For most of the radiopharmaceutical industry, the isotope conversation has increasingly revolved around two elements. Lutetium-177 has become the commercial workhorse of targeted radiopharmaceutical therapy, powering products including Pluvicto and Lutathera and establishing the infrastructure, clinical workflows and manufacturing networks on which much of the industry's expansion is being built. Actinium-225, meanwhile, has emerged as perhaps the industry's most closely watched next-generation isotope, driven by the extraordinary energy delivered by alpha particles and the possibility of treating disease that no longer responds adequately to beta-emitting therapies.
Yet between those two approaches sits another isotope whose characteristics are beginning to attract considerably more attention. Terbium-161.
For years, Tb-161 existed largely as an intriguing radiochemistry project. Its physical characteristics looked compelling on paper, academic groups demonstrated impressive preclinical results and researchers suggested that it could potentially improve upon Lu-177.
Now that proposition is moving into the clinic.
First-in-human prostate cancer data have demonstrated that Tb-161 radioligand therapy can be delivered at clinically meaningful activities with encouraging safety and antitumor activity. Neuroendocrine tumor studies are exploring the isotope as an alternative to Lu-177-based peptide receptor radionuclide therapy. Additional commercial programs have entered clinical development. At the same time, isotope producers and enrichment companies are beginning to build the Gd-160 supply chain required to manufacture Tb-161 at scale.
The result is an increasingly important question for nuclear medicine. Could terbium-161 become the next major therapeutic isotope?
The science suggests there is a credible case.
At first glance, Tb-161 looks remarkably similar to Lu-177. Tb-161 has a physical half-life of approximately 6.95 days, compared with roughly 6.6 days for Lu-177. Its average beta energy is approximately 154 keV, also reasonably close to Lu-177. It emits photons that can be used for SPECT imaging and dosimetry. And because terbium and lutetium are both lanthanides with similar chemistry, many of the same DOTA-based chelators already used with Lu-177 can potentially be used with Tb-161.
From a radiopharmaceutical-development perspective, that similarity matters. Tb-161 is not asking the industry to completely reinvent the radioligand therapy model. In principle, developers can take targeting molecules already familiar to nuclear medicine—PSMA ligands, somatostatin receptor ligands and potentially numerous antibodies, peptides and small molecules—and exchange one therapeutic radionuclide for another. But that is where the similarities largely end.
Tb-161 produces something Lu-177 produces in much smaller quantities: a substantial emission of low-energy conversion and Auger electrons.
Those electrons travel extremely short distances through tissue, depositing their energy within microscopic and, in some cases, subcellular dimensions. That creates a fundamentally different radiation field around the radiopharmaceutical. The beta particles emitted by Tb-161 can provide the familiar cross-fire effect useful for treating macroscopic tumors. The additional short-range electrons, however, can deliver highly localized radiation around individual radionuclide decay events.
In other words, Tb-161 effectively combines two radiation scales in the same isotope. A conventional beta component helps irradiate larger tumor volumes. And the conversion and Auger electron component may provide additional potency against microscopic disease.
That distinction could become increasingly important as radiopharmaceutical therapy moves earlier in cancer treatment.
Beta-emitting therapies benefit from what nuclear medicine calls the cross-fire effect. A radiopharmaceutical does not have to reach every cancer cell individually. Beta particles emitted from one targeted cell can travel far enough to irradiate neighboring cells.
That is particularly useful in large, heterogeneous tumors where uptake of the radiopharmaceutical may not be perfectly uniform. But there is another side to the equation. When a tumor becomes extremely small—or when only scattered malignant cells remain—the geometry changes.
There are fewer radioactive decay events occurring around each malignant cell, and some of the beta energy can leave the microscopic tumor without ever being absorbed within it. That is one reason researchers have long been interested in Auger emitters.
Auger electrons deposit energy across extraordinarily short distances. If a radiopharmaceutical can deliver those emissions sufficiently close to the cancer cell—and particularly near radiosensitive intracellular structures—the local energy deposition can become highly consequential.
Tb-161 creates an intriguing compromise. Instead of choosing between conventional beta therapy and an Auger-based strategy, the isotope delivers both.
Modeling studies cited in the Journal of Nuclear Medicine have estimated substantially greater absorbed doses from Tb-161 than Lu-177 in single-cell and small-cluster geometries, precisely where short-range electron emissions become most relevant. That could make Tb-161 particularly interesting for micrometastatic disease. And micrometastatic disease may ultimately be one of the biggest opportunities in radiopharmaceutical therapy.
The first generation of commercial radiopharmaceutical therapies largely entered cancer treatment relatively late. Patients typically had substantial disease burdens and had already received multiple systemic therapies. But that paradigm is changing. Radiopharmaceutical therapy is increasingly being studied—and in some cases approved—earlier in the treatment sequence.
As these therapies move upstream, physicians will increasingly encounter patients with lower-volume disease. That could change the characteristics of the ideal therapeutic radionuclide. An isotope optimized principally for large tumors may not necessarily be optimized for microscopic deposits or residual malignant cell populations.
