Liquid chromatographic determination of levodopa, levodopa-carbidopa, and related impurities in solid dosage forms

A rapid reverse phase liquid chromatographic method was developed for the determination of levodopa and levodopa-carbidopa in tablets and capsules. The method also separates these drugs from 3-(3,4,6-trihydroxyphenyl)alanine and methoxytyrosine, impurities of levodopa, and from methyldopa and 3-O-methylcarbidopa, impurities of carbidopa. The mobile phase was 3% acetic acid and the detection wavelength was 280 nm. The method was linear over the concentration range 0.05-0.40 mg levodopa/mL, 0.01-0.06 mg carbidopa/mL, 0.9-12.8 micrograms 3-(3,4,6-trihydroxyphenyl)alanine/mL, 0.7-3.1 micrograms methyldopa/mL, 5-20 micrograms methoxytyrosine/mL, and 0.5-3.3 micrograms 3-O-methylcarbidopa/mL. Mean recoveries (%) for spiked commercial tablets were: levodopa 100.3, carbidopa 100.4, 3-(3,4,6-trihydroxyphenyl)alanine 99.1, methoxytyrosine 100.0, methyldopa 100.0, and 3-O-methylcarbidopa 99.4.