Pfizer, which has offered to buy Durham's Icagen for $6 per share, wants to be clear: It won't pay a dime more.

In response to pressure from Icagen investors who are unhappy with the value of the offer, Pfizer sent a letter today to Icagen's board.

With the takeover offer set to expire on Aug. 31, "we thought it appropriate to reaffirm unambiguously, as we made clear when negotiating the merger agreement with you, that $6 per share is our best and final price," the letter states. "Pfizer will not pay more."

The so-called tender offer requires that a majority of Icagen investors agree to the deal. "If a majority of shares are not tendered, however, we still will not raise our offer," wrote Douglas Giordano, Pfizer's senior vice president of worldwide business development.

Billionaire and activist investor Carl Icahn isn't giving up his fight for control of Clorox, the parent of Durham-based Burt's Bees.

In a regulatory filing today, Icahn proposed replacing the company's entire board with his own 11 directors, including himself.

Clorox, which last month rejected Icahn's $10.7 billion takeover offer, said it would review the proposal, but doesn't support his efforts. "Mr. Icahn is nominating candidates solely to advance his own agenda," Clorox wrote in a statement.

Billionaire and activist investor Carl Icahn is pushing for a sale of Clorox, the consumer-products conglomerate that owns Durham-based Burt's Bees.

Icahn offered to buy Clorox for $76.50 a share, or more than $10 billion. But he also urged company officials to seek other "strategic buyers" including Procter & Gamble, Kimberly-Clark, Unilever and Colgate-Palmolive.

These potential acquirers could market Clorox's brands more aggressively overseas, Icahn wrote in a letter to Clorox CEO Donald Knauss.

"I would love to have this company," Icahn told CNBC today. But he added that he expects there will be competing offers.