Interactions of HIV protease inhibitors with ATP-dependent drug export proteins

We used renal proximal tubules from a teleost fish (killifish; Fundulus het eroclitus), fluorescent substrates and confocal microscopy to study the int eractions between human immunodeficiency virus protease inhibitors and drug -transporting ATPases. Both saquinavir and ritonavir inhibited luminal accu mulation of a fluorescent cyclosporin A derivative (a substrate for P-glyco protein) and of fluorescein methotrexate [a substrate for multidrug resista nce-associated protein 2 (Mrp2)]. Of the two protease inhibitors, ritonavir was the more potent inhibitor of transport by a factor of at least 20. Rit onavir was at least as good an inhibitor of P-glycoprotein- and Mrp2-mediat ed transport as cyclosporin A and leukotriene C4, respectively. Inhibition of P-glycoprotein- and Mrp2-mediated transport was not due to toxicity or i mpaired metabolism, because neither saquinavir nor ritonavir inhibited tran sport of fluorescein on the renal organic anion system. Experiments with a fluorescent saquinavir derivative showed strong secretion into the tubular lumen that was inhibited by verapamil, leukotriene C4, saquinavir, and rito navir. Together, the data demonstrate that saquinavir, and especially riton avir, are potent inhibitors of P-glycoprotein- and Mrp2-mediated transport. The experiments with the fluorescent saquinavir derivative suggest that th ese protease inhibitors may also be substrates for both P-glycoprotein and Mrp2.