Merck has returned global rights for the investigational clotbuster betrixaban to California-based Portola Pharmaceuticals, according to PharmaTimes.

Betrixaban is an oral Factor Xa inhibitor anticoagulant being evaluated for the prevention of stroke in patients with atrial fibrillation. In July 2009, Merck and Portola announced a collaboration for the drug that could have been worth $470 million plus royalties, and Merck made an upfront payment of $50 million.

Betrixaban completed the phase II EXPLORE-Xa trial in which it showed dose-dependent clinical activity with similar or lower rates of bleeding compared to warfarin. Luciano Rossetti, head of global scientific strategy at Merck, said, "working closely with our Portola collaborators we have advanced betrixaban to this phase III-ready stage."

However, the rights have been returned "as part of an ongoing prioritization of our late-stage pipeline," though he stressed that Merck "remains committed to delivering medicines for cardiovascular disease."

Portola CEO William Lis said regaining the rights "represents a transformational opportunity" for the firm. "We will work with our academic partners on options for an independent development plan to bring betrixaban to the market and intend to discuss these options with the FDA in the near future."