Discovery of ent-labdane derivatives from Andrographis paniculata and their anti-inflammatory activity

The plant Andrographis paniculata has a long history of cultivation in Southeast Asia, especially its extensive anti-inflammatory activity, and the famous natural antibiotic andrographolide comes from this plant. In China, A. paniculata, as the main crop, has become a major source of traditional Chinese medicine (TCM) for the clinical treatment of inflammation. To further explore the diverse diterpene lactones with better anti-inflammatory activity from A. paniculata, twenty-one ent-labdanes, including six undescribed compounds (andropanilides D-I), were isolated. Their structures with absolute configurations were thoroughly determined by comprehensive NMR spectroscopic data, HRESIMS analysis and quantum chemical calculations. All isolated compounds were evaluated for anti-inflammatory activities based on the Griess method. Meanwhile, after structure-activity relationships analysis, the anti-inflammatory activity of andropanilide D (1) (IC₅₀ = 2.31 μM) was found to be better than that of the positive control drug (dexamethasone, IC₅₀ = 6.52 μM) and andrographolide (IC₅₀ = 5.89 μM). Further mechanisms of activity indicated that andropanilide D significantly reduced the secretion of TNF-α, IL-6 and IL-1β and downregulated the protein expression of COX-2 and iNOS in LPS-induced RAW264.7 macrophages in a concentration-dependent manner based on Western blot and ELISA experiments. In conclusion, andropanilide D possesses potential medicinal value for the treatment of inflammation and further expands the material basis of the anti-inflammatory effect of A. paniculata.