Abstract P3-03-04: Establishment and characterization of ESR1-mutant breast cancer PDX models

Abstract The estrogen receptor (ER or ESR1) drives proliferation and growth of luminal type breast cancers. Endocrine therapies are highly effective in a majority of these cancers types; however, disease progression eventually occurs due to acquired resistance resulting in hormone-independent breast cancer. One mechanism of resistance is acquired mutations at codons 537 and 538 in the ligand binding domain of the receptor which are found in ∼12% of pretreated, ER+ patients. These mutations result in constitutive ER activation and hormone-independent progressive disease. Although ESR1 mutated breast cancers are insensitive to endocrine therapy, novel agents targeting these mutations may be effective; however few preclinical models of ESR1-mutant breast cancer are available for preclinical analysis. To this end we have established two patient derived xenograft (PDX) models harboring mutations at codon 537; ST941 with ESR1Y537S and ST1799 with ESR1Y537C. We have characterized these models using genomic analysis and compared with analysis from paired or serially collected clinical tissue and blood. Dependence on estradiol for model growth and sensitivity to endocrine therapies were also evaluated and compared with a hormone-dependent breast cancer models. The ESR1 mutations were not present in primary clinical tissue while several additional mutations were identified including in TP53 and PIK3CA genes. In vivo the ESR1 mutant models grew in the presence or absence of exogenous estradiol and demonstrated reduced sensitivity to endocrine therapies compared with hormone-dependent breast cancer models. ST941 treated with tamoxifen, letrozole or fulvestrant reported moderate tumor growth inhibition versus control while tumor stasis or regressions were reported in MCF-7 and hormone-dependent PDX breast models. Overall we have established and characterized two models of ESR1 mutant breast cancer which can be utilized for development of targeted therapies. Citation Format: Wick MJ, Helman E, Meade J, Clark MJ, Vaught T, Tolcher AW, Rasco D, Patnaik A, Lang A, Beeram M, Papadopoulos KP. Establishment and characterization of ESR1-mutant breast cancer PDX models. [abstract]. In: Proceedings of the Thirty-Eighth Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2015 Dec 8-12; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2016;76(4 Suppl):Abstract nr P3-03-04.