Analytical method for simultaneous quantification of levodopa and carbidopa in the injectable oleogel formulation by HPLC.

The developed method for simultaneous detection of levodopa and carbidopa was able to separate the peaks of the drug and sodium bisulfite in the in-vitro release samples and stability samples. Levodopa (LD), a pro-drug of dopamine, is used as the gold standard treatment for Parkinson's disease. It is usually prescribed with carbidopa (CD) to prevent the conversion of levodopa to dopamine peripherally, thus reducing undesirable side effects. Both drugs are unstable at pH 7.4 beyond 24 h due to their oxidation, therefore 0.2% sodium bisulfite is added to the formulation as an antioxidant. The separation was performed by gradient elution using the Luna-C18 column (250 × 4.6 mm, 5 µm) at a flow rate of 1 ml/min. The mobile phase was composed of mobile phase A 30 mM potassium phosphate and acetonitrile (95:5, v/v) with 35 mM tetrabutylammonium hydrogen sulphate and mobile phase B containing 30 mM potassium phosphate and acetonitrile (50:50 v/v). Drug peaks were detected at 280 nm with retention times of 3.05 ± 0.001 min for LD and 3.64 ± 0.001 min for CD. The validation of the method according to US FDA guidelines and results were found to be within acceptable limits. The method was linear from 10-100 µg/ml (r