GEMMABio Unveils Duchenne Muscular Dystrophy (DMD) Program and Presents Preclinical Data for Next‑Generation Gene Therapies for DMD and Spinal Muscular Atrophy Type 1 (SMA1) at the ASGCT 2026 Annual Meeting

-DMD preclinical data support candidate declaration for GB703, a potential DMD gene therapy designed to broaden mutation coverage and improve muscle delivery
-SMA1 preclinical data support the ongoing CHARISMA Phase 1/2 clinical trial for SMA1, which is advancing on schedule

PHILADELPHIA, May 14, 2026 /PRNewswire/ -- Gemma Biotherapeutics ("GEMMABio"), a clinical‑stage, global, genetic medicines company, today announced the presentation of preclinical data supporting candidate declaration for GB703, a novel, potential gene therapy for Duchenne muscular dystrophy (DMD). The company also publicized the presentation of comprehensive preclinical data for GB221, a next-generation, investigational gene therapy for spinal muscular atrophy type 1 (SMA1). Juliette Hordeaux, DVM, PhD, DECVP, Chief Scientific Officer at GEMMABio, will present the DMD data today at 8:15 a.m. ET and the SMA1 work at 3:30 p.m. ET during oral presentations at the American Society of Gene and Cell Therapy (ASGCT) 2026 Annual Meeting held in Boston, MA, and virtually.

"We are excited to announce Duchenne muscular dystrophy as our latest named program," said GEMMABio Chief Executive Officer James M. Wilson, MD, PhD, "Our research team developed an innovative, myotropic AAV capsid design and created a de-immunized, miniature transgene with the ultimate goal of evaluating whether these design features may improve delivery at low doses, reduce toxicity, and expand the group of mutations that can be safely treated with this investigational gene therapy in future clinical trials. At this time, we continue our intensive preclinical work on the DMD program, and we are continuing to execute on [investigational new drug] IND-enabling studies to bring it forward to the clinic as swiftly as possible."

Key Highlights from the DMD Presentation at ASGCT include:

• GB703 is designed with a de-immunized, therapeutic payload intended to potentially broaden the spectrum of dystrophin mutations that may be treated: The miniaturized transgene is a hybrid micro-utro/dystrophin that incorporates the self-utrophin-derived exon 8-11 region, potentially expanding patient eligibility to those with N-terminal deletions in dystrophin.
• GB703 deploys a novel myotropic capsid designed to optimize delivery at lower doses: Engineered AAV capsid, GCap104, targets skeletal and heart muscle in mice and nonhuman primates (NHPs), and was observed to support expression of hybrid micro-utro/dystrophin in NHPs at 3×10¹³ GC/kg by intravenous administration. It was well-tolerated in NHPs over a 60-day study.
• The hybrid micro-utro/dystrophin payload showed measurable biological effects in a mouse model of DMD: Therapeutic transgene expression was associated with restoration of the β-dystroglycan complex and nNOS sarcolemmal binding, as well as reduction in markers of muscle damage in 3-month-old mice.

Together, these design features are intended to address two major challenges in DMD gene therapy: achieving efficient muscle delivery at lower doses and potentially treating patients whose mutations may make them poor candidates for existing microdystrophin-based approaches. GEMMABio is advancing GB703 through IND-enabling studies designed to support future clinical trial initiation.

Key Highlights from the SMA1 Presentation at ASGCT include:

• Next-generation design with potential to support motor neuron function while reducing overexpression-related toxicities and sensory neuropathy in preclinical models: The GB221 investigational gene therapy comprises an AAV (AAVhu68) carrying a functional copy of a codon-optimized SMN1 gene under the control of a ubiquitous, moderate-strength human promoter. GB221 is designed with novel microRNA (miRNA) target site technology that silences transgene expression in dorsal root ganglia (DRG) neurons to reduce the risk of peripheral sensory neuropathy. As a one-time treatment delivered directly to the cerebrospinal fluid (CSF) via intra-cisterna magna (ICM) injection, it has demonstrated a favorable safety profile in preclinical animal models.
• GB221 improved survival and neuromotor function in a mouse model of severe SMA: GB221 administration resulted in >50% survival at 120 days compared to <15 days in untreated delta7 Smn^-/- mice, and GB221-treated mice exhibited improvement in strength (measured by righting reflex time) by Day 14 post-treatment. Normal strength and trunk control were demonstrated by all treated animals that survived to weaning age.
• GB221 achieved transgene expression without observed toxicities in NHPs: GB221 ICM administration resulted in expression in 20-90% of motor neurons without DRG toxicity or sensory neuropathy. No liver, cardiac, or other toxicities were observed at the maximum feasible dose (5.8×10¹³ GC/animal ~ 6.4×10¹¹ GC/g brain).

