Unraveling the Mechanism of HIV-1 Hypersusceptibility to Tenofovir Imparted by Islatravir Resistance Mutations

In response to the newly approved antiretroviral therapy (ART) islatravir (ISL), the M184V and A114S resistance mutations have emerged in the human immunodeficiency virus type 1 reverse transcriptase (HIV-1 RT). These mutations markedly hypersensitize RT to the globally administered ART tenofovir disoproxil fumarate (TDF). We have solved six structures - four by X-ray crystallography and two by cryo-EM - that capture the single- and double-mutant RTs during inhibitor incorporation and demonstrate the role of the mutations in altering protein-antiviral interactions. These snapshots reveal that the smaller, more flexible TDF diphosphate (TDF-DP) can better accommodate mutation-induced active site changes than ISL triphosphate (ISL-TP). Structural differences between the two inhibitors are consistent with biochemical determination of inhibitory constants (Kis), highlighting differences at the step of inhibitor incorporation. Virological evaluation of ISL and TDF combinations reveals additive inhibition of HIV-1. Given the converse ISL hypersusceptibility imparted by the TDF-resistant K65R mutation, we propose ISL and TDF as a combination that can inspire future therapeutic options.