The antiproliferative effects of phenoxodiol are associated with inhibition of plasma membrane electron transport in tumour cell lines and primary immune cells.

Although the redox-active synthetic isoflavene, phenoxodiol, is in Phase 3 clinical trials for drug-resistant ovarian cancer, and in early stage clinical trials for prostate and cervical cancer, its primary molecular target is unknown. Nevertheless, phenoxodiol inhibits proliferation of many cancer cell lines and induces apoptosis by disrupting FLICE-inhibitory protein, FLIP, expression and by caspase-dependent and -independent degradation of the X-linked inhibitor of apoptosis, XIAP. In addition, phenoxodiol sensitizes drug-resistant tumour cells to anticancer drugs including paclitaxel, carboplatin and gemcitabine. Here, we investigate the effects of phenoxodiol on plasma membrane electron transport (PMET) and cell proliferation in human leukemic HL60 cells and mitochondrial gene knockout HL60rho(o) cells that exhibit elevated PMET. Phenoxodiol inhibited PMET by both HL60 (IC(50) 32 microM) and HL60rho(o) (IC(50) 70 microM) cells, and this was associated with inhibition of cell proliferation (IC(50) of 2.8 and 6.7 microM, respectively), pan-caspase activation and apoptosis. Unexpectedly, phenoxodiol also inhibited PMET by activated murine splenic T cells (IC(50) of 29 microM) as well as T cell proliferation (IC(50) of 2.5 microM). In contrast, proliferation of WI-38 cells and HUVECs was only weakly affected by phenoxodiol. These results indicate that PMET may be a primary target for phenoxodiol in tumour cells and in activated T cells.