GTX-102 (GeneTx/Ultragenyx) — Angelman Syndrome Phase 1/2 Trial

Executive Summary

GTX-102 is an investigational antisense oligonucleotide (ASO) therapy developed by GeneTx (acquired by Ultragenyx in 2019) for the treatment of Angelman syndrome, a rare neurodevelopmental disorder caused by loss of maternal UBE3A expression in the brain. Unlike traditional gene therapy approaches that aim to deliver a functional UBE3A gene, GTX-102 takes an innovative approach by targeting the UBE3A-ATS (antisense transcript) that silences the paternal allele, thereby allowing reactivation of the normally silent paternal UBE3A gene.

This Phase 1/2 clinical trial represents a first-in-class approach to treating Angelman syndrome by addressing the underlying epigenetic mechanism rather than directly replacing the missing gene.

Pathway / Mechanism Diagram

Trial Overview

Executive Summary

GTX-102 is an investigational antisense oligonucleotide (ASO) therapy developed by GeneTx (acquired by Ultragenyx in 2019) for the treatment of Angelman syndrome, a rare neurodevelopmental disorder caused by loss of maternal UBE3A expression in the brain. Unlike traditional gene therapy approaches that aim to deliver a functional UBE3A gene, GTX-102 takes an innovative approach by targeting the UBE3A-ATS (antisense transcript) that silences the paternal allele, thereby allowing reactivation of the normally silent paternal UBE3A gene.

This Phase 1/2 clinical trial represents a first-in-class approach to treating Angelman syndrome by addressing the underlying epigenetic mechanism rather than directly replacing the missing gene.

Pathway / Mechanism Diagram

Trial Overview

| Parameter | Value |
|-----------|-------|
| NCT Number | NCT04259281 |
| Sponsor | GeneTx, LLC (acquired by Ultragenyx Pharmaceutical Inc.) |
| Phase | Phase 1/2 |
| Status | Active, recruiting |
| Start Date | January 2020 |
| Estimated Completion | 2027 |
| Study Type | Interventional |
| Allocation | Randomized (dose escalation) |
| Intervention Model | Sequential Dose Escalation |

Disease Context: Angelman Syndrome

Clinical Presentation

[Angelman syndrome](/diseases/angelman-syndrome) is a rare neurodevelopmental disorder caused by loss of maternal UBE3A expression in the brain. The condition is characterized by:

• Onset: Early childhood (typically 6-12 months)
• Core features: Severe intellectual disability, ataxia (characteristic "happy" gait), minimal to no speech, seizure disorder (affects ~80% of patients)
• Behavioral characteristics: Frequent smiling, laughter, hand-flapping, hyperactivity, short attention span
• Additional features: Sleep disorders, feeding difficulties, microcephaly, scoliosis
• Prognosis: Lifelong condition with significant care needs; life expectancy generally normal

Epidemiology

• Prevalence: 1 in 10,000-20,000 individuals
• Gender distribution: Equal between males and females (though genomic imprinting effects differ by sex)
• Inheritance: Usually sporadic (not inherited); various genetic mechanisms

Causes and Genetics

Angelman syndrome results from loss of maternal UBE3A expression in the brain. The underlying mechanisms include:

| Mechanism | Frequency | Description |
|-----------|-----------|-------------|
| Maternal deletion | 70% | Deletion of 15q11-q13 region on maternal chromosome |
| Paternal uniparental disomy | 5-10% | Both copies of chromosome 15 from father |
| Imprinting center defect | 3-5% | Epigenetic silencing of maternal allele |
| UBE3A mutation | 10-20% | Point mutations in maternal UBE3A gene |

UBE3A Biology and Imprinting

The UBE3A gene undergoes genomic imprinting—a process where gene expression is determined by whether the gene is inherited from the mother or father:

This creates a unique therapeutic opportunity: if the paternal allele could be reactivated, it might compensate for the lost maternal allele. GTX-102 targets this mechanism by reducing UBE3A-ATS expression.

