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

Quantitative Colorimetric Sensing of Carbidopa in Anti-Parkinson Drugs Based on Selective Reaction with Indole-3-Carbaldehyde

The quality of life of patients affected by Parkinson's disease is improved by medications containing levodopa and carbidopa, restoring the dopamine concentration in the brain. Accordingly, the affordable quality control of such pharmaceuticals is very important. Here is reported the simple and inexpensive colorimetric quantification of carbidopa in anti-Parkinson drugs by the selective condensation reaction between the hydrazine group from carbidopa and the formyl functional group of selected aldehydes in acidified hydroalcoholic solution. An optical assay was developed by using indole-3-carbaldehyde (I3A) giving a yellow aldazine in EtOH:H2O 1:1 (λmax~415 nm) at 70 °C for 4 h, as confirmed by LC-MS analysis. A filter-based plate reader was used for colorimetric data acquisition, providing superior results in terms of analytical performances for I3A, with a sensitivity ~50 L g-1 and LOD ~0.1 mg L-1 in comparison to a previous study based on vanillin, giving, for the same figures of merit values, about 13 L g-1 and 0.2-0.3 mg L-1, respectively. The calibration curves for the standard solution and drugs were almost superimposable, therefore excluding interference from the excipients and additives, with very good reproducibility (avRSD% 2-4%) within the linear dynamic range (10 mg L-1-50 mg L-1).

Sensing of Catecholamine in Human Urine Using a Simple Colorimetric Assay Based on Direct Melanochrome and Indolequinone Formation

We used the first enzyme-free synthesis and stabilization of soluble melanochrome (MC) and 5,6-indolequinone (IQ) derived from levodopa (LD), dopamine (DA), and norepinephrine (NE) oxidation to develop a simple colorimetric assay for catecholamine detection in human urine, also elucidating the time-dependent formation and molecular weight of MC and IQ using UV-Vis spectroscopy and mass spectrometry. The quantitative detection of LD and DA was achieved in human urine using MC as a selective colorimetric reporter to demonstrate the potential assay applicability in a matrix of interest in therapeutic drug monitoring (TDM) and in clinical chemistry. The assay showed a linear dynamic range between 5.0 mg L^-1 and 50.0 mg L^-1, covering the concentration range of DA and LD found in urine samples from, e.g., Parkinson's patients undergoing LD-based pharmacological therapy. The data reproducibility in the real matrix was very good within this concentration range (RSD_av% 3.7% and 6.1% for DA and LD, respectively), also showing very good analytical performances with the limits of detection of 3.69 ± 0.17 mg L^-1 and 2.51 ± 0.08 mg L^-1 for DA and LD, respectively, thus paving the way for the effective and non-invasive monitoring of dopamine and levodopa in urine from patients during TDM in Parkinson's disease.

Colorimetric determination of carbidopa in anti-Parkinson drugs based on 4-hydroxy-3-methoxybenzaldazine formation by reaction with vanillin

In this paper is reported the selective colorimetric detection and quantification of carbidopa, an inhibitor of aromatic amino acid decarboxylase, in the co-presence of levodopa as dopamine precursor in pharmaceutical formulations for the treatment of Parkinson's disease. The method is based on the selective condensation reaction between the hydrazine group from carbidopa and the formyl functional group of vanillin, a natural flavoring agent, in acidified alcoholic solution. The yellow color development (λmax ~ 420 nm) due to the formation of 4-hydroxy-3-methoxybenzaldazine (HMOB) was observed for carbidopa only, whereas levodopa, lacking the hydrazine group, did not color the solution, as expected. The calibration curves for two tablet formulations of levodopa in combination with carbidopa (4:1) were superimposable with levodopa/carbidopa (4:1), as well as carbidopa alone, in standard solution, i.e., the excipients and additives did not interfere with carbidopa determination, corresponding to a mean recovery about 105%. The linear dynamic range was between 5.00 and 50.0 mg L-1 with very good reproducibility within this range (CVav% about 3-4%) and very good sensitivity, with limits of quantification of about 1 mg L-1. The colorimetric method developed here is very simple, inexpensive, and effective for drug estimation and quality control of pharmaceutical formulations.

Colorimetric selective quantification of anthocyanins with catechol/pyrogallol moiety in edible plants upon zinc complexation

Here is examined the colour development from common anthocyanins (i.e., cyanidin, delphinidin, malvidin, and pelargonidin glycosides) and from anthocyanins-rich extracts (i.e., bilberries, strawberries, and raspberries), using zinc-anthocyanin complexes as molecular probe. We have observed the absorbance increase in the blue region in presence of large excess of zinc ion at acidic pH for cyanidin and delphinidin derivatives, likely due to quinoidal base stabilization from catechol and pyrogallol moiety. The assay condition were studied and applied to natural extracts containing these compounds. The same behaviour was observed for bilberry and, to a minor extent, for raspberry extracts, due to the larger cyanidin/delphinidin contents in the former than in the latter. Anthocyanin standard UV-Vis analysis in buffer has shown a very good linear correlation for cyanidin and delphinidin (R² = 0.995 and 0.997, respectively), good precision (CV% = 7.4% and 5.3% respectively), high sensitivity (Cyε_600nm = 8300 M^-1 cm^-1, LOD = 0.264 ± 0.005 mg L^-1, LOQ = 0.478 ± 0.007 mg L^-1, and Dpε_600nm = 15,900 M^-1 cm^-1, LOD = 0.143 ± 0.002 mg L^-1, LOQ = 0.478 ± 0.007 mg L^-1). The effectiveness of this colorimetric method for the selective quantification of catechol/pyrogallol-based anthocyanins has been demonstrated in the aforementioned complex real matrices and compared to LC-MS/MS analysis and pH-differential method, offering a valuable tool to characterize plant and food extracts particularly rich in zinc-coordinating anthocyanins.