The shortest journey in cancer medicine is the one a radioactive atom takes to reach a tumor. It is measured in centimetres. It still fails most of the time.
Ratio Therapeutics is a Boston company betting $70 million on that specific distance. On July 31, 2026, it closed a Series C round that pushed total capital past $240 million, with Bristol Myers Squibb and Eli Lilly among the investors. The money funds a Phase 1/2 trial in advanced sarcomas and the manufacturing capacity that this modality quietly depends on.
The asset, [Ac-225]RTX-2358, is an actinium-225 payload that binds to a protein called FAP on the surface of tumor cells. It is in the ATLAS Phase 1/2 trial for relapsed or refractory soft-tissue sarcomas.
The round, $70 million Series C closed July 31 2026, brings cumulative funding above $240 million. Bristol Myers Squibb stayed in; Eli Lilly and Catalio Capital Management came in as new investors.
The constraint is the isotope. Actinium-225 supply, not trial design, has been the field's bottleneck, and Ratio spent the last year signing supply agreements instead of marketing its pipeline.
Radiopharmaceuticals have a habit of looking transformative in the clinic and then disappointing at scale. Lutathera and Pluvicto proved the modality works, then revealed how fragile the supply chain under it actually is. That history is worth holding onto while reading a $70 million round for a molecule that has not yet left Phase 2.
A payload built to fail close to its target
Targeted alpha therapy is a different physics from the beta emitters that preceded it. A beta particle travels far enough to damage several cells at once, which is useful and imprecise in roughly equal measure. An alpha particle, the kind released by actinium-225, travels about 30 to 80 micrometres in tissue. That is a few cells wide. It deposits a large amount of energy in that short path, tears both strands of the DNA it meets, and stops.
The whole design therefore comes down to one number: how much radioactivity reaches the tumor versus how much leaks into healthy tissue. FAP, the target of RTX-2358, is a protein that sarcomas and many solid tumors overexpress, which is the company's reason for betting on it. Its chief scientific officer, John Babich, put the problem plainly.
It is important to have a substantial ratio between how much radioactivity goes into the tumor and how much of it goes everywhere else in the normal tissues, and that is a difficult problem to control.— John Babich, President and Chief Scientific Officer, Ratio Therapeutics
That is the science in one paragraph. The engineering is where the money goes.
The trial: sarcomas, six cycles, twelve months
The ATLAS study is a single Phase 1/2 trial running at Memorial Sloan Kettering, MD Anderson, and Princess Margaret. Patients with relapsed or refractory soft-tissue sarcomas receive the actinium-225 payload intravenously once every eight weeks, up to six cycles over twelve months. FAP expression is confirmed first with a companion imaging agent, so the trial does not dose patients whose tumors lack the target.
The first cohort was dosed in December 2025. That cadence matters: it means the Series C is funding the expansion phase, not a launch. The company says the proceeds will also move a next-generation radioligand candidate into the clinic and support a fifth investigational new drug (IND) filing.
As we march the ATLAS trial forward and prepare for our 5th IND filing, these proceeds are instrumental across the development and ultimately the supply of our targeted and PK-optimized radiopharmaceuticals.— Jack Hoppin, Chief Executive Officer, Ratio Therapeutics
The round: why $70 million now
Investor structure tells you which thesis is being funded. Bristol Myers Squibb is a strategic participant, consistent with its broader move into radioligand therapy. Eli Lilly arrived as a new investor in the round. Catalio Capital Management, the life-sciences investor, joined alongside family office capital. Sue Meng of Duquesne Family Office, the round's anchor existing investor, described the decision in execution terms.
Ratio is a leader in radiopharmaceutical innovation and it has backed up science with execution — hitting clinical milestones, deepening strategic partnerships, and building the manufacturing infrastructure this modality demands.— Sue Meng, Managing Director, Duquesne Family Office
The external validation is already on the balance sheet. A November 2024 license agreement with Novartis on a somatostatin-receptor-2 radioligand candidate carried a headline value up to $745 million. That deal, plus the Series C, gives Ratio a funding structure that does not depend on a single positive readout to survive.
The real constraint is the isotope
Actinium-225 has a half-life of ten days and no commercial reactor farm behind it. For years the field has run on a handful of production routes, mostly from thorium decay chains and, more recently, accelerator-based methods. Companies have signed multi-year supply deals the way data centers sign power purchase agreements: to guarantee a scarce input years in advance.
Ratio has been doing exactly that. It holds a supply agreement with TerraPower Isotopes signed in June 2025, added PanTera as a strategic supplier in June 2026, and expanded manufacturing collaboration with PharmaLogic in May 2026. On the manufacturing side it is building a 65,000 square foot facility in Salt Lake City slated to operate in the second half of 2027. As Babich told an interviewer, the supply picture looks far more promising than it did five years ago, thanks to scaling efforts from TerraPower Isotopes, PanTera, and Niowave.
That is the honest version of the story. The scarcity that once limited the field is being engineered away, which is good for the modality and uncomfortable for any single company's moat. If actinium-225 becomes a commodity, differentiation shifts entirely to targeting and manufacturing yield. Those are exactly the two places Ratio is spending the round.
What has to go right is now concrete. The ATLAS expansion cohort needs to reproduce the safety and target-engagement signals seen so far. The next-generation candidate needs a clean IND. The Salt Lake City facility needs to open on schedule. Each of those is a normal operational task for a clinical-stage company. None of them is guaranteed, and sarcomas are a small, difficult indication where the placebo effect and the noise floor both run high.
Watch the isotope, not the press release. A $70 million round says the investors believe supply will be there. The trial data will say whether the payload deserved it.
