The inhibition of DPP-4 by Plantago asiatica L. and the discovery of active compounds

The objective of this study was to investigate the inhibitory effects of Plantago asiatica L. on DPP-4 and to determine the active compounds responsible for this inhibition. Extracts were prepared from the root and aerial portions of the plant using various polar solvents. The DPP-4 inhibition of these extracts was analyzed in an extracellular assay and then assigned to either the high or low inhibition group. The chemical profiles of these extracts were analyzed using an LC-MS method. The ion fragments that were potent in the high inhibition extracts were selected through OPLS-DA and correlation analysis, and then identified and obtained for verification of DPP-4 inhibition in both the extracellular assay and the Caco-2 cell line. The extracts showed various inhibitory activities with an average inhibition of 28.4 %. A total of 20 compounds were selected from the extracts with higher inhibitory activity and identified. Six of them, baicalein, scutellarein, calceolarioside B, isoacteoside, verbascoside, and plantamajoside, exhibited DPP-4 inhibition with IC₅₀ values of 44.32, 59.90, 112.71, 157.02, 154.58, and 195.07 μg/mL, respectively, in extracellular assays. They also showed inhibition of DPP-4 on the Caco-2 cell line, with IC₅₀ values of 31.56, 34.71, 20.29, 33.29, 24.52, and 87.88 μg/mL, respectively. The extracts from PAL showed inhibitory activity towards DPP-4, and the related compounds were identified and verified.