This study compares a new drug, Saruparib (AZD5305), with a placebo in men with a type of prostate cancer called metastatic castration-sensitive prostate cancer (mCSPC). *Metastatic* means the cancer has spread, and *castration-sensitive* means it can be controlled by lowering testosterone. The study involves about 1800 men who will be split into two groups. They will be chosen at random to receive either the drug with a new hormonal agent (NHA) or a placebo with NHA. Participants will undergo regular scans to check if the cancer is growing. The study aims to see if Saruparib can help patients live longer without the cancer getting worse. Key Points:

• Study includes regular tumor scans and treatments continue until cancer progresses.
• Participants must be male, 18 or older, with specific cancer characteristics.
• Participants must not father children or donate sperm during and after the study for a set time.

Participants will be monitored for survival, and their safety will be checked by a group of experts. Eligibility includes having prostate cancer that fits specific criteria and being able to provide certain medical samples.