Mitomycin C (MMC) is regarded as the prototype bioreductive alkylating agent in clinical use. To elucidate the biochemical basis of MMC resistance, we isolated a drug resistant derivative (designated CHO-MMCr) of a Chinese hamster ovary cell line (CHO-K1) by exposure to progressively higher concentrations of MMC. CHO-MMCr cells exhibited a 17-fold increase in resistance to MMC and were 33-fold cross-resistant to the monofunctional derivative, decarbamoyl mitomycin C. In contrast, CHO-MMCr cells showed only a 2-fold level of resistance to BMY 25282, a more easily activated analogue of MMC, and exhibited parental sensitivity to MMC under radiobiologically hypoxic conditions. CHO-MMCr cells showed no increased resistance to a range of DNA damaging agents including several other alkylating agents (e.g., melphalan and methyl methanesulfonate). Cross-resistance to drugs associated with the multidrug resistant phenotype (e.g., Adriamycin and vincristine) was present only at very low levels.

Using a specific high performance liquid chromatography technique, we examined the rates of reduction of MMC and BMY 25282 in cell extracts from CHO-K1 and CHO-MMCr cells under both aerobic (air) and hypoxic (N2) conditions. Reduction rates for both drugs were at least 30-fold faster under nitrogen than in air. Metabolism of MMC was undetectable in air but was readily detectable under nitrogen and was 2–3-fold slower in CHO-MMCr cell extracts than in CHO-K1 cell extracts. Although BMY 25282 was more readily reduced under nitrogen, no difference was detected between extracts from CHO-K1 or CHO-MMCr cells in the rate of reduction of BMY 25282 under either air or nitrogen. The activity of NADPH:cytochrome P-450 (cytochrome c) reductase, an enzyme implicated in the bioreductive activation of MMC, was 3–4-fold lower in CHO-MMCr cells than in the parental line. These findings suggest that the resistance of CHO-MMCr cells to MMC under aerobic conditions may be due to impaired metabolic activation of the drug as a result of a decrease in NADPH:cytochrome P-450 reductase activity. This supports the view that decreased bioreductive enzyme activity may be a significant mechanism for acquired resistance to MMC in tumor cells in vivo and that more readily activated analogues may be potentially useful in overcoming this specific form of resistance.


Work supported by the North of England Cancer Research Campaign and the Medical Research Council.

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