Fluctuations of unbound and total phenytoin concentrations during the day in epileptic patients on valproic acid comedication.

The influence of daily fluctuations in the concentration of valproic acid (VPA) on the unbound and total concentration of phenytoin (PT) was examined in a prospective study. The serum concentrations of 28 patients with epilepsy (group PT + VPA) who were treated with PT and concurrently with VPA and 15 patients (group PT) who were treated with PT but without VPA comedication were determined at 8.00, 11.00, 14.00, 17.00, and in part at 20.00 h. The results show that there are significantly greater fluctuations in the total PT concentration among patients on VPA than those not on VPA. The fluctuations in the total PT concentration during the day correlated with the fluctuations in the VPA concentration. On the other hand, the fluctuations in the concentrations of unbound PT of patients on VPA were comparable with those not on VPA. In the absence of VPA, the diurnal fluctuations of the total PT concentration correlated highly significantly with the fluctuations of the unbound PT concentration. This is not the case in patients on VPA. Our data are a further indication of the smaller significance of the total PT concentration as compared with the unbound PT concentration in the combined treatment of PT and VPA.