Inclusion Criteria:

          -  Age ≥ 18 years of age

          -  Women can be peri- or postmenopausal, as defined by at least one of the following:

               -  Age ≥ 60 years;

               -  Age < 60 years and cessation of regular menses for at least 12 consecutive months
                  with no alternative pathological or physiological cause; and serum estradiol and
                  follicle-stimulating hormone (FSH) level within the laboratory's reference range
                  for postmenopausal females;

               -  Documented bilateral oophorectomy;

               -  Must receive a gonadotrophin-releasing hormone agonist beginning at least 4 weeks
                  prior to the first dose of study medication

          -  Histologically or cytologically confirmed diagnosis of advanced adenocarcinoma of the
             breast

          -  Estrogen receptor positive disease

          -  Human Epidermal Growth Factor Receptor 2 negative disease

          -  Measurable disease per Response Evaluation Criteria in Solid Tumors v1.1

        Exclusion Criteria:

          -  Prior therapy within the following windows:

               -  Tamoxifen, aromatase inhibitor, fulvestrant, or other anti-cancer endocrine
                  therapy < 14 days;

               -  Any investigational drug therapy < 28 days or 5 half-lives (whichever is shorter)

               -  Any prior systemic chemotherapy regardless of the stop date, but the subject must
                  have recovered to eligibility levels from prior toxicity

          -  Prior treatment with CDK4/6 inhibitors

          -  Unexplained symptomatic endometrial disorders (including, but not limited to,
             endometrial hyperplasia, dysfunctional uterine bleeding, or cysts)