SAT-338 Novel Preclinical Models of Estrogen Receptor Alpha Positive Metastatic Invasive Lobular Breast Carcinoma Using Dual Bioluminescent and Fluorescent Human Cell Line Xenografts

Invasive lobular breast carcinoma (ILC) is a unique but understudied subtype of breast cancer that accounts for 10-15% of all cases. Despite exhibiting favorable prognostic and predictive factors (>90% Estrogen Receptor alpha [ER]-positive, highly Progesterone Receptor [PR]-positive, low HER2 and Ki67 expression), ILC tumors paradoxically have frequent long-term recurrences, suggestive of endocrine resistance. While recent studies on the endocrine response and in vitro biological phenotypes of human ILC cell lines have proven them useful for modeling ILC in the laboratory, there is limited data on their utility when grown as xenografts in immunocompromised mice. Here we used dual bioluminescent and fluorescent labeled ER-positive human ILC cell lines and performed a comprehensive in vivo characterization of their growth orthotopically as primary tumors and at secondary sites following spontaneous or experimental metastasis. Consistent with the histology from patients with ILC, primary cell line xenograft tumors exhibited the characteristic dyscohesive, single-file growth of ILC cells. In vivo tracking of bioluminescent cells in NOD scid gamma (NSG) mice coupled with ex vivo fluorescence analysis revealed spontaneous metastases to the bone, brain, lungs and ovaries, closely mirroring the clinical patterns of human ILC tumor dissemination. In contrast, in models of experimental metastasis in athymic nude mice, tail vein injections did not lead to lung colonization, while only one cell line colonized bones following intracardiac injections. Similar to primary tumors, the metastatic lesions also harbored the classical histological features of ILC, including single strand growth, loss of E-cadherin-mediated adherens junctions and mislocalization of p120-catenin to the cytoplasm. Importantly, both the primary tumors and the metastases exhibited high ER expression and significant response to the anti-endocrine agent fulvestrant - a selective ER downregulator, proving the utility of human ILC cell line xenografts as preclinical models for lobular breast cancer. Ongoing work focused on genomic and transcriptional analyses of primary and metastatic lesions, as well as isolated cell lines will reveal additional mechanistic insights into the biology of ILC dissemination. This is the first report of a hormone responsive ER-positive metastatic xenograft model faithfully representing unique ILC features such as ovarian metastases, which will serve as a valuable pre-clinical platform for testing novel therapeutics and improving the clinical outcome of patients with this understudied subtype of breast cancer.