Species difference in stereoselective involvement of CYP3A in the mono-N-dealkylation of disopyramide

1. To determine which CYP isoenzyme is involved in the N-dealkylation of di sopyramide (DP) metabolism in human and dog, and to determine the stereosel ectivity of DP metabolism with human CYP and dog CYP isoenzymes, the follow ing in vitro metabolism studies of DP were conducted: correlation between h uman CYP isoenzyme activities and DP metabolism with human liver microsomes ; inhibition of DP metabolism in human and dog liver microsomes with chemic al inhibitors of CYP isoenzymes; inhibition of DP metabolism in human micro somes with human CYP antibodies; inhibition of DP metabolism in dog liver m icrosomes with human and dog CYP antibodies; metabolism of DP with human (C YP3A4) and dog (CYP3A12) cDNA-expressed isoenzymes; determination of K-m an d V-max of DP enantiomers by using cDNA-expressed CYP3A4 and CYP3A12. 2. In human liver microsomes, the formation of the mono-N-dealkylated disop yramide (MNDP) metabolite was best correlated with CYP3A4 activities. DP me tabolism was substantially inhibited by ketoconazole, troleandomycin (TA) a nd human CYP3A4 antibody. DP was metabolized by cDNA-expressed CYP3A isoenz ymes. In dog liver microsomes, DP metabolism was inhibited by ketoconazole, TA and dog anti-CYP3A12. DP was also metabolized by cDNA-expressed CYP3A12 . 3. CYP3A4 and CYP3A12 are the principal isoenzymes involved in DP metabolis m in human and dog respectively. There was no stereoselectivity in N-dealky lation of DP by human CYP3A4. However, there was notable stereoselectivity in the N-dealkylation by dog CYP3A12.