In recent years, pharmaceutical companies have paid increasing attention to quality risk management. Coupled with the improvement of regulations, guidelines, and standards, as well as strengthened supervision, the critical role of sterilizing filtration in the production of sterile products has been widely recognized. However, there is an important step before sterilizing filtration that is easily overlooked: Bacteria-Reduction Filtration, a key step preceding sterilizing filtration.
Sterilizing filters are widely used in the production of sterile pharmaceuticals. Based on the purpose of filtration, the process is divided into sterilizing filtration and bacteria-reduction filtration.
What is Bacteria-Reduction Filtration?
Unlike sterilizing filtration, bacteria-reduction filtration aims to reduce the microbial contamination level in the medium to be filtered to an acceptable degree through physical retention.
"Microbial control should be applied throughout the production process of sterile products to avoid microbial contamination. Before the final sterilizing filtration, the microbial contamination level of the medium to be filtered should generally be less than or equal to 10 CFU/100mL." — NMPA (National Medical Products Administration) No. 85, 2018, "Technical and Application Guide for Sterilizing Filtration"
Bacteria-reduction filtration is typically applied at two key points:
• One is before the filling of sterile products produced by a terminal sterilization process.
• The other is before the sterilizing filtration step for sterile products produced by a non-terminal sterilization process.
Its core purpose is to ensure that the microbial contamination level meets the expected standard before final sterilization or sterilizing filtration, thereby guaranteeing the cleanliness and safety of the filtration process.
Sterilizing Filtration vs. Bacteria-Reduction Filtration
Sterilizing and bacteria-reduction filtration have distinct roles in a filtration system but also share some control requirements.
Commonalities include:
• Both must limit the release of particulates and endotoxins to prevent re-contamination of the liquid.
• Both need to control extractables that may be released from the filter cartridge material to avoid affecting product quality.
Differences lie in:
• Sterilizing filtration aims to completely remove microorganisms and maintain product sterility, but its small pore size makes it prone to clogging.
• Bacteria-reduction filtration aims to reduce, not eliminate, microorganisms. Its larger pore size reduces the risk of clogging.
In summary, in practical applications, these two filtration processes are often used in combination. For example, in the production of sterile preparations, a bacteria-reduction filter may be used to pre-treat the drug solution to lower the microbial load, followed by a sterilizing filter to ensure the final product's sterility.
Bacteria-Reduction Filtration Process
In a terminal sterilization process, the material is first processed in a compounding tank, then undergoes initial filtration through a titanium rod filter (if applicable). The drug solution passes through two stages of pre-filtration to further remove larger particles, then through a bacteria-reduction filter to lower the microbial count. Finally, after filling and sterilization/packaging steps, the product's sterility is ensured.
In a non-terminal sterilization process, WFI (Water for Injection) and powder are mixed in a mixing tank, then passed through a bacteria-reduction filter to remove microorganisms. The drug solution is passed through a sterilizing filter for further sterility assurance.
In summary, bacteria-reduction filtration plays a crucial role in both processes. It not only reduces the microbial bioburden but also provides protection for the subsequent sterilizing filtration, thereby ensuring the sterility and safety of the pharmaceutical products. Through rational filtration process design, filtration efficiency can be improved, production costs can be reduced, and product quality can be guaranteed.
Bacteria-Reduction Filtration System Design
Bacteria-reduction filtration is generally designed to be implemented prior to the filling or sterilizing filtration stages. Its primary objective is to reduce the microbial load in the drug solution to an acceptable level, rather than to completely eliminate microorganisms. Nevertheless, bacteria-reduction filtration can effectively control microbial contamination and remove impurity particles. Furthermore, its application helps to lower the pre-sterilization microbial bioburden before moist heat sterilization, thereby reducing the pyrogen levels post-sterilization.
Filtration Process and System Design – Redundant Filtration
There are three common configurations for liquid filters, though other design approaches are also available (Figure 3).
Figure 3. Three Common Designs of Redundant Filtration Systems
Gas filters are typically installed in Grade A, B, C, or D cleanrooms according to specific product process requirements. If necessary, they can also be installed in unclassified areas, such as compressed air filters for autoclaves.
Hardware Configuration of Filtration Systems
Whenever conditions permit, fully enclosed systems capable of Clean-in-Place (CIP), Steam-in-Place (SIP), and in-situ integrity testing are the preferred choice for aseptic processing.
A typical filtration system comprises: a pre-filtration holding tank, a post-filtration holding tank, a pump, an integrity tester, filters (housings and cartridges), pressure measurement devices (pressure gauges or transmitters), temperature recording devices (thermometers or sensors), filtration piping, and condensate drainage devices (e.g., steam traps).
