Aromatase Inhibitor Therapy With or Without Fulvestrant for the Treatment of HR Positive Metastatic Breast Cancer With an ERS1 Activating Mutation, the INTERACT Study

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