Development of Peptidic Inhibitors for Triple-Negative and Estrogen Receptor-Positive Breast Cancer

Triple Negative Breast Cancer (TNBC) accounts for 20% of all breast cancer cases and is one of the most clinically challenging subtypes of breast cancer due to its aggressive nature. Recently, Histone Deacetylase associated transcriptional regulatory proteins SIN3A and SIN3B have emerged as potential molecular targets for TNBC therapeutics. Peptidic inhibitors and small molecules targeting the PAH2 domain, a well-characterized protein-interaction domain of the SIN3 homologs have shown induction of epigenetic reprogramming along with upregulation of differentiation markers and downregulations of epithelial-to-mesenchymal transition markers in TNBC cell lines. Our efforts have resulted in a library of hydrocarbon stapled peptidic inhibitors based on a known binding partner (MXD1) with various positions of cyclization. Fluorescence Anisotropy assays show increased binding affinity of these stapled peptides, as well as anti-proliferative effects when used as treatment with MDA-MB-231 and BT549 cell lines as compared to the wild type MXD1 peptide. These compounds will serve as potential therapeutic leads as well as chemical tools for elucidation of the mechanisms and action of SIN3A and SIN3B. While the prognosis for patients whose disease remains confined to the breast is good, nearly 30% of breast cancers become metastatic. Of those patients who develop metastatic breast cancer, 30-40% also acquire a mutation to the estrogen receptor ligand binding domain. Patients with these somatic ER mutations have been shown to have significantly shorter survival rates compared to those without (20.7 vs. 32.1 months). These mutations render ER constitutively active and are relatively resistant to known therapies, so that new ways of targeting mutant ERs are desperately needed. Previous work in our lab described hydrocarbon-constrained, high-affinity γ-substituted peptides that are able to directly block binding of the ER to steroid receptor coactivators and have selectivity for the most common ER mutation in patients: D538G. We aim to design and synthesize peptides that take advantage of the D538G mutation to create specific antagonists for the D538G estrogen receptor, by creation of a novel, unnatural amino acid capable of olefin metathesis and γ-functionalization. This novel building block offers the potential for site-selective SAR and other conjugations at the gamma position of a stapling residue and may be a valuable tool for peptide chemistry.