Sequence-dependent synergism between dichloromethotrexate and cisplatin in a human colon carcinoma cell line

We studied the cell killing effects of cisplatin (CDDP), 3',5'-dichloromethotrexate (DCM), and different combinations of these drugs in a human colon carcinoma line (HCT-8). Using a clonogenic assay, the ED50 values for DCM were 3, 0.3, and 0.05 microM after 4-, 24-, and 48-hour exposure, respectively, while cell kill induced by CDDP was less time-dependent (ED50 values: 16, 7.8, 3.3, and 2.8 microM after 2-hour, 4-hour, 24-hour, and continuous incubation, respectively). Sequential exposure of these cells to DCM followed by CDDP resulted in synergism at every dose studied. The synergy was maximal with short DCM pretreatment intervals and decreased with increasing lengths of exposure to the antifol. In contrast, simple additive effects were observed when DCM was given together with or following CDDP. Considering the significant hepatic inactivation of both these compounds, our in vitro data encourage the design of clinical protocols with the sequence DCM----CDDP for hepatic arterial infusion treatment of liver metastasis from colon cancer.