The ABC proteins P-glycoprotein (Pgp), multidrug resistance protein-1 (MRP-1) and breast cancer resistance protein (BCRP) mediate multidrug resistance in cancer cells, and Pgp and BCRP also play important roles in drug absorption and disposition. The immunosuppressive agents cyclosporine A, sirolimus (rapamycin) and tacrolimus (FK-506) are known to inhibit drug transport mediated by Pgp, and we compared their effects on MRP-1 and BCRP, as well as Pgp. Effects of cyclosporine A, rapamycin and FK-506 on transport of mitoxantrone, a substrate for Pgp, MRP-1 and BCRP, were studied in cell lines overexpressing these proteins, including HL60/VCR (Pgp), HL60/ADR (MRP-1) and 8226/MR20 (BCRP). Cellular content of mitoxantrone was measured by flow cytometry in each cell line following 30-minute incubation with mitoxantrone in the presence and absence of each of the three immunosuppressive agents, and histograms were compared by the Kolmogorov-Smirnov statistic, generating D-values. Cyclosporine A enhanced mitoxantrone uptake in cell lines overexpressing Pgp, MRP-1 and BCRP at the clinically achievable concentration of 2.5 μM. Rapamycin also enhanced mitoxantrone uptake in all three cell lines at 2.5 μM, but this concentration exceeds that achieved with doses used in current immunosuppressive regimens. However, pre-incubation with rapamycin for at least 3 hours at the clinically achievable concentration of 0.25 μM enhanced mitoxantrone uptake in HL60/VCR, HL60/ADR and 8226/MR20 cells, without effects on Pgp, MRP-1 or BCRP expression, consistent with modulation of transport. FK-506 enhanced mitoxantrone uptake in HL60/VCR, HL60/ADR and 8226/MR20 cells at 1 μM, but had no effect at the maximum clinically achievable concentration of 24 nM. In contrast to findings with rapamycin, pre-incubation with 24 nM FK-506 for up to 6 hours did not enhance mitoxantrone uptake in any of the three cell lines. In conclusion, cyclosporine A modulates drug transport by Pgp, MRP-1 and BCRP at pharmacologically relevant concentrations, while FK-506 also modulates all three proteins, but only at concentrations exceeding those achievable in the clinical setting, and rapamycin modulates at clinically achievable concentrations, but modulation requires prolonged exposure. Thus the three immunosuppressive agents commonly used in transplantation regimens differ in their effects on multidrug resistance mediated by Pgp, MRP-1 and BCRP, and in their effects on absorption and disposition of co-administered drugs that are substrates for these ABC proteins.

[Proc Amer Assoc Cancer Res, Volume 47, 2006]