compositions and methods for preventing and treating cancer are provided that comprise extracts of inula britannica or compounds isolated therefrom . the compounds of 1 - o - acetylbritannilactone or 1 , 6 - o , o - diacetylbritannilactone are isolated and used for inducing apoptosis in cells and for treating cancer in an animal . induction of apoptosis comprises contacting cells with a composition comprising the compounds in an amount sufficient to induce bcl - 2 protein phosphorylation . further , treatment of cancer in an animal comprises administering to an animal a composition comprising the compounds in an amount sufficient to induce bcl - 2 protein phosphorylation to treat the cancer .