Anti-HIV Drugs

1. Anti-HIV Drugs Cathy Molina November 11, 2004

2. Some HIV Facts • HIV – the Human Immunodeficiency Virus is the retrovirus that causes AIDS • HIV belongs to the retrovirus subfamily lentivirus. • HIV attaches to cells with CD4 receptors (T4 cells and macrophages).

3. HIV Life Cycle1 • Step 1: Attachment of virus at the CD4 receptor and chemokine co-receptors CXCR4 or CCR5 • Step 2: viral fusion and uncoating • Steps 3-5: Reverse transcriptase makes a single DNA copy of the viral RNA and then makes another to form a double stranded viral DNA • Step 6: migration to nucleus • Steps 7-8: Integration of the viral DNA into cellular DNA by the enzyme integrase • Steps 9-11: Transcription and RNA processing • Steps 12-13: Protein synthesis • Step 14: protease cleaves polypeptides into functional HIV proteins and the virion assembles • Step 15: virion budding • Step 16: Virion maturation

4. Anti- HIV Drug Targets2 Three types of drugs are currently in clinical use: • nucleoside and nucleotide reverse transcriptase (RT) inhibitors • non-nucleoside reverse transcriptase inhibitors • protease inhibitors (PIs)

5. Nucleoside and Nucleotide Analogs • Nucleoside analogs (NRTI) act as chain terminators or inhibitors at the substrate binding site of RT • NRTI’s must be phosphorylated (three steps) to their 5’-triphosphate form to become active inhibitors. • Nucleotide analogs (NtRTI) already contain a phosphate group and only go through 2 steps to become active. • The 5’-triphosphate of the NRTI’s compete with the 2’-deoxynucleoside’s 5’-triphosphate for binding to reverse transcriptase leading to viral DNA chain termination3.

6. Nucleoside Analogs • There are currently 7 FDA-approved NRTI’s and one nucleotide analog. • The first anti-HIV drug approved was the NRTI known as AZT or Zidovudine (1987). • AZT was discovered as a treatment of AIDS during a screening process for the identification of effective AIDS treatments4. • Antiviral selectivity due to higher affinity for HIV RT than human DNA polymerases.

7. Non-Nucleoside Analogs • Non-nucleoside analog reverse transcriptase inhibitors (NNRTI’s) inhibit viral DNA replication by binding at the allosteric non-bonding site of RT, causing a conformational change of the active site. • NNRTI’s do not require bioactivation by kinases. • Three NNRTI’s are currently approved for clinical use in combination therapy: nevirapine, delavirdine, and efavirenz

8. Non-Nucleoside Analogs5 Delavirdine Benzoxazinone Nevirapine

9. Protease Inhibitors • During the reproduction cycle of HIV a specific protease is needed to process GAG and POL polyproteins into mature HIV components. • If protease is missing noninfectious HIV is produced. • HIV protease inhibitors are specific to HIV protease because it differs significantly from human protease. • The 6 PI’s currently approved for clinical use were all designed by using structure-based drug design methods4.

10. HIV Protease6 • The crystal structure of HIV protease was first obtained at Merck Laboratories. • HIV protease is a 99 amino acid aspartyl protease that functions as a homodimer with one active site. • The active sites of protease are hydrophobic.

11. Protease Inhibitors7 • HIV PI’s target the peptide linkages in the gag and gag-pol polyproteins which must be cleaved by protease. • All approved PI’s contain a hydroxyethylene bond instead of a normal peptide bond. • The hydroxyethylene bond makes PI’s non-scissile substrate analogs for HIV protease

12. Protease Inhibitors7 • ABT-378 or lopinavir was approved in 2000 for use in combination with ritonavir (a PI) (Kaletra) • Ritonavir strongly inhibits the metabolism of ABT-378

13. Some Alternative Therapies • Virus adsorption inhibitors – interfere with virus binding to cell surface by shielding the positively charged sites on the gp-120 glycoprotein • Polyanionic compounds • Viral coreceptor antagonists – compete for binding at the CXCR4 (X4) and CCR5 (R5) coreceptors • bicyclams and ligands

14. Virus Adsorption Inhibitors • Cosalane was originally developed as an anti-cancer agent by researchers at Purdue University and the U.S. National Cancer Institute8. • Cosalane was developed from a chemical known as ATA (aurintricarboxylic acid), which has long been known to have anti-HIV activity8. • ATA is a mixture of different polymers. Chemists took one of the low molecular weight components of ATA, and attached it to a steroid molecule in order to target the substance more effectively to the surface of viruses and of cells. • The result was cosalane. • Cosalane binds to the HIV gp-120 protein.

