Synergistic effects of docetaxel and S‐1 by modulating the expression of metabolic enzymes of 5‐fluorouracil in human gastric cancer cell lines

We have recently demonstrated in a Phase I/II study that combination chemotherapy with docetaxel (TXT) and S-1 is active against metastatic gastric carcinomas. To elucidate the mechanisms underlying the synergistic effects of these drugs, both the growth inhibitory effects and the expression profiles of enzymes involved in fluorouracil (5-FU) metabolism were examined in vitro and in vivo. TXT alone and in combination with 5-FU inhibited the growth of each of the 5 gastric cancer cell lines that we examined (TMK-1, and MKN-1, -28, -45 and -74), in a time- and dose-dependent manner. Moreover, striking synergistic effects were observed in TMK-1 cells in vitro with IC50 values of between 4.73 and 0.61 nM 5-FU. Furthermore, in TMK-1 xenografts, 5-FU/TXT cotreatments exhibited synergistic antitumor effects. The combination of S-1 and TXT, however, exhibited greater growth-inhibitory effects than the 5-FU/TXT cotreatments. The mechanisms underlying these synergistic effects of S-1 and TXT were examined by expression and activity analyses of the 5-FU metabolic enzymes. The expression of thymidylate synthase (TS), and dihydropyrimidine dehydrogenase (DPD) were decreased 50 and 73% of control levels, respectively, and that of orotate phosphorybosyl transferase (OPRT) was increased by 3.9-fold at the protein level. These findings suggested that biochemical modulation of the 2 drugs had occurred, which was further confirmed by the results of the activity assays. These data strongly indicate that a combination chemotherapy of TXT and S-1 is effective against gastric carcinomas and is therefore a good candidate as a standard chemotherapeutic strategy in treating these tumors. © 2006 Wiley-Liss, Inc.