Melanochrome-based colorimetric assay for quantitative detection of levodopa in co-presence of carbidopa and its application to relevant anti-Parkinson drugs.

In this paper is reported the selective detection and quantification of levodopa in co-presence of carbidopa. The method took advantage of the spontaneous oxidation and color development of levodopa at basic pH here driven by alkaline earth cations and co-solvent in solution. We have shown for the first time the generation and stabilization of the purple melanochrome from levodopa, by using magnesium acetate and dimethyl sulfoxide, which was here exploited for the development of a quantitative colorimetric assay for the active principle ingredient in commercial drugs for the treatment of Parkinson's disease. The calibration curves of levodopa in the two tablet formulations, containing carbidopa as decarboxylase inhibitor, showed a common linear trend between 10 mg L and 40 mg L with levodopa alone or in combination with carbidopa in standard solutions, with very good reproducibility (CV%, 3.3% for both brand and generic drug) and very good sensitivity, with limit of quantification about 0.6 mg L in any case. The colorimetric method here developed is very simple and effective, appearing as a rapid and low-cost alternative to other methodologies, involving large and expensive instrumentations, for drug estimation and quality control of pharmaceutical formulations.