Study to Evaluate Safety and Dosimetry of [18F]GEH121224 in Patients With Locally Advanced or Metastatic Breast Cancer

Inclusion Criteria: Female patients aged ≥18 years Willingness and ability to comply with study procedures and signed and dated informed consent For women of child-bearing potential, negative urine pregnancy test at screening and o on the day of investigational medicinal product (IMP) administration (with the result known before IMP administration) and willingness to use contraception (barrier, abstinence, non-hormonal) for 3 weeks after injection of [18F]GEH121224 Life expectancy >3 months Diagnosis of locally advanced or metastatic breast cancer Target lesion diameter of ≥15 mm that has not been previously treated with radiotherapy Histologically confirmed locally advanced/metastatic breast cancer with a biopsy within the past 12 months confirming HER2-positive status by either IHC or FISH from primary tumor Previously received no more than 1 prior lines of systemic chemotherapy (including anti HER2 therapy) for metastatic breast cancer Eastern Cooperative Oncology Group (ECOG) performance status 0 to 2 Adequate organ function as defined by: Hemoglobin ≥10 g/dL White blood cell count ≥3.0 × 109/L Platelet count ≥75 × 109/L Serum creatinine ≤1.4 mg/dL Aspartate aminotransferase and alanine aminotransferase ≤2× upper limit of normal (ULN) Total bilirubin ≤2× ULN or 3.0 mg/dL in patients with Gilbert's syndrome At least one lesion is fluorodeoxyglucose (FDG)-avid Exclusion Criteria: Patient has a primary non-breast malignancy (small dermatological malignancies such as basal cell carcinoma <10 mm are allowed) Pregnancy or breast-feeding or, for women of child-bearing potential, unwillingness to use an acceptable form of birth control Chronically active hepatitis B or C Current history of drug or alcohol abuse or any active liver disease Administration of other IMP within 30 days of screening Systemic therapy including anti-HER2 therapy within 2 weeks before enrollment Ongoing toxicity > grade 2 from previous standard or investigational therapies, according to National Cancer Institute's Common Terminology Criteria for Adverse Events (CTCAE) version 5.0 History of cardiac disease (myocardial infarction, arrhythmias requiring therapy, symptomatic valvular disease, cardiomyopathy, or pericarditis) Evidence of significant medical condition, ongoing severe disease, or laboratory finding that, in the opinion of the Investigator, makes it undesirable for the patient to participate in the study Severe claustrophobia, inability to lie flat or fit into the scanner (e.g., body weight ≥350 lbs [160 kg]), or any other inability to tolerate the PET scan Known allergies to any product used in this study or any constituents of GEH121224 (18F) Injection