Simple SummaryAspirin has been studied for a long time in familial adenomatous polyposis (FAP), but the data on the prevention of colorectal adenoma formation and recurrence remain controversial. We conducted a biomarker-based clinical study in eight FAP patients treated with low-dose Aspirin to clarify the drug’s mechanism of action. Aspirin appropriately acetylated platelet cyclooxygenase (COX)-1 but left persistently high systemic thromboxane(TX)A2and prostaglandin (PG)E2biosynthesis (assessed by measuring their primary urinary metabolites, i.e., 11-dehydro-TXB2and PGEM) associated with the incomplete acetylation of COX-1 in colorectal mucosa and adenomas. Proteomics of colorectal adenomas of FAP patients showed that the vimentin upregulation and hemoglobin subunit beta downregulation distinguished the FAP patients in two groups with high vs. low residual 11-dehydro-TXB2levels, possibly identifying Aspirin nonresponders and responders, respectively. Novel chemotherapeutic strategies in FAP may involve drugs that affect TXA2and PGE2signaling in the colorectum with receptor antagonists, and clinical studies should be performed to verify the drugs’ impact.AbstractBackground: The results of Aspirin prevention of colorectal adenomas in patients with familial adenomatous polyposis (FAP) are controversial. Methods: We conducted a biomarker-based clinical study in eight FAP patients treated with enteric-coated low-dose Aspirin (100 mg daily for three months) to explore whether the drug targets mainly platelet cyclooxygenase (COX)-1 or affects extraplatelet cellular sources expressing COX-isozymes and/or off-target effects in colorectal adenomas. Results: In FAP patients, low-dose Aspirin-acetylated platelet COX-1 at Serine529 (>70%) was associated with an almost complete inhibition of platelet thromboxane (TX) B2generation ex vivo (serum TXB2). However, enhanced residual urinary 11-dehydro-TXB2and urinary PGEM, primary metabolites of TXA2and prostaglandin (PG)E2, respectively, were detected in association with incomplete acetylation of COX-1 in normal colorectal biopsies and adenomas. Proteomics of adenomas showed that Aspirin significantly modulated only eight proteins. The upregulation of vimentin and downregulation of HBB (hemoglobin subunit beta) distinguished two groups with high vs. low residual 11-dehydro-TXB2levels, possibly identifying the nonresponders and responders to Aspirin. Conclusions: Although low-dose Aspirin appropriately inhibited the platelet, persistently high systemic TXA2and PGE2biosynthesis were found, plausibly for a marginal inhibitory effect on prostanoid biosynthesis in the colorectum. Novel chemotherapeutic strategies in FAP can involve blocking the effects of TXA2and PGE2signaling with receptor antagonists.