Irinotecan Hydrochloride Administration Considering Dosing-Time Attenuates Delayed Diarrhea in Rats

Background: Irinotecan hydrochloride (CPT-11) is an important anticancer drug used in a wide range of regimens to treat colorectal and gastric cancers, and one of its severe side effects is delayed diarrhea. Therefore, based on known circadian variations in intestinal function and drug metabolism, we investigated whether CPT-11-induced delayed diarrhea may be attenuated by the time of dosing. Methods: When CPT-11 was administered to rats at 9:00 or 21:00, CPT-11-induced delayed diarrhea was assessed, and concentrations of CPT-11, its active metabolite SN-38, and SN-38 glucuronide (SN-38GL) in blood, intestinal tissues, and intestinal contents were measured. Results: The severity of diarrhea was significantly less in the 21:00 dosing group compared with the 9:00 dosing group. Blood SN-38 concentrations 8 h after the administration of CPT-11 were significantly higher in the 9:00 dosing group than in the 21:00 dosing group. SN-38, which exerts potent cytotoxic effects, circulates enterohepatically. When SN-38 is absorbed from the intestinal mucosa, intestinal tissues may be injured, resulting in delayed diarrhea. CPT-11 and SN-38 concentrations in intestinal tissues and contents 8 h after the administration of CPT-11 were significantly higher in the 9:00 dosing group than in the 21:00 dosing group at all measurement points. This was consistent with more severe CPT-11-induced delayed diarrhea in the 9:00 dosing group. Conclusions: Chronotherapy with CPT-11 may reduce CPT-11-induced delayed diarrhea. These differences in SN-38 concentrations in the intestinal tract at different dosing times may contribute to the time-dependent reduction in CPT-11-induced delayed diarrhea.