Interaction of pefloxacin and enoxacin with the human cytochrome P450 enzyme CYP1A2

Background and objectives: Pefloxacin is reported to cause clinically relevant inhibition of theophylline metabolism in vivo, but in vitro pefloxacin was only a weak inhibitor of the cytochrome P450 CYP1A2, mediating main theophylline biotransformation. We therefore further characterized the interaction between pefloxacin and CYP1A2. Methods: A randomized 3-period change-over study was conducted in 12 healthy young volunteers on the steady-state interactions between pefloxacin or enoxacin (400 mg twice a day) with caffeine (183 mg once daily), a validated marker of CYP1A2. Caffeine pharmacokinetics were estimated after its fifth dose. Studies in human liver microsomes were carried out to measure the effect of pefloxacin and norfloxacin on caffeine 3-demethylation, an in vitro CYP1A2 probe, and to identify the enzyme(s) that mediate pefloxacin N-4'-demethylation with selective inhibitors. Results: For the in vivo study, ANOVA-based point estimates (90% confidence intervals [CI]) for the ratios of caffeine pharmacokinetics with and without pefloxacin coadministration were 1.11 for maximal steady-state plasma concentrations (C-max,C-ss; 90% CI, 0.99 to 1.26), 0.53 for total clearance (CLt,ss; 90% CI, 0.49 to 0.58), and 1.04 for the beta-phase distribution volume (V(d)beta; 90% CI, 0.96 to 1.13). The values for enoxacin were 1.99 for C-max,C-ss (90% CI, 1.77 to 2.23), 0.17 for CLt,ss (90% CI, 0.16 to 0.19), and 1.01 for V(d)beta (90% CI, 0.90 to 1.13). Thus pefloxacin caused a 2-fold decrease in caffeine clearance, and enoxacin caused a 2-fold decrease in caffeine clearance. In vitro, norfloxacin and pefloxacin competitively inhibited CYP1A2, with inhibition constant (K-i) values of 0.1 and 1 mmol/L, respectively, and CYP1A2 was the only enzyme with a relevant contribution (approximately 50%) to pefloxacin N-4'-demethylation. Conclusions: Enoxacin and to a lesser extent pefloxacin may cause clinically relevant interactions with further CYP1A2 substrates. The data suggest that the pefloxacin interaction is partly mediated by its major metabolite norfloxacin.