Gene Therapy, RNA Therapeutics & Future Directions

The neuromuscular disease landscape has been transformed by the emergence of gene replacement therapy, RNA-targeting therapeutics, and targeted biologics. Over the past decade, the FDA has approved multiple gene therapies and antisense oligonucleotides for spinal muscular atrophy (SMA), Duchenne muscular dystrophy (DMD), and amyotrophic lateral sclerosis (ALS), ushering in an era of precision medicine for previously untreatable conditions. At the same time, complement inhibitors and neonatal Fc receptor (FcRn) blockers have redefined treatment of autoimmune neuromuscular disorders such as myasthenia gravis (MG) and chronic inflammatory demyelinating polyneuropathy (CIDP). These advances carry significant challenges including safety concerns, prohibitive costs, manufacturing limitations, and questions of equitable access that will shape the next generation of neuromuscular therapeutics.

Gene Therapy Platforms

AAV Vector Biology

Adeno-associated virus (AAV) vectors are the dominant gene delivery platform in neuromuscular medicine. AAV serotypes differ in tissue tropism: AAV9 crosses the blood-brain barrier and transduces motor neurons efficiently, making it the vector of choice for SMA; AAVrh74 and AAVrh10 show enhanced muscle tropism suitable for muscular dystrophies. AAV vectors carry single-stranded DNA packaged in a non-enveloped capsid and do not integrate into the host genome, instead persisting as episomal DNA. Key advantages include low immunogenicity relative to other viral vectors, broad tissue tropism, and long-term transgene expression in postmitotic cells. However, the limited packaging capacity (~4.7 kb) necessitates the use of truncated “micro” or “mini” transgenes for large genes such as dystrophin (full-length cDNA ~14 kb).

Onasemnogene Abeparvovec (Zolgensma) for SMA

Zolgensma, an AAV9 vector delivering a functional SMN1 transgene, was approved by the FDA in May 2019 for children under 2 years with SMA. It represented the first gene therapy approved for any neuromuscular disease. A single intravenous infusion (1.1 × 10¹⁴ vg/kg) achieves widespread transduction of motor neurons in the spinal cord. Clinical trials demonstrated that presymptomatic treatment produced near-normal motor milestones, while symptomatic infants achieved independent sitting and, in some cases, walking — outcomes historically impossible in SMA type 1. In November 2025, the FDA approved an intrathecally-delivered formulation (Itvisma) for patients aged 2 years and older, expanding the treatable population. Global sales reached $1.2 billion in 2024, though the list price of $2.125 million per patient remains controversial.

Delandistrogene Moxeparvovec (Elevidys) for DMD

Elevidys delivers a micro-dystrophin transgene via AAVrh74, aiming to provide a truncated but functional dystrophin protein that stabilizes the sarcolemma. It received accelerated FDA approval in June 2023 for ambulatory boys aged 4–5 years, followed by expanded approval in June 2024 for all patients aged ≥4 years (traditional approval for ambulatory; accelerated for non-ambulatory). In 2024, sales reached $821 million, approaching blockbuster status.

Gene Therapy for X-Linked Myotubular Myopathy: Lessons Learned

The ASPIRO trial evaluated AAV8-delivered myotubularin (AT132) for X-linked myotubular myopathy (XLMTM). While efficacy was remarkable — ventilator-dependent infants achieved independent breathing and motor milestones — four participants died of progressive cholestatic liver failure. Deaths occurred at both the lower dose (1.3 × 10¹⁴ vg/kg) and higher dose (3.5 × 10¹⁴ vg/kg), with all fatalities developing severe cholestatic liver injury followed by hepatic decompensation. Preclinical studies subsequently showed that AAV8 significantly increases susceptibility to cholestasis in XLMTM mouse models. These events underscore the critical need for hepatobiliary monitoring, pre-existing liver disease screening, and cautious dose selection in systemic AAV programs.

Antisense Oligonucleotides (ASOs)

Antisense oligonucleotides are short synthetic nucleic acid sequences (typically 18–25 nucleotides) that bind complementary pre-mRNA or mRNA to modulate gene expression. Two primary mechanisms are exploited in neuromuscular disease: splice modulation (redirecting exon inclusion or exclusion) and RNase H-mediated mRNA degradation (reducing toxic protein production).

Nusinersen (Spinraza) for SMA

Nusinersen, approved by the FDA in December 2016, was the first disease-modifying therapy for SMA. It is a 2’-O-methoxyethyl phosphorothioate ASO that binds an intronic splicing silencer in SMN2 pre-mRNA, promoting inclusion of exon 7 and increasing production of full-length, functional SMN protein. Administered intrathecally every 4 months (after loading doses), nusinersen demonstrated significant improvements in motor function and survival across SMA types 1–3 in pivotal trials (ENDEAR, CHERISH). Its success established the proof of concept for RNA-targeted therapy in neurodegenerative disease.

Tofersen for SOD1-ALS

Tofersen (Qalsody), approved in 2023 under accelerated approval, targets SOD1 mRNA for RNase H-mediated degradation in patients with ALS caused by SOD1 mutations (~2% of all ALS). Administered intrathecally every 4 weeks, tofersen reduced SOD1 protein and neurofilament light chain (NfL) concentrations in CSF. While the phase 3 VALOR trial did not meet its primary clinical endpoint (ALSFRS-R change at 28 weeks), biomarker reductions and long-term extension data supported accelerated approval. Tofersen represents the first genotype-targeted therapy in ALS and established a biomarker-guided treatment paradigm for motor neuron disease.

