Postmortem serum protein binding and brain concentrations of antiepileptic drugs in autoptic specimens from 45 epileptic patients

The free fraction of antiepileptic drugs can, in certain diseases, be greatly increased in the serum. In order to study the significance of this increase for the concentration in the brain, the postmortem concentrations of phenytoin (PT), phenobarbital (PB), carbamazepine (CBZ), and its metabolite carbamazepine-10,11-epoxide (CE) in the serum (total and free), as well as in specimens of the frontal cortex of 45 epileptic patients who died from various causes, were determined. The postmortem free fractions were higher than reported in the literature and varied considerably from subject to subject. For PT the free fraction was 21.7% (median), for PB 68.0%, for CBZ 33.4%, and for its metabolite CE 53.6%. The values for a control group of 236 otherwise healthy epileptic patients were in agreement with those given in the literature, namely 10.4% for PT, 55.6% for PB, 20.9% for CBZ, and 42.5% for CE. Using a nonparametric correlation coefficient (Kendall T), the concentrations in the frontal cortex of the autopsied patients correlated with the postmortem free serum concentrations, especially for the substances with high protein binding (PT and CBZ), better (PT r = 0.88, PB r = 0.86, CBZ r = 0.87, CE r = 0.79) than with the total concentrations (PT r = 0.69, PB r = 0.80, CBZ r = 0.77, CE r = 0.77). The study indicates that in critically ill patients the determination of the free concentration in serum is indispensable. If treatment is orientated solely on the total concentration, unexpectedly high concentrations in the brain and hence possible intoxication of the patient in the critical or final state can result.