Issues and challenges of subvisible and submicron particulate analysis in protein solutions

The analysis of particulates has been a longstanding challenge in biopharmaceutical drug product development and quality control because the active constituents themselves may form particulate matter as a degradation product that may be difficult to quantify. These analytical challenges were met with success as long as the definition of particulate matter remained well within the capabilities of the instruments and methods used to measure it. The current testing as per USP <788> for parenterals at ≤100 mL stipulates that the sample "passes" the test if the average number of particles present does not exceed 6,000 per container at ≥10 μm and does not exceed 600 per container at ≥25 μm. The new challenge, posed by regulatory direction and academic research, is to count and to characterize subvisible particulates that are ≤10 μm with the goal of providing higher resolution information about the particulate levels and potential consequences of this product quality attribute in vivo. The present discussion focuses on two parallel efforts: (a) to develop a model system for protein subvisible particulates in samples with high protein concentrations and (b) to evaluate the capabilities and limitations of different technologies available (at the time these studies were conducted) for subvisible and submicron particle (<1 μm in diameter) sizing and counting. Our findings illustrate the importance of using appropriate instrumentation that is adapted to the characteristics of the samples to be analyzed. Any sample manipulation to meet the capabilities and to accommodate the limitations of the analytical technique should be carefully evaluated.