Antivirals 2 Flashcards
What is meant by genetic barrier resistance?
- The genetic barrier to resistance refers to how easily a virus can develop resistance to a particular antiviral drug through genetic mutations.
- A high genetic barrier means that multiple mutations are required for the virus to become resistant to the drug: Ex. Integrase inhibitors; Highest genetic barrier of antiretroviral: protease inhibitors
- A low genetic barrier means that only one or a few mutations are sufficient to confer resistance, making resistance more likely to develop rapidly: Ex. NRTIs, NNRTIs
What step in the viral life cycle is affected by the drug classes?
- Nucleoside RT inhibitors, non-nucleoside RT inhibitors inhibit reverse transcription
- Maraviroc inhibits CD4 and chemokine receptors
- Enfuvirtide inhibits attachment and fusion
- Raltegravir inhibits integrase
- Protease inhibitors inhibit maturation
How are nucleoside RT inhibitors activated metabolically before enzyme inhibition can occur?
- Require 3 step phosphorylation by host cell kinases to become active - must be activated by cellular kinases to triphosphate form; thymidine kinase, thymidylate kinase, NDP kinase
- MOA: reverse transcriptase has 3 activities: RNA dependent DNA polymerase; ribonuclease H (chops up RNA); DNA-dependent DNA polymerase (makes complementary strand of DNA from 1st strand it synthesized; NRTIs interfere with 1st and 2nd strand DNA synthesis – act at 2 different activation steps: RNA-dependent DNA polymerase and DNA-dependent DNA polymerase
What is the MOA of nucleoside reverse transcriptase inhibitors?
- NRTIs are nucleoside analogs that lack the 3’ OH – competitive inhibitor of reverse transcriptase and DNA chain terminator, inhibit elongation; they outcompete endogenous nucleoside triphosphates
- Activated NRTIs compete with sATP, dCTP, dGTP, and dTTP to be incorporated into growing DNA chain by RT; NRTIs have higher affinity for HIV RT than for cellular DNA polymerases
What are the NRTIs?
abacavir, emtricitabine, tenofovir alafenamide, lamivudine
What are the SEs of NRTIs?
mitochondrial toxicity from drug’s affinity to mitochondrial DNA polymerase
* Some NRTIs inhibit human DNA polymerase-gamma leading to anemia, granulocytopenia, myopathy, peripheral neuropathy, and pancreatitis; lactic acidosis and hepatic steatosis; lipoatrophy (loss of body fat)
What are the key structural differences between these nucleoside analogues and the normal nucleosides used by reverse transcriptase?
- Tenofovir – no ribose ring
- Lamivudine: contains sulfur
- Emtricitabine: contains sulfur and fluorine
- Abacavir: double bond and triangle addition
What are the differences in structure and activation between tenofovir and the other nucleoside RT inhibitors. How does this contribute to the longer half-life of tenofovir?
- Tenofovir is a nucleotide analog rather than a nucleoside analog like the other NRTIs, so it already contains one phosphate group. Because tenofovir is a nucleotide analog of adenosine monophosphate (AMP), it bypasses the first phosphorylation step that nucleoside analogs require; only needs two phosphorylation steps for activation. Other NRTIs require phosphorylation through three steps (monophosphate → diphosphate → triphosphate), leading to more reliance on cellular kinases and potentially slower activation.
- This longer half-life of TFV-DP is due to: Stable incorporation into cells and reduced efflux from the cell. Efficient phosphorylation that results in sustained intracellular concentrations of the active drug. Resistance to degradation compared to other NRTI triphosphates.
What is the the activation pathways for tenofovir alafenamide?
- TAF must be processed to TFV by cellular enzymes before phosphorylation
- TAF is hydrolyzed by cathepsin A to form tenofovir; this step is selective and ensures most of the active drug is released within the target cells not the blood stream; inhibits HIV reverse transcriptase by causing chain termination, stopping viral replication
- Turned over to tenofovir diphosphate (only 2 phosphorylation steps) form in infective cells to prevent kidney toxicity, and this is the active form; TDF is converted to tenofovir in the plasma, leading to systemic exposure.
- TAF is more stable in the plasma and preferentially converted to tenofovir inside cells by the enzyme cathepsin A. This results in lower systemic concentrations and reduced toxicity.
What drug can cause a hypersensitivity reaction?
abacavir: Black box warning: malaise, dizziness, HA, GI disturbances; CD8+ cytotoxic T cell activation causes hypersensitivity reaction; highly associated with HLA-B*5701 allele, testing for this is needed before initiating therapy
How can HIV become resistant to the RT inhibitors? Does resistance to one RT inhibitor confer resistance to all drugs in this class? How can combinations of RT inhibitors be used to manage resistance?
- Resistant mutations arise quickly because HIV polymerase is error prone; RT inhibitors are unable to suppress viral replication > 90%
- Discriminatory mutations: selectively impair the ability of RT to incorporate analogues of DNA (helping RT to distinguish between normal dNTPs and NRTIs)
- Excision mutations: ATP molecule mediates the removal of NRTI after incorporation into chain; selected by thymidine analogs AZT and d4T (thymidine analog mutations, allow ATP to bind to RT)
- Some mutations confer resistance to a subset of NRTIs, but make RT more susceptible to inhibition by others – take advantage of this and use NRTIs in combo
How do the non-nucleoside RT inhibitors inhibit RT?
