Dose-Response and Pharmacokinetics of Gabapentin Enacarbil (GEn [XP13512 / GSK1838262]) in Restless Legs Syndrome

A Randomized, Double-Blind, Placebo-Controlled, Dose-Response Study to Assess the Efficacy, Safety, and Pharmacokinetics of XP13512 (GSK1838262) in Patients With Restless Legs Syndrome

The objective of the study was to generate the data necessary to determine the gabapentin exposure produced by 4 dose levels of GEn (600 mg, 1200 mg, 1800 mg, and 2400 mg) or placebo, and the corresponding relief of symptoms in subjects with Restless Legs Syndrome (RLS).

This was a multicenter, randomized, double blind, placebo controlled, parallel group study, comparing 4 doses of GEn (XP13512) with placebo given once daily to subjects with RLS. Eligible subjects were randomized in equal numbers into 1 of 5 treatment groups (GEn 600 mg, 1200 mg, 1800 mg, or 2400 mg or placebo) for 12 weeks of treatment. Data from this study will be utilized as part of a larger pharmacokinetic (PK) pharmacodynamic (PD) analysis of data from several studies (XP084/RXP111495) that are part of the GEn RLS clinical development program. Safety and tolerability were also assessed.

Inclusion Criteria:

• Men or women at least 18 years of age
• RLS, based on the IRLSSG Diagnostic Criteria
• History of RLS symptoms occurring at least 15 nights in the month prior to Screening or, if on RLS treatment, this frequency of symptoms must have been applicable prior to start of treatment
• Documented RLS symptoms for at least 4 of the 7 consecutive evenings/nights
• Total RLS severity score of 15 or greater on the IRLS Rating Scale
• If taking dopamine agonists, gabapentin, or other treatments for RLS (e.g., opioids, benzodiazepines) medications must have been discontinued at least 2 weeks prior to Screening;
• If taking any prescription medication, therapy must have been stabilized for at least 3 months prior to Screening with no anticipated changes for the duration of the study;
• Body Mass Index (BMI) of 34 or below
• estimated creatinine clearance of at least 60 mL/min

Exclusion Criteria:

• a sleep disorder (e.g., sleep apnea) that may significantly affect the assessment of RLS
• history of RLS symptom augmentation or end of dose rebound with previous dopamine agonist treatment
• neurologic disease or movement disorder (e.g., diabetic neuropathy, Parkinson's disease, multiple sclerosis, dyskinesias, and dystonias);
• other clinically significant or unstable medical condition or conditions which could affect RLS treatment efficacy assessments
• serum ferritin level below 20 ng/mL
• currently suffering from moderate or severe depression using the Diagnostic and Statistical Manual of Mental Disorders and Treatment IV (DSM IV TR)