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 µm) 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 µm, 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, >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.

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Supported by the National Cancer Institute of Canada and the Alberta Heritage Savings Trust Fund—Applied Research: Cancer. Flow cytometry was supported by a program grant from the Alberta Heritage Savings Trust Fund—Applied Research: Cancer. Portions of this work were presented previously (1).

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