Abstract 1618: OP-1250 prevents tumor spread in a model of metastatic mutant ERα+ breast cancer

Abstract

Abstract Estrogen receptor positive (ER+) breast cancers represent about 70-75% of all breast cancer. In general, these cancers are effectively treated with adjuvant endocrine therapies, with or without the addition of CDK4/6 inhibitors. However, many patients develop resistance and eventually progress to metastatic disease (MBC). Acquired mutations in ESR1, which encodes estrogen receptor alpha (ERα), contribute to 20-40% of endocrine therapy resistant MBC. In patients, breast cancer metastasis occurs in the liver, lungs, brain and bone. OP-1250 is a novel compound that is a complete ERα antagonist (CERAN). Previous studies have shown that OP-1250 effectively blocks both the AF-1 and AF-2 activation functions of ERα. In the current study, we used a mutant ERα MBC endocrine therapy resistant xenograft model to examine the efficacy of OP-1250 in a metastatic context. MCF7 cells engineered to express one of the most common and aggressive ERα mutations, Y537S, were labeled with luciferase and injected via the nipple (mammary intraductal MIND model) into NSG mice. Mice were treated with 3 and 10 mg/kg of OP-1250 alone or in combination with palbociclib at 70mg/kg and tumor growth was monitored via a Xenogen IVIS imager. At study end point, mice were sacrificed and excised organs were imaged ex-vivo and further processed for immunohistology analysis. OP-1250 at both doses inhibited primary tumor growth as well as metastasis to the lung, liver, brain and bone, with greater effect at 10 mg/kg, and was significantly more effective than Faslodex. When combined with palbociclib at 70 mg/kg, an additional significant decrease in metastasis was observed, compared to OP-1250 or palbociclib alone. Our results also suggest that the efficacy seen in the combination study was driven primarily by OP-1250. In total, these results demonstrate the potential use of OP-1250, both alone and in combination with a CDK4/6 inhibitor, to inhibit tumor growth and metastasis in a model of aggressive mutant ER+ MBC. Citation Format: Muriel Laine, Marianne E. Greene, Tiffany Leng, Sophia Li, Gopinath S. Palanisamy, Cyrus L. Harmon, Leslie Hodges-Gallagher, Peter J. Kushner, Geoffrey L. Greene. OP-1250 prevents tumor spread in a model of metastatic mutant ERα+ breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 1618.