Aims: Base dissociation constants of 30 model chemicals were investigated to constitute potential determinant
factors predicting the contributions of flavin-containing monooxygenases (FMOs).
Background: The contributions of FMOs to the metabolic elimination of new drug candidates could be underestimated
under certain experimental conditions during drug development.
Objective: A method for predicting metabolic sites and the contributions of FMOs to N-oxygenations is proposed
using a molecular descriptor, the base dissociation constant (pKa base), which can be estimated in silico
using commonly available chemoinformatic prediction systems.
Methods: Model drugs and their oxidative pathways were surveyed in the literature to investigate the roles of
FMOs in their N-oxygenations. The acid and base dissociation constants of the nitrogen moieties of 30 model
substrates were estimated using well-established chemoinformatic software.
Results: The base dissociation constants of 30 model chemicals were classified into two groups based on the reported
optimal in vitro pH of 8.4 for FMO enzymes as a key determinant factor. Among 18 substrates (e.g.,
trimethylamine, benzydamine, and itopride) with pKa (base) values in the range of 8.4-9.8, all N-oxygenated
metabolites were reported to be predominantly catalyzed by FMOs. Except for three cases (xanomeline;
L-775,606; and tozasertib), the nine substrates with pKa (base) values in the range 2.7-7.9 were only moderately
or minorly N-oxygenated by FMOs in addition to their major metabolic pathway of oxidation mediated by cytochrome
P450s. N-Oxygenation of T-1032 (with a pKa of 4.8) is mediated predominantly by P450 3A5, but
not by FMO1/3.
Conclusion: The predicted contributions of FMOs to the N-oxygenation of drug candidates can be simply estimated
using classic base dissociation constants.