Preclinical Models of Brain Metastases in Breast Cancer

Abstract

Breast cancer remains a leading cause of mortality among women worldwide. Brain metastases confer extremely poor prognosis due to a lack of understanding of their specific biology, unique physiologic and anatomic features of the brain, and limited treatment strategies. A major roadblock in advancing the treatment of breast cancer brain metastases (BCBM) is the scarcity of representative experimental preclinical models. Current models are predominantly based on the use of animal xenograft models with immortalized breast cancer cell lines that poorly capture the disease's heterogeneity. Recent years have witnessed the development of patient-derived in vitro and in vivo breast cancer culturing systems that more closely recapitulate the biology from individual patients. These advances led to the development of modern patient-tissue-based experimental models for BCBM. The success of preclinical models is also based on the imaging technologies used to detect metastases. Advances in animal brain imaging, including cellular MRI and multimodality imaging, allow sensitive and specific detection of brain metastases and monitoring treatment responses. These imaging technologies, together with novel translational breast cancer models based on patient-derived cancer tissues, represent a unique opportunity to advance our understanding of brain metastases biology and develop novel treatment approaches. This review discusses the state-of-the-art knowledge in preclinical models of this disease.

Keywords: animal imaging; brain metastasis; breast cancer; multimodal imaging; patient-derived xenografts; preclinical animal models.

Figure 1

Establishment of breast cancer brain metastasis from immortalized breast cancer cell lines. (I) Immortalized breast cancer cell lines are established from breast cancer primary tumors or metastases or (II) spontaneously developed breast cancer in the mouse model. Cells are then cultured in vitro (a) and introduced into mice (b) with the goal of developing brain metastasis. In some models, formed brain metastases are then dissociated to single cells and passaged in vitro (c) to generate a brain-seeking clone and then are reintroduced into the animal (d). Often multiple re-passaging cycles are used to establish brain-seeking clones until an efficient BCBM mouse model is generated (e) (see text). Created with BioRender.com.

Figure 2

Methods of introduction of cancer cells into an experimental animal to generate breast cancer brain metastatic models. Various introduction methods applied for (a) xenogeneic models, (b) syngeneic models, and (c) patient-derived xenograft models are presented (see text). Breast cancer cells are most commonly introduced into mice via intracranial, intracarotid, intracardiac, or mammary fat pad injections or implantation. More sophisticated approaches, such as ligation of the external and common carotid arteries during intracarotid injection, intracranial transplantation using pipette tip through burr hole, and bilateral subcutaneous injection using a trocar have also been described for PDX models. Created with BioRender.com.

Figure 3

Schematic of in vivo imaging methodologies. Breast cancer brain metastasis models can be imaged with fluorescence imaging (FLI), bioluminescence imaging (BLI), positron emission tomography (PET), computed tomography (CT), and magnetic resonance imaging (MRI) or combinations of these techniques (multimodality imaging) (see text for details). In FLI and BLI, cells are labeled with reporters and introduced into the animal. Substrates are then used to detect a bioluminescent signal in BLI, while no substrates are required for FLI. In PET imaging, radiotracers conjugated to a substrate (see text and Table 2) are used. In targeted PET imaging, radiotracers can be conjugated to antibodies against specific molecules expressed by cancer cells to improve sensitivity and specificity of detection of the metastases. CT and MRI can often use contrast enhancing molecules such as iodine (CT) or gadolinium (MRI) to improve image contrast and detection. MRI can also utilize iron particles (iron-based MRI) to improve cellular detection and allow for monitoring of the arrest, growth, and retention of cancer cells in vivo. Created with BioRender.com.