Identification of CYP3A4 as the enzyme involved in the mono-N-dealkylationof disopyramide enantiomers in humans

To identify which cytochrome P-450 (CYP) isoform(s) are involved in the maj or pathway of disopyramide (DP) enantiomers metabolism in humans, the in vi tro formation of mono-N-desalkyldisopyramide from each DP enantiomer was st udied with human liver microsomes and nine recombinant human CYPs. Substrat e inhibition showed that SKF 525A and troleandomycin potently suppressed th e metabolism of both DP enantiomers with IC50 values for R(-)- and S(+)-DP of <7.3 and <18.9 mu M, respectively. In contrast, only weak inhibitory eff ects (i.e., IC50 > 100 mu M) were observed for five other representative CY P isoform substrates [i.e., phenacetin (CYP1A1/2), sparteine (CYP2D6), tolb utamide (CYP2C9), S-mephenytoin (CYP2C19), and p-nitrophenol (CYP2E1)]. Sig nificant 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 beta-hydroxylation of testosterone. Conversely, n o significant correlations were observed between the catalytic activities f or DP enantiomers and those for the O-deethylation of phenacetin, 2-hydroxy lation of desipramine, hydroxylation of tolbutamide, and 4'-hydroxylation o f S-mephenytoin. Further evidence for involvement of CYP3A P450s was reveal ed by an anti-human CYP3A serum that inhibited the mono-N-dealkylation of b oth DP enantiomers and 6 beta-hydroxylation of testosterone almost complete ly (i.e., >90%), whereas it only weakly inhibited (i.e., <15%) CYP1A1/2- or 2C19-mediated reactions. Finally, the recombinant human CYP3A3 and 3A4 sho wed 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.