center dot Antiretroviral protease inhibitors such as lopinavir and saquinavir have been shown to be substrates of ABCB1. Co-administration with the potent ABCB1 and CYP3A4 inhibitor, ritonavir, has shown improved pharmacokinetics and subsequent therapeutic effects of protease inhibitors. Darunavir is a recently licensed protease inhibitor with potent antiretroviral effects but has yet to be characterized as a potential substrate for drug transporters.