To evaluate the safety, efficacy and pharmacokinetics of gabapentin enacarbil in moderate to severe primary RLS patients with moderate renal impairment and to confirm dosage and administration in such population.

This study is a multicenter, non-blinded, non-comparative study. Patients who meet the eligible criteria will undergo one week observation period followed by four week treatment period. In treatment period, patients will receive gabapentin enacarbil orally one daily after dinner. After the four-week treatment period with gabapentin enacarbil, a one-week post-observation period will be undertaken.

Inclusion Criteria:

• Patients who have been diagnosed with restless legs syndrome (RLS) according to the international RLS Study Group Diagnostic Criteria
• Patients whose score is 15 or greater on the International RLS (IRLS) Rating Scale on the first day of the pre-observation period.
• Patients whose creatinine clearance level at the first day of pre-observation period is ≥30 mL/min to <60mL/min estimated by Cockcroft-Gaul t equation
• Patients who provide written consent

Exclusion Criteria:

• Patients with sleep disorder such as sleep apnea syndrome, which affects the RLS evaluation
• Patients with history of augmentation and treatment rebound of RLS symptoms when using dopamine agonis
• Patients with neuropathy and movement disorder (diabetic neuropathy, Parkinson's disease, multiple sclerosis, dyskinesia, rheumatoid arthritis etc)
• Patients with uncontrolled diabetes [HbA1c>7.5% (NGSP value), in the past six months], iron deficiency anemia, or drug administration of hypnotic sedatives.
• Patients who attempted suicide within six months prior to the consent obtained, or who have been diagnosed as a suicide risk by investigator or sub-investigator
• Patients with moderate or severe depression
• Patients with alcohol dependence or drug intoxication or who have a history of abuse or drug dependence in the past one year
• Patients whose job is shift work, professional driver, or engaging in hazardous activities such as operating machinery
• Patients with history of hypersensitivity to gabapentin
• Patients who have experience to participate in other post-marketing clinical studies or clinical trials within 12 weeks prior to the start of pre-observation
• Others: patients who are concluded to be ineligible by an investigator or sub-investigator as ineligible

Access to anonymized individual participant level data will not be provided for this trial. Further details on Astellas' data sharing policy can be found at https://www.clinicaltrials.astellas.com/transparency/.