Chimeric Capsid Proteins Impact Transduction Efficiency of Haploid Adeno-Associated Virus Vectors

Current trends in gene therapy to treat inherited disorders of the brain

Gene therapy development, re-engineering, and application to patients hold promise to revolutionize medicine, including therapies for disorders of the brain. Advances in delivery modalities, expression regulation, and improving safety profiles are of critical importance. Additionally, each inherited disorder has its own unique characteristics as to regions and cell types impacted and the temporal dynamics of that impact that are essential for the design of therapeutic design strategies. Here, we review the current state of the art in gene therapies for inherited brain disorders, summarizing key considerations for vector delivery, gene addition, gene silencing, gene editing, and epigenetic editing. We provide examples from animal models, human cell lines, and, where possible, clinical trials. This review also highlights the various tools available to researchers for basic research questions and discusses our views on the current limitations in the field.

Subretinal Gene Therapy Drug AGTC-501 for XLRP Phase 1/2 Multicenter Study (HORIZON): 24-Month Safety and Efficacy Results

PURPOSE

To evaluate the safety and efficacy of subretinal gene therapy using AGTC-501 (rAAV2tYF-GRK1-RPGR) in male participants with X-linked retinitis pigmentosa (XLRP).

DESIGN

Phase 1/2, open-label, dose-escalation study.

METHODS

Setting: Four centers in the United States. Patient or Study Population: Twenty-nine males with XLRP and confirmed pathogenic RPGR variants. Mean age was 31.6 years (range 15-55).

INTERVENTION

Subretinal injection of AGTC-501 at doses ranging from 2.48 × 1010 to 1.99 × 1012 vg/eye administered in one eye per participant. Subretinal injection sites initially targeted the peripheral retina and then transitioned to the macula with successive cohorts.

MAIN OUTCOME MEASURES

Treatment-emergent adverse events (TEAEs), serious adverse events (SAEs), laboratory parameters, and immunological responses. Efficacy was evaluated by mesopic microperimetry mean sensitivity.

RESULTS

All 29 participants experienced ≥1 TEAE. Eleven (38%) experienced ≥1 grade 3 TEAE. Six (21%) experienced ≥1 ocular SAE related to AGTC-501, including retinal detachment (n = 4), subcapsular cataract (n = 1), and glaucoma (n = 1). Two (6.9%) experienced non-ocular treatment-emergent SAEs. Immunological findings did not indicate safety concerns. Three of 4 participants at the highest dose exhibited concerning retinal pigment epithelial changes. Half the participants at the highest tolerated dose (6.8 × 1011vg/eye) maintained ≥7 dB improvement in ≥5 loci at 24 months.

CONCLUSIONS

Subretinal AGTC-501 was generally well-tolerated. Despite all participants experiencing at least one TEAE, most of these events were mild in nature, exhibited complete resolution, and were associated with the subretinal injection procedure itself rather than the study agent. The highest dose exhibited an unfavorable risk-benefit profile due to the development of RPE changes. Although this group had the highest improvement in retinal sensitivity, our team has decided not to continue this dose in future clinical trials. Preliminary efficacy was observed at the maximum tolerated dose. Further studies are warranted to assess long-term safety and efficacy of AGTC-501 for XLRP treatment.

AAV Capsid Modification and Its Influence on Viral Protein Stoichiometry and Packaging Fitness: Current Understandings and Future Direction

The field of gene therapy has witnessed significant advancements in the utilization of Adeno-associated virus (AAV) owing to its inherent biological advantages. Targeted AAV vectors are generated through genetic or chemical modification of the capsid for user-directed purposes. However, this process can result in imbalances in viral protein sequence homogeneity, stoichiometry, and functional transduction vector units, thereby introducing new challenges. This mini review focuses on the ongoing efforts to develop targeted vectors, which inadvertently present unsolicited obstacles for clinical application and provided perspectives on future directions.

A robust and flexible baculovirus-insect cell system for AAV vector production with improved yield, capsid ratios and potency

Manufacturing of adeno-associated viruses (AAV) for gene and cell therapy applications has increased significantly and spurred development of improved mammalian and insect cell-based production systems. We developed a baculovirus-based insect cell production system-the SGMO Helper-with a novel gene architecture and greater flexibility to modulate the expression level and content of individual Rep and Cap proteins. In addition, we incorporated modifications to the AAV6 capsid sequence that improves yield, capsid integrity, and potency. Production of recombinant AAV 6 (rAAV6) using the SGMO Helper had improved yields compared to the Bac-RepCap helper from the Kotin lab. SGMO Helper-derived rAAV6 is resistant to a previously described proteolytic cleavage unique to baculovirus-insect cell production systems and has improved capsid ratios and potency, in vitro and in vivo, compared with rAAV6 produced using Bac-RepCap. Next-generation sequencing sequence analysis demonstrated that the SGMO Helper is stable over six serial passages and rAAV6 capsids contain comparable amounts of non-vector genome DNA as rAAV6 produced using Bac-RepCap. AAV production using the SGMO Helper is scalable using bioreactors and has improved yield, capsid ratio, and in vitro potency. Our studies demonstrate that the SGMO Helper is an improved platform for AAV manufacturing to enable delivery of cutting-edge gene and cell therapies.

