Subvisible particles are a concern in the biopharmaceutical industry due to their possible correlation with immunogenicity. The impact of subvisible particles on biologics is unknown but potentially adverse, and is complicated by the lack of technologies that can adequately measure them.

Subvisible particles are usually defined as those that are too big for analysis with size exclusion chromatography, but too small to be seen with the naked eye—generally ?0.1 ?m to <100 ?m. This definition, however, is not set in stone. “That’s the debate that’s been going on in the industry for years,” said Dr. Desmond Hunt, senior scientific liaison to the United States Pharmacopeia (USP) General Chapters—Packaging, Storage and Distribution Expert Committee. “Some say the cutoff is at 50 ?m; some say 100 ?m. I don’t think USP or our committee members have put a stake in the ground as far as where that lies.” Regardless of the specifications, there has been much talk within the industry and regulatory agencies in the past few years about the need for more accurate detection of subvisible particles. At a Chemistry, Manufacturing, and Controls Strategy Forum held in Washington, DC, last January, FDA Office of Biotechnology Products Division of Therapeutic Proteins Deputy Director Barry Cherney stressed the importance of limiting the amount of subvisible particles in biologics. “If there is a theoretical risk of uncertain significance, in the absence of convincing data that the attribute is not critical, you should assume it is critical and needs to be controlled,” he stated. Concerns are that subvisible particles could render a product that has been shown to be safe and effective in the clinical trial stage less so after it is on the market. Subvisible particles cannot be eliminated entirely from biologics, but the desire to make the safest, most effective products possible has led to the drafting of Chapter <787>, “Particulate Matter for Biopharmaceuticals Injections,” by the USP’s Dosage Forms Expert Committee. The chapter will apply specifically to biologics, which previously adhered to the guidelines of <788>, “Particulate Matter in Injections,” which specifies light obscuration and membrane microscopy as the methods for particle determination.

However, <788>, which was developed for small-molecule parenterals, has been problematic for biologics. The major issue the biopharmaceutical industry experienced with <788> was that it required a large volume of a product for testing. “If you’re in development, using 25 mL can be quite a large amount of your development product, so it was not practical for large volumes,” Dr. Hunt said. Other issues were biologics’ tendency to be viscous and <788>’s inclusion of sonication, which can introduce more particles into the product, he added.

Major aspects of <787> will be sample preparation, sampling volume and how to manage viscous products, as well as examining other particle size ranges, Dr. Hunt said. The USP specifies that the tests outlined in <788> are for particles ?10 ?m and ?25 ?m in size. “It is a big discussion within the industry about the immunogenicity of protein conglomerates less than 10 ?m, so there’s something in the chapter that touches upon that,” he said. This will likely be a relief to the industry. There is particular concern over the absence of standards for subvisible particles below 10 ?m, as multiple therapeutic protein studies have indicated that they are much more prevalent. The chapter has been written and approved and will be published in June 2012 in Pharmacopeial Forum. Dr. Hunt believed that the final version of the chapter will be close to what exists now.

Like Chapter <788>, Chapter <787> will include light obscuration as the primary method and membrane microscopy as the secondary method. A chapter above 1000, <1787>, will be developed to address other technologies for particle size analysis. “The question is, if we introduce [another technology] into the chapter at this time, what specifications we put around it,” Dr. Hunt explained. The other techniques will be added after <787> because the biotech industry wants a chapter out as soon as possible, Dr. Hunt told IBO. One technique that will be included in <1787> is flow microscopy. Since different instruments have different sensitivities and algorithms, different chapters to address different particle properties will be necessary.

Having <787> released in the meantime will likely benefit the biopharmaceutical industry in regard to FDA submissions. “If there’s a chapter already out, they can say, ‘We tested <787>, so it reduces their regulatory burden,’” he explained. “So we wanted to get the chapter out as quickly as possible before we started introducing new technologies and new specifications around them.”