Therapeutic Response Monitoring with ⁸⁹Zr‐DFO‐Pertuzumab in HER2‐Positive and Trastuzumab‐Resistant Breast Cancer Models

Immuno‐positron emission tomography (PET) has great potential to evaluate the target expression level and therapeutic response for targeted cancer therapy. Immuno‐PET imaging with pertuzumab, due to specific recognition in different binding sites of HER2, could be useful for the determination of the therapeutic efficacy of HER2‐targeted therapy, trastuzumab, and heat shock protein 90 (HSP90) inhibitor, in HER2‐expressing breast cancer. The aim of this study is to evaluate the feasibility of monitoring therapeutic response with ⁸⁹Zr‐DFO‐pertuzumab for the treatment of HER2‐targeted therapeutics, trastuzumab, or the HSP90 inhibitor 17‐DMAG, in trastuzumab‐resistant JIMT‐1 breast cancer models. We prepared an immuno‐PET imaging agent using desferoxamine (DFO)‐pertuzumab labeled with ⁸⁹Zr and performed the biodistribution and PET imaging in breast cancer xenograft models for monitoring therapeutic response to HER2‐targeted therapy. ⁸⁹Zr‐DFO‐pertuzumab was successfully prepared and showed specific binding to HER2 in vitro and clearly visualized HER2 expressing JIMT‐1 tumors. 8₉Zr‐DFO‐pertuzumab had prominent tumor uptake in HER2 expressing JIMT‐1 tumors. JIMT‐1 tumors showed trastuzumab‐resistant and HSP90 inhibitor sensitive characterization. In immuno‐PET imaging, isotype antibody‐treated JIMT‐1 tumors had similar uptake in trastuzumab‐treated JIMT‐1 tumors, but 17‐DMAG‐treated JIMT‐1 tumors showed greatly reduced uptake compared to vehicle‐treated tumors. Additionally, HER2 downregulation evaluated by immuno‐PET imaging was verified by western blot analysis and immunofluorescence staining which resulted in a significant reduction in the tumor’s HER2 level in 17‐DMAG‐treated JIMT‐1 tumors. ⁸⁹Zr‐DFO‐pertuzumab immuno‐PET may be clinically translated to select pertinent patients for HER2‐targeted therapy and to monitor the therapeutic response in HER2‐positive cancer patients under various HER2‐targeted therapeutics treatments.