Low Level Whole-Brain Radiation Enhances Theranostic Potential Of Single Domain Antibody Fragments For HER2-Positive Brain Metastases

Background Single-domain antibody fragments (aka VHH, ∼13 kDa) are promising delivery systems for brain tumor theranostics; however, achieving efficient delivery of VHH to intracranial lesions remains challenging due to the tumor-brain barrier. Here, we evaluate low-dose whole-brain irradiation as a strategy to increase the delivery of an anti-HER2 VHH to breast cancer-derived intracranial tumors in mice. Methods Mice with intracranial HER2-positive BT474BrM3 tumors received 10-Gy fractionated cranial irradiation and evaluated using non-invasive imaging methods. The anti-HER2 VHH 5F7 was labeled with 18F, administered intravenously to irradiated mice and controls, and PET/CT imaging was conducted at various intervals after irradiation. Tumor uptake of 18F-labeled 5F7 in irradiated and control mice was compared by PET/CT image analysis and correlated with tumor volumes. In addition, longitudinal dynamic contrast-enhanced MRI (DCE-MRI) was conducted to visualize and quantify the potential effects of radiation on tumor perfusion and permeability. Results Increased 18F-labeled 5F7 intracranial tumor uptake was observed with PET in mice that received cranial irradiation, with maximum tumor accumulation seen approximately 12 days post initial radiation treatment. No radiation-induced changes in HER2 expression were detected by Western blot, flow cytometry, or on tissue sections. DCE-MRI imaging demonstrated transiently increased tumor perfusion and permeability after irradiation, consistent with the higher tumor uptake of 18F-labeled anti-HER2 5F7 in irradiated mice. Conclusion Low-level brain irradiation induces dynamic changes in tumor vasculature that increase the intracranial tumor delivery of an anti-HER2 VHH, which could facilitate the use of labeled sdAb to detect, monitor, and treat HER2-expressing brain metastases. Key points Importance of the Study Improving the detection and treatment of brain metastases (BM) that overexpress human epidermal growth factor receptor type 2 (HER2) is an urgent medical need. Drug delivery to BM is confounded by their tumor vasculature, which is more restrictive than in GBM. Single domain antibody fragments, about one-tenth the size of antibodies, could be promising theranostic vectors for BM provided sufficient BM uptake could be achieved. In this study, we utilized longitudinal PET imaging to demonstrate that low-dose whole-brain irradiation (WBRT) significantly increased 18F-labeled HER2-specific 5F7 VHH uptake in intracranial HER2-positive tumors in mice. Combining low dose WBRT with 5F7 VHH labeled with α-or β-particle emitting radionuclides could provide an effective and specific targeted radiotherapeutic strategy for treating patients with HER2-expressing BM. ### Competing Interest Statement G.V., M.R.Z., and Z.Z. are inventors on a patent application describing the RL-III 18F labeling technology licensed by Zentera Alpha from Duke University. The other authors disclosed no potential conflict of interest.