• Max Lataillade, Nannan Zhou, Samit R Joshi, Sangil Lee, David A Stock, George J Hanna, Mark Krystal, and AI438011 study team.
• Currently, Research and Development, ViiV Healthcare, Wallingford, CT.
• J. Acquir. Immune Defic. Syndr. 2018 Mar 1; 77 (3): 299-307.

BackgroundFostemsavir is a prodrug of temsavir, an attachment inhibitor that binds to HIV-1 gp120, blocking viral attachment to host CD4 T-cells. The phase 2b trial AI438011 investigated the safety, efficacy, and dose-response of fostemsavir vs ritonavir-boosted atazanavir (ATV/r) in treatment-experienced, HIV-1-infected subjects.MethodsTwo hundred fifty-one treatment-experienced subjects with baseline (BL) susceptibility to study drugs [temsavir half-maximal inhibitory concentration (IC50) <100 nM, PhenoSense Entry assay] received fostemsavir or ATV/r, each with tenofovir disoproxil fumarate + raltegravir. Subjects meeting resistance-testing criteria were assessed for emergent viral drug resistance. Changes in temsavir IC50 from BL was given a conservative technical cutoff (>3-fold increase).Results66/200 fostemsavir and 14/51 ATV/r subjects had resistance testing performed; 44/66 and 9/14 were successfully tested using the PhenoSense GT assay. No subjects had emergent tenofovir disoproxil fumarate or ATV resistance. Six fostemsavir-treated subjects developed emergent raltegravir resistance. 29/66 fostemsavir-treated subjects had an evaluable phenotype using PhenoSense Entry (which tests for viral susceptibility to temsavir) and 13/29 exhibited >3-fold increase in temsavir IC50 from BL. gp120 population sequencing was successful in 11/13 subjects and 7 had emergent substitutions in gp120 associated with reduced temsavir susceptibility (S375, M426, or M434). However, 5/13 fostemsavir-treated subjects achieved subsequent suppression to <50 copies/mL before the week 48 database lock, regardless of key gp120 substitutions.ConclusionsResponse rates remained similar across study arms regardless of BL nucleoside reverse transcriptase inhibitor, nonnucleoside reverse transcriptase inhibitor, and protease inhibitor resistance-associated mutations. Emergent changes in viral susceptibility occurred more frequently with fostemsavir compared with ATV/r. However, the full impact of temsavir IC50 changes and emergent HIV-1 gp120 substitutions, and thus appropriate clinical cutoffs, requires further study. Fostemsavir is being evaluated in a phase 3 trial in heavily treatment-experienced subjects.

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