Defining the quality parameters that support AAV vector safety, potency, purity, and consistency

As gene therapy continues to advance, adeno-associated virus (AAV) has become one of the most important platforms for in vivo gene delivery. AAV vectors are widely used because they can efficiently deliver therapeutic genetic material to target tissues, support durable transgene expression in many applications, and have a relatively favorable safety profile. As AAV-based products progress from research to clinical and commercial manufacturing, robust quality control becomes increasingly critical.

For AAV gene therapy products, Critical Quality Attributes, or CQAs, are measurable product properties that must be controlled to help ensure safety, identity, purity, potency, and consistency. These attributes include vector genome titer, capsid content, empty/full ratio, purity, impurity profile, genome integrity, potency, infectivity or transduction activity, sterility-related safety tests, and stability. A strong analytical control strategy must therefore evaluate not only how much AAV is present, but also how much of it is functional, correctly packaged, pure, and consistent from batch to batch.

Key Critical Quality Attributes of AAV Gene Therapy Products

Purity is a central CQA for AAV drug substance and drug product. It describes the proportion of desired AAV vector relative to product-related and process-related impurities. Product-related impurities may include empty capsids, partially filled capsids, fragmented genomes, aggregates, or incorrectly packaged nucleic acids. Process-related impurities may include host-cell proteins, host-cell DNA, residual plasmid DNA, residual transfection reagents, nucleases, or chromatography-related contaminants. Controlling these impurities is essential because they may affect safety, immunogenicity, potency, and product consistency.

Titer is another key quality attribute, but it must be clearly defined. Vector genome titer, typically measured by qPCR or digital PCR, estimates the number of genome-containing particles. Capsid titer measures total capsid particles, including empty, partial, and full capsids. Infectious or transducing titer measures the functional ability of the vector to enter cells and deliver an expressible genome. Because these assays measure different aspects of the product, a single titer result cannot fully describe AAV quality.

Genome integrity is increasingly important in AAV product characterization. A therapeutically active AAV particle should contain the intended vector genome in the correct size and configuration. Truncated genomes, rearranged genomes, partial genomes, residual plasmid backbone DNA, or packaged host-cell DNA may affect potency, safety, and dose interpretation. For this reason, genome integrity analysis is becoming a key part of AAV quality assessment.

Capsid content, especially the ratio of empty, partially filled, and full capsids, is also a major AAV quality attribute. Empty capsids do not contain the intended therapeutic genome, while partial capsids may contain incomplete or unintended nucleic acid species. These populations can influence dose calculation, immune exposure, and batch consistency.

Potency and infectivity are among the most biologically relevant CQAs. Potency assays should reflect the intended biological activity of the product, such as target-cell transduction, transgene expression, or a disease-relevant functional readout. Because AAV products differ by serotype, promoter, transgene, target tissue, and mechanism of action, potency assays are often product-specific and require careful development.

Major Analytical and Quality Control Challenges

Accurate titer determination remains one of the most common challenges in AAV analytics. qPCR and digital PCR results can be affected by sample preparation, nuclease digestion, primer and probe design, reference standards, residual plasmid DNA, and genome secondary structure. Capsid titer methods may also vary depending on capsid serotype, antibody specificity, method calibration, and assay format.

Purity analysis is difficult because AAV preparations may contain many different impurity species at low levels. Analytical methods must be sensitive and specific enough to detect host-cell proteins, host-cell DNA, residual plasmid DNA, empty capsids, aggregates, and other contaminants that may influence product safety or performance.

Genome integrity analysis is another technical challenge. AAV vector preparations can contain a mixture of full-length genomes, truncated genomes, partial genomes, and non-vector DNA. Methods such as capillary electrophoresis, Southern blot, next-generation sequencing, long-read sequencing, analytical ultracentrifugation, and ddPCR-based assays can provide useful information, but each has limitations in sensitivity, resolution, throughput, or interpretability.

Potency and infectivity assays are particularly difficult to standardize. Ideally, these assays should reflect the product’s mechanism of action in clinically relevant target cells. However, target cells may be difficult to source, culture, or adapt for routine quality control. Surrogate cell lines may improve practicality but may not fully represent in vivo activity. This creates a balance between biological relevance, assay robustness, turnaround time, and suitability for release testing.

Strategies for Strengthening AAV Quality Control

A robust AAV quality control strategy should use orthogonal and phase-appropriate analytics. Instead of relying on one method, developers should combine complementary assays to evaluate the same quality attribute from different perspectives. For example, vector genome titer, capsid titer, and empty/full capsid analysis together provide a more complete view of dose and product composition.

Key strategies include:

• Developing more accurate methods for vector genome titer, capsid titer, empty/full ratio, genome integrity, purity, impurity profiling, aggregation, and potency.
• Establishing reference standards and well-characterized controls to improve comparability across laboratories, platforms, and manufacturing sites.
• Applying orthogonal methods to confirm critical results, especially for titer, purity, genome integrity, and capsid content.
• Building product-specific potency assays that are biologically meaningful, robust, and suitable for routine quality control.
• Linking analytical results with process understanding so CQAs can be connected to upstream production, downstream purification, formulation, storage, and stability.

Conclusion

AAV Critical Quality Attributes are fundamental to the safety, efficacy, and consistency of AAV gene therapy products. Key attributes such as purity, titer, genome integrity, capsid content, potency, infectivity, impurity profile, and stability must be carefully defined, measured, and controlled throughout development and manufacturing.

Although AAV analytics still faces important challenges, continued progress in assay development, standardization, reference materials, sequencing, and orthogonal characterization will strengthen AAV product quality control. A scientifically sound and well-integrated analytical strategy is essential for advancing AAV gene therapies from early development to reliable clinical and commercial application.