Determination of progabide and its main acid metabolite in biological fluids using high-performance liquid chromatography and electrochemical detection: Application to the measurement of blood/plasma partition ratio

A method for the measurement in plasma, blood and urine of progabide, its main acid metabolite, and the corresponding benzophenone is described. This assay allows the determination of progabide and its acid metabolite for therapeutic drug monitoring, and with a minimum detectable concentration of 1–10 ng/ml for progabide and its acid metabolite, it is sensitive enough for pharmacokinetic studies. Progabide and its metabolites are extracted from biological samples with toluene at pH 4.5. Following reduction of the imine bond with sodium borohydride, the reduced drugs are back-extracted into an aqueous phase at acid pH and reextracted by diethyl ether at alkaline pH. Progabide, its acid metabolite and the benzophenone are separated by high-performance liquid chromatography using a 3-μm ODS column with a quaternary solvent mixture of methanol-acetonitrile-phosphate buffer (0.033 M, pH 5.5)—sodium chloride (1.5 M) (30:30:40:9, v/v), and detected electrochemically at a potential of +850 mV vs. an Ag/AgCl electrode. Antiepileptic drugs like carbamazepine, carbamazepine epoxide, phenytoin, valproic acid and ethosuximide do not interfere with the assay. Blood/plasma partition ratios of 0.69 and 0.55 for progabide and its acid metabolite, respectively, indicate that the former but not the latter is present in red blood cells.