RTAD-04 LOW-DOSE RADIATION INDUCES HER2 UPREGULATION AND ENHANCES EFFICACY OF TRASTUZUMAB DERUXTECAN IN BREAST CANCER BRAIN METASTASES

Abstract

Breast cancer brain metastases (BCBM) remain therapeutically challenging despite advances in systemic therapy. The HER2 antibody-drug conjugate trastuzumab deruxtecan (T-DXd) has demonstrated activity in HER2-low disease, including in the CNS; however, treatment durability remains limited. ADC efficacy depends on cell surface HER2 availability. Strategies that enhance HER2 expression may improve therapeutic response. Prior studies suggest radiation induces HER2 expression. We hypothesized that low-dose radiation increases tumor cell surface HER2 in HER2-low BCBM, thereby enhancing T-DXd internalization and efficacy, while maintaining low toxicity. We conducted mechanistic studies in HER2-low breast cancer cell lines (T47D, MDA-MB-231), five patient-derived brain metastasis organoids, and orthotopic murine models. Multiple RT regimens were evaluated for effects on HER2 expression, ADC internalization, and cytotoxicity. HER2 expression was quantified by flow cytometry and immunohistochemistry, and in vivo modulation was assessed in orthotopic tumors. Single-cell RNA sequencing characterized radiation-induced signaling and immune changes. A retrospective cohort of 13 patients treated with T-DXd was analyzed based on receipt of RT within 3 months prior to therapy, with CNS progression-free survival assessed by Kaplan-Meier analysis. Low-dose RT (3.33Gy×4) increased the percent of cells with HER2 membrane expression by 20-50% in HER2-low cell lines and organoids compared with no RT or higher-dose regimens (20Gy×1). Orthotopic models confirmed HER2 upregulation in vivo. RT enhanced T-DXd internalization and cytotoxic efficacy in HER2-low models. Transcriptomic analyses demonstrated activation of HER2-related oncogenic pathways and distinct immune signatures. Clinically, RT within 3 months prior to T-DXd was associated with improved CNS-PFS (13.4 vs 2.6 months; HR 0.33, 95% CI 0.07-1.66).

In conclusion, low-dose RT reprograms HER2-low BCBM by increasing HER2 expression and ADC internalization, while minimizing off target impacts. These findings support a radiation-priming strategy to optimize T-DXd efficacy. Prospective studies are warranted to define optimal radiation dosing and sequencing to improve intracranial control.