Verlukast (MK-0679) conjugation with glutathione by rat liver and kidney cytosols and excretion in the bile.

Verlukast (MK-0679) is a potent leukotriene D4 antagonist that was under development for the treatment of bronchial asthma. A previously uncharacterized metabolite of verlukast was formed in incubations using rat liver cytosol fortified with glutathione (GSH). The metabolite was detected by HPLC and characterized by UV spectroscopy (photodiode array detection after HPLC) and capillary HPLC continuous flow-liquid secondary-ion mass spectrometry. After a large-scale incubation and isolation, it was further characterized by 500 MHz proton NMR. The metabolite is a 1,4-Michael addition product in which GSH has added to position 12 of the styryl quinoline double bond of verlukast. There is no apparent stereoselectivity because a mixture of the two possible isomers, in equal amounts, was observed by NMR. Although there was spontaneous chemical addition of GSH to verlukast (0.18 nmol/min), the reaction was shown to be enzyme-catalyzed in studies using three different preparations of rat liver cytosol at pH 7.4. Using Lineweaver-Burk analysis of experiments in which the effect of verlukast concentration on the rate of conjugation was studied, the apparent KM and Vmax were determined to be 107 +/- 22 microM (SD, N=3) and 0.66 +/- 0.21 nmol/min/mg protein, respectively. In similar studies with GSH as the variable substrate, the apparent KM and Vmax were 2.32 +/- 0.68 mM and 0.69 +/- 0.14 nmol/min/mg protein, respectively. Incubations with kidney cytosol produced the GSH, cysteinylglycine, and cysteine conjugates of verlukast. In bile collected from rats dosed intravenously with 50 mg/kg of verlukast, approximately 80% of the dose was recovered up to 4 hr postdose. The GSH conjugate accounted for 16.5% of the dose. The cysteinylglycine, cysteine, and N-acetylcysteine conjugates were observed and together accounted for 7.5%. Verlukast accounted for 14.5%, and the remainder of the metabolites (40.5%) were oxidation or acyl glucuronide metabolites.