HER2 Molecular Imaging With 89Zr-trastuzumab PET/CT as a Predictive Biomarker for Antibody-drug Conjugate Sequencing in Patients With Advanced HER2-positive Breast Cancer

ZEPHIR-02 is a multicentre, open-label phase II study that will enroll subjects with HER2-positive advanced/metastatic breast cancer (mBC) who have experienced disease progression under trastuzumab deruxtecan (T-DXd) in the metastatic setting. All subjects will undergo baseline biopsy, blood collection, FDG-PET/CT and 89Zr-trastuzumab PET/CT (HER2-PET/CT) and will be classified as HER2-PET/CT positive or negative, as previously described in the ZEPHIR trial. Focusing on a central visual "patient-based" classification that captures the entire disease burden, a side-by-side display will be used, comparing baseline FDG-PET/CT (which identifies all FDG-positive metastases regardless of their HER2-imaging status) and HER2-PET/CT. Subjects will be categorized into two HER2-PET/CT patterns (positive vs. negative) based on proportion of FDG-avid tumor load with significant 89Zr-trastuzumab uptake. Subjects classified as "positive" will receive T-DM1 as monotherapy, IV 3.6mg/kg every 3 weeks (21 days +- 3 days) until disease progression, unacceptable toxicity or request of the subject to withdraw from the study. FDG-PET/CT will be performed before cycle 2 of T-DM1 will serve as a research tool to correlate metabolic changes with clinical outcomes. Other FDG-PET/CT will be performed before cycle 4 of T-DM1 for assessment of response. Subjects who demonstrate a partial or complete response (responders) will continue treatment with T-DM1. Subjects who exhibit stable disease or disease progression (non-responders) will discontinue study treatment and enter the survival follow-up period. For responders, subsequent metabolic evaluations will be performed every 3 months, with FDG-PET/CT. Treatment response will be assessed according to metabolic response. For these subjects, mandatory blood samples will be obtained at all metabolic reassessments. Subjects with HER2-PET/CT classified as "negative" will receive treatment of physician's choice (TPC) as per the best local clinical practice and be out of the study. All enrolled subjects will undergo a mandatory biopsy during the pre-treatment period. The study also includes mandatory translational procedures (i.e. collection of tumour biopsy during pre-treatment period and blood samples at pre-specified time points) for exploratory molecular analyses.