Schedule-dependent synergism and antagonism between raltitrexed ("Tomudex") and methotrexate in human colon cancer cell lines in vitro

The folate-dependent enzymes are attractive targets for cancer chemotherapy , Methotrexate (MTX), which inhibits dihydrofolate reductase, has been wide ly used for the treatment of solid tumors and hematological cancers, Raltit rexed (" Tomudex"), which inhibits thymidylate synthase, is a novel antican cer agent active against colorectal cancer and some other solid tumors. We studied the optimal schedule of raltitrexed and MTX in combination against four human colon cancer cell lines Colo201, Colo320, LoVo, and WiDr, These cells were simultaneously exposed to raltitrexed and MTX for 24 h, or seque ntially exposed to raltitrexed for 24 h followed by MTX for 24 h, or vice v ersa. Cell growth inhibition after 5 days was determined by using 3-(4,5-di methylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay, The effect s of drug combinations at the concentrations of drug that produced 80% and 50% cell growth inhibition (IC80 and IC50) were analyzed by the isobologram method (Steel and Peckham, 1979), Cytotoxic interactions between raltitrex ed and MTX were schedule-dependent, The simultaneous exposure to raltitrexe d and MTX showed additive effects in Colo201, LoVo and WiDr cells and antag onistic effects in Colo320 cells, The sequential exposure to raltitrexed fo llowed by MTX produced additive effects in all four cell lines. The sequent ial exposure to MTX followed by raltitrexed produced synergistic effects in Colo201, LoVo and WiDr cells and additive effects in Colo320 cells. These findings suggest that the sequential administration of MTX followed by ralt itrexed produces more than the expected cytotoxicity and may be the optimal schedule at the cellular level, Further in vivo and clinical studies will be necessary to determine the toxicity and to test the antitumor effects of sequential administration of MTX followed by raltitrexed proposed on the b asis of the in vitro synergism.