GlaxoSmithKline and partner XenoPort have received approval from the US Food and Drug Administration for their restless legs syndrome drug Horizant.

The agency has backed Horizant (gabapentin enacarbil) extended-release tablets for the treatment of moderate-to-severe primary RLS in adults. Efficacy was demonstrated in two 12-week clinical trials in adults and it is the first medication in its class to be approved for the syndrome.

The road to approval has not been a particularly smooth one. In February last year, the FDA issued a complete response letter saying a preclinical signal of pancreatic acinar cell tumours in rats was “determined to be of sufficient concern to preclude approval of the Horizant New Drug Application” in its current form. Further preclinical data was submitted and the agency has now given the thumbs-up.

The drug, which is a modified version of Pfizer’s epilepsy treatment Neurontin, is also in development for diabetic neuropathy but recently failed to show any improvement over placebo as a potential treatment for migraine in a Phase IIb study.

The RLS approval is particularly good news for XenoPort , whose shares leapt 57% in after-hours trading. Analysts believe Horizant will bring in revenues of around $100 million.