onyx Pharmaceuticals has signed a definitive agreement to acquire Proteolix, a biopharmaceutical company focused on discovering and developing novel therapies for haematological malignancies and solid tumours.

The move will provide Onyx with access to Proteolix’s lead compound, carfilzomib, a proteasome inhibitor in multiple clinical trials, for patients with relapsed and refractory multiple myeloma.

Onyx will make a $276m cash payment on closing of the transaction. Additional payments include $40m payable in 2010 based on development milestones and up to $535m depending on whether carfilzomib is approved in the US and Europe.

Onyx president and chief executive officer N Anthony Coles said that this acquisition provides strategic expansion into the $16bn haematological malignancies market.

“The transaction structure reflects our approach to growing our business in a disciplined fashion, including earnout payments contingent on specific approval-based events,” Coles said.

Proteolix is developing several therapeutics that inhibit the cellular proteasome, a validated and well-characterised approach to treating certain haematological malignancies.

Of these treatments carfilzomib, a next-generation proteasome inhibitor, has already demonstrated strong response rates in multiple studies with an encouraging safety profile.