Critical Calibration of Mass Photometry for Higher-Mass Samples Such as Adeno-Associated Virus Vectors

Automated Mass Photometry of Adeno-Associated Virus Vectors from Crude Cell Extracts

Mass photometry (MP) is a fast and simple analysis method for the determination of the proportions of subpopulations in an AAV sample. It is label-free and requires minimal sample volumes between 5-10 µL, which makes it a promising candidate over orthogonal techniques such as analytical ultracentrifugation (AUC), cryo-transmission electron microscopy (Cryo-TEM) or charge-detection mass spectrometry (CDMS). However, these methods are limited in their application to purified samples only. Here we developed a purification step based on single-domain monospecific antibody fragments immobilised on either a poly(styrene-divinylbenzene) resin or on magnetic beads prior to MP analysis that allows the quantification of empty, partially filled, full and overfull AAV vectors in crude cell extracts. This is aimed at identifying potentially promising harvest conditions that yield large numbers of filled AAV vectors during the early stages of the viral vector development platform, e.g., the type of transfection reagent used. Furthermore, we provide a direct comparison of the automated and manual handling of the mass photometer with respect to the quantities of AAV subspecies, molar mass of the capsid and payload, and highlight the differences between the "buffer-free" sample measurement and the "buffer-dilution" mode. In addition, we provide information on which candidates to use for calibration and demonstrate the limitations of the mass photometer with respect to the estimation of the capsid titer.

Quantification of Empty, Partially Filled and Full Adeno-Associated Virus Vectors Using Mass Photometry

Adeno-associated viruses (AAV) are one of the most commonly used vehicles in gene therapies for the treatment of rare diseases. During the AAV manufacturing process, particles with little or no genetic material are co-produced alongside the desired AAV capsid containing the transgene of interest. Because of the potential adverse health effects of these byproducts, they are considered impurities and need to be monitored carefully. To date, analytical ultracentrifugation (AUC), transmission electron microscopy (TEM) and charge-detection mass spectrometry (CDMS) are used to quantify these subspecies. However, they are associated with long turnaround times, low sample throughput and complex data analysis. Mass photometry (MP) is a fast and label-free orthogonal technique which is applicable to multiple serotypes without the adaption of method parameters. Furthermore, it can be operated with capsid titers as low as 8 × 10¹⁰ cp mL^-1 with a CV < 5% using just 10 µL total sample volume. Here we demonstrate that mass photometry can be used as an orthogonal method to AUC to accurately quantify the proportions of empty, partially filled, full and overfull particles in AAV samples, especially in cases where ion-exchange chromatography yields no separation of the populations. In addition, it can be used to confirm the molar mass of the packaged genomic material in filled AAV particles.

Production and characterization of an AAV1-VP3-only capsid: An analytical benchmark standard

Adeno-associated viruses (AAVs) are non-enveloped ssDNA icosahedral T = 1 viruses used as vectors for clinical gene delivery. Currently, there are over 200 AAV-related clinical trials and six approved biologics on the market. As such new analytical methods are continually being developed to characterize and monitor the quality and purity of manufactured AAV vectors, these include ion-exchange chromatography and Direct Mass Technology. However, these methods require homogeneous analytical standards with a high molecular weight standard comparable to the mass of an AAV capsid. Described here is the design, production, purification, characterization, and the cryo-electron microscopy structure of an AAV1-VP3-only capsid that fulfills this need as a calibrant to determine capsid mass, charge, homogeneity, and transgene packaging characteristics.