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.
