Different substrate elimination rates of model drugs pH-dependently mediated by flavin-containing monooxygenases and cytochromes P450 in human liver microsomes.

The predicted contributions of flavin-containing monooxygenase 3 (FMO3) to drug candidate N-oxygenations can be estimated using classic base dissociation constants of the N-containing moiety. In this study, metabolic clearance values in human liver microsomes were experimentally determined for available model drugs. Typical metabolic clearance values (34-96 μL/min/mg protein) at pH 8.4 of trimethylamine, benzydamine, and itopride were two-to fourfold higher than those at pH 7.4. In contrast, the metabolic clearance of control drug midazolam at pH 8.4 was half that at pH 7.4. The ratios of clearance values at pH 8.4 to those at pH 7.4 and the substrate pKa (base) values of reported metabolic N-oxygenation sites of trimethylamine, benzydamine, clomipramine, chlorpromazine, tamoxifen, itopride, loxapine, xanomeline, tozasertib, dasatinib, and clozapine were significantly correlated (r = 0.60, p < 0.05, n = 11). These results suggested that the simple comparison of metabolic clearance values at pH 8.4 and at pH 7.4 could be useful for predicting the contributions of FMO3 to the N-oxygenations of new drug candidates. This method, along with in silico pKa (base) values > 8.4, could prove useful for predicting the contributions of FMO3 to N-oxygenations as part of drug development.