The objectives of the trial are to evaluate the long-term efficacy and safety of HORIZANT (Gabapentin Enacarbil) 600 mg daily, for the treatment of RLS in adolescents (13 to 17 years of age) diagnosed with moderate-to-severe primary RLS.

Patients who complete the 12-week daily dosing efficacy and safety study (XP109) will be allowed to enroll in the open-label extension (OLE) study (XP110), thereby providing exposure to HORIZANT for up to 48 weeks. Enrolled patients will take open-label HORIZANT 600 mg tablets once a day at approximately 5 PM with food, beginning at Week 0 of the OLE study up to and including Week 36. After the end of the treatment period, a follow-up visit will be included 14 days (± 3 days) after the last dose of HORIZANT. Patients will visit the clinical site on 6 different occasions, including a follow-up visit 14 days (± 3 days) after the last dose of study drug.

Inclusion Criteria:

Patients who completed participation in HORIZANT Study XP109.

• Negative pregnancy test for females of childbearing potential. Male patients able to father a child must agree to use a barrier method (male condom, female condom, diaphragm, or cervical cap) with spermicide for at least 30 days prior to dosing and throughout the study. Fertile, sexually active patients must agree to use 2 medically accepted methods of contraception
• Patients must be willing to refrain from using any drugs that are likely to affect RLS or sleep assessments for the duration of the study.
• Signed patient and parent (or legal guardian) Institutional Review Board (IRB)-approved assent and consent forms before any study procedures are carried out

Exclusion Criteria:

• Patients who, in the opinion of the investigator, would be noncompliant with the study visit schedule, procedures, or medication administration
• Patients who have developed clinically significant or unstable medical conditions, or who would otherwise be unsuitable for participation in a continuation study with gabapentin enacarbil