European immunology company argenx has agreed to buy Unnatural Products, a US developer of macrocyclic peptide drugs, in a deal worth up to about $1.15 billion.

argenx, which is listed on Euronext and Nasdaq under the ticker ARGX, announced the definitive agreement on 9 November. It already holds a minority stake in Unnatural Products and will now acquire all remaining shares in the Santa Cruz, California-based company.

Deal terms

Under the terms, argenx will pay $550 million in cash on hand when the deal closes, plus up to $595 million in payments tied to specified development and regulatory milestones. The full $1.15 billion would only be paid if those milestones are met.

The boards of both companies have approved the transaction, which is expected to close by the end of 2026, subject to customary closing conditions. Morgan Stanley is acting as exclusive financial adviser to argenx, with DLA Piper as its legal adviser.

Shares in argenx rose more than 4.5% on Monday as European markets rallied, with the wider Stoxx 600 index up 1.3% on the day.

What argenx is buying

Unnatural Products has built a discovery platform for macrocyclic peptides, ring-shaped molecules designed to combine some of the selectivity of antibodies with the drug-like properties of small molecules. Its platform pairs machine learning-guided design with massively parallel synthesis and direct-to-biology screening.

argenx said the platform was highly complementary to its existing antibody engineering work and would open new ways to pursue novel biology, including medicines that can be taken orally or given on a long-acting basis, two formats that are often difficult to achieve with antibody-based therapies alone.

Following the close, Unnatural Products' team and leadership will stay at its Santa Cruz site, which will become argenx's home for peptide discovery.

Building on an existing partnership

The acquisition formalises a relationship that began with a multi-target research collaboration announced in July 2025. The two companies have been working together on an oral inhibitor of FcRn, the neonatal Fc receptor targeted by argenx's established drug Vyvgart (efgartigimod).

No specific Unnatural Products programmes, molecular targets or indications were disclosed in the announcement, and none of its candidates was identified as being in clinical testing. The deal is therefore a bet on the platform itself rather than on a named drug.

The timing reflects growing industry interest in macrocyclic peptides. Two such medicines won US approval earlier this year, Merck's once-daily cholesterol tablet Lipfendra in July and Johnson & Johnson's oral psoriasis treatment Icotyde, giving argenx regulatory reference points for the modality it is buying into.

For investors, the deal adds a new technology arm to a company built around antibodies, at a moment when European biotech stocks are drawing fresh attention after a choppy few weeks for the sector.