Identification of Novel Epimer Metabolites of Sweroside in Rats With Liquid Chromatography/Time of Flight Mass Spectrometry Combined With Picolinoyl Derivatization

Sweroside (SRD) is an important iridoid compound that exhibits various pharmacological and biological activities. However, it has been suggested that the remarkable pharmacological activity is due to the metabolites of the iridoids rather than themselves. Therefore, it is crucial to identify the actual bioactive metabolites of iridoids in vivo. The in vivo metabolites of SRD were analyzed using ultra-high performance liquid chromatography-time of flight-mass spectrometry (UHPLC-TOF-MS) and picolinoyl derivatization. Two major exosomatic SRD metabolites derived from human intestinal bacteria, epinaucledal and naucledal, were incubated with liver microsomes, which were reduced to swerosimetabolin A (SMA) and a novel metabolite named swerosimetabolin B (SMB), respectively. Pharmacokinetic studies of SRD were performed Wistar rats. The mass detection sensitivity of the epimer metabolites SMA and SMB using UHPLC-TOF-MS increased apparently following picolinoyl derivatization. These two metabolites distinctly existed within the rat plasma after gavage administration of SRD. The concentrations of SMA and SMB in plasma were analyzed using UHPLC-TOF-MS in positive ion mode, with 3,4-dihydro-5-(hydroxymethyl) isochroman-1-one as an endogenous reference. SMA and SMB achieved maximal mean plasma concentrations of 61.98 ± 8.6 ng/mL (within 2 h) and 42.64 ± 9.6 ng/mL (within 4 h), respectively, following gavage administration of 20 mg/kg SRD. The area under the concentration-time curve for SMB was 1.5-fold higher than that of SMA. Furthermore, the SRD metabolism to the sulfate of SMA and SMB was deduced. This study reports the metabolism of SRD into SMA and SMB in vivo. These metabolites demonstrated high pharmacological activity, which can help clarify the effects of SRD.