[CAS NO. 107753-78-6]  Zafirlukast (ICI-204219)

Zafirlukast is similar to 3-methylindole because it contains an N-methylindole moiety that has a 3-alkyl substituent on the indole ring. Zafirlukast readily forms (10% conversion in 60 min) a GSH adduct with molecular mass of 880 Da upon incubation with horseradish peroxidase. Zafirlukast produces time-, concentration-, and NADP(H)-dependent inactivation of CYP3A4. Zafirlukast is shown to inhibit CYP3A enzyme activity in vitro (nonpreincubation inhibition) with a Ki of 2 μM in one report, but higher values have been published, and our KI for the inactivation process is 13.4 μM. Zafirlukast has high plasma protein binding (>99%), which would lower the free-drug concentration. Zafirlukast undergoes two successive one-electron oxidations to afford a highly electrophilic α,β-unsaturated iminium species, which reacts spontaneously with GSH to yield the observed adduct.