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