Inclusion Criteria:

          -  Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1

          -  Absolute neutrophil count (ANC) >= 1.5 x 10^9/L

          -  Platelets >= 50 x 10^9/L

          -  Hemoglobin (Hb) >= 9 g/dL

          -  Total serum bilirubin =< 2.0 mg/dL

          -  Alanine aminotransferase (ALT) and aspartate aminotransferase (AST) =< 2.5 x upper
             limit of normal (ULN) (=< 5 x ULN in patients with liver metastases)

          -  Serum creatinine =< 1.5 x ULN

          -  Activating ESR1 mutation (e.g. D538G, Y537S/N, S463P) identified on ctDNA. Novel ESR1
             alterations allowed as per discretion of principal investigator (PI)

          -  On AI with CDK4/6 inhibitor (palbociclib, ribociclib, or abemaciclib) as first line
             therapy for metastatic breast cancer (MBC) for at least 12 months without evidence of
             clinical progression

          -  Patients with histologically confirmed HR positive (estrogen receptor [ER] positive
             [+] and/or progesterone receptor [PR]+ [> 10%]), MBC

        Exclusion Criteria:

          -  Pregnant or lactating women

          -  Received prior therapy for MBC (except for AI use for up to 4 weeks prior to
             initiation of CDK4/6 inhibitor)

          -  Prior therapy with fulvestrant in the metastatic setting

          -  Corrected QT (QTc) interval > 480 msec, Brugada syndrome or known history of QTc
             prolongation or Torsade de Pointes

          -  Psychiatric illness which would limit informed consent

          -  Patients with de novo metastatic disease

          -  Patients who have any severe and/or uncontrolled medical conditions such as: unstable
             angina pectoris, symptomatic congestive heart failure, myocardial infarction =< 6
             months prior to enrollment, serious uncontrolled cardiac arrhythmia, or any other
             clinically significant cardiac disease, active (acute or chronic) or uncontrolled
             severe infection, liver disease such as cirrhosis, decompensated liver disease, and
             active and chronic hepatitis (i.e. quantifiable hepatitis B virus [HBV]-DNA and/or
             positive hepatitis B surface antigen [HbsAg], quantifiable hepatitis C virus
             [HCV]-ribonucleic acid [RNA]), severe hepatic impairment (Child-Pugh C)

          -  Human immunodeficiency virus (HIV)-positive patients on combination antiretroviral
             therapy

          -  Expected survival < 6 months

          -  Any serious medical illness, other than that treated by this study, which would limit
             survival to less than 1 month

          -  Patients with a history of non-compliance to medical regimens or who are considered
             potentially unreliable or will not be able to complete the entire study

          -  Patients who are currently part of or have participated in any clinical investigation
             with an investigational drug within 1 month prior to dosing

          -  Women of child-bearing potential (WOCBP), defined as all women physiologically capable
             of becoming pregnant, must use highly effective methods of contraception during the
             study and 8 weeks after the end of treatment. Highly effective contraception methods
             include combination of any two of the following: placement of an intrauterine device
             (IUD) or intrauterine system (IUS); barrier methods of contraception: condom or
             occlusive cap (diaphragm or cervical/vault caps) with spermicidal
             foam/gel/film/cream/vaginal suppository; total abstinence or; male/female
             sterilization

          -  Male patients whose sexual partner(s) are WOCBP who are not willing to use adequate
             contraception, during the study and for 8 weeks after the end of treatment