Kinetic analysis of active efflux of vincristine from multidrug-resistant P388 leukemia cells.

Kinetic analysis of vincristine transport in parental and multidrug-resistant P388 leukemia cells was attempted by indirect assessment of its efflux. Practically, the initial velocity and steady-state level of vincristine uptake by ATP-depleted cells, and its steady-state level in untreated cells, were measured. As a result, a saturable process of not only influx but also efflux of vincristine was observed for the first time with both cell lines, suggesting the existence of a carrier-mediated system for influx and efflux. With increasing extracellular drug concentrations, the contribution of the mediated transport to the total flux was decreased and that of the unsaturable process, that is, simple diffusion, was increased. It should be particularly noted that the Km and Vmax values of efflux in the resistant cells were significantly less and greater, respectively, than those of the sensitive cells, providing a biochemical basis for enhanced efflux as a mechanism of multidrug-resistance. No significant difference in kinetic parameters of vincristine influx and intracellular binding contributing to resistance was found between the two cell lines.