The particles' origins

Various types of particle smaller than 10 µm can be present in a parenteral drug, but regulators' primary concern is with proteinaceous particles because they are more likely to cause immunogenicity than extrinsic particles. Denaturation can lead to aggregation that causes the native monomeric therapeutic protein to form dimers, trimers, or polymers. During development, proper consideration of the formulation, the way the material is suspended in the formulation, and the formulation's dynamics over time can help prevent intrinsic particles from forming or agglomerating.

Stresses during product manufacture (e.g., freezing, thawing, agitation, and foaming) also could lead to the formation of aggregates. These concerns, too, can be addressed during formulation and process development, and the industry already knows a good deal about these concerns. "The risk for aggregate and potential proteinaceous particle formation is really product dependent. Certain molecules are more sensitive than others," says Satish K. Singh, research fellow at Pfizer. "It is imperative that the formulation-development scientist understand the weaknesses of the molecule and the impact a drug-product manufacturing process can have on it."

Although the potential for extrinsic contamination always exists, industry professionals believe that current controls for preventing it are adequate. "In aseptic processing, liquids pass through one or more 0.22-µm filter media. In our current environment, control of visible to subvisible particle content is believed to control the sub-10 µm content," says Aldrich.

The industry's perspective

Yet drugmakers are not necessarily convinced that the particles are always hazardous. Companies agree with regulators about the need to monitor protein-based particles smaller than 10 µm because of their potential immunogenicity. But human clinical data on the connection between protein aggregates and immunogenicity are equivocal because a multitude of factors determines a patient's immune reaction, and it is difficult to identify the effect of a specific quality parameter, says Singh. "Animal-model data indicate that immunogenicity can be triggered by protein aggregates or particles, but it is also clear that not all aggregates or particles cause immunogenicity," he says.

Most of the standard, small-volume parenterals probably are not a cause for concern, says Cherris. Many of them either are not proteins or are proteins that do not cause problems. For example, vaccines generally have not been indicated in any of the reported immunogenicity problems, he adds.

Whether these subvisible particles can affect drugs' efficacy also is a matter of debate. "These particles cannot decrease the effectiveness of protein drugs directly because they often represent only nanograms or micrograms of protein" and would not measurably decrease the amount of available drug, says Singh. If the particles cause an immune reaction, however, the resulting antibodies can either have no effect, have an indirect effect through pharmacokinetics or pharmacodynamics, or directly negate the drugs' efficacy. "The worst-case scenario is if these antibodies are neutralizing against a nonredundant endogenous protein also, in which case, the antibodies become a safety risk," says Singh.

The nature of the subvisible particle may determine whether it causes adverse reactions. Not every biological product necessarily is associated with immunogenicity, says Cherris. "I would say that there are only subtle indicators of that [problem] in specific product cases, many of which in the biotech world are still in development," he adds.

Others agree that the risk of immunogenicity appears so far to be associated with certain products. "Specific characteristics in the aggregates, probably related to the structure of the protein in the aggregate and the nature of the exposed epitopes, cause some aggregates to be immunogenic whereas others are not, says Singh. Likewise, the risk that particles smaller than 10 µm will decrease a drug's efficacy also is likely to be limited to specific products or formulations.

Some drugmakers point out that no injectable solution is required to be completely free of subvisible particles, which have been present in marketed products for some time. Nevertheless, the link between immunogenicity and particles smaller than 10 µm has not been studied thoroughly, and our understanding of what these particles do is limited.

"I think it would be in everyone's best interest for people to develop an understanding of subvisible particles, their generation, and analysis, so as to respond if we determine that they present any type of risk," says Morrey Atkinson, chief scientific officer and vice-president of research and development at Cook Pharmica. Companies can begin by generating the data during product development, while the drug is still in the clinic. If these subvisible particles are stable over time, they are not likely to pose a risk.