Abstract
Rationale
Epilepsy is one of the most common chronic neurological disorders worldwide. About a third of patients suffering from seizures do not become seizure-free with current treatment options. One approach to restore the balance between excitation and inhibition in the epileptic brain is to increase GABAergic inhibition by replacing degenerated endogenous inhibitory neurons by transplantation. Transplantation of medial ganglionic eminence (MGE) forebrain-type GABAergic interneurons into the brain and spinal cord has been shown to restore balance to hyper-excitable neural networks in preclinical rodent models. I was part of a company, which is developing a human MGE-type interneuron cell therapy derived from a human pluripotent stem cell (hPSC) line, for clinical investigation in patients with drug-resistant temporal lobe epilepsy (TLE). The human cell therapy can be reproducibly manufactured to derive post-mitotic interneurons from hPSCs with >85% efficiency. The cells express markers of an MGE-type cortical/hippocampal interneuron lineage.

Methods
The efficacy of human interneurons was evaluated after transplantation into the intra-hippocampal kainate mouse model of mesial TLE, which is a model for pharmacoresistant seizures and resembles many aspects of human TLE pathology. Following kainate-induced status epilepticus, mice developed spontaneous recurrent electrographic seizures. In this chronic epileptic stage, animals received hippocampal transplants of human interneurons, or control injections of vehicle. Mice were monitored for seizures with hippocampal electrodes at several time points for up to 10 months post-transplant (PT) and cell persistence, fate and distribution were evaluated at the end of the study. We compared different production batches as well as different doses to identify a minimally efficacious dose and maximum feasible dose, which included multiple behavioral assays to detect potential abnormalities.

Results
The transplanted cells matured into interneuron subtypes, which were distributed throughout the epileptic mouse hippocampus, and the human interneurons persisted for at least 1 year PT, the latest time point studied. Transplantation reproducibly led to long-term seizure suppression with 70-80% fewer electrographic seizures in the mice that received cell transplants than in age-matched vehicle-injected mice across multiple studies and cell batches. The cumulative duration of seizures was also reduced significantly. Overall, two thirds of mice that received cell-transplants became seizure-free by 6 months PT. The most severe neurodegeneration and granule cell dispersion in this model is found close to the kainate injection site in the ipsilateral rostral CA1. Pathology was significantly alleviated in the rostral hippocampus in the mice that were transplanted with cells. Transplants with a higher dose also decreased damage in less affected areas caudal from the kainate injection site. The higher doses of cells resulted in comparable seizure suppression to the lower reference dose studies.
In the dose escalation studies, no ectopic human tissues or teratomas were found and no adverse effects were detected across a battery of behavioral assays.

Conclusions
Human interneurons persist long-term in the epileptic mouse hippocampus, result in stable seizure freedom and reduce hippocampal pathology for most animals, and are well tolerated at high doses. These findings support further development of inhibitory interneuron cell therapy for drug-resistant TLE.

Funding:
This research was supported in part by a grant from the California Institute for Regenerative Medicine (CIRM DISC2-10525; TRAN1-11611) and was performed at Neurona Therapeutics Inc., San Francisco.