Hydrophilic interaction liquid chromatography-tandem mass spectrometry analysis of oligosaccharides and iridoid glycosides in rat plasma: Pharmacokinetic characterization of previously overlooked oligosaccharides from Radix Rehmanniae

Radix Rehmanniae (RR) is a widely used herb in traditional Chinese Medicine with properties of tonifying the kidneys and nourishing the blood. Both raw and processed RR are effective for the treatment of diabetes in clinical practice. Oligosaccharides and iridoid glycosides are the primary active components responsible for the anti-diabetic effects of RR. In this study, a rapid and sensitive hydrophilic interaction liquid chromatography-tandem mass spectrometry (HILIC−MS/MS) method was developed for simultaneous determination of oligosaccharides (raffinose, manninotriose and stachyose) and iridoid glycosides (catalpol and ajugol) in rat plasma. Significant analytical challenges were encountered during method development, including distinct retention behaviors of oligosaccharides and iridoid glycosides, low ionization and extraction efficiency of oligosaccharides, thermal instability of catalpol and reduced column performance. The strategies to overcome these challenges were presented by optimizing chromatographic separation, mass spectrometric detection and sample preparation. The best separation was achieved using an Accucore-150-Amide-HILIC column (100 mm × 2.1 mm, 2.6 μm) at 50 °C with mobile phase consisted of acetonitrile and ammonium acetate (2.5 mM) under gradient elution. Ammonium adduct ions produced by positive electrospray ionization were chosen as precursor ions for multiple reaction monitoring transitions. The established HILIC−MS/MS method exhibited good linearity (r > 0.9937) with the lower limits of quantification of 0.01–0.2 μg/mL using only 50 µL of plasma sample. The method was successfully applied to pharmacokinetic characterization of oligosaccharides and iridoid glycosides in normal and type 2 diabetic rats following intragastric administration of raw and processed RR extracts.