Cell survival in B16 melanoma after treatment with combinations of cytotoxic agents: lack of potentiation.

The extent of tumour, cell kill, produced by treating B16 melanomas with vincristine, cyclophosphamide, 5-fluorouracil and gamma-rays, alone and in combination, was determined using an in vitro colony assay. Cell kill by vincristine was revealed as a reduction in the yield of cells obtained by trypsinization, and as a decrease in the colony-forming ability of the extracted cells. The reduction in cell yield was interpreted as evidence of rapid cell lysis. Cyclophosphamide and gamma-rays also reduced both cell yield and surviving fraction, but in this case the small decrease in cell yield was due to an increase in cell volume. FU had no effect on cell yield, but surviving fraction was reduced. Tumour weight was also measured, and used in conjunction with cell yield and surviving fraction data to calculate the fraction of surviving cells per tumour following treatment with the agents. In combination studies, single doses of two different cytotoxic agents were given either simultaneously, or up to 24 h apart in either sequence, and assays were performed 24 h after the second drug was given. Combinations of vincristine + cyclophosphamide and 5-fluorouracil + gamma-rays were chosen because they had been shown by other workers to exhibit marked schedule dependency, including considerabl potentiation, against leukaemic cell lines. However, in the B16 melanoma there was no evidence of schedule-dependent cell killing with either of these combinations. For all sequences studied, the fraction of surviving cells per tumour was slightly greater than the predicted additive response calculated from single-drug controls.