Tb-161's physical profile makes it particularly interesting in that environment. Its beta emissions retain the cross-fire advantages that have helped make Lu-177 successful. Its conversion and Auger electrons potentially increase radiation delivery as the target becomes smaller.
Rather than competing with Lu-177 simply by delivering “more radiation,” Tb-161 may offer something more strategically important: radiation that is distributed differently at the cellular level. However, the distinction remains a hypothesis requiring randomized clinical validation. But it is a scientifically plausible hypothesis—and early human data have now made it considerably harder to dismiss.
One of the most important developments for Tb-161 came from the VIOLET study evaluating [161Tb]Tb-PSMA-I&T in men with metastatic castration-resistant prostate cancer.
Thirty patients received escalating activities followed by treatment at 7.4 GBq, with therapy administered every six weeks for as many as six cycles. The recommended Phase 2 dose was established at 7.4 GBq.
The early efficacy signals were notable. Seventy percent of patients achieved a PSA decline of at least 50%, while 40% achieved PSA declines of at least 90%. Median radiographic progression-free survival was approximately 11.1 months. Treatment-related severe adverse events were relatively uncommon, and investigators reported no treatment-related deaths.
Those results should be interpreted carefully.
VIOLET was a small, single-center, single-arm Phase I/II study. It was not designed to prove that Tb-161 is superior to Lu-177, and cross-trial comparisons between radioligand therapies are inherently unreliable. But it answered a much more fundamental question. Tb-161 works in humans.
The isotope can be produced, incorporated into a PSMA radiopharmaceutical, administered repeatedly at therapeutically relevant activities and followed with conventional nuclear medicine imaging and dosimetry. And that moves Tb-161 from an interesting theoretical radionuclide into a legitimate clinical-development platform.
Prostate cancer may be generating much of the attention, but the potential opportunity extends well beyond PSMA. Researchers have also been investigating Tb-161 in neuroendocrine tumors using somatostatin receptor-targeted radiopharmaceuticals. The Phase 0 BETA PLUS program is evaluating [161Tb]Tb-DOTA-LM3, a somatostatin receptor subtype 2 antagonist, relative to Lu-177-based therapy.
Early dosimetry has been striking. Published work reported that [161Tb]Tb-DOTA-LM3 produced a median tumor absorbed dose approximately 7.6 times that observed with [177Lu]Lu-DOTATOC in the evaluated patients, although part of that difference results from the properties of the DOTA-LM3 antagonist itself rather than the isotope alone. That qualification is crucial. Tb-161 development illustrates something the industry increasingly understands about radiopharmaceutical therapy: the isotope cannot be evaluated independently from the targeting molecule.
Ligand internalization, receptor density, tumor retention, subcellular localization, pharmacokinetics and clearance can all influence whether short-range electrons become therapeutically meaningful. A poorly targeted Tb-161 molecule will not become a great therapy simply because Tb-161 has attractive physics. But when the biology and isotope are matched correctly, the combination could be powerful.
This is where the commercial implications become larger. An isotope becomes economically important not because one drug succeeds, but because multiple radiopharmaceutical developers begin designing pipelines around it. Lu-177 has demonstrated that phenomenon. Its success extends far beyond any individual product. An entire industrial ecosystem has emerged around enriched Yb-176, reactor irradiation, isotope purification, radiolabeling, contract manufacturing, nuclear pharmacies, specialized logistics, treatment centers and dosimetry.
Tb-161 could potentially follow a similar path.
PSMA is already being explored. Somatostatin receptor-targeted therapy is already being explored. Radiopharm Theranostics has advanced RAD402, an anti-KLK3 monoclonal antibody labeled with Tb-161, into clinical development for prostate cancer. ClinicalTrials.gov now lists a Phase 1 study designed to evaluate its safety, biodistribution, pharmacokinetics, dosimetry and preliminary antitumor activity.
Additional targets are likely to follow if the isotope continues producing favorable clinical data. That is when the market opportunity becomes much larger. Tb-161 would no longer be competing as simply another isotope. It would become part of the toolkit developers use when engineering radiopharmaceuticals around specific tumor biology.
The other reason Tb-161 deserves attention is manufacturing. Many promising therapeutic isotopes face severe production challenges. Tb-161 is not trivial to manufacture, but the pathway is reasonably well understood.
One principal production route begins with enriched gadolinium-160. Gd-160 captures a neutron in a reactor to form Gd-161, which subsequently beta decays to Tb-161. Because the product is a different element from the starting target, Tb-161 can then be chemically separated from the gadolinium.
The U.S. Department of Energy highlighted research from the University of Utah and University of Missouri demonstrating the production of medically suitable, high-purity Tb-161 using even relatively low-power research reactors. That does not mean global commercial capacity exists today. It does not. One of the principal historical constraints has been access to sufficiently enriched Gd-160.
But that bottleneck is now attracting investment.
