AAV Vector Manufacturing Advances Hold the Key to Accessible Gene Therapy

Adeno-associated virus (AAV) vectors have become the delivery vehicle of choice for modern gene therapy, carrying therapeutic genes into cells with broad tissue tropism, a favorable safety profile, and durable expression. More than 10,000 rare disorders have been described worldwide, roughly 80 percent of them genetic in origin, yet only about 5 percent of affected patients currently have treatment options. Approved AAV-based products such as Luxturna for inherited retinal dystrophy, Zolgensma for spinal muscular atrophy type 1, Hemgenix and Bequez for hemophilia B, Roctavian for severe hemophilia A, Elevidys for Duchenne muscular dystrophy, and Kebilidi for aromatic L-amino acid decarboxylase deficiency illustrate the clinical reach of the platform. But a new review published in Frontiers in Molecular Medicine argues that the bottleneck for the next wave of therapies is not vector design or clinical science. It is manufacturing.

The review, authored by N. Charan S. S. Kowshik of Ocugen India and Pushpendra Singh of Ocugen, lays out an end-to-end framework for establishing robust AAV manufacturing processes, from plasmid production and cell culture through transfection, purification, and fill-finish. The authors contend that process variability, low yields, and poor scalability remain the central constraints on translating AAV’s therapeutic promise into accessible treatments. Their prescription is a data-driven, quality-by-design approach in which every unit operation is characterized, modeled, and controlled before a process is validated and submitted to regulators.

At the heart of conventional AAV production is transient transfection of human embryonic kidney 293 (HEK293) cells. Three plasmids are typically prepared: a cis-acting plasmid carrying the therapeutic gene flanked by inverted terminal repeats, a trans-acting plasmid encoding AAV replication and capsid proteins, and a helper plasmid enabling viral replication in the host cell. These are introduced into cells expanded from master and working cell banks, and after two to three days in the bioreactor the cells are harvested for downstream purification. Each step in this chain introduces variability. Inconsistent plasmid uptake by host cells leads directly to variation in the ratio of empty to full capsids, a critical quality attribute that determines potency, dose consistency, and immunogenicity.

The upstream innovations highlighted in the review attack this variability from several directions. High-density perfusion cultures maintain cells in an optimal growth phase by continuously supplying fresh nutrients, extending the transfection window and enhancing genome replication and packaging. In one reported setup, an N-1 perfusion process for suspension HEK293 cells achieved cell densities of 35 million cells per milliliter with greater than 95 percent viability after seven days, supporting production bioreactors of 2,000 to 5,000 liters. For adherent systems, single-use fixed-bed perfusion bioreactors such as the iCELLis platform have demonstrated eight-fold higher productivity through post-transfection media exchange. Optimized transfection parameters, including plasmid amount and DNA-to-polyethyleneimine ratio, can push unit yields as high as 2 x 10¹² vector genomes per milliliter.

Plasmid system design is evolving just as rapidly. Traditional triple-plasmid transfection, while fast and efficient, often produces inconsistent co-delivery of the required genetic elements. Advanced dual-plasmid platforms that combine the transgene, AAV rep and cap genes, and accessory elements on one plasmid, with adenoviral helper functions on a second, have shown improved productivity and full capsid proportions across multiple serotypes and scales up to 50-liter bioreactors. The AAVone single-plasmid system consolidates all necessary components into one construct, yielding a two- to four-fold increase in vector production, reduced batch variability, and lower DNA impurities, characteristics the authors consider especially suitable for GMP-grade manufacturing. Design-of-experiments optimization of triple-plasmid ratios has nearly doubled genome titers for AAV2 and AAV9 while reducing empty capsid formation, and engineered helper plasmids with deletions in adenoviral E4 and E2a genes have delivered more than two-fold productivity gains.

Plasmid quality itself emerges as a critical starting-material attribute. The supercoiled percentage of plasmid DNA directly influences transfection performance: highly supercoiled plasmids form smaller, more compact complexes with transfection reagents, which are taken up more efficiently during endocytosis. The review notes that regulatory expectations commonly call for a defined lower limit above 80 percent supercoiled DNA in bulk release specifications. Achieving that standard requires careful control of bacterial cultivation, harvesting during logarithmic growth phase, and precise alkaline lysis and neutralization, followed by anion-exchange or monolith-based chromatography to separate supercoiled plasmids from open-circular and linear species. Next-generation transfection reagents, including FectoVIR-AAV, the TransIT-AAViator system with its RevIT enhancer, and modified linear PEI formulations, offer reported productivity gains of two-fold or more over conventional PEI while reducing reagent and plasmid consumption.

