Phase 1 Study of 68Ga-R11228 and 177Lu-R11228 in Patients With Advanced Breast Cancer

Inclusion Criteria: Part A * Pathologically confirmed estrogen and/or progesterone receptor (ER/PR) positive and HER2 negative locoregionally recurrent or metastatic breast cancer * At least one target or non-target lesion per RECIST v1.1 criteria. * Male or non-pregnant, non-lactating female subjects age ≥18 years. Part B * Pathologically confirmed estrogen and/or progesterone receptor (ER/PR) positive and HER2 negative locoregionally recurrent or metastatic breast cancer * Refractory to endocrine therapy. Note: there is no limit on prior number of lines of endocrine therapy or prior treatments with CDK4/6, AKT, PI3K and/or mTOR inhibitors. * Received or declined at least one line of chemotherapy or antibody drug conjugate in the locoregionally recurrent or metastatic setting * Progressive disease or intolerance to last treatment. * At least one target lesion per RECIST v1.1 criteria. * Male or non-pregnant, non-lactating female subjects age ≥18 years. * Eastern Cooperative Oncology Group (ECOG) Performance Status of 0 or 1. * Life expectancy of at least six months. * Adequate bone marrow reserve, hepatic function and renal function. Exclusion Criteria: Part A * Study participant has not recovered from clinically significant adverse event(s) resulting from most recent anticancer therapy/intervention. * Known central nervous system (CNS) disease, except for those subjects with treated brain metastasis who are stable for at least 1 month, having no evidence of progression or hemorrhage after treatment and no ongoing requirement for corticosteroids, as ascertained by clinical examination and brain imaging (magnetic resonance imaging \[MRI\] or computed tomography \[CT\]) during the screening period. * Radiotherapy for breast cancer ≤ 28 days prior * Received a radionuclide within a period of less than 10 physical half-lives of the administered radionuclide prior to dosing with 68Ga-R11228. * Any condition that precludes the proper performance of imaging procedures required in this study. Part B * Study participant has not recovered from clinically significant adverse event(s) resulting from most recent anticancer therapy/intervention * Treatment with anticancer therapy or with an investigational drug or device within 21 days or 5 half-lives of the agent (whichever is shorter) * Known central nervous system (CNS) disease, except for those subjects with treated brain metastasis who are stable for at least 1 month, having no evidence of progression or hemorrhage after treatment and no ongoing requirement for corticosteroids, as ascertained by clinical examination and brain imaging (magnetic resonance imaging \[MRI\] or computed tomography \[CT\]) during the screening period. * Radiotherapy for breast cancer ≤ 28 days * Prior systemic radionuclide therapeutic treatment. * Received a radionuclide within a period of less than 10 physical half-lives of the administered radionuclide prior to dosing with 68Ga-R11228. * Any unresolved NCI-CTCAE Version 5.0 Grade 2 or higher toxicity (except alopecia and Grade 2 platinum-therapy related neuropathy) from previous breast cancer treatment and/or from medical/surgical procedures or interventions. * Any condition that precludes the proper performance of imaging procedures required in this study.