Regeneron Couldnt Crack the Undruggable. It Just Paid $125M for Someone Who Did

Regeneron just admitted it couldn't solve one of biology's hardest drug puzzles and paid $125 million for someone else's answer.

Parabilis Medicines, a Harvard-born biotech that has spent nearly a decade trying to crack beta-catenin, filed its S-1 with the Securities and Exchange Commission on May 19, 2026, one day after signing a deal with Regeneron Pharmaceuticals. The timing was not coincidental. Regeneron, a company famous for building its own drug pipeline from start to finish, paid $125 million upfront plus another $75 million in equity to partner with Parabilis before the IPO price was even set. The deal could reach $2.3 billion if Parabilis hits all its milestones, according to Regeneron's announcement. The company declined to comment beyond its press release.

The pattern is unusual. Regeneron built its flagship drugs, including the eye treatment Eylea and the COVID therapy REGEN-COV, largely from its own laboratories without acquiring external science. It has historically preferred to develop targets in-house rather than license them. The decision to pay a premium for pre-IPO access to Parabilis's beta-catenin program is a departure from that habit.

The target is beta-catenin, a signaling protein that sits at the center of the so-called Wnt pathway, a network that tells cells when to grow, divide, and differentiate. When beta-catenin goes wrong, it is implicated in a range of cancers and fibrotic diseases. For thirty years, the pharmaceutical industry tried to block it with small molecules and never got a drug approved. Beta-catenin is what researchers call "undruggable" by conventional means: the target lives inside cells, and most drugs cannot get there.

Parabilis thinks it has solved that problem. Its platform, called Helicon, attaches cell-permeable peptides to antibodies, letting the payload slip past cellular defenses that have defeated every prior attempt. Helicon appears to be the first platform applying this combination to intracellular protein-protein interactions, according to the company's S-1 filing. The lead candidate, zolucatetide, formerly called FOG-001, is the first direct inhibitor of the interaction between beta-catenin and the T-cell factor, or TCF, family of transcription factors, according to Parabilis. Independent peer review of Phase 1 data has not yet been published.

The Phase 1 data are preliminary but suggestive. In a study in desmoid tumors, rare non-cancerous growths in connective tissue that can be disabling or life-threatening when they press on organs, zolucatetide shrank or stabilized tumors in most patients who received it. The FDA granted fast-track designation for desmoid tumors in November 2025, clearing a path for faster review. Parabilis plans to launch a Phase 3 trial funded by the IPO proceeds.

If zolucatetide succeeds in Phase 3, it would be the first approved drug that directly blocks the beta-catenin/TCF interaction in humans. That matters beyond desmoid tumors: beta-catenin is implicated in colorectal cancer, hepatocellular carcinoma, and a range of fibrotic diseases. A validated Helicon approach would suggest the platform could be applied to other historically undruggable targets, potentially opening a new front in drug discovery. If it fails, the thirty-year history of beta-catenin as a dead end stands.

Regeneron is contributing its VelocImmune antibodies, the same platform behind its flagship drugs, to extend the Helicon platform across multiple targets beyond the initial five in the collaboration. Tiered royalties run up to low double-digits on net sales of approved medicines, according to the deal terms.

Parabilis is not a startup running on hope. It has raised more than $800 million over its lifetime, including a $305 million Series F in January 2026, and it entered the Regeneron deal with roughly $329 million in cash, Fierce Biotech reported. Its largest outside shareholder is Fidelity Management, at 11.3 percent. The company employs 145 full-time staff and 31 consultants. It has spent nearly $600 million on research and development across its history, according to its IPO filing.

That financial cushion is precisely why the IPO is hard to read. The filing shows a placeholder goal of $100 million, a ceiling figure, not a guaranteed raise. With $329 million already in the bank and $125 million incoming from Regeneron, Parabilis was not facing a capital crunch. What it was facing, according to people familiar with the matter cited by BioPharma Dive, was pressure from long-term investors to provide liquidity after more than a decade of private funding. The IPO window has been quiet since late February 2026, with no new biotech pricings in the second quarter. Going public in that environment suggests Parabilis prioritized its investors' exit over optimal market timing.

The company applied to list on the Nasdaq under the ticker PBLS. What Regeneron bought is not a drug. Zolucatetide is still in Phase 2. It bought optionality on a platform that may have finally solved one of the oldest hard problems in drug hunting.