Title: Next Generation Rat Models of ER+ Breast Cancer Abstract: Despite the prevalence of estrogen receptor-positive (ER+) breast cancer (BCa), there are no mammalian models that are immunocompetent and metastasize spontaneously to clinically relevant organs such as bones. ER+ BCs comprise ~70% of BCa cases and are treated with surgical resection and adjuvant endocrine therapies. Although the 5-year survival rate is high, risk of metastatic recurrence persists for dec- ades, and 20-40% of ER+ BCa patients eventually succumb to metastatic disease, most often in bones. Our limited knowledge of ER+ BCa has been largely derived from oversimplified cell line models and in vitro experiments. Our available xenograft models exclude the critical immune component while genetically engineered mouse models (GEMMs) rarely stably maintain ER expression and rely on ER signaling – and none metastasize to bone, the most frequent site. As a result, very little is known about the initial tumori- genesis process and bone metastasis of ER+ BCa, which greatly impedes our effort to develop new prevention strategies or adjuvant therapeutics that can block/treat ER+ BCa bone metastasis. We have developed a series of rat ER+ BCa models by intraductal injection of viruses to introduce genetic alterations. Our preliminary data demonstrated that these rat ER+ mammary tumors: 1) developed in immunocompetent hosts, 2) showed endocrine therapy sensitivity or resistance, and 3) readily formed ER+ metastases including in bone – all of which are key properties that are lacking in existing BCa models. Based on these, we hypothesize that rat-based intraductal injection models (RIIMs) of breast cancer faithfully recapitulate human ER+ BCa in terms of tumor initiation, metastasis, and therapeutic responses. We will pursue the following three aims: (1) To characterize early progression of ER+ BCa in RIIM models. (2) To characterize the metastatic behaviors of ER+ BCa in RIIM models. (3) To credential ER+ RIIM models in recapitulating therapeutic responses and resistance mechanisms of human ER+ BCa.