The purpose of this study is to examine the safety and effectiveness of USL255 as adjunctive therapy in patients with refractory partial onset-seizures.

Inclusion Criteria:

• Subject has a confirmed diagnosis of partial-onset seizures with or without secondary generalization for at least 12 months prior to Visit 1.
• Currently on a stable dosing regimen of 1 to 3 AEDs for at least 4-weeks prior to Visit 1 (12 weeks for phenobarbital and primidone).
• Have a minimum of 8 partial-onset seizures and no more than 21 consecutive seizure free days, during the 8-week baseline.

Exclusion Criteria:

• Have a history of seizure episodes lasting less than 30 minutes in which several seizures occur with such frequency that the initiation and completion of each individual seizure cannot be distinguished, within 3 months prior to Visit 1.
• Have a history of pseudoseizures, or status epilepticus, within 3 months prior to Visit 1.
• Have a history of metabolic acidosis, nephrolithiasis, ureterolithiasis, or narrow angle glaucoma.
• Have a history of suicidal attempts, suicidal ideation, or uncontrolled psychiatric illness within 2 years of Visit 1.
• Currently taking, or have taken felbamate within the past 18 months, or have taken vigabatrin in the past.
• Have taken topiramate within the past 6 months.