We have evaluated the antitumor activity of XELOX (a combination therapy of capecitabine (Xeloda®) and oxaliplatin) in human colorectal and gastric cancer xenograft models. In human colorectal cancer model CXF280, antitumor activity of the combination at two-thirds of the maximum tolerated dose (MTD) was superior to that of each monotherapy at MTD. Furthermore, in human colorectal cancer model COL-05-JCK and human gastric cancer xenograft model GXF 97, the combination also showed at least additive antitumor activity. In addition, toxicity was not augmented with the combination therapy in these three models. As demonstrated using ELISA or immunohistochemistry, oxaliplatin in xenograft model tumors up-regulated the level of thymidine phosphorylase (dThdPase), a key enzyme for the metabolism of capecitabine to 5-fluorouracil. These results suggest that oxaliplatin might potentiate the antitumor activity of capecitabine by up-regulating the tumor level of dThdPase. Based on these results, clinical trials of XELOX against colorectal and gastric cancers are warranted.

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