General requirements for stainless steel filter housings used in sterile product filtration include:
l Parts in contact with the product fluid must be made of 316L stainless steel.
l High pressure and temperature resistance (e.g., 121°C, 150 psi / approx. 10 bar).
l Sanitary clamp connections for product inlet and outlet.
l Sanitary hose connections for venting and draining.
l Easy drainage of condensate.
l Easy venting of cold air.
l Minimal product fluid hold-up (dead volume).
l Mechanically polished or electropolished surface finish.
l Gaskets and O-rings with excellent chemical compatibility.
Figure 4. Valve and Piping Design of a Redundant Filtration System
Post-Filtration Tank Positioning Requirements: Within a filtration system, there are multiple configurations for the placement of the post-filtration or buffer tank. For specific details, please refer to Figure 5.
Figure 5. Three Designs of Filter and Tank Configurations with Their Advantages and Disadvantages
Bacteria-reduction filtration systems should utilize 0.45 μm or 0.22 μm (or smaller) filters to reduce microbial contamination. During design, factors such as filter sizing, fluid volume, filtration time, and pressure drop must be considered to ensure process control. Additionally, the microbial level of the fluid must be monitored throughout the filtration process, as the fluid is non-sterile both before and after filtration.
Reuse of Bacteria-Reduction Filters
Liquid bacteria-reduction filters are designed for single-use or continuous multi-batch production. In practice, they are sometimes used for multiple production runs of the same product. Reuse generally refers to employing the same filter for multiple batches of the same liquid product. The following scenarios are considered reuse:
l Flushing between batches.
l Flushing and sterilization between batches.
l Cleaning, storage, and sterilization between batches.
The decision to reuse a filter should be based on a thorough understanding of product and process risks, determined through a formal risk assessment. Risk factors to consider include:
l Microbial breakthrough (bacterial penetration).
l Filter integrity defects.
l Increased extractables.
l Suitability of the cleaning method for removing all product components.
l Quality risks posed to subsequent batches by product residues (or derivatives formed after sterilization).
l Premature filter clogging.
l Performance alterations caused by aging of filter components.
Filter reuse must be validated, taking into account fluid characteristics (e.g., pH, concentration), filtration and sterilization conditions, the number of use cycles, and batch size. Testing should be conducted under the most challenging (worst-case) conditions to ensure the safety and efficacy of the filter. Refer to Figure 6 for the validation items required for filter reuse.
Figure 6. Validation Items for Filter Reuse
In summary, when employing a bacteria-reduction filtration process, the pre-sterilization microbial contamination level must be maintained within acceptable limits. Filter reuse should not compromise the level of microbial contamination control.
Filter reuse must remain within validated process parameters. During operation, critical parameters such as the number of sterilization cycles, filtered batch volume, upstream/downstream pressure drop, temperature, total number of filter uses, and integrity test results must be continuously monitored.
Bacteria-Reduction Filtration System Filter Validation
When performing filtration process validation, the chemical compatibility between the filter cartridge material and the product must be considered, as well as the impact of substances that may migrate from the filter cartridge on the terminal preparation. At the same time, according to the requirements of NMPA's "Technical and Application Guide for Sterilizing Filtration" and EU GMP Annex-1, the validation process should include tests for chemical compatibility, extractables/leachables, and adsorption.
"The normal operation of bacteria-reduction filtration is an important measure to ensure that the microbial contamination level of the product before final sterilization (or before sterilizing filtration) meets an acceptable level. Bacteria-reduction filtration process validation should include chemical compatibility, extractables/leachables, and adsorption." — 2018 NMPA No. 85, "Technical and Application Guide for Sterilizing Filtration"
"8.81 The selection of components for the filtration system and their interconnection and arrangement within the filtration system, including pre-filters, should be based on the critical quality attributes of the product, justified and documented." — 2022 EU GMP Annex-1
In summary, whether for managing process risks or meeting regulatory requirements, the validation of bacteria-reduction filters is a consideration.
For detailed validation content, please refer to the upcoming sharing on "Qualification and Validation of Liquid Sterilizing Filtration," including sterilizing filter qualification, filtration process validation, qualification and validation recommendations, and validation items.
Conclusion
Bacteria-reduction filtration is a key technology in the production of sterile pharmaceuticals. It provides strong assurance for product quality and patient safety by precisely controlling microbial contamination levels. With the development of pharmaceutical technology, bacteria-reduction filtration will continue to play its important role in the production of sterile pharmaceuticals.
Regulatory References
• Pharmaceutical GMP Guide (2023 Revision) — Sterile Preparations, Volume I
• PDA Technical Report No. 26, Revised 2008, Sterilizing Filtration of Liquids
• EU Guidelines to Good Manufacturing Practice, Annex 1 — Manufacture of Sterile Medicinal Products, 2022