At $9.2 billion, the Cidara acquisition lands among the top 5 largest deals of the year.
Merck is making another big-ticket purchase, dropping around $9.2 billion to acquire California’s Cidara Therapeutics and its late-stage antiviral asset.
Merck has offered to purchase all of Cidara’s outstanding shares for $221.50 apiece. The boards of directors of both companies have already signed off on the deal, and pending all clearances, the transaction is set to close in the first quarter of 2026.
The centerpiece of Friday’s deal is Cidara’s lead asset CD388, an investigational drug-Fc conjugate consisting of zanamivir, the main ingredient of GSK’s inhalable flu drug Relenza, attached to a human antibody fragment. “It is not a vaccine, but a long-acting antiviral drug,” the biotech claims on its website. In particular, Cidara is proposing CD388 as a “single-dose, universal preventative agent” against all flu virus strains.
In a statement on Friday, Dean Li, president of Merck Research Laboratories, called CD388 “important,” noting its “strain-agnostic properties” that the pharma plans to leverage to prevent symptomatic flu in high-risk patients. Last month, Cidara presented Phase IIb data for the asset at ID Week, touting 57.7% prevention efficacy for a 150-mg dose against influenza A and B in healthy unvaccinated adults.
CD388’s benefits improved with increasing doses: prevention efficacy jumped to 61.3% at the 300-mg level and to 76.1% at 450 mg. The biotech also found “no apparent safety issues” with CD388. The candidate is currently being tested against a placebo in a Phase III study of around 6,000 adults and adolescents who are at high-risk of developing flu complications. The trial is currently recruiting participants with an estimated completion date in 2027.
Aside from CD388, Cidara is working on investigational therapies for solid tumors, though none have entered the clinic to date.
Merck has made several big-ticket deals this year, led by its $10 billion buyout of Verona Pharma in July, giving the pharma the chronic obstructive pulmonary disease drug Ohtuvayre. The Verona takeover is so far the second largest acquisition of 2025, trailing Johnson & Johnson’s $14.7 billion play for Intra-Cellular Therapies in January.
Merck’s Cidara deal likewise lands in the top 5, just coming up short of the current third-placer, Sanofi’s $9.5 billion bet for Blueprint Medicines in June.
Other high-value deals for Merck this year include its potential $2 billion partnership with China’s Jiangsu Hengrui for a small-molecule lipoprotein(a) inhibitor and the $493 million collaboration with Cyprumed for the technology to make peptide therapies orally available.
