Clean Versus Sterile
It is recognized that any medical device that punctures the skin or any medication that enters the bloodstream must be sterile, and sterilization procedures are reasonably well-established in the pharmaceutical and medical device industries. What may not be as well understood is that devices and medications should also be essentially free of particles (i.e. clean). Parenteral products (medication or nutrition introduced into the body via infusion, injection or implantation) must be both sterile and clean. Controlling unwanted particulate matter in products requires knowledge of potential sources of particles and processes that might generate particles to avoid potential problems. It is important to monitor the products through their lifecycle to maintain their integrity.
Sterilization is the process used to remove or kill any microbiological activity such as fungi, bacteria, viruses and spores on surfaces or in solution. Steam sterilization and sterile filtration are most commonly used in the pharmaceutical industry. The sterile filtration process involves passing the product through a 0.2 µm filter and has the added benefit of removing unwanted particulate that may be present. The filtration and filling processes are performed in cleanroom facilities to further reduce the chance of introducing particles. The containers (vials or syringe cartridges) are cleaned prior to filling. These processes are usually well-controlled and are very effective in reducing or eliminating unwanted particles in parenteral products. However, particles do slip through these stringent procedures, and on these occasions, the source of the problem needs to be identified.
Getting to the Source of the Problem
Sources of particles include environmental debris, such as fibers and fiber fragments. Processing equipment may generate foreign particles. Particles may also originate from the container or packaging itself or interaction of the product with the packaging. Less commonly, the product itself may react and form particulate matter over time. These types of particles often manifest themselves during accelerated stability studies in the drug development process.
Parenteral containers are inspected for the presence of particles, either manually by trained inspectors and/or using automated methods. Products with high rejection rates are often submitted for particle identification to identify the root cause and prevent further problems. The size range for visible particles has not been fully established, but is usually defined as greater than 50 µm. Parenteral products are tested for the presence of subvisible particles using the USP <788> method. Part I of the method outlines the light obscuration method to test for particles in the >10 µm and >25 µm size ranges. The second part of USP <788> is a microscopic method utilizing filtration of the product onto a gridded filter membrane, then using a light microscope to obtain a particle count. The second part of the method is often used when particle counts are high, or the product is not suitable for the light obscuration method.
Silicone oil, used in pharmaceutical containers as a lubricant for vial stoppers and cartridge (syringe) plungers, is a common source of particulate; it is also applied to the tops of the stoppers as an aid in processing. Under normal conditions, the silicone used in the packaging does not present a problem. However, if higher amounts of silicone are present in the samples, it interferes with the USP <788> light obscuration method and can result in high particle counts. The samples are filtered through the membrane and the particles are counted using a microscope. The oil is absorbed into the filter and is not observed as particles. The silicone oil can be confirmed by filtration of the product onto a different type of filter membrane - usually a polycarbonate filter membrane. The oil, if present, can be observed more easily and can be extracted from the membrane and analyzed using Fourier Transform Infrared Spectroscopy (FTIR) to confirm the presence of silicone. High amounts of silicone may also be detected visually as a haze in the solution. Silicone may also interact with protein-based drugs and form amorphous stringy particulate that collapses onto the filter membrane. The amorphous residue is not defined or counted as a particle using the USP <788> microscopic method and is not readily observed on the gridded filter membranes. Using proper filters and lighting methods, the protein/silicone residue can be isolated and confirmed using FTIR.
Particles can also be generated from the glass containers. Glass provides a relatively stable material for sterile packaging, but it is not an entirely inert material and can be subject to degradation. In severe cases, glass delamination flakes (or lamellae) appear in solution. These flakes are very thin and vials are often described as having a “twinkling” appearance when examined using a fiber optic light source. However, the flakes themselves are difficult to see on the filter membrane, so if glass delamination is suspected, it is crucial to use the proper filter membrane and lighting to see the flakes. It is also important to inspect the inside surfaces of the vial to look for evidence of delamination. Glass delamination may occur due to irregularities in the glass container or interactions with the product. Processing of the vials and products may also lead to delamination. Some active ingredients may attack the glass.
By the time the glass flakes appear in solution, the problem is hard to fix and can result in product recalls and possible shortages of vital drugs. It is important to check for product interactions with glass during the early stages of new drug development. Vial manufacturers offer many different types of glass vials that can be tested for compatibility with the drug product as part of the stability study.
Particles can also be generated during the use of the product. When a drug product is administered as an infusion, the contents of the product vial are removed using a syringe with a hypodermic needle, then added to the infusion bag. The needle may remove a portion of the stopper rubber, which is added to the bag along with the drug. The stopper coring particle is then observed in the infusion bag as a potential contaminant, and the product is not administered to the patient. The coring particle can be isolated and analyzed to confirm that it was related to the packaging and is not an extraneous particle.
As previously mentioned, vials are cleaned prior to sterile filling and these processes are carried out in cleanroom environments. On rare occasions, fibers may be introduced during filling and observed during the inspection of the vial. The most common type of fibers observed are cotton and paper fibers and fiber fragments (linters). Polyester fibers may also be observed. Glass vials are cleaned prior to use and then heated at high temperature (depyrogenation) to kill endotoxins. If fibers or plastic wrap are not removed from the vials prior to depyrogenation, they are charred and may appear in the filled vials as charred organic material. The charring may prevent the identification of the original material, but the presence of charred material usually indicates that it was present prior to depyrogenation.
Preventing Future Problems
In summary, pharmaceutical products and devices that enter the blood stream must be sterile and should also be essentially particle-free. However, sterilization does not guarantee that products are clean. Monitoring for particles is important and should be included in all phases of drug development and stability studies, as the identification of particles is necessary to determine the source (root cause), prevent future problems and help ensure patient safety.