ROLE OF HEPATIC FLAVIN-CONTAINING MONOOXYGENASE-3 IN DRUG AND CHEMICAL METABOLISM IN ADULT HUMANS

In conjunction with asymmetric chemical syntheses and spectral, chirop tical, chromatographic and stereochemical correlation methods, we have developed procedures for the quantification of sulfoxide enantiomers and tertiary amine N-oxide diastereomer metabolites arising from the a ction of the adult human liver and other flavin-containing monooxygena ses (FMOs). The parallel nature of the metabolic in vitro-in vivo stud ies and the use of chemical model oxidation systems allowed us to iden tify the FMO isoform involved. We investigated the enantioselective S- monooxygenation of cimetidine and the diastereoselective tertiary amin e N-1'-oxygenation of (S)-nicotine as stereoselective functional probe s of adult human liver FMO action, In both cases, the majority of evid ence points to adult human liver FMO3 as the principal enzyme responsi ble for cimetidine S-oxygenation and (S)-nicotine N-1'-oxygenation in vitro and in vivo. The excellent agreement between the absolute config uration of the major cimetidine S-oxide and (S)-nicotine N-1'-oxide me tabolites isolated from human urine and the major metabolite formed in the presence of adult human liver microsomes suggests that in vitro h epatic preparations may serve as a useful model for the in vivo condit ion. Further, that adult human liver cDNA-expressed FMO3 in Escherichi a coil also gave the same absolute stereoselectivity (i.e. for (S)-nic otine N-1'-oxygenation) confirms the identity of the monooxygenase in vivo. Although we cannot rule out the involvement of minor contributio ns of cytochrome P-450 monooxygenases in cimetidine and (S)-nicotine o xidation, the majority of the data support the fact that cimetidine S- oxygenation and (S)-nicotine N-1'-oxygenation are stereoselective func tional probes of adult human liver FMO3 activity. Finally, because the stereochemistry of the principal metabolite of cimetidine and (S)-nic otine in small experimental animals is distinct from that observed in humans, it is likely that species variation in predominant FMO isoform s exist and this may have important consequences for the choice of exp erimental animals in human preclinical drug design and development pro grams.