Inclusion Criteria:

          1. be ≥18 years of age;

          2. have a diagnosis of metastatic ER+ breast cancer in patients who received and
             progressed on at least two lines of endocrine therapy, with one that included a
             CDK4/CDK6 inhibitor (e.g., palbociclib or ribociclib);

          3. have metastatic disease evaluable on imaging studies;

          4. have a histologically confirmed diagnosis of ER+ breast cancer;

          5. have an Eastern Cooperative Oncology Group (ECOG) performance status of ≤1 (Appendix
             II);

          6. have adequate hepatic function, defined as having a serum total bilirubin
             concentration ≤1.5mg/d, or ≤2 x the upper limit of normal (ULN) if associated with
             hepatobiliary metastases or Gilbert syndrome, and having serum aspartate
             aminotransferase (AST) and alanine aminotransferase (ALT) concentrations ≤2.5 × ULN,
             or ≤5 x ULN for patients with known hepatic metastases;

          7. have adequate renal function, defined as having a serum creatinine ≤1.5 × ULN and a
             creatinine clearance ≥40mL/min (estimated using the Cockcroft-Gault formula);

          8. have adequate hematologic function, defined as having a hemoglobin ≥8g/dL, an absolute
             neutrophil count (ANC) ≥1.0 × 109/L, and platelet count ≥75.0 x 109/L;

          9. be willing and able to comply with study visits and procedures;

         10. have read, understood and signed the informed consent form (ICF) approved by the
             Institutional Review Board (IRB);

         11. if a woman of childbearing potential (WOCP), not be pregnant (confirmed by a negative
             serum pregnancy test within 14 days of study entry and re-confirmed by a urine
             pregnancy test on the morning of Study Day 1, prior to the start of study treatment)
             and/or not be breast-feeding; [Note: Women who are postmenopausal for at least 1 year
             or surgically sterile (bilateral tubal ligation, bilateral oophorectomy or
             hysterectomy) are not considered to be a WOCP.]

         12. If a WOCP, agree to use during the study and for at least one month after the last
             dose of study drug a medically acceptable method of birth control [such as an oral,
             implantable, injectable, or transdermal hormonal contraceptive, an intrauterine device
             (IUD), a double barrier method (condoms, sponge, diaphragm, or vaginal ring with
             spermicidal jellies or cream), or total abstinence.];

         13. if male (and whether or not surgically or medically sterile), agree to use during the
             study and for at least one month after the last dose of study drug a double barrier
             method of birth control (in addition to any other birth control method practiced by
             his partner) while engaging in sexual intercourse with a partner who is pregnant,
             possibly pregnant or able to become pregnant.

        Exclusion Criteria:

          1. have received chemotherapy, hormonal therapy, biologic therapy, immunotherapy or
             radiation therapy within 14 days prior to the planned start of study treatment.

          2. have inadequate recovery* from adverse events resulting from previously-administered
             anti-cancer therapies; [*Note: Unless more specifically defined in Inclusion Criteria
             6, 7 and 8 above, adequate recovery is defined as improvement to ≤ Grade 2 for any
             other hematologic toxicity and for peripheral neuropathy, and improvement to ≤ Grade 1
             for any other non-hematologic toxicity.]

          3. have a history of venous thromboembolism, including deep vein thrombosis, pulmonary
             embolism or retinal vein thrombosis, unless currently on anticoagulant therapy;

          4. have impending visceral crisis that requires chemotherapy;

          5. have known uncontrolled or symptomatic CNS metastases;

          6. have any clinically significant infection, defined as any acute viral, bacterial, or
             fungal infection that requires systemic treatment or have received any anti-infective
             treatment within 7 days prior to the screening visit;

          7. have any other severe, uncontrolled medical condition, including unstable congestive
             heart failure (Stage III-IV of the New York Heart Association Functional
             Classification (Appendix III))

          8. have a known or suspected allergy to the study drug or any study drug component;

          9. have any other medical or psychiatric condition or laboratory abnormality that may
             increase the risk associated with study participation, that may interfere with the
             interpretation of study results or that otherwise would, in the opinion of the
             Investigator, make study participation inappropriate;

         10. have any non-healing wound, fracture, or ulcer within 28 days prior to the planned
             start of study treatment;

         11. have received any other investigational drug within 28 days (or 5 half-lives, if
             longer) prior to the start of study screening.