Abstract C109: Cytomegalovirus reactivation drives brain metastasis in breast cancer and reveals a targetable therapeutic axis

Abstract Background: Brain metastases (BM) are a major cause of mortality in metastatic breast cancer, often occurring in advanced stages with poor prognosis. While tumor-intrinsic mechanisms are well studied, the role of latent viral infections remains underexplored. Notably, cytomegalovirus (CMV) reactivation has been detected in ∼90% of breast cancer BM lesions, suggesting a potential contribution to metastasis. Methods: We developed a CMV-infected breast cancer BM mouse model to evaluate the impact of CMV and test the therapeutic efficacy of the anti-CMV drug foscarnet, alone and in combination with capecitabine. To define underlying mechanisms, we performed single-cell spatial transcriptomics and multiome (ATAC + RNA) profiling on brain tissues from non-infected controls, CMV-infected vehicle-treated, and CMV-infected foscarnet-treated mice. Clinical relevance was assessed through survival and immune profiling of BM samples from 20 breast cancer patients stratified by CMV reactivation status. Results: CMV infection significantly shortened survival in vehicle-treated mice compared to controls (P = 0.0063). Foscarnet monotherapy improved survival (P = 0.026), with further benefit from the combination with capecitabine (P < 0.0001). Spatial profiling revealed enrichment of immunosuppressive CXCR3+C1q+ microglia at the tumor margin in CMV-infected, vehicle-treated mice—a population reduced by foscarnet or absence of infection. Transcriptomic and in vitro analysis demonstrated that CMV reprograms microglia toward an M2-like phenotype, marked by increased TGF-β, ARG1, IL-10, PPARG, and CHIL3 expression. CMV-infected 4T1 tumor cells also upregulated CXCL9 (P = 4×10-10) and CCL2 (P = 6×10-6), promoting microglial recruitment and M2 polarization. In patient samples, high CMV reactivation was associated with significantly shorter survival compared to low/no reactivation (P = 0.005). Conclusion: CMV reactivation promotes brain metastasis by reprogramming microglia into an M2-like, immunosuppressive state, while infected tumor cells further sustain this environment via chemokine signaling. These findings position CMV as a previously underrecognized driver of BM progression and resistance. Although anti-CMV treatment is not standard for BM, our data support its therapeutic potential. A Phase II clinical trial has been initiated to evaluate anti-CMV therapy in CMV-reactivated BM patients. Targeting this viral-immunologic axis may represent a novel and impactful strategy for improving outcomes in breast cancer brain metastases. Citation Format: Wenjuan Dong, Shiruo Wong, Puri Akshjot, Jenny Chang, Hong Zhao, Stephen Wong. Cytomegalovirus reactivation drives brain metastasis in breast cancer and reveals a targetable therapeutic axis [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference on Molecular Targets and Cancer Therapeutics; 2025 Oct 22-26; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2025;24(10 Suppl):Abstract nr C109.