Markedly altered membrane transport and intracellular binding of vincristine in multidrug‐resistant Chinese hamster cells selected for resistance to vinca alkaloids

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Abstract
Studies of a multidrug‐resistant variant (DC‐3F/VCRd‐5L) of Chinese hamster lung cells selected for resistance to vinca alkaloids revealed marked alterations in transport and intracellular binding of [3H]vincristine compared to parental DC‐3F cells. Influx of [3H]vincristine in DC‐3F cells appears to be an equilibrating, but mediated, process. Although saturation kinetics for [3H]vincristine influx were not demonstrated, an extremely high temperature‐dependence (Q1027–37°C = 5–6) and trans‐inhibition of influx following preloading of cells with nonradioactive vincristine argue in favor of a carrier‐mediated process. Efflux of [3H]vincristine from parental cells conformed to first‐order kinetics (t1/2 37° = 3.6 ± 0.4) and exhibited a lower temperature‐dependence (Q10 27–37°C = 3–3.5) than influx. In variant vs. parental cells, influx of [3H]vincristine was reduced 24‐fold and efflux was increased twofold, accounting for the large (approximately 48‐fold) reduction in steady‐state level of exchangeable drug accumulating in variant cells. Otherwise, transport of [3H] vincristine in these cells showed characteristics similar to parental DC 3F cells. Also, the rate and amount of intracellular binding of [3H]vincristine in variant cells was almost 40‐fold lower than in parental cells. These alterations in influx and efflux of [3H]vincristine and its intracellular binding appear to account, at least to a major extent, for the high level of resistance (2,750‐fold) of this variant to vinca alkaloids. In contrast, cross‐resistance of this variant to daunomycin (178‐fold) could be explained only minimally by a transport alteration. Only a two‐fold increase in efflux of [3H]daunomycin was demonstrated in variant vs. parental cells along with some decrease in intra‐cellular binding. Influx of [3 H]daunomycin was unaltered. In view of these results, we conclude that these two agents most likely do not share the same route for entry in these cells but might share the same efflux route.