XenoPort and GlaxoSmithKline reported positive top-line results from a phase III trial of XP13512, a gabapentin pro-drug with enhanced absorption, for the treatment of moderate to severe restless legs syndrome. This 12-week, double-blind, placebo-controlled study, dubbed XP053, enrolled 325 subjects. The endpoints were met with statistical significance in both dose groups when compared to placebo.

The subjects received placebo or 600 mg or1, 200 mg of XP13512, given once per day. The primary endpoints were the change from baseline for the International Restless Legs Syndrome (IRLS) rating scale score at end of treatment and the percentage of subjects showing significant improvement on the Investigator Clinical Global Impression of Improvement (CGI-I) scale at end of treatment.

In the 600 mg dose group, the unadjusted mean reduction in the IRLS scale score was -13.8 (p less than 0.0001). 

At the end of treatment, 73% of subjects treated with 600 mg of XP13512 were reported as “much improved” or “very much improved” on the CGI-I scale (p less than 0.0001 compared with placebo). In the 1,200 mg dose group, the unadjusted mean reduction in the IRLS scale score was -13.0 versus -9.8 for placebo (p=0.0015).

At the end of treatment, 78% of subjects treated with 1,200 mg XP13512 were reported as “much improved” or “very much improved” on the CGI-I scale compared with 45% treated with placebo (p less than 0.0001). Treatment was generally well tolerated, with adverse events mild to moderate.

An New Drug Application (NDA) filing is planned for the third quarter of this year.

In February 2007, GlaxoSmithKline and XenoPort entered into an exclusive development and commercialization collaboration for the drug.