Major Inclusion Criteria:

          1. Signed written informed consent.

          2. >= 20 years.

          3. Any menopausal status:

             Pre and Post menopausal defined according to standard criteria in the protocol.

          4. Histological or cytological confirmation of adenocarcinoma of the breast.

          5. Documented positive estrogen receptor status of primary or metastatic tumor tissue,
             according to the local laboratory parameters. HER-2 negative.

          6. Metastatic or locoregionally recurrent disease and radiological or objective evidence
             of progression on or after the last systemic therapy prior to starting IMP.

          7. Prior chemotherapy, endocrine therapy and other therapy in the advanced setting is
             restricted as follows:

               1. No more than 2 lines of chemotherapy for advanced disease.

               2. Recurrence or progression on at least one line of endocrine therapy in the
                  advanced/metastatic disease setting.

               3. There is no limit on the number of lines of prior endocrine therapies.

               4. Prior treatment with CDK4/6 inhibitors is permitted.

          8. At least one lesion (measurable and/or non-measurable, as per RECIST 1.1) that can be
             accurately assessed at baseline and is suitable for repeated assessment by computed
             tomography (CT), magnetic resonance imaging (MRI), or plain X-ray; or clinical
             examination.

          9. Eastern Cooperative Oncology Group (ECOG)/World Health Organisation (WHO) performance
             status 0 to 1

        Major Exclusion Criteria:

          1. Intervention with any of the following:

               1. Any cytotoxic chemotherapy, investigational agents, or other anti-cancer drugs
                  for the treatment of advanced breast cancer from a previous treatment regimen or
                  clinical study within 14 days of the first dose of study treatment.

               2. Medications or herbal supplements known to be strong inhibitors/inducers of
                  cytochrome P450 (CYP) 3A4/5 sensitive CYP2B6 substrates and drugs which are
                  substrates of CYP2C9 and/or CYP2C19.

               3. Drugs that are known to prolong QT and have a known risk of Torsades de Pointes.

               4. Radiotherapy with a limited field of radiation for palliation within one week of
                  the first dose of IMP, radiotherapy to more than 30% of the bone marrow or a wide
                  field of radiation within 4 weeks of the first dose of IMP.

               5. Major surgical procedure or significant traumatic injury.

          2. Any unresolved toxicities from prior therapy greater than CTCAE Grade 1 at the time of
             starting IMP.

          3. Presence of life-threatening metastatic visceral disease.

          4. Evidence of severe or uncontrolled systemic diseases, including uncontrolled
             hypertension and active bleeding diatheses.

          5. 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.

          6. History of another primary malignancy.

          7. Male subjects are excluded from this study.

          8. History of hypersensitivity to active or inactive excipients of AZD9833.

          9. The following cardiovascular criteria: QTcF >470 ms, resting heart rate <45 bpm,
             clinically significant abnormalities of resting electrocardiogram, uncontrolled
             hypertension, symptomatic hypotension, factors that increase the risk for QTc
             prolongation, left ventricular ejection fraction <50%.

         10. Inadequate bone marrow reserve or organ function

         11. Involvement in the planning and conduct of the study.

         12. Judgment by the investigator that the patient should not participate in the study.