Selective Cyclooxygenase-2 Inhibitors and Familial Adenomatous Polyposis (FAP): Investigation of Colonic COX-2 as a Target for Chemoprevention in Pirc Rats

Objective: The objective of this project is to evaluate the chemoprevention efficacy of locally bioavailable and selective COX-2 inhibitor 6a1 by using Pirc rat model with DSS-accelerated polyps multiutility, and then to evaluate if epithelial COX-2 is a good target for preventing polyps development and growth in Pirc rats. Methods：The individual turnover number of COX-1 and COX-2 proteins were derived by PGE2 production and absolute quantification of protein amounts using newly developed LC-MS method. Taken together of the local protein amounts and turnover numbers of COX-1 and COX-2, the individual contribution of COX-1 and COX-2 to colon PGE2 production was assessed and compared. Absolute amount of COX-1 and COX-2 proteins and PGE2 levels were determined in Pirc rats at different ages. Pirc rats were treated with 3% DSS to induce COX-2 expression and accelerate polyps multiplicity. Celecoxib and 6a1 were given orally, and the local PGE2 level and COXs levels were determined by LC-MS methods. The COX/PGE2 inhibitory effects, polyps suppression efficacy of locally bioavailable and selective inhibitor 6a1 were calculated and evaluated in DSS/Pirc rat model. Results：An absolute quantification LC-MS method for COX-1 and COX-2 proteins were successfully developed and fully validated. In Pirc rats, colon epithelial COX-1 protein levels were 5- to 15-fold higher than COX-2 protein levels at all tested ages, and polyps protein levels of COX-1 and COX-2 were 1-fold and 3-fold higher than epithelium levels, respectively. Unexpectedly, both COX-2 protein and PGE2 levels were not increased with polyps development. 3% DSS treatment resulted in a 59-fold increment of epithelial COX-2 but no change in epithelial COX-1 protein levels. The contribution of elevated epithelial COX-2 protein caused a 10-fold increase of epithelium PGE2 levels, and then accelerated the polyps multiplicity up to 10-fold in Pirc rat model. Orally administration of a locally bioavailable and selective COX-2 inhibitor 6a1 can further diminish the COX-2 expression but failed to suppress epithelium PGE2 level in DSS-induced Pirc rats, and as a result failed to suppress polyps development and growth. Orally administration of a systemic and selective COX-2 inhibitor celecoxib at pharmacological doses (5, 20 and 70 mg/kg) to DSS-induced Pirc rats were too toxic to be tolerated by the Pirc rats. Conclusion：In Pirc rat model, epithelial COX-1 protein levels were always higher than epithelial COX-2 levels at different ages. They both remained stable with age or when polyps burden increased, suggesting high epithelial COX-2 level may not be required for polyps development in Pirc rats. Inhibition of COX-2 activities by a locally bioavailable COX-2 inhibitor 6a1 did not reduce PGE2 level nor polyps growth in DSS-induced or non-DSS treated Pirc rat models, suggesting the chemopreventive efficacy of 6a1 was not significant though it could effectively suppress COX-2 expression level. These data demonstrated that suppression of epithelial COX-2 alone is not enough for the prevention of polyps growth in DSS-induced or non-DSS treated Pirc rat models, invalidating colon epithelial COX-2 as a target for polyps chemoprevention.