Targeting mutant estrogen receptor driven breast cancers

Project Summary/AbstractA majority of Breast cancers (BC) express estrogen receptor alpha (ERα). While endocrine therapies targeting either estrogen production or ERα are effective, acquired resistance is common. Sequencing of metastatic endocrine therapy-resistant (ETR) tumors has shwown that ERα mutations are frequent (30-40%), do not respond to endocrine therapies and are the molecular drivers of ETR-BC. Thus, drugs designed to specifically target these ERα mutations in ETR-BC represent a significant unmet clinical need.We leveraged the recent structural characterization of the mutant ERα ligand binding domain (LBD) to rationally design bis-benzamides to fit the binding pocket. Following iterative rounds of modeling, synthesis, testing and optimization with rt2000 bis-benzamides, we have identified a lead compound, ERX-315, that binds more avidly to ERα LBD. Our preliminary studies showed that ERX-315 has potent anti-proliferative activity against mutant ERα-driven tumors, as seen in genetically modified cell lines in vitro, patient derived explants (PDEs) ex vivo and patient derived xenografts (PDXs) in vivo. Importantly, a methylated version of ERX-315, ERX-314 binds the mutant ERα poorly and does not affect proliferation of these tumors. Ultrastructural and molecular studies reveal that ERX-315, but not ERX-314 (a methylated version of ERX-315, does not bind ERα, serves as a negative control) induces significant endoplasmic reticulum stress, leading to a shutdown of de novo protein synthesis and apoptotic cell death in BC. Importantly, ERX-315 does not induce endoplasmic reticulum stress or cell death in normal cells and is non-toxic in animal models. We have shown that this capacity of ERX- 315 to induce endoplasmic reticulum stress is unique among drugs targeting ERα, including selective ERα modulators and degraders, such as GDC-0180, AXD-9496 and fulvestrant.The objective of this direct Phase II SBIR proposal is to enable EtiraRx to perform IND-enabling studies for clinical translation of ERX-315 in ETR-BCs. We have shown that ERX-315 has favorable pharmacologic parameters for clinical translation and is amenable to good manufacturing practice manufacturing. In Aim 1, we will we will perform dose-ranging finding studies in two species and define maximum tolerated dose and toxicity at that dose. In Aim2, we will synthesize large-scale batches of ERX-315 and will perform single- and multi-dose pharmacokinetic studies and evaluate tissue biodistribution. We will define 30-day toxicity in rats and dogs with a 2-week recovery, and evaluate functional recovery, cardiovascular and respiratory safety. In Aim 3, We will test the efficacy of clinical grade ERX-315 in biologically and clinically relevant preclinical models of ETR-BC, including patient derived xenografts and patient derived explants. The intellectual property around ERX-315 is protected by multiple patents licensed to EtiraRx. a novel small molecule targeting mutant ERα, and with an unique ability to induce endoplasmic reticulum stress and apoptotic cell death. If successful, our proposed studies will enable within 2 years the first-in-class studies with ERX-315 in women with ETR-BC.