Merck and Immune Design said they've struck an agreement that will pay $5.85 per share in cash for Immune Design, for an approximate value of $300 million.

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Shares of Seattle biotechnology company Immune Design leapt 310 percent Thursday after pharmaceutical giant Merck announced it would acquire the developer of cancer-vaccine technology.

Merck and Immune Design said Thursday they’ve struck a definitive agreement that will pay $5.85 per share in cash for Immune Design, headquartered in the Eastlake neighborhood, for an approximate value of $300 million.

“Scientists at Immune Design have established a unique portfolio of approaches to cancer immunization and adjuvant systems designed to enhance the ability of a vaccine to protect against infection, which could meaningfully improve vaccine development,” Dr. Roger Perlmutter, president of Merck Research Laboratories, said in a statement. “This acquisition builds upon Merck’s industry-leading programs that harness the power of the immune system to prevent and treat disease.”

Immune Design went public in 2014 at $12 per share, raising only $60 million — a small sum in the biotech world, where hundreds of millions of dollars are table stakes in the drug-development game.

Its shares trended downhill since, and this year have traded at $1.50 or below.

News of the deal sent the stock soaring from Wednesday’s close of $1.42 to near the announced acquisition price.