Abstract Metastatic breast cancer (MBC) is the most advanced stage of breast cancer, diagnosed as stage IV. Typically, MBC metastases occur in the bones, liver, brain, and lungs. Bone metastases cause significant morbidity and mortality in late-stage breast cancer patients. At present, there is no efficient curative or preventive treatment for bone metastases. Therefore, establishment and validation of well characterized in vivo models of breast cancer bone metastasis is of utmost importance. The aim of this study was to validate an intracardiac model of bone MBC utilizing GFP tagged triple negative (estrogen receptor; ER, progesterone receptor; PR and human epidermal growth factor receptor 2; HER2 negative) human breast cancer cells, which could be used to study efficacy of new potential treatments. Female athymic nude mice (Hsd: Athymic Nude-Foxn1nu) were used in this study. Bone seeking MDA-MB-231(SA)-GFP human breast cancer cells were inoculated into the left cardiac ventricle of 4-6 weeks old female mice. Allocation to treatment groups (n=15) was performed by stratification procedure based on animal weight prior to the beginning of the study. To validate the model, the current standard-of-care (SOC) treatment (Paclitaxel) was administered between study days 13-17 at the therapeutic dose. Serum bone biomarker TRACP5b was measured before cancer cell inoculation and at sacrifice. Planar X-ray of hind limbs and ex vivo fluorescence imaging of the whole body were performed at sacrifice. The observed tumor take rate was 100%. 33% of mice in the vehicle group were cachectic, but mice treated with SOC exhibited only mild and temporary weight loss. Tumors were observed mainly in skeletal sites but also in soft tissues. The SOC treatment decreased both total and skeletal tumor burden. Based on X-ray images, tumor induced osteolytic bone lesions were smaller in the SOC treated group compared to vehicle though no differences were observed in total bone lesion area. Serum TRACP5b was increased in the vehicle group as a result of osteolytic lesions, and decreased with the SOC treatment. In conclusion, an intracardiac model of MBC was successfully validated with the SOC treatment paclitaxel that efficiently inhibited disease progression. Therefore, this intracardiac model provides a promising tool for testing new treatments against bone and soft tissue MBC in vivo. Citation Format: Justyna Zdrojewska, Mari I. Suominen, Katja M. Fagerlund, Jussi M. Halleen, Jenni Bernoulli, Johann Zimmermann. Validation of a preclinical model for bone metastatic triple negative breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 2007.