Drug giant Pfizer (NYSE: PFE) is acquiring Triangle-based Icagen (Nasdaq: ICGN) in a deal valued up $56 million, the companies announced Wednesday.

The companies were already drug development partners. Pfizer said in an SEC filing last month that it was considering a buyout of Icagen.

Pfizer also owned 11 percent of Icagen’s shares before the deal was announced.

Pfizer said it would pay $6 for each of Icagen’s 8.3 million shares.

The news sent Icagen shares down sharply – more than 20 percent – to under $6 in early morning trading.Icagen had traded as high as $8.40 on Tuesday.

In anticipation of a deal, investors had driven Icagen shares up from around $2.40 when Pfizer made the SEC filing.

The two firms have been partners since a potential $1 billion deal was announced in 2007.

Pfizer and Icagen have been working together on three drug development deals that could have meant more than $41 billion to the RTP firm. Pfizer commited to buying $5 million in Icagen stock most recently in September 2010. It purchased $5 million in stock shortly after the partnership was announced in 2007.

“We’re excited that Icagen, a global leader in pain research, will join Pfizer, further strengthening our innovative core,” said Ruth McKernan, senior vice president of Pfizer’s Pain & Sensory Disorders and Regenerative Medicine unit, known as Neusentis, in a statement.

“Icagen’s capabilities and core ion channel technology will help to further expand Pfizer’s position in the pain relief disease area and our ability to develop potential first-in-industry drugs for the treatment of pain and related disorders,” she added.

The companies expect the deal to close before the end of the year.

“During the nearly four years since the initiation of our collaboration, each side has developed a mutual appreciation of the expertise and capabilities of the other,” said Icagen Chief Executive Officer Kay Wagoner. “By joining forces in a more integrated manner, we believe that our joint efforts towards the identification and development of novel pharmaceuticals targeting specific ion channels will be significantly enhanced.”

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