• Inclusion criterion was revised to indicate that hormonal contraception is considered an acceptable method of contraception when used with effective nonhormonal contraception and to clarify that male participants with bilateral orchiectomy are required to use a condom with spermicide. • Specified that known potent inhibitors or inducers of cytochrome P3A4 (CYP3A4) enzymes (other than antiseizure medications [ASMs]) are not permitted during the study. • A statement was added that participants should avoid exposure to excessive sunlight or artificial ultraviolet light during the study and be advised to use sunscreen/sunblock and/or wear protective clothing when exposure could not be avoided. • Added that study treatment discontinuation was required for corrected QT interval using Fridericia’s formula (QTcF) change from baseline >60 millisecond (msec) (cardiologist verified) and confirmed with repeated electrocardiogram (ECG) measure. • Added that study treatment discontinuation was required for QTcF of >500 msec (cardiologist verified) on any ECG tracing. • Added a statement that the electronic data capture (EDC) and study-specific electronic case report forms (eCRFs) will comply with European Union General Data Protection Regulation.

The protocol was revised to allow enrollment of participants who did not enroll directly from FOS2021. A Screening Period of up to 8 weeks and a 1-week open label (OL) Dose Titration Period was added. • Removed statement that participants should avoid excessive sunlight and ultraviolet light exposure during the study. • Added a statement that the Sponsor will store biosamples in a manner compliant with applicable national and local regulations and the requirements of the European Union General Data Protection Regulation. • Added that entries into the daily seizure diary will be reviewed with the participant by qualified study staff personnel who have completed the study-specific training. • Revised methods for measuring body temperature. • Added a statement that study visits may be conducted remotely if in-person visits were not possible for COVID-19-related reasons. • Updated to state that SAEs must be reported immediately and no later than 24 hours of knowledge of the event under any circumstances. • Added that pregnancies in partners of male participants must be reported to Neurocrine Biosciences, Inc (NBI) and that NBI will ask to follow the partner’s pregnancy. • Updated Sponsor contact information for reporting SAEs and pregnancies. • Removed the requirement for postbaseline safety data from the definition of the safety analysis set.

• The secondary objective was recategorized as an ‘other’ objective. • Changes were made to the study endpoints, and the safety and efficacy analyses were updated to reflect the changes to the endpoints. • Clarified that a Taper Visit is not applicable for participants who withdraw from the study if they have dose de-escalation before the Early Termination Visit. • Increased the frequency of pregnancy testing for female participants of childbearing potential during the OL treatment period and removed the requirement for a confirmatory serum pregnancy test to be conducted at the study center. • Added that the local regulatory authority will be notified if urgent safety measures are required. • Updated statements about the security of information technology systems and Sponsor compliance with applicable laws and regulations for disclosure of study results. • Updated the Schedule of Assessments to clarify timing of seizure diary collection and dispensation.