Synergistic interaction of cyclosporin A and verapamil on vincristine and daunorubicin resistance in multidrug-resistant human leukemia cells in vitro.

We studied the effects of cyclosporin A and verapamil on the modulation of vincristine and daunorubicin resistance in a multidrug-resistant subline of human T-cell acute lymphatic leukemia GM3639. Our results show that cyclosporin A is more effective than verapamil as a modulator of the high degree of primary vincristine resistance and the low degree of daunorubicin cross-resistance expressed by this cell line. Isobologram analysis revealed that the combined modulators act synergistically in correcting both vincristine and daunorubicin resistance.