A Study to Investigate Efficacy and Safety With Oral AZD9833 Compared With Intramuscular Fulvestrant in Post-menopausal Women at Least 18 Years of Age With Advanced ER-positive HER2 Negative Breast Cancer

This study is looking for post-menopausal women, aged 18 or older, who have advanced breast cancer. Specifically, your cancer needs to be 'ER-positive' (meaning it's affected by hormones) and 'HER2-negative'. This means your cancer has either spread to other parts of the body or has come back in the nearby area after previous treatments. You must also have evidence that your cancer has progressed or gotten worse despite your last cancer treatment.

There are also some important points that might mean you can't join the study. For example, you shouldn't have had certain other breast cancer medications (like Fulvestrant or similar drugs) before. You also need to be generally well enough to take part, as assessed by your medical team. Your doctors will check your full medical history and current health to make sure this study is right and safe for you. They'll also make sure you're not taking any medications that might interfere with the study drugs.

If you join this study, it will involve several stages. First, you'll have a 'screening visit' where doctors will check if you meet all the requirements. If you're eligible, you'll be randomly assigned to one of the treatment groups: either different doses of the new drug AZD9833 (taken as tablets) or the existing drug Fulvestrant (given as an injection). You'll take your study treatment in cycles, usually lasting four weeks each.

Throughout the treatment period, you'll have regular scheduled visits to the clinic. During these visits, the medical team will monitor your health, check for any side effects, and assess how well the treatment is working. This will involve physical examinations, blood tests, and scans. You'll continue with the treatment cycles until your cancer shows signs of progressing or if you decide with your doctor to stop treatment.

Once you stop taking the study treatment, you'll have at least two safety follow-up visits: one when you stop treatment and another about 28 days later. After these visits, the study team will continue to follow your health and survival over a longer period, even if you are no longer taking the study drug. The total duration of your participation will vary depending on how long you receive treatment and the length of the follow-up period.