In vitro and in vivo distribution and binding of phenytoin to rat brain.

The distribution and binding of phenytoin (PHT) were studied in rat brain using anatomically intact tissue. The pattern and kinetics of PHT distribution in vivo were examined with quantitative carbon-14 autoradiography. Initially gray matter levels were greater than white matter levels, but after 30 min the opposite condition was found. At a given time point, the levels of PHT among various gray matter structures as a group or among white matter structures as a group were uniform. The ratio of radioactivity in white matter to that in gray matter was 3:1 120 min after injection of radiolabeled PHT. Thin-layer chromatography showed only PHT in the brain 15 min after injection of PHT and both PHT and its hydroxylated metabolite [5-(p-hydroxyphenyl)-5-phenylhydantoin] 120 min after injection. The proportions of PHT and 5-(p-hydroxyphenyl)-5-phenylhydantoin were the same in white as in gray matter. Direct chemical measurements of brain samples obtained after coinjections of tracer amounts of radiolabeled PHT with increasing doses of unlabeled PHT corroborated the autoradiographic findings and revealed no displacement of the tracer by pharmacologic doses of the unlabeled drug. In vitro binding of PHT was investigated with sections obtained from frozen brains and with physiologically intact brain slices. Both high (4-10 nM) and low (1 microM) affinity conditions were examined. In no case was specific binding detected. Binding was greater in gray matter than in white matter in sections, whereas binding was greater in white matter than in gray matter in slices. We conclude that PHT distribution and binding reflect physico-chemical factors, such as lipid content, and physiological factors, such as blood flow and selective partitioning into white matter in living tissue, rather than specific receptors.