Chiral liquid chromatography-tandem mass spectrometry assay to determine that dexpramipexole is not converted to pramipexole in vivo after administered in humans

Dexpramipexole (DEX) was being investigated in clinical studies for the treatment of amyotrophic lateral sclerosis (ALS). To monitor the potential chiral interconversion of dexpramipexole to pramipexole (PPX) in vivo, a highly sensitive and selective chiral LC-MS/MS assay was developed and qualified for the detection of pramipexole in the presence of dexpramipexole in human plasma. In this assay, plasma samples were extracted by protein precipitation coupled with solid phase extraction (SPE). The analyte PPX was separated from its enantiomer DEX using a chiral HPLC method. The assay was qualified with a dynamic range of 0.150-1.00 ng/mL. The lower limit of quantitation (LLOQ) for PPX was 0.150 ng/mL in the presence of up to 1000 ng/mL of DEX. The qualified method was used to analyze plasma samples from a DEX clinical study. No PPX was detected in humans at pharmacologically significant levels after administration of dexpramipexole at single doses up to 600 mg per day.