A Comparative Study of AZD9833 Plus Palbociclib Versus Anastrozole Plus Palbociclib in Patients With ER-Positive HER2 Negative Breast Cancer Who Have Not Received Any Systemic Treatment for Advanced Disease

INCLUSION CRITERIA Full list of inclusion criteria * Pre-/peri-menopausal women or men can be enrolled if amenable to be treated with concomitant, approved LHRH agonists for the duration of the study treatment. * De novo Stage 4 disease, or recurrence from early stage disease after at least 24 months of standard adjuvant endocrine therapy. Note that at least 12 months must have elapsed since the patient's last dose of adjuvant AI therapy without disease progression on treatment. Note that a 2-week washout period is required after the last dose of tamoxifen prior to randomisation. * Histologically or cytologically documented diagnosis of ER+, HER2-negative breast cancer based on local laboratory results. * Previously untreated with any systemic anti-cancer therapy for their locoregionally recurrent or metastatic ER+ disease. * Measurable disease as defined per RECIST v.1.1 OR at least one lytic or mixed (lytic + sclerotic) bone lesion with a soft tissue component that can be assessed by CT or MRI. * Eastern Cooperative Oncology Group performance status of 0 or 1. * Adequate organ and marrow function. * Willingness and ability to comply with scheduled visits, treatment plan, laboratory tests, and other study procedures. INFORMATION FOR TRIAL PARTICIPANTS Participants can join the trial if they: * Have breast cancer that cannot be treated with surgery or radiation * Have breast cancer that has already spread into other parts of the body at the time of diagnosis, or has come back after at least 2 years of a standard endocrine treatment * Have ER proteins but not overexpression of HER2 protein in their tumors * Have never received any type of cancer therapy that affects the whole body for advanced breast cancer * Are able to do their daily activities EXCLUSION CRITERIA Full list of exclusion criteria * Previous neoadjuvant or adjuvant treatment with an AI treatment +/- CDK4/6 inhibitor with disease recurrence while on or within 12 months of completing treatment. * Prior exposure to AZD9833, other investigational SERDs/endocrine agents or fulvestrant. * Participation in another clinical study with a study treatment or investigational medicinal device administered in the last 4 weeks prior to randomization or concurrent enrollment in another clinical study, unless it is an observational (non-interventional) clinical study or during the follow-up period of an interventional study. * Advanced, symptomatic, visceral spread, that are at risk of life-threatening complications in the short term and/or impending visceral crisis * Known active uncontrolled or symptomatic CNS metastases, carcinomatous meningitis, or leptomeningeal disease. * Any clinically important and symptomatic heart disease . * Currently pregnant (confirmed with positive pregnancy test) or breast-feeding. * As judged by the investigator, any evidence of diseases (such as severe or uncontrolled systemic diseases, renal transplant and active bleeding diseases) which, in the investigator's opinion, makes it undesirable for the participant to participate in the study or that would jeopardize compliance with the protocol. * Any concurrent anti-cancer treatment. * Active infection including tuberculosis, HBV and HCV. INFORMATION FOR TRIAL PARTICIPANTS Participants cannot join the trial if they: * Have uncontrolled cancer that has spread to the brain or the spinal cord * Have received certain treatments for cancer in the past but the cancer came back within 1 year * Had certain types of tumors in the past, which the study doctors think could come back * Are currently taking any treatment for cancer or are taking medications or supplements that affect certain proteins in the body * Have any major health problem, infection, or surgery that could make it difficult or dangerous to participate in this trial, such as tuberculosis, HIV, heart problems, or a kidney transplant The above information is not intended to contain all considerations relevant to a patient's potential participation in a clinical trial.