Effect of Oral (Z)-Endoxifen in Premenopausal Women With Measurable Breast Density

Inclusion Criteria: 1. Premenopausal women (defined as having at least one menstruation during the prior 12 months; women with an intrauterine device are considered premenopausal if \<52 years) 2. Women of childbearing potential using a highly effective method of birth control\* throughout the study period and willing to comply with monthly pregnancy testing 3. Screening mammogram performed within 3 month of study inclusion 4. Mammographic density assessed as BI-RADS® score B, C, or D 5. 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 Exclusion Criteria: 1. Mammographic BI-RADS® malignancy code ≥3 at baseline mammography 2. Any previous diagnosis of breast cancer, including carcinoma in situ, or any other cancer (non-melanoma skin cancer and in situ cancer of the cervix are not exclusion criteria) 3. A history of breast surgery, e.g., reduction or enlargement, which might affect mammographic density measurements 4. Current medical conditions: 1. APC (activated protein C) resistance, an inherited coagulation disorder 2. Systolic pressure higher than 160 mm Hg or diastolic higher than 100 mm Hg 3. Cataract(s) 4. Uncontrolled diabetes (defined as HbA1c \>50 mmol/mol) 5. Abnormal lab values deemed clinically significant by Investigator 5. BMI \> 30 6. Women who have an increased risk of venous thrombosis due to immobilization, e.g., using wheelchair 7. Agents that have the potential to decrease endoxifen levels through increased metabolic clearance: 1. Certain anti-epileptic therapies (carbamazepine, fenytoin, fenobarbital, lamotrigine) 2. Certain antibiotics (rifamycins) 3. St John's wort (in Swedish: johannesört) 4. Certain HIV medications (efavirenz, ritonavir) 8. Lactating, pregnant, or plan to become pregnant in the next year 9. History of thromboembolic disease such as embolus, deep vein thrombosis, stroke, TIA (transient ischemic attack) or myocardial infarction 10. Allergy to endoxifen or any of its components 11. Previous treatment with an anti-oestrogen, including experimental drug studies (e.g., Karma CREME, Karisma-1 or Karisma-2) 12. Prescribed and regular use of anticoagulants (defined as substances included in group B01A in the ATC-system) 13. Participation in another investigational clinical trial in the last 6 months 14. Not willing or able to understand the study information and/or informed consent