Pfizer’s (NYSE:PFE) $56 million bid to acquire Icagen (Nasdaq:ICGN) faces a big hurdle – two of Icagen’s largest shareholders.

Merlin Nexus and New Leaf Venture Partners, two firms holding Icagen stock, submitted a July 28 letter to the Icagen board contesting the deal, saying they “believe the purchase price dramatically undervalues Icagen’s assets and is not in the best interests of all stockholders.”

Pfizer, which owns 11 percent of Icagen stock, offered to buy the remaining shares for $6 per share. That’s below the stock’s 52-week high of $8.40 per share reached earlier this month as speculation of a Pfizer acquisition spurred investor interest in the Durham, North Carolina pharmaceutical company.

Pfizer is acquiring Icagen to feed its pipeline of pain drugs. (Read about the deal here.)

The two companies have partnered for the last four years to develop Icagen’s technology, which focuses on compounds that modulate ion channels, protein structures found in the cells of the human body. Icagen’s lead candidate ICA-105665, a compound to treat pain and epilepsy, is currently in mid-stage clinical trials.

While private equity firm Merlin Nexus and healthcare investor New Leaf cite ICA-105665′s progress, they also note the potential of its pre-clinical compounds to address the multibillion-dollar pain market. PF-05089771, another pain compound, is expected to start clinical trials in the next 12 months, they said.

The firms say that data from early studies on the compound have yet to be presented, but “the facts that the trial continues to escalate to higher doses, that Pfizer initiated a second phase 1 multiple ascending dose trial and that Pfizer has entered into a definitive agreement to acquire Icagen, all suggest that the data thus far remain very positive.”

Merlin Nexus owns 10.4 percent of Icagen, according to securities filings, just below Pfizer’s stake in the company. But Merlin Nexus and New Leaf say they aim to rally other shareholders to seek a better price.

“We believe that the board should have sought a greater valuation for Icagen and at the present time, we do not intend to tender our shares under the current acquisition terms,” they wrote. “We continue to explore our options, including potentially contacting other stockholders to share our concerns.”

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