Carl Icahn sued Amylin Pharmaceuticals on Monday to extend a deadline for nominating directors, as the activist investor pushes to win enough board seats to engineer a sale of the drugmaker.

Icahn, whose 8.94 percent stake has made him Amylin's third-largest shareholder, faulted the company for failing to reveal that it had rejected a roughly $3.5 billion takeover bid from Bristol-Myers Squibb.

In a complaint filed in Delaware Chancery Court, the billionaire said it would be unfair to deny shareholders another chance to nominate directors who could push for a sale, given the developments since the original Jan. 25 deadline.

Based in San Diego, Amylin makes diabetes medications that include Bydureon, Byetta and Symlin.

"Amylin is at a crossroads -- it can be sold, now, at a significant premium, or it can continue to attempt to market its products, which may be lucrative if successful, but which entails substantial risk," the complaint said. "Stockholders of Amylin should be given the choice."

The Bristol-Myers bid valued Amylin at $22 per share, a person familiar with the matter said last week.

Shares of Amylin have risen more than 50% to above that level since news of the bid surfaced on March 28, and Icahn said the company "can and should" be sold at a premium.

"We believe Mr. Icahn's lawsuit is without merit," Amylin said in a statement. "Amylin's board is fully aware of its fiduciary duties, and is committed to always acting in the best interests of all stockholders."

Icahn has a long history of activism in healthcare, including engineering a $6.5 billion sale of ImClone Systems to Eli Lilly and winning two board seats at biotechnology company Biogen Idec.

In the complaint, Icahn attacked Amylin's board for publicly offering 13 million shares on March 8 for $15.62 each without revealing the Bristol-Myers bid.

He also objected to a March 6 grant of stock options to various executives, including 300,000 to Chief Executive Daniel Bradbury, allowing them buy shares at $16.02 each, 27.2 percent below the Bristol-Myers offer.

Amylin has told brokers it plans to hold its annual meeting on May 11, but has not formally announced a date, the complaint said.

Icahn separately said he would file another lawsuit if Amylin does not by Wednesday provide copies of books and records that he has demanded.

The case is Icahn Partners LP et al v. Amylin Pharmaceuticals Inc et al, Delaware Chancery Court, No. 7404.