Thrombosis, brain metastasis formation, and the perivascular metastatic niche—Novel insights from the tumor microvascular circulation
Year: 2024
Type: article
Abstract: Significant attention has been devoted to enhancing our understanding of brain metastases, as they remain a dreaded end-stage complication of multiple tumor histologies. Numerous studies have attempted to investigate the defining features of the metastatic brain tumor microenvironment, in search of common cooccurring immunologic, genetic, metabolic, and neuronal features, with an ultimate goal of developing therapeutic targets to prevent brain metastasis formation.1 Despite these advances, unresolved questions remain regarding the initial steps within the tumor microvasculature leading to the intracranial metastatic cascade. Studies exploring brain tumor seeding and angiogenesis have focused on the unique hemodynamic interplay between tumor cells and the microvascular network within the brain.2 While various cellular mediators have been linked to this metastatic process, Angiopoietin-2 (Ang-2) and vascular endothelial growth factor (VEGF) have been implicated as key pro-angiogenic regulators that impair the integrity of the blood-brain barrier, promote cellular transmigration into the central nervous system, and drive metastatic seeding within the perivascular niche.3–5 Leveraging these insights, Roesler et al.6 carried out a series of elegant studies with multiple metastatic tumor models and a detailed cranial window 2-photon microscopy technique to investigate the hemodynamic features of circulating tumor cells within the brain, their impact on cerebral microcirculation, and subsequent tumor colony formation. The authors find evidence that the critical initial metastatic step involves capillary vessel occlusion from metastatic cell emboli and downstream alterations in blood velocity and microinfarctions. Supporting the observation that circulating tumor cells within the brain vasculature lead to ischemic events that promote metastatic seeding, Roesler et al. demonstrated colocalization of tumor-induced microinfarctions with subsequent sites of brain metastasis formation. Furthermore, the process of tumor cell-mediated vessel occlusion resulted in focal hypoxia within the brain metastatic niche and subsequent tumor cell colonization. This process resulted in Ang-2 upregulation and VEGF expression, findings reminiscent of the initial steps of embolic stroke formation.7 To further link Ang-2 expression and brain metastasis formation, the authors utilized an intracardiac injection model with the human 99LN breast cancer cell line in transgenic Ang-2 gain of function (GOF) mice. Using this approach, they demonstrated that Ang-2 GOF mice exhibited increased extravasation and tumor formation. These findings suggested metastatic tumor cell seeding and outgrowth within the brain is, at least in part, promoted by Ang-2 expression. After the authors demonstrated increased expression of Ang-2 within the brain microvasculature and VEGF in tumor cells within the hypoxic pre-metastatic niche, they carried out a series of exciting additional experiments to test the hypothesis that early inhibition of these targets would alter the hypoxic pre-metastatic niche and prevent metastatic tumor growth in the brain. Utilizing an Ang-2 inhibitor (AMG 386) or a VEGF-trap (Aflibercept), they studied the ability to suppress brain metastasis formation using an intracardiac injection model with both H1_DL2 human melanoma and JIMT-1 human breast cancer cells. Using this approach, the authors demonstrated a significant reduction in cancer cell extravasation, overall quantity of brain metastasis (~50% reduction), and brain metastasis volume in H1_DL2 mice pretreated with AMG 386 and Aflibercept. In contrast, JIMT-1 mice treated with AMG 386 and Aflibercept resulted in a significant reduction solely in tumor volumes. These findings suggest that therapeutic targeting Ang-2/VEGF has the potential to alter the hypoxic, perivascular pre-metastatic niche. Due to variation in response between tumor types, further studies will be needed to explore the underlying contribution across different histologies of Ang-2, VEGF, and potentially other factors that mediate microvascular occlusion and tumor outgrowth. Additional studies can also explore whether this therapeutic strategy limits the overall number of cancer-induced microinfarcts or the magnitude of response to the infarct itself. These additional investigations will be critical to develop treatment strategies that achieve even greater magnitude of intracranial tumor cell reduction across multiple tumor histologies. Lastly, the authors utilized a tissue microarray consisting of 191 patients from multiple tumor histologies to link Ang-2, CD31, and Ki-67 with clinical outcomes. While Ang-2/CD31 expression levels varied across tumor types, they dichotomized the brain metastasis cohort into Ang-2/CD31 high versus low tumors and incorporated Ki67 expression. Integrating these parameters, they observed the greatest reduction in overall survival amongst patients with high Ki67 expression and an elevated Ang-2/CD31 ratio. While these results provide a generalized association between Ang-2 expression, cellular proliferation, and the perivascular cellular composition in a heterogenous post-resection brain metastasis cohort of patients, future studies with extensive clinical correlates will be needed to solidify the link between Ang-2 mediated brain metastasis formation and clinical outcomes. Taken together, the study by Roesler et al. leverages extensive in vivo brain metastasis modeling to elucidate the mechanistic process of tumor cell-mediated microvascular occlusion, subsequent infarction-like micro-environmental reaction, and downstream upregulation of Ang-2/VEGF. Upregulation of both Ang-2 and VEGF has been shown in non-brain metastasis models to promote metastasis formation and targeting these pathways reverses this process.3,4,8 Additionally, encouraging preclinical studies have been reported combining Nintedanib, a tyrosine kinase inhibitor targeting VEGF, and BI836880, a bi-specific anti-VEGF-A/Ang-2 antibody, to prevent lung cancer brain metastases following intracardiac injection.9 However, the work by Roesler et al. is the first study to pursue early dual inhibition of Ang-2/VEGF using AMG 386 and Aflibercept to prevent the downstream hypoxic cascade within the intracranial pre-metastatic perivascular niche. While this work provides novel insights into the impact of cerebral microvascular occlusion from circulating tumor cells, there are a number of key areas for future investigation. For instance, radiation serves as a critical component of the treatment armamentarium for brain metastases and is received by nearly all brain metastases patients. Future studies will need to explore the impact of radiation on the cerebral microvasculature, Ang-2/VEGF expression, and downstream tumor cell extravasation. In addition, the studies reported herein, while elegant, utilize xenograft models, which can limit micro-environmental analyses. Also, the impact of targeting these perivascular components on immune cell infiltration/activation warrants further investigation. While aspects of these questions are being explored in the non-brain metastasis context (NCT03239145), they warrant further exploration in brain metastases. Finally, this work strongly supports the proposal that targeting Ang-2/VEGF represents a tool for limiting tumor cell transmigration into the CNS. While clinical trials test this approach as a brain metastasis prevention technique, it will likely need to be combined with other therapeutic modalities amongst patients with preexisting brain metastases.
Source: Neuro-Oncology
Language: English
Subfield: Pulmonary and Respiratory Medicine
Field: Medicine
Domain: Health Sciences
Open Access status: bronze
OpenAlex ID: W4402003049