Open-label Extension to Protocol 1042-0500

Inclusion Criteria: * Have completed all scheduled clinical study visits in the previous Protocol 1042 0500 and have been deemed eligible (no SAEs thought to be drug related and had a response to treatment) by the Investigator. * Be diagnosed with IS regardless of etiology. Diagnostic Criteria: Seizures consisting of single or repetitive short muscular contractions leading to flexion or extension. Spasms may be characterized as tonic or myoclonic contractions, may occur singly or in clusters, and typically occur bilaterally and symmetrically. The EEG pattern must be consistent with the diagnosis of IS (hypsarrhythmia, modified hypsarrhythmia, multifocal spike wave discharges, etc). * Have a 24 hour vEEG recording confirming the diagnosis of IS. * Have had a magnetic resonance imaging (MRI) performed to determine any possible causes of IS. * Have been previously treated with 3 AEDs or fewer. * Have a parent/guardian who is properly informed of the nature and potential risks and benefits of the clinical study, is willing and capable of complying with all clinical study procedures, and has given informed consent in writing prior to entering the clinical study. Exclusion Criteria: * Current treatment with more than 2 concomitant AEDs. * Have an active CNS infection, demyelinating disease, degenerative neurological disease, or CNS disease deemed progressive (with the exception of tuberous sclerosis) as evaluated by brain MRI. * Have any disease or condition (medical or surgical…