Gabapentin enacarbil is a prodrug of the anticonvulsant gabapentin. The efficacy and safety of gabapentin enacarbil for the treatment of moderate to severe primary restless legs syndrome (RLS) has been evaluated in several clinical trials in the United States and Japan. Although most clinical trials assessed gabapentin enacarbil at doses greater than 600 mg/day and demonstrated the overall safety and efficacy (defined as improvements in the coprimary endpoints of the international RLS rating scale [IRLS] total score and Clinical Global Impression-Improvement response), the US Food and Drug Administration approved the 600 mg once-daily dosage because doses higher than 600 mg/day were considered to provide no additional benefits and were associated with higher rates of adverse events, such as somnolence and dizziness. Nonetheless, the results of clinical trials and post hoc meta-analyses have indicated that the 1,200 mg once-daily dosage was the most validated gabapentin enacarbil treatment for not only subjective RLS symptoms but also severe sleep disturbance associated with RLS. A Japanese dose-finding study showed that 900 mg/day, the intermediate dose between 600 and 1,200 mg, failed to show a significant improvement in IRLS total score, probably because many of the patients who discontinued treatment did so early, suggesting that a half-landing dose may cause more adverse effects than favorable ones in some RLS patients early in the treatment. Gabapentin enacarbil may have two distinct therapeutic doses for the treatment of RLS: 600 mg/day or lower doses for the treatment of subjective RLS symptoms and 1,200 mg/day or higher doses for the treatment of both subjective RLS symptoms and associated problems such as severe sleep disturbances.
Keywords: dose-finding; gabapentin enacarbil; meta-analysis; restless legs syndrome.