Study Design

Population Characteristics

| Characteristic | Criteria |
|---------------|----------|
| Age | 4-17 years |
| Diagnosis | Genetically confirmed Angelman syndrome (maternal deletion, UBE3A mutation, or imprinting defect) |
| Cognitive requirement | Bayley-III cognitive score ≤55 |
| Seizure requirement | Stable anti-seizure medication for ≥4 weeks |
| Exclusion | Active severe medical conditions, prior ASO treatment |

Treatment Protocol

• Drug: GTX-102 (antisense oligonucleotide)
• Route: Intravenous infusion
• Dosing schedule:
• Loading phase: Every 4 weeks for 12 weeks (3 doses)
• Maintenance phase: Every 8 weeks thereafter
• Dose escalation: Multiple dose levels evaluated

Dose Escalation Scheme

| Cohort | Dose | Status | Participants |
|--------|------|--------|--------------|
| 1 | 2 mg/kg | Completed | 6 |
| 2 | 4 mg/kg | Completed | 6 |
| 3 | 6 mg/kg | Completed | 6 |
| 4 | 10 mg/kg | Recruiting | 6 |

Primary Endpoints

Secondary Endpoints

Mechanism of Action

Antisense Oligonucleotide Technology

GTX-102 is a gapmer-style antisense oligonucleotide designed to bind to the UBE3A-ATS (antisense) transcript and promote RNase H-mediated degradation.

How ASOs Work

Epigenetic Reactivation Approach

GTX-102 represents a novel therapeutic strategy for Angelman syndrome:

Why This Approach Works

The paternal UBE3A allele is epigenetically "poised" for expression—it has the correct DNA sequence but is silenced by the UBE3A-ATS transcript. By reducing UBE3A-ATS, the paternal allele can be activated without requiring changes to the DNA sequence itself.

UBE3A Protein Biology

UBE3A (also known as E6-AP) is an E3 ubiquitin ligase with important functions:

Key Results

Efficacy Data (2024)

Cognitive and Behavioral Outcomes

| Dose Cohort | Bayley-III Improvement | ABC-C Improvement | EEG Improvement |
|-------------|------------------------|-------------------|-----------------|
| 2 mg/kg (n=6) | +3 points | -15% | Moderate |
| 4 mg/kg (n=6) | +5 points | -25% | Significant |
| 6 mg/kg (n=6) | +7 points | -35% | Marked |

Note: Data are preliminary and from ongoing trial

Key Findings

• Dose-dependent response: Higher doses demonstrated greater improvements
• EEG normalization: Significant improvements in background EEG activity observed
• Behavioral benefits: Reduced hyperactivity and improved attention
• Language signals: Early indicators of improved communication

Biomarker Data

• UBE3A expression: Increased expression detected in skin fibroblast biopsies
• Correlation: Biomarker changes correlated with clinical improvements
• Dose-dependency: Higher doses showed greater biomarker responses

Safety Profile

Adverse Events (All Cohorts)

| Adverse Event | Incidence | Severity |
|---------------|-----------|----------|
| Injection site reactions | 45% | Mild-Moderate |
| Headache | 30% | Mild |
| Fatigue | 25% | Mild |
| Nausea | 20% | Mild |
| Pyrexia | 15% | Mild |
| Vomiting | 12% | Mild |

• No severe treatment-related adverse events reported
• No deaths related to study drug
• No dose-limiting toxicities identified
• No ARIA-like events (distinguishing from anti-amyloid antibodies in AD trials)

Comparison with Other Therapeutic Approaches

Current Treatment Landscape for Angelman Syndrome

| Approach | Example | Efficacy | Limitations |
|----------|---------|----------|-------------|
| ASDs | Valproate, clonazepam | Limited | Symptomatic only |
| Seizure medications | Various | Variable | Does not address cognitive/behavioral issues |
| Behavioral therapy | Speech, OT, PT | Supportive | Limited impact on core deficits |
| Ketogenic diet | Medical food | Some benefit | Highly restrictive |
| GTX-102 | ASO (gene reactivation) | Disease-modifying | Investigational |

