Updated from 11:20 a.m. EST

Bristol-Myers Squibb ( BMY) said Tuesday it is "extremely disappointed" that ImClone Systems ( IMCL) has chosen to reject its demands over the experimental cancer drug Erbitux, but the drugmaker pledged to work with its partner, at least temporarily.

Bristol-Myers' response came in the form of a letter to ImClone, released publicly just before 3 p.m. EST. Earlier in the day, ImClone said it would not accept the drugmaker's demands to rework terms of the partnership for the experimental cancer drug Erbitux.

Bristol-Myers had demanded a bigger cut of the profits from the drug and the temporary removal of ImClone CEO Sam Waksal and his brother Harlan, who is the firm's COO.

"We are extremely disappointed with your response to our proposal of Feb. 5th," Bristol-Myers' letter states. "Clearly, we have a fundamentally different view of the serious issues created by U.S. Food and Drug Administration's refusal to file letter, subsequent events and the appropriate course of action to take, including who should lead the effort going forward."

ImClone's management has told Bristol-Myers that it intends to take the lead at an FDA meeting on Feb. 26 to discuss the Erbitux approval application. Bristol says it will continue to provide ImClone support on a voluntary basis to prepare for the FDA meeting, and that its executives intend to attend the meeting.

But Bristol-Myers didn't commit to working with ImClone beyond the Feb. 26 FDA meeting.

"Finally, I must consider the best interests of Bristol-Myers Squibb Co. and its shareholders. Accordingly, we are considering our business and legal options with respect to our relationship with ImClone but will wait until after the FDA meeting to determine what further actions we may take," the letter states.

Shares of ImClone closed up 82 cents, or 4.9%, to $17.65 a share in Tuesday's trading. Shares of Bristol-Myers rose 21 cents to $45.38 a share.

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