The effect of tenidap sodium on the disposition and plasma protein binding of phenytoin in healthy male volunteers.

1. The effects of tenidap sodium 120 mg day‐1 at steady state and placebo on the plasma protein binding and pharmacokinetics of phenytoin were compared in this randomised, double‐blind, placebo‐controlled, parallel‐group study, involving 12 healthy young men, conducted over 34 days. 2. Single oral doses of phenytoin 200 mg were given on days 1‐3 and 29‐31, and intravenous phenytoin, 250 mg infused over 20 min, was given on days 4 and 32. Tenidap (120 mg day‐1), or matching placebo, was administered as single oral daily doses from days 8 to 34 inclusive. 3. The plasma protein binding of phenytoin was determined immediately before oral phenytoin administration on days 1 and 29. Pharmacokinetic parameters were estimated from the serum phenytoin concentration‐time curves derived on days 4 and 32 following the phenytoin infusions. The differences between the pre‐ and post‐ treatment mean percentage of unbound plasma phenytoin and mean pharmacokinetic parameters were compared between treatment groups. 4. Tenidap sodium 120 mg day‐1, at steady state, increased the percentage of unbound phenytoin in plasma by approximately 25%, but did not significantly affect AUC(0,48h) or Cmax. 5. Since tenidap increases the percentage of unbound phenytoin in plasma, when monitoring phenytoin plasma concentrations free concentrations of phenytoin should be considered. 6. Tenidap was well tolerated throughout the study. 1995 The British Pharmacological Society