Phenytoin and Folic Acid Interaction

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Abstract
The effect of folic acid (1 mg/day orally) on phenytoin steady-state pharmacokinetics was studied in 4 male folate-deficient epileptic patients who were treated with only 1 anticonvulsant. Each patient served as his own control before and after starting folic acid replacement therapy. The Michaelis-Menten parameters, Vmax [maximum metabolic rate] and Km, were calculated for each patient, and compliance with the single anticonvulsant drug (phenytoin) regimen was documented. Blood and urine samples were collected just before (day 1) and after 180 or 300 days of vitamin administration. Total and free phenytoin were measured in plasma; and phenytoin, 5-(p-hydroxyphenol)-5-phenylhydantoin (p-HPPH), 5-(3,4-dihydroxyphenyl)-5-phenylhydantoin (CAT) and 5-(3,4-dihydroxy-1,5-cyclohexadienyl)-5-phenylhydantoin (DHD) were measured in 24-h urine. After the addition of folic acid, total phenytoin plasma concentration decreased 7.5-47.6% in 3 of the 4 patients, and the extent of this change correlated with Km (r2 = 0.99). Ratios of urinary metabolites to parent drug increased in those patients showing a decrease in plasma phenytoin caused by folic acid supplementation. This indicated that a folic acid-associated increase in phenytoin oxidative metabolism had occurred.

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