Lucanthone is a novel inhibitor of autophagy that induces cathepsin D-mediated apoptosis. To investigate the anticancer activity of Lucanthone, cell viability is measured by MTT assay. Lucanthone reduces cell viability to a similar extent in a panel of seven breast cancer cell lines. In addition, a direct comparison reveals that Lucanthone is significantly more potent than Chloroquine (CQ) at reducing breast cancer cell viability with a mean IC 50 of 7.2 μM versus 66 μM for CQ. Measurement of cell viability in two representative cell lines (MDA-MB-231 and BT-20) by ATPlite assay and trypan blue exclusion reveals comparable results.