Direct Identification and Quantification of Recombinant Adeno-Associated Virus in Crude Cell Lysate and Conditioned Medium by Mass Photometry

Recombinant adeno-associated virus (rAAV) has attracted attention as a gene therapy vector. Monitoring the percentage of full particles (FPs) to the sum of empty particles (EPs) and FPs (F/E ratio) is required to optimize the rAAV production conditions; however, there is a lack of analytical methods to identify FPs and EPs and quantify the F/E ratio of rAAV without purification. Here, we established a direct analysis method for identifying FPs and EPs and quantifying the F/E ratio and genomic titer of unpurified rAAV in crude cell lysate and conditioned medium by mass photometry (MP). MP can detect the events of both molecules that bind to the glass surface and molecules that unbind from the glass surface. Few unbinding molecules were detected in the cell lysate and conditioned medium, but unbinding particles were as prevalent as binding particles in rAAV. By analyzing the unbinding side of the histogram, the F/E ratio of rAAV in the cell lysate was directly quantified with accuracy comparable to that of purified rAAV, which showed there was no interference from impurities. The genomic titer of rAAV in cell lysate was also estimated using particle counts of the unbinding side. This method can successfully determine the F/E ratio and estimate genomic titers of rAAV in crude cell lysate and conditioned medium during the manufacturing process. Direct quantification by MP is a convenient, rapid, and accurate method for quantifying unpurified rAAV and will be useful for improving rAAV production processes, for example, by screening manufacturing conditions.