Beyond Tamoxifen: How (Z)-Endoxifen May Transform ER+ Breast Cancer Treatment — and Offer New Hope in Duchenne Muscular Dystrophy

22 Jul Beyond Tamoxifen: How (Z)-Endoxifen May Transform ER+ Breast Cancer Treatment — and Offer New Hope in Duchenne Muscular Dystrophy

Publications discussed: The rise of selective estrogen receptor modulators (SERMs) in breast cancer therapy: a promising horizon and (Z)-Endoxifen as a Potential Modulator of Utrophin Pathways in Duchenne Muscular Dystrophy: A Mechanistic and Transcriptomic Perspective

Dr. Steven C. Quay

MedicalResearch.com: What is the background for these studies? What are the main findings of the reviews?

Dr. Quay: Breast cancer remains the most common cancer diagnosis and the second leading cause of cancer death among women in the United States. Around 70% of these cases are estrogen receptor-positive (ER+), meaning the tumor cells use the body’s natural estrogen as fuel to multiply and grow. While traditional endocrine therapies like tamoxifen have saved countless lives, patients frequently face severe side effects or eventually develop resistance to the treatment.

To bridge this gap, these studies show the promise of next-generation selective estrogen receptor modulators (SERMs), specifically (Z)-endoxifen. The main findings indicate that this active-form drug delivers more consistent, well-tolerated exposure to keep pace with a patient’s unique biology. We are looking at the whole picture — from cancer prevention in high-risk patients to shrinking tumors before cases worsen. Ultimately, this approach builds on tamoxifen’s legacy and blocks estrogen signaling to provide patients with more effective therapy options. For context on risk factors in breast cancer prevention, see this overview of breast cancer risk and dense breast tissue.

MedicalResearch.com: How does Z-Endoxifen differ from the usual forms of estrogen?

Dr. Quay: Natural estrogen acts like a key that unlocks estrogen receptors in cells. In estrogen receptor-positive (ER+) breast cancers, this “unlocking” acts as cellular fuel, signaling the cancer cells to multiply and grow. (Z)-endoxifen is fundamentally different because it is a selective estrogen receptor modulator (SERM). Instead of fueling the cell, it blocks natural estrogen from attaching, which effectively cuts off the fuel supply and stops the cancer cells from growing.

MedicalResearch.com: Why might Z-Endoxifen be useful for such disparate entities as breast cancer and muscular dystrophy?

Dr. Quay: Z-endoxifen is best known in the context of breast cancer because it is the active metabolite of tamoxifen and can block estrogen receptor signaling, which many breast cancers use as a growth signal. What is less obvious is that estrogen receptor biology is also relevant in skeletal muscle. In Duchenne muscular dystrophy, the genetic cause is loss of dystrophin, but much of the disease burden comes from downstream muscle injury, inflammation, impaired repair, fibrosis, and loss of force. That creates a rationale for testing whether a drug like Z-endoxifen could help modify the muscle environment, even though it does not replace dystrophin.

This idea is supported by prior tamoxifen studies in DMD mouse models, where tamoxifen improved muscle pathology and function. More recently, Z-endoxifen data in mdx5Cv dystrophic mice showed improvements in motor performance and muscle-force measures, with trends toward reduced muscle damage. So while breast cancer and DMD are very different diseases, they may share hormone-responsive pathways that can be targeted in different ways. Notably, the FDA granted (Z)-endoxifen a Rare Pediatric Disease Designation for DMD in late 2025, recognizing its potential as a broader platform therapy. According to the American Cancer Society, breast cancer remains the most diagnosed cancer in U.S. women, with ER-positive cases comprising the majority — underscoring the importance of advances in endocrine therapy.

MedicalResearch.com: What should readers take away from your report?

Dr. Quay: The main takeaway is that we need to stop thinking of molecules as “single-disease” treatments. When looking deeper at how drugs interact with our genetics, we can find completely unexpected solutions for entirely different conditions. With (Z)-endoxifen, the big shift is realizing its potential goes far beyond oncology. In breast cancer, it is a more efficient and consistent way to cut off the estrogen fuel supply that tumors thrive on. But in Duchenne muscular dystrophy, it acts through a completely separate genetic pathway to boost a backup protein that stabilizes failing muscle cells. Ultimately, this report shows how next-generation medicine can cross traditional boundaries by offering new hope to both cancer patients and families facing rare genetic diseases.

MedicalResearch.com: What recommendations do you have for future research as a result of this study?

Dr. Quay: Moving forward, future research should focus on two clear tracks. First, for Duchenne muscular dystrophy, we need to move quickly from preclinical models into targeted human clinical trials. Research should prioritize establishing clear, quantifiable biomarkers — specifically tracking how effectively (Z)-endoxifen upregulates and localizes the utrophin protein in human muscle tissue, and mapping out the exact dosing required to stabilize muscle cell membranes without causing adverse hormonal side effects. Second, for oncology, future studies should continue optimizing the drug’s delivery throughout the entire care continuum. This includes expanding trials on its use as a preventive therapy for high-risk women with dense breast tissue, and gathering more long-term data on how its consistent, active-form exposure can prevent treatment resistance and improve daily quality of life compared to traditional therapies.

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Last Updated on July 22, 2026 by Marie Benz MD FAAD