Biliary excretion of irinotecan and its metabolites.

PURPOSE: The aim of this study was to investigate the excretion of irinotecan hydrochloride (CPT-11) and its metabolites into the gastrointestinal lumen via the biliary route after intravenous administration of lactone and carboxylate forms of CPT-11. METHODS: Biliary excretions of CPT-11 and its metabolites, SN-38 and SN-38-glucuronide, were investigated by an in vivo administration study using rats. The biliary excretion profiles for both the lactone and carboxylate forms of CPT-11 and its metabolites were determined. RESULTS: After the i.v. injection of the lactone form of CPT-11, the cumulative biliary excretion of SN-38-glucuronide was much greater than that of CPT-11 and SN-38, and biliary excretion of SN-38 was less than that of CPT-11. Further, CPT-11 and SN-38 were mainly excreted into bile as carboxylate forms. After the administration of the CPT-11 carboxylate form, biliary excretion of SN-38-glucuronide was significantly smaller than that after the administration of CPT-11 lactone form. On the other hand, biliary excretion of CPT-11 and SN-38 was greater after dosing with the CPT-11 carboxylate form than that after the CPT-11 lactone form. CONCLUSIONS: The results suggest that the rate of conversion of lactone to carboxylate forms of CPT-11 and its metabolites plays a major role in the biliary excretion of these compounds.