Schedule-dependent synergism and antagonism between paclitaxel and methotrexate in human carcinoma cell lines

Paclitaxel and methotrexate are active against a variety of solid tumors. Because of differences in their mechanisms of action and toxicity profiles, the combination of these two agents has clinical potential. Clinical studies of this combination are in progress. We studied the optimal schedule of paclitaxel and methotrexate in combination at various schedules in vitro using human lung cancer A549, breast cancer MCF7, ovarian cancer PA1, and colon cancer WiDr cells. Cells were simultaneously exposed to paclitaxel and methotrexate for 24 h and sequentially exposed to paclitaxel for 24 h followed by methotrexate for 24 h or vice versa. Cell growth inhibition after 5 days was determined by a 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. The effects of drug combinations at the concentration of drug that produced 80% cell growth inhibition (the IC80 level) were analyzed by the isobologram method. The simultaneous exposure to paclitaxel and methotrexate produced additive to antagonistic effects in the A549 and PA1 cells, and antagonistic effects in the MCF7 and WiDr cells. The sequential exposure to paclitaxel followed by methotrexate produced additive effects in all four cell lines. The reverse sequence produced synergistic effects in the A549, MCF7, and WiDr cells, and additive effects in the PA1 cells. These findings suggest that a sequential administration of methotrexate followed by paclitaxel may be the appropriate schedule for this combination. On the basis of the observed in vitro synergism, further in vivo and clinical studies are necessary to clarify the toxicity and proposed antitumor effects of this schedule.