Two large Icagen investors say that Pfizer's $56 million takeover offer for the Durham drug-development company is too low and will push for a higher price.

Pfizer announced on July 20 that it planned to buy the remaining shares of Icagen it doesn't already own for $6 each.

But Merlin Nexus and New Leaf Venture Partners believe that Icagen's shares could be worth up to three time as much, the investment firms wrote in a letter last week to Icagen's board and chairman Charles Sanders. The firms disclosed the letter in a Securities and Exchange Commission filing on Friday.

"We believe the purchase price dramatically undervalues Icagen’s assets, and is not in the best interests of all stockholders," they wrote.