(Reuters) - Merck ( MRK ) will acquire Cidara Therapeutics ( CDTX ) in a nearly $9.2 billion deal, the companies said on Friday, gaining access to an experimental drug for flu prevention.

Merck ( MRK ) is looking to diversify its revenue beyond Keytruda as its patents for the blockbuster drug begin to expire later this decade.

The drugmaker will pay $221.50 per share in cash for Cidara, 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.

Merck ( MRK ), 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.

Since 2021, Merck ( MRK ) has nearly tripled its late-stage pipeline, combining in-house development with deals such as the $11.5  billion purchase of Acceleron in 2021, which netted the pulmonary arterial hypertension drug Winrevair.

In July, Merck ( MRK ) signed its $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 is developing its long-acting antiviral drug CD388, which has the potential to be a single-dose, universal prevention against all flu strains.

"We are confident that CD388 has the potential to be another important driver of growth through the next decade," said Merck ( MRK ) 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 who are at higher risk of developing complications from influenza.

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.

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

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