2-DIMENSIONAL HIGH-PERFORMANCE LIQUID-CHROMATOGRAPHIC METHOD TO ASSAY PARA-HYDROXYPHENYLPHENYLHYDANTOIN ENANTIOMERS IN BIOLOGICAL-FLUIDS AND STEREOSELECTIVITY OF ENZYME-INDUCTION IN PHENYTOIN METABOLISM

A two-dimensional high-performance liquid chromatographic method was developed to assay the enantiomers of a major phenytoin metabolite, p-hydroxyphenylphenylhydantoin (p-HPPH). Racemic p-HPPH was first separated from phenytoin and other interfering peaks by a reversed-phase column and monitored by an ultraviolet detector. At the retention time of p-HPPH, the racemic p-HPPH peak was automatically transferred to a chiral ligand-exchange column to separate R-p-HPPH and S-p-HPPH by a time-programmed column-switching valve. The ratio of enantiomers was measured by a second ultraviolet detector. The method can be used to assay R- and S-p-HPPH enantiomers with reasonable sensitivity and reproducibility. By using this method, the steroselectivity of enzyme induction and inhibition of phenytoin metabolism was investigated. Male rats were treated with phenobarbital, 3-methylcholanthrene, acetone, Aroclor 1254, pregnenolone-16-alpha-carbonitrile, dexamethasone and isosafrole. Microsomes were prepared from the rat liver and phenytoin hydroxylation was measured. Pretreatment with phenobarbital, pregnenolone-16-alpha-carbonitrile or acetone induced phenytoin metabolism non-stereoselectively. Pretreatment with dexamethasone decreased R-p-HPPH formation without affecting the formation of S-p-HPPH. Liver microsomes from female rats showed a higher S-p-HPPH formation whereas R-p-HPPH formation remained the same. Various inhibitors were added to inhibit phenytoin metabolism by control microsomes. Sulphaphenazole, ketoconazole, 4,4-di(p-methoxyphenyl)hydantoin, cimetidine and diazepam inhibited the formation of R- and S-p-HPPH. Quinidine, tolbutamide and mephenytoin showed no significant inhibitory activity. None of these inhibitors showed steroselectivity.