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The purpose of this study is to test the safety and effectiveness of the sequence of two investigational drugs (trastuzumab deruxtecan followed by datopotamab deruxtecan, or datopotamab deruxtecan followed by trastuzumab deruxtecan) to learn whether the treatment works in treating HER2-negative (HER2-low or HER2-0) metastatic breast cancer.

The names of the study drugs involved in this study are:

* Datopotamab deruxtecan (a type of antibody drug conjugate)
* Trastuzumab deruxtecan (a type of antibody drug conjugate)

This open-label research study is studying (Z)-endoxifen as a possible treatment for pre-menopausal women with ER+/HER2- breast cancer. (Z)-endoxifen belongs to a group of drugs called selective estrogen receptor modulators or "SERM", which help block estrogen from attaching to cancer cells. This study has two parts: a pharmacokinetic part and a treatment part.

The PK part (how the body processes the drug) will enroll about 18 participants. All participants will take (Z)-endoxifen capsules daily. Twelve participants will be randomly assigned (50/50 chance) to take (Z)-endoxifen alone or (Z)-endoxifen with a monthly injection of goserelin a drug that temporarily stops the ovaries from making estrogen. This part will help determine the best dose of (Z)-endoxifen by measuring the drug levels in the blood and how long the body takes to remove it.

The Treatment Cohort has been simplified to a single study arm (Z)-endoxifen + goserelin. Up to 20 participants will be enrolled that have a baseline Ki-67 10% and 45 participants will be enrolled that have a baseline Ki-67\>10%.

A key goal of the study is to see if (Z)-endoxifen can slow down or stop tumor growth as measured by a reduction in Ki-67 levels. Tumor tissue samples will be taken by breast biopsy after about 4 weeks of treatment to check levels of this biomarker. If the tumor shows signs of response, participants can continue treatment for up to 24 weeks or until they have surgery.

Study participation is up to 6 months (24 weeks of treatment) followed by surgery and a one-month follow up visit.

This ComboMATCH patient screening trial is the gateway to a coordinated set of clinical trials to study cancer treatment directed by genetic testing. Patients with solid tumors that have spread to nearby tissue or lymph nodes (locally advanced) or have spread to other places in the body (advanced) and have progressed on at least one line of standard systemic therapy or have no standard treatment that has been shown to prolong overall survival may be candidates for these trials. Genetic tests look at the unique genetic material (genes) of patients' tumor cells. Patients with some genetic changes or abnormalities (mutations) may benefit from treatment that targets that particular genetic mutation. ComboMATCH is designed to match patients to a treatment that may work to control their tumor and may help doctors plan better treatment for patients with locally advanced or advanced solid tumors.

This research study is looking to see how well male breast cancer responds to preoperative treatment with endocrine therapy and which endocrine therapy regimen is the most effective treatment for male breast cancer.

The drugs used in this study are:

* Tamoxifen
* Anastrozole
* Degarelix
* Abemaciclib