Pre-Use/Post-Sterilization Integrity Testing (PUPSIT) continues to be a much-discussed topic in the manufacture of sterile pharmaceutical and biopharmaceutical products. This article summarizes the outputs of a multi-year consortium between BioPhorum and the Parenteral Drug Association (PDA) on the topic.
This “Sterile Filtration Quality Risk Management” (SFQRM) consortium was formed in 2017 with a mission to explore the risk of product contamination due to sterile filtration failures, to identify the actions needed to prevent such failures, to determine under what conditions it may be appropriate to deploy PUPSIT, collect best practices for performing PUPSIT. Underlying this effort were the principles of using objective scientific data and ultimately assurance of product sterility and patient safety.
It is generally recognized that post-use filter integrity testing is sufficient to detect filter failure and ensure patient safety unless there is a possibility that a filter passing the post-use test could have allowed bacterial penetration during filtration. This possibility is the phenomenon referred to as filter “flaw masking”, hypothesized to occur when, for example, a filter is damaged during sterilization such that it allows bacterial penetration, but that the damage becomes plugged during the filtration process to such an extent that it allows the filter to exhibit a passing post-use integrity test result. Two workstreams within the SFQRM consortium were designed specifically to evaluate the risk of this filter flaw masking and to understand in what conditions it might occur: Masking Studies, and Bacterial Challenge Test (BCT) Data Mining.
Masking Studies
The objective of the Masking Studies workstream was to determine if the hypothesized masking phenomena can occur and under what conditions. To directly evaluate the possibility of filter flaw masking, this workstream executed tests where marginally flawed filters were challenged with a proteinaceous solution to plug the defects and create a passing post-use integrity test.
The following sets of flawed filters were used for the tests:
- Cartridge filters rejected from filter manufacturing lines due to marginally out-of-specification integrity test results
- Disc filters with intentionally created defects generated by laser-drilling 10 μm holes in 47mm flat disc membranes.
The results of these studies were shared in the masking studies article, “Test Process and Results of Potential Masking of Sterilizing Grade Filters”.1
Subscribe to our e-Newsletters
Stay up to date with the latest news, articles, and events. Plus, get special offers
from American Pharmaceutical Review – all delivered right to your inbox! Sign up now!
Of the 24 cartridge filters tested with 24g/L foulant concentration and 90%+ flow decay (Set 1), only two demonstrated apparent “flaw masking” with a pre-use integrity test failure followed by a post-use integrity pass. As for the laser-drilled filter discs (Set 2), an automated integrity tester was able to detect the damaged filters in all of the test conditions whether challenged with 0.8 g/L or 24 g/L foulant solutions at any blockage level (i.e. flow rate decay) up to 75%. No flaw masking could be identified as all integrity tests failed the post-use test as well as the pre-use test. At blockage levels above 75%, manual bubble point tests were performed and only two of the 27 test conditions demonstrated passing post-use integrity test results.
The results of the masking studies workstream show that flaw masking can be made to occur, but it is not likely to occur under typical drug manufacturing conditions. These studies also demonstrate some criteria for evaluating the risk of masking: If companies manufacture products with unusually high foulant concentrations and use filters to levels that approach blockage conditions, then the risk of masking may be relevant. However, if filtration processes use systems that are appropriately sized, experience minimal flow decay and filter blockage, then the risk of flaw-masking will be minimal.
Bacterial Challenge Test (BCT) Data Mining
The consortium identified an additional way to evaluate the risk of flaw masking which doesn’t require finding (or creating) defective filters. Any fluid with a flaw-masking mechanism should also cause an increase in the bubble point value of an integral filter. In other words, the relative movement of a bubble point value between a pre-use test and a post-use test of an integral filter can indicate whether a flawmasking mechanism is present (Figure 1).
The “BCT Data Mining” workstream used this concept to evaluate historical integrity test results from over 2,000 filters used in bacterial challenge tests (the bacterial retention validation performed on sterilizing filters). These historical tests provide an opportunity to evaluate whether a bubble point inflation mechanism (and thus a risk for fl aw masking) exists for any given fluid and filter combination.
The tests start with an initial confirmation of filter integrity (pre-use). Then the filters are exposed to the product, plus a high concentration of bacterial challenge organism. After the completion of the test, the filtrate is evaluated for sterility, and the integrity of the filter is also checked (post-use). The SFQRM consortium recognized that historical test reports could be data-mined to compare post-challenge integrity test results to pre-challenge integrity test results and determine whether a bubble point inflation mechanism exists for a particular filter/fluid combination.
A “bubble point ratio” was calculated by dividing the post-use integrity test result by the pre-use integrity test result. A ratio above 1.0 represents bubble point inflation, while a ratio below 1.0 represents bubble point depression. When the data were evaluated in aggregate, the mean ratio was exactly 1.00 (to 2 decimal places) for all test filters and 0.99 for all control filters (0.45 μm rated filters) with narrow distributions around these means. This indicates that there is no wholesale trend of bubble point inflation in the industry, and the same was true when separately evaluating each filter membrane material, and each type of fluid (drug product, buffer, biologic, etc.)
When looking at individual fluid/filter combinations, a small fraction (less than 1.5%) of the filter test conditions demonstrated ratios well above 1.0. These outliers represent fluid/filter combinations with a theoretical possibility of a flaw masking mechanism, at least when performed under the worst-case processing conditions and with the addition of the challenge organism. The trend identified by the Masking Studies team was confirmed here: flaw masking or significant bubble point inflation is rare, and the few fluids with ratios notably above 1.0 tended to be those for which significant fouling (flow decay) was observed. The results of these studies were shared in detail in “Datamining To Determine The Influence Of Fluid Properties On The Integrity Test Values”.2
Evaluating the Risk of Flaw Masking
It should be noted that the occasional possibility for bubble point inflation identified by the two workstreams above does not represent a risk to product safety by itself. A bubble point inflation mechanism must be combined with the use of a flawed filter in the first place to create a risk of microbial contamination of the filtrate. Not only must the filter be flawed, but it must be marginally flawed in such a way that a bubble point could still be obtained and inflated, yet sufficiently flawed to permit microbial passage. Typical use and sterilization of these filters are validated by the supplier and/or end user to remain integral under the routine processing and sterilization conditions.
