First Patient With Breast Cancer Dosed With Novel Radioactive Medicine

The first patient in the U.S. has been dosed with a novel radioactive medicine to treat advanced breast cancer as part of an international clinical trial.

The first patient in the United States has been dosed with a novel radioactive medicine to treat their advanced breast cancer as part of an international multisite clinical trial, according to a press release shared by the Mayo Clinic.

In the clinical trial, patients are set to be treated with actinium-225, a highly potent alpha-emitting radiopharmaceutical therapy. The agent is meant to work by passing through the blood to stick to cancer cells, delivering a powerful and precise radiation without harming healthy cells.

The treatment was originally developed to treat a subtype of gastroenteropancreatic neuroendocrine tumors. These tumors are rare; they form in the pancreas and the gastrointestinal tract.

Mayo Clinic researchers are the first in the United States to administer this therapy to a patient. The phase 1b/2 open-label clinical trial which the patient was enrolled on is underway at all three Mayo Clinic campuses, including Rochester, Minnesota; Phoenix, Arizona; and Jacksonville, Florida. Additionally, there are approximately 20 more sites nationwide. The first patient received treatment at the Florida location.

The news release goes on to explain that beta-emitting radiopharmaceuticals deliver low-level radiation using beta particles; these are small subatomic particles. In contrast, alpha-emitting radiopharmaceuticals use alpha particles, which are 8,000 times more massive and travel only about three cell diameters from their emission point.

“This means alpha emitters can deliver a much more powerful impact over a shorter distance. If you consider killing a cancer cell is like knocking down a brick wall, then the difference is like throwing a 10-pound dumbbell (beta) at the wall versus a fully loaded Mack truck (alpha),” Dr. Geoffrey Johnson explained in the press release. "The alpha emitter's potential lies in its power and in its ability to precisely kill even a single cancer cell without injuring surrounding healthy tissue, making it a next-generation therapy."

Johnson is a professor of radiology at the Mayo Clinic, and a leader in radiopharmaceutical therapies, the release notes. Johnson went on to say that these are innovative cancer treatments which use radioactive medicines designed to target and kill cancer cells with high precision.

The news release concluded by emphasizing that preclinical study data show that the investigative therapy demonstrated feasibility and potential efficacy for treatment of estrogen receptor-positive metastatic breast cancer.

More Information on the Ongoing Trial

The phase 1b/2 open-label trial is evaluating patients with estrogen receptor-positive, HER2-negative locally advanced and unresectable or metastatic breast cancer following progression on antibody-drug conjugates and/or chemotherapy.

To be eligible for enrollment, patients must have an ECOG performance status of 2 or less; histologically confirmed estrogen receptor-positive, HER2-negative, locally advanced and unresectable or metastatic breast cancer that is not amenable to curative treatment; and at least one tumor lesion measurable that is SSTR-PET positive. Additionally, at least 80% of all RECIST v1.1-measurable tumor lesions must be SSTR-PET positive. Subjects must also have sufficient renal function, as well as adequate hematologic and hepatic function.

Subjects will be excluded from the study if they have received prior radiopharmaceutical therapy, including radioembolization; have unresolved toxicities from prior treatments; or have significant cardiovascular disease. Additional exclusion criteria include the presence of uncontrolled or symptomatic central nervous system metastases, carcinomatous meningitis, spinal cord compression, or leptomeningeal disease.

What Are Radiopharmaceuticals?

Radiopharmaceuticals provide a localized delivery of radionuclides to targeted areas in the body for the diagnosis and treatment of various cancers, according to research published in Signal Transduction and Targeted Therapy, a biomedical journal published by Nature Research.

Radiopharmaceutical therapy, which induces direct, irreversible damage to the targeted — in this case, cancer — cells, is gaining attention for treating refractory diseases unresponsive to existing therapies. Targeted radiopharmaceutical-based treatment approaches are increasingly being adopted in clinical practice, marking the beginning of a new era in radiopharmaceutical development, the investigators wrote in their research.

“The new generation of radiopharmaceuticals... [is making it] possible to improve the efficiency and biosafety of tumor diagnosis and therapy. Numerous studies have focused on developing novel radiopharmaceuticals targeting a broader range of disease targets, demonstrating remarkable in vivo performance. These include high tumor uptake, prolonged retention time, and favorable pharmacokinetic properties that align with clinical standards,” researchers wrote.

Overall, this means that these newer agents are enabling safer, more effective cancer diagnosis and treatment by targeting a wider range of cancer cells with improved precision.

References

1. “Mayo Clinic treats first person in the US with a novel radiopharmaceutical therapy for breast cancer,” by Brittany Cordeiro. Mayo Clinic News Network. August 1, 2025.
2. “Trial of 225Ac-DOTATATE (RYZ101) in Subjects with ER+, HER2-negative Unresectable or Metastatic Breast Cancer Expressing SSTRs. (TRACY-1),” by ClinicalTrials.gov. Last updated July 17, 2025.
3. “Radiopharmaceuticals and Their Applications in Medicine,” by Siqi Zhang, et al. Signal Transduction and Targeted Therapy.

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