Scrutinizing the Subvisible

Regulators question whether particles that they can't see hurt patients.
Apr 02, 2011
Volume 35, Issue 4

Protein aggregates in biological drugs have the potential to trigger an immune response in the patient, which, at the very least, can decrease the drug's effectiveness, and at worst could cause potentially harmful side effects. Part of the industry's efforts to limit the presence of aggregates in their therapies entails testing for subvisible particles, which potentially are the most immunogenic aggregates. Subvisible protein particles are relatively large assemblies that contain anywhere from thousands to millions of protein molecules. US Pharmacopeia <788> "Particulate Matter in Injections" limits the number of particles equal to or larger than 10 µm that are allowed per container of drug (1).

The disregarded particles

USP does not address particles smaller than 10 µm in parenteral drugs, however. This omission drew little comment until it was mentioned in a 2009 article by John F. Carpenter, associate professor of pharmaceutical sciences at the University of Colorado Health Sciences Center. "If only particles > 10 µm were quantified in a given product, there could be gaps in understanding of important degradation products and in product-quality assessment," wrote Carpenter (2).

Carpenter and his coauthors, including eight officials at FDA's Center for Drug Evaluation and Research, raised the possibility that particles smaller than 10 µm could affect the safety and efficacy of therapeutic protein products over their shelf lives. Manufacturing operations sometimes create hundreds of thousands of particles 1.5–3 µm in size, the authors noted, and protein particles can accumulate over time during storage of the final product. Yet recommendations for detecting such particles are lacking.

Carpenter called for industry and academia to define current particle-counting instruments' capabilities to observe particles as small as 0.1 µm, recognizing the potential need for new instruments. The effect of protein aggregates on immunogenicity also should be examined, including "studies of the role of protein class, amount of aggregate, size of aggregates, and protein conformation in aggregates," wrote Carpenter (2).

The regulators' response

Carpenter's article attracted the attention of drug companies and regulatory bodies around the world. To gather information about the topic, USP held a discussion about particle characteristics and their effect on liquid and aerosol products during their workshop on particle detection and measurement on Dec. 8–10, 2010.

Participants spoke about sampling techniques and methods for data expression and interpretation, according to Scott Aldrich, principal consultant for Ultramikro and member of the 2010–2015 USP Dosage Forms expert committee. Much of the workshop discussion focused on the difficulties in adequately measuring the sizes and concentrations of particles in the sub-10-µm population for biotherapeutic formulations.

USP is assembling an expert panel to determine whether to establish a new particle-limits chapter for biotherapeutic pharmaceutical injections. "We anticipate any new chapter to provide methods tailored to the sensitivities of these formulations, with options for methodologies and a discussion of typical effects upon particle size, yet with no plan for limits specific to the biomolecular formulation," says Aldrich.

FDA has not published regulations about particles smaller than 10 µm, partly because so few data are available. But the agency has asked firms to start assessing the background and control of subvisible particles in that size range, which previously had been ignored. FDA is interested in determining whether particles smaller than 10 µm correlate with adverse reactions reported through USP's or FDA's medical-awareness systems, says Cherris. The data also could help the agency decide whether to issue a guidance.

FDA primarily is approaching biopharmaceutical firms because they must monitor their products for intrinsic protein-based particles and control the extrinsic particles that might infiltrate their process, according to Roy Cherris, managing partner of Bridge Associates International. Large biopharmaceutical companies are highly interested in trying to determine whether particles smaller than 10 �m are a major cause for concern, says Cherris.

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