GlaxoSmithKline and XenoPort Inc. said Thursday the Food and Drug Administration asked the company additional questions on their potential restless leg syndrome treatment Horizant.
The agency sent the companies a "Complete Response" letter and is concerned about the laboratory findings that showed pancreatic cell tumors in rats.
In November, the agency extended the review date to Feb. 9. in order to assess a risk evaluation and mitigation plan on the drug candidate.
The companies said they are assessing their next step in the regulatory process.
XenoPort is based in Santa Clara, Calif., and GlaxoSmithKline is headquartered in London.