Onyx Pharma (Nasdaq: ONXX) shares fell today following headlines that pharmaceutical giant Pfizer (NYSE: PFE) won't likely place a bid for the company. Headlines came from Reuters this afternoon.

It was disclosed earlier in July that Amgen (Nasdaq: AMGN) put in a bid at $120 per share for Onyx. Onyx's Board said that Amgen's bid significantly undervalued Onyx and its prospects, and was not in the best interest of Onyx or its shareholders... at the time.

Shareholders agreed, pushing Onyx higher over the last few weeks on ">expectations of a stronger offer from a larger pharma, maybe one just like Pfizer. Other bidders might include Bayer, which is partners with Onyx on two drugs already, Novartis (NYSE: NVS), and Bristol-Myers Squibb (NYSE: BMY).

One of Onyx's key cancer drugs is Kyprolis. The drug is expected to have sales of $257 million this year, rising to $2.3 billion by 2019.

Pfizer may have decided that the recent gain in share price put valuation at little to rich at this point. Amgen is now the front-runner for a takeover of Onyx, and shares of Onyx are down 2.2 percent on the session after peaking into positive territory this afternoon.