• Anti-depressant imipramine blocks the growth and progression of breast cancers.
• Imipramine induces cell cycle arrest and blocks heightened DNA repair activities in breast cancer cells.
• Imipramine blocked the ER-α signaling and improved the efficacy of anti-estrogen in ER + breast cancers.
• Imipramine improved the efficacy of PARP inhibitor in TNBC.
Aberrant activities of various cell cycle and DNA repair proteins promote cancer growth and progression and render them resistant to therapies. Here, we demonstrate that the anti-depressant imipramine blocks the growth of triple-negative (TNBC) and estrogen receptor-positive (ER+) breast cancers by inducing cell cycle arrest and by blocking heightened homologous recombination (HR) and non-homologous end joining-mediated (NHEJ) DNA repair activities. Our results reveal that imipramine inhibits the expression of several cell cycle- and DNA repair-associated proteins including E2F1, CDK1, Cyclin D1, and RAD51. In addition, we show that imipramine inhibits the growth of ER + breast cancers by inhibiting the estrogen receptor- α (ER-α) signaling. Our studies in preclinical mouse models and ex vivo explants from breast cancer patients show that imipramine sensitizes TNBC to the PARP inhibitor olaparib and endocrine-resistant ER + breast cancer to anti-estrogens. Our studies suggest that repurposing imipramine could enhance routine care for breast cancer patients. Based on these results, we designed an ongoing clinical trial to test the efficacy of imipramine for treating patients with triple-negative and estrogen receptor-positive breast cancer. Since aberrant DNA repair activity is used by many cancers to survive and become resistant to therapy, imipramine could be used alone and/or with currently used drugs for treating many aggressive cancers.