This open-label research study is studying (Z)-endoxifen as a possible treatment for pre-menopausal (still having periods) women with ER+/HER2- breast cancer. This study includes a pharmacokinetic part (PK, how the drug works in your body) and a treatment part. The primary purpose of the study is to see how (Z)-endoxifen works on tumor cell growth by taking a biopsy after 4 weeks of treatment to measure Ki-67. Ki-67 is a cancer marker that indicates how well the treatments work to slow cancer cell growth. Overall, this study will help determine if (Z)-endoxifen can effectively treat premenopausal women with ER+/HER2- breast cancer without the need for monthly injections of goserelin which is a medication given to block the ovaries from making estrogen (also called ovarian suppression). Studies have shown harmful long-term effects of ovarian suppression in premenopausal women. The PK part of the study will be enrolled first, enrolling about 6 study participants who will all receive oral once daily (Z)-endoxifen treatment. This part of the study will help select the dose of (Z)-endoxifen to use in the treatment part by measuring the levels of (Z)-endoxifen in the blood stream and determine how long it takes for the body to remove it. About 160 study participants will be enrolled in the treatment part. The treatment part will help to determine how oral once daily (Z)-endoxifen, when taken by itself, compares to oral once daily exemestane (a medication that decreases the amount of estrogen in the body, also known as an aromatase inhibitor) and monthly injections of goserelin. Exemestane and goserelin taken together is a standard treatment regimen for premenopausal patients with ER+/HER2- breast cancer. Study participants are randomly assigned to treatment with an equal (50/50) chance to be assigned to (Z)-endoxifen or standard treatment. Study participation is up to 24 weeks of treatment followed by surgery.
2 Primary · 23 Secondary · Reporting Duration: Baseline Assessment up to Cycle 3 (each cycle is 28 days) or up to 12 weeks
174 Total Participants · 5 Treatment Groups
Primary Treatment: PK Cohort · No Placebo Group · Phase 2
Age 18+ · Female Participants · 8 Total Inclusion CriteriaMark “Yes” if the following statements are true for you: