Pfizer's (NYSE: PFE) $56 million offer for Icagen (Nasdaq: ICGN) has been extended one more day as the pharmaceutical giant remains just shy of the minimum shares needed to execute the acquisition.

Unless there is another extension, Pfizer's Icagen offer now expires midnight tonight. As of midnight Aug. 31, the 49 percent of diluted shares tendered was 107,431 shares short of the majority needed to approve the deal.

The agreement for a Pfizer acquisition of Icagen was announced on July 20, a cash deal valued at $6 per share. That price was a more than 22 percent discount to the July 19 close of $7.75. Icagen shares traded at around $2.40 per share until Pfizer's interest in acquiring Icagen became public in June. Icagen disclosed in securities filings that it had been seeking a buyer for more than four years with no success until Pfizer approached the company with an offer in April.

But Pfizer's offer for Icagen has come under scrutiny from some Icagen shareholders who claim it grossly undervalues the company and its drug pipeline of pain drug candidates. Investment firms Merlin Nexus and New Leaf Venture Partners say Pfizer had access to positive phase 1 data for a pain compound, data that has not yet been made public. The firms estimate that Icagen's value is between $11 and $19 per share -- up to $165 million.

Last week, Pfizer sent Icagen's board of directors a letter stating firmly that the offer of $6 per share was the company's "best and final price." The company added that if a majority of shares are not tendered for the sale, Pfizer will still not raise its offer.

Pfizer said today that if all of the conditions of the tender offer are satisfied, it will move forward with the acquisition, and Icagen shareholders who do not tender their shares of Icagen common stock will not be paid for their shares until after the deal is completed..