Merck (MRK) agreed on Thursday to return all rights for its investigational antistroke medication betrixaban to partner company Portola Pharmaceuticals following a review of its portfolio.

Luciano Rossetti, senior vice president of global scientific strategy at Merck said the revelation was part of an ongoing prioritization of its late-stage pipeline.

“Regaining full rights to betrixaban represents a transformational opportunity for Portola,” said William Lis, chief executive of Portola. “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.”

The two teamed in July 2009 to develop and commercialize the drug, which is an oral inhibitor anticoagulant being evaluated for the prevention of stroke in patients with atrial fibrillation.

The medication most recently completed Phase II testing showing dose dependent clinical activity with similar or lower rates of bleeding compared to warfarin, another antistroke drug

Follow Jennifer Booton on Twitter at @Jbooton