Onyx Pharmaceuticals Inc. (ONXX) said it is seeking potential buyers after rejecting an unsolicited acquisition offer from Amgen Inc. of around $8.7 billion, based on shares outstanding, potentially setting the stage for a bidding war for the cancer-drug developer.

Onyx said in a statement that Amgen's offer "significantly undervalued Onyx and its prospects, and was not in the best interest of Onyx or its shareholders."

Amgen's bid valued the biotech company at $120 a share, or about 38% more than its closing price Friday of $86.82. Including notes that convert to equity and stock options, the offer is worth around $10 billion, according to a person familiar with it.

Amgen declined to comment.

Lori Melancon, an Onyx spokeswoman, said the company had received indications of interest in the past. That, combined with Amgen's offer, led the board to authorize the company's financial adviser to contact other potential acquirers. Onyx has a "fiduciary responsibility to our shareholders' to explore the possibility of a merger, she said.

Onyx's principal attraction is its blood-cancer treatment Kyprolis, a potential blockbuster therapy. One virtue of the drug is that Onyx doesn't have to share revenue from it with a partner in most of the world, according to a person familiar with the matter. But a big global drug company might be able to distribute Kyprolis more effectively than Onyx could on its own.

Kyprolis could help Amgen offset expected declines for some of its core assets as they face patent expirations and the possibility of new generic competition. Amgen's sales force has decades of experience selling cancer-related drugs like Neulasta, which helps boost the body's resistance to infection in patients receiving chemotherapy, said Mark Schoenebaum, an ISI Group analyst.

Acquiring Onyx also could make strategic sense for other large pharmaceutical and biotechnology companies looking to gain access to new cancer medications. Such drugs can cost more than $100,000 annually but have faced relatively little resistance from insurers.

Kyprolis, which was approved in the U.S. last year and which Onyx is in the process of launching globally, had sales of $64 million in the quarter ended March 31, and could reach nearly $2 billion in annual sales by 2018, according to analysts' projections.

Among the companies--besides Amgen---for which Onyx could be a good strategic fit are Pfizer Inc. (PFE) and Bayer AG (BAYRY, BAYN.XE), as well as big cancer players such as Novartis AG (NVS, NOVN.VX), Roche Holding AG (RHHBY, RO.EB, ROG.VX) and Celgene Corp. (CELG).

Pfizer, Bayer and Novartis declined to comment. Roche and Celgene didn't respond to requests for comment.

Onyx has a collaboration and royalty licensing agreement with Bayer that allows the German drug giant to share in the profits of two cancer drugs. It also has rights to an experimental cancer treatment being developed by Pfizer, the rights to which could be sold for a hefty profit by any company that purchases Onyx. That's because the industry's appetite for acquiring royalty streams has increased recently, Mr. Schoenebaum said.

One of Onyx's agreements with Bayer is for Nexavar, a liver- and kidney-cancer treatment for which the companies are trying to expand sales. Onyx reported recently that Nexavar helped to nearly double the number of months that patients with a rare type of advanced thyroid cancer lived without their disease worsening. Patients with treatment-resistant thyroid tumors have no other treatment options, study investigators said at the time. If Nexavar is approved it would be the first new drug for this type of thyroid cancer in 40 years.

If other suitors do emerge, however, it isn't clear whether they come up with an offer that would be acceptable to Onyx's board.

If Onyx remains independent, "it will be viewed as a strategic blunder by their investment bankers," Mr. Schoenebaum said. "However, I don't think that's going to happen."

Until its statement Sunday, Onyx had publicly maintained that it wanted to remain independent and was looking to expand its own development pipeline through acquisitions. Onyx is considered one of the biotechnology sector's most promising companies, even as research and development costs have kept it from achieving profitability.

"There is every indication that we should just keep going and going on our own," N. Anthony Coles, Onyx's chairman and chief executive, said in a March interview. "We do believe, though, that the way to build a successful sustainable business is through acquisition, either companies or products, even licensing deals."

Over the past 12 months, Onyx shares have traded between $63 and $101.57. Their value has grown since the approval of Kyprolis last summer and on the potential for expanded sales of Nexavar.

After the market's close on Friday, Canada's Financial Post reported details of Amgen's offer, leading Onyx shares to jump more than 25%, to $109.01 in after-hours trading Friday.

Centerview Partners LLC is acting as financial adviser to Onyx, and Goodwin Procter LLP is acting as legal adviser.

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