Pharmacokinetics And Metabolism Of A Cysteinyl Leukotriene-1 Receptor Antagonist From The Heterocyclic Chromanol Series In Rats: In Vitro-In Vivo Correlation, Gender-Related Differences, Isoform Identification, And Comparison With Metabolism In Human Hepatic Tissue

CP-199,331 is a potent antagonist of the cysteinyl leukotriene-1 (LT1) receptor, targeted for the treatment of asthma. The pharmacokinetic/metabolism properties of CP-199,331 were studied in rats and compared with those in human liver microsomes/hepatocytes. In vitro biotransformation of CP-199,331 in rat and human hepatocytes was similar, consisting primarily of CP-199,331 O-demethylation. Marked sex-related differences in plasma clearance (CLp) of CP-199,331 were observed in rats: 51 and 1.2 ml/ min/kg in males and females, respectively. This difference in CLp was attributed to gender differences in metabolizing capacity because V-max and K-m values for CP-199,331 metabolism were 30-fold higher and 8-fold lower, respectively, in male rat liver microsomes compared with female microsomes. Scale-up of the in vitro microsomal data predicted hepatic clearance (CLh) of 64 and 2.5 ml/min/kg in male and female rats, respectively. These values were in close agreement with the in vivo CLp, suggesting that CP-199,331 CLp in male and female rats was entirely due to hepatic metabolism. Studies with rat recombinant cytochromes P450 and anti-rat cytochrome P450 (CYP) antibodies revealed the involvement of male rat-specific CYP2C11 in the metabolism of CP-199,331. In contrast, CP-199,331 metabolism in human liver microsomes was principally mediated by CYP3A4. The projected human clearance in liver microsomes and hepatocytes varied 6-fold from low to moderate, depending on CYP3A4 activity. Considering that O-demethylation is the major route of elimination in humans, the in vivo clearance of CP-199,331 may exhibit moderate variability, depending on CYP3A4 abundance in the human population.