EMERYVILLE, Calif. ( TheStreet) -- Onyx Pharmaceuticals ( ONXX) is acquiring privately held cancer drugmaker Proteolix in an all-cash deal that adds a mid-stage blood cancer drug to the company's portfolio. Under terms of the agreement, Onyx will pay $276 million upfront for Proteolix. Another $575 million in payments from Onyx to Proteolix's shareholders are contingent on the future development and success of Proteolix's lead multiple myeloma drug, carfilzomib, the company said Monday. Wall Street has been waiting with mixed feelings for Onyx to acquire another cancer drugmaker ever since the company raised about $300 million in August. Onyx's reliance on its partnership with the German drug giant Bayer and the cancer drug Nexavar for its sole source of profit is widely seen by investors as a liability. But investors also have worried about Onyx's ability to pull off a smart acquisition that would deepen its product portfolio without overpaying. Onyx shares are down 21% year to date. By buying Proteolix, Onyx CEO Tony Coles may have successfully threaded the needle of investor expectations. Proteolix is a private company, but its main drug carfilzomib is fairly well-known to biotech investors and analysts. Data from phase II studies presented at medical meetings last December and again in June showed the drug to be a potentially strong competitor to two well-established multiple myeloma drugs - Takeda's Velcade and Celgene's ( CELG) Revlimid. The structure of the transaction is back-end loaded, so Onyx doesn't have to pay a significant portion of the $851 million total purchase price if carfilzomib's development falters.