Similarity of metabolism for CAI (NSC 609974) in human liver tissue in vitro and in humans in vivo

Metabolism of a new antitumor agent, CAI (NSC 609974), was investigated in human liver tissue in vitro and in plasma and urine of patients receiving CAI. Metabolites identified by HPLC following C-labeled CAI incubation with human liver microsomes and reported as percentage of total metabolites formed were M1 (0-3.4%), M2 (4. 5-33%), M3 (60-79%), and M4 (7.2-19%). Ketoconazole, an inhibitor of cytochrome P450 3A4, prevented formation of M1, M3, and M4 (concentration of drug that inhibited metabolite formation by 50% when compared to maximum uninhibited activity, 3.0 &mgr;M), but only weakly inhibited formation of M2 (concentration of drug that inhibited metabolite formation by 50% when compared to maximum uninhibited activity, >50&mgr;M). CAI incubated with recombinant human P450 3A4 microsomes produced metabolites M3 and M4. Conjugation of M3, most likely a glucuronide, was observed after incubation of C-labeled CAI and UDP-glucuronic acid with human liver 13,000 x g supernatant. Plasma samples from patients receiving CAI contained CAI (3.1-5.0 &mgr;g/ml), M1 (0.9-2.6 &mgr;g/ml), and M2 (1. 0-2.2 &mgr;g/ml). CAI and M3 but not M4 were observed in the urine samples. After incubation of the urine samples withbeta-glucuronidase, CAI concentrations increased 67%, M3 increased up to 9-fold, and M4 was detected. CAI is metabolized in vitro and in vivo by both Phase I and Phase II enzymes and is metabolized to M3 and M4 by P450 3A4. These studies suggest that elevated levels of CAI may result from P450 3A4 inhibition by ketoconazole if these two drugs are coadministered. Correlation between CAI metabolism in vitro and results obtained in patients demonstrates the usefulness of liver metabolism studies in vitro in the early stages of drug development.