Background
Fostemsavir (FTR), approved by the FDA in July 2020, is a first-in-class small molecule for treating multi-drug resistant HIV-1 infection. It is the prodrug of its active form, temsavir (TMR). TMR is classified as an attachment inhibitor and is thought to stabilize a native state of HIV Env, the state recognized by most broadly neutralizing antibodies, while also influencing Env glycosylation and proteolytic cleavage. However, the precise mechanism(s) by which TMR affects Env processing and conformation, particularly on cell-associated versus virus-incorporated forms of Env, remains unclear.
Methods
In this study, we measured the impact of FTR on antibody binding to Env using a cell-based assay in two systems: a tetherin-high system presenting Env on virus particles and a tetherin-low system presenting cell-surface Env. Using this system, we examined binding with and without TMR across various HIV-1 Env forms, including lab-adapted and primary strains, using several bNAbs targeting CD4, V2, and V3 regions. Additionally, we assessed bNAb binding to wildtype, hypercleavable (RR), and uncleaved (RS) Env mutants of QH0692 HIV-1 T/F strain over time.
Results
TMR increased V3 region recognition of bnAbs for wildtype viruses (YU2, T/F QH0692), with more pronounced effects on cell-associated (CA) versus virus-incorporated (VI) forms of Env. For the uncleavable Env QH0692 RS mutant an overall reduction of bnAbs binding was observed, whereas bnAb binding specifically to the V3 region, was dramatically enhanced. While this effect was observed for CA and VI forms of Env, enhancement was more marked on CA forms, suggesting a more pronounced effect on cell-associated forms of gp160 Env. Additionally, enhancement of V3 directed bnAb binding after 15 mins of TMR treatment with the RS mutant (gp160), suggests that TMR may directly influence Env conformation rather than indirectly affecting it through processing or glycosylation inhibition.
Conclusions
These data indicate that FTR does not primarily affect all strains by decreasing Env expression, and may be used in combination with certain V3 bnAb treatments to potentially enhance the effectiveness of bnAb-based therapies.
Table or Figure
