Excretion into gastrointestinal tract of irinotecan lactone and carboxylate forms and their pharmacodynamics in rodents

Purpose. To investigate the excretion of irinotecan hydrochloride (CPT-11) and its active metabolite, SN-38, into the gastrointestinal lumen via the biliary and/or intestinal membrane route after dosing with lactone and carboxylate forms of CPT-II, and to evaluate the toxic and antitumor effects of the two forms. Methods. The excretions of CPT-11 and SN-38 were. investigated by the in situ perfusion technique using rats. The incidence of delayed diarrhea was evaluated after i.v. dosing (60 mg/kg) with CPT-IZ lactone and carboxylate forms for 4 days. Antitumor activity and changes in body weight were investigated in mice with Meth A tumors. Results, The excretion of CPT-11 into bile was greater in dosing with CPT-II carboxylate than that with its lactone form, whereas the exsorption across intestinal membrane was greater in dosing with CPT-11 lactone than that with its carboxylate form. Dosing with CPT-11 lactone dose-dependently inhibited the increase in tumor weights in Meth A tumor mice, whereas the dosing with its carboxylate form reduced the antitumor effect. Conclusions. The decreased antitumor effect caused by dosing with the CPT-II carboxylate form could be due to less accumulation in the tissue including tumor cells resulting from the rapid elimination of the form in the body.