Vol. 28, Issue 8, 937-944, August 2000

Identification of CYP3A4 as the Enzyme Involved in the Mono-N-Dealkylation of Disopyramide Enantiomers in Humans

Hirotoshi Echizen, Michika Tanizaki, Jun Tatsuno, Kan Chiba, Teresa Berwick, Masayoshi Tani, Frank J. Gonzalez, and Takashi Ishizaki

Department of Pharmacotherapy, Meiji Pharmaceutical University (H.E.); Department of Pharmaceutical Science, Science University of Tokyo (M.T.); Mitsubishi Chemical Corporation (J.T.); Division of General Surgery, International Medical Center of Japan (T.B.), Tokyo; Laboratory of Biochemical Pharmacology and Toxicology, Chiba University (K.C.), Chiba; Department of Pharmacology and Therapeutics, Graduate School of Clinical Pharmacy, Kumamoto University, Kumamoto (T.I.), Japan; and Laboratory of Metabolism, National Cancer Institute, National Institute of Health, Bethesda, Maryand (F.J.G.)

To identify which cytochrome P-450 (CYP) isoform(s) are involved in the major pathway of disopyramide (DP) enantiomers metabolism in humans, the in vitro formation of mono-N-desalkyldisopyramide from each DP enantiomer was studied with human liver microsomes and nine recombinant human CYPs. Substrate inhibition showed that SKF 525A and troleandomycin potently suppressed the metabolism of both DP enantiomers with IC₅₀ values for R()- and S(+)-DP of <7.3 and <18.9 µM, respectively. In contrast, only weak inhibitory effects (i.e., IC₅₀ > 100 µM) were observed for five other representative CYP isoform substrates [i.e., phenacetin (CYP1A1/2), sparteine (CYP2D6), tolbutamide (CYP2C9), S-mephenytoin (CYP2C19), and p-nitrophenol (CYP2E1)]. Significant correlations (P < .01, r = 0.91) were found between the activities of 11 different human liver microsomes for mono-N-dealkylation of both DP enantiomers and that of 6-hydroxylation of testosterone. Conversely, no significant correlations were observed between the catalytic activities for DP enantiomers and those for the O-deethylation of phenacetin, 2-hydroxylation of desipramine, hydroxylation of tolbutamide, and 4'-hydroxylation of S-mephenytoin. Further evidence for involvement of CYP3A P450s was revealed by an anti-human CYP3A serum that inhibited the mono-N-dealkylation of both DP enantiomers and 6-hydroxylation of testosterone almost completely (i.e., >90%), whereas it only weakly inhibited (i.e., <15%) CYP1A1/2- or 2C19-mediated reactions. Finally, the recombinant human CYP3A3 and 3A4 showed much greater catalytic activities than seven other isoforms examined (i.e., CYP1A2, 2A6, 2B6, 2C9, 2D6, 2E1, and 3A5) for both DP enantiomers. In conclusion, the metabolism of both DP enantiomers in humans would primarily be catalyzed by CYP3A4, implying that DP may have an interaction potential with other CYP3A substrates and/or inhibitors.