No abstract

Cytotoxic chemotherapy is an effective treatment for invasive breast cancer. However, experimental studies in mice also suggest pro-metastatic effects of chemotherapy. Primary tumours release extracellular vesicles (EVs), including exosomes, that can facilitate the seeding and growth of metastatic cancer cells in distant organs, but the effects of chemotherapy on tumour-derived EVs remain unclear. Here we show that two classes of cytotoxic drugs broadly employed in pre-operative (neoadjuvant) breast cancer therapy, taxanes and anthracyclines, elicit tumour-derived EVs with enhanced pro-metastatic capacity. Chemotherapy-elicited EVs are enriched in annexin-A6 (ANXA6), a Ca2+-dependent protein that promotes NF-kB-dependent endothelial cell activation, Ccl2 induction, and Ly6C+CCR2+ monocyte expansion in the pulmonary pre-metastatic niche to facilitate the establishment of lung metastasis. Genetic inactivation of Anxa6 in cancer cells, or Ccr2 in host cells, blunts the pro-metastatic effects of chemotherapy-elicited EVs. ANXA6 is detected, and potentially enriched, in the circulating EVs of breast cancer patients undergoing neoadjuvant chemotherapy.

Most deaths from cancer are explained by metastasis, and yet large-scale metastasis research has been impractical due to the complexity of in vivo models. Here, we introduce an in vivo barcoding strategy capable of determining the metastatic potential of human cancer cell lines in murine xenografts at scale. We validated the robustness, scalability and reproducibility of the method, and applied it to 500 cell lines 1 , 2 spanning 21 solid cancer types. We created a first-generation Metastasis Map (MetMap) that reveals organ-specific patterns of metastasis and allows relating those patterns to clinical and genomic features. We demonstrated the utility of MetMap by exploring the molecular basis of breast cancers capable of metastasizing to the brain - a principal cause of death in these patients. We found that breast cancers capable of metastasizing to the brain had unexpected evidence of altered lipid metabolism. Perturbing lipid metabolism curbed brain metastasis development, suggesting a therapeutic strategy to combat the disease and demonstrating the utility of MetMap as a public resource to support metastasis research.

A hallmark of metastasis is the adaptation of tumor cells to new environments. Metabolic constraints imposed by the serine and glycine-limited brain environment restrict metastatic tumor growth. How brain metastases overcome these growth-prohibitive conditions is poorly understood. Here, we demonstrate that 3-phosphoglycerate dehydrogenase (PHGDH), which catalyzes the rate-limiting step of glucose-derived serine synthesis, is a major determinant of brain metastasis in multiple human cancer types and preclinical models. Enhanced serine synthesis proved important for nucleotide production and cell proliferation in highly aggressive brain metastatic cells. In vivo , genetic suppression and pharmacologic inhibition of PHGDH attenuated brain metastasis, but not extracranial tumor growth, and improved overall survival in mice. These results reveal that extracellular amino acid availability determines serine synthesis pathway dependence, and suggest that PHGDH inhibitors may be useful in the treatment of brain metastasis. SIGNIFICANCE:Using proteomics, metabolomics, and multiple brain metastasis models, we demonstrate that the nutrient-limited environment of the brain potentiates brain metastasis susceptibility to serine synthesis inhibition. These fi ndings underscore the importance of studying cancer metabolism in physiologically relevant contexts, and provide a rationale for using PHGDH inhibitors to treat brain metastasis.

Summary Brain metastasis is an end stage in breast cancer progression. Traditional treatment options have minimal efficacy, and overall survival is on the order of months. The incidence of brain metastatic disease is increasing with the improved management of systemic disease and prolongation of survival. Unfortunately, the targeted therapies that control systemic disease have diminished efficacy against brain lesions. There are reasons to be optimistic, however, as emerging therapies have shown promise in preclinical and early clinical settings. This review discusses recent advances in breast cancer brain metastasis therapy and potential approaches for successful treatment.

Although targeted therapies are often effective systemically, they fail to adequately control brain metastases. In preclinical models of breast cancer that faithfully recapitulate the disparate clinical responses in these micro-environments, we observed that brain metastases evade phosphatidylinositide 3-kinase (PI3K) inhibition despite drug accumulation in the brain lesions. In comparison to extracranial disease, we observed increased HER3 expression and phosphorylation in brain lesions. HER3 blockade overcame the resistance of HER2-amplified and/or PIK3CA-mutant breast cancer brain metastases to PI3K inhibitors, resulting in marked tumor growth delay and improvement in mouse survival. These data provide a mechanistic basis for therapeutic resistance in the brain microenvironment and identify translatable treatment strategies for HER2-amplified and/or PIK3CA-mutant breast cancer brain metastases.

AUTHOR CONTRIBUTIONS G.F., A.A. and A.L. performed experiments and helped analyze all data with supervision from R.K.J. and M.G.V.H. D.P.K., D.B. and D.F. contributed in identifying metabolic signatures of brain metastasis; K.LA. and A.W. contributed to in vitro work and GCMS analysis. A.D. and C.B.C. performed LCMS lipidomics and helped with analysis; L.B., B.P. and V.A.D performed IMS imaging and analysis; X.J. and T.R.G. contributed to extracellular fluid isolation and provided input on data interpretation. J.M.P., N.I.L. and E.B. collected clinical samples and contributed to analysis of patient tumor sections; J.C. and D.G.D. performed ultrasound imaging of liver tumors; C.R.C. and S.M.D. contributed to in vivo glucose tracing studies. Z.A. performed flow cytometry analysis. R.F. and J.N. helped with analysis of lipidomics data. I.C., C.N. and D.E.H. analyzed human expression databases. K.N. performed analysis of Affymetrix array. M.D. and S.R. contributed to CRISPR Cas9 methodology and animal implantations.

Blood-brain/blood-tumor barriers (BBB and BTB) and interstitial transport may constitute major obstacles to the transport of therapeutics in brain tumors. In this study, we examined the impact of focused ultrasound (FUS) in combination with microbubbles on the transport of two relevant chemotherapy-based anticancer agents in breast cancer brain metastases at cellular resolution: doxorubicin, a nontargeted chemotherapeutic, and ado-trastuzumab emtansine (T-DM1), an antibody-drug conjugate. Using an orthotopic xenograft model of HER2-positive breast cancer brain metastasis and quantitative microscopy, we demonstrate significant increases in the extravasation of both agents (sevenfold and twofold for doxorubicin and T-DM1, respectively), and we provide evidence of increased drug penetration (>100 vs. <20 µm and 42 ± 7 vs. 12 ± 4 µm for doxorubicin and T-DM1, respectively) after the application of FUS compared with control (non-FUS). Integration of experimental data with physiologically based pharmacokinetic (PBPK) modeling of drug transport reveals that FUS in combination with microbubbles alleviates vascular barriers and enhances interstitial convective transport via an increase in hydraulic conductivity. Experimental data demonstrate that FUS in combination with microbubbles enhances significantly the endothelial cell uptake of the small chemotherapeutic agent. Quantification with PBPK modeling reveals an increase in transmembrane transport by more than two orders of magnitude. PBPK modeling indicates a selective increase in transvascular transport of doxorubicin through small vessel wall pores with a narrow range of sizes (diameter, 10-50 nm). Our work provides a quantitative framework for the optimization of FUS-drug combinations to maximize intratumoral drug delivery and facilitate the development of strategies to treat brain metastases.