AAV vectors: The Rubik's cube of human gene therapy

Defective genes account for ∼80% of the total of more than 7,000 diseases known to date. Gene therapy brings the promise of a one-time treatment option that will fix the errors in patient genetic coding. Recombinant viruses are highly efficient vehicles for in vivo gene delivery. Adeno-associated virus (AAV) vectors offer unique advantages, such as tissue tropism, specificity in transduction, eliciting of a relatively low immune responses, no incorporation into the host chromosome, and long-lasting delivered gene expression, making them the most popular viral gene delivery system in clinical trials, with three AAV-based gene therapy drugs already approved by the US Food and Drug Administration (FDA) or European Medicines Agency (EMA). Despite the success of AAV vectors, their usage in particular scenarios is still limited due to remaining challenges, such as poor transduction efficiency in certain tissues, low organ specificity, pre-existing humoral immunity to AAV capsids, and vector dose-dependent toxicity in patients. In the present review, we address the different approaches to improve AAV vectors for gene therapy with a focus on AAV capsid selection and engineering, strategies to overcome anti-AAV immune response, and vector genome design, ending with a glimpse at vector production methods and the current state of recombinant AAV (rAAV) at the clinical level.

Dexamethasone Transiently Enhances Transgene Expression in the Liver When Administered at Late-Phase Post Long-Term Adeno-Associated Virus Transduction

Glucocorticoids have anti-inflammatory and immunosuppressive functions and have commonly been used for preventing liver toxicity after the systemic application of a high dose of adeno-associated virus (AAV) vector for gene therapy. Clinical studies have reported that glucocorticoids have rescued factor IX (FIX) expression in patients with hemophilia B who showed a reduced FIX expression at 6 to 10 weeks post-AAV vector administration. In this study, we explored whether glucocorticoids could affect transgene expression in AAV targeted livers in animal models. When dexamethasone was applied before AAV9/FIX vector administration in the wild-type C57BL/6 mice, FIX expression was much higher than that of the control mice at any time point. More importantly, FIX expression transiently increased after dexamethasone was administered at week 6 or later post-AAV injection regardless of the various dexamethasone treatments applied. The transient enhancement in transgene expression was observed once there were one to several consecutive dexamethasone treatments completed. A similar result was also achieved in other wild-type BALB/c and hemophilia B mice that were treated with AAV9/FIX and dexamethasone. This mechanism study demonstrated that the administration of dexamethasone did not change either AAV genome copy number or transgene expression at the transcription level but transiently decreased interferon beta (IFN-β) and tumor necrosis factor alpha (TNF-α) expression in the livers of mice at a later time after AAV injection. Next, we studied the effect of dexamethasone on late transgene expression in hemophilia B dogs. Dexamethasone was administered 1 year after AAV9/FIX injection. Inconsistent with the results in mice, no significant change of FIX expression was observed in hemophilia B dogs. In summary, the results from this study indicate that dexamethasone may have various effects on transgene expression in AAV-transduced livers in different species, which provides valuable information about the rational application of dexamethasone in future clinical studies.

Adeno-Associated Virus (AAV) Gene Delivery: Dissecting Molecular Interactions upon Cell Entry

Human gene therapy has advanced from twentieth-century conception to twenty-first-century reality. The recombinant Adeno-Associated Virus (rAAV) is a major gene therapy vector. Research continues to improve rAAV safety and efficacy using a variety of AAV capsid modification strategies. Significant factors influencing rAAV transduction efficiency include neutralizing antibodies, attachment factor interactions and receptor binding. Advances in understanding the molecular interactions during rAAV cell entry combined with improved capsid modulation strategies will help guide the design and engineering of safer and more efficient rAAV gene therapy vectors.

Bound Protein- and Peptide-Based Strategies for Adeno-Associated Virus Vector-Mediated Gene Therapy: Where Do We Stand Now?

Recombinant adeno-associated virus (rAAV) vectors have become one of the most promising and efficacious delivery vehicles for human gene therapy; however, low infectivity remains a major ongoing obstacle in the clinical application of rAAV vectors. Multiple strategies, including rAAV capsid modification and the application of pharmacological reagents, have been explored to enhance rAAV vector gene delivery. Recently, a new strategy using native proteins or various peptides has shown promise for increasing rAAV transduction locally or globally. This review summarizes the current status of protein- and peptide-based strategies and mechanisms to modulate rAAV transduction. We also provide a potential insight regarding the design of effective approaches for rAAV transduction enhancement in future clinical studies.