Antiretroviral Drugs Flashcards
List the Antiretrovirals
NRTI – zidovudine, abacavir NNRTI – efavirenz Protease Inhibitor – ritonavir, atazanavir Membrane Fusion – enfuvirtide CCR5 (entry inhibitor) – maraviroc Integrase Inhibitor - raltegravir
Nucleoside reverse transcriptase inhibitors (NRTIs)
Ex: Zidovudine, Abacavir
Normal HIV-1 RT:
Components:
• 2 subunits (p66 & p55) derived from proteolytic cleavage of p66 homodimer
• N-terminal of p66 = polymerase
• C-terminal of p66 = endonuclease (RNaseH) activity to degrade RNA strand of RNA/DNA intermediate
• P55 = structural role
Functions:
• Synthesize negative strand of DNA (uses viral RNA as template)
• Hydrolyzes positive strand viral RNA
• Synthesizes the positive strand of DNA
NRTIs = nucleoside analogs that lack 3’OH needed for chain elongation
• When incorporated = terminate chain elongation
• Note: need to be phosphorylated to be active
Non-nucleoside reverse transcriptase inhibitors (NNRTIs)
Ex: Efavirenz
Binds hydrophobic pocket adjacent to active site in p66 → conformational change → blocks RT activity
o Do NOT need phosphorylation to be active
Protease Inhibitors (PIs)
Active HIV-1 protease = homodimer
PIs mimic natural cleavage sites of HIV-1 protease
• Bind active site with high affinity
• Prevents protease cleavage of Gag-Pol and Pol polyproteins into mature/functional units
Ex: lopinavir/ritonavir, atazanavir
• Small amount of Ritonavir used to inhibit CYP3A4 metabolism of Lopinavir = make Lopinavir viable
Fusion Inhibitors (FIs)
o Normal HIV fusion requires interactions between HIV gp41 and gp120 and host cell CD5, CCR5, and CXCR4 receptors
o Note: if mutant CCR5 receptor = HIV-1 infection is blocked
• Physiologically normal but 32 bp deletion in coding region → frameshift → non-functional receptor → no HIV-1 entry and infection
Ex: Enfuvirtide
• Binds to gp41 → blocks gp41-mediated membrane fusion
• Competitively inhibits folding of HR1 to HR2 (segments on gp41) → disrupts formation of 6-helix bundle needed for membrane fusion
Entry Inhibitors (EIs)
o Ex: Maraviroc
- Targets host protein
- Blocks gp120-CCR5 → prevents entry of CCR5-tropic (R5) virus
- BUT does NOT prevent entry of CXCR4-tropic (X4) virus
Integrase Inhibitors (IIs)
o Ex: Raltegravir
o Docks in acceptor DNA binding site = interferes with strand transfer
Explain how phosphorylation of NRTIs can cause drug interactions among NRTIs.
- For activity = require phosphorylation by cellular enzymes in cytoplasm
- Can have common or different pathways of phosphorylation → drug interactions
• Lack of phosphorylation → increased absorbance of NRTIs (relative to nucleotide analogs)
Negative drug interaction:
o Thymidine kinase has higher affinity for zidovudine than stavudine
o Result: zidovudine antagonizes effects of stavudine
How to detect resistance in anti-HIV drugs?
Phenotypic testing
Sample of HIV is grown in the laboratory
• Add dose of 1 drug
The growth rate of the HIV is compared to the rate of wild type virus
• If the sample grows more than wild type = it is resistant to the drug.
Phenotypic resistance is reported as “fold” resistance.
Genotypic testing
o The sequence of the sample virus is compared to the wild type virus
Resistance in NRTIs
Mutated RT that is able to discriminate against NRTIs during DNA synthesis
• Associated with decreased HIV-1 replication in vitro
Mutations in RT that promotes hydrolytic removal (primer unblocking) of chain-terminating NRTI = able to continue DNA synthesis
• Can lead to cross-resistance
Resistance in NNRTIs
o Single mutation in NNRTI-binding pocket
o With monotherapy treatment = rapid resistance emerges
Resistance in PIs
Mutations in binding in protease pocket → cross-resistance within class
Individual “signatiure” mutations to drugs (not cause resistance to entire class)
• Allow sequential use of other PIs as resistance emerges
Resistance in FIs
o Mutations in region of gp41 envelope protein
Resistance in EIs
o From emergence of mixed-tropic or CXCR4 tropic viruses
Resistance in IIs
o Mutations in integrase gene
