Alleviation of side effects induced by irinotecan hydrochloride (CPT-11) in rats by intravenous infusion

Irinotecan hydrochloride (CPT-11) is a potent topoisomerase I inhibitor and is established and used widely as an antitumor agent. However, it sometimes causes severe side effects such as myelosuppression and diarrhea. These dose-limiting toxicities prevent the adoption of CPT-11 in aggressive chemotherapy. Thus we sought to determine in a rat model whether extending the period of infusion of CPT-11 would ameliorate the adverse reactions. CPT-11 was administered intravenously (i.v.) to rats at a dose of 60 mg/kg per day for four consecutive days as a bolus injection or as 3-, 8- or 24-h infusions, and then blood cell counts and the incidence of acute and delayed-onset diarrhea were monitored. Serious acute and delayed-onset diarrhea and marked decreases in the number of neutrophils and lymphocytes were observed in the bolus injection group. These symptoms were alleviated in the infusion groups with the degree of alleviation dependent on infusion time. In the bolus injection group, mucosal impairment of the cecal epithelium including wall thickening, edema, a decrease in the number and size of crypts, and the formation of a pseudomembrane-like substance was observed, whereas these changes were less severe in the infusion groups. Areas under the plasma concentration-time curves (AUCpla) of CPT-11 and its metabolite, 7-ethyl-10-hydroxycamptothecin (SN-38), differed little among the bolus injection group, and the 3-h and 8-h infusion groups. However, the AUCpla values of CPT-11 and SN-38 were significantly decreased and increased, respectively, in the 24-h infusion group. The maximum plasma concentrations (Cmax) of CPT-11 decreased with increasing infusion time, but those of SN-38 did not. It was confirmed that the side effects of CPT-11 were alleviated by extending the infusion time. The pharmacokinetic parameters suggested that the Cmax of CPT-11 is closely related to the incidence and severity of adverse reactions such as myelosuppression and acute and delayed-onset diarrhea.