WEEK 8 - Antiretroviral Drugs-focus on HIV

Question 1

MoA for HIV Fusion Inhibitors

e.g. Enfuvirtide

Answer 1

Prevents HIV from entering host cell by inhibiting fusion

• Long synthetic peptide chain
• Can NOT be given ORALLY (would be degraded by proteases)
  - 90mg BD via SC injection
• High cost treeatment

NOTE:
- Used in combination therapies
- Useful for multi-drug resistant HIV infection

Question 2

MoA for HIV NucleosideReverse Transcriptase Inhibitors (NRTI)

e.g. Abacavir, Lamivudine, Emtricitabine

Answer 2

NRTIs STOP chain elongation of viral DNA
- by binding to / blocking deoxy-nucleotide site of RT preventing nucleotide triphosphate substrate binding
- lack of 3-hydroxy stops replication

NRTIs are substrates / competitive inhibitors for viral DNA polymerase
- NRTIs have similar structure to the natural deoxy-nucleotide triphosphate substrate of RT

MoA:
1. Nucleoside drug undergoes 1st phosphorylation using viral kinase = pro-drug formed
- phosphate is too negtaive / polar to cross membrane hence WHY need to form pro-drug
2. Pro-drug undergoes 2nd and 3rd phosphorylation using host kinase
- pro-drug is lipophilic = can cross cell membrane
3. 3rd phosphorylation of nucleoside drug is what interacts with RT
- a.k.a. nuceloside triphosphate drug = active drug

NOTE:
- Nucleoside drug is not useful on its own / not phosphorylated
- NRTIs must NOT have 3-hydroxyl group present (OH= group allows DNA replication)
- Inhibitor MUST be selective for viral enzymes

Question 3

MoA for HIV Nucleotide Reverse Transcriptase Phosphonate Prodrugs

e.g. Tenofovir disoproxil, Tenofovir alafenamide

Answer 3

Both examples are prodrugs of TAF (tenofovir)
- both are hydrolysed to TAF when in hepatocytes

1. TAF is a monophosphate
   - formed after 1st hydrolysis
2. TAF can eneter hepatocytes via active transport (OATP) or passive permeaility
3. TAF is hydrolysed by CES1 = carboxylate anion formed
   
   • 1st phosphorylation
4. Anion is hydrolysed further to produce phosphate di-anion = TFV
   
   • 2nd phosphorylation
5. TFV is further phosphorylated to form TFV diphosphate (TFV-DP)
6. TFV-DP is the nucelotide triphosphate (neucleotide host kinase substrate)
   
   • this triphosphonates is incorporated into the growing DNA strand
7. Replication stops due to LACK of HYDROXYL group

NOTE:
- Host kinases are used for further hydrolysis

Question 4

MoA for HIV Non-Nucleoside Reverse TranscriptaseInhibitors (NNRTI)

e.g. Rilpivirine

Answer 4

NNRTIs bind to NNRTI site (secondary site) indirectly inhibiting natural deoxy-nucelotide triphosphate substrate
- Stops / slows down DNA synthesis

NNRTIs are non-competitive inhibitors
- do NOT have similar structure to nucleoside substrate of RT
- do NOT bind to deoxy-nucletoide binding site
- when bind to laternative site = conformational changes that distort the position of the deoxy binding sites occur

NOTE:
- NOT phosphorylated

Question 5

MoA for HIV Integrase Inhibitors

e.g. Cabotegravir, Raltegravir, Elvitegravir, Dolutegravir

Answer 5

Inhibits integrase (enzyme)
- prevents insertion of viral DNA into host genome / DNA
- prevents HIV infection progressing
- prevents virus using host to make viral proteins / virions

Question 6

MoA for HIV Protease Inhibitors

e.g. Atazanavir, Darunavir

Answer 6

Prevent protease from cleaving (hydrolysing) peptide bonds between long protein chains into specific parts
- inhibits viral proteins / virions production
- inhibits maturation of virus
- inhibits virus budding + infecting other cells

MoA:
1. H2O attacks the carbonyl of the Phe-Pro substrate = a high energy tetrahedral carbon intermediate with two hydroxy groups is formed
- a.k.a. sp3 intermediate (transition state)
- amide hydrolysis reaction
2. Protease inhibitors designed MIMIC this tetrahedral TRANSITION STATE
3. Inhibitor binds tightly to protease active site + prevents protease cleaving pro-form of viral proteins
- inhibitor forms hydrogen bonds with protease

HIV Protease INFO:
- is a symmetrical dimer
- has 2 identical subunits
- is an ASPARTYL PROTEASE ~ has 2 aspartic acid active site carboxylate residues

Question 7

Combination Therapy

w.g. HAART (Highly Active Anti-Retroviral Therapy)

Answer 7

Research prove that combination therapy is essential + effective

• NEED combination therapy for HIV treatment to be effective
• Giving 2 drugs targeting 2 diff. targets reduced mortality massively
• AIM to target min. 3 targets
  -there is a total of 4 targets available

Question 8

What treament is given to treatment-naivepatients

Answer 8

Treatment-naivepatients = patients who have just acquired HIV

1. TWO NRTIs
   - TAF (Tenofovir)
   - Abacavir
2. ONE of the FOLLOWING:
   - NNRTIs
   - Integrase inhibitor (Cabtogreavir)
   - Protease inhibitor

Question 9

What treatment is given to HIV-1 infected patients

Answer 9

Cabotegravir (Integrase Inhibitor) + Rilpivirine (NNRTI)

Either:
- Oral daily tablets
- Separte bi-montly IM injections

Question 10

What treatment is given to drug resistant HIV-1 patients

Answer 10

1. TWO NRTIs
   - TAF or TDF
   - Lamivudine or Emtrictabine
   If resisant to above use Zidovudine (alternative)
2. TWO of the FOLLOWING:
   - Dolutegravir or Darunavir (DRV)
   - integrase or protease inh.
   - Boosted with Cobicistat (COBI) or Ritonavir

If further resistance
- Use Maraviroc (MVC) (CCR5 antagonist)

USE treatment with higher barriers to resistance

Question 11

Example Exam Questions

Answer 11

1. List four druggable targets for the HIV lifecycle. Describe the MOA of one target. Give an example of a drug that inhibits this target. (5 marks)
2. Combination therapy is needed for the treatment of HIV. Discuss, including drug examples. (5 marks)