SOUTH SAN FRANCISCO ( TheStreet) -- Onyx Pharmaceuticals ( ONXX) has rejected Amgen's ( AMGN) takeout offer of $120 per share in cash and said it will seek additional bidders. I believe there are five key potential suitors for Onyx, each with a unique motivation to gain control of the company's most important asset -- the blood cancer drug Kyprolis. Here, then, is a rundown of the five companies I believe will seek to acquire Onyx in the coming weeks: 1. Amgen While Amgen has oncology products, a bid for Onyx is less about synergies and more about seeking growth. Right now, Amgen's only late-stage blood cancer product is blinatumamab, which is in phase II studies for acute lymphoblastic leukemia. Acquiring Onyx would make Amgen an instant and significant player in the blood cancer treatment market, as well as further expand its existing footprint in the solid tumor market. Amgen is also the slowest-growing large-cap biotech company, which is especially problematic as peer companies Biogen Idec ( BIIB), Celgene ( CELG), and Gilead Sciences ( GILD) are all growing much faster or about to enter high-growth periods. In the eyes of many investors, Amgen isn't even considered a biotech company any longer, meaning it isn't rewarded with the growth multiple of its competitors. An acquisition of Onyx might change that perception for the better. 2. Bayer Bayer is already partnered with Onyx. The two companies co-market the liver and kidney cancer drug Nexavar and the colon cancer drug Stivarga. This gives Bayer a significant advantage over other potential suitors because it stands to gain the most from potential cost-saving synergies. It also means Bayer might be able to offer a higher price for Onyx and keep the dilutive effect on earnings to a minimum. Of course, the "Bayer is going to buy Onyx" speculation has been chatted up by Wall Street investors for years without a deal ever being coming to fruition. One has to wonder of Bayer is really that motivated to participate in the auction process for Onyx. 3. Celgene Celgene has the most complementary group of cancer-drug assets of all the potential Onyx suitors. Celgene already has a significant presence in blood cancer with the currently approved drugs Thalomid, Revlimid and Pomalyst. In addition, Celgene has shown a recent interest in acquiring new blood-cancer drugs with differing mechanisms of action like the purchase of Avila Therapeutics for its BTK inhibitor and the recent collaboration with Morphosys to gain access to a monoclonal antibody that targets CD38.