Trial of 225Ac-DOTATATE (RYZ101) in Subjects With ER+, HER2-negative Unresectable or Metastatic Breast Cancer Expressing SSTRs.

Inclusion Criteria Subjects must meet all the following criteria for enrollment in the study: * Histologically confirmed, ER+, HER2-negative, locally advanced and unresectable or metastatic breast cancer not amenable to treatment with curative intent. * Adequate renal, hematologic, and hepatic function * Additional inclusion criteria for Monotherapy arm: * Measurable disease with at least 80% SSTR-positive lesions by SSTR-PET imaging * Eastern Cooperative Oncology Group (ECOG) performance status ≤2 * Documented radiologic progression after at least 2 and no more than 4 prior chemotherapy or ADC regimens in the metastatic setting * Additional inclusion criteria for ET Combination arms: * Prior treatment with CDK4/6 inhibitors in combination with endocrine therapy in the advanced or metastatic setting. * Eastern Cooperative Oncology Group (ECOG) performance status ≤1 * Bone only or bone dominant disease. At least 1 bone lesion must be SSTR-positive on SSTR-PET imaging. For bone dominant disease, at least 50% of soft tissue measurable lesions must be SSTR-positive on SSTR-PET imaging. At least one bone lesion must be positive on both SSTR-PET and on FDG-PET imaging * Brain metastases are allowed if definitively treated and clinically stable as confirmed by interval imaging Exclusion Criteria Subjects who meet any of the following criteria will be excluded from the study: * Prior radiopharmaceutical therapy, including radioembolization. * Any toxicities from prior treatments that have not recovered to CTCAE Grade ≤1, except for alopecia. * Significant cardiovascular disease * Known active uncontrolled or symptomatic central nervous system (CNS) metastases, carcinomatous meningitis, spinal cord compression, or leptomeningeal disease. * History of hypersensitivity or allergy to 225Ac, 68Ga, 64Cu, octreotate, or any of the excipients of DOTATATE imaging agents. * Prior or concurrent malignancy whose natural history or treatment has the potential to interfere with the study safety or efficacy assessments. * Pregnancy or breastfeeding * Additional exclusion criteria for ET Combination arms: * Presence of liver metastases or symptomatic metastatic visceral disease * Prior PARPi exposure or are candidates for PARPi per local guidelines