Adenosine Pathway Activation Defines Genetically Linked Immunosuppressive Subtypes in Solid Tumor Brain Metastases

Introduction: Brain metastases represent a major clinical challenge due to a distinct immunosuppressive microenvironment and limited, heterogeneous efficacy of PD-1/PD-L1 immune checkpoint inhibition. The adenosine pathway, mediated by the ectonucleotidases CD39 and CD73, has emerged as an alternative immune escape mechanism, yet its relevance in brain metastases across tumor entities remains insufficiently characterized. Methods: We conducted targeted panel sequencing of brain metastases from multiple primary tumor entities and evaluated compartment-resolved expression of CD39, CD73, and PD-L1 by immunohistochemistry, distinguishing tumor cell and immune cell expression. Tumor mutational burden (TMB), recurrent gene alterations, and gene fusions were analyzed and integrated with immune marker profiles to define immunogenomic subtypes. Results: Brain metastases displayed a heterogeneous mutational landscape with recurrent alterations including TP53, KRAS, PIK3CA, and APC. CD39 and CD73 expression was frequent and highly variable, occurring on both tumor cells and tumor-infiltrating immune cells, and only partially overlapping with PD-L1 expression. A substantial subset of cases exhibited an adenosine-high phenotype despite low or absent PD-L1. Marker-associated enrichment analyses identified distinct genetic correlates, including enrichment of KRAS alterations in tumors with CD39/CD73 positivity on malignant cells, and APC/PIK3CA-associated patterns linked to immune compartment marker expression. TMB did not significantly differ across major tumor entity groups. Gene fusions were detected in a subset of tumors but were largely independent of immune phenotypes. Conclusions: Adenosine pathway activation is a frequent, genetically associated immune escape feature of brain metastases that complements PD-L1-based stratification. Integrating CD39/CD73 with PD-L1 enables actionable immunogenomic subtyping and supports rational immunotherapy strategies targeting adenosine-mediated immunosuppression.