Encysive ( ENCY) was hammered Monday after the Food and Drug Administration failed to come through with approval for Thelin, its treatment for pulmonary arterial hypertension.

The agency requested more clinical work on the drug after raising "concerns and observations that must be satisfied prior to achieving approval," Encysive said.

In recent trading, Encysive shares lost $4.18, or 46%, to $4.90.

"We will work in close collaboration with the FDA to clarify the path forward," Encysive said. "We are hopeful that this can be accomplished without the need for additional clinical work, but that will require discussion with the agency before we can be sure."

"We remain confident in Thelin's value, and look forward to the opportunity to provide PAH patients with a new treatment option," the company said.