| A significant number of cancer patients treated with the topoisomerase inhibitor, irinotecan, experience severe, delayed-onset diarrhea. Clinicians will typically reduce subsequent doses of irinotecan in affected patients, which may result in diminished effectiveness of the anti-cancer agent. The active metabolite of irinotecan, 7-ethyl-10-hydroxy-camptothecin (SN-38), has been implicated as the colon-toxic agent, by causing direct injury to colonic epithelial cells. SN-38 is consequently detoxified to SN-38 glucuronide (SN-38G) by UDP-glucuronosyltransferase (UGT) isozymes expressed not only in the liver, but also in the gastrointestinal tract. Although luminal SN-38 has been identified as an important mediator of diarrhea, the influence of intestinal UGTs at the site of toxicity has not been addressed. Thus, intestinal SN-38 glucuronidation was studied to address the hypothesis that UGT activity in the intestine is a critical factor in modulating SN-38 toxicity, as enteric cells have the ability to prevent SN-38 toxicity locally through UGT expression.; The primary human and rat isoforms that glucuronidate SN-38 were identified, many of which are localized in the intestine. SN-38 carboxylate was found to be a suitable, more soluble surrogate for the active lactone form for in vitro glucuronidation, as the rank order of UGT isoform catalysis was similar for the two forms, despite different rates of conjugation. Intersubject SN-38 glucuronidation in human intestinal tissue from duodenal, jejunal, and colonic regions was variable, and was highly correlated with both UGT1A1 and 1A9 probe substrate glucuronidation, indicating a primary role for these isoforms in intestinal SN-38G formation. The hypothesis that intestinal UGTs modulate SN-38-induced diarrhea was directly tested in a Gunn rat model with reconstituted hepatic UGT activity via adenoviral gene delivery. These rats, devoid of intestinal UGT1A activity, were highly susceptible to intestinal damage and diarrhea after irinotecan administration, compared to control heterozygote rats, which possessed both hepatic and intestinal UGTs. Collectively, this research advanced the knowledge of the metabolic detoxification of SN-38 in the intestine by establishment and identification of appropriate in vitro reaction conditions, isoform specificity and expression, and variability in glucuronidation. These studies have also supported the hypothesis that intestinal UGT expression modulates the incidence of gastrointestinal toxicity to SN-38. |
