Abstract P6-03-01: Development of patient-derived xenograft tumor model with organ-specific metastatic potential for evaluation of new therapeutics for hormone receptor-positive advanced breast cancer
Authors: Qiang W , Zhong Z , Gerratana L , Zhang Y , Zhang Q , Gursel D , Wei J-J , Bleher R , James C , O'Halloran T , Cristofanilli M 
Published: May 2019

Abstract
Background: Breast cancer (BC) is a heterogeneous disease with most common metastatic sites of liver, lung, brain, and bone. Endocrine resistance in hormone receptor-positive (HR+) advanced BC (ABC) cancer is a clinical challenge. ESR1 mutations are a key mechanism in acquired resistance, primarily occurs after exposure to endocrine therapy such as aromatase inhibitors but also selective estrogen modulators and degraders (i.e. Tamoxifen and Fulvestrant). Circulating tumor cells (CTCs) enumeration is a prognostic biomarker in ABC but the relation between the onset of ESR1 mutations and CTCs status is still unclear. Aim of this project is to define the clinical behavior of ESR1 mutated ABC in terms of metastasizing potential, through CTC enumeration and pattern; and to establish ESR1 mutated HR+ ABC PDX models able to recapitulate these characteristics.
Methods: CTCs and circulating tumor DNA (ctDNA) were characterized in 55 HR+ ABC patients. ESR1 mutations status from 55 patient plasma cell-free DNA were generated using Guardant Next Generation Sequencing. Samples were also examined for numbers of CTCs by CellSearch. Association of ESR1 mutations with sites of distant organ metastasis and with CTC enumeration was analyzed by Chi square test and Kruskal–Wallis test, respectively. In preclinical model development, six samples of pleural effusion-derived tumor cells from Stage IV HR+ ABC patients were collected to establish HR+ ABC with ESR1 mutation PDX tumor model and its derived 3D organoid/spheroid cultures
Results: ESR1 mutations were identified in 10 out of 55 patients (4 Y537S variant and 3 D538G variant, 4 other variants, 1 patient with both variants). In 55 patients, 72 visceral vs 27 bone metastatic incidences were observed; the data indicated 9 observed vs 4.5 expected in ESR1 mutated and 16 observed vs 20.5 expected in wild type (WT) (P=0.003) for liver metastasis; 10 observed vs 7.1 expected in ESR1 mutated and 29 observed vs 31.9 expected in WT (P=0.026) for bone metastasis. Further liver metastasis analysis of individual hot spot mutation site indicated 4 observed vs 1.8 expected in Y537S and 21 observed vs 23.2 expected in WT (P=0.037); and 3 observed vs 1.4 expected in D538G and 22 observed vs 23.6 expected in wild type (P=0.088). The analysis of correlation/distribution between CTCs numbers and ESR1 mutated suggested CTCs median of 13 (IQR 7-49) in ESR1 mutated and 0 (IQR 0-4) in WT HR+ patients (P=0.0044). Four ABC PDX tumor models were developed in immunodeficient NSG female mice demonstrated by pathology to have highly heterogeneous characteristics and metastatic features of the origin patient tumor, in particular, breast fat pad xenografted PDX tumor can result in metastasis to liver and lung tissue. In addition, two patient 3D tumor organoid/spheroid cultures were successfully established.
Conclusions: ESR1 mutated ABC is associated with more aggressive (Stage IV) clinical behavior demonstrated by association with visceral metastases and CTCs detection. ESR1-mutated PDX models recapitulate aggressive features of the disease and can be used for preclinical testing of novel agents in endocrine resistant disease.
Citation Format: Qiang W, Zhong Z, Gerratana L, Zhang Y, Zhang Q, Gursel D, Wei J-J, Bleher R, James C, O'Halloran T, Cristofanilli M. Development of patient-derived xenograft tumor model with organ-specific metastatic potential for evaluation of new therapeutics for hormone receptor-positive advanced breast cancer [abstract]. In: Proceedings of the 2018 San Antonio Breast Cancer Symposium; 2018 Dec 4-8; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2019;79(4 Suppl):Abstract nr P6-03-01.