Herein we report ErSO-TFPy as a small molecule that induces quantitative or near-quantitative regression of tumors in multiple mouse models of breast cancer with a single dose...Mechanistically, these tumor regressions are a consequence of rapid induction of necrotic cell death in the tumor and are immune cell independent. If successfully translated to human cancer patients, the benefits of such an anticancer drug that is effective with a single dose would be significant.