Saturable process involved in active efflux of vincristine as a mechanism of multidrug resistance in P388 leukemia cells.

Kinetic analysis of vincristine (VCR) efflux in multidrug-resistant and parental P388 leukemia cells was performed to investigate the difference in activity between the two cell lines. Efflux velocities of VCR were directly determined from the slope of the initial release of drug induced by resuspending the preloaded cells in VCR-free medium, representing unidirectional efflux from intracellular free or loosely bound drug pools. Further, the equilibrium binding of VCR to whole-cell homogenates was analyzed by ultrafiltration to estimate intracellular unbound drug concentrations. A two-site binding model was found to fit the data best for both cell lines, and depletion of ATP by the addition of apyrase decreased binding. The binding parameters were similar between the two cell lines. A Hofstee plot of efflux demonstrated the existence of both linear and saturable transport of VCR in both cell lines. The greater maximum velocity observed with VCR efflux in the resistant cells suggests that an increased number of transporters causes greater activity of this process in the resistant cells.