The purpose of this Phase III study is to assess the long-term safety, tolerability and efficacy of flexibly dosed retigabine Immediate Release (IR) as adjunctive therapy in adult subjects with partial-onset seizures. In addition, those subjects who successfully completed 20 weeks of adjunctive treatment with retigabine IR in the parent study, RGB113905, and who were thought to have benefitted from treatment will be provided continued access to retigabine IR.