Purpose: To determine the most effective combination chemotherapy with S-1 against pancreatic cancer and to clarify the mechanism of synergy between S-1 and the partner drug.
Methods: We tested a combination of S-1 with the following antitumor drugs in an in vitro MTT assay against pancreatic cancer cell line MIA PaCa-2: gemcitabine (GEM), cisplatin (CDDP), irinotecan (CPT-11), mitomycin C, adriamycin, and paclitaxel. The efficacy of S-1, GEM, and a combination of S-1 and GEM was also tested in vivo by administering S-1 (10 mg/kg) orally to nude mice five times a week for 3 weeks, and GEM (100 mg/kg) intravenously every 2-3 days for a total of six times. A treated-to-control ratio (T/C) of relative mean tumor weight values less than 50% was determined to be effective. Furthermore, we investigated the mechanism of the synergistic effect of S-1 and GEM on the cell cycle by flow cytometry, because both S-1 and GEM are known as antimetabolic drugs. To verify cell death induced by a change in the distribution of the cell cycle phases, we investigated apoptosis by sub-G1 analysis and a TUNEL assay.
Results: From classical isobolography analysis of the in vitro MTT assay, the combination of S-1 plus GEM was found to be the most effective of the combinations tested. In vivo, T/C (percentage) with the combination of S-1 plus GEM was 48.2%, which was lower than that of S-1 or GEM alone, and the combination enhanced antitumor activity. Cell cycle analysis showed greater cell cycle delay with the combination treatment (S-1 plus GEM) than for each single drug treatment, and apoptotic cells were detected only in treatments including GEM.
Conclusion: The combination chemotherapy of S-1 and GEM appears to be useful for pancreatic cancer. Both cycle delay by S-1 plus GEM and apoptosis induced by GEM are involved in this synergistic mechanism.