Simple SummaryBreast cancer is the most commonly diagnosed cancer and the second leading cause of cancer deaths among women globally. Due to its biological heterogeneity, breast cancer treatment and prognosis are highly variable among patients. In particular, the triple-negative subtype (TNBC) has a poorer prognosis with a lack of targeted therapies. With this in mind, we sought to develop a potentially more effective, novel receptor-targeting strategy. This study describes measuring the expression of the putative biomarker, interleukin-13 receptor (IL-13R)α2, in breast cancer (including TNBC) and its therapeutic targeting using a novel hybrid cytolytic peptide (Pep-1-Phor21) approach. We have shown in this manuscript that IL-13Rα2 exhibits potential as a therapeutic target, particularly for TNBC types of breast cancer. Importantly, drug-induced breast cancer cell lysis could be enhanced by treatment with epigenetically active anti-cancer compounds, suggesting that a combination adjuvant therapy of Pep-1-Phor21 with such compounds may be a particularly productive strategy for TNBC.AbstractHighly metastatic breast cancers, such as triple-negative subtypes (TNBC), require the most effective treatments. Since interleukin-13 receptor (IL-13R)α2 is reportedly over-expressed in some cancers, we investigated here its expression and the feasibility of therapeutically targeting this receptor in breast cancer using a novel hybrid cytolytic peptide (Pep-1-Phor21) consisting of IL-13Rα2-binding (Pep-1) and cytolytic (Phor21) domains. This study demonstrates that particularly TNBC tissues and cells display the prominent expression of IL-13Rα2. Furthermore, Pep-1-Phor21 induced the rapid necrosis of tumor cells expressing cell-surface IL-13Rα2. Notably, IL-13Rα2 expression was found to be epigenetically regulated in breast cancer cells in that the inhibition of histone deacetylase (HDAC) or DNA methyltransferase (DNMT) upregulated IL-13Rα2 expression, thereby sensitizing them to Pep-1-Phor21. IL-13Rα2-negative non-malignant cells were refractory to these epigenetic effects. Consistent with its cytolytic activity, Pep-1-Phor21 readily destroyed IL-13Rα2-expressing breast cancer spheroids with HDAC or DNMT inhibition, further enhancing cytolytic activity. Therefore, the Pep-1-Phor21-mediated targeting of IL-13Rα2 is a potentially novel therapeutic strategy for TNBC. Given that tumor cells can be selectively sensitized to Pep-1-Phor21 via the epigenetic up-regulation of IL-13Rα2, a combined adjuvant approach involving Pep-1-Phor21 and epigenetic inhibitors may be an effective strategy.