TRENTON, N.J. — Bristol- Myers Squibb Co. on Thursday offered $4.5 billion in cash for its cancer drug partner, biotechnology company ImClone Systems Inc., saying the two are “a natural fit.”

The proposal offers ImClone stockholders $60 per share, a 30 percent premium to the New York-based company’s closing price of $46.44 Wednesday. New York-based Bristol-Myers, the world’s No. 14 drugmaker by revenue, already owns about 17 percent of ImClone.