Vincristine is a microtubule-destabilizing antimitotic drug that has been used in cancer therapy for over 40 years. However, the knowledge on vincristine-induced cell death pathways is still sparse. Here, we show that vincristine induces dramatic changes in the lysosomal compartment and sensitizes cells to lysosomal membrane permeabilization. In HeLa cervix carcinoma cells, vincristine induced mitotic arrest and massive cell death associated with an early increase in the lysosomal volume and lysosomal leakage followed by the activation of the intrinsic apoptosis program. In contrast, the majority of vincristine-treated MCF-7 breast carcinoma cells resisted apoptosis. Instead, they adapted to the spindle assembly checkpoint and escaped the mitotic arrest as micronucleated and senescent cells with an increase in the volume and the activity of their lysosomal compartment. Consistent with its substantial effects on the lysosomes, vincristine greatly sensitized cultured cancer cells as well as orthotopic breast cancer xenografts in mice to the cytotoxicity induced by siramesine, a sigma-2 receptor ligand that kills cancer cells by destabilizing their lysosomes. Importantly, the combination of nontoxic concentrations of vincristine and siramesine resulted in massive cell death even in MCF-7 cells that were capable of escaping vincristine-induced spindle assembly checkpoint and cell death. Similar synergism was observed when siramesine was combined with a semisynthetic vincristine analogue, vinorelbine, or with microtubule-stabilizing paclitaxel. These data strongly suggest that combination therapies consisting of microtubule-disturbing and lysosome-destabilizing drugs may prove useful in the treatment of otherwise therapy-resistant human cancers. [Cancer Res 2007;67(5):2217–25]