Avermectins inhibit multidrug resistance of tumor cells.

The modification of the sensitivity of Hep-2 and P388 tumor cells to taxol and vincristine, substrates of multidrug resistance proteins, by naturally occurring avermectins and the effect of avermectins on the accumulation of calcein in cells and the efflux of rhodamine 123 were studied. While avermectins did not affect the sensitivity of tumor cells to hydrogen peroxide and cisplatin, they significantly enhanced the sensitivity of cells of both wild-type and resistant strains to taxol and vincristine. The coefficients of modification for resistant strains were substantially higher.