Antivirals for HIV Yasir Waheed, PhD

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).

HIV Life Cycle 1 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

Anti- HIV Drug Targets 2 Three types of drugs are currently in clinical use: 1.nucleoside and nucleotide reverse transcriptase (RT) inhibitors 2.non-nucleoside reverse transcriptase inhibitors 3.protease inhibitors (PIs)

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 termination 3.

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 treatments. Antiviral selectivity due to higher affinity for HIV RT than human DNA polymerases.

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.

Non-Nucleoside Analogs Delavirdine Benzoxazinone Nevirapine

Protease Inhibitors During the reproduction cycle of HIV a specific protease is needed to process the polyproteins into mature HIV components. If protease is missing non-infectious 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 methods.

HIV Protease 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.

Protease Inhibitors 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.

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

Virus Adsorption Inhibitors Cosalane was originally developed as an anti- cancer agent by researchers at Purdue University and the U.S. National Cancer Institute. Cosalane was developed from a chemical known as ATA (aurintricarboxylic acid), which has long been known to have anti-HIV activity. The result was cosalane. Cosalane binds to the HIV gp-120 protein.

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 activity. One such compound is AMD3100.

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

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). Other side effects of anti-HIV drugs include pancreatitis, myopathy, anemia, peripheral neuropathy, nausea, and diarrhea.

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.

18 10 Million Patients on Antiretroviral Therapy 2013 Global AIDS Response Progress Reporting (WHO/UNICEF/UNAIDS)

19 Principles of HIV Drug Resistance Not all drug failure is due to resistance Partial HIV suppression promotes resistance Resistance may fade but not disappear when a drug is stopped

20 Principles of Resistance (2) Some mutations allow certain viruses to resist the effects of one or more antiretroviral drugs. Each infected person has a mixture of viruses, some of which are resistant to some medications. The drug resistant virus usually grows faster and better than the drug susceptible virus. The drug resistant virus replaces the drug susceptible virus in the patient.

21 Resistance Testing Two types: – Genotyping Less expensive Can usually be completed in 1-2 weeks – Phenotyping More expensive Generally takes 2-3 weeks to complete

22 Suspect Resistance in the Setting of Treatment Failure Due to HIV’s high transcription error rate and high level of replication, mutant HIV variants constantly generated. These variants often contain mutations that confer variable levels of resistance to antiretroviral agents. Poor adherence or suboptimal regimens can lead to resistance and ‘viral breakthrough’.

23 How Does Resistance Develop? Results from changes (mutations) in the genetic information in the virus. These changes occur whenever HIV is replicating. Every possible mutation occurs tens of thousands of times each day.

24 Resistance Mutations For some drugs (NNRTIs and 3TC), a single mutation causes high-level resistance. – Resistance to these drugs occurs very quickly For other drugs (most NRTIs and PIs), many mutations must occur before high-level resistance is observed. – Resistance to these drugs occurs more slowly

25 Cross-Resistance Resistance to one drug can cause resistance to others of the same class – NNRTI: complete cross-class resistance – NRTI: partial cross-class resistance – PI: partial cross-class resistance Partly overcome by ritonavir boosting

26 Minimize Emergence of Viral Resistance Never prescribe ARVs in the absence of adherence counseling and support Never prescribe monotherapy or dual therapy Ensure optimal serum drug concentrations – Avoid drug interactions – Diagnose and manage malabsorption If ARV medications are to be discontinued, stop all drugs at the same time – Possible exception: NNRTI-based regimen

vs WHO Guidelines

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