Breast cancer (BC) is the most prevalent malignancy in women, with HER2 amplification detected in 25–30% of metastatic cases. While HER2-targeted therapies such as trastuzumab have significantly improved patient survival, their efficacy in HER2+ brain metastases (BrM) is often compromised by acquired resistance and limited blood-brain barrier (BBB) permeability. This study investigates the potential of trastuzumab radiolabeled with the β⁻-emitting radionuclide ¹⁷⁷Lu ([¹⁷⁷Lu]Lu-DOTA-Trastuzumab) as a strategy to overcome resistance in HER2+ BrM.

HER2+ BC cell lines and their brain-tropic derivatives were evaluated for HER2 expression and sensitivity to trastuzumab and [¹⁷⁷Lu]Lu-DOTA-Trastuzumab. In vivo models were established via orthotopic implantation of HER2+ BC cells for primary tumor development and intracardiac injection to generate BrM. Tumor progression was monitored using magnetic resonance imaging (MRI), while HER2 expression was assessed through [⁸⁹Zr]Zr-DFO-Trastuzumab positron emission tomography (PET) imaging. BBB permeability across metastatic lesions was evaluated using dynamic contrast-enhanced MRI (DCE-MRI).

Brain-tropic HER2+ cells maintained HER2 expression but exhibited resistance to trastuzumab. In contrast, [¹⁷⁷Lu]Lu-DOTA-Trastuzumab induced significant DNA damage and cytotoxicity in vitro. PET imaging confirmed specific radiotracer uptake in HER2+ primary tumors and BrM. A single dose of [¹⁷⁷Lu]Lu-DOTA-Trastuzumab effectively suppressed primary tumor growth and led to complete BrM remission in 40% of treated animals. BBB permeability varied across metastatic lesions, potentially influencing radiotracer uptake and therapeutic response.

These findings highlight [¹⁷⁷Lu]Lu-DOTA-Trastuzumab as a promising therapeutic strategy to overcome trastuzumab resistance in HER2+ BrM. Its potent anti-tumor effects and ability to target brain metastases underscore its potential to improve outcomes in patients with metastatic HER2+ BC.