Fostemsavir is a prodrug and is hydrolysed to the active compound temsavir in the small intestine. Temsavir is mainly metabolized by esterase-mediated hydrolysis with a small contribution of CYP3A4. Coadministration of fostemsavir (600 mg twice daily) with darunavir/cobicistat (800/150 mg once daily) increased temsavir Cmax, AUC and Cmin by 79%, 97% and 124% (n=15). These changes are not considered to be clinically significant and do not warrant a dose adjustment of fostemsavir.