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Background Fostemsavir is a prodrug of temsavir, an attachment inhibitor that binds directly to HIV-1 gp120, blocking initial viral attachment and entry into host CD4+T-cells. Efficacy, safety and dose-response data of fostemsavir in treatment-experienced, HIV-1-infected subjects, through week 48, are reported.Methods AI438011 is an ongoing Phase IIb, randomized, active-controlled trial (NCT01384734). Subjects were randomized 1:1:1:1:1 into five arms: fostemsavir (400 mg twice daily, 800 mg twice daily, 600 mg once daily or 1,200 mg once daily) and a reference arm (ritonavir-boosted atazanavir [ATV/r] 300/100 mg once daily), each with a backbone of raltegravir 400 mg twice daily plus tenofovir disoproxil fumarate 300 mg once daily.Results In total, 251 subjects were treated. Through week 48, the proportion of fostemsavir subjects with HIV-1 RNA <50 copies/ml was 61–82% and 77–95% (modified intent-to-treat [mITT] and observed analysis, respectively); 71% and 88% for ATV/r subjects (mITT and observed). Observed virological response rates were 74-100% versus 96% (fostemsavir versus ATV/r) in subjects with baseline viral load <100,000 copies/ml and 60-91% versus 71% when baseline viral load was =100,000 copies/ml. Across fostemsavir arms, median CD4+T-cell count increases from baseline were 145-186 cells/µl and 142 cells/µl for the ATV/r arm. Fostemsavir doses were generally well tolerated and no fostemsavir-related adverse events led to discontinuation.Conclusions Through week 48, fostemsavir continued to be well tolerated and showed similar efficacy to ATV/r. These results support the ongoing Phase III trial in heavily treatment-experienced adults with limited therapeutic options (=2 classes of active antiretrovirals remaining). ClinicalTrials.gov identifer: NCT01384734.