These findings support GEMMABio's approach of maintaining SMN1 expression in motor neurons while reducing expression in tissues associated with sensory neurotoxicity risk.

"Our priorities are always safety, efficacy, and access, and we create every therapy with those principles in mind. Our DMD candidate, GB703, and our SMA1 investigational therapy, GB221, were created as next-generation gene therapies that incorporate novel design elements," said Dr. Juliette Hordeaux, who leads the scientific teams responsible for innovative new approaches at GEMMABio. "We anticipate that GB703, which integrates a self-utrophin derived exon 8-11 region, has the potential, if successful in the clinic, to expand access to the ~15% of DMD patients who are currently ineligible for all other approved and investigational microdystrophin-based products due to risk of chronic immune responses. For our SMA program, GB221 was designed with original microRNA silencing technology to suppress SMN1 transgene expression in dorsal root ganglia neurons, and we expect that this will reduce the risk of peripheral sensory neuropathy that may be associated with other approved SMA gene therapies."

SMA1 program clinical update:
The CHARISMA Phase 1/2 clinical trial is underway in Brazil and continues to progress according to plan, with the study designed to evaluate safety, tolerability, and early efficacy signals in pediatric participants under 12 months of age.

About DMD
Duchenne muscular dystrophy (DMD) is a degenerative neuromuscular disease that leads to progressive weakness in skeletal muscles and heart. DMD is caused by certain loss-of-function mutations in the dystrophin gene (DMD), and it follows an X-linked, recessive pattern of inheritance. In the absence of functional dystrophin protein, muscles sustain ongoing damage and deterioration marked by limited muscle growth, strength, and flexibility, and there are additional adverse impacts on respiratory and cardiac function. DMD mostly affects males, and the global incidence of DMD is approximately 1:5,000 live male births.

DMD is the most common type of muscular dystrophy worldwide. It is typically diagnosed between the ages of 2 and 5, and it often presents with developmental delays in sitting, walking, and speech though progression of symptoms varies for each patient. Patients experience progressive loss of ambulation at ~12 years of age, and median survival is 25-30 years.

About GB703
GB703 is an investigational, novel gene therapy for DMD designed to express a micro-utro/dystrophin hybrid gene in muscle cells throughout the body, including skeletal and cardiac muscle. The absence of functional dystrophin protein in muscle cells is the underlying cause of DMD. GB703 consists of an engineered, myotropic, adeno‑associated virus (AAV) capsid, GCap104, encasing a de-immunized, miniaturized, utro/dystrophin hybrid gene and is intended to potentially address a broad spectrum of dystrophin mutations, including N-terminal region deletions. It is delivered by intravenous infusion.

About SMA1
Spinal muscular atrophy (SMA) is a progressive neurodegenerative disease that affects lower motor neurons in the spinal cord. Progressive muscle weakness impairs breathing, eating, crawling/walking, and other activities. SMA is caused by a mutation in the survival motor neuron gene 1 (SMN1), and it follows an autosomal recessive pattern of inheritance. The global incidence of all types of SMA, whose severity is modulated by the number of SMN2 gene copies, is approximately 1:10,000 live births.

SMA Type 1 (SMA1) is a very severe form with early onset and diagnosis occurring at less than 6 months of age. Untreated SMA1 patients do not survive beyond 18 months of age. SMA1 patients typically have 1-2 copies of the SMN2 gene. The global incidence of SMA1 is approximately 1:17,000 live births.

About GB221
GB221 is an investigational, clinical stage, next‑generation gene therapy for SMA1 developed to express SMN1 in motor neurons. The absence of functional SMN1 protein in motor neurons is the underlying cause of SMA. GB221 is a one-time gene therapy that consists of an adeno‑associated virus (AAV) human isolate, AAVhu68, encasing a functional copy of a codon‑optimized SMN1 gene in a modified transgene expression cassette designed to reduce overexpression and sensory neurotoxicity. It is delivered directly to the cerebrospinal fluid (CSF) via intra-cisterna magna (ICM) injection.

About CHARISMA
CHARISMA (NCT07070999) is a first‑in‑human clinical study of GB221 in SMA1 that evaluates safety, tolerability, and efficacy in pediatric participants under 12 months of age. Additional details can be found at ClinicalTrials.gov.

About GEMMABio 

GEMMABio is a clinical-stage therapeutics company focused on speeding the research of and global access to life-changing advanced therapies for those living with rare diseases. The company conducts research and product development functions to bring gene therapy discoveries from the bench to the bedside faster and affordably. GEMMABio is led by gene therapy industry pioneer Jim Wilson and his team of experts, who previously conducted their work in academia. Learn more about GEMMABio at gemmabiotx.com and follow us on LinkedIn.

SOURCE GEMMA Biotherapeutics