The Tumor Microenvironment in Non-Small Cell Lung Cancer Brain Metastases: Composition, Dynamics, and Therapeutic Implications
Brain metastases (BM) are a common and clinically significant complication of non-small cell lung cancer (NSCLC), representing a major contributor to its high mortality. Among all metastatic sites, involvement of the central nervous system (CNS) is associated with a particularly profound deterioration in patients' quality of life. The incidence of BM varies considerably across different histological subtypes and molecularly defined groups of NSCLC. The development of BM is a multistep process involving primary tumor invasion, hematogenous dissemination, blood-brain barrier (BBB) transmigration, and successful colonization of the brain. This process is strongly shaped by reciprocal interactions between the tumor cells and the unique brain tumor microenvironment (TME). Brain endothelial cells, pericytes, astrocytes, microglia, neurons, and recruited peripheral immune and stromal cells collectively regulate BBB permeability, immune evasion, extracellular matrix remodeling, and metastatic outgrowth. Compared with primary lung tumors, BM displays a distinct immune landscape characterized by reduced lymphocyte infiltration, enrichment of immunosuppressive myeloid populations, impaired antigen presentation, and extensive crosstalk with CNS-resident cells. These features contribute to therapeutic resistance and help explain the heterogeneous intracranial efficacy of systemic treatments. While immune checkpoint inhibitors and chemotherapy combinations provide benefit in selected patients, the most pronounced intracranial responses are observed with CNS-penetrant targeted therapies in molecularly defined subsets. Emerging strategies aim to directly target the metastatic niche, including myeloid cells, tumor vasculature, immune checkpoints, and cellular immunotherapies. A deeper understanding of the brain metastatic ecosystem may enable the development of more effective, biology-driven therapeutic approaches for NSCLC BM.