The duration of study participation in Part A will be approximately 26 weeks, which includes a 1- to 3-week Screening Period, 4-week Baseline Observation Period, 14-week Dose Optimization Treatment Period, 10-day Taper Period, and a Safety Follow-up Period (4 weeks after end of taper visit). Participants will be randomized centrally in a 1:1 ratio to receive either GWP42003-P or matching placebo. Randomization will be stratified by clobazam use (on/off) and age of seizure onset (3 years of age and younger or older than 3 years of age). Upon completion of the double-blind phase (Part A), participants will have an option to continue in a 54-week open-label extension (Part B).
The primary aim of Part A of the study to assess the efficacy and tolerability of GWP42003-P compared to placebo as an adjunctive treatment for children with Epilepsy with myoclonic-atonic seizures (EMAS) -associated seizures.~Part B of this study will be conducted to evaluate the long-term safety and tolerability of GWP42003-P in participants with EMAS.