AbstractBreast cancer is the most common cancer among women worldwide, yet successful treatment remains a clinical challenge. Ivermectin, a broad-spectrum antiparasitic drug, has recently been characterized as a potential anticancer agent due to observed antitumor effects. However, the molecular mechanisms involved remain poorly understood. Here, we report a role for ivermectin in breast cancer suppression by activating cytostatic autophagy both in vitro and in vivo. Mechanistically, ivermectin-induced autophagy in breast cancer cells is associated with decreased P21-activated kinase 1 (PAK1) expression via the ubiquitination-mediated degradation pathway. The inhibition of PAK1 decreases the phosphorylation level of Akt, resulting in the blockade of the Akt/mTOR signaling pathway. In breast cancer xenografts, the ivermectin-induced cytostatic autophagy leads to suppression of tumor growth. Together, our results provide a molecular basis for the use of ivermectin to inhibit the proliferation of breast cancer cells and indicate that ivermectin is a potential option for the treatment of breast cancer. Cancer Res; 76(15); 4457–69. ©2016 AACR.