Abstracts

Rationale:
Dravet Syndrome (DS) is a severe, treatment-resistant developmental and epileptic encephalopathy that typically presents with frequent and prolonged seizures in the first year of life, followed by significant cognitive delays, motor impairment, sleep problems, and profound behavioral difficulties that resemble autistic spectrum disorders, obsessive compulsive disorder and attention deficit hyperactivity disorder. Up to 20% of patients die before adulthood. DS is frequently caused by single allele SCN1A mutations that result in loss-of-function of the Nav1.1 voltage-gated sodium channel, leading to dysfunction of GABAergic inhibitory neurons and runaway excitation. Currently approved therapies for DS aim at reducing seizure frequency, but none address the underlying genetic cause and neurodevelopmental impact of the disorder. Encoded has developed ETX101, an investigational GABAergic interneuron-selective AAV9 expressing an engineered transcription factor designed to upregulate endogenous SCN1A. ETX101 has shown to increase Nav1.1 protein expression, prolong survival, and decrease the occurrence of spontaneous and hyperthermia-induced seizures in a DS mouse model. Here we describe the biodistribution, activity, and safety of ETX101 in non-human primates (NHPs) after a one-time intracerebroventricular (ICV) injection.
Method:
ETX101 at doses 4.8E13 -8.0E13 vg/animal (n=4) and vehicle (n=2) were administered to NHPs via one-time, unilateral ICV injection. Blood chemistry and clinical examinations were assessed at baseline and weekly after dosing for the duration of this 28-day study. Biodistribution of ETX101 vector genomes and transgene mRNA expression were examined in neural and peripheral tissues using digital droplet polymerase chain reaction (PCR). Histopathology evaluation was performed in all major tissues. AAV9 neutralizing antibodies (NAb) were measured pre/post-dosing from serum and cerebral spinal fluid (CSF) samples.
Results:
One-time, unilateral ICV injection of ETX101 in NHPs led to widespread vector biodistribution and robust transgene expression throughout the brain, including key structures involved in complex cognitive behaviors, such as cerebral cortex and hippocampus. Importantly, transgene mRNA expression was limited to the central nervous system (CNS), reaching similar levels within the different CNS structures after 28 days. ETX101 was well tolerated with no detectable changes in clinical findings, such as body weight or body temperature. No macroscopic or microscopic findings were observed in the tissues examined, including brain, liver, dorsal root ganglion, and spinal cord. All animals survived until necropsy. Serum titers of AAV9 NAbs increased 28 days post ETX101 administration while CSF titers of AAV9 NAbs were indistinguishable from pre-treatment levels.
Conclusion:
These results indicate that ETX101 is well tolerated, supporting its continued clinical development to treat the genetic cause of SCN1A+ DS. Additionally, ETX101 leads to broad CNS biodistribution, providing further evidence that unilateral ICV injection is the appropriate delivery route for this AAV-mediated gene therapy to treat SCN1A+ DS.
Funding:
:Encoded Therapeutics