Initial dose of the study treatment will be 10 mg/kg/day. After 2-weeks on 10 mg/kg/day, the dose will be up-titrated to 20 mg/kg/day (maximum 1200 mg/day). After 2 weeks on 20 mg/kg/day, dose will be up-titrated to 30 mg/kg/day (maximum 1200 mg/day) and patients will receive this dose for 8 weeks. If intolerable adverse events (AEs) occur, the patient can be down-titrated to the previous dose (only 1 down-titration step will be allowed) or discontinued. After the 8-week maintenance period, the study treatment will be tapered off in 10 mg/kg/day 2 week steps. However, if a patient experiences an increase in seizure frequency (e.g. more than 100% increase vs. baseline) during tapering-off, the patient can proceed directly to the open-label part of the study (Part II).