A Study of [177Lu]Lu-A9-0631 and [225Ac]Ac-A9-0642 With [68Ga]Ga-A9-6217 or [177Lu]Lu-A9-0631 Imaging in GRPR+ Solid Tumors

Inclusion Criteria: * Participant has provided written informed consent prior to any study-specific procedure. * Age ≥ 18 years old. * Eastern Cooperative Oncology Group (ECOG) performance status of 0 to 1. * Have measurable target lesion per RECIST v1.1. * Progression from, intolerant to, or ineligible for (due to unavailability or contraindication) local standard of care therapies and have one of the following locally advanced or metastatic tumor types: prostate cancer; HR+/HER2- breast cancer; colorectal cancer; and GRPR-positive solid tumors. * Have GRPR-expressing disease as confirmed by PET / CT or SPECT imaging. * Adequate organ function within 14 days of first administration of investigational therapeutic product. * At least 4 weeks from prior major surgery. Exclusion Criteria: * Received anticancer treatment including but not limited to chemotherapy, antibody therapy, immunotherapy, gene therapy, vaccine therapy, angiogenesis inhibitors, or experimental drugs ≤ 14 days prior (≤ 28 days prior in case of checkpoint inhibitor therapy and other antibody therapies) or 5 half-lives, (t1/2) whichever is shorter, to the administration of therapeutic investigational product. * History of uncontrolled allergic reactions and/or known or expected hypersensitivity to a peptide-based imaging or therapeutic agent or any excipient present in either of the therapeutic investigational products. * Major surgery within 4 weeks of planned first dose of the therapeutic investigational product. * Prior external beam radiation therapy to more than 25% of the bone marrow. * Pregnant, breastfeeding, or unwilling to practice birth control during participation in the study. * Any active infection.