FILE PHOTO: Merck logo is seen in this illustration taken March 26, 2025. REUTERS/Dado Ruvic/Illustration/File Photo

(Reuters) -Merck will acquire Cidara Therapeutics in a nearly $9.2 billion deal, gaining access to an experimental drug for flu prevention in its effort to diversify ahead of patent loss for its blockbuster cancer drug Keytruda.

The drugmaker will pay $221.50 per share in cash for Cidara, the companies said on Friday, a premium of 108.9% from its last closing price.

Shares of Cidara, which has a market capitalization of $3.3 billion, doubled in value to $216.05 in premarket trading, while Merck slipped 1.6%.

Merck, which is set to lose patents for Keytruda later this decade, has nearly tripled its late-stage pipeline since 2021 by combining in-house development with big deals such as the $11.5  billion purchase of Acceleron for pulmonary arterial hypertension drug Winrevair.

The company had in July signed a $10 billion buyout of UK-based Verona Pharma gaining Ohtuvayre, a newly approved drug for chronic obstructive pulmonary disease, commonly known as "smoker's lung".

Cidara's experimental drug CD388 is a long-acting antiviral drug which has the potential to be a single-dose, universal prevention against all flu strains. It aims to protect those at higher risk of flu, with the potential to provide season-long protection.

CD388 is not a vaccine and is expected to be efficacious in individuals regardless of immune status, the company said.

"We are confident that CD388 has the potential to be another important driver of growth through the next decade," said Merck CEO Robert Davis.

It belongs to a class known as drug-Fc conjugates that links a drug to a human antibody fragment and is being studied in late-stage trial in adults and adolescents at higher risk of developing complications from influenza.

Cidara aims to achieve target enrolment of 6,000 participants by December, it said earlier this month. The drug has received breakthrough designation from the U.S. Food and Drug Administration, which could speed up its approval process.

A single dose of the antiviral gave up to 76% protection from symptomatic influenza over 24 weeks compared to placebo, in mid-stage trial studying the drug in healthy unvaccinated adults aged 18 to 64.

Merck, through a subsidiary, will acquire all of Cidara's outstanding shares. The transaction has an equity value of $6.96 billion, according to a Reuters calculation.

The deal is expected to close in the first quarter of 2026.

(Reporting by Sriparna Roy in Bengaluru; Editing by Arun Koyyur)