['P-glycoprotein is a 170-kDa plasma membrane multidrug transporter that actively exports cytotoxic substances from cells. Overexpression of P-glycoprotein by tumor cells is associated with a multidrug-resistant phenotype. Immunosuppressive agents such as cyclosporins and macrolides, have been shown to attenuate P-glycoprotein activity. However, the mechanism by which some immunosuppressants inhibit P-glycoprotein function has not been determined. Since cyclosporin and macrolide immunosuppressants inhibit calcineurin (CaN) phosphatase and FKBP12 peptideprolyl isomerase (FKBP12 PPI) activity, studies were conducted to determine if these effects are directly related to the inhibitory effects these immunosuppressants have on P-glycoprotein function.', 'Western blot analysis was performed to assess CaN and FKBP12 protein levels in P-glycoprotein-negative (MCF-7) and -positive (MCF-7/Adr) breast cancer cell lines. P-glycoprotein function was determined by intracellular doxorubicin accumulation and/or cytotoxicity assays before and after CaN and FKBP12 were independently inhibited by pharmacological antagonists.', 'CaN and FKBP12 levels were similar in MCF-7 and MCF-7/Adr cells. P-glycoprotein function was not affected by treatment of P-glycoprotein-expressing MCF-7/Adr cells with CaN and FKBP12 antagonists.'] Is immunosuppressant inhibition of P-glycoprotein function independent of drug-induced suppression of peptide-prolyl isomerase and calcineurin activity?