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Previous studies have shown that prostaglandin E2 (PGE2) is involved in intestinal carcinogenesis through its binding to the PGE2 receptor subtypes EP1 and EP4 and activation of downstream pathways. ONO-8711 and ONO-AE2–227, prostaglandin E receptor subtype EP1- and EP4-selective antagonists, respectively, are known to suppress formation of intestinal polyps in adenomatous polyposis coli gene-deficient mice. The present study was designed to investigate the combined effects of EP1 and EP4 antagonists on spontaneous polyp formation in APC1309 mice in order to determine the contribution of each receptor to intestinal tumorigenesis. APC1309 mice were treated with 400 ppm of ONO-8711 alone, 400 ppm of ONO-AE2–227 alone or both in combination in the diet for 6 weeks. The mean area of polyps found in the intestine, calculated as the longer diameter × the shorter diameter ×π, was reduced by 12%, 43% (P<0.01) and 56% (P<0.01) of the mean control value (8.8 mm2) in the ONO-8711 alone, ONO-AE2–227 alone and combination treatment groups, respectively, suggesting clear additive effects of the combination. The same additive tendency for suppression was also observed with respect to the numbers of polyps in the intestine. Polyp size reduction was more remarkable with the EP4 antagonist, while the number reduction was more pronounced with the EP1 antagonist. Our results indicate that EP1 and EP4 may have separate intrinsic roles and, to some extent, contribute to polyp formation independently. Thus, combination treatment has potential for the chemoprevention of colon carcinogenesis.