Abstract

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 ¹H NMR analysis of the(S)-ketotifen N-glucuronides, the conformation of the piperidylidene ring differs between the two isomers. Enzymatic hydrolysis with Escherichia coliβ-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 μM) 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 apparentK_M 15 μM andV_max 470 pmol/min/mg protein. The two(S)-ketotifen glucuronides were formed by two-enzyme kinetics with K_M1 1.3 μM andK_M2 92 μM andV_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 ofN-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 betweenN-glucuronide isomer ratios in vitro and in vivo.