Characterization and inhibition of mephenytoin 4-hydroxylase activity in human liver microsomes

The in vivo metabolism in humans of the anticonvulsant mephenytoin exhibits stereoselectivity as well as genetic polymorphism of the 4-hydroxylation pathway. The characteristics of the involved cytochrome P-450 isozyme are, however, not known completely. Accordingly, the ability of human liver microsomes to metabolize mephenytoin and its enantiomers was investigated in vitro, and the ability of related anticonvulsants and other compounds to inhibit 4-hydroxylation was studied. Marked stereoselectivity was observed in the conversion of S-mephenytoin to its 4-hydroxy metabolite, but N-demethylation was essentially similar for both enantiomers. The intrinsic clearance (Vmax/Km) for 4-hydroxymephenytoin formation showed an almost 10-fold range in five livers and was 150- to 1000-fold greater than that for N-demethylation. Competitive inhibition of 4-hydroxylation was observed with ethotoin, mephobarbital, methsuximide and phensuximide, but not other commonly used anticonvulsants such as ethosuximide, phenobarbital, phenytoin and primidone. However, synthetic N-alkyl analogs of the latter compounds were found to be inhibitory. An aryl residue alpha to the carbonyl carbon of an N-alkyl lactam in a 5- or 6-membered ring, therefore, appears to be a minimal requirement for strong interaction with the 4-hydroxylase. Warfarin, but not diazepam, ketoconazole or iodochlorohydroxyquin, were also competitive inhibitors, but at much higher concentrations than the anticonvulsants. Competitive inhibition at concentrations similar to the Km of 4-hydroxymephenytoin formation (30-350 microM) may indicate that the isozyme is involved in the metabolism of the substrates under consideration and, therefore, their in vivo metabolism may be regulated to some extent by the same genetic factor(s) that determine mephenytoin's biotransformation.