Simultaneous high-performance liquid chromatographic analysis of carbidopa, levodopa and 3-O-methyldopa in plasma and carbidopa, levodopa and dopamine in urine using electrochemical detection.

Two assay procedures are described for the analysis of levodopa, carbidopa and 3-O-methyldopa in plasma and levodopa, carbidopa and dopamine in urine. The methods are suitable for quantifying the analytes following therapeutic administration of levodopa and carbidopa. Both were based on reversed-phase high-performance liquid chromatography (HPLC) with electrochemical detection and with methyldopa as the internal standard. Plasma samples were prepared by perchloric acid precipitation followed by the direct injection of the supernatant. Urine was prepared by alumina adsorption, and the analytes were desorbed with perchloric acid solution containing disodium EDTA and sodium metabisulfite prior to injection into the HPLC system. The methods have been utilized to evaluate the pharmacokinetics and bioavailability of oral dosage forms containing levodopa and carbidopa.