Pfizer (NYSE: PFE) will not raise its offer to acquire Icagen (Nasdaq: ICGN).

The pharmaceutical giant sent a letter to Icagen’s board of directors on Friday holding firm on its purchase price of $6 per share, which amounts to $56 million.

Douglas Giordano, senior vice president, worldwide business development, wrote that with the approaching approaching Aug. 31 expiration of Pfizer’s offer to purchase Icagen, the company wants to “reaffirm unambiguously” that $6 per share, is Pfizer’s “best and final price.”

Giordano added that if a majority of shares are not tendered, the company will still not raise its offer. Approval of a majority of shareholders is required in order for the Pfizer-Icagen deal to go through.

In June, speculation that Pfizer was interested in buying drug partner Icagen sent the Durham, North Carolina-based developmental stage pain drug company’s stock from $2.40 per share to a July 19 peak close of $7.75 per share. Pfizer’s acquisition of Icagen at $6 per share was announced July 20.

Investment firms Merlin Nexus and New Leaf Venture Partners have been among the most vocal shareholder opponents of the deal, claiming the price undervalues Icagen and its early-stage assets in particular. Merlin Nexus and New Leaf continue to claim that Pfizer had access to positive phase 1 data for a pain compound, data that has not yet been made public. They add that had Icagen had allowed the pain programs to reach milestones, their value would be higher. Merlin Nexus and New Leaf offer their own estimates of Icagen’s pain program; they price the stock at between $11 and $19 per share – up to $165 million.

A report from consultancy Campbell Alliance suggests that big pharma is showing increasing interest in licensing or acquiring earlier stage compounds. But much of that activity is focused on compounds in phase 2 clinical development. Pfizer offered no comment on the Merlin Nexus and New Leaf claims.

Icagen has been angling for an acquirer for four years with no offers until Pfizer approached the company earlier this year. Pfizer initially offered between $4 and $5 per share. Icagen disclosed in securities filings that two other bids emerged but neither approached Pfizer’s final cash offer of $6 per share. If a majority of shareholders approve the deal, it is expected to close by the end of 2011.

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