Sterilization By Filtration For Parenteral & Biologic Products
What is sterilization?
Sterilization is any process that removes, kills, or deactivates all forms of life. Sterilization is related to the term sterile, which means a complete absence of viable microorganisms or microbes that have the potential to reproduce. There are seven primary methods for medical device sterilization. These methods are steam sterilization, radiation sterilization, dry heat sterilization, sterilization by filtration, gas sterilization (such as ethylene oxide sterilization), vapor sterilization, and liquid sterilization. This article takes a deeper look at sterile filtration, sterile filtration FDA guidances, and sterile filtration for bioprocessing. First, we will cover “what is sterile filtration?”
What is sterile filtration?
Sterilization by filtration (also known as sterile filtration) is a “cold” method of sterilization that removes microbes instead of killing them. Some of us use low-level sterile filtration methods at home to filter our tap water. Since sterilization by filtration works by removing microbes, sterile filtration is the only sterilization method that doesn’t rely on an elevated temperature, toxic chemicals, or another form of energy (such as gamma radiation) to destroy microorganisms. As a result, sterile filtration is excellent for products that cannot be sterilized with heat or products containing a biological agent, such as an antibody or enzyme. Sterile filtration for biologics is also known as sterile filtration for bioprocessing. Sterile filtration for bioprocessing is the only sterilization method for many medical therapies containing proteins and biomolecules.
How is Sterile Filtration performed?
Simply speaking, sterile filtration for bioprocessing (and other products) is performed by flowing a non-sterile product through a sterile filter. The sterile filter then removes particulate matter and microorganisms from the non-sterile liquid formulation. In removing particulate matter and microorganisms, the filter sterilizes the product. Filters sterilize through a combination of sieving, screening, entrapment, impaction, and electrostatic attraction of particles (including microbes). Particles are collected on the surface of the filter during sieving and screening. In contrast, entrapment occurs when particles smaller than filter pores lodge themselves within the filter’s passageway. Electrostatic attraction absorbs particles of opposite charge to the filter surface. Note that entrapped particles, or particles held with an electrostatic charge, can be dislodged and end up in the product filtrate when flow rates or pressure is increased or varied. Keep this in mind for viscous products.
Membrane filters are used for sterilizing solutions because of their ability to be nonreactive with most products, retain particles, and not shed debris. While filter membranes are mostly nonreactive, products with peptides should use filters with polysulfone and polyvinylidene difluoride to prevent accidental protein adsorption to the filter membrane. Often filter membranes are composed of cellulose esters, nylon, polysulfone, polycarbonate, PVDF, or polytetrafluoroethylene. Structurally, filters are designed to increase surface area and thus increase the flow rate. Standard filter designs are flat membranes, pleated cylinders, and cartridge structures. During use, the product enters the outside of the filter cartridge with applied positive pressure forcing the fluid inward through the filter. The sterile filtrate then exits from the center of the cartridge. The filter and housing are steam sterilized before product filtration, typically by steam-in-place (SIP) systems. Pressurization during SIP sterilization must be gradual to maintain filter integrity. Often filters are dried with compressed gas after sterilization and before use.
Factors affecting sterile filtration efficiency & particle retention:
- Type of particle—Particle source (metal, microbe, etc.), shape, charge, and size.
- Filter material—Filter composition affects the charge-related attraction of particles, including microorganisms. Van der Waals forces, hydrogen bonding, and hydrophobic attraction are filter properties affecting charge-related particle attraction.
- Filter membrane thickness—Filter thickness slows the fluid flow and affects the particle adsorption. However, a rough and thick membrane can be just as efficient as a fine, thin one depending upon the product filtered.
- Filter porosity—The smaller the porosity, the greater the retention of microorganisms and the slower the flow rate. Product-filter incompatibilities increase as filter porosity decreases.
- Temperature—Higher temperatures increase microbial proliferation and viability, thus increasing the possibility of microbial adsorption with the filter’s pore walls.
