Key Inclusion Criteria:

          -  Progressive metastatic or locally advanced or metastatic breast cancer.

          -  Clinically confirmed as postmenopausal.

          -  Eastern Cooperative Oncology Group (ECOG) score of 0 to 1 at screening.

        Key Exclusion Criteria:

          -  HER2 positive patients by local laboratory testing.

          -  Triple negative breast cancer.

          -  Any chemotherapy within the 28 days prior to the first dose of study drug.

          -  Any non-chemotherapy anti-cancer drug less than 5 half-lives (30 days for biologics)
             or less than 14 days for small molecule therapeutics, or if half-life is not known.

          -  Tamoxifen and aromatase inhibitors within 14 days prior to the first dose of study
             drug.

          -  Fulvestrant within 30 days prior to first dose of study drug.

          -  Any investigational drug therapy within 5 half-lives of the previous investigational
             study drug or 30 days, whichever is shorter.

          -  Radiation therapy for breast cancer within 2 weeks of dosing and planning to have
             radiation therapy during participation in this study.

          -  Known history of human immunodeficiency virus infection (HIV) or hepatitis C or active
             hepatitis B infection, unless the patient was diagnosed >10 years prior to enrollment
             and no evidence of active liver disease.

          -  Currently taking testosterone, methyltestosterone, oxandrolone (Oxandrin),
             oxymetholone, danazol, fluoxymesterone (Halotestin), or testosterone-like agents.

          -  Untreated or uncontrolled brain metastasis.

          -  Diagnosed with or treated for cancer within the previous 2 years, other than breast
             cancer or non-melanoma carcinoma of the skin.

          -  Pregnant and nursing females.