Quantification of sifuvirtide in monkey plasma by an on-line solid-phase extraction procedure combined with liquid chromatography/electrospray ionization tandem mass spectrometry

A simple, automated and rapid method has been developed for the determination of a novel antiviral peptide sifuvirtide in monkey plasma. Raw plasma samples were directly loaded onto an on-line solid-phase extraction (SPE) column, which removes the time-consuming and laborious sample pretreatment. Following a timed valve-switching event, the analyte was eluted on-line to a reversed-phase high-performance liquid chromatography (RP-HPLC) column and subsequently introduced into a linear ion trap mass spectrometer, LTQ-MS, via an electrospray ionization (ESI) interface. The multiply charged peptides were specified and quantitatively analyzed using selective reaction monitoring (SRM). A highly pure four iodine-sifuvirtide was synthesized using an optimized iodogen method and proved to be a suitable internal standard (IS). A single analysis run takes about 18 min. Validation of the method demonstrated that the linear calibration curves covered the range of 4.88-5000 ng/mL, and the correlation coefficients were above 0.9923. The limit of detection (LOD) with the signal-to-noise (S/N) ratio higher than 12 was calculated as 1.22 ng/mL. The intra- and inter-batch precisions were less than 12.7% and 9.1%, and the mean accuracy ranged from -5.2% to 3.6%, respectively. Any carry-over effect from the system was negligible. In a pharmacokinetic (PK) study of sifuvirtide after a single intravenous or subcutaneous dose in monkeys, the on-line SPE-LC/MS/MS system was successfully utilized to determine hundreds of samples with only one extraction column, which indicated the feasibility and the reliability of this method for application in preclinical and clinical PK studies of peptide drugs.