If the deal is approved by both companies' boards and regulators, Abgenix shareholders will receive $22.50 a share — a 54 percent premium over its closing stock price of $14.65 Wednesday.

"That's a bargain," said Matt Murray, an analyst with Rodman & Renshaw. If Abgenix shareholders and regulators approve the deal, Amgen will acquire all rights to panitumumab, a promising colon-cancer drug the two companies are developing.

Abgenix had earlier sold half the drug's future profits to Seattle's Immunex, which Amgen acquired in 2002.

Amgen said Wednesday that it believes sales of the still-experimental drug could exceed $2 billion a year.

"We think Amgen's low-balling," said Murray, who believes annual sales could reach $3.2 billion.

Amgen CEO Kevin Sharer said the company made the offer after the colon-cancer drug was found to worked better than expected during a large human trial last month.

"We paid a strong premium because we see significant upside," Sharer said. He said the deal will be profitable for Amgen once panitumumab sales reach $1 billion.

The results showed the drug slowed the progression of the disease by 46 percent in those receiving the drug compared to those who didn't in the 463-patient experiment. A skin rash was the most common side effect. The companies had expected the disease to progress 33 percent slower in those receiving the drug.

Panitumumab is produced by mice genetically engineered with human genes to make molecules that help prevent the cancer from growing in patients. Abgenix is also developing an osteoporosis drug.

In aftermarket trading, Abgenix shares soared nearly 50 percent, or $7.25, on news of the deal. Amgen shares rose $1.40, or 1.8 percent, in late trading. Earlier, the stock fell 68 cents to close at $76.78 on the Nasdaq Stock Market.

Amgen said it hopes the deal will close in the first quarter next year.

The amount of debt Amgen will assume was not immediately disclosed.

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