Abstract Unmet needs in the breast cancer field include the development of effective treatments for 1) drug-resistant HER2+ breast cancers, 2) Triple-Negative (HER2-, ER-, PR-) Breast Cancers (TNBCs), and 3) Inflammatory Breast Cancers (IBCs). Treatment-refractory HER2+ tumors retain HER2 expression after acquiring drug resistance, and large fractions of TNBCs and IBCs overexpress either HER2 or EGFR. Thus, drugs that selectively kill breast tumors that overexpress EGFR or HER2 could benefit patients suffering from these three classes of treatment-refractory tumors. Our team has identified a class of compounds termed Disulfide bond Disrupting Agents (DDAs) that selectively kill cancer cells that overexpress HER2 or EGFR. DDA-mediated cancer-selective cytotoxicity results from A) downregulation of the EGFR/HER2/HER3 proteins, B) inactivation of the Akt oncogene, and C) activation of the Unfolded Protein Response (UPR). Importantly DDAs only elicit these responses in cancer cells that overexpress EGFR or HER2. DDAs exhibit striking anticancer activity in human HER2+ xenograft tumors grown orthotopically in mice, with no evidence of toxicity. DDAs function through mechanisms distinct from those of HER2 specific monoclonal antibodies and tyrosine kinase inhibitors, and biochemical studies indicate that DDAs are complementary with Lapatinib with respect to blockade of HER2 driven proliferative and survival signaling. Current efforts are directed toward identifying mechanism-based drugs that synergize with DDAs to potentiate HER1-3 downregulation, Akt inhibition, and UPR, and to kill HER2+/EGFR+ patient-derived xenograft tumors that are refractory to current therapies. Citation Format: Brian K. Law, Mary E. Law, Renan B. Ferreira, Mengxiong Wang, Bradley J. Davis, Ronald K. Castellano, Coy D. Heldermon. Multifaceted targeting of drug-resistant EGFR+ and HER2+ breast tumors [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 4050. doi:10.1158/1538-7445.AM2017-4050