[This article was originally published on PharmTech.com in January 2009. It was later published in the April 2009 issue of Pharmaceutical Technology; Vol. 33, Issue 4, pp. 74-79.]

Silicone oil, because of its inert biologic properties, is widely used in the pharmaceutical industry as a lubricant for rubber stoppers, plungers, and prefilled syringes (1). Silicone oil is applied in these syringes to facilitate easy movement of the plungers within the barrels. One consequence of using silicone oil is the potential for microdroplets to shed from the coating into the drug formulation. Biopharmaceutical products are sensitive to silicone oil because it can interact with protein-based drugs or active ingredients producing particles (2–7). The presence of silicone oil droplets in parenteral formulations is also suspected of causing protein aggregation (4, 7). The potential impact of silicone oil contaminants on drug products is possible in some cases for certain sensitive therapeutic proteins and antibodies, especially with active biopharmaceutical drug products that may contain mere femptograms of active ingredient. As a result, quantification of silicone oil droplet populations in chemical and pharmaceutical products may be important. When these products also contain populations of particles composed of other materials such as protein aggregates, this is a challenging assignment.

United States Pharmacopeia General Chapter <788> and the harmonized versions in the European and Japanese Pharmacopoeias (8–10) set limits and cite enumeration methods for subvisible, foreign particulate matter in parenteral products. USP <788> does not consider intrinsic sources of particulate matter such as protein aggregates, silicone oil droplets, air bubbles, micelles, or precipitates. However, the test methods cited in the section, which include automatic light obscuration and filter membrane microscopic analysis, might also in principle be applied to mixed silicone droplet and aggregate populations. However, both particle types are highly transparent and not easily measured by either of these techniques. In addition, automated light obscuration provides no capability, and membrane microscopy limited capability, to differentiate between these two highly transparent particle types.

Micro-Flow Imaging (MFI, Brightwell Technologies Inc., Ottawa, ON, Canada) uses digital imaging of a flowing sample stream for analyzing particles in suspension. Brightfield images of individual particles are captured as a sample stream passes through a flow-cell centered in the field of view of a custom magnification system having a well-characterized and extended depth of field. Images are analyzed by the system software to extract each particle. The software compiles a database containing count, size, and concentration as well as morphological parameters like intensity, circularity, maximum Feret's diameter and aspect ratio. This database is analyzed using the system software to produce particle distributions and isolate particle subpopulations using histograms and scatter plots of particle statistics (11). The instrument has a particle size range of 2 μm to 300 μm for low magnification (400 μm flow cell) and 0.75 μm to 75 μm for high magnification (100 μm flow cell) systems. Particle subpopulations in parenteral samples are isolated, quantified, and characterized by applying custom-designed morphological filters using parameters such as aspect ratio, circularity, and intensity. MFI complements <788> techniques by assisting in classifying the different extrinsic and intrinsic particle types present in injectable solutions.