Abstract
Cytochrome P450 (CYP) involved in the two major pathways of imipramine (IMI) was reappraised using human liver microsomes phenotyped forS-mephenytoin 4′-hydroxylation in vitro and 11 recombinant human CYP isoforms. Individual Eadie-Hoffstee plots for IMIN-demethylation and 2-hydroxylation showed a monophasic profile in microsomes obtained from three putativeS-mephenytoin poor metabolizer (PM) livers, whereas the plots gave a biphasic relationship (except for one case in 2-hydroxylation) in those from the three extensive metabolizer (EM) livers. Effects of CYP-selective inhibitor/substrate probes on the two metabolic reactions were examined at the two IMI concentrations (2 and 400 μM) with microsomes obtained from the two PM and three EM livers.S-mephenytoin inhibited IMI N-demethylation by 50% at the low concentration in microsomes from the EM livers with no discernible effect on this pathway in those from the PM livers. Furafylline inhibited the N-demethylation by about 60% at the low and high substrate concentrations in microsomes from both the EM and PM livers. Quinidine abolished the 2-hydroxylation at the low and high concentrations in microsomes from both the EM and the PM livers. Among the recombinant human CYPs, CYP2C19, 2C18, 2D6, 1A2, 3A4 and 2B6 in rank order catalyzed the N-demethylation, whereas CYP2D6, 2C19, 1A2, 2C18 and 3A4 catalyzed the 2-hydroxylation. TheK m values obtained from recombinant CYP2C19 and 1A2 approximated those of the high- and low-affinity components from human liver microsomes for IMI N-demethylation, respectively. For IMI 2-hydroxylation, the respectiveK m values obtained from recombinant CYP2D6 and 2C19 were close to those of the high- and low-affinity components from human liver microsomes. Our human liver microsomal study using the near-therapeutic IMI concentration (2 μM) suggests that 1) CYP2C19 and 1A2 are involved in the N-demethylation and the 2-hydroxylation is mediated exclusively by CYP2D6 and partially by CYP2C19 in the EM livers, and 2) CYP1A2 and 2D6 play a major role in IMI N-demethylation and 2-hydroxylation, respectively, in the PM livers. Our recombinant human CYP isoform study, in general, supports this conclusion.
Footnotes
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Send reprint requests to: Takashi Ishizaki, M.D., Ph.D., Department of Clinical Pharmacology, Research Institute, International Medical Center of Japan, Toyama 1-21-2, Shinjuku-ku, Tokyo 162, Japan.
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↵1 This study was supported by a grant-in-aid from Drug Innovation Science Project (1-2-10) and from the Ministry of Human Health and Welfare, Tokyo, Japan.
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↵2 Present address: Department of Clinical Pharmacology, Research Institute, International Medical Center of Japan, Tokyo, Japan.
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↵3 Present address: Laboratory of Biochemical Pharmacology and Toxicology, Faculty of Pharmaceutical Sciences, Chiba University, Chiba, Japan.
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↵4 Present address: Division of General Surgery, International Medical Center of Japan, Tokyo, Japan.
- Abbreviations:
- CYP
- cytochrome P450
- ECD
- electrochemical detection
- DMI
- desipramine
- EM
- extensive metabolizer
- IMI
- imipramine
- PM
- poor metabolizer
- rac
- racemate
- R/S
- enantiomeric ratio
- TAO
- troleandomycin
- 2-OH-IMI
- 2-hydroxyimipramine
- 2-OH-DMI
- 2-hydroxydesipramine
- Received September 30, 1996.
- Accepted February 4, 1997.
- The American Society for Pharmacology and Experimental Therapeutics
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In this issue
Reappraisal of Human CYP Isoforms Involved in ImipramineN-Demethylation and 2-Hydroxylation: A Study Using Microsomes Obtained from Putative Extensive and Poor Metabolizers ofS-Mephenytoin and Eleven Recombinant Human CYPs
