Advances in technology and research have pushed the field to the brink of a revolutionary era of treatment.
TARGETED GENETIC THERAPIES are revolutionizing the field of neuromuscular medicine. Previously, patients were limited regarding pharmacologic approaches to treatment and relied mainly on supportive care. The inevitable progression of disease led to anticipated trajectories of declining strength and function. But now, new therapeutic strategies are changing this trajectory, providing hope that leaves the future less clearly delineated.
Gene-transfer therapy utilizes a viral vector to deliver a transgene (a single-stranded DNA genome). Administration of the product leads to widespread biodistribution and release of the transgene in cell nuclei, where its promoter drives sustained protein expression.1 Disorders of loss of protein function are well suited to this therapeutic approach. Although protein expression is modified by the therapy, the individual’s genome is largely unchanged (which is distinct from gene-editing technologies). In the neuromuscular field, gene-transfer therapies have been explored most rigorously in spinal muscular atrophy (SMA) and more recently in Duchenne muscular dystrophy (DMD).
Novel therapeutics, including gene transfer, have radically changed life expectancy and functional status for patients with SMA, a neurodegenerative disease resulting from a biallelic deletion/mutation in the survival motor neuron 1 (SMN1) gene. SMN1 directs production of survival motor neuron (SMN) protein, which is essential to maintain the integrity of the motor neuron.2 Without sufficient protein production, motor neurons irreversibly degenerate, leading to progressive weakness.3 SMA affects 1 in 11,000 live births and has been a leading genetic cause of infant deaths.4 The most common and severe phenotype demonstrates symptoms in the first few months of life, including low muscle tone and failure to meet early motor milestones. Patients with this phenotype never achieve independent sitting and usually do not survive past 2 years without treatment.
Onasemnogene abeparvovec (Zolgensma; Novartis) is a gene-transfer therapy that is FDA approved in the US for the treatment of children younger than 2 years with SMA.5 It is made up of an AAV9 capsid that carries a recombinant single-stranded human SMN transgene. After 1-time intravenous administration, onasemnogene abeparvovec is widely distributed in peripheral and central nervous system tissues as it crosses the blood-brain barrier to target motor neuron nuclei in the anterior horn of the spinal cord.6 The most common adverse effects (AEs) associated with its use include elevated liver enzymes and vomiting.
The efficacy of onasemnogene abeparvovec has been established in clinical trials, and real-world experience is rapidly evolving. The first clinical trial to demonstrate benefit was the phase 1 START trial (NCT02122952), which began in 2014 and included 15 patients with infantile SMA.7 At the last follow-up visit 24 months after administration, the mean Children’s Hospital of Philadelphia Infant Test of Neuromuscular Disorders motor function score in the high-dose cohort was 56.5 points, compared with 5.3 points in the historic comparison.8 The long-term follow-up study (NCT04042025) includes 13 of the patients enrolled in START, with all 10 from the high-dose cohort alive and free from ventilatory support now more than 6 years after administration. All motor milestones achieved in START have been maintained, supporting durability of efficacy.9
