A Long-term Safety and Tolerability Study of USL261 in Patients With Seizure Clusters

Inclusion Criteria: 1. Has a competent, adult caregiver(s) who is able to recognize and observe the subject's seizure cluster episodes 2. Has an established diagnosis of partial or generalized epilepsy that includes all the following: * A documented history of seizure clusters lasting a minimum of 10 minutes, seizure cluster pattern is observable, stereotyped, and recognizably different from the subject's other non-cluster seizure activity (if any) * A second seizure in the seizure cluster typically occurs within 6 hours from the time of recognition * A seizure cluster pattern composed of multiple (≥ 2) partial or generalized seizures * A seizure cluster pattern established \>3 months before Visit 1 * A frequency of ≥ 3 stereotyped seizure clusters during the year before Visit 1 * At least 1 stereotyped seizure cluster occuring ≤ 4 months before Visit 1 3. Currently on a stable regimen of AED(s) that includes a benzodiazepine 4. Weight is 40 kg to 125 kg, inclusive Exclusion Criteria: 1. Has a neurological disorder that is likely to progress in the next year 2. Has a severe chronic cardio-respiratory disease 3. Has a psychogenic, non-epileptic seizure(s) within the 5 years before Visit 1 4. Has a history of their stereotypical seizure cluster progressing to status epilepticus within 2 years before Visit 1 5. Has a history of acute narrow-angle glaucoma 6. Has had active suicidal plan/intent or active suicidal thoughts in the 6 months before Visit 1 or a suicide attempt in the past 5 years