Bcl-2 is an intracellular membrane protein that prevents cells from undergoing apoptosis in response to various cell-death signals. It negatively regulates mitochondrial outer membrane permeabilization, which is responsible for the release of apoptogenic factors and the subsequent activation of caspases. A microbial metabolite, aranorosin, was identified as an inhibitor of the anti-apoptotic function of Bcl-2. Based oil its structure, a more potent derivative, K050, was synthesized. Apoptosis could be induced in a cell line that overexpressed Bcl-2 when cells were treated with an anti-Fas antibody in addition to K050, at sub-micromolar concentrations. Furthermore, K050 inhibited anti-apoptotic functions regulated by Bcl-2, resulting in a Fas-triggered mitochondrial transmembrane potential loss, the activation of caspase-9, and a morphological change to apoptosis. Inhibition of cell-based function of Bcl-2 and its antiapoptotic effects could serve as useful pharmacological effects. Thus, a novel aranorosin derivative, K050, could be a potent therapeutic agent against Bcl-2-overexpressing human malignancies. (C) 2008 Elsevier Inc. All rights reserved