Two phase 3 STR1VE trials (STR1VE-US [NCT03306277] and STR1VE-EU [NCT03461289]) examined onasemnogene abeparvovec administration in patients with symptomatic infantile SMA in the United States and Europe. The results demonstrated rapid and sustained increases in motor function scores as early as 1 month following treatment, with clinically meaningful benefits observed regarding survival and motor milestone achievement compared with natural history.10,11 SPR1NT (NCT03505099) was an open-label, single-arm, phase 3 study evaluating administration of onasemnogene abeparvovec in genetically diagnosed infants younger than 6 weeks with no observable clinical signs of SMA. There were 29 patients enrolled, and results demonstrated that all patients were alive and free from permanent ventilation as of the last cutoff date. All patients in the cohort of infants with 2 copies of SMN2— who are predicted to likely have a more severe phenotype—were able to achieve sitting without support at the 18-month assessment, including 11 who achieved this motor milestone in an age-appropriate time period.12 Eight of 15 patients in the cohort with 3 copies of SMN2 achieved the primary efficacy end point of standing without support within a normal developmental window. The remaining 7 patients were younger than the age cutoff for this motor skill. Six of 15 patients in the 3-copy cohort also achieved walking alone within the normal developmental time period. The remainder of patients were younger than the age cutoff for this motor skill.13 The efficacy of early treatment administration in genetically diagnosed but not yet symptomatic infants in this clinical trial supported the efforts for newborn screening programs throughout the United States, many of which have begun implementation.14
Future and ongoing studies are addressing intravenous onasemnogene abeparvovec administration in older and heavier patients in the phase 3b SMART trial (NCT04851873). An intrathecal formulation has been evaluated in patients aged 6 to 60 months in the phase 1 STRONG study (NCT03381729), examining patients with SMA who sit but are nonambulatory. This study was placed on clinical hold because of preclinical concerns for dorsal root ganglia toxicity, but the hold has since been lifted. The primary efficacy end point was met, with 92% of patients demonstrating a 3-point-or-greater increase in the Hammersmith Functional Motor Scale Expanded assessment score at a postbaseline visit.15 Intrathecal administration in treatment-naïve patients with SMA who have achieved sitting but are nonambulatory and are between ages 2 and 17 years will be assessed in the new phase 3 STEER trial (NCT05089656).16 The potential for additive benefit with combination or sequential therapy has been considered and is being explored in the RESPOND study (NCT04488133), in which safety and efficacy of nusinersen (Spinraza; Biogen) administration in patients aged 2 to 36 months who have already received onasemnogene abeparvovec is being investigated.17
DMD is a neuromuscular disease for which gene-transfer therapy is also being explored. DMD affects 1 in 3500 male births and is an X-linked disorder characterized by progressive muscular weakness that typically leads to loss of ambulation by aged 13 years, as well as neuromuscular restrictive lung mechanics with respiratory failure and cardiac failure leading to early death, usually by the third or fourth decade of life.18
The DMD gene encodes the protein dystrophin, which serves to anchor the actin cytoskeleton to the extracellular matrix and maintains muscle structural integrity during contraction. Patients with DMD have mutations that disrupt the production of full-length dystrophin protein, leading to muscle breakdown and infiltration of the muscle with fat and fibrosis over time. The DMD gene is the largest gene in the body at 14 kb, making it too large to package completely in a viral capsid for gene transfer. Although, the description of a gentleman with a very mild case of Becker muscular dystrophy who was ambulatory at age 61 years, despite deletion of 46% of the coding region of the DMD gene, invoked consideration of the potential for critical regions of the dystrophin gene.19 Clinical trial programs have created “micro” or “mini” dystrophin transgenes that include some of these particular critical regions packaged into AAV vectors. Companies exploring this therapeutic strategy have selected different AAV serotypes, unique components of the transgene, and specific promoters for their products. Sarepta, Solid Biosciences, and Pfizer have explored administration of their products in patients with DMD in clinical trials.
