DDDR-81. A Ribonucleotide Reductase Targeted Strategy Enhances Radiosensitivity in Breast Cancer Brain Metastases via Triapine

Abstract BACKGROUND AND OBJECTIVE Breast cancer brain metastases (BCBM) are a frequent and devastating complication of advanced disease, particularly in triple-negative breast cancer (TNBC), where standard therapies such as whole-brain radiotherapy (WBRT) offer limited benefit due to dose-limiting toxicity and intrinsic tumor radio-resistance. Our lab has previously shown that ribonucleotide reductase regulatory subunit M2 (RRM2), a critical regulator of de novo dNTP synthesis, is mechanistically linked to enhanced DNA repair and therapeutic resistance in glioblastoma. This study investigated whether targeting RRM2 with Triapine, a CNS-penetrant ribonucleotide reductase inhibitor, could disrupt radiation-induced DNA repair in BCBM and sensitize tumors to radiotherapy. METHODS To examine the clinical relevance of RRM subunits in BCBM, we analyzed expression patterns across breast cancer subtypes using publicly available RNA-seq datasets. MDA-MB-231BR, a brain-tropic triple-negative breast cancer (TNBC) cell line, was used to model BCBM. The effect of Triapine on radiation response was assessed using immunofluorescence staining for γH2AX, clonogenic survival assays, cell viability assays, and LC-MS/MS-based quantification of intracellular dNTP pools. Ongoing in vivo experiments using orthotopic BCBM models will further elucidate Triapine-induced changes in tumor burden and survival. RESULTS On analyzing publicly available datasets, we found that RRM2 expression was elevated in TNBC and negatively correlated with overall survival (P < 0.05). Immunofluorescence studies using MDA-MB-231BR cells showed a significant increase in γH2AX foci formation in the Triapine plus radiotherapy group compared to radiation alone (P < 0.001), indicating enhanced DNA damage. Similarly, a marked reduction in clonogenic survival and cell viability (P < 0.01) was observed in the combination group versus radiation alone, demonstrating therapeutic synergy. CONCLUSION This approach represents a novel repositioning of Triapine, a clinically characterized, CNS-penetrant, FDA-approved RRM inhibitor, as a targeted radiosensitizer for BCBM, offering a compelling therapeutic avenue by disrupting radiation-induced DNA repair in brain metastases.