Downstream processing confronts a different challenge: separating the genome-containing capsids that constitute the active drug substance from empty and partially filled particles that contribute nothing to efficacy and may trigger unwanted immune responses. Affinity chromatography has advanced considerably with serotype-agnostic peptide ligands that enable broad, high-yield capture across all major AAV serotypes under milder elution conditions. The POROS CaptureSelect AAVX resin, for example, achieves recovery rates of 65 to 80 percent across divergent serotypes while supporting repeated regeneration. Density gradient ultracentrifugation with cesium chloride or iodixanol remains a workhorse for impurity removal, typically requiring two to three cycles for enhanced purity, though its scalability is limited. Continuous-flow ultracentrifugation systems with rotors accommodating up to 50 liters have reported recovery rates exceeding 55 percent in a single run, beginning to address that constraint. Polishing chromatography then exploits the difference in surface charge between full and empty capsids: two-pass anion-exchange strategies, membrane-based ion exchange optimized through design of experiments, and dual salt elution gradients have achieved full capsid enrichment of 70 to 80 percent or higher at production scale.

The review places quality-by-design at the center of the entire manufacturing enterprise. Process characterization begins with risk assessment, using failure mode effects analysis and cause-and-effect matrices to identify potential critical process parameters and evaluate their impact on critical quality attributes such as vector titer, capsid integrity, full-to-empty ratio, and aggregation. Validated scale-down models, built by matching power per unit volume, oxygen transfer coefficients, and shear stress between commercial-scale bioreactors of 50 to 2,000 liters and small-scale stirred tanks, allow multiple experiments to be run efficiently. Statistical tools including multivariate data analysis, principal component analysis, and equivalence testing confirm that the scale-down model faithfully represents the commercial process. Design of experiments then maps the design space, from which proven acceptable ranges and normal operating ranges are defined. Variations within the design space are not regarded as changes requiring supplementary regulatory submissions, and once a parameter exceeds its proven acceptable range during commercial manufacturing, the batch is considered a failure.

Process validation follows the three-stage framework outlined in the US FDA guidance: process design, process qualification, and ongoing process verification. Because AAV products are manufactured in limited batches during development and early commercial phases, the authors emphasize a risk-based, statistically justified approach to determining the number of process performance qualification batches. The overall residual risk depends on product knowledge, process understanding, and manufacturing experience; tightly controlled processes may require fewer batches to produce statistically meaningful data, while highly variable processes demand multiple campaigns. No harmonized industry standard currently exists for justifying batch number, though three consecutive qualification batches remain the common recommendation for processes under control.

Looking ahead, the authors see artificial intelligence and continuous manufacturing as pivotal to the next generation of AAV therapies. AI-driven soft sensing with predictive modeling can estimate critical quality attributes such as viral titer and capsid integrity in real time from readily available process variables, enabling continuous monitoring, early deviation detection, and predictive quality assessment. Mechanistic modeling of empty-full capsid separation using anion-exchange membrane chromatography has already demonstrated that AI-enhanced models can predict recovery yields with remarkable fidelity. Advanced analytical technologies, including cryoelectron microscopy, mass spectrometry-based proteomics, and molecular dynamics simulations, coupled with AI and predictive modeling, are expected to illuminate capsid behavior and host interactions, empowering the engineering of capsids optimized for stability, immune evasion, and targeted tissue delivery. The worldwide AAV vector market is expected to expand at a compound annual growth rate of approximately 14 percent until 2035, and the authors argue that standardized orthogonal analytics, traceable reference materials, and multi-laboratory ring trials to harmonize potency and genome titer assays will be essential to bring manufacturing standards closer to accessible, safer biologics. Automated platforms integrating high-throughput screening with synthetic biology, they conclude, could ultimately permit fully customizable AAV vectors designed for specific diseases or tailored to individual patients, transforming the platform into a remarkably precise and adaptable instrument for the coming era of personalized gene therapy.

Subject of Research: Advancing AAV vector manufacturing: challenges, innovations, and future directions for gene therapy

Article Title: Advancing AAV vector manufacturing: challenges, innovations, and future directions for gene therapy

Article References: Kowshik, N. C. S. S., & Singh, P. (2025). Advancing AAV vector manufacturing: challenges, innovations, and future directions for gene therapy. Frontiers in Molecular Medicine, 5, Article 1709095. https://doi.org/10.3389/fmmed.2025.1709095

Image Credits: AI Generated

DOI: 10.3389/fmmed.2025.1709095

Keywords: Advancing, vector, manufacturing, challenges, innovations, future, directions, gene, therapy, scientific research