Radiopharmaceutical developer Ratio Therapeutics has raised $70 million in Series C funding to advance a class of cancer drugs that deliver radiation directly to tumours.

The round, announced August 3, 2026, drew new investors Catalio Capital Management, Eli Lilly and Wasatch Group, alongside existing backers the Duquesne Family Office and Bristol Myers Squibb — a notable show of big-pharma interest in targeted radiotherapy.

What radioligand therapy does

These are molecules that seek out markers on cancer cells and carry a radioactive payload to destroy them while sparing healthy tissue.

The construct has three parts: a targeting molecule binding a protein on the tumour surface, a radioactive isotope providing the killing, and a chelator holding the isotope securely to the rest.

The advantage over external radiation is geometric. A radiation beam must pass through healthy tissue to reach a tumour and can only treat what can be aimed at, whereas a radioligand circulates and accumulates wherever the target is expressed — including metastases too small or numerous to target individually.

Why the chelator matters as much as the isotope

The pipeline is built on two in-house platforms — the Trillium pharmacokinetic-tuning technology and the Macropa chelator system.

A chelator failing is the central safety risk in this field. If the isotope detaches in circulation, it distributes according to its own chemistry rather than the targeting molecule’s — and several isotopes accumulate in bone, delivering radiation to marrow, which is dose-limiting and potentially serious.

The requirement is stringent because the timescales are long relative to chemistry: the construct must hold together in blood for hours to days, in the presence of proteins and ions competing for the metal.

Pharmacokinetic tuning addresses the complementary problem — how long the construct circulates. Too fast and insufficient reaches the tumour; too slow and healthy tissue is irradiated while it clears.

The lead programme

Lead candidate [Ac-225]RTX-2358 targets fibroblast activation protein (FAP) and is being tested in advanced sarcomas in the Phase 1/2 ATLAS trial.

FAP is an interesting target because it is expressed not on tumour cells themselves but on cancer-associated fibroblasts — the supporting cells tumours recruit to build their surrounding stroma.

That makes it broadly applicable across cancer types, since many solid tumours have such stroma regardless of tissue of origin. It also means the therapy attacks the tumour’s support structure rather than the malignant cells directly, which works with radiation because the emitted particles damage cells nearby — the payload does not need to be attached to the cell it kills.

Why actinium-225

The Ac-225 designation names an alpha emitter, which differs meaningfully from the beta emitters used in earlier radiopharmaceuticals.

Alpha particles deposit far more energy over a much shorter distance — a few cell diameters — causing double-strand DNA breaks that cells struggle to repair. That combination gives greater killing power with less damage to surrounding tissue.

Actinium-225 is also famously scarce. Global supply has historically been limited to small quantities produced from decaying stockpiles, and expanding it has been a recognised constraint on the entire alpha-emitter field.

Where the money goes

Proceeds will fund the ATLAS trial, move a next-generation candidate into the clinic, broaden discovery to new targets including GRPR and, through a Novartis partnership, SSTR2 — and scale up manufacturing, a persistent bottleneck in a field depending on short-lived radioactive isotopes.

That bottleneck is unlike anything in conventional pharmaceuticals. The product decays continuously from the moment it is made, so it cannot be stockpiled, and every dose must be produced, released and delivered within a window set by physics.

Why big pharma is buying into this field

The presence of both Eli Lilly and Bristol Myers Squibb on the cap table reflects a broader repositioning that followed clinical validation of the modality.

Radiopharmaceuticals were a niche for decades — used diagnostically at scale, therapeutically only in narrow settings such as radioactive iodine for thyroid disease. The change came when targeted radioligand therapies demonstrated survival benefit in prostate cancer and neuroendocrine tumours in randomised trials.

That converted the modality from promising to proven, and large companies responded by acquiring the specialists. Several multi-billion-dollar acquisitions of radiopharmaceutical developers followed in short succession, and the remaining independent companies became correspondingly more valuable.

Investing early through a venture round is the cheaper version of the same move. It buys visibility into the technology and a relationship with the company, without committing to an acquisition price before the clinical data exist.

The supply constraint shapes that calculation too. A company with secured isotope access and validated manufacturing holds something that cannot be replicated quickly with capital alone — which is an unusual position in pharmaceuticals, where manufacturing is normally the commoditised part.

“This financing reflects investor confidence in progress made and opportunities ahead,” chief executive Jack Hoppin said, calling the proceeds “instrumental across development and supply of targeted radiopharmaceuticals.” Business news, not investment advice.