Brain metastasis is a prevalent malignancy associated with a very poor prognosis1,2. The standard of care for brain metastasis patients is multimodal, consisting of neurosurgery, whole brain radiotherapy and/or stereotactic radiosurgery depending on the patients' general health status, extracranial disease burden, and the number and location of tumors in the brain3,4. Patients with up to three intracranial lesions are eligible for surgical resection or stereotactic radiosurgery, while whole-brain radiation therapy is recommended for patients with multiple lesions to avoid the risk of surgery-related infection and edema5. However, whole brain radiotherapy can inflict damage on radiosensitive brain structures, contributing to poor quality of life6.
Systemic therapy is a non-invasive alternative and logical approach to treat patients with multiple lesions7. However, it is less considered due to the long standing notion that systemic therapies have poor efficacy because the passive delivery of cytotoxic drugs via the bloodstream cannot achieve therapeutic levels in the brain without the risk of unsafe toxicity8. This paradigm is starting to change with the recently U.S. Food and Drug Administration (FDA)-approved systemic therapy (tucatinib with trastuzumab and capecitabine indicated for metastatic HER2+ breast cancer brain metastasis)9,10,11,12 and the update in treatment guidelines to include consideration of systemic therapy options for brain metastasis patients13,14.
In this context, developments in the field of molecular targeted therapy, immunotherapy, and alternative drug-delivery systems, such as a targeted nano-drug carrier, can potentially overcome the challenges of brain metastasis treatment15,16,17,18. In addition, chemical and mechanical approaches to improve drug delivery via permeabilization of the brain-tumor barrier are also being investigated19,20. To study and optimize such approaches to be fit for purpose, it is crucial to use preclinical models that not only mirror the complex physiology of brain metastasis but also allow for objective analysis of intracranial drug response.
Broadly, the current approaches to model brain metastasis in vivo involve intracardiac (left ventricle), intravenous (usually tail vein), intracranial, or intracarotid (common carotid artery) injection of cancer cells in mice21,22,23,24,25,26,27. Apart from tumor engraftment strategies, genetically engineered mouse models where tumor formation is triggered by the removal of tumor suppressor genes or activation of oncogenes are useful for tumor modeling. However, only a few genetically engineered mouse models are reported to produce secondary tumors and even fewer that reliably produce brain metastases28,29,30.
Engraftment methods such as intracardiac (left ventricle) and intravenous (usually tail vein) injection mimic the systemic dissemination of cancer. These models typically produce lesions in multiple organs (e.g., brain, lungs, liver, kidneys, spleen) depending on the capillary bed that traps most tumor cells during their circulatory 'first pass'31. However, inconsistent rates of brain engraftment will require more animals to achieve the sample size for the desired statistical power. The number of tumor cells that eventually get established in the brain via these intracardiac and intravenous injection methods is variable. Hence, brain metastasis tumor burden can vary between animals and the difference in progression can make standardizing the experimental timeline and interpretation of results a challenge. The extracranial tumor burden can lead to non-brain metastasis mortality, rendering these models unsuitable for evaluating intracranial efficacy. Brain-tropic cell lines have been established using artificial clonal selection processes to reduce extracranial establishment, but take rates have been inconsistent, and the clonal selection process can reduce the heterogeneity normally found in human tumors32.
Brain-specific engraftment methods such as the intracranial and intracarotid injection allow for more consistent and efficient brain metastasis modeling. In the intracranial method33, cancer cells are typically injected into the frontal cerebral cortex, which generates quick and reproducible tumor outgrowth with low systemic involvement. While the procedure is well tolerated with low mortality33, the caveats are that it is a relatively crude approach that rapidly introduces a (localized) bolus of cells in the brain and does not model early brain metastasis pathogenesis. The needle damages brain tissue vasculature, which then causes localized inflammation5,34. From experience, there is a tendency for tumor cell injectate to reflux during removal of the needle, leading to leptomeningeal involvement. Alternatively, the intracarotid method delivers cells into the common carotid artery with brain microvasculature as the first capillary bed to be encountered, modeling survival in circulation, extravasation, and colonization24. In agreement with others25, our experience with this method found that it can result in facial tumors due to unintentional delivery of cancer cells via the external carotid artery to capillary beds in these tissues (unpublished data). It is possible to prevent facial tumors by first ligating the external carotid artery before common carotid artery injection (Figure 1). In the rest of the article, this method is referred to as the 'internal carotid artery injection'. From experience, the internal carotid artery injection method consistently generates brain metastasis with very few systemic events and has been successful in generating brain metastasis models of different primary cancers (e.g., melanoma, breast, and lung cancers) (Figure 1). The disadvantages are that it is technically challenging, time-consuming, invasive, and requires careful optimization of cell numbers and a monitoring timeline. In summary, both the intracranial and internal carotid artery injection methods produce mouse models suitable for evaluating therapeutic impact on brain tumor-related survival benefit.
This protocol describes the internal carotid artery injection method to produce a mouse model of brain metastasis with almost no systemic involvement and therefore suitable for preclinical evaluation of drug distribution and efficacy of experimental therapeutics.

Figure 1: Schematic representation of internal carotid artery injection protocol for brain metastasis. Internal carotid artery injection with external carotid artery ligation can reliably produce a brain metastasis model from various primary cancers. In this protocol, three ligatures are placed on the carotid artery (annotated as L1-L3 in the figure). Please click here to view a larger version of this figure.