Sesquiterpenoids from Inula britannica and Their Potential Effects against Triple-Negative Breast Cancer Cells

Flowers of Inula britannica commercially serve as pharmaceutical herbs in the manufacturing of medicinal products. In the current study, sesquiterpenoids of I. britannica flowers’ extract and their potential effects against triple-negative breast cancer (TNBC) cells were investigated. Eight structurally diverse sesquiterpenoids, including one sesquiterpenoid dimer (1) and seven sesquiterpenoid monomers (2–8) were isolated from this source. The structures of all compounds were elucidated by 1D/2D NMR data, and their absolute configurations were discerned by single crystal X-ray diffraction. All of the compounds were tested for their potential effects against TNBC. Specifically, 5 displayed strong antiproliferative potency against TNBC cells with a high selective index (SI) on MCF-7 cells (SI > 4 of IC50 on MDA-MB-468/IC50 on MCF-7), and dimer 1 (IC50 = 8.82 ± 0.85 μM) showed better antiproliferative potency against MCF-7 cells than the other monomers did (2–8) (IC50 > 20 μM). To our best knowledge, compound 5 is the first sesquiterpenoid targeting TNBC cells.