Serum protein binding of 25 antiepileptic drugs in a routine clinical setting: A comparison of free non–protein‐bound concentrations

Summary Objective Given that only the free non–protein‐bound concentration of an antiepileptic drug (AED) crosses the blood–brain barrier, entering the brain and producing an antiepileptic effect, knowledge and measurement of the free drug fraction is important. Such data are sparse, particularly for newer AEDs, and have arisen from the use of disparate methodologies and settings over the past six decades. We report on the protein binding of 25 AEDs that are available for clinical use, along with two pharmacologically active metabolites (carbamazepine‐epoxide and N‐desmethyl clobazam), using standardized methodology and under set conditions. Methods The protein binding of the various AEDs was undertaken in sera of 278 patients with epilepsy. Separation of the free non–protein‐bound component was achieved by using ultracentrifugation (Amicon Centrifree Micropartition System) under set conditions: 500 μl serum volume; centrifugation at 1,000 g for 15 min, and at 25°C. Free and total AED concentrations were measured by use of fully validated liquid chromatography/mass spectroscopy (LC/MS) techniques. Results Gabapentin and pregabalin are non–protein‐bound, whereas highly bound AEDs (≥88%) include clobazam, clonazepam, perampanel, retigabine, stiripentol, tiagabine, and valproic acid as well as the N‐desmethyl‐clobazam (89%) metabolite. The minimally bound drugs (