title: Stereoselective N-demethylation of chlorpheniramine by rat-liver microsomes and the involvement of cytochrome P450 isozymes.
abstract: Previous studies have suggested that degradation of the two stereoisomers of chlorpheniramine in the liver might be catalysed by different types of cytochrome P450. Stereoselective N-demethylation of chlorpheniramine and the involvement of cytochrome P450 (CYP) isozymes have, therefore, been investigated in the liver microsomes of eight-week-old male rats. Incubation of racemic chlorpheniramine with liver microsomes from the male rat resulted in the formation of both enantiomers of monodesmethylchlorpheniramine (DMChp). Further metabolism of DMChp to didesmethylchlorpheniramine (DDMChp) did not, however, occur. The S/R enantiomeric ratio for intrinsic clearance (Vmax/Km) was approximately 2.0, suggesting that the N-demethylation was stereoselective for S-(+)-chlorpheniramine. On the other hand, although the Vmax/Km value for the formation of S-(+)- and R-(-)-DMChp increased with phenobarbitone-inducible rat-liver microsomes, there was no difference between the rates of N-demethylation of the enantiomers. In contrast, 3-methylcholanthrene reduced the intrinsic clearance of S-(+)-chlorpheniramine by N-demethylation and increased its value for R-(-)-chlorpheniramine, showing no stereoselectivity for the N-demethylation of chlorpheniramine. The difference between the intrinsic clearance of the two enantiomers by N-demethylation was because of differences in affinity for the catalysing enzyme. This is indicative of stereoselective involvement of the main enzyme concerned in the N-demethylation of the enantiomers, considered to be CYP 2C11. Anti-CYP 2C11 also partially inhibited the N-demethylation of racemic chlorpheniramine in rat-liver microsomes exposed to phenobarbitone and 3-methylcholanthrene. That CYP 2B1 was involved in the N-demethylation of both enantiomers was also supported by results from an experiment using phenobarbitone-inducible rat-liver microsomes. CYP1A1 did not, however, catalyse the N-demethylation of either enantiomer. These results indicate that N-demethylation of the S-(+)-enantiomer of chlorpheniramine occurs preferentially in the microsomes, demonstrating the stereoselective contribution of CYP2C11. Immunoinhibition studies suggest, moreover, that the N-demethylation of both chlorpheniramine enantiomers is catalysed by CYP2B1, but not by CYP1A1.
authors.affiliation: Department of Pharmaceutics I, Tohoku College of Pharmacy, Sendai, Japan.
authors.forename: A
authors.initials: A
authors.lastname: Nomura
authors.forename: E
authors.initials: E
authors.lastname: Sakurai
authors.forename: N
authors.initials: N
authors.lastname: Hikichi
mesh_terms: D000818:Animals; D000906:Antibodies / Q000276:immunology; D001189:Aryl Hydrocarbon Hydroxylases; D002744:Chlorpheniramine / Q000493:pharmacokinetics*; D019363:Cytochrome P-450 CYP1A1 / Q000276:immunology / Q000378:metabolism; D019362:Cytochrome P-450 CYP2B1 / Q000276:immunology / Q000378:metabolism; D019392:Cytochrome P-450 CYP2E1 / Q000276:immunology / Q000378:metabolism; D003577:Cytochrome P-450 Enzyme System / Q000276:immunology / Q000378:metabolism*; D000072467:Cytochrome P450 Family 2; D006634:Histamine H1 Antagonists / Q000493:pharmacokinetics*; D066298:In Vitro Techniques; D007527:Isoenzymes / Q000276:immunology / Q000378:metabolism*; D008297:Male; D008748:Methylcholanthrene / Q000494:pharmacology; D008862:Microsomes, Liver / Q000187:drug effects / Q000201:enzymology*; D010634:Phenobarbital / Q000494:pharmacology; D051381:Rats; D017208:Rats, Wistar; D013237:Stereoisomerism; D039181:Steroid 16-alpha-Hydroxylase; D013250:Steroid Hydroxylases / Q000276:immunology / Q000378:metabolism
chemical_list: D000906:Antibodies; D006634:Histamine H1 Antagonists; D007527:Isoenzymes; D002744:Chlorpheniramine; D008748:Methylcholanthrene; D003577:Cytochrome P-450 Enzyme System; D013250:Steroid Hydroxylases; D019392:Cytochrome P-450 CYP2E1; D001189:Aryl Hydrocarbon Hydroxylases; C450279:CYP2C11 protein, rat; D019363:Cytochrome P-450 CYP1A1; D019362:Cytochrome P-450 CYP2B1; D000072467:Cytochrome P450 Family 2; D039181:Steroid 16-alpha-Hydroxylase; C022657:steroid hormone 6-beta-hydroxylase; D010634:Phenobarbital