Reversal by Two Dihydropyridine Compounds of Resistance to Multiple Anticancer Agents in Mouse P388 Leukemia in vivo and in vitro

We investigated whether two representative 1,4‐dihydropyridine derivatives, NK‐250 and NK‐252, could potentiate the antitumor activity of multiple anticancer agents including vincristine (VCR), vinblastine, vindesine and actinomycin D in drug‐resistant tumor cells and their parental drug‐sensitive tumor cells. NK‐250 and NK‐252 at 5–10 μM almost completely reversed VCR resistance in cultured VCR‐resistant P388/VCR cells derived from the mouse drug‐sensitive P388/S leukemia cell line and also potentiated the cytocidal activity of VCR in drug‐sensitive P388/S cells. NK‐250 and NK‐252 at 1–10 μM inhibited the photoaffinity labeling by [(3)H]azidopine of the cell‐surface 170,000‐molecular‐weight P‐glycoprotein. In chemotherapeutic experiments with leukemia‐bearing mice, NK‐250 or NK‐252 was orally administered in combination with different drugs of the MDR phenotype administered intraperitoneally. The antitumor activity of the various combinations was found to be augmented in mice bearing P388/S‐ and P388/VCR‐leukemia. Among the combinations examined, the combination of NK‐250 and VCR was the most effective. These two 1,4‐dihydropyridines, NK‐250 and NK‐252, are unique compounds because they were effective not only in circumventing the drug resistance, but also in potentiating the action of antitumor drugs against drug‐sensitive tumors.