Regeneron Pharmaceuticals (Nasdaq: REGN) has entered a research collaboration with private Cambridge, Massachusetts-based Parabilis Medicines to discover and develop a new class of therapeutics called Antibody-Helicon conjugates, combining Regeneron's antibody engineering infrastructure with Parabilis's stabilized, cell-penetrant alpha-helical peptide platform. The deal carries up to approximately USD 2.325 billion in total potential value, including a USD 50 million upfront payment, a USD 75 million equity commitment, and up to USD 2.2 billion in milestone payments across five initial targets. Parabilis, formerly known as Fog Pharmaceuticals, has raised over USD 800 million in total funding, including a USD 305 million Series F held in January this year.
Under the agreement, Regeneron will also be eligible to add targets beyond the initial five through additional option payments, with tiered royalties up to the low double-digits payable to Parabilis on net sales of any approved products.
Deal context
The Helicon peptide platform, developed by Parabilis, produces conformationally stabilized alpha-helical peptides built predominantly from custom non-canonical amino acids. The stabilization — achieved through covalent crosslinks that lock the peptide backbone into a helical geometry — addresses two longstanding limitations of peptide therapeutics: rapid unfolding in solution and inability to cross cell membranes. Parabilis describes the resulting molecules as capable of penetrating cells and engaging intracellular protein-protein interaction surfaces that are inaccessible to conventional antibodies and largely intractable for small molecules.
The Antibody-Helicon conjugate format pairs these cell-penetrant peptide payloads with antibodies derived from Regeneron's VelocImmune platform, using the antibody component to direct delivery to specific cell types while the Helicon payload engages intracellular targets. The structure is conceptually analogous to an antibody-drug conjugate, but substitutes a cytotoxic small molecule payload with a peptide designed to modulate protein-protein interactions rather than kill cells indiscriminately.
Parabilis has one clinical-stage asset generated from the platform: zolucatetide (FOG-001), a direct inhibitor of the β-catenin:TCF4 interaction that has received US FDA Orphan Drug and Fast Track designations for desmoid tumors and has demonstrated in-tumor target engagement in early human data. The company has also disclosed preclinical programs targeting ERG, pan-RAS, and androgen receptor interactions. The programs covered under the Regeneron collaboration are at the discovery stage; no candidate names, specific target identities, or clinical timelines have been disclosed.