Fostemsavir, a prodrug of human immunodeficiency virus attachment inhibitor temsavir (TMR), is in phase III development in combination with other antiretroviral agents for the treatment of human immunodeficiency virus type I (HIV-1) infection in heavily treatment-experienced adults with multidrug-resistant HIV-1 infection for whom it is otherwise not possible to construct a suppressive antiviral regimen due to resistance, intolerance, or safety considerations. The proarrhythmic potential of fostemsavir was studied in a thorough QT study and exposure-response modeling was performed at therapeutic and supratherapeutic concentrations of TMR. Fostemsavir 1,200 mg b.i.d. did not result in a clinically meaningful change from placebo in baseline-adjusted Fridericia-corrected QTc (ddQTcF); however, at a supratherapeutic dose of 2,400 mg b.i.d., the upper bound of the two-sided 90% confidence interval (CI) of ddQTcF was 13.2 msec, exceeding the clinically important 10 msec threshold. A linear model of ddQTcF as a function of TMR plasma concentrations described these observations. Based on simulations with this model, TMR concentrations up to 7,500 ng/mL are expected to have an upper 90% CI bound for QTcF ≤ 10 msec. This concentration is 4.2-fold higher than the geometric mean TMR peak plasma concentration (Cmax ) of 1,770 ng/mL in heavily treatment-experienced HIV-1 infected patients administered fostemsavir 600 mg b.i.d. in the phase III BRIGHTE study (NCT02362503).
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