Irinotecan (CPT-11) is an anticancer prodrug. It is converted by carboxylesterase to yield an active metabolite, 7-ethyl-10-hydroxycamptothecin (SN-38), which acts as a topoisomerase I inhibitor. Several oxidative metabolites of CPT-11 have been identified in humans, including 7-ethyl-10-[4-N-(5-aminopentanoic acid)-1-piperidino]carbonyloxycamptothecin (APC) and 7-ethyl-10-(4-amino-1-piperidino)carbonyloxycamptothecin (NPC), generated by cytochrome P-450 3A4 (CYP3A4). Other minor metabolites in which metabolic pathways and biologic activities have not been identified also exist. To further investigate the metabolism of CPT-11 in human liver, we analyzed metabolites of CPT-11 in human hepatic microsomes using a high-performance liquid chromatography/mass spectrometry (HPLC/MS) system and detected a new metabolite that was the major one produced in the microsomal system. HPLC-tandem mass spectrometry (HPLC/MS/MS) analysis indicated that this compound was an oxidation product formed by the loss of two hydrogen atoms from the terminal piperidine ring. Kinetic analyses indicated that a single enzyme generated the metabolite, and we have identified this enzyme in two in vitro systems. The formation of the new metabolite was significantly inhibited by SKF525A, ketoconazole, and an anti-CYP3A4 antibody and catalyzed specifically by CYP3A4 expressed in insect microsomes. A significant correlation was observed between the generation of this metabolite and the CYP3A4 content in individual human hepatic microsomes. These findings indicate that this newly detected metabolite is a CYP3A4-generated product that may be produced in hepatic microsomes of patients treated with CPT-11.