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 exam ined the involvement of CYP1A1 in this reaction in more detail. It is concluded that this reaction is catalyzed exclusively by CYP1A1 in rat s, mice, and humans based on the following criteria: 1) the epoxidatio n of Verlukast is negligible in uninduced rats, which express CYP1A2 b ut not CYP1A1; 2) Verlukast epoxidation is highly inducible by BNF tre atment (60- to 200-fold); 3) Verlukast epoxidation in BNF-treated rat microsomes was inhibited by a-naphthoflavone (ANF) treatment, indicati ng that this activity was mediated by the CYP1A subfamily; 4) >95% of Verlukast epoxidation in BNF-treated rat microsomes was inhibited by a ntibodies raised against CYP1A1; and 5) Verlukast was epoxidized by hu man CYP1A1 but not CYP1A2. Thus, Verlukast epoxidation appears to be s pecific for rat, mouse, and human CYP1A1. Additional studies showed th at Verlukast was metabolized to Verlukast dihydrodiol in microsomes fr om uninduced rhesus monkeys. This reaction was inhibited by nanomolar concentrations of ANF in rhesus monkey microsomes implicating the invo lvement of the CYP1A subfamily. In addition, the 8-hydroxylation of R- warfarin, a pathway that is selective for rodent and human CYP1A1 acti vity, was also catalyzed at significant rates by rhesus monkey microso mes. These findings indicate that, unlike rats, mice, and humans, whic h have very low constitutive levels of hepatic CYP1A1 activity, the un induced rhesus monkey is able to catalyze reactions specific to CYP1A1 in rodents and humans. However, only CYP1A2, but not CYP1A1, was immu nologically detectable in these monkey microsomes. Based on these resu lts, two possibilities exist: 1) the monkey constitutively expresses a CYP1A1 that is not immunologically detectable with antibodies to rode nt CYP1A1, or 2) the monkey constitutively expresses another P-450 (CY P1A2), which has the catalytic properties of rodent and human CYP1A1. Verlukast dihydrodiol was formed in incubations of Verlukast with live r slices from rhesus monkeys and was present in plasma from Verlukast- treated rhesus monkeys. Thus, this CYP1A1-like activity may play a sig nificant role in the disposition and biological effects of xenobiotics in the uninduced rhesus monkey.