GlaxoSmithKline and partner XenoPort have suffered a setback after regulators in the USA rejected Horizant, a potential treatment for restless legs syndrome.

The companies have received a complete response letter from the US Food and Drug Administration about Horizant (gabapentin enacarbil) extended-release tablets for moderate-to-severe primary RLS. In the letter, the FDA says that a preclinical finding of pancreatic acinar cell tumours in rats was “of sufficient concern to preclude approval” of Horizant, a modified version of Pfizer’s epilepsy blockbuster Neurontin, for RLS at this time.

GSK says the agency has acknowledged that similar findings were known for gabapentin at the time of its approval for epilepsy (it was approved at the end of 1993), but concluded that the seriousness and severity of that disease justified the potential risks.

GSK and XenoPort, which submitted the New Drug Application over a year ago, said they are “assessing the appropriate next steps and will be communicating with the FDA”. Analysts have previously forecast that Horizant, formerly referred to as Solzira, could be a $500 million-a-year treatment for RLS which is characterised by unpleasant and sometimes painful sensations in the legs that result in a compelling urge to move.

The drug is also in mid-stage trials for painful diabetic neuropathy and migraine and is partnered with Astellas Pharma in Japan. The FDA’s decision is likely to hurt XenoPort’s shares severely when the US markets open later today.