TPS256 Background: Pembro, a PD-1 inhibitor, and olaparib, a PARP inhibitor, have some single-agent antitumor activity in mCRPC. The phase 1b/2 KEYNOTE-365 study (NCT02861573) showed promising activity with pembro and olaparib in pts with mCRPC unselected for homologous recombination deficiency, warranting further investigation. Methods: KEYLYNK-010 (NCT03834519) is a phase 3 trial to evaluate efficacy and safety of pembro + olaparib in molecularly unselected enza- or abi-pretreated pts with mCRPC who progressed with taxane chemotherapy. An estimated 780 pts will be randomly assigned 2:1 to receive pembro 200 mg IV every 3 wk plus olaparib 300 mg orally twice daily or abi 1000 mg orally once daily + prednisone/prednisolone 5 mg orally twice daily (enza-pretreated pts) or enza 160 mg/day orally (abi-pretreated pts). Stratification will be by prior treatment (abi/enza) and measurable disease (yes/no). Adults (≥18 y) with histologically confirmed mCRPC who progressed on androgen deprivation therapy ≤6 mo of screening, and an ECOG PS of 0 or 1 are eligible. Pts previously received either abi or enza (but not both) as well as 1 docetaxel-based regimen. Pts are required to provide tumor tissue for biomarker analysis. Responses will be assessed by CT/MRI and radionuclide bone imaging per Prostate Cancer Working Group 3 (PCWG3)–modified RECIST v1.1 by blinded independent central review (BICR) every 9 wk during the first year and every 12 wk thereafter. Treatment will continue with up to 2 y of pembro (35 cycles) and olaparib or abi/enza until disease progression, unacceptable toxicity, or consent withdrawal. Primary end points are OS and rPFS. Secondary end points are time to initiation of subsequent anticancer therapy; ORR, and DOR per PCWG3-modified RECIST v1.1 by BICR; time to PSA progression; time to first symptomatic skeletal event; time to radiographic soft tissue progression; time to pain progression; and safety. Exploratory end points are identification of molecular (genomic, metabolic, or proteomic) biomarkers indicative of response. Accrual began May 2, 2019. Clinical trial information: NCT03834519.