Sarepta’s product SRP-9001 (microdystrophin) was first administered to 4 boys aged 4 to 6 years in the 101 trial (NCT03375164) who demonstrated a mean improvement of 7.5 points in the North Star Ambulatory Assessment (NSAA) over 3 years. This was in comparison to what is known of natural history, whereby NSAA peaks at aged 6 years with a rate of decline of 3 points per year thereafter.20 The 102 clinical trial (NCT03769116) examined administration of microdystrophin to boys aged 4 to 7 years at enrollment. The biopsy from week 12 post treatment demonstrated 23.8% microdystrophin expression by Western blot. Functional end point data at 48 months was examined in the 4-to-5-year-old group as well as the 6-to- 7-year-old group. The 4-to-5-year-old boys who received SRP-9001 showed a statistically significant difference in NSAA score at 48 months post treatment compared with placebo. The 6-to- 7-year-old boys were randomized for age, but not NSAA score, and were ultimately found to be not well matched at baseline, making comparison of NSAA score at 48 months challenging when examining treatment vs placebo.21
The next study of SRP-9001, 103 ENDEAVOR study (NCT04626674), began enrollment in December 2020, examining administration of gene transfer with commercially processed material. The study is ongoing and including ambulatory as well as nonambulatory boys. Data for the first 11 boys aged 4 to 7 years at enrollment demonstrated a 55.4% change in microdystrophin expression from baseline by Western blot at 12 weeks post treatment. There was a mean improvement in NSAA score of 3 points from baseline at 6 months in these first 11 boys as well. The most common AEs included vomiting and transaminitis. No clinically relevant complement activation has been observed.22 SRP-9001 301 EMBARK (NCT05096221) is the next phase of Sarepta’s clinical trial program, and is now open for enrollment.
Solid Biosciences’ product SGT-001 has been administered at enrollment in the phase 1 IGNITE DMD trial (NCT03368742) including boys aged 4 to 17 years. An interim analysis of the 3 high-dose subjects demonstrated widespread distribution of microdystrophin-positive muscle fibers. The average microdystrophin protein level in subjects in the high-dose cohort via Western blot at day 90 was approximately 10% of normal dystrophin. Differences in NSAA scores over 1 year following treatment were noted relative to trajectories typically seen in natural history. There were improvements in 6-minute walk test distance in both the low-dose and high-dose groups at 1 year.23 A clinical hold that was placed on the trial program after concern for complement activation leading to patient hospitalization has since been lifted. No new drug-related safety findings have been identified and all previously reported serious AEs have fully resolved. The most common drug-related AEs include nausea, fever, and vomiting.
Pfizer’s product, PF-06939926, has been evaluated in a phase 1 clinical trial (NCT03362502) of minidystrophin. Expression of minidystrophin was evaluated at 2 and 12 months post treatment, demonstrating sustained protein expression. NSAA scores demonstrated improvement 1 year after treatment compared with natural history.24 The most common AEs were vomiting, nausea, decreased appetite, and pyrexia. Serious AEs included persistent vomiting resulting in dehydration, acute kidney injury with atypical hemolytic uremic syndrome-like complement activation, and thrombocytopenia with complement activation. The FDA placed a clinical hold on the trial program December 20, 2021, after the death of a patient in the phase 1 study.
Safety remains a priority, and mitigating AEs involves great consideration for immune responses related to gene transfer. Deaths in the ASPIRO clinical trial (NCT03199469) for patients with another neuromuscular disease, X-linked myotubular myopathy, revealed evidence of comorbid hepatobiliary disease at baseline that appeared to increase risk for worsening cholestasis and liver failure, with some fatal liver dysfunction observed in that clinical trial, prompting a clinical hold.25,26 Hepatotoxicity with transaminitis has been most commonly observed in the SMA and DMD programs and appears to be steroid responsive when related to cytolysis rather than synthetic liver dysfunction and failure. To optimize safety, programs have focused on evaluating baseline liver function, ensuring no recent or current infection at the time of drug administration, and administering corticosteroids prior to and following drug administration.
Great enthusiasm surrounds the promise of gene transfer therapies, both within and beyond neuromuscular medicine. The efficacy of the SMA gene transfer program has marked a significant milestone in the journey to alter natural history for patients with neuromuscular disease. What was once a goal to just survive has shifted to a focus on how to optimize and thrive. With scientific advancement has also come a respect for the accompanying risk and the unknowns that persist. There is still much to be learned about gene transfer. Setting expectations for potential therapeutic outcomes remains an important consideration as opportunities evolve. In addition, although these pharmacologic strategies change the model of treatment, comprehensive multidisciplinary care remains a core component of the therapeutic strategy for patients with nerve and muscle disease.