Safety, Tolerability, Pharmacokinetics, and Efficacy of Acapatamab in Subjects With mCRPC

All Parts Inclusion Criteria: * Subject has provided informed consent prior to initiation of any study specific activities/procedures * Subjects with histologically or cytologically confirmed mCRPC who are refractory to a novel antiandrogen therapy (abiraterone, enzalutamide, and/or apalutamide) and have failed at least 1 (but not more than 2) taxane regimens (or who are deemed medically unsuitable to be treated with a taxane regimen or have actively refused treatment with a taxane regimen). Progression on novel antiandrogen therapy may have occurred in the non-metastatic CRPC setting * Subjects must have undergone bilateral orchiectomy or must be on continuous ADT with a gonadotropin releasing hormone (GnRH) agonist or antagonist * Total serum testosterone \</= 50 ng/dL or 1.7 nmol/L * Evidence of progressive disease, defined as 1 or more PCWG3 criteria: * PSA level \>/= 1 ng/mL that has increased on at least 2 successive occasions at least 1 week apart * nodal or visceral progression as defined by RECIST 1.1 with PCGW3 modifications * appearance of 2 or more new lesions in bone scan * Eastern Cooperative Oncology Group (ECOG) performance status of 0 - 1 * Life expectancy \>/= 6months Exclusion Criteria: * Any anticancer therapy or immunotherapy within 4 weeks of start of first dose, not including luteinizing hormone-releasing hormone agonist (LHRH)/GnRH analogue (agonist/antagonist). Subjects on a stable bisophosphonate or denosumab regimen for \>/= 30 days prior to randomization are eligible * Prior PSMA-targeted therapy (subjects on prior therapy may be eligible if discussed with Amgen medical monitor prior to enrollment) * Central nervous system (CNS) metastases, leptomeningeal disease, or spinal cord compression * Active autoimmune disease or any other diseases requiring immunosuppressive therapy while on study * Needing chronic systemic corticosteroid therapy (prednisone \> 10 mg per day or equivalent) or any other immunosuppressive therapies (including anti-tumor necrosis factor alpha \[TNF alpha\] therapies) unless stopped 7 days prior to start of first dose * Myocardial infarction, unstable angina, cardiac arrhythmia requiring medication, and/or symptomatic congestive heart failure (New York Heart Association \> class II) within 12 months of first dose of acapatamab Part 2 only: * Subjects on a prior PD-1 or PD-L1 inhibitor who experienced a Grade 3 or higher immune-related adverse event prior to first day of dosing * History or evidence of interstitial lung disease or active, non-infectious pneumonitis Part 3 only: \- Evidence of active tuberculosis on chest radiograph within 3 months prior to the first dose of investigational product Part 6 only: Subjects are excluded from this cohort if any of the following additional criteria apply: * Subjects taking strong OAT3 inhibitors (eg, probenecid) or adjust the dosing to 1 mg PO QD. * Subjects with latent or active tuberculosis at screening