Abstract
Treatment of murine L1210 cells with methotrexate (MTX) followed by 5-fluorouracil (FUra) produced synergistic cytotoxicity, but only in media containing serum with low concentrations of hypoxanthine, such as horse serum and dialyzed fetal calf serum. Addition of hypoxanthine (1 to 10 µm) during drug exposure reduced the synergism of sequential MTX (1 to 100 µm)-FUra (30 to 300 µm) treatment. The reduction of synergy by hypoxanthine varied with the MTX concentration, higher hypoxanthine concentrations being required to prevent synergy at higher MTX concentrations. The cytotoxicity produced by sequential MTX (10 µm)-FUra (30 to 300 µm) treatment was also reduced if thymidine was added to the regrowth media following drug exposure. The rescue by thymidine was concentration dependent, but as little as 0.5 µm thymidine was sufficient to substantially reduce the synergistic cytotoxicity. These results indicate that both hypoxanthine and thymidine are critical determinants of sequential MTX-FUra synergy and call into question the relevance of experiments in low-thymidine- and low-hypoxanthine-containing media to the clinical situation, where plasma hypoxanthine and thymidine concentrations are often greater than 1 and 0.5 µm, respectively.
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