Inclusion Criteria:

        Part A:

          -  Patients at least 18 years of age at the time of signing the informed consent.

          -  Patients must have histologically or cytologically confirmed ER+ and HER2- advanced
             breast cancer for which standard curative therapy is no longer effective or does not
             exist.

          -  Patients must have measurable or non-measurable disease by RECIST criteria
             (version1.1), with radiologic tumor assessments performed within 28 days of the first
             dose of therapy.

          -  Patients must have received at least 2 prior endocrine regimens in any setting
             (neoadjuvant, adjuvant or advanced/metastatic) a CDK4/6 inhibitor and up to 3 prior
             regimens of cytotoxic chemotherapy in the locally advanced or metastatic setting.

          -  Patients must be willing to undergo a biopsy of accessible tumor within 4 weeks prior
             to the initiation of study treatment and a follow-up biopsy on treatment for ER IHC
             testing and PD studies. (Patients without accessible tumor tissue may be eligible
             after discussion with the Medical Monitor.)

          -  Women must be postmenopausal due to surgical or natural menopause.

        Part B:

          -  Patients must have received at least 1 prior endocrine regimen for a minimum of 6
             months in the locally advanced or metastatic setting; if more than 1 prior endocrine
             regimen has been administered, only one of the regimens must have been administered
             for a minimum of 6 months in the locally advanced or metastatic setting

          -  Patients must have received a CDK4/6 inhibitor

          -  Patients must have received 1 prior regimen of cytotoxic chemotherapy in the locally
             advanced or metastatic setting

          -  Women must be postmenopausal due to surgical or natural menopause.

        Part C:

          -  Patients must have received no more than one prior endocrine therapy in the advanced
             setting unless the patient recurred during adjuvant treatment or within the 12 months
             of completion of adjuvant endocrine therapy in which case no prior endocrine therapy
             is required in the advanced setting.

          -  Patients must have received no more than one prior chemotherapy regimen for advanced
             disease.

          -  Women must be postmenopausal due to surgical or natural menopause.

        Exclusion Criteria:

        Part A:

          -  Patients with known symptomatic brain metastases requiring steroids (above physiologic
             replacement doses). Patients with previously diagnosed brain metastases are eligible
             if they have completed their treatment and have recovered from the acute effects of
             radiation therapy or surgery prior to first dose of study drug, have discontinued
             high-dose corticosteroid treatment for these metastases for at least 4 weeks and are
             neurologically stable as judged by the Investigator.

          -  Patients who have received 4 or more regimens of chemotherapy for locally advanced or
             mBC.

          -  Receipt of prior anti-cancer or other investigational therapy within 14 days prior to
             the first administration of study drug.

          -  Radiation therapy within 4 weeks of first dose of study drug or prior irradiation to
             >25% of the bone marrow. Palliative radiation for the alleviation of pain due to bone
             metastasis will be allowed during the study.

        Part B:

          -  Patients who have received more than 1 regimen of chemotherapy for locally advanced or
             mBC.

        Part C:

          -  Patients have received more than one regimen of chemotherapy for advanced/metastatic
             disease (regardless of prior adjuvant chemotherapy use) in addition to endocrine
             therapy.