CYP1A1 specificity of Verlukast epoxidation in mice, rats, rhesus monkeys, and humans.

It has previously been shown that Verlukast is converted to Verlukast dihydrodiol in microsomes from beta-naphthoflavone (BNF)-treated, but not uninduced Swiss Webster mice and Sprague-Dawley rats. We have examined the involvement of CYP1A1 in this reaction in more detail. It is concluded that this reaction is catalyzed exclusively by CYP1A1 in rats, mice, and humans based on the following criteria: 1) the epoxidation of Verlukast is negligible in uninduced rats, which express CYP1A2 but not CYP1A1; 2) Verlukast epoxidation is highly inducible by BNF treatment (60- to 200-fold); 3) Verlukast epoxidation in BNF-treated rat microsomes was inhibited by alpha-naphthoflavone (ANF) treatment, indicating that this activity was mediated by the CYP1A subfamily; 4) > 95% of Verlukast epoxidation in BNF-treated rat microsomes was inhibited by antibodies raised against CYP1A1; and 5) Verlukast was epoxidized by human CYP1A1 but not CYP1A2. Thus, Verlukast epoxidation appears to be specific for rat, mouse, and human CYP1A1. Additional studies showed that Verlukast was metabolized to Verlukast dihydrodiol in microsomes from uninduced rhesus monkeys. This reaction was inhibited by nanomolar concentrations of ANF in rhesus monkey microsomes implicating the involvement of the CYP1A subfamily. In addition, the 8-hydroxylation of R-warfarin, a pathway that is selective for rodent and human CYP1A1 activity, was also catalyzed at significant rates by rhesus monkey microsomes. These findings indicate that, unlike rats, mice, and humans, which have very low constitutive levels of hepatic CYP1A1 activity, the uninduced rhesus monkey is able to catalyze reactions specific to CYP1A1 in rodents and humans.(ABSTRACT TRUNCATED AT 250 WORDS)