Differential cellular retention of vincristine and vinblastine by cultured human promyelocytic leukemia HL-60/Cl cells: the basis of differential toxicity.

Differential toxicity of vincristine and vinblastine against cells of a cloned subline of human promyelocytic leukemia (HL-60/Cl) was dependent on exposure conditions. During continuous exposures of 48 h, vincristine and vinblastine were equitoxic with drug concentrations that inhibited proliferation rates by 50% of 7.6 and 8.1 nM, respectively. When cells were subjected to 4-h exposures and transferred to drug-free medium, the drug concentration of vinblastine that inhibited proliferation rates by 50% (1.1 microM) was significantly greater than that of vincristine (41 nM). Analysis by flow cytometry of the effects of equitoxic drug exposures on cell-cycle progression suggested that vincristine and vinblastine acted by the same mechanism (G2-M phase inhibition). [3H]Vincristine and [3H]vinblastine were bound to serum proteins in growth medium to the same extent (25%) over a wide range of concentrations, and the amounts of "free" extracellular drug did not decrease during prolonged exposures. Analysis by high-performance liquid chromatography of extracts of cultures incubated with growth-inhibitory concentrations of [3H]vincristine or [3H]vinblastine indicated little, if any, metabolism of either drug by cells or culture fluids; after 24 h, 85-95% of radioactivity was recovered from cells or growth medium as unchanged vincristine or vinblastine. At concentrations from 6 nM to 6 microM, vinblastine entered cells rapidly, reaching maximum levels within 0.5-2 h, and the relationship between maximal cell-associated drug and extracellular free vinblastine was linear. Although uptake of vincristine was slower than that of vinblastine, the cellular content of vincristine reached that of vinblastine during prolonged (12-24 h) exposures, and the amounts of cell-associated drug, relative to extracellular drug concentrations, indicated considerable "concentrative" accumulation (intra: extracellular ratios, greater than 100). When drug exposures were ended by transfer of cells to drug-free medium, vinblastine was released from cells more rapidly and to a greater extent than vincristine, independent of whether exposures were 4 or 24 h. Rates of uptake and release of vinblastine (50 nM) were unaffected by depletion of cellular adenosine triphosphate, suggesting that rapid release was not mediated by an energy-dependent efflux system.