New research proposes using an effectively stable gadolinium precursor to overcome a decades-old isotope supply bottleneck and enable Tb-149 production on widely available medical cyclotrons.

SAN FRANCISCO, Sept. 24, 2026 /PRNewswire/ -- Marathon Fusion today announced a novel approach for large-scale production of terbium-149 (Tb-149), a radioactive isotope that researchers have studied for decades as a promising candidate for targeted cancer therapy, but have never been able to produce in quantities sufficient for clinical use.

"[Tb-149] is the radionuclide of choice in all aspects except production," wrote researchers affiliated with the University Hospital of Geneva and CERN in a 1996 paper examining the isotope's potential for targeted cancer therapy. Tb-149 was discovered in 1950 and has been considered for targeted alpha therapy for more than three decades.

Marathon's new paper, Scalable Terbium-149 Production from Highly Enriched Gadolinium-150 Targets, introduces a novel approach to break this supply constraint and enable the full potential of Tb-149-based cancer therapies.

The challenge of Tb-149 production

Highlighting both the isotope's desirability and limited production, researchers in a 2025 European multidisciplinary review of targeted alpha therapy wrote: "Due to the valuable combination of physical characteristics… Terbium-149 is one of the most promising radionuclides. [However,] the radionuclide's restricted availability has prevented the start of clinical trials… The unconventional production of Terbium-149 was the main reason why Terbium-149 had not yet reached clinical trials."

Tb-149 has a rare combination of properties for targeted radiotherapy. It emits an alpha particle capable of destroying nearby cancer cells while traveling only a short distance through tissue. Unlike other alpha emitters under development, Tb-149 also produces a positron signal that could allow physicians to image the tumor while treating it.

Despite promising preclinical work with targeting agents including PSMA, rituximab and somatostatin-receptor ligands, Tb-149 has not previously progressed to human trials. Only a handful of specialized facilities have produced Tb-149 with current facilities achieving far less production than is necessary to complete a clinical trial.

Because Tb-149 has a half-life of only 4.1 hours, conventional centralized production faces extreme logistical challenges. This is contrasted with the 66-hour half-life of Mo-99, or the 6.6-day half-life of Lu-177, radioisotopes which are successfully produced centrally.

Marathon's scalable production method

Inspired by the fluorine-18 production pathway used to enable millions of PET scans each year in the United States alone, Marathon proposes focusing on the production of a rare precursor isotope to unlock onsite production of Tb-149, mitigating challenges associated with its short half-life.

In fluorine-18 production, a precursor isotope called oxygen-18 is irradiated to produce fluorine-18, an isotope with a half life of less than 2 hours. This is made possible by the centralized offsite enrichment of oxygen-18, a rare isotope which makes up just 0.2% of naturally occurring oxygen. By producing a stockpile of a rare precursor isotope, oxygen enrichment enables the decentralized, onsite production of the short-lived fluorine-18.

In Marathon's proposed production pathway, the precursor gadolinium-150 (Gd-150) is produced centrally. Unlike oxygen-18, Gd-150 is not technically stable, but its 1.8-million-year half-life enables stockpiling and shipment without meaningful losses.

Gd-150 could then be irradiated locally by a distributed network of cyclotrons, using existing equipment, to generate Tb-149 onsite.

"Terbium-149 has been clinically interesting for a long time, we just haven't had enough of it to get drugs through clinical trials, or to feel like there will be a meaningful market even after approval," said Adam Rutkowski, CTO and Co-founder of Marathon Fusion. "Our work shows that there's a practical solution leveraging existing infrastructure to not only get Tb-149-based drugs through clinical trials, but to reach patients all over the world."

To produce the Gd-150 precursor, an abundant isotope called Europium-151 is irradiated and transmuted. Marathon's analysis finds that existing technology could produce enough Gd-150 to support tens of thousands of Tb-149 doses per year.

"We've seen a recent confluence of factors: progress in targeting driven by years of clinical research, evidence for the efficacy of alpha therapy demonstrated through dozens of clinical trials, and a vibrant ecosystem capable of bringing new treatments to market," said Kyle Schiller, CEO and Co-founder of Marathon Fusion. "If we can show that there's a scalable production pathway for terbium-149, an isotope clinicians have wanted to use for decades, that'll bring all the pieces together to take a leap forward in cancer treatment options."

At a large scale, Marathon's researchers calculate that small fusion reactors could irradiate enough europium-151 to produce the precursor stockpile necessary for millions of annual doses of Tb-149, creating a promising connection between the emerging fusion industry and the supply chain for next-generation cancer medicines.

Radiotherapy: driving the early market for nuclear fusion

While several fusion startups are targeting power-plant demonstrations in the 2030s, fusion reactors could be used to produce isotopes for radiotherapy even earlier, and without needing to hit the size or performance requirements necessary for power production.

"Fusion reactors produce an extraordinary number of high-energy neutrons," said Dr. Jason Parisi, Principal Research Scientist at Marathon Fusion who previously worked as a Staff Research Scientist at the Princeton Plasma Physics Lab. "Electricity is the application everyone associates with fusion, but those neutrons can also be used to mass-manufacture medical isotopes, creating a new supply chain for this critical industry."

A scientific hypothesis now moving toward experiment

Models predict a reaction rate based on decades of nuclear data and advances in computational tools, but there has not been a direct experimental measurement of the nuclear cross section at the heart of Marathon's production method. Marathon's methods rely on established nuclear-reaction models, benchmarked experimentally against neighboring isotopes.

"With the model in place, it's now critical to move forward with experimental confirmation," said Dr. Jason Parisi. "If demonstrated, this would validate a previously overlooked route with all the right characteristics to solve Tb-149 supply. The important thing now is to test it."

About Marathon Fusion

Marathon Fusion is an isotope production company, enabling an abundant supply of radioisotopes for the radiopharma industry, and developing isotope technology to deliver clean fusion energy more quickly with better economics. Marathon is supported by leading institutions including the U.S. Department of Energy's ARPA-E, Breakthrough Energy Fellows, 1517 Fund and Übermorgen Ventures. For more information, visit www.marathonfusion.com.

CONTACT: Kyle Schiller; kyle@marathonfusion.com

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SOURCE Marathon Fusion, Inc.