Methodological validation of a cryo-TEM-based detection method for empty capsid ratio in recombinant adeno-associated...

Recombinant adeno-associated virus (rAAV) is one of the most promising vectors for gene therapy. It consists of a protein capsid that encapsulates the genetic material. However, during the production process, except for the full particles containing the genome, a variety of types of particles are also produced concomitantly, such as empty particles (no DNA encapsulated in the capsid), viral fragments, damaged viruses, and viral aggregates. Empty particles have been reported to induce unnecessary immune response and reduce transduction efficiency. Therefore, a quantitative method that objectively assesses the content of rAAV particles containing the genome is crucial for quality measurement. In this study, we developed a technique to detect the proportion of empty capsids in rAAV samples by using cryogenic transmission electron microscopy (cryo-TEM). This method not only accurately quantifies the percentage of empty capsids, but also allows for the characterization and analysis of other different particle types within the sample. Our comprehensive evaluation of specificity, precision, accuracy, linearity, and limit of quantitation (LOQ) demonstrates the advantages of cryo-TEM technology for the quantitative analysis of rAAV empty capsid ratio. Moreover, this research offers a thorough, multidimensional approach to enhance the understanding and implementation of quality control for rAAV.