The primary objective of this trial is to assess the efficacy of XP13512 taken once daily compared to placebo for the treatment of patients suffering from Restless Legs Syndrome (RLS).

Inclusion Criteria:

• Patients with primary RLS, based on the International RLS Study Group Diagnostic Criteria.
• History of RLS symptoms at least 15 nights in the prior month or, if on treatment, this frequency of symptoms before treatment was started;
• Documented RLS symptoms for at least 4 of the 7 consecutive evenings/nights during the Baseline study period;
• Total RLS severity score of 15 or greater on the International Restless Legs Syndrome Study Group Rating Scale (IRLS rating scale) at Visit 1 and at Visit 2;
• Discontinuation of dopamine agonists and/or gabapentin at least 2 weeks prior to Baseline;
• Discontinuation of other treatments for RLS (e.g., opioids, benzodiazepines) at least 2 weeks prior to Baseline;
• If female of child-bearing potential, the subject must agree to use clinically accepted birth control through completion of the study;
• Body Mass Index 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;
• Neurologic disease or movement disorder (e.g., diabetic neuropathy, Parkinson's Disease, Multiple Sclerosis, dyskinesias, and dystonias);
• Abnormal laboratory results, electrocardiogram (ECG) or physical findings;
• Pregnant or lactating women;
• Women of childbearing potential who are not practicing an acceptable method of birth control.