Interactions of P-glycoprotein with several analogues and metabolites of cyclosporin A were studied to gain a better understanding of this immunosuppressant's mechanism of excretion and nephrotoxicity. Incorporation of [3H]azidopine into human renal P-glycoprotein in the presence of various concentrations of different cyclosporins was quantitated. Competitive [3H]azidopine photolabeling and 3H drug transport assays of CHRC5 multidrug-resistant cells were also conducted to evaluate effects of cyclosporins on P-glycoprotein function. Cyclosporins A [half-maximal inhibition constant (K0.5) = 20 nM] and G (K0.5 = 40 nM) blocked [3H]azidopine photolabeling of renal P-glycoprotein at very low concentrations, whereas higher concentrations of cyclosporin C (K0.5 = 500 nM) and metabolites 1, 17, and 21 (K0.5 = 200 nM) were required to inhibit photolabeling. Metabolites H and 8 were ineffective in inhibition of [3H]azidopine photolabeling of human renal P-glycoprotein. Similarly, cyclosporins A, C, and G were the best inhibitors of [3H]azidopine photolabeling of P-glycoprotein in multidrug-resistant C5 cells; the various metabolites were less effective. Cyclosporins A, C, and G also enhanced cellular accumulation of [3H]cyclosporin A and several other 3H-labeled compounds known to be transported by P-glycoprotein in multidrug-resistant C5 cells. Differential affinities of cyclosporin A metabolites for P-glycoprotein suggest considerable drug-binding site specificity. Our current hypothesis is that cyclosporin A may be more nephrotoxic than its metabolites by virtue of its superior ability to bind to and competitively inhibit urinary excretion of an endogenous P-glycoprotein substrate. Our findings provide the basis for future design and testing of new cyclosporin derivatives that have immunosuppressive activity yet may be less nephrotoxic because of their poor interaction with renal P-glycoprotein.