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Efforts to innovate in the fight against influenza include multiple failed universal flu vaccines. Merck is aiming for universality with a drug, not a vaccine, via the $9.2 billion acquisition of Cidara Therapeutics, a biotechnology company whose lead program brings properties of both small molecule and biologic therapies to offer long-acting protection against viral pathogens.
The deal announced Friday comes as San Diego-based Cidara continues Phase 3 testing of its influenza drug candidate, CD388. For Merck, the deal diversifies its pipeline by adding a late-stage asset with blockbuster potential along with a platform technology that could support the pharmaceutical giant’s drug R&D work in infectious disease and beyond.
Cidara’s drugs have two main components, each with a different mechanism of action. The first part is a small molecule designed to bind to a target on the surface of the pathogen to inhibit viral proliferation. The small molecule is conjugated to the second component, the Fc region of the human antibody IgG1. This protein fragment extends a Cidara drug’s half-life and engages the immune system to provide an additional response. Cidara calls this new type of therapy a drug-Fc conjugate, or DFC.
CD388 incorporates multiple copies of small molecule inhibitors of neuraminidase, an enzyme found on the surface of flu viruses. Neuraminidase inhibitors are already widely available in flu antiviral products such as Tamiflu from Roche and Relenza from GSK. In Cidara’s drug, the neuraminidase inhibitors are conjugated to an Fc fragment. Cidara is developing this DFC for the prevention of influenza infection in those at higher risk of flu complications. The company has said that because CD388 is not a vaccine, it does not rely on an immune response and is expected to work in those with weaker immune systems. Also, its long-acting effect offers the potential for protection during the entire flu season.
This past June, Cidara reported preliminary data from a placebo-controlled Phase 2b study that enrolled about 5,000 unvaccinated adults. Results showed the subcutaneously injected drug met the study’s main goal, demonstrating statistically significant prevention efficacy for each of the three doses tested. The study drug was safe and well tolerated.
In September, Cidara began a Phase 3 study whose start was accelerated by six months in order to overlap with the 2025 flu season in the Northern Hemisphere. The program will continue into the spring of next year to enroll participants in the Southern Hemisphere. Merck said an interim analysis is planned for the first quarter of 2026 to assess trial size and powering assumptions, and also to determine whether to increase enrollment beyond the 6,000 participants planned for the study.
With the Cidara acquisition, Merck is getting an asset that Johnson & Johnson abandoned. In 2021, Cidara entered a partnership with J&J subsidiary Janssen Pharmaceuticals, which secured global rights to CD388 and took over Phase 2 development. But in 2023, Janssen stopped work on much of its infectious disease pipeline, including the Cidara antiviral. Last year, Cidara reacquired rights to CD388 and raised funding to continue mid-stage clinical development of the program.
Merck will also get Cloudbreak, the proprietary Cidara platform technology that yielded CD388. Cloudbreak has additional applications in oncology and it has produced DFCs for targets in solid tumors, where the biotech said this type of therapy could have advantages over antibody drug conjugates. Cidara has been exploring business opportunities for these preclinical programs, but Merck might choose to keep them for its own cancer pipeline.
Merck’s portfolio is headlined by the cancer immunotherapy Keytruda, which accounted for $29.4 billion in revenue in 2024 and is currently the company’s top-selling product. But Keytruda faces patent expiration in 2028, so Merck has been striking business deals for late-stage drugs with the potential to replace Keytruda’s revenue. Earlier this year, Merck paid $10 billion to buy Verona Pharma, whose blockbuster prospect Ohtuvayre is approved for treating chronic obstructive pulmonary disease and has potential applications in other respiratory disorders. Merck’s business development efforts in 2024 included deals that brought blockbuster prospects for cancer and eye diseases.
The $9.2 billion acquisition price for Cidara breaks down to $221.50 in cash per share. That’s more than double the stock’s Thursday closing price. When Cidara went public in 2015, the then preclinical biotech priced its shares at $16 apiece. Cidara’s stock price has been climbing steadily since the June report of Phase 2 data for CD388.
“This acquisition expands and complements our respiratory portfolio and pipeline,” Dean Li, president of Merck Research Laboratories, said in a prepared statement. “Influenza continues to pose a significant global health threat, causing widespread illness, morbidity and death each year especially in older adults and immunocompromised individuals, such as those with cancer and chronic diseases.”
The Merck and Cidara boards of directors have approved the transaction, which is expected to close in the first quarter of 2026. Merck has scheduled a Monday 8 a.m. investor call to discuss the deal.
Image by Flickr user quapan via a Creative Commons license
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