Of the PIs, ritonavir is the most potent inhibitor, followed by indinavir, nelfinavir, amprenavir, and saquinavir, in decreasing order.4 Additionally, the PIs are substrates and inhibitors of P-glycoprotein, a major drug transmembrane efflux protein that transports PIs and other drugs out of the cell, which can lead to a decrease in the bioavailability of PIs.3