Onyx Pharmaceuticals Confirms Receipt of Unsolicited Acquisition Proposal From Amgen

SOUTH SAN FRANCISCO, CA–(Marketwired – Jun 30, 2013) – Onyx
Pharmaceuticals, Inc. (
NASDAQ:
ONXX) today confirmed that it has
received and rejected an unsolicited proposal from Amgen Inc. to
acquire all of Onyx’s outstanding shares and share equivalents for
$120 per share in cash, subject to due diligence and other
conditions. The Board of Directors of Onyx had evaluated the
proposal made by Amgen with the assistance of its financial and
legal advisors and concluded that the price proposed by Amgen
significantly undervalued Onyx and its prospects, and was not in
the best interest of Onyx or its shareholders. Onyx communicated
this determination to Amgen on Friday, June 28, 2013.

Based upon expressions of interest received from other third
parties and the recent proposal from Amgen, the Onyx Board has
authorized its financial advisor to contact potential acquirers who
may have an interest in the Company in a transaction that is in the
best interest of Onyx shareholders.

“Onyx has tremendous momentum and, with the expansion of our
pipeline and two successful product launches, the Company and our
talented employees have created significant value for Onyx
shareholders,” said Dr. N. Anthony Coles, Chairman and CEO. “The
Board and the management team remain focused on the opportunities
in front of us, including the potential to expand the use of our
existing therapies in different types of cancer and across
different lines of therapy, as a result of several ongoing Phase 3
studies. We are actively exploring the potential to combine Onyx
with another company as an option to create additional value for
Onyx shareholders.”

Onyx does not intend to communicate further regarding the Amgen
proposal or the process by which the Board of Directors will
consider other proposals. There can be no assurance that an
improved proposal will be made by Amgen or any other entity, that a
definitive agreement will be executed relating to any proposed
transaction, or that any transaction will be approved or
consummated.

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

About Onyx Pharmaceuticals, Inc.

Based in South San Francisco, California, Onyx
Pharmaceuticals, Inc. is a global biopharmaceutical company engaged
in the development and commercialization of innovative therapies
for improving the lives of people with cancer. The company is
focused on developing novel medicines that target key molecular
pathways. For more information about Onyx, visit the company’s
website at
www.onyx.com.

Onyx Pharmaceuticals is on Twitter. Sign up to follow our
Twitter feed @OnyxPharm at

http://twitter.com/OnyxPharm.

Cautionary Statement Concerning Forward Looking Statements

This news release may include certain statements
that are not descriptions of historical facts, but are
forward-looking statements. These forward-looking statements can be
identified by terminology such as “will,” “expects,” “anticipates,”
“future,” “intends,” “plans,” “believes,” “estimates” and similar
statements. Forward-looking statements involve risks, uncertainties
and other factors that could cause actual results to differ
materially from those contained in any such statements. Potential
risks and uncertainties include, but are not limited to, risks
discussed in Onyx’s filings with the U.S. Securities and Exchange
Commission. Onyx does not undertake any obligation to update any
forward-looking statement, except as required under applicable
law.