In vitro prediction and in vivo verification of enantioselective human tofisopam metabolite profiles.
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.
Cameron
Michael D
MD
Scripps Florida, Department of Molecular Therapeutics, 5353 Parkside Dr., Jupiter, FL 33458, USA. cameron@scripps.edu
Wright
Jason
J
Black
Christopher B
CB
Ye
Naidong
N
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
D011994:Recombinant Proteins; D001569:Benzodiazepines; D003577:Cytochrome P-450 Enzyme System; C012582:tofisopam