title: In vitro prediction and in vivo verification of enantioselective human tofisopam metabolite profiles.
abstract: In vitro studies were conducted to elucidate the metabolic profiles of and the enzymes responsible for the metabolism of (R)- and (S)-tofisopam (1-(3,4-dimethoxyphenyl)-5-ethyl-7,8-dimethoxy-4-methyl-5H-2,3-benzodiazepine). Large differences were observed between the two enantiomers. The major metabolite in incubations of 500 ng/ml (approximately 1.3 microM) (R)-tofisopam in human liver microsomes corresponded to demethylation of the methoxy group at the 4-position of the phenyl ring (M3). Incubating (R)-tofisopam with recombinant cytochrome P450 (P450) or with human liver microsomes and isoform-selective P450 chemical inhibitors indicated that M3 was primarily catalyzed by CYP2C9. Similar incubations with S-tofisopam indicated that the primary metabolite was due to demethylation of the methoxy group at the 7-position of the benzodiazepine ring (M1), and this reaction was catalyzed primarily by CYP3A4. The primary metabolites of both enantiomers were further demethylated to form a common didemethylated metabolite (M5) where the methoxy groups at positions 4 and 7 are demethylated. Analysis of plasma and urine samples from human clinical trials confirmed the in vitro observations. Subjects orally treated with 200 mg b.i.d. (R)-tofisopam had a 2-h M1/M3 plasma ratio of 1:29 and a ratio of 1:123 in urine, whereas patients orally administered (S)-tofisopam at 150 mg/kg t.i.d. had opposite M1 to M3 ratios of 8:1 in plasma and 6:1 in urine.
authors.affiliation: Scripps Florida, Department of Molecular Therapeutics, 5353 Parkside Dr., Jupiter, FL 33458, USA. cameron@scripps.edu
authors.forename: Michael D
authors.initials: MD
authors.lastname: Cameron
authors.forename: Jason
authors.initials: J
authors.lastname: Wright
authors.forename: Christopher B
authors.initials: CB
authors.lastname: Black
authors.forename: Naidong
authors.initials: N
authors.lastname: Ye
mesh_terms: D000328:Adult; D001569:Benzodiazepines / Q000097:blood / Q000737:chemistry / Q000493:pharmacokinetics* / Q000652:urine; D003577:Cytochrome P-450 Enzyme System / Q000235:genetics / Q000378:metabolism*; D005260:Female; D006801:Humans; D008297:Male; D008862:Microsomes, Liver / Q000378:metabolism*; D008875:Middle Aged; D011994:Recombinant Proteins / Q000378:metabolism*; D013237:Stereoisomerism
chemical_list: D011994:Recombinant Proteins; D001569:Benzodiazepines; D003577:Cytochrome P-450 Enzyme System; C012582:tofisopam