15. Viral Coreceptor Antagonists • Bicyclams are a type of viral coreceptor antagonist. • They are very specific and potent X4 coreceptor antagonists. • Bicyclams belong to a class of macrocyclic polyamines consisting of two cyclam units linked by an aliphatic bridge • Bicyclams with an aromatic linker apparently had higher antiviral activity10. • One such compound is AMD3100.

16. Combination Therapy • Combination therapy often called HAART is standard care for people with HIV. • Monotherapy created virus resistance to the individual drug. Some combination therapies increase the time it takes for the virus to become resistant. • Combinations of a PI or NNRTI with one or two NRTI’s is often recommended. • Combination therapy may reduce individual drug toxicity by lowering the dosage of each drug

17. Combination Therapy • The combination of drugs chosen is based on the history of each individual patient and synergistic drug interactions. • Some drugs compete with each other for binding sites or enzymes. • Example: zidovudine and stavudine • both nucleoside analogs compete for the same kinase. Stavudine is not phosphorylated because zidovudine is preferred5.

18. Combination Therapy and Drug Resistance • Some drug combinations can restore sensitivity of the virus to drugs it was previously resistant to. • Example: lamivudine and zidovudine • The HIV M184V mutation is resistant to lamivudine but restores sensitivity to zidovudine resistant virus mutants5.

19. Drug Toxicity and Side Effects • All available antiretroviral drugs are toxic. • Side effects of nucleoside analogs are lactic acidosis and severe hepatomegaly with steatosis (enlarged fatty liver)11. • Other side effects of anti-HIV drugs include pancreatitis, myopathy, anemia, peripheral neuropathy, nausea, and diarrhea.

20. Reducing Drug Toxicity • The use of combination therapy: • Combining agents with favorable synergistic properties allows a decrease in dose or dosing frequency • Ritonavir alone cause gastrointestinal side effects but when used in combination with other PI’s it can be administered at a lower dose.

21. Conclusions • An effective anti-HIV therapy is still needed. • Several possible targets are being studied and tested. • The area of anti-HIV drugs has more room for growth and the future for the discovery of new effective drugs is promising.

22. References • NIAID HIV Life Cycle. http://www.niaid.nih.gov/daids/dtpdp/virpage1.htm (accessed Oct 2004). • De Clerq, E. New anti-HIV agents and targets. Med. Res. Rev. 2002, 22(6), 531-565. • El Kouni, M. H. Trends in the design of nucleoside analogues as anti-HIV drugs. Current Pharmaceutical Design.2002,8(8), 581-593. • Block, J. H.; Beale, J. M. Antiviral Agents, Wilson and Gisvold’s Textbook of Organic Medicinal and Pharmaceutical Chemistry, 11th ed; Lippincott Williams & Wilkins: Maryland, 2004; pgs 379, 943. • De Clerq, E.; Vandamme, A-M. Combination Therapy of AIDS. Birkhauser Verlag: Germany, 2004. • Brik, A.; Wong, C-H. HIV-1 protease: mechanism and drug discovery. Organic & Biomolecular Chemistry. 2003, 1(1), 5-14. • De Clerq, E. New Developments in Anti-HIV Chemotherapy. Current Medicinal Chemistry. 2001, 8, 1543-1572. • cosalane website – look up • Ruell, J. A.; De Clercq, E.; Pannecouque, C. Synthesis and Anti-HIV Activity of Cosalane Analogues with Substituted Benzoic Acid Rings Attached to the Pharmacophore through Methylene and Amide Linkers. J. Org. Chem. 1999, 64, 5858-5866. • Labrosse, B.; Brelot, A.; Heveker, N.; Sol, N. Determinants for Sensitivity of Human Immunodeficiency Virus Coreceptor CXCR4 to the Bicyclam AMD3100. J. Virol. 1998, 6381–6388. • Simple FactSheet from the AIDS Treatment Data Network. http://www.atdn.org/simple/abac.html (accssed Nov 2004).