title: Influence of substrate configuration on chlorpheniramine N-demethylation by hepatic microsomes from rats, rabbits, and mice.
abstract: Measurements of formaldehyde formation in parallel incubations containing either (S)-(+)- or (R)-(-)-chlorpheniramine (CPA) and rat liver microsomes demonstrated that the active antihistamine, (S)-(+)-CPA, is N-demethylated about 35% faster than the inactive (R)-(-)-enantiomer. The KM values for the enantiomers were the same. Phenobarbital pretreatment increased Vmax values without affecting the stereoselectivity. N-Demethylation occurred at a several-fold faster rate with rabbit than with rat liver microsomes, but stereoselectivity was the same. N-Demethylation of CPA enantiomers were studied in microsomes prepared from each of four inbred strains of mice. These experiments demonstrated that stereoselectivity is species-dependent, as no significant differences in metabolism rates of CPA enantiomers could be detected with these microsomes. Pseudoracemic mixtures containing equal quantities of deuterated (S)-(+)-CPA and unlabeled (R)-(-)-CPA were incubated with microsomes from three species. Formation of the enantiomers of N-desmethyl- and N,N-didesmethyl-CPA (DMCPA and DDMCPA, respectively) were measured by GC/MS techniques. With microsomes from rats and mice, the ratio of (S)-DMCPA to (R)-DMCPA was essentially the same as that determined by measuring the formaldehyde formed in separate incubations of (S)-(+)- or (R)-(-)-CPA. Stereoselectivity with rabbit liver microsomes and pseudoracemic CPA was substantially higher than that determined in incubations with the separate enantiomers. The results suggest either that (S)-(+)-CPA inhibits the N-demethylation of (R)-(-)-CPA under these conditions, or that DMCPA undergoes further biotransformation by a route(s) which is stereoselective, favoring the (R)-enantiomer. Formation of DDMCPA could only be detected with rabbit microsomes and was found to occur with approximately the same stereoselectivity as that determined for the formation of DMCPA.
mesh_terms: D000818:Animals; D002744:Chlorpheniramine / Q000378:metabolism*; D008401:Gas Chromatography-Mass Spectrometry; D007700:Kinetics; D008297:Male; D051379:Mice; D008807:Mice, Inbred BALB C; D008809:Mice, Inbred C3H; D008810:Mice, Inbred C57BL; D008811:Mice, Inbred DBA; D008862:Microsomes, Liver / Q000201:enzymology*; D010089:Oxidoreductases, N-Demethylating / Q000378:metabolism*; D011817:Rabbits; D051381:Rats; D013045:Species Specificity; D013237:Stereoisomerism; D013379:Substrate Specificity
chemical_list: D002744:Chlorpheniramine; D010089:Oxidoreductases, N-Demethylating