Two New Sesquiterpenoid Dimers from Inula japonica and Their In Vitro Anti-Hepatocellular Carcinoma Activity
The inflorescences of Inula japonica Thunb. (Asteraceae) are a traditional Chinese medicine used for treating cough, phlegm, and vomiting. Sesquiterpenoid dimers, formed via Diels-Alder, hetero-Diels-Alder, [2+2] cycloaddition, or radical coupling, exhibit potent anti-inflammatory, neuroprotective, and antitumor activities. However, their low natural abundance and structural complexity hinder isolation and development. This study isolated six sesquiterpenoid dimers from the ethyl acetate fraction of a 90% ethanol extract of I. japonica using multiple chromatographic techniques. Their structures were elucidated by HRESIMS, NMR, IR, UV, and calculated NMR/ECD. Compounds 1 (inujaponolide T) and 3 (inujaponolide U) are new: a eudesmane-guaiane dimer and a 1,10-seco-eudesmane-guaiane dimer, respectively. The other four were identified as inujaponolide E (2), inujaponolide D (4), inujaponolide I (5), and japonicone X (6). In vitro anti-hepatocellular carcinoma activity was evaluated against HepG2 cells using MTT and colony formation assays. All compounds exhibited potent cytotoxicity with IC50 values of 2.61–13.94 μmol/L; compound 6 was most active (IC50 = 2.61 μmol/L). A preliminary structure-activity relationship indicated a positive correlation between the number of acetoxy substituents and antitumor activity. Compounds 1 and 3 inhibited cell viability and reduced colony formation in a dose-dependent manner. These findings expand the chemical diversity of I. japonica and provide a basis for developing these dimers as anti-hepatocellular carcinoma lead compounds.