The U.S. Pharmacopeia (USP) has officially recognized mass photometry as a key orthogonal method for characterizing new adeno-associated virus (AAV) reference standards, marking a significant advancement in gene therapy quality control. The technology, developed by Refeyn, is now referenced in the new draft USP general chapter <1067> and included in USP's Gene Therapy Analytical Guide.
New Standards Address Critical Quality Control Gap
AAVs serve as crucial delivery vehicles for gene therapies, making precise measurement of the empty/full capsid ratio a critical quality attribute (CQA) essential for ensuring product safety and efficacy. Until now, the field has lacked method harmonization and physical standards to support accurate quantitation of full and empty capsids.
USP has addressed this challenge by releasing new standards for measuring empty and full AAV capsids using five different analytical methods from a comprehensive multi-laboratory study. Mass photometry was employed alongside SEC-MALS, AUC, UV-vis, and CD-MS to characterize USP's new AAV8 (Empty Capsids) and AAV8 (Full Capsids) reference standards.
Recognition in Official USP Guidelines
Mass photometry is 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 notably partially filled capsids, recommending the use of multiple orthogonal techniques for accurate capsid assessment.
Refeyn's SamuxMP mass photometer, detailed on the USP's reference standard certificates for AAV8 capsids, is optimized for AAV characterization to offer quick and easy empty/full capsid ratio measurements with minimal sample requirements. The instrument can quantify partially filled and overfilled capsids, and detect impurities and aggregation across all AAV serotypes.
Growing Regulatory Acceptance
This USP recognition follows similar acknowledgment by the British Pharmacopeia in 2024, which included mass photometry in its Advanced Therapy Medicinal Products Guidance on Characterization of the Capsid Particle Population in rAAV Products. The British Pharmacopeia described mass photometry as a "viable option for at-line process monitoring" in its section on empty/full characterization methods.
"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."
Industry Implementation and Best Practices
A recent webinar presented by scientists from USP and Pharmaron Biologics discussed the need for harmonized best practices and test methods for AAV testing from raw material qualification through product release. The presentation included insights into the new USP standards and analytical characterization of AAV content utilizing mass photometry and orthogonal techniques, with Pharmaron scientists highlighting their use of mass photometry for AAV capsid optimization.
The webinar, titled "USP standards for AAV testing and characterization using mass photometry," was presented by Dr. Anthony Blaszczyk, Senior Scientist II at USP, along with Dr. Lauren Tomlinson, Scientist, and Dr. Paul Getty, Senior Technical Specialist from Pharmaron Biologics.
