Identification of a novel cardenolide (2′′-oxovorusharin) from Calotropis procera and the hemisynthesis of novel derivatives displaying potent in vitro antitumor activities and high in vivo tolerance: Structure-activity relationship analyses - Van Quaquebeke, E; Simon, G; Andre, A; Dewelle, J; El Yazidi, M; Bruyneel, F; Tuti, J; Nacoulma, O; Guissou, P; Decaestecker, C; Braekman, J C; Kiss, R; Darro, F (J Med Chem, 2005)