• Have a diagnosis of epilepsy with partial-onset (focal) seizures (POS) with or without secondarily generalized seizures according to the International League Against Epilepsy's (ILAE) Classification of Epileptic Seizures. A diagnosis should have been established at least 12 months prior to Visit 1 (Screening) by clinical history and an electroencephalogram (EEG) that is consistent with the diagnosis; normal interictal EEGs will be allowed provided that the participant meets the other diagnosis criterion (i.e., clinical history)

• Male or female participant, from age 2 to less than 18 years at the time of informed consent/assent (dates including informed consent in YKP3089C039)

• Have a minimum weight of 10.0 kilograms (kg) (22.0 pounds \[lb\])

• Have had a brain imaging (e.g., magnetic resonance imaging \[MRI\] scan or computed tomography (CT) within 10 years before Visit 1 (Screening) that ruled out a progressive cause of epilepsy.

• For subjects new to Study YKP3089C040, participants must have had at least 1 POS seizure during the 28-day Baseline Period. Only simple POS with motor signs, complex POS, and complex POS with secondary generalization are counted toward this inclusion for POS

• Are currently being treated with stable doses of 1 to a maximum of 3 approved antiepileptic drugs (AEDs). Doses must be stable for at least 4 weeks before to Visit 1 (Screening). A vagal nerve stimulator \[VNS\] will not be counted as one of the 3 allowed AEDs but the settings should be stable for at least 4 weeks prior to Visit 1 (Screening).

• Investigator believes subject could benefit from new or continued exposure to study drug

• Subjects entering from study YKP3089C039 must continue to meet all of the inclusion criteria from the YKP3089C039 study

• Subjects receiving felbamate as a concomitant AED must meet the following criteria:

∙ Have a 12-month history of felbamate use and a history of a fixed dosing regimen for a minimum of 60 days prior to Visit 1 (Screening).

‣ No prior or known history of hepatotoxicity or hematologic disorder due to felbamate.

⁃ Subjects following a ketogenic diet will be allowed as long as the diet has been stable for at least 30 days prior to Visit 1 (Screening) and will remain stable for the duration of the study