Adeno-associated viruses (AAV) as viral vectors in gene therapy allow for the targeted delivery of a transgene to elicit a therapeutic effect. However, during manufacture not all AAV particles contain the intended genetic payload. Typically, some empty capsids, bearing no genetic material, are produced. The relative abundance of these empty capsids compared to those bearing the transgene (full capsids) is considered a critical quality attribute with direct impact on treatment efficacy and patient safety.

Mass Photometry is positioned to generate E:F (empty:full) capsid ratios for AAV samples whilst requiring minimal material, time and avoiding the need for expert analysts. The results obtained were comparable to results generated by, the current gold standard Analytical Ultra‑Centrifugation (AUC), for E:F capsid ratio generation.  Additionally, a range of commercially available purified AAV samples with known E:F capsid ratios were analysed by mass photometry with expected vs. measured E:F capsid ratios plotted. The results showed good agreement and linearity, over the range tested, with an r² value >0.96.

Finally, fractions were taken from various steps throughout Pharmaron’s AAV downstream  purification process and measured for E:F capsid ratio. The data indicated cumulative clearance of empty capsids. Using this methodology, it was possible to identify which fractions contained the lowest E:F capsid ratio with very low sample volume, concentration, and analysis time. Mass photometry can therefore help overcome the key challenge associated with material availability for AAV in‑process testing especially when assessing small scale production runs and therefore compliments Pharmaron’s existing world-class AAV manufacturing and analytical capabilities.