The intention of the study is to demonstrate superiority of Saruparib (AZD5305) + physician's choice NHA relative to placebo + physician's choice NHA by assessment of radiographic progression-free survival (rPFS) in participants with mCSPC.

Approximately 1800 adult participants with mCSPC will be assigned to one of two cohorts (550 HRRm and 1250 non-HRRm) and randomized in a 1:1 ratio to receive either Saruparib (AZD5305) with NHA or placebo with NHA. They will receive their assigned treatment and regular tumor evaluation scans until disease progression, or until treatment is stopped for another reason.

All patients will be followed for survival until the end of the study. Independent data monitoring committee (DMC) composed of independent experts will be convened to confirm the safety and tolerability of Saruparib (AZD5305) + physicians choice NHA.

Inclusion Criteria:

• Male ≥ 18 years of age.
• Histologically documented prostate adenocarcinoma which is de novo or recurrent and castration-sensitive. Participants with pathologic features of small cell, neuroendocrine, sarcomatoid, spindle cell, or signet cell histology are not eligible.
• Metastatic disease as documented by the investigator prior to randomisation, with clear evidence of ≥ 1 bone lesion and/or ≥ 1 soft tissue lesion that is suitable for repeated assessment with CT and/or MRI.
• Participant is receiving ADT with a GnRH analogue or has undergone bilateral orchiectomy starting ≥ 14 days and < 4 months prior to randomisation.
• ECOG performance status of 0 or 1 with no deterioration over the 2 weeks prior to randomisation.
• Provision of FFPE tumour tissue sample and blood sample (for ctDNA).
• Confirmed HRRm status by central tumour tissue and/or ctDNA test is required to determine cohort eligibility.
• Adequate organ and bone marrow function as described in study protocol.
• Participants must not father children or donate sperm from signing ICF, during the study intervention and for 6 months after the last dose of study intervention.
• Participants must use a condom from signing ICF, during study intervention, and for 6 months after the last dose of study drug, with all sexual partners.

Exclusion Criteria:

• Participants with a history of MDS/AML or with features suggestive of MDS/AML (as determined by prior diagnostic investigation). In case there is no clinical MDS/AML suspicion, no specific screening for MDS/AML (by BM/bone biopsy) is required.
• Participants with any known predisposition to bleeding.
• Any history of persisting (> 2 weeks) severe cytopenia.
• Refractory nausea and vomiting, chronic gastrointestinal diseases, inability to swallow the formulated product or previous significant bowel resection that would preclude adequate absorption of AZD5305 and/or the assigned NHA.
• History of another primary malignancy, with exceptions.
• Persistent toxicities (CTCAE Grade ≥ 2) caused by previous anticancer therapy.
• Spinal cord compression or brain metastases unless asymptomatic, stable, and not requiring steroids for at least 4 weeks prior to start of study intervention.
• Cardiac criteria, including history of arrhythmia and cardiovascular disease.
• Any prior anticancer pharmacotherapy or surgery for metastatic prostate cancer, with exceptions.
• Prior treatment within 14 days with blood product support or growth factor support.
• Participants who are unevaluable for both bone and soft tissue progression.

Male participants

Qualified researchers can request access to anonymized individual patient-level data from AstraZeneca group of companies sponsored clinical trials via the request portal. All request will be evaluated as per the AZ disclosure commitment: https://astrazenecagrouptrials.pharmacm.com/ST/Submission/Disclosure.

AstraZeneca will meet or exceed data availability as per the commitments made to the EFPIA Pharma Data Sharing Principles. For details of our timelines, please rerefer to our disclosure commitment at https://astrazenecagrouptrials.pharmacm.com/ST/Submission/Disclosure.

When a request has been approved AstraZeneca will provide access to the de-identified individual patient-level data in an approved sponsored tool. Signed Data Sharing Agreement (non-negotiable contract for data accessors) must be in place before accessing requested information. Additionally, all users will need to accept the terms and conditions of the SAS MSE to gain access. For additional details, please review the Disclosure Statements at https://astrazenecagrouptrials.pharmacm.com/ST/Submission/Disclosure.

Double-blind