Intracarotid injection-based brain metastasis models without disrupting the blood-brain barrier for preclinical drug evaluation

Brain metastases are one of the most common and serious complications of solid tumors, affecting 20-40% of patients. They are common in lung, breast, and melanoma cancers and are associated with a poor prognosis and limited treatment options. Key challenges include the blood-brain barrier (BBB), which restricts penetration of many therapeutic agents, and the brain's historically immune-privileged status, limiting immune responses against tumor cells. Establishing in vivo models that accurately replicate the natural metastatic process to the brain is essential for understanding disease mechanisms and advancing therapeutic research. Most current preclinical brain metastasis models rely on intracranial implantation of tumor cells directly into the brain or systemic delivery via intracardiac injection. Although intracranial implantation enables rapid local tumor formation, this approach disrupts the BBB integrity and limits physiological relevance for studying natural metastatic progression and evaluating compounds with specific BBB-crossing properties. In contrast, intracardiac injection allows systemic dissemination of tumor cells and can lead to brain metastases in ~50% of cases using MDA-MB-231 cells and 70%-80% using JIMT-1 cells while likely preserving BBB integrity. However, these models are limited by suboptimal brain metastasis rates or early euthanasia due to aggressive extracranial tumor growth, hindering study of brain-specific disease progression over time. In this study, we present a more physiologically relevant brain metastasis model based on intracarotid injection. This approach enhances the targeted delivery of tumor cells to the brain while preserving BBB integrity, increasing translational relevance. We report take rate and intracranial tumor growth kinetics of human breast cancer cell lines following intracarotid injection and compare these outcomes with those from conventional intracardiac injection. The intracarotid technique yields a high rate of brain metastasis and confines tumor growth to the brain region. To further validate the model, we evaluate efficacy of standard-of-care (SOC) therapies within this system, providing insights into therapeutic responsiveness under conditions closely mimicking clinical scenarios. Additionally, we present data on the growth of 4T1 syngeneic tumor cells in the brain, enabling the study of immune-tumor interactions in an immunocompetent host. This syngeneic model adds another dimension by allowing exploration of immunological mechanisms and testing of immunotherapies in a controlled, biologically relevant setting. In conclusion, these findings suggest that the intracarotid injection model is a valuable tool for preclinical evaluation of novel therapies targeting brain metastases.