Discovery of and mechanistic insights into anti-inflammatory sesquiterpenoids from flowers of Inula japonica Thunb.

Flowers of Inula japonica Thunb., a rich source of sesquiterpenoids, have long been used in traditional medicine to improve inflammatory conditions. To explore previously unreported anti-inflammatory sesquiterpenoids, isolation and structural elucidation were performed on this plant material, yielding 18 compounds (1-18), including 6 previously undescribed sesquiterpenoids (1-3 and 6-8). The structures of these compounds, including their absolute configurations, were assigned based on one- and two-dimensional nuclear magnetic resonance experiments, high-resolution electrospray ionization mass spectrometry, electronic circular dichroism spectral calculations and measurements, and chemical derivatization. Compounds 3, 8-11, 14, and 18 significantly inhibited NO production in lipopolysaccharide-stimulated macrophages. Network pharmacology analysis suggested the involvement of the MAPK signaling pathway, along with key targets such as AKT1, EGFR, JUN, CASP3, CCND1, and IL1B, in the biological action of the active compounds. This outcome is supported by a molecular docking model, which proposes binding patterns between the active compounds and key targets. The results of this study improve our understanding of the anti-inflammatory mechanisms of I. japonica flowers and contribute to the structural diversity of bioactive sesquiterpenoids.