Isotopia has said it has already been manufacturing Tb-161 weekly for clinical studies. The company has entered supply agreements designed to increase its access to enriched Gd-160, including arrangements involving Kinectrics and ASP Isotopes. ASP's agreement calls for four years of enriched Gd-160 supply beginning in 2026.
That activity is strategically significant. Isotope markets frequently begin developing before the drug market itself becomes obvious.
Enrichment companies secure precursor supply. Reactor operators test irradiation pathways. Separation chemistry is optimized. GMP processes are established. Developers begin clinical programs. And only afterward does demand become visible at scale.
Tb-161 appears to be entering that intermediate stage now.
Tb-161's roughly seven-day half-life may also be commercially important.
It sits in a familiar logistics window. The isotope is long-lived enough to support centralized manufacturing and regional or potentially international distribution while remaining short-lived enough to provide clinically useful therapeutic activity.
That is very different from isotopes requiring near-immediate local production. And because the half-life closely resembles Lu-177, healthcare systems accustomed to Lu-177 radioligand therapy may find many aspects of Tb-161 treatment familiar: Treatment scheduling, radiopharmacy operations, radiation-safety procedures, patient release considerations, waste management and SPECT-based post-treatment imaging.
None will necessarily be identical, and regulatory approvals will still be required for individual products. But Tb-161 does not require nuclear medicine to invent an entirely new operating model. And that could lower one of the biggest barriers facing new therapeutic isotopes: clinical adoption.
Calling Tb-161 a potential blockbuster should not be confused with predicting the disappearance of Lu-177. That is unlikely. Lu-177 has enormous advantages. Its supply chain is increasingly mature. Multiple approved products depend on it. Manufacturers have invested heavily in production capacity. Hospitals understand how to use it. Regulators understand it. Radiopharmaceutical developers have years of experience working with it.
Most importantly, there are circumstances where the cross-fire properties of a beta emitter like Lu-177 may be exactly what clinicians want. Tb-161 therefore does not need to replace Lu-177 to become commercially important. It only needs to demonstrate superior performance in certain disease settings. Micrometastatic disease could be one. Small lesions could be another. Specific internalizing ligands could create another. And treatments intended to eradicate minimal residual disease could ultimately represent another.
The radiopharmaceutical market may eventually become less about identifying the single “best” isotope and more about matching radiation characteristics to tumor architecture. That would create room for multiple therapeutic radionuclides: Lu-177, Ac-225, Pb-212, Tb-161. And potentially others.
Despite the excitement, one critical piece of evidence is still missing. Tb-161 has not yet demonstrated in a randomized clinical trial that its additional conversion and Auger electrons translate into better patient outcomes than an equivalent Lu-177 therapy. That is the experiment that matters.
Physics can predict an advantage. Dosimetry can measure an advantage. Preclinical models can demonstrate an advantage. Single-arm clinical studies can suggest an advantage. But commercial medicine ultimately requires evidence that patients benefit.
That could mean higher response rates. Longer progression-free survival. Better eradication of microscopic disease. Lower administered activity for equivalent tumor control. Improved therapeutic index. Or some combination of those outcomes.
Until those comparisons are made, claims that Tb-161 is inherently “better than Lu-177” remain premature. Therefore, the more interesting conclusion is that Tb-161 may not have to be universally better. It simply has to be meaningfully better somewhere.
The rise of Tb-161 reflects a larger transition occurring across radiopharmaceutical therapy. The industry's first major question was whether targeted radiation could produce transformative cancer medicines. Lu-177 helped answer that.
The next question is more sophisticated. Which type of radiation should be delivered to which biological target, at what stage of disease, and at what microscopic distance from the cancer cell?
That is a much larger scientific design space. Alpha particles may be optimal for some targets. Beta particles may remain preferable for others. Short-range electron emissions may create advantages in entirely different settings.
Tb-161 is particularly interesting because it occupies more than one of those worlds simultaneously. It offers the established therapeutic characteristics of a medium-energy beta emitter while adding a substantial short-range electron component capable of depositing additional radiation at microscopic dimensions. But that does not guarantee clinical success. The isotope has moved considerably beyond theoretical promise.
Human studies are underway. Early prostate cancer results are encouraging. Neuroendocrine tumor dosimetry has strengthened the scientific case. Commercial developers are building Tb-161 programs. Producers are expanding precursor supply. And the manufacturing pathway increasingly looks compatible with the infrastructure the nuclear medicine industry is already constructing. For an isotope that was largely confined to radiochemistry laboratories only a few years ago, that represents remarkable progress.
Lu-177 helped establish radiopharmaceutical therapy as a major pharmaceutical category. Ac-225 has captured the industry's imagination as the next frontier in alpha therapy. Terbium-161 may be quietly building a different proposition. Not necessarily as the isotope that replaces either one. But potentially the isotope that fills the enormous space between them. And if radiopharmaceutical therapy continues moving toward earlier-stage, lower-volume and increasingly microscopic disease, that space could become very valuable indeed.
