Chromatographic methods development, validation and degradation characterization of the antithyroid drug Carbimazole

The current paper reports the development and validation of stability-indicating HPLC and HPTLC methods for the separation and quantification of main impurity and degradation product of Carbimazole. The structures of the degradation products formed under stress degradation conditions, including hydrolytic and oxidative, photolytic and thermal conditions, were characterized and confirmed by MS and IR analyses. Based on the characterization data, the obtained degradation product from hydrolytic conditions was found to be methimazole-impurity A of Carbimazole as reported by the British Pharmacopeia and the European Pharmacopeia. A stability-indicating HPLC method was carried out using a Zorbax Eclipse Plus CN column (150 × 4.6 mm i.d, 5 μm particle size) and a mobile phase composed of acetonitrile-0.05 m KH₂ PO₄ (20: 80, v/v) in isocratic elution, at a flow rate of 1 mL/min. The method was proved to be sensitive for the determination down to 0.5% of Carbimazole impurity A. Additionally, a stability-indicating chromatographic HPTLC method was achieved using cyclohexane-ethanol (9:1, v/v) as a developing system on HPTLC plates F₂₅₄ with UV detection at 225 nm. The proposed HPLC and HPTLC methods were successfully applied to Carbimazole^® tablets with mean percentage recoveries of 100.12 and 99.73%, respectively.