Effects of phenytoin on the in vivo kinetics of thiamine and its phosphoesters in rat nervous tissues.

The in vivo effects of chronic (30 days) and subchronic (10 days) intragastric treatment with phenytoin (PHT) (500 mg/kg) b.wt., suspended in 10% arabic gum water solution) on the uptake and metabolism of thiamine (T), T monophosphate (TMP) and T pyrophosphate (TPP) were evaluated in rat nervous regions (cerebral cortex, brainstem, cerebellum and sciatic nerve) by determining the radioactivity of T and its phosphoesters in plasma and tissues at fixed time intervals (0.25-240 h) after an i.p. injection of thiazole-[2-14C]thiamine (30 micrograms: 1.25 microCi). A nutritionally adequate diet containing T in excess was given to the animals in order to produce a virtually stable content of T compounds in the tissues. Analytical data were processed by using a compartmental model which allowed the calculation of fractional rate constants (FRC), turnover rates (TR) and turnover times. Compared with vehicle-treated controls, animals treated chronically with PHT exhibited lower levels of radiolabelled T compounds in all nervous regions except for the cerebral cortex. These alterations were not found in animals receiving subchronic treatment. Evaluation of FRC values indicated that PHT-induced effects on T metabolism differed depending on the length of PHT treatment and the nervous region considered. Overall, PHT appeared to interfere mainly with T and TMP uptake, TPP dephosphorylation to TMP and TPP turnover times, these effects being particularly prominent in the cerebellum and in the brainstem of chronically treated animals. Since all changes in T uptake and metabolism were observed in the absence of overt behavioural toxicity, these findings may have potential clinical relevance in highlighting possible mechanisms by which PHT therapy can alter brain metabolism.