ETX101 (Encoded Therapeutics) — Dravet Syndrome Gene Activation Therapy

Executive Summary

ETX101 is an investigational gene therapy developed by Encoded Therapeutics using an AAV9 vector to deliver a CRISPR-activation (CRISPRa) system that selectively upregulates the wild-type [SCN1A](/genes/scn1a) allele in patients with [Dravet syndrome](/diseases/dravet-syndrome). Unlike antisense oligonucleotide approaches that require precise allele targeting, ETX101 aims to increase expression of the healthy SCN1A copy by activating its endogenous promoter, potentially restoring normal Nav1.1 sodium channel levels in inhibitory neurons without requiring direct gene replacement or precise targeting of disease-causing variants[@encoded2023].

The therapy represents a novel approach in the gene therapy landscape for monogenic epilepsy: rather than delivering a corrected gene or silencing the mutant allele, it amplifies the patient's own functional gene copy. This approach could benefit patients with diverse SCN1A variant types, including missense, nonsense, and splice-site mutations, since these all result in reduced SCN1A expression through haploinsufficiency rather than production of a toxic protein.

Trial Overview

Executive Summary

ETX101 is an investigational gene therapy developed by Encoded Therapeutics using an AAV9 vector to deliver a CRISPR-activation (CRISPRa) system that selectively upregulates the wild-type [SCN1A](/genes/scn1a) allele in patients with [Dravet syndrome](/diseases/dravet-syndrome). Unlike antisense oligonucleotide approaches that require precise allele targeting, ETX101 aims to increase expression of the healthy SCN1A copy by activating its endogenous promoter, potentially restoring normal Nav1.1 sodium channel levels in inhibitory neurons without requiring direct gene replacement or precise targeting of disease-causing variants[@encoded2023].

The therapy represents a novel approach in the gene therapy landscape for monogenic epilepsy: rather than delivering a corrected gene or silencing the mutant allele, it amplifies the patient's own functional gene copy. This approach could benefit patients with diverse SCN1A variant types, including missense, nonsense, and splice-site mutations, since these all result in reduced SCN1A expression through haploinsufficiency rather than production of a toxic protein.

Trial Overview

| Parameter | Value |
|-----------|-------|
| Sponsor | Encoded Therapeutics, Inc. |
| Phase | IND-enabling / Preclinical (2025-2026) |
| Indication | Dravet syndrome (SCN1A haploinsufficiency) |
| Vector | AAV9 |
| Delivery Route | Intra-cisterna magna (ICM) |
| Mechanism | CRISPR-activation (CRISPRa) of wild-type SCN1A allele |
| Target Population | Pediatric patients ages 2-8 |
| Status | IND-enabling studies |

Disease Context: Dravet Syndrome

Overview

[Dravet syndrome](/diseases/dravet-syndrome) is a catastrophic developmental and epileptic encephalopathy caused by heterozygous loss-of-function mutations in [SCN1A](/genes/scn1a), encoding the Nav1.1 sodium channel. The disease typically manifests in the first year of life with prolonged febrile seizures, followed by progression to multiple seizure types, developmental plateauing, and progressive cognitive impairment.

SCN1A Haploinsufficiency

The primary disease mechanism in Dravet syndrome is SCN1A haploinsufficiency — reduced expression of the Nav1.1 channel in inhibitory GABAergic interneurons. This leads to impaired sodium currents, reduced neuronal excitability control, and seizure susceptibility[@catterall2020]. Key points:

• ~60% of variants are loss-of-function (nonsense, frameshift, splice-site)
• ~40% of variants are missense — produce reduced or non-functional protein
• Both classes result in reduced Nav1.1 protein levels
• Targeting wild-type allele upregulation addresses all variant types

Treatment Landscape Challenges

Current treatments (ASDs, CBD, fenfluramine) manage symptoms but do not address the underlying genetic cause. Gene therapy approaches include:

| Approach | Mechanism | Advantages | Challenges |
|----------|-----------|-------------|-------------|
| ASO (STK-001) | Allele-specific silencing | Precise targeting | Requires variant-specific design |
| Gene replacement | Full SCN1A delivery | Complete correction | Gene size (~6kb) at AAV limit |
| Gene activation (ETX101) | Upregulate WT allele | Variant-agnostic | Efficiency, durability |

Mechanism of Action

CRISPR-Activation (CRISPRa) Platform

ETX101 uses a catalytically dead Cas9 (dCas9) fused to transcriptional activator domains (VP64, p65, Rta) delivered via AAV9. The dCas9-activator complex is guided to the SCN1A promoter region by a single-guide RNA (sgRNA), where it recruits endogenous transcriptional machinery to increase SCN1A mRNA transcription from the wild-type allele[@bhatt2022].

