NEW YORK – Drug developer Bristol-Myers Squibb Co. offered $4.5 billion in cash for its cancer drug partner ImClone Systems Inc. Thursday, though the biotechnology company's board has yet to comment on the proposal.

The offer, for $60 per share, marks a 30 percent premium to New York-based ImClone's closing price of $46.44 Wednesday. New York-based Bristol-Myers already owns about 17 percent of ImClone and is a longtime U.S. partner on the colon and head and neck cancer drug Erbitux.

ImClone shares surged beyond Bristol's offering price in premarket trading, gaining $18.29, or 39.4 percent, to $64.73.

“In my view, and in the view of our board of directors, this transaction makes compelling business sense for both of our companies and is in the best interests of our respective shareholders and the cancer patients for whom our companies' life saving medicines are so important,” Bristol-Myers Chairman and Chief Executive James M. Cornelius said in a letter to ImClone's board.

He categorized the offer as attractive and looks forward to a “prompt response” from ImClone.

Cornelius called ImClone Chairman Carl C. Icahn with the offer Thursday and delivered the letter, which is included in a Securities and Exchange Commission filing.

Icahn, a billionaire activist investor, won control of ImClone in 2006 after a proxy battle that led to the departure of CEO Joseph L. Fischer and four board members. He was named chairman and created an executive committee to lead the company. John H. Johnson was named CEO in August of 2007.

At the time of the takeover, Icahn said he planned to investigate why ImClone's relationship with Bristol-Myers had “seriously deteriorated” following a public feud over Erbitux.

Shares of Bristol-Myers closed at $21.51 Wednesday.