This will be a Phase 2 clinical trial to assess somatostatin receptor (SSTR) expression in Stage IV estrogen receptor positive (ER+) breast cancer using Gallium-68 DOTATATE or Copper-64 DOTATATE PET/CT.

Subjects with metastatic breast cancer are planned to be enrolled to undergo Gallium-68 DOTATATE or Copper-64 DOTATATE PET/CT imaging at one time point. SSTR uptake in metastatic lesions will be evaluated by independent central review (ICR). FDG PET and bone scans will also be performed within +/- 3 weeks of DOTATATE imaging.

The primary objective of this study is to evaluate uptake of Gallium-68 DOTATATE or Copper-64 DOTATATE in metastatic ER+ breast cancer lesions.

Inclusion Criteria:

• Age of at least 18 years at the time of signing the informed consent.

• Biopsy proven, ER+, HER2 any, clinically progressive, Stage IV breast cancer requiring restaging.

• For women of childbearing potential (WOCBP):

  a. Negative serum pregnancy test within 48 hours of Ga-68-DOTATATE injection. A women is considered to be of childbearing potential if she is post-menarchal, has not reached postmenopausal state (≥ 12 continuous months of amenorrhea [no menstrual bleeding of any kind, including menstrual period, irregular bleeding, spotting, etc.] with no identified cause other than menopause), and has not undergone surgical sterilization (total hysterectomy, or bilateral tubal ligation or bilateral oophorectomy at least 6 weeks before taking study drug).

• Willingness and ability to provide written informed consent prior to any study-specific assessments and procedures commence.

Exclusion Criteria:

• Known hypersensitivity to Gallium-68 DOTATATE or Copper-64 DOTATATE, octreotate, or any of the excipients of Gallium-68 DOTATATE or Copper-64 DOTATATE
• Current high-dose glucocorticoid (≥ 20 mg prednisone daily or equivalent); long acting SSA within the last 28 days; short acting SSA that cannot be interrupted for 24 hours.
• Unable to perform PET/CT scans according to technical specifications and local guidelines.
• Concurrent primary malignancy, except adequately treated carcinoma in situ, non-melanoma carcinoma of the skin or any other curatively treated malignancy that has achieved complete response and is not expected to require treatment for recurrence during participation in the study.
• Current somatic or psychiatric disease/condition that may interfere with the objectives and assessments of the study.
• Unable or unwilling to comply with the requirements of the study protocol.
• Prior participation in any interventional clinical study within 30 days prior to SSTR PET/CT scan.