Data from Estrogen Receptor Mutations as Novel Targets for Immunotherapy in Metastatic Estrogen Receptor–positive Breast Cancer

Abstract

<div>Abstract<p>Estrogen receptor–positive (ER<sup>+</sup>) breast cancer is not considered immunogenic and, to date, has been proven resistant to immunotherapy. Endocrine therapy remains the cornerstone of treatment for ER<sup>+</sup> breast cancers. However, constitutively activating mutations in the estrogen receptor alpha (<i>ESR1</i>) gene can emerge during treatment, rendering tumors resistant to endocrine therapy. Although these mutations represent a pathway of resistance, they also represent a potential source of neoepitopes that can be targeted by immunotherapy. In this study, we investigated <i>ESR1</i> mutations as novel targets for breast cancer immunotherapy. Using machine learning algorithms, we identified <i>ESR1</i>-derived peptides predicted to form stable complexes with HLA-A*0201. We then validated the binding affinity and stability of the top predicted peptides through <i>in vitro</i> binding and dissociation assays and showed that these peptides bind HLA-A*0201 with high affinity and stability. Using tetramer assays, we confirmed the presence and expansion potential of antigen-specific CTLs from healthy female donors. Finally, using <i>in vitro</i> cytotoxicity assays, we showed the lysis of peptide-pulsed targets and breast cancer cells expressing common <i>ESR1</i> mutations by expanded antigen-specific CTLs. Ultimately, we identified five peptides derived from the three most common <i>ESR1</i> mutations (D538G, Y537S, and E380Q) and their associated wild-type peptides, which were the most immunogenic. Overall, these data confirm the immunogenicity of epitopes derived from <i>ESR1</i> and highlight the potential of these peptides to be targeted by novel immunotherapy strategies.</p>Significance:<p>Estrogen receptor (ESR1) mutations have emerged as a key factor in endocrine therapy resistance. We identified and validated five novel, immunogenic ESR1-derived peptides that could be targeted through vaccine-based immunotherapy.</p></div>