From the data presented above, it becomes clear that the majority of filter/fluid combinations present negligible risk of bubble point inflation, and thus negligible risk of filter flaw masking. Moreover, a review of terminal sterilizing grade filtration applications and their lack of excessive flow decay further minimize masking risks. Even if the sterility risk of flaw masking were very remote, but there are no off setting risks encountered by implementing and executing PUPSIT, then PUPSIT might be recommended in all cases. As was shown by two additional workstreams and consortium papers described below, however, performing PUPSIT is not free from risk itself. Instead, we find a trade-off of risks. This demonstrates why it is so critical for filter endusers to perform a robust evaluation of the risk of flaw-masking for their particular fluid and filter combination.
Minimizing the Risks of Performing PUPSIT
The Risk Assessment workstream executed detailed risk assessments of the entire sterilizing filter life cycle as described in “Points to Consider for Risks associated with Sterilizing Grade Filters and Sterilizing Filtration”.3 This effort identified potential faults/failure modes that could compromise the integrity of a filter or otherwise add risk to the sterile product manufacturing process. Many of these potential failure modes occur during the execution of PUPSIT itself, which was confirmed by the results of a BioPhorum survey performed in summer 2019 by 21 drug manufacturing sites, representing 17 BioPhorum member companies.
The survey responses demonstrated that incorporating PUPSIT into a filtration process increases the complexity of the process, and with increased complexity comes risks such as:
- Requiring the system to maintain much higher pressures (60psi+) increasing the risk of sterile boundary leaks. Deviations and leaks were reported in classified areas due to burst tubing junctions in such cases.
- Longer process times associated with filter wetting, blowdowns, and need to adapt to non-standard situations (such as re-wetting the filter, re-orienting the filter to effectively remove air, etc.).
- System manipulations on the sterile side of the filter.
- Exponential increase in the complexity when using a redundant filtration design.
A similar survey performed in 2017 identified one drug product manufacturer that actually reported a process simulation (media fill) failure that was traced to a root-cause associated with performing PUPSIT.
The message taken from the survey was that with careful process design and development, filter users can mitigate (yet not completely eliminate) the increased complexity, product discard, and turnover time associated with PUPSIT.
It was with this in mind that the final deliverable of the SFQRM consortium was developed: “Points to Consider for Implementation of Pre-use Post-Sterilization Testing (PUPSIT)”.4 This paper shares PUPSIT best practices identified over the years by filtration subject matter experts at filter supplier firms and end-users, so that when PUPSIT is performed, it is performed in such a way that it reduces the risk as much as possible. This also ensures that risk assessments are not biased: When comparing risks of not performing PUPSIT with risks when performing PUPSIT, the PUPSIT case will be presented fairly, with as many best practices incorporated as possible.
A reader of this comprehensive document will become intimately aware of the challenges of implementing PUPSIT such as:
- Filter reorientation
- Sterile blow down after integrity test
- Performing PUPSIT on filters inside an isolator
- PUPSIT with redundant filters
- The further addition of new critical sterilizing gas and liquid filters required to maintain the sterile boundary and allow PUPSIT
- Manipulations of the sterile side of the filter
Recommendation
Through the work of this team, the flaw-masking phenomenon was shown to be at least theoretically possible through observation of rare flaw masking under extreme process conditions. Yet data demonstrates that for most of the fluid and filter combinations under normal processing conditions, there is no flaw masking risk. This is especially true when considering the need for a sterilizing grade filter to be properly sized to filter the entire fluid batch at hand. Based on this data, and the strong case for added process risk accompanying the performance of PUPSIT, we recommend that end-users should take a risk-based approach to the implementation of PUPSIT in filtration processes.
Because the need for PUPSIT depends so strongly on the potential for flaw masking, we recommend that any sterile product manufacturer considering a process without PUPSIT should perform a process-specific evaluation of the risk their fluid / filter combination has for flaw masking. Approaches can evaluate factors such as:
- Knowledge of fluid constituents (relevant for filtration of WFI or some other solutions where no flaw-masking mechanism can be imagined)
- The level of flow decay encountered during the process
- Direct assessment of a product’s ability to inflate the bubble point
- The use of pre-filtration, to remove flaw-masking components from the fluid stream before it encounters the final sterilizing filter.
If there is a reasonable risk of flaw masking that cannot be adequately reduced using process controls, the default position should be to perform PUPSIT. Any relative risk evaluation of “PUPSIT” and “No PUPSIT” process designs must ensure that the best possible PUPSIT scenario is compared to the No PUPSIT scenario.
References
- Ferrante, S., et al. “Test Process and Results of Potential Masking of Sterilizing Grade Filters.” PDA Journal of Pharmaceutical Science and Technology Accepted Article (Published online May 28, 2020)
- Thome, B., et al. “Datamining To Determine The Influence Of Fluid Properties On The Integrity Test Values.” PDA Journal of Pharmaceutical Science and Technology Accepted Article (Published online May 28, 2020).
- Waldron, K., et. al. “Points to Consider for Risks Associated with Sterilizing Grade Filters and Sterilizing Filtration” (Published online July 2020).
- Ensign, S., et al. “Points to Consider for Implementation of Pre-Use Post-Sterilization Integrity Testing (PUPSIT)” (Published online August 2020).