This clinical trial is designed to test whether a single image-guided intracerebral administration of inhibitory nerve cells, called interneurons (NRTX-1001), into subjects with drug-resistant unilateral mesial temporal lobe epilepsy (MTLE), with or without mesial temporal sclerosis (MTS), is safe (frequency of adverse events) and effective (seizure frequency). NRTX-1001 comprises human interneurons that secrete a neurotransmitter, gamma-aminobutyric acid (GABA).

Subjects will undergo a single CT or MRI-guided intracerebral administration of human interneurons that secrete the inhibitory neurotransmitter, gamma-aminobutyric acid (GABA), into the temporal lobe region of the brain where the seizures are thought to arise. NRTX-1001 is intended to suppress the onset and spread of seizures. Safety, efficacy, tolerability, and effects on reducing seizure frequency and epilepsy disease symptoms will be assessed at regular intervals for 2 years after the administration of NRTX-1001. After the two-year period, subjects will be followed with quarterly phone calls and annual visits in years 3 through 5, and then annual visits in years 6 through 15. Subjects will be placed on an immunosuppressant medication regimen for a duration of one year to partially suppress the subjects' immune system to promote the intended long-term persistence of NRTX-1001. This immunosuppressant medication is intended to be discontinued after the first year; however, the NRTX-1001 cells are intended to persist long-term.

Key Inclusion Criteria:

1. Male or Female, age ≥18 to ≤75
2. Focal seizures, clinically defined as unilateral MTLE
3. Has failed to achieve seizure control despite adequate trials of at least 2 ASDs at appropriate doses
4. Currently on stable doses (at least 1 month prior to Screening) of approved ASDs
5. Single seizure focus confirmed within one hippocampus
6. Seizure frequency averages ≥4 per 28-day period, including at least 2 clinical focal seizures per 28-day period with objective manifestations or more severe types, over the 6 months prior to the Screening Visit. (Phase 1/2 only)
7. Disabling seizure frequency of ≥2 per 28-day period averaged over 3 months prior to the Screening visit and over a prospective 10-week baseline period prior to Randomization (Phase 3 Only)

Key Exclusion Criteria:

1. Epilepsy due to other medical conditions and/or progressive neurologic disease
2. Evidence of seizure focus outside of the hippocampus or evidence of seizures of non- focal origin.
3. Significant other medical conditions which would impair safe participation
4. History of status epilepticus in the 3 years prior to screening.
5. Primary or secondary immunodeficiency
6. Suicide attempts in the past year 3 years
7. Severe psychiatric disorders
8. Prior lobectomy or LITT procedure
9. MRI indicating potential malignant lesion
10. Pregnancy, or currently breastfeeding.

This is a two-phase study. Phase 1/2 is open-label and unmasked. Phase 3 is blinded with participant, part of investigator team, and outcomes assessor masked to treatment assignment.