34. Development and Validation of Identity and Homogeneity Assays for AAV Preparations

Adeno-associated virus (AAV) vectors have emerged as key clinical candidates for gene therapy. Yet, the efficiency and safety of these 20-25 nm biological nanoparticles remain difficult to harmonize across pre-clinical studies due to the limitations of current analytical tools. The presence of residual DNA, protein contaminants, empty particles and VP subunits resulting from incomplete capsid assembly are variables that can strongly modulate the reliability of in vivo data and that, therefore, need to be closely monitored in AAV research laboratories. In this work, 70 AAV preparations, obtained with various production (baculo/Sf9 and triple transfection system) and purification (iodixanol gradient and double cesium-chloride gradient) techniques were analyzed using a thermal shift assay based on the fluorescent dye Sypro® Orange. The fluorescence fingerprint obtained did not only allow to discriminate various AAV serotypes based on their capsid melting temperatures, but also enabled to probe the homogeneity and purity of AAV vector preparations, investigated in parallel using dynamic light scattering (DLS) and polyacrylamide gel electrophoresis. In particular, a double fluorescence transition indicated the presence of capsid-associated protein contaminants whereas a high initial fluorescence background correlated with the presence of free protein contaminants and capsid subunits, possibly resulting from capsid degradation during vector purification or storage. The variability, sensitivity and precision of this assay were further investigated in two different AAV research laboratories. This simple, fast (analysis of 94 preps in ~6 hrs) and low-cost assay emerges as a relevant tool for characterization of AAV vector preparations and will help to increase the reliability of in vivo gene transfer studies.