Three big names in the pharmaceutical industry, Pfizer Inc. (PFE), AstraZeneca (AZN) and Novartis AG (NOVN), are among the companies preparing offers for Onyx Pharmaceuticals. Prior to this, the San Francisco-based company recently rejected an unofficial bid from Amgen Inc. (AMGN) according to two sources familiar with the deal.

After signing the confidentiality agreements, the three companies had been given the chance to make initial bids this week, according to the sources who asked to remain undisclosed due to privacy issues regarding the deal.

Onyx Pharmaceuticals announced during the previous month that it is teaming up with advisers in choosing bidders after turning down Amgen's $120 per share offer.

The rejection did not put off Amgen from bidding for Onyx however, three sources familiar with the matter have said, as it continues to show interest in the matter and is waiting to resume talks of a deal between the two companies.

Market value of Onyx Pharmaceuticals as of July 19 is US$9 billion.