Biomarkers to Detect Endocrine Therapy Resistance

Inclusion Criteria:

1. Willing to provide informed consent

2. Individuals at least 18 years of age

3. Have biopsy-proven ER/PR-positive (defined as ER ≥1 percent and PR ≥1 percent byIHC) and HER2-negative advanced or metastatic LBC starting new standard of careendocrine therapy

4. Adequate organ function as indicated by standard laboratory tests (CBC, liverfunction tests or CMP) allowing for systemic breast cancer treatment per treatingoncologist

5. Patients with evaluable bone-only disease that is lytic or mixed lytic-sclerotic areeligible

6. Willing to comply with all study procedures and be available for the duration of thestudy

7. Disease may be measurable by RECIST 1.1 criteria or non-measurable. Lesion size mustbe at least 1cm. If only bone lesions present, they should be lytic or mixedlytic-sclerotic. If only liver lesions present, patient is not eligible.

Exclusion Criteria:

1. Patients with active brain metastases

2. Patients with liver-only disease are not eligible due to high background liveractivity related to the radiopharmaceutical's hepatobiliary route of elimination

3. Unable to lie flat during or tolerate PET/CT

4. Patients with a history of allergic reaction attributable to compounds of similarchemical or biologic composition to FFNP

5. Presence of liver failure as judged by patient's treating physician

6. Individuals who are pregnant, lactating, or planning on becoming pregnant during thestudy

7. Not suitable for study participation due to other reasons at the discretion of theinvestigators

8. Patients with progesterone-receptor negative disease defined as PR <1 percent by IHC