- Type of fluid being filtered—Highly viscous products will require applied pressure to move the product through the filter. Thus, microbes or therapeutic biologics present in viscous products will experience higher shear forces during filtration. Viscous products will disrupt particle adsorption to the filter membrane but will not affect size exclusion. Surface-active agents in products will bind to solid surfaces and may reduce or prevent bacterial adsorption to the filter. However, surface-active agents will not affect filter membrane structure or microbe size.
- Applied pressure, flow rate, and usage time—Pressure, flow rate, or usage time increases can harm filter integrity and affect microbe sizes. Higher pressures, flow rates, and usage times decrease filter integrity, especially when outside the manufacturer’s recommendations. Generally, commercially used membrane sterilizing filters can be used for up to one week without changes in functionality.
What affects product-filter compatibility?
According to sterile filtration FDA guidances, the product must not adversely affect the particle retention of the filter. Also, the product must not cause the filter to leach any materials into the product. Filter manufacturers provide information on the liquid volume a filter can flush before oxidizable substances are released to ensure sterile filtration FDA guidances are met. Further, filter manufacturers provide data on extractables obtained with exposure to various solvents. Common filter extractables include oligomers, mold release agents, antioxidants, wetting agents, manufacturing debris, plasticizers, and 0-ring material. Sometimes protein biologics will bind to the filter material. In this case, a pre-flush step may be used before filtration to occupy any available binding sites for the proteins and remove any potential extractables. Polyethersulfone and polyvinylidene fluoride filters are best for proteinaceous products as they are low protein-binding filters.
Data for filter-product compatibility qualification include:
- Limits for flow rates, temperature, and pressure
- Ensure that the filter meets the non-fiber releasing criteria from Code of Federal Regulations Title 21, Section 210.3
- Procedures for filter sterilization
- Diffusion rate for the in-process integrity tests
- Correlation of the integrity test value and the amount of test organism ( diminuta) retained
- Specifications and instructions for the filter integrity test
What are the problems with sterilization by filtration?
- Defining appropriate “worst-case” conditions for filter validations
- The need to validate the removal of the smallest types of organisms, for example, mycoplasma and viruses
- Effects of the filter on the product
- Bacterial deformation
- Air entrapment potential during filter sterilization by steam
- Issues with 0.2 µm filters not removing all possible microbial contamination necessitating double filtration.
Summary
Overall, sterilization by filtration is a heat-free sterilization method that works well for products that cannot undergo traditional moist heat or chemical sterilization methods. There are several factors affecting sterile filtration efficiency & particle retention. These factors include particle type, filter material, filter membrane thickness, filter porosity, filtration temperature, the type of fluid being filtered, filtration pressure, filtration rate, and filter usage time. Developing worst-case protocols for sterilization validations of sterilization by filtration processes can be tricky. Ensure you choose a contract testing organization that can provide appropriate sterilization validations for your product needs.
Ethide Labs is a contract testing organization specializing in Sterilization Validations & Sterility Testing. Ethide Labs also offers EO Residual Testing, Microbiology Testing, Cytotoxicity Testing, Bacterial Endotoxin Testing, Bioburden Testing, Package Integrity Testing & Environmental Monitoring services for medical device companies and allied industries. Ethide is an ISO 13485 certified facility.
References
International Organization for Standardization. Sterilization of health care products- Moist heat- Part 1: Requirements for the development, validation, and routine control of a sterilization process for medical devices. Geneva (Switzerland): ISO; 2006. (ISO 17665-1:2006/(R)2016).
Michael J. Akers. Sterile Drug Products Formulation, Packaging, Manufacture, and Quality. Drugs and the Pharmaceutical Sciences. Informa Healthcare. 2010.
United States Pharmacopeial Convention. <1115> Bioburden Control of Non-Sterile Drug Substances and Products. Rockville, MD, USA. 2021. (USPC <1115>).
United States Pharmacopeial Convention. <1116> Microbiological Control & Monitoring of Aseptic Processing Environments. Rockville, MD, USA. 2021. (USPC <1116>).
United States Pharmacopeial Convention. <1211> Sterility Assurance. Rockville, MD, USA. 2021. (USPC <1211>).
