A new metabolite of irinotecan in which formation is mediated by human hepatic cytochrome P-450 3A4

Irinotecan (CPT-11) is an anticancer prodrug. It is converted by carboxyles terase to yield an active metabolite, 7-ethyl-10-hydroxy-camptothecin (SN-3 8), which acts as a topoisomerase I inhibitor. Several oxidative metabolite s of CPT-11 have been identified in humans, including 7-ethyi-10-[4-N-(5-am inopentanoic acid)-1-piperidino]carbonyloxycamptothecin (APC) and 7-ethyl-1 0-(4-amino-1-piperidino)carbonyloxycamptothecin (NPC), generated by cytochr ome P-450 3A4 (CYP3A4). Other minor metabolites in which metabolic pathways and biologic activities have not been identified also exist. To further in vestigate the metabolism of CPT-11 in human liver, we analyzed metabolites of CPT-11 in human hepatic microsomes using a high-performance liquid chrom atography/ 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 oxi dation product formed by the loss of two hydrogen atoms from the terminal p iperidine ring. Kinetic analyses indicated that a single enzyme generated t he 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 CY P3A4 expressed in insect microsomes. A significant correlation was observed between the generation of this metabolite and the CYP3A4 content in indivi dual human hepatic microsomes. These findings indicate that this newly dete cted metabolite is a CYP3A4-generated product that may be produced in hepat ic microsomes of patients treated with CPT-11.