Simultaneous determination of levodopa, carbidopa, 3-O-methyldopa and dopamine in plasma using high-performance liquid chromatography with electrochemical detection.

An analytical method is described which allows a fast, reliable and precise determination of levodopa and its metabolises 3-O-methyldopa and dopamine, as well as the peripheral aromatic amino acid decarboxylase inhibitor carbidopa, in a single 1 ml plasma sample of Parkinsonian patients. The compounds are quantitatively isolated on small Sephadex G-10 columns and determined by HPLC with electrochemical detection. Dihydroxybenzylamine or alpha-methyldopa are used as the internal standard. An example of therapeutic drug monitoring in a patient with fluctuations in motor performances is given. It is confirmed that interference with absorption of levodopa from the stomach by food can be partly responsible for these observed fluctuations.