The goal of this trial is to see if active surveillance monitoring and hormonal therapy in patients diagnosed with ductal cell carcinoma in situ (DCIS), an early stage of breast cancer, can be an effective management of the disease.

Participants will be asked to receive control hormonal therapy or an investigational hormonal therapy treatment. Participants will be asked to return for evaluation with MRI at three months and six months. Depending on the evaluation participants will have the option to continue on the treatment. If the evaluation suggests surgery is recommended, the participant will discontinue the study treatment and will undergo surgery. In addition to the treatment and MRI evaluation, participants will be asked to provide blood sample to understand their immune status, provide saliva sample for genetic testing, provide the study with a portion of the tissue or slides generated from tissue removed during surgery performed as part of their standard of care.

The goal of this trial is to see if active surveillance monitoring and hormonal therapy in patients diagnosed with Ductal cell Carcinoma In Situ (DCIS), an early stage of breast cancer, can be an effective management of the disease. The current management of most patients with DCIS involves surgical intervention with or without radiation, similar to more aggressive breast cancers. These treatments can come with some significant health effects.The main question this study aims to answer is: to determine whether novel endocrine therapy increases the fraction of patients who will be suitable for long-term active surveillance.

Participants will be asked to take one of three investigational study medication (z-Elacestrant, Testosterone + Anastrazole, or Endoxifen) or receive control hormonal therapy (Tamoxifen or an aromatase inhibitor), depending on the treatment to which they have been randomized. Participants will be asked to return for evaluation with MRI at three months and six months. Depending on the evaluation, participants will have the option to continue on the treatment, with follow up evaluations of Mammogram and MRI at 6 month intervals. If the evaluation suggests surgery is recommended, the participant will discontinue the study treatment and will undergo surgery. In addition to the treatment and MRI evaluation, participants will be asked to:

• Provide blood sample to understand their immune status
• Provide saliva sample for genetic testing
• Provide the study with a portion of the tissue or slides generated from tissue removed during surgery performed as part of their standard of care.

Participants will be followed annually for 10 years.

Inclusion Criteria:

A. Female, at least 18 years old

B. Previous diagnosis of HR+ DCIS (at least 50% ER or PR; biopsy will have been performed previously at diagnosis) with or without microinvasion

• Patients with a diagnosis of hormone positive DCIS who have undergone surgery with positive margins that have not been re-excised are candidates to enroll in the trial.

C. Patients who have previously received endocrine therapy should have a washout period of at minimum 4-6 weeks prior to the screening MRI on the RECAST-DCIS trial

D. Bilateral mammogram performed within up to 6 months (180 days) of the start of trial treatment may be used for screening evaluation. If a bilateral mammogram has been performed within 1 year (12 months) of the start of trial treatment, then a diagnostic unilateral mammogram within 6 months (180 days) of the start of trial treatment will be acceptable for screening evaluation.

E. MRI performed on an I SPY (RECAST) approved scanner within 2 months (60 days) of the start of trial treatment for lesion evaluation may be used for screening evaluation.

F. CBC w/ diff, CMP, and Lipid Panel within normal limits within a year of the start of trial treatment. Abnormal labs to be repeated within 60 days prior to the start of trial treatment. Patients will be considered eligible for screening labs that are abnormal or out-of-range if the investigator has deemed the lab results not-clinically significant

G. Negative urine or serum pregnancy test within 1 month of the start of trial treatment

H. Controlled HIV positive patients are allowed as long as their current medication does not contraindicate the study's investigational agent

I. Willingness and ability to provide tumor samples for research

Exclusion Criteria:

A. Pregnant or actively breastfeeding women

B. History of allergic reactions attributed to compounds of similar chemical or biologic composition to study agent based on review of the medical record and patient history

C. Invasive carcinoma or identification of a mass on MRI that is subsequently biopsied and found to be invasive cancer

D. Co-enrollment in clinical trials of pharmacologic agents requiring an IND

E. Ongoing treatment for DCIS other than what is specified in this protocol

F. Uncontrolled intercurrent illness, including psychiatric conditions, that would limit compliance with study requirements

G. Medical history or ongoing gastrointestinal disorders potentially affecting the absorption of investigational agent and/or tamoxifen. Active inflammatory bowel disease or chronic diarrhea, known active hepatitis A/B/C*, hepatic cirrhosis, short bowel syndrome, or any upper gastrointestinal surgery including gastric resection or banding procedures

*Active hepatitis, defined as: A (positive HA antigen or positive IgM); B (either positive HBs antigen or positive hepatitis B viral DNA test above the lower limit of detection of the assay); C (positive hepatitis C antibody result, and quantitative hepatitis C (HCV) ribonucleic acid (RNA) results greater than the lower limits of detection of the assay)

H. Participants who are unable to swallow normally or unable to take tablets and capsules. Predictable poor compliance with oral treatment

Female