Inclusion Criteria:  ≥ 18 years of age. Metastatic CRPC with castrate levels of testosterone (≤ 50 ng/dL or ≤ 1.7 nmol/L). Eastern Cooperative Oncology Group (ECOG) performance status of 0 to 1. Progressed (by PSA or radiologic criteria) during or following treatment with a novel androgen receptor signaling inhibitor (ARSI, eg, abiraterone, enzalutamide, apalutamide, darolutamide), which may have been given for either hormone-sensitive prostate cancer or CRPC. Received 1 to 2 prior lines of taxane chemotherapy, unless the physician and patient believe the patient is medically ineligible or the patient refuses (ineligibility or refusal must be documented in the source documents). Progressed by PSA or radiologic criteria on or during last therapy for prostate cancer.  Measurable or non-measurable disease as per radiographic evaluation. Lesions situated in a previously irradiated area are considered evaluable if progression has been demonstrated in such lesions since radiation.  • Note: If disease is considered non-measurable, a minimum PSA of 1 ng/dL is required with at least 1 confirmed rise at a minimum of a 1-week interval.  Adequate hematologic function, defined as absolute neutrophil count ≥ 1.5 × 109/L, hemoglobin ≥ 9.0 g/dL, and platelet count ≥ 100 × 109/L. Transfusions are permitted to meet hemoglobin and platelet criteria. However, the patient must have a stable hemoglobin level and platelet count for ≥ 2 weeks prior to dosing without transfusion. Adequate renal function, defined as serum creatinine clearance ≥ 30 mL/min per Cockcroft-Gault formula. Total bilirubin ≤ 1.5 × upper limit of normal (ULN) (≤ 3 × ULN if elevated because of Gilbert's syndrome, and ≤ 2 × ULN for patients with known liver metastases). Aspartate aminotransferase and alanine aminotransferase < 2.5 × ULN (< 5 × ULN if liver metastases present). Prothrombin time (PT) or international normalized ratio (INR) and activated partial thromboplastin time (aPTT) ≤ 1.5 × ULN unless the patient is receiving anticoagulant therapy, in which case PT/INR or aPTT must be within therapeutic range of intended use of anticoagulants.  Exclusion Criteria:  Currently participating in or has participated in a trial of an investigational device or has used an investigational device within 21 days before the first dose of study drug. Any component of small cell or neuroendocrine histology. Previously received an anti-CD39 antibody, anti-CD39 targeted therapy, or other agent targeting the adenosine pathway. Prior treatment with programmed death-ligand 1 (PD-L1)/programmed death receptor-1 (PD-1) inhibitors. Prior treatment with ≥ 3 lines of taxane chemotherapy administered as a single agent or as part of a combination regimen. Symptomatic or untreated brain metastases (including leptomeningeal metastases). Patients previously treated for brain metastases must be at least 4 weeks from completion of radiation treatment with follow-up imaging showing no progression. Current pneumonitis with or without steroid requirement or history of pneumonitis requiring steroids. Another malignancy other than prostate within 2 years of trial entry, except for those with a low risk of spreading or negligible risk of death such as non-melanoma skin cancer or Ta superficial bladder cancer. Active autoimmune disease that has required systemic treatment in past 2 years (ie, with use of disease modifying agents, corticosteroids, or immunosuppressive drugs).  Medical conditions requiring chronic steroid (ie, > 10 mg/day of prednisone or its equivalent).  • Note: Replacement therapy (eg, levothyroxine, insulin, or physiologic corticosteroid replacement therapy for thyroid, adrenal, or pituitary insufficiency) is allowed.  Administration of a live attenuated vaccine within 6 weeks before the first dose of study drug.  • Exception: Health Authority approved COVID-19 vaccines are permitted.  Any gastrointestinal condition that would preclude the use of oral medications (eg, difficulty swallowing, nausea, vomiting, or malabsorption).