effort has been expended over the past three decades to discover and develop novel, anxioselective BZ ligands that have improved side effect profiles. (Comprehensive Medicinal Chemistry II Chapter 6.04)
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New Approaches towards GABAA - Partial Agonists
Clinical trialsfor GAD andpanic disorderbut sedative
No tolerance to anxiolytc effect, no withdraw, lower abuse potential
EC50 = 3-10 MSimilar to diazepamPhase III but livertoxicity
Low nM N
N
N
O
Ambien (Zolpidem)3/2/11 CHEM E-120
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New Approaches towards GABAA - Subtype Selective
Ki similar but no efficacy at 1
1,2,3 selective1,5 partial agonist2,3 full agonist
Low nM at 1,2,3,53 full agonist1,2,5 partial agonist
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2/3 selective example - Merck
Ki (nM) Efficacy (1.0)1 = 1.5 -7% (antagonist)2 0.123 = 8.5 0.445 = 12.1 0.01
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Serotonergic System in Anxiety
Agonists for 5-HT1A receptor presynaptic (autoreceptor) in raphe nuclei could contribute to delayed onset.
Buspirone (Buspar) is a 5-HT1A partial agonist approved for GAD. NonsedativeIneffective in patients who have previously taken benzodiazepines.Delayed onsetBuspirone has no significant affinity for benzodiazepine receptors and does not affect GABA binding in vitro or in vivo when tested in preclinical models.Buspirone has moderate affinity for brain D2-dopamine receptors. Some studies do suggest that buspirone may have indirect effects on other neurotransmitter systems.
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5-HT1A Selective Compound
J. Med. Chem 2001, 44, 186
Selectivity problems versus 1-anderergic receptorHigh homology (45%) in transmembrane amino acid sequences of 5-HT1A and 1-anderergic receptors
To design compounds with selectivity for 5-HT1A versus 1 synthesized a training set of 32 compounds based on the structure below. The structural field of compounds VI is defined with 6 parameters:
three indicator variables (IA, IB and In) size of the A and B rings and nlipophilic ()electronic σo- or σm- steric (MR)
Used to pick the R groups from a set of 387 substituents.
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5-HT1A Selective Compound
J. Med. Chem 2001, 44, 1863/2/11 CHEM E-120
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5-HT1A Selective Compound
J. Med. Chem 2001, 44, 186, 198
n = 4m = 0,1
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5-HT1A Selective Compound
J. Med. Chem 2001, 44, 186, 198
Meta-position m-bromo or m-amino group (VW(Br) = 17.2 Å3; VW(NH2) = 11.4 Å3) leads to compounds with the same affinity at both receptors. A trifluoromethyl group (VW = 24.2 Å3) at this position leads to an increase in the 5-HT1A selectivity (compound 28: 26-fold).
Best selectivity ratio is reached with the most voluminous group, a m-NHCOPri (VW = 70.8 Å3) which leads to the most selective compounds 20 and 32 (98- and 59-fold, respectively).
n = 4 favors 5-HT1A
ring sizeno effect
ortho no effect
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5-HT1A Selective Antagonist
J. Med. Chem 2001, 44, 186
QSAR
Pharmacology
No effect of mouse rectal tempBlocked hypothermia induced by 8-OH-DPAT (5-HT1A agonist)No affect on behavioral tests but blocked effects of 8-OH-DPAT
5-HT1A antagonistD2 antagonist
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5-HT1A Selective Antagonist
J. Med. Chem 2001, 44, 198
Developed a QSAR model based on Hansch Analysis and Neural Networks to design compounds with selectivity for 5-HT1A versus 1
Used the training set of 32 compounds. Each chemical structure defined with 9 descriptors:
three indicator variables (IA, IB and In) size of the A and B rings and nlipophilic (o and m)electronic [field (F) and resonance (R)] position and nature of R.steric van der Waals (Vo, Vm)
Hansch Analysis 5-HT1A pKi = 7.55 – 0.251IA – 0.762IN=3 + 1.73F – 0.0292Vo – 0.0193Vm + 1.26o
1 pKi = 7.68 – 0.272IB + 1.32F – 0.0306Vo – 0.0417Vm + 1.17o
lead to:
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5-HT1A Selective Agonist
5-HT1A Ki = 27 nM antagonist 1 > 1000 nM
5-HT1A Ki = 4.1 nM agonist 1 > 1000 nM
JMC 2005, 48, 2548
affect 1
5-HT1A more tolerant
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5-HT1A Selective Agonist
J. Med. Chem 2005, 48, 2548
(S)-9
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5-HT1A Selective Agonist
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5-HT1A Selective Agonist
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5-HT1A Selective Agonist
Ionic piperizine N - Asp
H-bonds Thr, Ser, Trp
Van der waalsaromatic-aromaticTrp and Phe
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Future Targets
Transient Receptor Potential Channel 5 (TRPC5) mice in which TRPC5 gene was deleted showed diminished fear levels in anxiety models. Highly expressed in amygdalaCell. 2009 May 15;137(4):761-72
CRF1 antagonists – shown to produce anxiolytic effects
Neurokinin-2 (NK2) antagonist – Saredutant (SR48968) in Phase 3 trials for GAD. Less side effects than SSRI’s but relapse not significantly reduced (MDD)
Metabotropic (GPCR) glutamate receptors – mGlu 2 agonists, mGlu 1 antagonists
beta 3 receptor agonist – given after trauma, reduce memory of traumatic events (PTSD)
V1b receptor antagonist – vasopression peptide in HPA, found in amygdala
NK3 – NK2 receptor antagonist
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