• Adult female or male \>= 18 years of age at the time of informed consent

• Histologically and/or cytologically confirmed diagnosis of estrogen-receptor positive with ER \>10% (regardless of progesterone receptor (PgR) expression) breast cancer by local laboratory testing (based on the most recently analyzed tissue sample)

• HER2 negative breast cancer defined as a negative in situ hybridization test or an immunohistochemistry (IHC) status of 0, 1+ or 2+. If IHC is 2+, a negative in situ hybridization (e.g. fluorescence in situ hybridization (FISH), chromogenic in situ hybridization (CISH), or silver in situ hybridization (SISH)) test is required by local laboratory testing (based on the most recently analyzed tissue sample)

• Participant has advanced (loco regionally recurrent not amenable to curative therapy (e.g. surgery and/or radiotherapy) or metastatic) breast cancer

• Participants may be:

⁃ relapsed with documented evidence of relapse on or within 12 months from completion of (neo)adjuvant endocrine therapy (+/- CDK4/6 inhibitor) with no treatment for advanced disease OR

⁃ relapsed with documented evidence of relapse more than 12 months from completion of (neo)adjuvant endocrine therapy and then subsequently progressed with documented evidence of progression after one line of endocrine therapy (except fulvestrant) (+/- CDK4/6 inhibitor) for advanced disease OR

⁃ advanced breast cancer at diagnosis that progressed with documented evidence of progression after one line of endocrine therapy (except fulvestrant) (+/- CDK4/6 inhibitor) Note: Participant who relapsed with documented evidence of relapse on/or within 12 months from completion of (neo)adjuvant endocrine therapy and then subsequently progressed with documented evidence of progression after one line of endocrine therapy (with either an antiestrogen or an aromatase inhibitor) for advanced disease will NOT be included in the study. At least one target lesion (i.e., a measurable lesion as per RECIST 1.1) in the baseline stand-alone CT or MRI, showing \[68Ga\]Ga-NeoB uptake on PET/CT or PET/MRI scoring 2 or higher, based on the Visual Scoring Scale.

∙ Adequate bone marrow and organ function as defined by the laboratory values.

‣ Standard 12-lead ECG values defined as the mean of the triplicate ECGs and assessed locally:

‣ QT interval corrected by Fridericia's formula (QTcF) interval at screening \< 450 msec

‣ Mean resting heart rate 50-90 bpm (determined from the ECG)

‣ Eastern Cooperative Oncology Group (ECOG) performance status 0 or 1