Inclusion Criteria:

          -  Age ≥ 18 years of age who are 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
                  FSH level within the laboratory's reference range for postmenopausal females;

               -  Documented bilateral oophorectomy;

               -  Medically confirmed ovarian failure.

          -  Histologically or cytologically confirmed diagnosis of advanced (metastatic or
             locoregionally recurrent) adenocarcinoma of the breast, not amenable to any potential
             curative intervention

          -  Estrogen Receptor (ER) positive disease

          -  Human Epidermal Growth Factor Receptor 2 (HER2) negative disease

          -  Radiographic documentation of disease progression on the last (most recent) treatment
             administered prior to enrolling in the study.

          -  Documented prior response to endocrine therapy for metastatic disease (SD, PR, or CR
             by RECIST v1.1 criteria) lasting > 6 months

          -  Measurable disease meeting the criteria specified by RECIST v1.1. Tumor lesions
             previously irradiated or subjected to other locoregional therapy will only be deemed
             measurable if progression at the treated site after completion of therapy is clearly
             documented.

        (Note: subjects with bone-only metastatic cancer may be enrolled in the dose escalation
        portion of the study but must have a lytic or mixed lytic-blastic lesion that can be
        accurately assessed by CT or MRI.)

        Exclusion Criteria:

          -  Subjects who have received only neoadjuvant or adjuvant therapy for breast
             adenocarcinoma

          -  Monotherapy: Prior anticancer or investigational drugs for the treatment of ER+/HER2
             negative advanced breast cancer within the following windows:

               -  Tamoxifen, aromatase inhibitor, or other anti-cancer endocrine therapy < 14 days
                  before first dose of study treatment

               -  Any chemotherapy < 28 days before first dose of study

               -  Any investigational drug therapy < 28 days or 3 half-lives (whichever is longer)
                  prior to first dose of study treatment

          -  Combination Therapy: Any prior treatment with any CDK4/6 inhibitor (combination
             cohorts only)

          -  Any prior treatment with immunotherapy (e.g. ipilimumab, nivolumab, pembrolizumab,
             atezolizumab)

          -  Patients with bone-only disease (Dose Expansion cohorts only) Note: Patients with
             bone-only disease may be enrolled in the Dose Escalation portion of the study, but
             must have a lytic or mixed lytic-blastic lesion that can be accurately assessed by CT
             or MRI

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