Lecture 12 Chem 195

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Lecture 12 - Chem 195

• HIV Drug Discovery and the Coupled Evolution of
  Therapeutics and Viral Resistance

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Housekeeping

• Today - HIV - next week Molecularly Based Cancer
  Therapeutics
• Will not be available for office hours next
  Tuesday
• References Wlodawer.pdf, newtargetsHIV.pdf

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HIV and Drug Discovery

• Virus first sequenced in early 1980s
• Member of the retrovirus family
• Replicates RNA -gt DNA (provirus) -gt RNA
• Two major subtypes, HIV-1 and HIV-2
• Many variants
• Error prone replication (1104 error frequency)
• Rapid selection of resistant mutant viruses IF
  replication is allowed to proceed
• As for HBV, balance in mutations between
  replication competence and drug resistance

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HIV Drug Discovery

• Targets
• Virally encoded enzymes
• Protease
• Reverse Transcriptase
• Integrase
• Other steps in viral replication
• Cell fusion
• Cellular receptors for viral entry
• CD4
• Co-receptors - CCR5, CXCR4 (chemokine/GPCRs)

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The HIV Genome and Proteome
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The HIV Infection Cycle
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HIV Genome Targets

• Two validated virally encoded enzymes
• HIV reverse transcriptase
• Active site directed inhibitors are nucleoside
  chain terminators
• For HIV-1 there are another class called NNRTIs
  (non-nucleoside RT inhibitors)
• HIV protease
• Aspartyl protease analogous to renin, pepsin
• Phe-Pro specificity
• Viral maturation occurs in viral particles
• Peptidomimetic based inhibitors (active site)

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Approved Drugs for HIV

• 5 approved RT nucleoside inhibitors
• 6 approved protease inhibitors
• 3 approved NNRTIs
• None are used today as monotherapy - the present
  therapeutic modalities, Highly active
  anti-retroviral therapy (HAART), are all
  combinations of 2-3 drugs

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HIV Protease with Inhibitors Bound
Symmetrical Dimer of 99 amino acids per subunit
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Some HIV Protease Inhibitors
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HIV Protease Inhibitors

• First generation
• Peptidomimetics
• Lipinskis rules?
• Transition state analogues (hydroxyethylene
  isosteres)
• Symmetry

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Evolution of Protease Inhibitors
Ritonavir is not very potent (ca. 10 nM IC50) but
inhibits P450 based metabolism Lopinavir is very
potent (ca. 10 pM IC50) and is effective against
mutant protease (V82X) but rapidly
metabolized Combination (Kaletra) low dose
Ritonavir Lopinavir is an excellent second
generation protease inhibitor
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Evolution of Protease Inhibitors II
Screening of compound collection led to initial
hit Co-crystal structure obtained Molecule
elaborated based on structure-based medicinal
chemistry First non-peptidic protease inhibitor
in clinical trials
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Structure of initial lead complex
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HIV Protease Summary

• Symmetrical dimer 99 amino acids
• Aspartyl protease
• Hydrophobic recognition sequence (Phe-Pro)
• First inhibitors were peptidomimetics
• A major triumph for structure based drug
  discovery
• Resistance - up to 1/3rd of the residues in HIV
  Pr mutate upon drug treatment
• Second generation drugs are smaller and more
  potent as well as with better ADME

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HIV Reverse Transcriptase

• Combination of polymerase and RNAase H activities
• Structurally related to all polymerases
• Thumb-palm-fingers motifs
• Conserved Gly-Asp-Asp
• First inhibitors were nucleoside chain terminators

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The Two Domains of HIV RT
p66
p51
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Structures of HIV RT
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Template Binding and Active Site of HIV RT
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HIV RT Nucleoside Inhibitors
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Resistant Mutations of HIV RTNucleosides
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Second Generation Nucleosides

• Principal mutation in Lamivudine treated patients
  is M184V
• This prevents resistance to AZT (additional AZT
  resistant mutation causes non-functional RT)
• Combivir (AZT lamivudine) now used in HAART

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Non-Nucleoside RT Inhibitors

• Third class of approved anti-HIV drugs
• Unique binding site found proximal to active site
  - only present in HIV-1 RT
• 3 drugs approved
• Only useful in combination therapy very rapid
  resistance as monotherapy

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NNRTIs
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Efavirenz and wild type RT
Second Generation NNRTIs accommodate to RT
mutations
Efavirenz and K103N RT
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Approaches to Resistance

• Two inhibitors targeted to different enzymes
• Two inhibitors to the same enzyme which generate
  different resistance patterns
• Metabolic inhibition (ritonavir) with a second
  inhibitor
• New inhibitors targeted to resistance enzymes or
  additional targets

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Next Generation Drugs

• Fusion - gp41 inhibitors
• Only peptidic inhibitors to date (T-20)
• CD4, CCR5, CXCR4 inhibitors
• Latter 2 are GPCRs, CCR5 well validated,
  mutations in gp120 can change co-receptor tropism
• Integrase inhibitors
• For all these targets there are molecules in
  clinical or preclinical trials