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

Shares of Cidara Therapeutics soared 104% on the news.

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

Merck will pay $221.50 per share in cash for Cidara, a premium of 108.9% from its last closing price of $105.99. Cidara shares doubled in value to $216.05 in premarket trading.

Since 2021, Merck 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 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.

Cidara's CD388 belongs to a class known as drug-Fc conjugates that links a drug to a human antibody fragment.

It is being studied in late-stage trial and aims to protect those facing the greatest risk of flu such as people with compromised immune systems or high-risk comorbidities, as its efficacy does not depend on a functional immune system.