AMT-260 Gene Therapy Study in Adults With Unilateral Refractory Mesial Temporal Lobe Epilepsy

Inclusion Criteria: * Diagnosis of unilateral refractory MTLE * History of seizures with an average of ≥ 2 documented focal impaired awareness seizures or focal to bilateral tonic-clonic seizures per 30-day period over the 3 month Retrospective Period. * On a stable type and dose regimen of up to a maximum of 4 approved Anti Seizure Drugs for at least 1 month prior to the Retrospective Period. * Confirmed unilateral hippocampal pathology and concordant unilateral seizure focus * No evidence of focal neurocognitive dysfunction, inconsistent with disease pathology- related MRI and/or (18F)FDG-PET findings. * Women of childbearing potential (WOCBP) and fertile male subjects must be willing and able to use highly effective methods of birth control consistently and correctly throughout the study. * For WOCBP only: Negative pregnancy test. Exclusion Criteria: * Implanted devices that would contraindicate MRI; MRI-compatible devices must be implanted ≥3 months prior to Screening (vagus nerve stimulation devices will be up to discretion of the Investigator). * Any other contraindications to general anesthesia, surgery, or intra-operative magnetic resonance imaging (MRI). * Medications that could confound clinical (e.g., antipsychotic medication and anti-viral therapy) and laboratory evaluations or could affect a participant's safety or their ability to undergo the neurosurgical procedure or comply with the procedures and study visit schedule. * Bilateral or multifocal epilepsy; or inability to confidently lateralize seizure onset. * Dementia or other progressive neurological disorders and progressive brain lesions. * Previous major disease-unrelated neurosurgical intervention due to intracranial tumor, trauma, or bleeding and/or history of previous intracranial surgery for treatment of epileptic seizures, not including diagnostic stereo-EEG. * MRI evidence of epileptogenic, extra-temporal lesions, or dual temporal lobe pathology. * Status epilepticus within the year prior to Screening.