Simultaneous quantitation of levodopa and 3- O-methyldopa in human plasma by HPLC–ESI-MS/MS: Application for a pharmacokinetic study with a levodopa/benserazide formulation

A sensitive and simple method was developed for the quantitation of levodopa and its metabolite 3- O-methyldopa, in human plasma, after oral administration of tablet formulations containing levodopa (200 mg) and benserazide (50 mg). The analytes were extracted by a protein precipitation procedure, using carbidopa as an internal standard. A mobile phase consisting of 0.2% formic acid and acetonitrile (94:6, v/v) was used and chromatographic separation was achieved using ACE C 18 column (50 mm × 4.6 mm i.d.; 5 μm particle size). Selected reaction monitoring was performed using the fragmentation transitions m/ z 198 → m/ z 107, m/ z 212 → m/ z 166 and m/ z 227 → m/ z 181 for levodopa, 3- O-methyldopa and carbidopa, respectively. Calibration curves were constructed over the range 50.0–6000.0 ng/mL for levodopa and 25.0–4000.0 ng/mL for 3- O-methyldopa. The method shown to be specific, precise, accurate and provided recovery rates higher than 85% for all analytes. No matrix effect was detected in the samples. The validated method was applied in a pharmacokinetic study with a levodopa/benserazide tablet formulation in healthy volunteers.