BET Protein Degrader ZBC260 Induces Tumor Vascular Reprogramming Through Endothelial Cell Remodeling.

PURPOSE: Endothelial cells regulate angiogenesis, matrix remodeling, and immune responses within the tumor microenvironment, yet endothelial cell heterogeneity across breast cancer subtypes and its regulation by bromodomain and extraterminal domain proteins remain unclear. This study profiled endothelial cells at single-cell resolution in two immunocompetent murine breast cancer models with distinct tumor immune microenvironments: PyMT_FVB/N and D2A1_BALB/c tumors. METHODS: Mice were treated intravenously with the bromodomain and extraterminal domain degrader (ZBC260) at 10mg/kg 3 days/week until tumors reached humane endpoints and were harvested. Single-cell RNA sequencing was performed on vehicle and ZBC260 treated tumors (n=3 per condition), and endothelial cell populations were identified and analyzed using Seurat package in R. RESULTS: Six transcriptionally distinct endothelial subpopulations were identified across both breast cancer models, with D2A1 tumors dominated by tip-like and proliferative endothelial cells with strong angiogenic and collagen-producing signatures. ZBC260 reprogrammed endothelial landscape by reducing proliferative, tip-like, and arterial endothelial cells while enriching venous and lymphatic endothelial cells. ZBC260 suppressed hypoxia, glycolysis, and mTORC1 signaling, downregulated angiogenesis and matrix associated pathways and was accompanied by reduced collagen signatures and increased leukocyte adhesion. Cell-cell communication analysis demonstrated that ZBC260 disrupted endothelial cells interactions with tumor and immune cells, by disrupting ligand-receptor communications and attenuating NOTCH, TGF-β, and VEGF signaling networks that sustain angiogenesis and immune suppression. CONCLUSION: Together, these findings highlight endothelial diversity across breast cancer models and identify ZBC260 as a regulator of tumor endothelial states. ZBC260 reprograms endothelial heterogeneity and remodels the tumor vascular landscape. The transcriptomic and cell-cell communication changes observed are consistent with the establishment of a more immune-permissive tumor microenvironment, supporting the potential of BET degradation as a strategy to complement immunotherapy and improve therapeutic responsiveness in breast cancer.