SMA gene therapy EXG001-307 extends survival in model mice through regulated expression and cerebrospinal fluid delivery
Coordinating SMN expression levels and delivery site
The spinal muscular atrophy (SMA) gene therapy candidate EXG001-307 improved survival and motor function in neonatal SMA model mice in a dose-dependent manner. Median survival in the highest tested dose group was 373 days, compared with 15 days in the vehicle control group. Published in Molecular Therapy, this preclinical study suggests the potential of adjusting tissue-specific expression levels and the route of administration together, rather than simply increasing therapeutic gene expression.
Existing SMA gene therapies have shown clinical benefit, but safety concerns such as hepatotoxicity have also been reported. The researchers focused not only on the amount of viral vector administered but also on excessive expression of the SMN protein produced by the delivered gene. EXG001-307 combines a human synapsin 1 (hSyn1) promoter and a CMV enhancer as gene-expression regulatory elements in an adeno-associated virus serotype 9 (AAV9) vector. Its design goal was to achieve the expression needed in the central nervous system while reducing excessive expression in peripheral tissues.
A candidate with higher expression was not always advantageous
The researchers compared several gene constructs in SMNΔ7 mice and used survival as the primary screening measure. EXG001-340, which incorporates codon optimization of the SMN gene in EXG001-307 to increase expression, had a median survival of 205 days at an intravenous dose of 2 × 10¹⁴ vg/kg, longer than the 35 days for EXG001-307. Here, vg denotes the number of vector genomes.
However, at 4 × 10¹⁴ vg/kg, median survival with EXG001-340 fell to 29.5 days, while median survival with EXG001-307 was 191 days. The researchers interpreted this as supporting the possibility of harm from excessive SMN expression. However, they did not conduct comprehensive necropsy and histopathology assessments in the high-dose EXG001-340 group, so neither the cause of early death nor a safe expression range was established.
Mouse efficacy and primate toxicity studies should be considered separately
In the dose-finding study, SMA model mice received a single intracerebroventricular injection of EXG001-307 at one of four doses on postnatal day 1. Median survival was 23 days at the lowest dose, 2 × 10¹⁰ vg per mouse, and 373 days at the highest dose, 2 × 10¹¹ vg per mouse; the longest survival was 501 days. Weight gain, the righting reflex, and locomotor ability also improved.
In a separate comparison of administration routes, intracerebroventricular delivery produced more favorable results than intravenous delivery. However, survival data from another experiment were reused in the comparison, and the doses differed, making it difficult to attribute the difference to route alone.
Safety studies in rats and juvenile cynomolgus monkeys used lumbar intrathecal administration, the route intended for clinical development. Both intracerebroventricular and lumbar intrathecal administration deliver the vector into the cerebrospinal fluid, but the injection sites differ. In addition, cerebrospinal fluid delivery does not confine the vector to the brain and spinal cord. Vector genomes were also detected in peripheral tissues, including the liver and heart, in monkeys.
Transient abnormal findings and limits of the clinical evidence
In rats, no treatment-related lesions were found in the spinal cord or dorsal root ganglia after 16 weeks of observation. In the toxicity study of 32 monkeys, minimal or mild lesions were observed at week 4 in the dorsal root ganglia (DRG), where sensory neuron cell bodies are located. Similar findings were present in control animals, although the degree of axonopathy was slightly greater in some animals in the high-dose group. At necropsy at week 26, these lesions had resolved.
In one high-dose monkey, the liver enzymes AST and ALT transiently rose on day 15 after dosing to 7.96-fold and 2.6-fold their respective baseline levels, then recovered by day 27. Thus, it would be inaccurate to say there were no liver-enzyme elevations or DRG lesions. Based on the overall assessment, the researchers considered the highest tested dose to be the no-observed-adverse-effect level (NOAEL).
The paper states that three patients had already received treatment in an investigator-initiated clinical trial (NCT06888661). However, it provides no detailed patient characteristics, endpoints, or follow-up results, so efficacy in humans or superiority over existing treatments cannot be assessed. The monkey study also did not directly compare the candidate with a control vector based on an existing treatment design. Some authors disclosed equity holdings in or employment with the developer, as well as interests in related patents.
The study design and results can be reviewed in the full paper and the bibliographic and abstract record.
Spinal muscular atrophy (SMA) is a severe neurogenetic disorder and the leading inherited cause of infant mortality. Although the available gene therapy has shown substantial efficacy in treating SMA, safety concerns, including hepatotoxicity underscore the need for optimization. In this study, our findings challenge the prevailing "more-is-better" paradigm, demonstrating that both insufficient and excessive transgene expression are suboptimal. To maximize therapeutic benefit while minimizing risk, we extensively optimized the expression cassette in AAV-based constructs and selected EXG001-307, an intra-CSF delivered AAV9 gene therapy engineered for tissue-selective and quantitatively controlled SMN expression . Comparative studies demonstrated that intra-CSF administered EXG001-307 may offer improved efficacy and safety relative to the other candidates. In subsequent IND-enabling studies, EXG001-307 demonstrated dose-dependent improvements in survival, body weight gain, and motor function in SMA model mice. Safety evaluations in rats and juvenile nonhuman primates further confirmed a favorable safety profile, with no evidence of systemic toxicity or sustained DRG pathology. Analyses in vector genome biodistribution and transgene expression revealed robust CNS transduction consistent with therapeutic benefit. These findings support clinical translation of EXG001-307 and highlight the importance of vector genome engineering and targeted delivery for safe and effective gene therapies.
This study provides evidence that vector dose, strength of gene expression, and tissue distribution should be considered together when evaluating SMA gene therapy candidates. The expression-enhanced variant performed better at a lower dose but worse at a higher dose, showing the difficulty of selecting candidates on the basis of increased expression alone. However, the specific mechanism by which overexpression may have caused early death and the safe range of expression require further investigation. Improvements in survival and motor function in mice, together with toxicity and distribution assessments in monkeys, support clinical development but do not establish long-term safety in patients or an advantage over existing treatments. Follow-up clinical studies should assess both efficacy and liver and sensory nervous system safety after lumbar intrathecal administration.