Conjugation of the enantiomers of ketotifen to four isomeric quaternary ammonium glucuronides in humans in vivo and in liver microsomes.
The antiallergic drug ketotifen is chiral due to a nonplanar seven-membered ring containing a keto group. Earlier studies have revealed glucuronidation at the tertiary amino group as a major metabolic pathway in humans. Chemical synthesis of glucuronides from racemic ketotifen now led to four isomers separable by HPLC of which two each could be ascribed to (R)-(+)- and (S)-(-)-ketotifen by synthesis from the enantiomers. According to (1)H NMR analysis of the (S)-ketotifen N-glucuronides, the conformation of the piperidylidene ring differs between the two isomers. Enzymatic hydrolysis with Escherichia coli beta-glucuronidase proceeded at a lower rate with the slower eluting (S)-ketotifen glucuronide than with the three other isomers. On incubation of the ketotifen enantiomers (0.5-200 microM) with human liver microsomes in the presence of UDP-glucuronic acid and Triton X-100, the N-glucuronides of (R)-ketotifen were produced with an apparent K(M) 15 microM and V(max) 470 pmol/min/mg protein. The two (S)-ketotifen glucuronides were formed by two-enzyme kinetics with K(M1) 1.3 microM and K(M2) 92 microM and V(max) values of 60 and 440 pmol/min/mg protein. After ingestion of 1 mg of racemic ketotifen, 10 healthy subjects excreted in urine 17 +/- 5% of the dose in the form of N-glucuronides. The (R)-ketotifen glucuronide isomers contributed one-sixth only, whereas the remainder consisted primarily of the (S)-ketotifen glucuronide isomer, which eluted last. Differential hydrolysis or membrane transport may be responsible for the discrepancy between N-glucuronide isomer ratios in vitro and in vivo.
Mey
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U
Department of Toxicology, University of Tuebingen, Tuebingen, Germany.
Wachsmuth
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Breyer-Pfaff
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U
D018926:Anti-Allergic Agents / Q000737:chemistry / Q000493:pharmacokinetics*; D002851:Chromatography, High Pressure Liquid; D004355:Drug Stability; D020719:Glucuronides / Q000737:chemistry*; D006801:Humans; D006868:Hydrolysis; D007665:Ketotifen / Q000737:chemistry / Q000493:pharmacokinetics*; D008862:Microsomes, Liver / Q000378:metabolism*; D000644:Quaternary Ammonium Compounds / Q000737:chemistry*; D013237:Stereoisomerism
D018926:Anti-Allergic Agents; D020719:Glucuronides; D000644:Quaternary Ammonium Compounds; D007665:Ketotifen