Schedule‐dependent in vitro combination effects of methotrexate and 5‐fluorouracil in human tumor cell lines

Because of conflicting reports of clinical synergy, we used the tetrazolium‐based colorimetric (MTT) assay to test in vitro combination effects of methotrexate (MTX) plus 5‐fluorouracil (FUra) in 4 schedules on 2 human non‐small‐cell lung cancer cell lines (adenocarcinoma, NCI‐H23; bronchioalveolar‐cell carcinoma, NCI‐H358), and I human colorectal adenocarcinoma cell line (SNU‐CI). The complete 3 dimensional set of isoboies in the dose range under study was generated by a microcomputer‐based method. We found that the combination effects of 8‐hr sequential FUra‐MTX, simultaneous administration of MTX‐FUra, and 8‐hr sequential MTX‐FUra were clearly antagonistic for all 3 cell lines. In contrast, the combination cytotoxic effects of 24‐hr sequential MTX‐FUra were much more active. Our in vitro model thus clearly shows that MTX‐FUra interactions are highly schedule‐dependent. This provides a rational basis for testing sequential MTX‐FUra with a longer administration interval than usually employed clinically.