BIOSYNTHESIS OF AN AMINOPIPERIDINO METABOLITE OF IRINOTECAN PIPERIDINO)-1-PIPERIDINO]CARBONYLOXYCAMPTOTHECINE] BY HUMAN HEPATIC MICROSOMES

Irinotecan piperidino)-1-piperidino]carbonyloxycamptothecine] is a wat er-soluble analogue of camptothecine used in the second-line treatment of advanced colon cancer. Recently, we identified, in the plasma of p atients and in human liver microsomal incubations, the presence of a n ew metabolite of irinotecan, -10-(4-amino-1-piperidino)carbonyloxycamp tothecine (NPC), which is produced by cleavage of the distal piperidin e ring of irinotecan. The kinetics of biotransformation of the lactone and carboxylate forms of irinotecan into NPC were studied using human liver microsomes. The formation of NPC was characterized by the follo wing parameters: K-M = 48.2 +/- 6.8 and 273 +/- 122 mu M and V-max = 7 4.1 +/- 4.9 and 78.6 +/- 27.7 pmol/min/mg of protein for the lactone a nd carboxylate forms of irinotecan, respectively. Interestingly, there was no formation of NPC from 7-ethyl-10-[4-N-(5-aminopentanoic acid)- 1-piperidino]carbonyloxycamptothecine, a major metabolite of irinoteca n that has an open distal piperidine ring and could be considered a po ssible metabolic precursor of NPC. The transformation of irinotecan in to NPC was found to be catalyzed principally by cytochrome P450 (CYP) 3A, based on three key results, as follows: 1) the CYP3A-selective inh ibitors ketoconazole (1 mu M) and troleandomycin (100 mu M) inhibited NPC formation by 99 and 100%, respectively; 2) of a series of microsom al preparations from transfected lymphoblastoid cells expressing speci fic CYPs, only those from CYP3A4 cDNA-transfected cells transformed ir inotecan into NPC; and 3) incubations with 15 individual preparations of human liver microsomes yielded highly significant correlations betw een the formation of NPC and both immunoreactivity with anti-CYP3A ant ibodies and testosterone 6 beta-hydroxylation tan activity specificall y mediated by CYP3A). The effects of 11 drugs (used at 100 mu M) on th is metabolism were studied with irinotecan lactone (25 mu M). Although ondansetron, loperamide, and racecadotril inhibited this pathway by 7 5, 95, and 95%, respectively, the concentrations used may not be clini cally achievable. However, significant inhibition by ketoconazole and troleandomycin indicates that NPC formation in patients may be influen ced by coadministration of drugs with known anti-CYP3A activities.