Merck makes big move in antivirals, acquiring Cidara's late-stage flu prevention drug for $9 billion
Innovative efforts to combat influenza include multiple failed universal flu vaccines. Merck aims to bring drugs, not vaccines, to the table with its $9.2 billion acquisition of Cidara Therapeutics, a biotech company whose leading programs bring small molecule and biologic therapeutics with properties that can provide long-lasting protection against viral pathogens.
Friday's announcement of the deal comes as San Diego-based Cidara continues Phase 3 testing of its flu drug candidate, CD388. For Merck, the deal diversifies its pipeline by adding late-stage assets with blockbuster potential and platform technologies that can support the pharma giant's drug development efforts in infectious diseases and beyond.
Cidara's drug has two main ingredients, each with a different mechanism of action. The first part is a small molecule designed to bind to targets on the surface of pathogens to inhibit viral proliferation. This small molecule is conjugated to a second component, the Fc region of human antibody IgG1. This protein fragment extends the half-life of the drug Cidara and stimulates the immune system to provide an additional response. Cidara calls this new type of therapy drug-Fc conjugate (DFC).
CD388 contains multiple copies of a small molecule inhibitor of neuraminidase, an enzyme found on the surface of influenza viruses. Neuraminidase inhibitors are already widely used in influenza antiviral products, such as Roche's Tamiflu and GlaxoSmithKline's Relenza. In Cidara's drug, the neuraminidase inhibitor is conjugated to an Fc fragment. Cidara is developing this DFC to prevent influenza infection in people who are at increased risk of influenza complications. Because CD388 is not a vaccine, it does not rely on an immune response and is expected to be effective in people with weakened immune systems, the company said. Additionally, its long-lasting action provides protection throughout the flu season.
In June, Cidara reported preliminary data from a placebo-controlled Phase 2b study enrolling about 5,000 unvaccinated adults. Results showed that the subcutaneously administered drug met the study's primary objective, demonstrating statistically significant preventive efficacy at each of the three doses tested. The study drug was safe and well tolerated.
In September, Cidara began a Phase 3 study that was initiated six months earlier to overlap with the 2025 flu season in the northern hemisphere. The program will run until next spring, enrolling participants from the Southern Hemisphere. Merck said it plans to conduct an interim analysis in the first quarter of 2026 to assess trial size and robust hypotheses and determine whether to increase enrollment beyond the 6,000 participants planned for the study.
By acquiring Cidara, Merck acquired the assets that Johnson & Johnson gave up. In 2021, Cidara entered into a partnership with Johnson & Johnson subsidiary Janssen Pharmaceuticals, which acquired global rights to CD388 and took over phase 2 development. But in 2023, Janssen halted work on much of its infectious disease pipeline, including its Cidara antiviral drug. Last year, Cidara regained rights to CD3888 and raised funding to continue the program's mid-stage clinical development.
Merck will also gain access to Cloudbreak, the proprietary Cidara platform technology that generates CD388. Cloudbreak, which has other applications in oncology, already produces DFCs targeting solid tumor targets, and the biotech says this type of therapy may have advantages over antibody-drug conjugates. Cidara has been exploring commercial opportunities with these preclinical programs, but Merck may choose to keep them within its own cancer pipeline.
Merck's product portfolio is led by cancer immunotherapy Keytruda, which has 2024 revenue of $29.4 billion and is currently the company's best-selling product. But Keytruda's patent expires in 2028, so Merck has been striking commercial deals for late-stage drugs that could potentially replace Keytruda's revenue. Earlier this year, Merck spent $10 billion to acquire Verona Pharma, whose promising Ohtuvayre is approved to treat chronic obstructive pulmonary disease and has potential applications in other respiratory conditions. Merck's business development efforts in 2024 include deals that hold significant promise for cancer and eye diseases.
Cidara's acquisition price was $9.2 billion, or $221.50 per share in cash. That's more than double the stock's closing price on Thursday. When Cidara went public in 2015, the preclinical biotech's stock price was $16 a share. Cidara stock has been climbing steadily since the release of CD388 phase 2 data in June.
“This acquisition expands and complements our respiratory portfolio and product lines,” Dean Li, president of Merck Research Laboratories, said in a prepared statement. “Influenza continues to pose a significant threat to global health, causing widespread illness, morbidity and mortality each year, particularly among older adults and immunocompromised individuals, such as those with cancer and chronic diseases.”
The boards of directors of Merck and Cidara have approved the transaction, which is expected to close in the first quarter of 2026. Merck has scheduled a conference call with investors at 8 a.m. Monday to discuss the deal.
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