Gene therapy based on adeno-associated virus (AAV) has come a long way and has been used for more than 20 years in over 2,000 patients. While AAV based gene therapy is regarded as generally safe, the field has experienced several setbacks recently: some trials were halted for safety reasons and product approvals were delayed.
The reasons for these developments are complex, but gene therapy is nowadays not only applied to a narrow spectrum of rare diseases with no other treatments available but is also beginning to be used as a therapeutic option in more common diseases. Consequently, many more patients are being treated and several limitations have surfaced:
- The quality of the clinically used vector material is often, despite cGMP manufacturing requirements, insufficient to prevent adverse events
- For difficult to access tissues, site-specific administration and delivery of gene therapy vectors can be an important barrier
- The database to demonstrate long lasting therapeutic effects in clinical trials is often thin and the interpretation of results can be difficult
- Patients experience serious adverse events after escalating to high AAV doses
The lack of AAV materials with high and reproducible quality, particularly the presence of substantial amounts of empty capsids is, at times, making the interpretation of controlled clinical trials difficult. Insufficient product quality in combination with very high AAV vector doses often leads to a massive presence of capsids in the liver, causing inflammatory responses and raising safety concerns. In addition, AAV vectors with a desired tropism towards specific tissues or organs and a high yield manufacturability are scarce, thereby limiting the applicability of AAV gene therapy. Yet another complication is that current AAV-based gene therapy technology cannot offer a once in a lifetime treatment in many cases, requiring a repeated dosing (if feasible) in case of therapeutic effects diminishing over time.
Adding to these issues is the fact that about 50 percent of the population already has some immunity to AAV so that not all affected patients will benefit from AAV-based gene therapies.
How to improve AAV-based therapies?
Evotec has identified and addressed a number of issues which were presented in our recent webinar titled “AAV Gene Therapy: Closing the Translational Gap”. According to Hanspeter Rottensteiner, VP, Head of In Vitro Gene Therapy of Evotec Austria, the most important needs are to:
- Improve the delivery vectors by developing capsids with better tissue tropism and translating into improved efficacy and safety. Also, promoters and the respective transgene need to be optimized to improve cell-type specificity, controllability, and durability
- Gain better insights into the effects of gene therapy by developing better biomarkers
Improve tracking and analyzing of side effects, for instance by tracking protein expression and metabolic fluxes with proteomic and metabolomic platforms and via high-throughput sequencing of RNA and DNA in target and non-target tissue and cells
Improved specificity
Dirk Grimm, Professor for Viral Vector Technologies at Heidelberg University, explains in the webinar that - thanks to new powerful technologies - knowledge about the host factors that influence AAV vector transduction has increased tremendously. In addition, many new technologies allow for the rational design of next-generation AAV capsids. These novel vectors not only avoid neutralizing anti-AAV antibodies circulating in the human population but also outperform wild-type capsids in terms of potency and / or specificity. Grimm demonstrated as an example that rationally designed myotropic AAV capsids with a tropism exclusively for muscle cells did not accumulate in the liver at all. They demonstrate a substantial improvement in terms of specificity as compared to capsids in use today. The latest technology advancement – barcoding of capsids and nucleic acids - allows for tracking capsids not only to specific organs and tissues but can also tell whether a functional transduction has taken place. Grimm added that the increased use of artificial intelligence will also make animal experiments largely obsolete. In his view, it will be possible in the future to design capsids tailormade to individual patients and their disease.
Filling the gaps
Werner Höllriegl, VP, Head of In Vivo Gene Therapy at Evotec GT GmbH, detailed in the webinar the current gaps in translational efficacy and safety that Evotec is addressing when preparing first-in-human trials. Great efforts are made to assess toxicity in liver, cardiovascular and nervous systems, but also for evaluation of potential oncogenic effects.
Höllriegl also mentioned gaps that cannot be filled by Evotec alone but require a concerted effort by the gene therapy community. Among them are lacking industry standards for AAV titers and dose assessments and guidance from the regulatory agencies on the design of preclinical studies that can best inform IND decisions.
He stressed, however, that even more research at basic and translational science level is needed to tackle specific efficacy or safety issues.
Comprehensive safety assessment
Evotec also brings its already long-established strong expertise in prediction of drug-induced liver injury to the table – an experience gained with small molecules, as explained by Rüdiger Fritsch, Principal Scientist, Metabolic Diseases at Evotec. Among other safety assessments, the Company uses a wealth of technologies and expertise to identify transcriptomic fingerprints and applies its proprietary and continuously growing database of tox-related data from hundreds of tested compounds. AI-based modeling can predict mechanisms and probabilities for AAV-induced liver toxicity. Moreover, using single nucleic RNA-sequencing tools, Evotec can study and profile the expression and activity of the transgene at single cell level – not only in liver cells and organoids but in a growing portfolio of tissues from brain, muscle, lung, heart, and kidney. Evotec thereby is successfully translating its toxicology expertise from the small molecule space to gene therapy research and development, offering a 21st century omics-approach to provide for safe and efficacious gene therapy vectors.
Stream the webinar