Potentiation of vincristine by vitamin A against drug-resistant mouse leukaemia cells.

Vitamin A has been shown to potentiate the cytotoxic action of anticancer agents like vincristine (VCR) against drug resistant mouse P388 leukaemia cells. In vitro tests showed enhancement by retinyl acetate of cytocidal activities of VCR against drug-sensitive leukaemia (P388/S) and VCR-resistant leukaemia (P388/VCR) cells in culture; retinyl acetate rather specifically potentiated VCR against cultured P388/VCR cells than P388/S cells. The cellular accumulation of radioactive VCR was significantly enhanced in cultured P388/VCR cells when retinyl acetate was present. The efflux of VCR from drug-resistant cells was blocked by retinyl acetate. The effect of the combination of vitamin A and VCR was also tested in vivo on the life-span of mice bearing P388/S or P388/VCR. Intraperitoneal administration of retinyl palmitate at 41.75 or 83.5 mg kg-1 was effective to potentiate the antileukaemic activity of VCR against P388/S bearing mice, and it also overcame vincristine-resistance in P388/VCR bearing mice.