Phenytoin (PHT) exhibits linear and Michaelis-Menten pharmacokinetics. PHT decreases serum folate; the vitamin folic acid (FA) is hypothesized to be a cofactor in the metabolism of PHT. The depletion of serum folate may explain the unpredictability of measured total serum PHT concentrations and time to steady state as compared with the Michaelis-Menten predictive calculations. We examined PHT pharmacokinetics before and after FA supplementation in 13 healthy male volunteers. The study was divided into two phases. Phase I determined V(max) (mg/day) and Km (micrograms/ml) of PHT to calculate PHT doses needed for the second phase. Phase II was a four-way cross-over study to examine the effect of 1 and 5 mg FA on total serum PHT concentrations 1 microgram/ml less and 5 micrograms/ml greater than the subject's Km, Km - 1 and Km + 5, respectively. Predicted versus measured total serum PHT concentrations, t90% (days to steady state), and the effect of FA were calculated for Km - 1 and Km + 5 before and after 1 or 5 mg FA. The measured total serum PHT concentration was always greater than the calculated concentration (p less than 0.05), and t90% was always longer than the calculated t90% (p less than 0.05) for Km - 1 before FA (all subjects decreased serum FA); the same was observed for Km + 5. If folate is assumed to be a cofactor in PHT metabolism, these results are expected, because depletion of the vitamin would indicate less folate to drive the metabolism of PHT, resulting in higher total serum PHT concentrations and longer time to reach steady state.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of phenytoin (PHT) on serum folate and the effect of additional oral folic acid (FA) on serum folate during continued treatment with PHT were studied in 13 healthy male subjects 20-35 years of age. The study was divided into two phases: Phase I determined Vmax (mg/kg/day) and Km (microgram/ml) of PHT in order to calculate the PHT doses needed for the second phase. Phase II was a four-way cross-over study to examine the effect of 1 and 5 mg FA on total serum PHT concentrations 1 microgram/ml less and 5 micrograms/ml greater than the subject's Km, Km-1 and Km+5, respectively. Both phases examined the effect of PHT on serum folate. In Phase I, serum folate decreased by a mean and standard deviation of 42.15 +/- 21.44% after an average of 24.15 +/- 5.63 days of PHT administration, with a mean steady-state total serum PHT concentration of 8.45 +/- 2.70 micrograms/ml. Mean percentage decreases in serum folate before the addition of 1 and 5 mg FA in Phase II were 12.80 +/- 31.45% and 23.24 +/- 21.24% for Km-1 and Km+5, respectively. The average numbers of days of PHT administration and total serum PHT concentrations before FA administration were 9.52 +/- 3.34 and 15.84 +/- 7.02 days, and 2.60 +/- 2.18 and 8.64 +/- 3.44 micrograms/ml, for Km-1 and Km+5, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

The purpose of this two-part review is to explain current drug treatment in part I and discuss investigational drug therapy and miscellaneous drugs in the management of parkinsonism in part II. The medical approach to this disease is still based on the imbalance between a deficiency of dopamine and a functional increase in acetylcholine. Anticholinergic agents are used to treat the tremors in the early stages of the disease. Drug In/ell Cfin Pharm 1987;2/:10-21.

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