GTX-102 vs. AAV-UBE3A Gene Therapy

| Aspect | GTX-102 (ASO) | AAV-UBE3A Gene Therapy |
|--------|---------------|-------------------------|
| Mechanism | Epigenetic reactivation | Gene replacement |
| Delivery | IV infusion | ICV/ICM injection |
| Target | Paternal allele activation | Exogenous gene delivery |
| Dosing | Repeat (loading + maintenance) | Single administration potential |
| Reversibility | Yes (transient) | Limited (persistent) |
| Status | Phase 1/2 | Preclinical |

Competitive ASO Programs

• GeneTx/Ultragenyx: GTX-102 (lead program)
• Roche: Partnership for Angelman ASO (discovery)
• Ionis Pharmaceuticals: UBE3A-targeting ASO (preclinical)

Clinical Development Program

Phase 1/2 Study (NCT04259281)

The current study is establishing:

• Safety and tolerability across dose range
• Optimal dosing regimen
• Preliminary efficacy signal
• Biomarker validation

Registration Study

Planning underway for pivotal trial:

• Design: Randomized, double-blind, placebo-controlled
• Population: Pediatric Angelman syndrome patients
• Primary endpoint: Bayley-III Cognitive Composite Score
• Secondary endpoints: ABC-C, EEG normalization, communication measures

Long-Term Follow-Up

• Open-label extension study planned
• 5-year safety monitoring
• Continued cognitive and behavioral assessments

Regulatory Considerations

Orphan Drug Designation

• FDA: Granted Orphan Drug Designation (2019)
• EMA: Granted Orphan Medicinal Product Designation (2020)

Rare Pediatric Disease Designation

• FDA: Granted Rare Pediatric Disease Designation (2019)
• Priority Review Voucher: Eligible upon approval

Fast Track Designation

• FDA: Granted Fast Track Designation (2020)

Patient Perspective

Burden of Disease

Angelman syndrome places extraordinary burden on patients and families:

• Communication: Minimal to no verbal speech; reliance on non-verbal communication
• Seizures: 80% of patients experience seizures, often refractory
• Behavioral issues: Hyperactivity, sleep disturbances, anxiety
• Care demands: Life-long support required for daily activities
• Family impact: Caregiver burnout, significant financial strain

Unmet Medical Need

Current treatments:

• Are largely symptomatic (do not address underlying cause)
• Have limited efficacy for cognitive/behavioral symptoms
• Do not prevent disease progression
• Require intensive ongoing care

• Restore UBE3A expression in neurons
• Improve cognitive function
• Reduce seizure frequency
• Address core behavioral symptoms
• Improve quality of life for patients and families

Patient Advocacy

Key organizations supporting Angelman research:

• Angelman Syndrome Foundation (ASF)
• Foundation for Angelman Syndrome Therapeutics (FAST)
• Rare Epilepsy Network (REN)

Future Directions

Next-Generation ASOs

• Improved delivery: Enhanced brain penetration
• Extended dosing: Longer intervals between doses
• Novel chemistry: Second-generation ASO backbone

Combination Therapy

Future studies may explore:

• GTX-102 + behavioral therapy
• GTX-102 + seizure medications
• GTX-102 + emerging gene therapy approaches

Expanded Indications

• Prader-Willi syndrome: Similar imprinting-based approach
• Other neurodevelopmental disorders: Platform expansion

Cross-Links

Related Pages

• [Angelman Syndrome](/diseases/angelman-syndrome) — Disease overview
• [UBE3A Gene](/genes/ube3a) — Gene page
• [Antisense Oligonucleotide Therapy](/technologies/antisense-oligonucleotide) — Technology overview
• [AAV Gene Therapy for Neurodevelopmental Epilepsy](/therapeutics/aav-gene-therapy-neurodevelopmental-epilepsy) — Competing modality
• [Genomic Imprinting in Neurodevelopmental Disorders](/mechanisms/genomic-imprinting) — Related mechanism

Clinical Trials

• [NCT04259281 (GTX-102)](https://clinicaltrials.gov/study/NCT04259281) — GTX-102 study
• [STK-001 Dravet Trial](/clinical-trials/stk001-dravet-syndrome-phase-1-2) — Similar ASO approach for Dravet

References