Open-label Extension Study in Paediatric Patients Who Have Completed the MEX-NM-301 Study.

"Patients will be enrolled sequentially into 2 cohorts.~Cohort 1 - (patients aged 12 to \< 18 years):~approximately 8 weeks - 4 weeks of dose titration period + 4 weeks of maintenance period.~Cohort 2- (patients aged 6 to \< 12 years,):~approximately 8 weeks - 4 weeks of dose titration period + 4 weeks of maintenance period. Enrolment for Cohort 2 will begin after initial pharmacokinetics (PK), safety and efficacy are confirmed in this population, of patients in Cohort 1"