Purpose: We investigated whether dipyridamole and various calcium channel blockers are inhibitors and/or substrates of breast cancer resistance protein (BCRP).
Methods: The effect of dipyridamole and the calcium channel blockers on mitoxantrone efflux by BCRP-overexpressing human embryonic kidney (HEK) cells was determined by flow cytometry. The ability of some of these compounds to reverse BCRP-mediated mitoxantrone resistance was measured by cytotoxicity assays. Transport studies were performed using radiolabeled compounds.
Results: Dipyridamole, nicardipine, nitrendipine, and nimodipine effectively inhibited BCRP-mediated mitoxantrone efflux; however, bepridil, diltiazem, and verapamil had no significant effect. Nifedipine is a much weaker BCRP inhibitor compared with other dihydropyridines tested. Nicardipine and dipyridamole were the most potent BCRP inhibitors among the compounds tested with IC50 values of 4.8 +/- 1.3 and 6.4 +/- 0.9 microM, respectively. Nicardipine and dipyridamole also effectively reversed BCRP-mediated mitoxantrone resistance in HEK cells. [3H]Nitrendipine was found not to be transported by BCRP. However, the transport of [3H]dipyridamole by BCRP was observed in both HEK and Madin-Darby canine kidney cells stably expressing the transporter, and this transport was completely abolished by fumitremorgin C, a known BCRP inhibitor.
Conclusions: Dipyridamole and several dihydropyridines are effective BCRP inhibitors, but bepridil, diltiazem, and verapamil are not. We also identified a new BCRP substrate, dipyridamole.