Monitoring of subvisible particles in therapeutic proteins

The unintended presence of particulate matter in injectable products is an indicator of the quality of the product. Subvisible particulates have historically been monitored through methods such as light obscuration and membrane microscopy, as outlined in the United States Pharmacopeia (USP) General Chapter <788> or the equivalent Ph Eur 2.9.19 and the Japanese Pharmacopeia General Chapter 20. These methods were designed to protect patients against the risk of capillary occlusion though the infusion of "foreign" particulate matter. With the development and commercialization of protein therapeutics, the application of these methods has to be adapted to the special requirements posed by such products. Apart from the "foreign" particulates, therapeutic protein products may also contain particulates that are inherent to the product, arising as a consequence of protein self-association or aggregation. The nature of these inherent "proteinaceous" particulates is generally different than the traditional "foreign" particulates. Proteinaceous particulates tend to be of an amorphous irregular morphology, soft, with a refractive index resulting in low contrast against an aqueous background, making them more difficult to detect and count compared to the "foreign" particulates. The growing realization of the importance of monitoring all (foreign as well as inherent) particulates in therapeutic protein products has led to a number of developments in this area including techniques of measurement. Here, we summarize a number of the procedures used for subvisible particle measurements, with new techniques as well as refinements to existing techniques.