Key design features:

Why Gene Activation for Dravet?

The haploinsufficiency model makes Dravet syndrome particularly suited for CRISPR-activation:

• Only one functional copy exists but produces ~50% of normal protein levels
• Boosting wild-type expression can compensate for lost mutant expression
• This is fundamentally different from dominant-negative mutations
• Works regardless of specific SCN1A variant type

Comparison to Other Approaches

| Feature | ETX101 (CRISPRa) | ASO (STK-001) | Gene Replacement |
|---------|------------------|---------------|----------------|
| Target | Wild-type allele | Mutant allele | Both alleles |
| Variant flexibility | High (all types) | Allele-specific | Limited by size |
| Delivery | AAV9 (ICM) | Intrathecal | AAV (dual-vector) |
| Dosing | Single | Repeat | Single |
| Mechanism | Activation | Silencing | Replacement |

Preclinical Data

Animal Model Evidence

Published preclinical studies in Dravet syndrome mouse models demonstrate:

Development Milestones

| Year | Milestone |
|------|-----------|
| 2021 | Series B ($70M) — Platform development |
| 2021 | Roche/Neurocrine partnership ($150M+ upfront) |
| 2022 | Key preclinical publications |
| 2023 | Series C ($135M) — IND-enabling studies |
| 2025 | IND filing expected |
| 2026 | Phase 1/2 trial start |

Trial Design Considerations

Target Population

• Age: Pediatric patients ages 2-8, prioritizing early intervention before severe developmental regression
• Diagnosis: Genetically confirmed Dravet syndrome (SCN1A mutation)
• Seizure history: Documented seizure frequency, prior treatment history
• Exclusion: Severe developmental delay, active status epilepticus

Primary Endpoints

• Safety: Incidence and severity of adverse events
• Tolerability: Dose-limiting toxicities
• Pharmacodynamics: SCN1A mRNA expression in surrogate tissue

Secondary Endpoints

• Efficacy: Seizure frequency reduction at 12 months
• Developmental: Vineland-3 or Bayley-3 scores
• Electrophysiological: EEG normalization
• Quality of life: CGI-C, PedsQL

Biomarker Strategy

| Biomarker | Sample | Purpose |
|-----------|--------|---------|
| SCN1A mRNA | iPSC-derived neurons, blood | Target engagement |
| Nav1.1 protein | Brain tissue (preclinical), skin biopsy | Pharmacodynamic |
| CSF cytokines | CSF | Safety monitoring |
| EEG metrics | Scalp EEG | Efficacy signal |

Regulatory Considerations

FDA Pathway

As of 2025, ETX101 is on an IND-enabling path with several regulatory tailwinds:

International Regulatory

• EMA: Parallel scientific advice ongoing
• PMDA (Japan): Early engagement for future filing

Competitive Landscape

ETX101 competes in the Dravet gene therapy space with:

| Company | Program | Approach | Phase |
|---------|---------|----------|-------|
| Stoke Therapeutics | STK-001 | ASO | Phase 1/2 |
| Encoded Therapeutics | ETX101 | CRISPRa | IND-enabling |
| Roche/Neurocrine | AAV-SCN1A | Gene replacement | Discovery |
| Academia | Various | Various | Preclinical |

Competitive Advantages of ETX101

Competitive Challenges

Key Open Questions

Financial & Corporate Information

Company Background

• Headquarters: South San Francisco, CA
• Founded: 2019
• Funding: $205M+ total (Series B + C)
• Partnerships: Roche/Neurocrine (SCN1A, $150M+ upfront)

Development Timeline

| Timeline | Milestone |
|-----------|-----------|
| Q3 2025 | IND submission |
| 2026 | Phase 1/2 trial initiation |
| 2027 | Phase 1/2 data readout |
| 2028 | Potential pivotal trial |
| 2029-2030 | BLA filing |

References