ANTIRETROVIRAL RESISTANCE IN CLINICAL PRACTICE innovirInstitute bringing science to life ANTIRETROVIRAL RESISTANCE IN CLINICAL PRACTICE Steven Miller Medical Director

Classes of ARVs NRTIs - AZT, d4T, ddI, abacavir, 3TC, FTC, tenofovir (Target enzyme: Reverse Transcriptase) NNRTIs - nevirapine, efavirenz, etravirine, rilpivirine (Target enzyme: Reverse Transcriptase) Integrase Inhibitors - raltegravir, dolutegravir (Target enzyme: Integrase) Protease Inhibitors - lopinovir, atazanavir, darunavir, ritonavir Entry and fusion inhibitors - maraviroc, enfuvirtide (Target: Cell surface receptors)

Goals of ART Established: - Reduce HIV related morbidity and mortality - Improve quality of life - Restore and preserve immune function - Maximally and durably suppress viraemia (<50 cp/mL) Emerging: - Decrease risk for IRIS - Decrease morbidity and mortality from non-HIV related disease - Achieve normal life expectancy - Decrease lifetime risk of transmission to others

Defining Resistance Clinically safe doses of antiretroviral medication/s lose their antiviral effect

Adequate Response Phase 1: Rapid (6-8 week) decline > 1 log; inhibition of virus in activated CD4’s Phase 2: Slower (8-24 week) decline to < 50 cp/mL; inhibition of virus in resting CD4 cells and macrophages

Types of treatment failure Category Criterion Action Virological 50 cp/mL at ≥ 24 wks Switch regimen Immunological Failure to gain 50 cells in 1st yr Assess for other causes Clinical New illnesses May not warrant switch Check for IRIS Check prophylaxis

Virological failure Serial determination of Viral Load: - early: VL decline < 1 log within 8 wks - late: VL above 50 cp/mL at 24 wks; or VL increase from <50 cp/mL to ≥ 50 cp/mL persistently (? > 200/1 000)

Pathways to Resistance Drug-Resistant Variants Pre-existing Transmitted Selected Subinhibitory Drug Levels Host Immune Failure Recombination (Re-infection) Persistent viral replication Evolution of drug resistance Drug failure Source: Adapted from Hirsch, et al. JAMA 1998; 279:1984.

Mechanisms - Drugs no longer able to bind to their target site (steric exclusion) - Removal of drug from an incomplete DNA copy of the viral genome

Protease Structure

PI Binding Site

Removal of Drug G - C - T - A - G DNA G - C - AZT - Incomplete DNA

Consequences of Resistance Rebound in plasma and tissue viraemia Alterations in viral fitness Impact on the susceptibility to other ARVs, even if not yet used (cross-resistance) Changes in viral tropism

Primary & Secondary Mutations Primary: - at, or near, the active site - significant impact on susceptibility Secondary: - may be pre-existing (“polymorphisms”) - have less impact on the level of resistance Both primary and secondary mutations accumulate over time

Mutations accumulate Resistance following single mutation: - 3TC, FTC, NNRTIs, indinavir, atazanavir, tenofovir Resistance following accumulation of mutations: other NRTIs, Aluvia, other boosted PIs Mutations accumulate proportional to the time on virologically failing regimen For first-line therapy, this can result in almost total NRTI resistance as well as NNRTI resistance

Resistance is irreversible Every genetic variant of HIV is archived in the human immune system Archiving 1: - viral DNA persists within long-lived cells (e.g. resting CD4, memory CD4 cells) - replication competent - not necessarily present in plasma - may cause “blips” - re-emerges as dominant population under selective pressure 1. Persaud, et al. J I D 2007 (March)

3TC/FTC Resistance A conundrum If initial regimen contains 3TC/FTC fails, resistance to this agent is almost universal This appears to be of little clinical relevance when switching ART provided: - a virologically active, boosted PI is used - a virologically active NRTI forms part of the combination 1. Persaud, et al. J I D 2007 (March)

NNRTI Resistance Complete cross-resistance between Nevirapine and Efavirenz Cannot sequence these two drugs 1. Persaud, et al. J I D 2007 (March)

Resistance Tests Techniques: - Genotype - Phenotype Purpose: - to distinguish between virological and non-virological causes of resistance - to optimise further ART Techniques: - Genotype - Phenotype Caution: - cellular mechanisms of resistance not assessed - requires an adequate level of viraemia

Indications for Resistance Testing Primary infection Failed prophylaxis - includes infants Treatment naïve - if >5% prevalence of primary transmitted resistance Treatment experienced - women with prior NVP monotherapy (MTCTp) - if patient on prolonged failing regimen - following second regimen failure

Benefits Avoid unnecessary switching of drugs Exclude adherence problems Perform directed treatment switches Use active drugs durably Avoid unnecessary toxicities from inactive drugs

Nomenclature for Mutations Reverse Transcriptase Wild Type Amino Acid (methionine) RT M184V Mutated Amino Acid (valine) Position of Amino Acid in RT

Truvada AZT + tenofovir Never use tenofovir + ddI How to switch NRTIs Initial NRTI Option 1 Option 2 Truvada Continue Truvada** AZT + tenofovir AZT + abacavir Abacavir + 3TC Truvada AZT + tenofovir AZT + ddI Abacavir + AZT/ddI AZT/d4T + 3TC Truvada Tenofovir + abacavir abacavir + ddI AZT/d4T + ddI tenofovir + Never use tenofovir + ddI

How to switch NNRTIs and PIs Initial Agent Replacement NNRTI Boosted PI (Aluvia, or Atazanavir/ritonavir PI Get expert opinion

Survival benefit of HAART Fully suppressive HAART expected to yield normal lifespan Emergence of resistance remains the greatest impediment to achieving this goal

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