Cucurbitacin B is a plant-derived triterpene that has the classic four-ring structure of mammalian steroids. It has diverse effects on mammalian cells, most notably inducing cell cycle arrest or apoptosis in a range of cancer cell lines.^1,2,3 Cucurbitacin B inhibits the growth of several breast cancer cell lines (IC50s = 18-50 nM).⁴ It suppresses the growth of tumors developed from MDA-MB-231 and 4T-1 breast cancer cells in mice.⁴ Cucurbitacin B blocks the inflammatory response of macrophages treated with lipopolysaccharide.⁵ In Drosophila, it serves as an ecdysteroid receptor antagonist (Kd = 5 µM).⁶