Although the treatment of early stage and metastatic breast cancers continues to improve, breast cancer brain metastases (BCBMs) remain a significant clinical challenge. This is especially true for patients with the HER2-positive (HER2+) subtype of breast cancer where up to half of patients with metastatic disease eventually develop brain metastases. Currently, patients with HER2+ BCBMs have a median survival of only 18 months. Although many HER2-targeting treatments are effective for early and even metastatic HER2+ breast cancer outside the brain, few drugs are effective once breast cancer has metastasized to the brain.

Due to the presence of the natural, protective blood/brain barrier, therapies that might block growth of HER2+ cells may either not enter the brain or may be ineffective against metastases within this privileged site. In addition, the brain metastases may interact with the brain environment in a way that leads to additional mechanisms of resistance. Thus, drug-resistant, HER2+ metastases in the brain can grow unchecked. In addition to the difficulty of delivering treatment to the brain, cancer biologists have had difficulty understanding brain metastasis behavior and testing novel therapies because accurate models of BCBM were lacking. We have recently met this challenge by creating mouse models in which tissues derived from patient BCBM are grown within the brains of mice so that these model tumors more closely resemble conditions within the patient. Our biochemical, molecular, and histochemical analyses of these transplanted BCBM tumors (termed orthotopic patient-derived xenograft or PDX models) demonstrate that they strongly recapitulate the properties of the original BCBM, providing faithful experimental models for elucidating the biological properties of these tumors and testing potential therapies.

Using these models, we have discovered differences in metastatic brain cancer cells that reveal vulnerabilities that may serve as targets for novel therapies. For example, we identified that the protein CDK4/6, which regulates cancer cell replication, is frequently deregulated in HER2+ BCBMs. Strikingly, this pathway is involved in proliferation of tumor cells and regulates the interaction of tumor cells with the immune system. Thus, inhibition of the CDK4/6 pathway may reduce the ability of BCBMs to proliferate and, at the same time, enhance antitumor immune activity against BCBM.

In this study, we propose to use PDX models and genetically engineered mouse models (GEMMs) of HER2+ BCBM to test novel CDK4/6-based combination therapies for activity against HER2+ BCBM. By using anti-CDK4/6 drugs with other targeted therapies, we will be able test the potential synergy of drug combinations against HER2+ BCBM. We have a wide range of PDX models that include HER2+ BCBM with various molecular alterations that occur in breast cancer to more accurately model the range of patients affected by this disease. We will assess the responses of these tumors to rationally designed combinations. Analysis of the PDX tumors before and after these various treatments will reveal biomarkers that may serve in the future to identify patients most likely to benefit. As the CDK4/6 pathway can affect the regulation of the interaction of tumor cells with the immune system, we will also use our unique GEMMs of brain metastases to assess whether inhibition of the CDK4/6 pathway, in combination with immunotherapy, can enhance therapeutic outcomes. Given that the brain and primary site microenvironments differ drastically and that little is known regarding the antitumor immune response against HER2+ BCBM, we will use GEMMs to ascertain whether the microenvironment influences the antitumor immunity of CDK4/6 inhibition and if inhibition of this pathway can enhance the ability of "immune checkpoint blockade" agents to increase antitumor activity.

As these therapeutic agents we will test on our HER2+ BCBM models are already in clinical development to treat patients without brain metastases, positive findings from our work can quickly inform the design of one or more clinical trials in patients with HER2+ BCBMs. In addition, we will be poised to validate whether the findings in our preclinical models translate to patients and whether the markers we identify that predict response or resistance to treatment hold true in patients as well. Thus, while this work aims to uncover how CDK4/6 inhibitors or CDK4/6 inhibitor-based drug combinations are effective in preclinical models of HER2+ BCBM, the path from these models to a positive impact on breast cancer patients' lives is potentially very short.