A modified gas chromatographic procedure for the determination of unconjugated and conjugated 4-hydroxydiphenylhydantoin (4-OH-DPH) in urine has been developed. Unconjugated 4-OH-DPH is determined after selective extraction with toluene-ether (1:1). For the assay of conjugated 4-OH-DPH, the urine is pre-extracted with isoamylalcohol before acid hydrolysis to avoid interference by the dihydrodiol metabolite of DPH. The sensitivity of the method is 0.1 mug per ml. The method has been used to determine the urinary metabolites in two adult volunteers, during steady state plasma concentrations of DPH and in the elimination phase.

In Dutch-belted rabbits, pregnancy caused several-fold decrease of in vitro hepatic microsomal aminopyrine, benzphetamine, and hexobarbital biotransformations. In pregnant Sprague-Dawley rats, various kinds of expressing the in vitro rates of hexobarbital biotransformation (per mg of microsomal protein, g of liver, 100 g of body weight) indicated unchanged or slightly elevated microsomal enzyme activity. In vivo, the course of hexobarbital blood levels after i.p. hexobarbital sodium, 100 mg/kg, indicated that the fate of hexobarbital was not primarily determined by the small changes of microsomal enzyme activity but, rather, by changed hexobarbital distribution. Different ways of expressing in vitro rates of aniline biotransformation showed decreased or unchanged enzyme activity during pregnancy and in vivo experiments indicated that these changes did not affect aniline metabolism in living rats. The results pointed out marked species differences in the effect of pregnancy on drug metabolism. Interpretation of in vitro biotransformation data for living animals suggested that with different substrates, microsomal enzyme activity and distribution, respectively, may exert different effects playing either significant or apparently minor role in drug disposition.

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