A Study to Investigate the Biological Effects of AZD9833 in Women With ER-positive, HER2 Negative Primary Breast Cancer

Inclusion criteria: * Provision of written informed consent prior to study entry * Female participants aged at least 18 years * Post-menopausal status defined as meeting at least one of the following criteria: 1. Have undergone a bilateral oophorectomy 2. Age ≥ 60 years 3. Age ≥ 50 and \< 60 years and with cessation of menses ≥ 12 months and follicle-stimulating hormone and oestradiol levels in the post-menopausal range and with an intact uterus in the absence of oral contraception or hormone replacement therapy prior to the diagnosis of breast cancer * Female participants with newly diagnosed primary breast cancer scheduled to undergo treatment with curative intent by surgery and irrespective of clinical node status * Histologically confirmed invasive breast cancer involving a palpable tumour of any size, or a tumour with an ultrasound assessed diameter of ≥ 1.0 cm * Participants with adequately treated non-melanoma skin cancer, curatively treated in-situ cancer of the cervix, or other solid tumours curatively treated with no evidence of disease for ≥ 3 months can be considered for the study * According to the local laboratory participants must have: 1. ER positive breast cancer 2. HER2-negative breast cancer * Eastern Cooperative Oncology Group (ECOG) performance status 0 to 1 Exclusion Criteria: * Previous systemic or local treatment for the new primary breast cancer currently under investigation (including surgery, radiotherapy, cytotoxic and endocrine treatments) * Intervention with any of the following: 1. Use of sex-hormone-containing drugs within 6 months prior to the first dose of study treatment 2. Medications or herbal supplements known to be strong inhibitors/inducers of CYP3A4/5, sensitive CYP2B6 substrates and drugs which are substrates of CYP2C9 and/or CYP2C19 which have a narrow therapeutic index 3. Drugs that are known to prolong QT and have a known risk of torsades de pointes * Inflammatory breast cancer * Any evidence of severe or uncontrolled systemic diseases which in the investigator's opinion makes it undesirable for the participant to participate in the study * Any of the following cardiovascular criteria: Mean resting QTcF \> 470 msec; resting heart rate of \< 50 bpm for stages 1 and 2 at screening;resting heart rate \<60 bpm at screening for Stage 3; any clinically important abnormalities in rhythm, conduction, or morphology of resting ECG; any factors that increase the risk of QTc prolongation or risk of arrhythmic events; known left ventricular ejection fraction \< 50%; significant cardiovascular procedure or event within the last 6 months; uncontrolled hypertension or symptomatic hypotension * Inadequate bone marrow reserve or organ function * Refractory nausea and vomiting, uncontrolled chronic gastrointestinal diseases, inability to swallow the formulated product, or previous significant bowel resection that would preclude adequate absorption of AZD9833 * History of hypersensitivity to active or inactive excipients of AZD9833 * Previous randomisation in the present study * Judgement by the Investigator that the participant should not participate in the study if the participant is unlikely to comply with study procedures, restrictions and requirements