Quantitation of levodopa and carbidopa in rat plasma by LC-MS/MS: The key role of ion-pairing reversed-phase chromatography

A simple and selective bioanalytical method was developed for simultaneous determination of levodopa and carbidopa in rat plasma by LC-MS/MS. Levodopa and carbidopa are small polar molecules, posing challenges in the development of selective and efficient chromatography conditions. Perfluoropentanoic acid (PFPA), a volatile ion-pairing agent, was utilized to enhance chromatographic characteristics of both compounds in the reversed-phase mechanism. The ion-pairing chromatography played an essential role in mitigating matrix effects and achieving adequate separation between interfering background peaks and those of the analytes of interest, especially for levodopa. A 96-well based, automated liquid-liquid extraction, via the use Hamilton NIMBUS liquid handlers, was developed. Butyl alcohol, when mixed with ethyl acetate, greatly increased the recovery of both levodopa and carbidopa. The addition of PFPA further enhanced recovery for both analytes. Sodium metabisulfite, an antioxidant, was used to stabilize levodopa and carbidopa in rat plasma. The method was validated in the ranges of 50-10,000 ng/mL and 25-5000 ng/mL for levodopa and carbidopa, respectively, using levodopa-d3 and carbidopa-d3 as internal standards. The validated method was successfully applied to analyze rat plasma samples from in-life studies.