Lilly agrees to acquire SiteOne Therapeutics for up to $1 billion

Eli Lilly, the maker of Mounjaro and Zepbound, has agreed to acquire SiteOne Therapeutics, a private biotech company developing small molecule pain drugs, in a deal worth up to $1 billion ^[1]. The announcement was made May 27, 2025 ^[1]. This deal is about pain treatment, not about GLP-1 weight-loss or diabetes drugs, but it shows how Lilly is spending money it earns from products like Zepbound to expand into other disease areas.

The centerpiece of the deal is a drug called STC-004, described as a "Phase 2 ready" Nav1.8 inhibitor being studied for chronic pain ^[1]. Nav1.8 is a type of sodium channel found in nerve cells that carry pain signals. SiteOne's broader work targets several ion channels, including Nav1.7, to treat pain without the addiction risk tied to opioid medications ^[1]. The company has also been working on treatments for chronic cough and chronic eye surface pain, using the same approach of targeting specific nerve channels ^[1].

Under the agreement, SiteOne shareholders could receive up to $1.0 billion in cash, made up of an upfront payment plus additional payments if the drug hits certain regulatory and sales milestones ^[1]. That structure means Lilly is not committing the full billion dollars up front, and the final payout depends on how STC-004 performs in future testing and, if approved, in the market ^[1]. Mark Mintun, a Lilly research and development executive, said the company wants to keep developing STC-004 as part of its push for "addiction-free pain therapies" ^[1]. SiteOne co-founder and CEO John Mulcahy said the company has spent more than a decade working toward safer, non-opioid pain treatments, and that Lilly's resources should help move the drug forward faster ^[1].

The deal still needs to close under standard conditions, and Lilly said it will account for the transaction under standard accounting rules once it does, folding the effects into its financial results and future guidance ^[1].

Why it matters for patients

This acquisition does not change anything about Ozempic, Wegovy, Mounjaro, or Zepbound. It is a separate line of research aimed at chronic pain, not obesity or diabetes. For people currently on GLP-1 medications, there is no direct effect from this news.

The deal does matter for the millions of Americans living with chronic pain, since it adds one more non-opioid drug candidate to a small and closely watched field. STC-004 is still in early stages, described as ready to enter Phase 2 trials, meaning it has not yet been tested in the larger trials needed for U.S. approval ^[1]. It is not yet known how long that testing will take or whether the drug will prove effective and safe enough to reach patients.

For patients and investors watching Lilly as a company, the purchase is a sign of where some of the profits from its diabetes and obesity drugs are being redirected. Lilly describes itself as working across several disease areas beyond diabetes and obesity, including pain, and this purchase fits that broader strategy ^[1].

What happens next

The transaction is subject to customary closing conditions, and Lilly has not given a specific closing date ^[1]. Once completed, Lilly plans to continue developing STC-004, though no timeline for Phase 2 trial results or a potential U.S. approval filing was included in the announcement ^[1]. Details on trial design, timing, or results are not yet available in the sources reviewed here.