The interaction between P-glycoprotein (P-gp) from membranes isolated from multidrug-resistant Chinese hamster ovary cells and cyclosporin A (CsA) analogues and its metabolites was characterized. Screening of these latter as chemosensitizers was performed using three different assays: (i) vinblastine uptake, (ii) photoaffinity labeling by [125I]iodoaryl azidoprazosin, and (iii) P-gp ATPase activity. Oxidation of the hydroxyl group at position I of CsA (200-096), CsG (215-834), or CsD (PSC-833) increased their inhibition of P-gp. CsA analogues (208-032, 208-183) modified at position 11 retained their ability to inhibit P-gp while analogues modified at position 2 (CsC and CsD) lost their efficiency. The inhibitions induced by metabolites of CsA were also compared to those obtained with CsG metabolites. From all the molecules tested, PSC-833 and 280-446 peptolide were the strongest inhibitors. Our results indicate that modifications of CsA analogues at position 1 and 2 are critical for their interaction with P-gp and that CsA metabolites retain a portion of the inhibitory activity of the parent drug.