Published in Human Gene Therapy on July 11, 2025
Download article PDF here: https://www.liebertpub.com/doi/epub/10.1177/10430342251359668

The Responsible Development of
Adeno-Associated Virus Gene Therapies

James M. Wilson 1 and Arthur L. Caplan 2

1 GEMMA Biotherapeutics Inc., Philadelphia, Pennsylvania, USA, 2 NYU Grossman School of Medicine, New York, New York, USA.
*Correspondence: Dr. James M. Wilson, GEMMA Biotherapeutics Inc., Suite 1200 TRL, 125 S. 31st Street, Philadelphia, PA 19104, USA. E-mail: jim.wilson@gemmabiotx.com

Abstract

Several unexpected fatalities in patients who received adeno-associated virus (AAV)-based gene therapies have recently occurred. These tragic events have cast a pall over the entire sector with some stakeholders suggesting that AAV is patently unsafe as a gene delivery platform and ought not to be pursued. This conclusion is not warranted.

Virtually all the reported fatalities and other non-lethal systemic toxicities have occurred in trials that require the injection of large doses of AAV into the circulation to target central nervous system, heart, and muscle tissues. The only exception is a fatality in a patient that received high-dose vector into the cerebral spinal fluid, with enough leaking into the periphery to cause systemic toxicity.1–3 This contrasts with the relatively good safety profile of the broader portfolio of AAV products that deliver lower doses of systemic vector or use localized routes of administration. In fact, AAV is remarkable for being less proinflammatory than most other viral and non-viral delivery systems and, in some settings, capable of suppressing adaptive immune responses.4–6

Prior experience with toxicity following systemic delivery of adenoviral vectors resulted in severe and potentially lethal systemic inflammation due to an acute activation of innate immunity.7 Manifestations of systemic toxicity following high doses of AAV in humans have been more heterogeneous with a full range of clinical syndromes being described that includes acute liver failure,8,9 atypical hemolytic uremic syndrome, thrombotic microangiopathy, hyperinflammatory syndrome, and capillary leak syndrome.10–12

Critical to developing effective mitigation strategies for these toxicities is an understanding of the inciting host-vector interactions that activate destructive downstream pathways. Our work in non-human primates13 and in some human gene therapy recipients14 indicates that the most proximal event is injury of endothelial cells that activates several host inflammatory pathways. This dose-dependent endothelial cell injury can explain many of the clinical findings such as thrombocytopenia due to sequestration of platelets on the surface of activated endothelial cells, disseminated intravascular coagulation, liver damage and fibrosis, increased vascular permeability, and complement activation.

In an attempt to reduce systemic toxicity, sponsors have utilized a variety of immune-modulating strategies, with prophylactic corticosteroids being the mainstay. More complex drug cocktails were deployed prophylactically as toxicities in clinical trials continued unabated. In addition to steroids, rapamycin and tacrolimus have been used to dampen T cells,15,16 Rituximab to suppress B cells and Eculizumab and Pegcetacoplan to inhibit complement. It is generally believed that aggressive immune modulation can improve the safety profile of high-dose AAV, although it has been difficult to clinically prove this. Furthermore, there is concern about the safety of prolonged immune suppression and the potential to pervert host defenses in a way that may worsen vector toxicity. This was exemplified in the AAV gene therapy trial for Danon disease where co-treatment with complement inhibitors possibly contributed to a lethal episode of capillary leak syndrome.17 However, the use of interleukin-6 inhibitors in the treatment of cytokine release syndrome associated with chimeric antigen receptor T-cell therapies illustrates how impactful targeted immune modulation can be in significantly improving the safety of cell and gene therapy products.18

It should not be surprising that AAV, like any biological product, is associated with dose-limiting toxicities. Unfortunately, it is necessary to push the dose of first-generation vectors close to the limit of tolerability when attempting to target the brain, muscle, and heart. Diseases involving trials that utilize high doses of AAV, such as Duchenne muscular dystrophy and inherited cardiomyopathies, are associated with a substantial need, allowing for a higher tolerance of risk for patients and families. However, it is crucial to find ways to improve the safety profile of these products and do so quickly, since these rapidly degenerating diseases do not afford patients the privilege of time.

Genetic medicines provide unique opportunities to leverage experiences within platforms to inform critical issues such as probabilities of success, risks, and benefits. For this to occur, sponsors must commit to more transparency about clinical data especially when it involves adverse events that could read through to patient safety across programs. However, there are limitations to the extrapolation of clinical experiences across different AAV products and different baseline patient populations even when data are shared. Just because AAV worked in treating an inherited retinopathy does not mean it will work when treating a liver metabolic disease. Likewise, one should not assume that the lethal systemic toxicity observed following high-dose AAV is relevant to applications that utilize much lower doses of vector. Broad generalizations about genetic platforms that are not based on shared scientific evidence do a disservice to scientific understanding and to the communities and patients we try to serve.

AUTHOR DISCLOSURE
J.M.W. is an employee and shareholder of GEMMA Biotherapeutics, Inc., and a professor at the University of Pennsylvania. J.M.W. and GEMMABio also have relationships with and may receive payments from several companies in the gene therapy space, including but not limited to iECURE, Inc., Passage Bio, Inc., the Center for Breakthrough Medicines, Franklin Biolabs, Ceva Santé Animale, and MavriX Bio LLC.

FUNDING INFORMATION
No funding was received for this article.

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