METABOLISM OF IRINOTECAN (CPT-11) BY HUMAN HEPATIC MICROSOMES - PARTICIPATION OF CYTOCHROME-P-450 3A AND DRUG-INTERACTIONS

Irinotecan (CPT-11) is a water-soluble analogue of camptothecin showin g activity in colon cancer, Recently, we identified a major metabolite of CPT-11 in patients' plasma, 7-ethyl-10-[4-N-(5-aminopentanoic acid )-1-piperidino] carbonyloxycamptothecin (APC), which is produced by th e oxidation of the distal piperidine ring (P. Rivory et al., Cancer Re s., 56: 3689-3694, 1996). As with all active camptothecin derivatives, CPT-11 is subject to spontaneous interconversion between a lactone an d a carboxylate form in aqueous media. The kinetics of biotransformati on of the two forms of CPT-11 into APC was studied using pooled human liver microsomes. The formation of APC was characterized by the follow ing parameters: K-m = 18.4 +/- 1.4 and 39.7 +/- 11.6 mu M; and V-max = 26.0 +/- 0.6 and 13.4 +/- 1.7 pmol/min/mg protein for the lactone and carboxylate forms of CPT-11, respectively. This reaction was found to be catalyzed principally by cytochrome P-450 (CYP) 3A because of thre e key results: (a) the CYP 3A-selective inhibitors ketoconazole (1 mu M) and troleandomycin (100 mu M) inhibited APC formation by 98 and 100 %, respectively, mostly in a competitive way; (b) using microsomes fro m transfected lymphoblastoid cells expressing specific CYPs, we found that only those from CYP 3A4 cDNA-transfected cells transformed CPT-11 into APC; and (c) using 15 individual preparations of human liver mic rosomes, we observed highly significant correlations between the activ ity of CPT-11 metabolism into APC and both immunoreactivity with anti- CYP 3A antibodies and testosterone 6 beta hydroxylation, an activity s pecifically mediated by CYP 3A. The effect on this metabolism of 11 dr ugs used at 100 mu M was studied with CPT-11 lactone at 25 mu M. Amika cin, Bactrim, ciprofloxacin, rocephine, 5-fluorouracil, metoclopramide , morphine, and paracetamol had no effect, but ondansetron, loperamide , and racecadotril inhibited this pathway by 25, 50, and 50%, respecti vely. These concentrations exceed those expected in vivo, APC formatio n in patients may thus be influenced by coadministered ketoconazole th erapy and may decline after administration of CPT-11 because of the la ctonolysis of the latter.