- Bind directly to site on RT (not active site), hydrophobic pocket near catalytic site; binding affects flexibility of enzyme, slows down and inhibits its activity; block RNA and DNA dependent DNA polymerase activities
- Nucleoside triphosphate and template bind to hydrophobic pocket of RT conformational change blocks polymerizations
- Do NOT compete with nucleotides for binding – noncompetitive inhibitors
do NOT bind to cellular DNA polymerases
Do NNRTIs need to be phosphorylated?
no
What are 1st generation NNRTIs?
Nevirapine, efavirenz, delavirdine
* Nevirapine AEs: hepatotoxicity, stevens-johnson syndrome
* Efavirenz AEs: neuropsychiatric, teratogenic
What are 2nd gen NNRTIs?
etravirine, rilpivirine: can bind in multiple orientations, binds to mutants that are resistant to other NNRTIs
NNRTIs are all metabolized by what?
- All metabolized by CYP3A – CYP3A4 inducers (rifampin) can reduce levels, CYP3A4 inhibitors can increase levels
Where are the non-nucleoside reverse transcriptase inhibitors binding to RT, and what are the consequences of the binding are.
- Bind to hydrophobic pocket of RT, inhibit RT; do NOT bind to cellular DNA polymerases, so not as many SEs
How does HIV become resistant to the non-nucleoside RT inhibitors?
- Single mutation in binding site can promote resistance; block/disrupt binding of NNRTI to hydrophobic pocket; mutations that confer resistance to NNRTIs do NOT cause resistance to NRTIs
What are the integrase inhibitors?
bictegravir, dolutegravir, raltegravir, and elvitegravir
What is the mechanism by which bictegravir, dolutegravir, raltegravir, and elvitegravir inhibit HIV integrase?
- Inhibits insertion of HIV DNA into human genome; integrase inhibitors block the strand transfer step; uses divalent metal ions to catalyze insertion, chelates both metal ions and stabilizes enzyme-DNA complex
- Resistance caused by primary mutations that reduce INI susceptibility.
- Elvitegravir metabolized by CYP3A4; only available in coformulation with cobicistat (this inhibits CYP3A4)
- Bictegravir: raises serum creatinine levels
What are the key structural feature of the HIV integrase inhibitors that enables these compounds to inhibit HIV integrase?
- The defining structural characteristic of HIV integrase inhibitors is the presence of a chelating motif that contains oxygen-containing functional groups (such as hydroxyl or carbonyl groups). This chelating motif has a triad of oxygen atoms that can bind to divalent metal ions (Mg²⁺) within the active site of the HIV integrase enzyme. HIV integrase requires two Mg²⁺ ions in its active site to facilitate the strand transfer step, where viral DNA is inserted into the host DNA. The chelating groups in the integrase inhibitors bind tightly to these Mg²⁺ ions, disrupting the enzyme’s function.
What are the protease inhibitors?
atazanavir, tipranavir, and darunavir
Why is the structure of darunavir unique?
- Darunavir has 2 unique features: makes extensive H bonds with protease backbone (less affected by changes in amino acid side chains; inhibits HIV protease dimerization
How does the structure of tipranavir, a non-peptidomimetic PI, differ?
nonpeptidic, does NOT mimic the substrate
What is the mechanism by which protease inhibitors block HIV protease activity?
- HIV protease inhibitors are transition state mimetics – peptidomimetic, bind to active site, don’t allow cleavage in bond, protease inhibited
- Amide bond is replaced by non-cleavable linkages inhibitor binding causes a conformation change in protease “flaps” close
How does resistance to protease inhibitors arise?
- Resistance: mutations can be in active site or far away; modify contacts between protease and inhibitor – reduce affinity of protease for inhibitor, ex. V82A; PIs bind tightly to protease, natural substrates are less tight, resistance mutations have greater effect on PI
- Some mutations confer resistance to some PIs, but increase susceptibility to others: ex. I50L confers resistance to atazanavir, increased susceptibility to other PIs
How do darunavir and atazanvir differ from other protease inhibitors with regards to PI resistance mutations?
- Darunavir can inhibit both wildtype and mutants that are resistant to other PIs.
- Atazanavir: has different resistant mutation profile
How can the effect of protease inhibitors on CYP3A4 be exploited for anti-retroviral therapies (i.e., what is PI-boosting and how is it used?).
- All are substrates and some are inhibitors of CYP3A4 – high potential for drug interactions, can cause increases in levels of other CYP3A4 metabolized drugs
Why do the structures of ritonavir and cobicistat act on CYP3A4 to act as PI boosters?
- Low doses of ritonavir inhibit CYP3A4: block metabolism of other PIs, increases serum concentrations, increases trough levels, reduces emergence of resistant viruses
- Lopinavir and tipranavir only used in combo with ritonavir
What are the current recommended initial anti-retroviral drug regimens?
- Biktarvy: bictegravir, emtricitabine, tenofovir alafenamide
- Triumeq: dolutegravir, abacavir, lamivudine
- Dovato: doletugravir, lamivudine
- Genvoya: elvitegravir/cobicistat, emtricitabine, tenofovir alafenamide
Long acting injectable therapies: cabotegravir and rilpivirine; lenacapavir – capsid inhibitor, prevents virus assembly of capsids into stable structure
