1448. Forgiveness of BIC/FTC/TAF: In Vitro Simulations of Intermittent Poor Adherence Find Limited HIV-1 Breakthrough and High Barrier to Resistance

BACKGROUND: Short lapses in adherence to ARVs can lead to virologic failure and emergence of resistance. Previous in vitro studies of regimen “forgiveness” simulated drug exposures of perfect adherence or short-term suboptimal adherence with bictegravir+emtricitabine+tenofovir alafenamide (BIC+FTC+TAF) and with dolutegravir and lamivudine (DTG+3TC). Here, viral breakthrough (VB) and resistance development were evaluated under alternating high and low drug exposures simulating variable adherence levels. METHODS: Wild-type HIV-1 (IIIb)-infected MT-2 cells were exposed to drug combinations and monitored for VB. Experiments alternated between high and low drug concentrations of either BIC+FTC+TAF or DTG+3TC (Table 1). Drug concentrations for each regimen were determined using human plasma-free adjusted clinical trough concentrations (C(min)), at simulated C(min) after missing 2 or 4 consecutive doses (C(min)-2 and C(min)-4) based on drug half-lives. Emergent HIV-1 were genotyped by deep sequencing and a 2% threshold. RESULTS: In these experiments, constant drug concentrations corresponding to full adherence (C(min)) did not lead to VB. Using C(min) concentrations for one week followed by constant C(min)-2 exposures for 4 weeks, DTG+3TC had VB and emergence of M184V/I in reverse transcriptase (RT) but there was no VB for BIC+FTC+TAF. Using alternating drug exposures of C(min) (weeks 1 and 3) and C(min)-2 or C(min) -4 (weeks 2, 4, and 5), VB was not observed with BIC+FTC+TAF, and VB was decreased or delayed with DTG+3TC compared to DTG+3TC held at C(min)-2 or C(min)-4. Resistance development was observed in some cultures with VB: 1 culture with BIC+FTC+TAF had G163R in IN and 19 cultures with DTG+3TC had INSTI and RT resistance including 10 with M184V/I. Table 1. Summary of Breakthrough Frequency and Resistance Development [Image: see text] CONCLUSION: BIC+FTC+TAF has high in vitro forgiveness and consistent protection against emergence of drug resistance during simulations of short lapses in adherence. Higher DTG+3TC exposure, whether constant or intermittent, was better at preventing or delaying VB than lower DTG+3TC exposures, but DTG+3TC was less forgiving than BIC+FTC+TAF. Prevention of viral replication and resistance development is necessary to maintain lifelong viral suppression, particularly in the real world where drug adherence is often imperfect. DISCLOSURES: Andrew Mulato, BS, MBA, Gilead Sciences, Inc. (Employee, Shareholder) Rima K. Acosta, BS, Gilead Sciences, Inc. (Employee, Shareholder) Stephen R. Yant, PhD, Gilead Sciences, Inc. (Employee, Shareholder) Tomas Cihlar, PhD, Gilead Sciences, Inc. (Employee, Shareholder) Kirsten L. White, PhD, Gilead Sciences, Inc. (Employee, Shareholder)