Stereoselective inhibition of diphenylhydantoin metabolism by p ‐hydroxyphenyl‐phenylhydantoin enantiomers in rats

Different doses of rac ‐ p ‐HPPH (0.4 and 4 mg/h) were given repeatedly to rats infused with [ 14 C]phenytoin. The serum levels of 14 C‐labeled and unlabeled p ‐HPPH, and [ 14 C]phenytoin were measured by an HPLC method and radiometric analysis. The clearance of phenytoin and p ‐HPPH was determined by rate of dosing divided by the steady‐state concentration. The phenytoin clearance was significantly lower in the high dose p ‐HPPH injection group than in the low dose group (87 versus 262 ml/h), whereas p ‐HPPH clearance showed no difference. The formation clearance of [ 14 C] p ‐HPPH was also significantly lower in rats injected with high dose of p ‐HPPH (35 versus 169 ml/h). The clearance of other elimination pathways was also lower in rats with high dose of p ‐HPPH (53 versus 89 ml/h). The serum protein binding of phenytoin was lower in rats injected with high dose of p ‐HPPH. The result indicated that injections of rac ‐ p ‐HPPH mainly inhibited on the formation of p ‐HPPH itself. The formation of (R)‐ p ‐HPPH and (S)‐ p ‐HPPH in microsomal preparation was measured by a ligand‐exchange chromatographic method. The formation of (S)‐ p ‐HPPH or (R)‐ p ‐HPPH was not only inhibited by the enantiomer itself, but also cross‐inhibited by the other enantiomer. To the formation of either (S)‐ p ‐HPPH or (R)‐ p ‐HPPH, (S)‐ p ‐HPPH showed a higher inhibitory activity. The use of rac ‐ p ‐HPPH to inhibit phenytoin metabolism in vivo involved several mechanisms. In addition to binding displacement on the serum proteins, one is the product inhibition by the respective p ‐HPPH metabolite, the other is the competitive inhibition by the other p ‐HPPH enantiomer.