Potentiation of doxorubicin cytotoxicity by the calcium antagonist bepridil in anthracycline-resistant and -sensitive cell lines. A comparison with verapamil.

The ability of the calcium channel blocker bepridil (Bp) to potentiate doxorubicin (Dx) cytotoxicity and enhance its accumulation in anthracycline-sensitive and -resistant human ovarian carcinoma cells (A2780 and 2780AD) and Chinese hamster ovarian (CHO) cells (AUXB1 and CHRC5) was compared with that of verapamil (Vp). A continuous exposure (48-72 h) to Bp as well as Vp potentiated Dx cytotoxicity in 2780AD cells. In short-term incubations (2 h Dx and 4 h calcium channel blocker) the same effects were observed: 4 microM Bp (4Bp) and Vp (4Vp) were equipotent, but at concentrations of 1 and 2 microM, Vp was more active (4Vp = 4Bp greater than 2Vp greater than 2Bp greater than 1Vp greater than 1Bp). In CHRC5 cells the corresponding sequence was: 4Vp greater than 4Bp greater than 2Vp greater than 2Bp greater than 1Vp greater than 1Bp. At high (marginally inhibitory) concentrations, Bp and Vp reversed Dx resistance completely in CHRC5 cells and partly in 2780AD cells. No significant potentiation of Dx cytotoxicity by Bp or Vp was found in A2780 cells, but both were active in AUXB1 cells. In studies with radiolabelled Dx (2 microM), Bp and Vp (16.5 microM) stimulated accumulation in CHRC5 cells almost up to the level in AUXB1 cells. Dx accumulation in 2780AD cells (52 pmol/10(6) cells) could be stimulated to a maximum of about 90 pmol/10(6) cells (drug level in A2780 cells was 153 pmol/10(6) cells). Also, Bp- and Vp-induced stimulation of Dx accumulation was observed in AUXB1, but not in A2780 cells. Bp effected a dose-dependent inhibition of Dx efflux from preloaded 2780AD cells, but not from A2780 cells. We conclude that Bp is more effective than Vp in reversing resistance when both compounds are used in vitro at concentrations which are clinically achievable in plasma.