Saruparib (AZD5305) Plus Camizestrant Compared With CDK4/6 Inhibitor Plus Endocrine Therapy or Plus Camizestrant in HR-Positive, HER2-Negative (IHC 0, 1+, 2+/ ISH Non-amplified), BRCA1, BRCA2, or PALB2m Advanced Breast Cancer

The primary objective of the study is to measure efficacy of saruparib (AZD5305) plus camizestrant compared with physician's choice CDK4/6i plus ET in patients with BRCA1, BRCA2, or PALB2m, HR-positive, HER2-negative (defined as IHC 0, 1+, 2+/ ISH non-amplified) advanced breast cancer

Approximately 2,620 participants will be screened to achieve approximately 500 participants randomised to study intervention. Participants will be randomised in a 2:2:1 ratio to one of the following intervention groups: * Arm 1: saruparib (AZD5305) plus camizestrant * Arm 2: Physician's choice CDK4/6i plus physician's choice ET * Arm 3: Physician's choice CDK4/6i plus camizestrant Treatment continues until BICR-confirmed disease progression, unacceptable toxicity occurs, or the participant withdraws consent.

Inclusion Criteria: * Adult females, pre/peri-menopausal and/or post-menopausal, and adult males * Histologically or cytologically documented diagnosis of HR-positive, HER2-negative breast cancer * Advanced breast cancer with either locally advanced disease not amenable to curative treatment or metastatic disease * ECOG performance status of 0 or 1 with no deterioration over the previous 2 weeks * FFPE tumour tissue from each participant * Documented germline tumour loss of function mutation in BRCA1, BRCA2, or PALB2 * Adequate organ and marrow function Exclusion Criteria: * Participants with history of MDS/AML or with features suggestive of MDS/AML * Participants with any known predisposition to bleeding * Any history of persisting severe cytopenia * Any evidence of severe or uncontrolled systemic diseases or active uncontrolled infections * Refractory nausea and vomiting, chronic GI disease, inability to swallow the formulated product, or previous significant bowel resection * History of another primary malignancy * Persistent toxicities (CTCAE Grade ≥ 2) caused by previous anti-cancer therapy excluding alopecia * Spinal cord compression, brain metastases, carcinomatous meningitis, or leptomeningeal disease * Evidence of active and uncontrolled hepatitis B and/or hepatitis C * Evidence of active and uncontrolled HIV infection * Active tuberculosis infection * Cardiac criteria, including history of arrythmia and cardiovascular disease * Concurrent exogenous reproductive hormone therapy or non-topical hormonal therapy for non-cancer-related conditions * Major surgical procedure or significant traumatic injury within 4 weeks of the first dose of study intervention or an anticipated need for major surgery during the study * Palliative radiotherapy with a limited field of radiation within 2 weeks or with wide field of radiation or to more than 30% of the bone marrow within 4 weeks before the first dose of study treatment * Prior treatment with systemic anti-cancer therapy for locoregionally recurrent or metastatic disease is not permitted, apart from treatment with ET up to 28 days before randomisation * Prior treatment within 28 days with blood product support or growth factor support * Any systemic concurrent anti-cancer treatment * Concomitant use of the following types of medications or herbal supplements within 21 days or at least 5 half-lives of randomisation: 1. Strong and moderate CYP3A4 inducers/inhibitors 2. Sensitive CYP2B6 substrates 3. Substrates of CYP2C9 and/or CYP2C19 which have a narrow therapeutic index, eg, warfarin (and other coumarin-derived vitamin K antagonist anticoagulants) and phenytoin. * Concomitant use of drugs that are known to prolong QT and have a known risk of TdP * Systemic use of atropine * The following exclusion criteria apply to treatments administered for early breast cancer: 1. Disease progression ≤ 84 days following the last dose of neo-adjuvant or adjuvant chemotherapy 2. Disease progression ≤ 1 year (365 days) from the last dose of treatment with a PARPi and/or platinum agent for early breast cancer 3. Disease progression ≤ 1 year (365 days) from the last dose with a CDK4/6i in the adjuvant setting 4. Disease progression ≤ 1 year (365 days) from the last dose of an oral SERD including camizestrant.