Beyond AAV: How Extracellular Vesicles Are Opening a New Chapter in Duchenne Gene Therapy

Spot Biosystems launched in June 2026 with $40 million in financing and a notable first: the non-viral delivery of full-length dystrophin into human patients with Duchenne muscular dystrophy. The company's platform uses extracellular vesicles, the body's own natural delivery system, to sidestep the cargo size limits and immunogenicity concerns that have constrained AAV-based gene therapies. In an ongoing investigator-initiated clinical trial, two pediatric patients with DMD showed dystrophin increases of more than 1,000% and 2,000%, along with functional muscle improvements lasting six months after dosing stopped.

The animal data underpinning the platform was published in Nature Biomedical Engineering, and the company's approach has drawn attention for its potential to enable repeat dosing, something AAV-based therapies currently cannot offer. NeurologyLive spoke with Andrew Lee, MD, physician-scientist, founder, and CEO of Spot Biosystems, to discuss what this platform could mean for the DMD treatment landscape.

In this Q&A, Dr. Lee explains the rationale behind using extracellular vesicles over AAV, what delivering full-length dystrophin offers over microdystrophin approaches, what the early clinical findings suggest, and how the company envisions non-viral gene delivery fitting into the broader future of DMD treatment.

NeurologyLive: Can you discuss the rationale behind using extracellular vesicles as a delivery platform, and how this approach may address some of the limitations associated with AAV-based gene therapies in Duchenne muscular dystrophy?

Andrew Lee, MD: For a disease like Duchenne, AAV falls short in several ways. The first has to do with cargo size: AAV can only carry roughly the smallest 20% of human genes. The second concerns the inherent immunogenicity and toxicity of the AAV vector itself, which has resulted in a number of adverse reactions and patient deaths over the past decade. Third, patients can mount neutralizing antibodies to the AAV capsid, which effectively limits treatment to one dose per lifetime.

Extracellular vesicles let us sidestep these issues. EVs are the body's own delivery machinery, developed over thousands of years of evolution to deliver genetic cargo from cell to cell. That means there is no viral capsid to provoke an immune response, and they can accommodate large genetic cargo like full-length dystrophin. Because they are non-viral, we can also dose EVs repeatedly, which matters for a progressive disease that needs sustained protein replacement rather than a one-shot fix.

What potential advantages does full-length dystrophin offer compared with microdystrophin approaches, from both a biologic and clinical perspective?

Full-length dystrophin has been out of reach for a simple reason: the gene is one of the largest in the human genome, far too big to fit inside an AAV vector. Our Nature Biomedical Engineering paper showed, for the first time, that the full-length protein could be delivered to skeletal muscle non-virally, which has reframed what is possible. Microdystrophin was a response to the cargo constraints of AAV, and it retains a functional subset of the full-length protein's domains. But the trimmed domains are not all inert. Some carry functions tied to muscle signaling, for example, so microdystrophin provides only partial functionality compared to the full-length protein.

Because our EVs are not limited by cargo size, we can deliver the entire sequence. Whether that beats microdystrophin clinically is still open, and more studies will give us the answer. But delivering the whole gene and therefore the whole protein has been the ideal therapy for many decades, and we finally have a system that makes it feasible.

The initial clinical experience showed substantial increases in dystrophin expression along with functional improvements. What have these early findings suggested about the potential of this platform, and what questions still need to be answered?

Our preliminary clinical results are very encouraging. In the first two pediatric patients dosed, we achieved safe delivery of full-length dystrophin to skeletal muscle, which is the first time that has been reported in humans. We also observed substantial increases in dystrophin expression after a month of dosing and functional muscle improvement that persisted for up to six months after dosing stopped.

But we are still in an early stage. This is a two-patient, first-in-human cohort. What these results do suggest is that the platform's core capabilities, first demonstrated in animals, are now showing early signs of success in patients. We continue to study durability beyond our current follow-up, how the response behaves across a larger group of DMD patients, and how the safety profile holds up in greater numbers.

Safety remains a major consideration for gene therapies. How might a non-viral delivery strategy influence repeat dosing potential, immunogenicity, or long-term treatment durability?

With AAV, immune recognition of the capsid and immunogenicity to the viral load itself are the main limiting factors. Currently, patients are only allowed one AAV dose per lifetime. The larger challenge is viral immunogenicity resulting from the viral load of AAV as a whole, which has resulted in a number of patient deaths due to liver, cardiac, and blood toxicities over the past decade.

Our vesicles are allogeneic, GMP-grade extracellular vesicles, so there is no viral capsid to react against. In mice, primates, and our first two human patients, repeated intravenous dosing resulted in sustained dystrophin expression with no observable toxicity in the liver, heart, blood, or other organs. That means serial dosing is not off the table the way it is with AAV.

For a disease where you are replacing a structural protein that turns over, redosing arguably matters more than any single administration, because durability becomes a function of a repeatable regimen. It is worth noting that we did administer tacrolimus during the dosing period and for one month afterward to minimize immune rejection of the full-length dystrophin itself, as our patients' immune systems may recognize certain protein motifs as foreign. We would not require tacrolimus if we were delivering EVs without the dystrophin cargo, as the EV delivery vectors themselves are low immunogenicity.

Looking ahead, what milestones will be most important as this program advances, and how do you envision non-viral gene delivery fitting into the future treatment landscape for Duchenne muscular dystrophy?

Near term, we are looking at more patients and longer follow-ups for our trial in Shanghai, then using that data to inform and accelerate a future US regulatory pathway. We hope a redosable, non-viral approach will ultimately provide more options to DMD patients whom the current toolkit does not serve well.

Longer term, the platform is not specific to dystrophin. We started with DMD because the need is acute and it is where we have proof of concept. But this technology offers a non-viral method to carry larger types of genetic cargo that AAV cannot, and it is relevant to many conditions needing repeated protein replacement. Get it right in DMD, and we would be opening a door for an entirely new type of gene therapy that could treat diseases which AAV and LNPs currently struggle to reach.

REFERENCE
1. Tian Y, Liu Y, Tong Y, et al. Skeletal-muscle-targeted non-viral delivery of full-length DMD mRNA for Duchenne muscular dystrophy. Nature Biomedical Engineering. Published online June 11, 2026. doi:10.1038/s41551-026-01689-5