Licochalcone A Derivatives as Selective Dipeptidyl Peptidase 4 Inhibitors with Anti-Inflammatory Effects.

A set of 22 analogs of licochalcone A was designed and synthesized to explore their potentials as dipeptidyl peptidase 4 (DPP4) inhibitors with anti-inflammatory effects. The anti-DPP4 effects of these analogs were evaluated using the fluorescent substrate Gly-Pro-N-butyl-4-amino-1,8-naphthalimide (GP-BAN). The nitro-substituted analogue 27 exhibited the most potent activity (Ki = 0.96 µM). A structure-activity relationship investigation revealed that 4-hydroxyl and 5-chloro substituents are essential for DPP4 inhibition, while the 3'-nitro substituent improved both DPP4 inhibition and microsomal stability. Furthermore, compound 27 demonstrated good selectivity for DPP4 over other proteases, including dipeptidyl peptidase 9 (DPP9), thrombin, prolyl endopeptidase (PREP), and fibroblast activation protein (FAP). The cytotoxic effect of 27 was evaluated in cancer cell lines HepG-2 and Caco-2 and in somatic RAW264.7 cells and RPTECs. Compound 27 showed no toxicity to normal cells and weak toxicity to cancer cells. In a living cell imaging assay, 27 blocked the dipeptidase activity of DPP4 in both Caco-2 and HepG-2 cells. This compound also dose-dependently suppressed the expression levels of the chemokines tumor necrosis factor-α (TNF-α), interleukin-6 (IL-6), and interleukin-1ß (IL-1ß).

Discovery of anthraquinones as DPP-IV inhibitors: Structure-activity relationships and inhibitory mechanism.

Dipeptidyl peptidase IV (DPP-IV) is an integrated type II transmembrane protein that reduces endogenous insulin contents and increases plasma glucose levels by hydrolyzing glucagon-like peptide-1 (GLP-1). Inhibition of DPP-IV regulates and maintains glucose homeostasis, making it an attractive drug target for the treatment of diabetes II. Natural compounds have tremendous potential to regulate glucose metabolism. In this study, we examined the DPP-IV inhibitory activity of a series of natural anthraquinones and synthetic structural analogues on DPP-IV using fluorescence-based biochemical assays. The inhibitory efficiency differed among anthraquinone compounds with different structures. Alizarin (7), aloe emodin (11), emodin (13) emerged the outstanding inhibitory potential for DPP-IV with IC50 values lower than 5 µM. To clarifying the inhibitory mechanism, inhibitory kinetics were performed, which showed that alizarin red S (8) and 13 were effective non-competitive inhibitors of DPP-IV, while alizarin complexone (9), rhein (12), and anthraquinone-2-carboxylic acid (23) were mixed inhibitors. Emodin was determined as inhibitor with the strongest DPP-IV-binding affinity determined via molecular docking. Structure-activity relationship (SAR) demonstrated that hydroxyl group at C-1 and C-8 sites and hydroxyl, hydroxymethyl or carboxyl group at the C-2 or C-3 site were very essential for DPP-IV inhibition, replacement of hydroxyl group with amino group at C-1 could led to an increase of the inhibitory potential. Further fluorescence imaging showed that both compounds 7 and 13 significantly inhibited DPP-IV activity in RTPEC cells. Overall, the results indicated that anthraquinones would be a natural functional ingredient for inhibiting DPP-IV and provided new ideas for searching and developing potential antidiabetic compounds.