Cyclosporin A (CSA) is an effective inhibitor of the P-glycoprotein (P-gp) activity and has been shown to modulate multidrug resistance (MDR) in in vitro experimental models. During degradation of CSA, the metabolites arising from the parental compound reach high levels in the serum of patients, and it is not clear whether these metabolites maintain the reversal activity of the parental compound, like the metabolites of verapamil. In an in vitro experimental model, we compared the reversal activity of CSA and 3 CSA metabolites (M1, M17, and M21) in the range of concentrations obtained in whole blood during a clinical trial with CSA used as a revertant agent. As experimental model we used LoVo-resistant cells. Our in vitro studies indicated that the metabolic hydroxylation and demethylation of CSA lead to molecules that greatly differ from the parent drug in their reversal activity. In the range of concentration detected in the whole blood of the patients (1-3 microM), CSA had a significant reversal activity. It decreased the IC50 of antineoplastic drugs involved in MDR (vincristine, taxol, doxorubicin and etoposide) but not the IC50 of platinum or methotrexate. CSA increased intracellular doxorubicin content and inhibited P-gp 3[H]azidopine photolabeling. Conversely, CSA metabolite concentrations superimposable to those observed in the patients (0.5-2.2 microM) had no sensitizing effects on the cytotoxicity of MDR-related anti-neoplastic drugs, nor did they affect 3[H]azidopine photolabeling or doxorubicin uptake. This study demonstrates that, during degradation of CSA, metabolite derivatives arise that have a very different reversal activity from that of the parental compound.