Standardisation in the Analytical Characterization of Adeno-Associated Virus (AAV) Vectors.

Adeno-associated virus (AAV) has become a leading vector for in vivo gene therapy, with eight products currently holding marketing authorization. As the field rapidly evolves, the need for robust analytical methods to characterize critical quality attributes (CQAs)-including capsid titer, genome titer, capsid content (empty/full ratio), identity, and purity-continues to grow. Reference Standard Materials (RSMs) play a pivotal role by providing well-characterized, standardized AAV batches that serve as universal benchmarks. RSMs facilitate the validation of emerging analytical technologies, ensure the accuracy and reproducibility of routine assays, and enable inter-laboratory comparability. However, developing universal AAV RSMs is fundamentally constrained by the complex biology, diversity of serotypes, vector genomes, and engineered capsid variants, necessitating serotype-specific and application-specific standards. Recent advances, including the release of pharmacopeial AAV8 reference standards characterized by multiple orthogonal methods, represent meaningful progress toward measurement harmonisation. This review addresses the critical need for RSMs in AAV gene therapy, evaluates the currently available pharmacopeial and commercial standards, and outlines practical strategies for in-house RSM development. Establishing robust, serotype-specific AAV RSMs and harmonised standard operating protocols (SOPs) are essential for advancing AAV gene therapy and ensuring accuracy, reproducibility, and safety across research, development, and clinical manufacturing.