Abstract 860: ESR1 coregulator binding inhibitor (ECBI): a novel agent for treating hormone therapy-resistant breast cancer
Authors: Vadlamudi Ratna K. , Sareddy Gangadhara Reddy , Viswanadhapalli Suryavathi , Lee Tae-Kyung , Ma Shi-Hong , Lee Wan Ru , Mann Monica , Krishnan Samaya Rajeshwari , Gonugunta Vijay , Liu Yang , Strand Douglas W. , Tekmal Rajeshwar Rao , Ahn Jung-Mo , Raj Ganesh V. 
Published: July 2016

Abstract
Background: Estrogen contribute to the progression of breast cancer via estrogen receptor 1 (ESR1) and current therapies involve either antiestrogens (AE) or aromatase inhibitors (AI). However, most patients develop resistance to these drugs. In resistant tumors, activation of ESR1 in the absence of ligand or mutations in ESR1 allow interaction between the ESR1 and coregulators leading to sustained ESR1 signaling and proliferation. Here we, developed a novel ESR1 coregulator binding inhibitor (ECBI) that targets persistent ESR1 signaling that commonly occur in therapy resistant breast tumors.
Methods: Using rational design, we synthesized and evaluated a small organic molecule (ECBI) that mimics the ESR1 coregulator nuclear receptor box motif. Mechanistic studies were conducted using reporter gene assays, RT-qPCR., ChIP, and RNA-Seq analysis. Xenografts and patient derived tumors were used for preclinical evaluation and toxicity.
Results: In estrogen induced proliferation assays using several ESR1+ve model cells, ECBI significantly inhibited growth and promoted apoptosis. Importantly, ECBI showed little or no activity on ESR1 negative cells. Further, ECBI also reduced the proliferation of several ESR1 positive hormonal therapy resistant cells. Mechanistic studies showed that ECBI interacts with ESR1, efficiently blocks ESR1 interactions with coregulators and reduces the ESR1 driven ERE reporter gene activity. Further, ECBI directly interacted with mutant-ESR1 with high affinity and significantly inhibited mutant-ESR1 driven oncogenic activity. RNA sequencing analysis revealed that ECBI blocks multiple ESR1 driven pathways, likely representing the ability of a single ECBI compound to block multiple ESR1-coregulator interactions. Treatment of ESR1-positive and therapy resistant as well as syngeneic xenograft tumors with ECBI (10 mg/kg/day/oral) significantly reduced the tumor volume compared to control. Using human primary breast tissue ex vivo cultures, we have provided evidence that ECBI has potential to dramatically reduce proliferation of human breast tumors.
Conclusions: The ECBI is a novel agent that targets ESR1 with a unique mechanism of action. ECBI has distinct pharmacologic advantages of oral bioavailability, in vivo stability, and is associated with minimal systemic side effects. Remarkably, ECBI block both native and mutant forms of ESR1 and have activity against therapy resistant breast cancer cell proliferation both in vitro and in vivo and against primary human tumor tissues ex vivo. This first-in-class agent with its novel mechanism of action overcomes the limitations of current therapies.
Citation Format: Ratna K. Vadlamudi, Gangadhara Reddy Sareddy, Suryavathi Viswanadhapalli, Tae-Kyung Lee, Shi-Hong Ma, Wan Ru Lee, Monica Mann, Samaya Rajeshwari Krishnan, Vijay Gonugunta, Yang Liu, Douglas W. Strand, Rajeshwar Rao Tekmal, Jung-Mo Ahn, Ganesh V. Raj. ESR1 coregulator binding inhibitor (ECBI): a novel agent for treating hormone therapy-resistant breast cancer. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 860.