A new HILIC-MS/MS method for the simultaneous analysis of carbidopa, levodopa, and its metabolites in human plasma

• This HILIC-MS/MS method is the first for simultaneous determination of levodopa, carbidopa, 3-o-methyldopa, and dopamine in human plasma.
• A fast and simple sample preparation procedure using protein precipitation was also developed.
• The method was fully validated according to US FDA and ANVISA bioanalytical method validation guidelines.
• The method has been used successfully for therapeutic drug monitoring.

Monitoring of the plasmatic levels of levodopa (LEV) and carbidopa (CAR) is necessary to adjust the dose of these drugs according to the individual needs of Parkinson's disease patients. To support drug therapeutic monitoring, a method using HILIC mode and LC–MS/MS was developed for the simultaneous determination of carbidopa, levodopa, and its metabolites (3-o-methyldopa (3-OMD) and dopamine (DOPA)) in human plasma. A triple quadrupole mass spectrometry was operated under the multiple reaction-monitoring mode (MRM) using the electrospray ionization technique. After straightforward sample preparation via protein precipitation, an Atlantis HILIC (150 × 2.1 mm, 3 μm, Waters, USA) column were used for separation under the isocratic condition of acetonitrile/water (79:21, v/v) containing 0.05% formic acid and 3 mmol/L ammonium formate and the total run time was 7 min. Deuterated LEV was used as internal standard for quantification. The developed method was validated in human plasma with a lower limit of quantitation of 75 ng/mL for LEV, 65 ng/mL for CAR and 3-OMD, and 20 ng/mL for DOPA. The calibration curve was linear within the concentration range of 75–800 ng/mL for LEV, 65–800 ng/mL for CAR and 3-OMD, and 20–400 ng/mL for DOPA (r > 0.99). The assay was accurate and precise, with inter-assay and intra-assay accuracies within ±13.44% of nominal and inter-assay and intra-assay precision ≤ 13.99%. All results were within the acceptance criteria of the US FDA and ANVISA guidelines for method validation. LEV, CAR, 3-OMD and DOPA were stable in the battery of stability studies, long-term, bench-top, autosampler, and freeze/thaw cycles. Samples from patients undergoing treatment were analyzed, and the results indicated that this new method is suitable for therapeutic drug monitoring in Parkinson's disease patients.