Exon-Skipping ASOs for DMD

Four exon-skipping ASOs have received FDA approval for DMD, each targeting specific exons to restore the reading frame of the dystrophin gene and produce a truncated but partially functional dystrophin protein (analogous to the milder Becker phenotype):

All four were granted accelerated approval based on dystrophin expression as a surrogate endpoint. Collectively, these ASOs are amenable to approximately 30% of DMD patients depending on deletion mutation. Limitations include modest dystrophin restoration (typically <5% of normal), requirement for weekly intravenous infusions, and ongoing debate regarding meaningful clinical benefit.

Small Molecule RNA Modulators

Risdiplam (Evrysdi) for SMA

Risdiplam, approved by the FDA in August 2020, is the first oral disease-modifying therapy for SMA. It is a small molecule that modifies SMN2 pre-mRNA splicing by stabilizing the transient double-stranded RNA structure at the exon 7 5’ splice site, promoting exon 7 inclusion and increasing full-length SMN protein production systemically. Administered as a once-daily oral liquid, risdiplam is approved for patients aged ≥2 months across all SMA types.

The success of risdiplam has spurred interest in small molecule splicing modifiers for other neuromuscular diseases, including myotonic dystrophy type 1 (DM1) and limb-girdle muscular dystrophies (LGMDs), though these remain in early pipeline stages.

Monoclonal Antibodies in Neuromuscular Disease

Complement C5 Inhibitors

Terminal complement inhibition has become a cornerstone of treatment for AChR antibody-positive generalized myasthenia gravis (gMG). Three complement C5 inhibitors are now approved:

• Eculizumab (Soliris): Humanized monoclonal antibody binding C5 to prevent cleavage into C5a and C5b; approved for AChR-positive gMG in 2017. Administered IV every 2 weeks. Requires meningococcal vaccination
• Ravulizumab (Ultomiris): Engineered from eculizumab with extended half-life; administered IV every 8 weeks. First patients treated in April 2024 for gMG. Demonstrated equivalent efficacy with less frequent dosing
• Zilucoplan (Zilbrysq): A macrocyclic peptide C5 inhibitor with dual mechanism — prevents C5 cleavage and sterically blocks C5b-C6 binding, inhibiting membrane attack complex assembly. FDA approved October 2023. Administered as daily subcutaneous self-injection. A 2025 comparative study showed all three C5 inhibitors reduced MG-ADL, QMG, and MGC scores similarly, with steroid-sparing effects (37–62% dose reduction)

FcRn Inhibitors

Neonatal Fc receptor (FcRn) blockers reduce pathogenic IgG levels by preventing IgG recycling, offering a novel mechanism for antibody-mediated neuromuscular diseases:

• Efgartigimod (Vyvgart): Approved for AChR-positive gMG (IV, December 2021; SC, June 2023). In June 2024, became the first FcRn blocker approved for CIDP (SC weekly). Prefilled syringe approved for at-home administration
• Rozanolixizumab (Rystiggo): Approved for gMG (SC) in the USA (June 2023) and EU (January 2024). Indicated for both AChR-positive and MuSK-positive gMG
• Nipocalimab: FDA approved in April 2025 for gMG, becoming the third FcRn blocker in this indication

Anti-CD20 and Other Biologics

Rituximab (anti-CD20) is used off-label in MuSK-positive MG (often considered first-line) and in refractory inflammatory neuropathies. Inebilizumab (anti-CD19) is approved for NMOSD and under investigation for other antibody-mediated neuromuscular conditions. These B-cell depleting therapies offer complementary mechanisms to complement and FcRn-targeted approaches.

Emerging Therapeutic Approaches

Approved Gene & RNA Therapies for Neuromuscular Disease

Challenges & Future Directions

Cost and Affordability

Gene therapies represent the most expensive single-dose treatments in medicine. Zolgensma carries a list price of $2.125 million; Elevidys was priced at $3.2 million at launch; and the most expensive gene therapy to date, Lenmeldy (for metachromatic leukodystrophy), costs $4.25 million per dose. Through 2024, Novartis reported cumulative Zolgensma revenues exceeding $6.4 billion globally. The U.S. payer system is not structured to absorb single large expenditures for treatments with benefits accruing over years or decades. Emerging payment models include the Centers for Medicare and Medicaid Innovation’s Cell and Gene Therapy Access Model, which features upfront payments with 5-year outcome tracking and performance-based rebates.

Durability and Redosing

AAV gene therapy is designed as a “one-and-done” treatment, but the durability of transgene expression remains uncertain beyond 5–7 years. Because systemic AAV delivery elicits robust neutralizing antibody responses against the capsid, redosing with the same serotype is currently not feasible. Strategies under investigation include capsid engineering to evade pre-existing immunity, immunomodulatory protocols (e.g., IgG-degrading enzymes such as imlifidase), and alternate serotype sequential dosing.

Manufacturing Scalability

AAV manufacturing is technically demanding, requiring large-scale cell culture, vector purification, and stringent quality control. Sarepta executives have noted that their projected Elevidys demand would require more gene therapy manufacturing capacity than existed globally at the time of calculation. This bottleneck constrains supply, delays patient access, and contributes to high pricing. Advances in suspension cell culture, baculovirus production systems, and continuous manufacturing may help address capacity constraints.

Equitable Global Access

More than five years after Zolgensma’s approval, it remains unavailable to children in many low- and middle-income countries. Barriers include insufficient regulatory infrastructure for advanced therapies, limited manufacturing capabilities, concentration of treatment centers in metropolitan academic hospitals, and the absence of sustainable funding mechanisms. In high-income countries, geographic disparities persist, with many patients traveling over 50 miles to reach qualified treatment centers. International initiatives for technology transfer, tiered pricing, and regional manufacturing hubs are being explored but remain nascent.

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