Inclusion Criteria:

          -  Age ≥ 18 years of age

               -  Women can be 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;

               -  Can be peri- or premenopausal, however, they 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 (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

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

          -  Evaluable or measurable disease 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.

        Exclusion Criteria:

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

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

               -  Any chemotherapy < 28 days before first dose of study, except for Phase 2
                  monotherapy which requires no prior chemotherapy treatment.

               -  Any investigational drug therapy < 28 days or 5 half-lives (whichever is shorter)
                  prior to first dose of study treatment

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