The SRB cytotoxicity assay was used to screen plant extracts, in a collaborative multinational OAS project involving Argentina, Bolivia, Colombia, Costa Rica, Guatemala, Nicaragua and Panama, against breast (MCF-7), lung (H-460), and central nervous system (SF-268) human cancer cell lines. Out of 310 species tested, 23 (7.4%) plants showed cytotoxic activity at GI50 values ≤10 µg/ml. The most active plants were Thevetia ahouai., Physalis viscosa., Piper jacquemontianum., Piper barbatum., Senna occidentalis., Tovomita longifolia., and Lippia cardiostegia.. Blepharocalyx salicifolius. and Senna occidentalis. were selectively active against one cell line, SF-268 or MCF-7, respectively. Within the framework of this project, 14 compounds have been isolated, 5 new (4 benzophenones, coumarin) and 9 known to the literature. But only the bioassay-guided fractionation of the active extract of Piper barbatum. leaves, which led to the isolation of three known compounds: (2′E., 6′E.)-2-farnesyl-1,4-benzoquinone (1), (2′E., 6′E.)-2-farnesylhydroquinone (2), and dictyochromenol (3), is reported here. The chemical structures of 1 and 2 were determined by spectral means (1D, 2D NMR, MS) and chemical data. Among these three, (2′E., 6′E.)-2-farnesyl-1,4-benzoquinone was the most active (MCF-7 GI50 = 1.8 µg/ml; H-460 GI50 = 4.8 µg/ml; SF-268 GI50 = 3.5 µg/ml).
