Mass Photometry Recognized by U.S. Pharmacopeia in New AAV Reference Standards for Gene Therapy Quality

USP employs mass photometry as key orthogonal method for empty/full characterization of AAV capsids

 Refeyn’s SamuxMP mass photometer for AAV analytics - used for characterizing the U.S. Pharmacopeia’s new adeno-associated virus (AAV) reference standards.
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Oxford, UK (18 June, 2025) – Refeyn, the company behind pioneering mass photometry technology, announces that mass photometry has been recognized by the U.S. Pharmacopeia (USP)—and referenced in the new draft USP general chapter <1067>—as a key orthogonal method for characterizing USP’s new adeno-associated virus (AAV) reference standards. As a major advancement in gene therapy quality, USP has recently released new standards for measuring empty and full AAV capsids using five different methods from a comprehensive multi-laboratory study.

AAVs play a key role in delivering gene therapies, and precise measurement of the empty/full capsid ratio—a critical quality attribute (CQA)—is crucial for developers and manufacturers of AAV-based gene therapies to ensure product safety and efficacy. Until now, there has been a lack of method harmonization and physical standards to support accurate quantitation of full and empty capsids. USP has addressed this challenge with the launch of a solutions package to help AAV manufacturers standardize their quality assessment and control. 

Developed in collaboration with leading scientists across multiple laboratories, USP’s new AAV standards tackle the essential need for consistency and reliability in AAV assessments. Mass photometry was used alongside SEC-MALS, AUC, UV-vis, and CD-MS to characterize USP’s new AAV8 (Empty Capsids) and AAV8 (Full Capsids) reference standards, and is now included in USP’s Gene Therapy Analytical Guide.

Mass photometry is also specifically referenced in the new USP general chapter <1067>, “Best Practices for the Manufacture and Quality Control of Recombinant Adeno-Associated Virus Gene Therapy Products”, which is currently open for public comment. The chapter highlights mass photometry—alongside AUC and TEM—as an orthogonal method capable of distinguishing between empty, full, and also notably partially filled capsids; and recommends using multiple orthogonal techniques for accurate capsid assessment.

Detailed on the USP’s reference standard certificates for AAV8 (Full Capsids) and AAV8 (Empty Capsids), Refeyn’s SamuxMP mass photometer is optimized for AAV characterization to offer quick and easy empty/full capsid ratio measurements with minimal sample. In addition to quantitative characterization of AAVs of all serotypes, the SamuxMP can quantify partially filled and overfilled capsids, and detect impurities and aggregation. Refeyn’s MassFerence® P2 Calibrant was also used alongside the SamuxMP for the certificates’ method parameters.

USP’s recognition of mass photometry follows similar acknowledgement by another pharmaceutical standards authority, the British Pharmacopeia, in 2024. The British Pharmacopeia included mass photometry in its Advanced Therapy Medicinal Products Guidance on Characterisation of the Capsid Particle Population in rAAV Products. In its section on empty/full characterization methods, mass photometry is described as a “viable option for at-line process monitoring.” 

“We are thrilled that mass photometry has been recognized by the USP for the characterization of empty, full, and—critically—partially filled AAV capsids, and that it is specifically referenced in the new USP general chapter <1067> as a recommended orthogonal method,” said Gabriella Kiss, Market Development Director at Refeyn. “Along with prior recognition by the British Pharmacopeia, this demonstrates that mass photometry is being accepted as a valuable part of the AAV analytics toolbox. And it has a lot to offer because it is a simple, easy-to-use method for measuring a significant CQA while ensuring consistent and reliable analytics.”

A recent webinar presented by scientists from the USP and Pharmaron Biologics discussed the need for best practices and test methods that are harmonized for AAV testing from raw material qualification through to product release. This includes insight into the new USP standards to support AAV quality testing and analytical characterization of AAV content utilizing mass photometry and orthogonal techniques. The Pharmaron scientists also highlighted how they use mass photometry for AAV capsid optimization.

To view the webinar ‘USP standards for AAV testing and characterization using mass photometry’—presented by Dr. Anthony Blaszczyk, Senior Scientist II, United States Pharmacopeia (USP) and from Pharmaron Biologics, Dr. Lauren Tomlinson, Scientist, and Dr. Paul Getty, Senior Technical Specialist, see https://refeyn.com/resources/usp-aav-standards-to-support-quality-testing-and-characterization  

To find out more about Refeyn and its mass photometry technology, please visit: https://www.refeyn.com 

About Refeyn www.refeyn.com
Refeyn Ltd. specializes in the development, production, and distribution of mass photometry for industry and academia. Mass photometry and macro mass photometry are unique bioanalytical technologies pioneered by Refeyn. By quantifying light scattering from particles in solution, mass photometry enables mass measurement at the single-particle level while macro mass photometry enables simultaneous measurement of viral vector size and contrast (a proxy for mass). 

Spun out of Oxford University in 2018, the company aims to make its technology accessible to all in the scientific community, enabling users to achieve new and better outcomes. Refeyn has been recognized for its unique approach to biomolecular analysis through multiple awards, including the Royal Society of Chemistry Emerging Technologies Competition. Applications of mass photometry include: Characterization of samples of biomolecules and adeno-associated viral (AAV) vectors, and analysis of protein oligomerization, biomolecular interactions, and antibody aggregation. Utilizing mass photometry and macro mass photometry, Refeyn’s products are capable of accurate analysis of single particles in solution, in their native state, and without the need for labels, opening up new possibilities for bioanalytics.