(Z)-Endoxifen for the Treatment of Premenopausal Women With ER+/HER2- Breast Cancer

Inclusion Criteria: 1. Female sex assigned at birth. Female to male transgender individuals who have not had any hormonal therapy may be considered for the trial after review and approval from the medical monitor and study sponsor. 2. Age 18 years or older 3. Not lactating, pregnant, or planning to become pregnant in the next year and agrees to take adequate steps to prevent becoming pregnant beginning at informed consent, during treatment and for 9 months after last dose and agree to not breast feed during treatment and for 3 months after last dose. 4. Must agree to use at least one non-hormonal highly effective method of contraception for the entire duration of study participation beginning at informed consent. Highly effective methods of birth control are defined as those, alone or in combination, that resulted in a low failure rate of \<1% per year when used consistently and correctly such as intrauterine devices (IUDs, non-hormonal such as copper IUD), bilateral tubal occlusion, sexual abstinence or vasectomized partner 5. Premenopausal defined as any female who: 1. is menstruating or 2. is not menstruating (last menstrual period \> 3 months prior to registration) but has a plasma estradiol in the premenopausal range as assessed locally 6. Pathologic confirmation of strongly estrogen receptor positive (ER+) (defined as estrogen receptor \[ER\] ≥ 67% or Allred Score 6-8) by local institution protocol 7. Eastern Cooperative Oncology Group ECOG Performance Status (ECOG PS) of 0 to 2 8. Nottingham (Elston-Ellis) Grade 1 or 2 9. HER2- breast cancer (histologically confirmed) using American Society of Clinical Oncology (ASCO)/College of American Pathologists (CAP) guidelines 10. Clinical T2 or T3 invasive breast cancer (per American Joint Committee on Cancer \[AJCC\] 8th edition clinical staging) 11. Clinical N0 or N1 invasive breast cancer (per American Joint Committee on Cancer \[AJCC\] 8th edition clinical staging) 12. MRI ≤ 35 days of registration 13. Must have given written informed consent before any study-related activities are carried out and must be able to understand the full nature and purpose of the trial, including possible risks and adverse effects 14. Willing to provide blood and breast tissue samples for research purposes at specified timepoints for the duration of their participation in the trial. Exclusion Criteria: 1. Bilateral invasive breast cancer; Inflammatory breast cancer defined as clinically significant erythema of the breast and/or documented dermal lymphatic invasion or bilateral invasive breast cancer (patients with pre-malignant disease or DCIS/LCIS in contralateral breast are eligible) 2. Prior diagnosis or treatment for breast cancer, including carcinoma in situ, or history of any other active malignancy within the past 2 years prior to study entry with the exception of: 1. Adequately treated in situ carcinoma of the cervix uteri 2. Adequately treated basal cell carcinoma or localized squamous cell carcinoma of the skin 3. Any other malignancy with a life expectancy of less than 2 years 3. Any uncontrolled intercurrent illness including, but not limited to: 1. Ongoing or active infection requiring systemic treatment with strong inhibitors/inducers of CYP450 enzymes (including bacterial infection, fungal infection, or detectable viral infection). 2. Symptomatic congestive heart failure, 3. Unstable angina pectoris, 4. Uncontrolled symptomatic cardiac arrhythmias 5. Uncontrolled hypertension 6. Uncontrolled diabetes (Hemoglobin A1c \[HbA1c\] \>7%) 7. Marked baseline prolongation of QT/QTc interval (e.g., repeated demonstration of a QTc interval \> 470 milliseconds \[msec\]) using Fridericia's QT correction formula seen ≤ 28 days of registration 4. Any of the following co-morbid conditions: 1. Known cataracts or retinopathy 2. History of deep vein thrombosis (DVT)/pulmonary embolism (PE) 3. Known activated protein C (APC) resistance, an inherited coagulation disorder 4. End stage kidney disease requiring dialysis 5. Evidence of the following laboratory abnormalities ≤ 28 days prior to registration: 1. Total bilirubin ≥ 1.5 x upper limit of normal (ULN) 2. Aspartate aminotransferase (AST) or alanine amino transferase (ALT) ≥ 2.5 x ULN 3. Platelet count (PLT) ≤ 75,000/mm3 4. Hemoglobin (Hb) ≤ 10 g/dL 6. Hormonal therapies including birth control and hormone replacement therapy, or prior use of androgen-based therapy during the study or within 1 week of registration. If subject has a prior medical history of Depo-Provera®, it is recommended that the last dose of 3-month contraceptive agents are \> 2.5 months from registration. 7. Allergy to endoxifen, goserelin, or exemestane or any of their components 8. Participation in another investigational clinical trial ≤ 6 months of registration 9. Known metastatic disease