HUMAN LIVER MICROSOMAL METABOLISM OF THE ENANTIOMERS OF WARFARIN AND ACENOCOUMAROL - P450 ISOZYME DIVERSITY DETERMINES THE DIFFERENCES IN THEIR PHARMACOKINETICS

1 To explain the large differences in (the stereoselectivity of) the clearances of the enantiomers of warfarin and acenocoumarol (4'-nitrowarfarin) their human liver microsomal metabolism has been studied and enzyme kinetic parameters determined. The effects of cimetidine, propafenone, sulphaphenazole, and omeprazole on their metabolism has been investigated. 2 The 4-hydroxycoumarins follow similar metabolic routes and are mainly hydroxylated at the 6- and 7-position (accounting for 63 to 99% of the metabolic clearances). 3 Due to the lower K(m) values of R- and S-acenocoumarol and higher V(max) values of S-acenocoumarol, the overall metabolic clearances of R/S acenocoumarol exceed those of R/S warfarin 6 and 66 times respectively. 4 The metabolism of both compounds is stereoselective for the S-enantiomers, which is 10 times more pronounced in the case of acenocoumarol. 5 Except for the 7-hydroxylation of the R-enantiomers (r=0.90; P<0.025), the 6- and 7-hydroxylation rates of R/S warfarin do not correlate with those of R/S acenocoumarol. 6 Sulphaphenazole competitively inhibits the 7- and in some samples partly (up to 50%) the 6-hydroxylation of S-warfarin as well as the 7-hydroxylation of R- and S-acenocoumarol and the 6-hydroxylation of S-acenocoumarol (K(i)s ranging from 0.5- 1.3 mum). 7 Omeprazole partly (40-80%) inhibits the 6- and 7-hydroxylation of R-warfarin (K(i) = 99 and 117 muM) and of R- (K(i) = 219 and 7.2 mum) and S-acenocoumarol (K(i) = 6.1 and 7.7 mum) but not S-warfarin in a competitive manner. 8 Differences in the partial (up to 40%) inhibition of the metabolism of the enantiomers of the 4-hydroxycoumarins were also observed for the relatively weak inhibitors, propafenone and cimetidine. 9 The results suggest that the coumarin ring hydroxylations of both compounds are catalysed by different combinations of P450 isozymes. The 7-hydroxylation of R/S acenocoumarol and the 6-hydroxylation of S-acenocoumarol are at least partly conducted by (a) P450 isozyme(s) of the 2C subfamily different from P450 2C9 (the main S-warfarin 7- and 6-hydroxylase).