DIFFERENTIAL INTERACTION OF HUMAN RENAL P-GLYCOPROTEIN WITH VARIOUS METABOLITES AND ANALOGS OF CYCLOSPORINE-A

Interactions of P-glycoprotein with several analogues and metabolites of cyclosporin A were studied to gain a better understanding of this i mmunosuppressant's mechanism of excretion and nephrotoxicity. Incorpor ation of [H-3]azidopine into human renal P-glycoprotein in the presenc e of various concentrations of different cyclosporins was quantitated. Competitive [H-3]azidopine photolabeling and H-3 drug transport assay s of CH(R)C5 multidrug-resistant cells were also conducted to evaluate effects of cyclosporins on P-glycoprotein function. Cyclosporins A [h alf-maximal inhibition constant (K-0.5) = 20 nM] and G (K-0.5 = 40 nM) blocked [H-3]azidopine photolabeling of renal P-glycoprotein at very low concentrations, whereas higher concentrations of cyclosporin C (K- 0.5 = 500 nM) and metabolites 1, 17, and 21 (K-0.5 = 200 nM) were requ ired to inhibit photolabeling. Metabolites H and 8 were ineffective in inhibition of [H-3]azidopine photolabeling of human renal P-glycoprot ein. Similarly, cyclosporins A, C, and G were the best inhibitors of [ H-3]azidopine photolabeling of P-glycoprotein in multidrug-resistant C 5 cells; the various metabolites were less effective. Cyclosporins A, C, and G also enhanced cellular accumulation of [H-3]cyclosporin A and several other H-3-labeled compounds known to be transported by P-glyc oprotein in multidrug-resistant C5 cells. Differential affinities of c yclosporin 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 superio r ability to bind to and competitively inhibit urinary excretion of an endogenous P-glycoprotein substrate. Our findings provide the basis f or future design and testing of new cyclosporin derivatives that have immunosuppressive activity yet may be less nephrotoxic because of thei r poor interaction with renal P-glycoprotein.