How to advance AAV-based gene therapies

AAV virus

Sara Donnelly, Director of Research Planning and Business Development at PhoenixBio USA explores why the right pre-clinical model is essential for teams wanting to advance adeno-associated virus vector-based gene therapies.  

Adeno-associated virus (AAV) vector-based gene therapies hold exceptional promise across a range of disease areas. We’re beginning to see their potential come to fruition, with the FDA having approved three treatments as of January 2023 — for retinal dystrophy1 spinal muscular atrophy,2 and haemophilia B.3The world’s confidence in the future of these therapies is also clear, as the global AAV vector manufacturing market has a projected compound annual growth rate (CAGR) of 22.5% by 2030.4  

Unfortunately, many AAV-based gene therapies fail during clinical trials, despite them appearing safe and efficacious in preclinical studies. One of the most significant roadblocks is successfully translating preclinical data to clinical trials, as commonly used preclinical animal models struggle to accurately predict transduction efficiency, biodistribution, and toxicity in humans.  

To streamline the path to clinical trials and increase the chances of securing regulatory approval, assessing which preclinical model enables better prediction of how your therapy will behave in humans is vital.  

This article looks at the challenges that AAV vector-based gene therapies face, examines the different preclinical model options available, and explores how choosing the right one can help developers progress only the most promising treatments to clinical trials.  

Promising therapies are stumbling in clinical trials

AAV vector-based gene therapies use modified viruses as drug delivery vehicles to effectively introduce DNA sequences into cells via transduction. Such therapies have become an appealing drug modality for multiple conditions, including monogenic disorders, cancers, and infectious diseases. Several factors make AAV vector-based gene therapies promising:  

  • An ability to infiltrate hard-to-reach cells with high specificity  
  • Apparent biosafety  
  • Potential long-term therapeutic effects 

However, AAV vector-based gene therapies are not living up to their full potential, as what seems to work well in preclinical models doesn’t always translate effectively into human trials. For example, researchers may find that an AAV vector-based gene therapy displays excellent transduction efficiency in a preclinical model, only to see the efficiency drop in clinical trials. This disconnection between preclinical and clinical outcomes poses one of the most significant challenges for AAV vector-based gene therapy development today.  

Preclinical model choice makes or breaks a study

Preclinical animal model choice is crucial in the development of AAV vector-based gene therapies, as the wrong one may not accurately predict human therapeutic outcomes. But two of the most frequently used models — non-human primates (NHPs) and mouse models — struggle to predict this.  

NHPs are a useful model for many researchers, owing to their physiological and metabolic similarity to humans. However, NHP models cannot fully predict AAV transduction in the human liver. What’s more, they are expensive, not readily available, and in many countries, there are a host of stringent regulations, as well as ethical and legal considerations around their use.  

Mouse models, on the other hand, are cost-effective and readily available, making them a favourite for many researchers. Additionally, they are globally accepted for use in preclinical studies. However, significant species-level differences between mouse and human mean that these models often show large discrepancies in AAV distribution, efficacy and hepatotoxicity between preclinical study and clinical trial data. For example, AAV vector-based gene therapies often show high transduction efficiency in wild-type mouse strains, such as C57BL/6, but low levels of human hepatocyte transduction in clinical trials,5 limiting the model’s usefulness in preclinical studies.  

Overall, while NHP and mouse models both have different strengths, neither model expresses human genes, mRNA, or proteins — meaning they fail to accurately reflect distribution, efficacy, and toxicity in humans. Consequently, researchers are progressing treatments that are unlikely to succeed at clinical trials, while rejecting potentially powerful ones. As such, companies urgently need more predictive models that enable them to progress the most promising AAV vector-based gene therapies to clinical trials. 

Boost predictive power with humanized liver mouse models

A chimeric mouse model with a humanized liver can bridge the gap between preclinical studies and clinical trials. Particularly, they can help researchers assess late-stage development candidate transduction efficiency, and give them confidence in efficacy and safety study data. But why are these models more accurate than traditional ones? 

Chimeric mice with humanized livers are engrafted with human hepatocytes,6 and in some cases feature up to a 95% engraftment rate. As such, they offer highly stable expression of human genes, mRNA, and proteins. Their livers even secrete human albumin, and possess functional human enzymes and transporters, closely mirroring the physiological conditions of a human liver (Figure 1).  

Figure 1: Chimeric mice with humanized livers closely mirror the physiological conditions within a human liver.

Accordingly, chimeric mouse models with humanized livers can provide more physiologically relevant data, overcoming the challenges researchers face with traditional preclinical models. The greater data translatability gives developers the confidence to progress the most promising treatments and reduces clinical failures, thereby slashing project costs and timelines. What’s more, rodents are used widely across preclinical R&D, so most drug development labs already have the infrastructure to utilise them. The incorporation of chimeric mouse models in research programs is therefore straightforward compared to undertaking NHP or other large animal studies. 

Accelerate your gene therapy project

Developing AAV vector-based gene therapies can be challenging. To progress only the most promising treatments to clinical trials, companies need to overcome data discrepancies between preclinical studies and clinical trials. Choosing the right animal model for preclinical studies is a vital step to reduce this discrepancy and improve data translatability.  

Chimeric mouse models with humanized livers can provide more physiologically relevant data during preclinical studies, better reflecting transduction efficiency in clinical trials. As such, drug developers can progress only the most promising AAV vector-based gene therapies to clinical trials, and ultimately become one step closer to realising the full potential of these treatments.  

Author Biography

Sara Donnelly is Director, Research Planning and Business Development at PhoenixBio USA, where she oversees a portfolio of studies involving humanized liver chimeric mice (the PXB-mouse). She received a Ph.D. in Cell Biology and Biochemistry from University College London and was a post-doctoral fellow at Albert Einstein College of Medicine in New York City. 

References

  1. U.S. Food & Drug Administration (2017) FDA approves novel gene therapy to treat patients with a rare form of inherited vision loss. Accessed 26 Jul 23. Available at https://www.fda.gov/news-events/press-announcements/fda-approves-novel-gene-therapy-treat-patients-rare-form-inherited-vision-loss 
  2. U.S. Food & Drug Administration (2019) FDA approves innovative gene therapy to treat pediatric patients with spinal muscular atrophy, a rare disease and leading genetic cause of infant mortality. Accessed 26 Jul 23. Available at https://www.fda.gov/news-events/press-announcements/fda-approves-innovative-gene-therapy-treat-pediatric-patients-spinal-muscular-atrophy-rare-disease 
  3. U.S. Food & Drug Administration (2022) FDA Approves First Gene Therapy to Treat Adults with Hemophilia B. Accessed 26 Jul 23. Available at https://www.fda.gov/news-events/press-announcements/fda-approves-first-gene-therapy-treat-adults-hemophilia-b 
  4. Grand View Research, Adeno Associated Virus Vector Manufacturing Market Size, Share & Trends Analysis Report By Scale Of Operations (Clinical, Commercial), By Method, By Application, By Therapeutic Area, By Region, And Segment Forecasts, 2023 – 2030. Accessed 26 Jul 23. Available at https://www.grandviewresearch.com/industry-analysis/adeno-associated-virus-vector-manufacturing-market-report 
  5. Lisowski, L. et al. (2014) Selection and evaluation of clinically relevant AAV variants in a xenograft liver model, Nature, 506(7488), 382–386. doi:10.1038/nature12875. 
  6. Sugahara, G. et al. (2020) Art of making artificial liver: Depicting human liver biology and diseases in mice, Seminars in Liver Disease, 40(02), 189–212. doi:10.1055/s-0040-1701444. 

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