Gene Therapy Advances: A Meta-Analysis of AAV Usage in Clinical Settings

Abstract

Adeno-associated viruses (AAVs) are the safest and most effective gene delivery vehicles to drive long-term transgene expression in gene therapy. While animal studies have shown promising results, the translatability of AAVs into clinical settings has been partly limited due to their restricted gene packaging capacities, off-target transduction, and immunogenicity. In this study, we analysed over two decades of AAV applications, in 136 clinical trials. This meta-analysis aims to provide an up-to-date overview of the use and successes of AAVs in clinical trials, while evaluating the approaches used to address the above challenges. First, this study reveals that the speed of novel AAV development has varied between therapeutic areas, with particular room for improvement in Central Nervous System disorders, where development has been slow. Second, the lack of dose-dependent toxicity and efficacy data indicates that optimal dosing regimes remain elusive. Third, more clinical data on the effectiveness of various immune-modulation strategies and gene editing approaches are required to direct future research and to accelerate the translation of AAV-mediated gene therapy into human applications.

Keywords: adeno-associated virus; clinical trials; gene therapy; promoters; tropism.

Figure 1

Adeno-associated virus (AAV) vector biology. The natural AAV serotype contains three open reading frames flanked by two inverted terminal repeats (ITR)—Cap (capsid) and Rep (replication). In recombinant AAVs (rAAV), the viral genome is replaced by a synthetic expression cassette containing a promoter, transgene of interest and a terminator such as polyadenylation (polyA) sequence, flanked by the ITRs. Cap and rep sequences are supplied as a plasmid in trans, which will produce the viral capsid that packages the expression cassette. To create a pseudo-typed vector (e.g., AAV2/1), the cap genes from another AAV serotype (e.g., AAV1) can be used to package the recombinant genome of another serotype (e.g., AAV2). rAAVs can be customised at the capsid and promoter level. The capsid gives AAV its tissue tropism, meaning the cells it infects, while the promoter drives either ubiquitous or tissue-specific expression of the transgene.

Figure 2

Overview of the experimental design used in this analysis. Two hundred forty-seven clinical trials were first identified through a keyword search for “AAV” on ClinicalTrials.gov. Two hundred seven interventional studies were screened, during which irrelevant studies, false positives and duplicates were removed manually. One hundred thirty-six trials were ultimately included for the meta-analysis.

Figure 3

Distribution of adeno-associated virus (AAV)-mediated gene therapy in clinical trials. Each panel represents number of clinical trials by (A) current status of the clinical phase, (B) therapeutic area, and (C) gene therapy approaches. BD, Blood disorders; CNS, Central Nervous System; ED, Eye Disorders; LSD, Lysosomal storage disorders; NMD, Neuromuscular Disorders.

Figure 4

Adeno Associated Virus (AAV) capsid usage and frequency in clinical trials. (A) Overall AAV capsid type usage across all clinical trials. (B) Capsid design across therapeutic areas. BD, Blood disorders; CNS, Central Nervous System; ED, Eye Disorders; LSD, Lysosomal storage disorders; NMD, Neuromuscular Disorders. The list of capsid descriptions can be found in Supplementary Table 2, Supplementary Figure 1.

Figure 5

A summary of promoter choice for AAV gene therapy delivery in clinical trials. (A) Overall promoter distribution in all clinical trials. (B) Promoter type across therapeutic areas. BD, Blood disorders; CNS, Central Nervous System; ED, Eye Disorders; LSD, Lysosomal storage disorders; NMD, Neuromuscular Disorders. The list of promoter abbreviations can be found in Supplementary Table 3, Supplementary Figure 2.

Figure 6

A summary of dosage regime of AAV administered in clinical trials. Dosages were reported in viral genomes (vg) administrated per patient per dose, via either systemic or targeted administration. (A) Range of doses in systemic administration, (B) Percentage of clinical trials with specific AAV dose windows for systemic administration. (C) Range of doses in targeted administration (if disclosed). (D) Percentage of clinical trials with specific AAV dose windows for targeted administration. All data presented as a total injected dose per patient. For trials with dosages given in vg/kg body weight, the dose was calculated for an average body weight of adult person (70 kg).

Figure 7

Summary of strategies to avoid immunogenicity induced by AAV vector in clinical trials. (A) Percentage of trials that exclude patients with pre-existing anti-capsid NAbs by therapeutic area. (B) Percentage of trials that reported (1) potential use of steroids during the study period, (2) exclude patients with pre-existing anti-AAV capsid neutralising antibodies (NAbs), and (3) exclude immunosuppressed patients (such as those with concurrent use of immunosuppressants or with immunosuppressive disorders). BD, Blood disorders; CNS, Central Nervous System; ED, Eye Disorders; LSD, Lysosomal storage disorders; NMD, Neuromuscular Disorders.

Figure 8

Trends in adeno-associated vector (AAV)-mediated gene therapy in clinical trials. (A) Cumulative AAV-mediated gene therapy clinical trials completed in the past 20 years. (B) Expected number of of clinical trial completions in the next 5 years or more (see Supplementary Table 5 for details).