Inclusion Criteria:  Male or female Age ≥ 18 years Menopausal Status [Female subjects] Histologically or cytologically confirmed diagnosis of advanced 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 Refractory to or intolerant of established therapy(ies) known to provide clinical benefit for their malignancy Prior Hormonal Therapy: Documented prior response to endocrine therapy for advanced or metastatic disease (SD, PR, or CR) lasting > 6 months24 weeks or disease recurrence after at least 24 months of adjuvant endocrine treatment. Prior Chemotherapy: Up to 2 prior lines of chemotherapy for the treatment of advanced breast cancer Prior treatment with a CDK4/6 inhibitor is allowed Evaluable or measurable disease per RECIST v1.1. Eastern Cooperative Oncology Group (ECOG) Performance Status of ≤ 1 All acute toxic effects of any prior anti-tumor therapy resolved to Grade ≤ 1 or baseline (with the exception of alopecia [any grade permitted]) Adequate organ function [Premenopausal and perimenopausal female subjects]: Negative serum pregnancy test Male and female subjects of childbearing potential or partners of subjects who engage in intercourse must agree to use protocol specified method(s) of contraception.  Exclusion Criteria:  Any of the following within the specified window prior to the first dose of study drug Prior hematopoietic stem cell or bone marrow transplantation Prior radiotherapy to > 25% of bone marrow Brain metastases that require immediate treatment or are clinically or radiologically unstable (i.e., have been stable for < 1 month). If receiving steroids, subjects must be receiving a stable to decreasing corticosteroid dose during at least 1 week before enrollment. Leptomeningeal disease that requires or is anticipated to require immediate treatment. Presence of life-threatening metastatic visceral disease or symptomatic pulmonary lymphangitic spread Other known active cancer(s) likely to require treatment in the next year that would impact the assessment of any study endpoints [Female subjects]: Pregnant or breast-feeding Unexplained symptomatic endometrial disorders (including, but not limited to endometrial hyperplasia, dysfunctional uterine bleeding, or cysts) Impairment of gastrointestinal (GI) absorption for oral medications Nausea, vomiting, or diarrhea > Grade 1 Myocardial infarction, symptomatic congestive heart failure (NYHA > Class II), unstable angina, or serious uncontrolled cardiac arrhythmia within the last 6 months QTc interval > 480 msec (based on the mean value of the triplicate ECGs), family or personal history of long or short QT syndrome, Brugada syndrome or history of Torsade de Pointes Concurrent use of food or drugs known to be moderate or strong CYP3A or CYP2C9 inducers and moderate or strong CYP3A4 or CYP2C9 inhibitors. Positive serum virological tests (HBsAg, HCV-AB, HIV-AB, TP-AB) at screening stage will be excluded. Any clinically significant disorder, condition, or disease that, in the opinion of the Investigator or Medical Monitor would pose a risk to subject safety or interfere with the study evaluations, procedures, or completion