Impacts of Storage Conditions and Synergistic Degradation on the Stability of Levodopa and Carbidopa in Aqueous Solutions

Levodopa, which is usually used in combination with carbidopa, is the gold standard medication in relieving motor symptoms of Parkinson’s disease. There is substantial evidence showing the drugs' stability issues; hence, understanding kinetics and factors affecting drug degradation is of interest. This study highlighted the effects of antioxidants, heat, pH, buffer types, and illumination on the degradation of the two compounds in aqueous solutions. Experimental data showed that levodopa and carbidopa were stable at pH ≤ 5.5 and ≤ 3.0, respectively. Illumination, heat, and neutralized pH accelerated the degradation of the drugs. Carbidopa stability was enhanced when antioxidants (e.g. sodium thiosulfate, disodium ethylenediamine tetraacetic acid) were introduced. The two drugs accelerated the degradation of each other, whereas carbidopa was more susceptible to degradation than levodopa. The presence of levodopa increased carbidopa degradation rate constant by approximately 4 times and 2 times under light shielding and illumination, respectively.