Sterile Filtration and PUPSIT - Questions and Answers at a Glance (Part 3)
Sterile filtration is a key step in the aseptic manufacture of pharmaceutical products. It serves to remove microbiological contamination and ensure the sterility of the end product. A closely related topic is PUPSIT (Pre-Use Post-Sterilization Integrity Testing), which checks the integrity of the filter after sterilization but before the actual filtration. PUPSIT is not an easy system to implement. Many companies face technical, spatial, and organizational challenges. Existing systems in particular often cannot be easily adapted for PUPSIT implementation. While some manufacturers have already successfully implemented PUPSIT, others are still experiencing difficulties or need to conduct a thorough risk analysis to justify an alternative approach or the complete avoidance of PUPSIT.
To understand the relevance and complexity of this topic, it is essential to consider the regulatory background. The revised Annex 1, which came into force in 2023, introduced significantly more detailed and stringent requirements for the manufacture of sterile medicinal products.
Despite its importance from a regulatory perspective and in terms of patient safety, implementing PUPSIT in practice can be complex. Many systems in aseptic production environments were not developed in accordance with the new Annex 1 requirements. Significant redesigns, investments, and operational adjustments are often necessary to adapt these systems to enable reliable and validated PUPSIT procedures. In addition, space constraints in cleanrooms, the need for additional equipment, and changes to SOPs and training programs add to the burden.
We therefore collected questions about sterile filtration from seminar participants, discussed them with an expert, and compiled them. Read the third of the three sets of questions below.
1. PUPSIT – as much as it makes sense – this requirement can make a sterile filter set-up very complicated – which in turn entails risks. E.G.: Ready to use, already assembled, gamma-sterilized double sterile filter system. What is your opinion on this?
I agree and for operations to perform PUPSIT can be even easier than doing pre-use pre-sterilization integrity test. However, at the end it is depending on the setup. I'm pretty sure there are ways to design it in that way that it might be complex but automated. However, this also comes with huge costs and is usually interesting if you build a new line. To re-construct an old line might be not so easy.
2. Can you make specific recommendations for preconditioning the filters? (e.g. time, temp)
This is really depending on the filter and product as well as what pressure/flow options you have in the process. I would suggest to connect with the filter supplier to get more guidance. They are very experienced with their wide range filter portfolio used by their customers.
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3. How do you assess the requirement for filter validation with "worst case" filters from the manufacturer?
I guess you mean the "minimum specification filter" which shall be close to the integrity limit during a bacterial challenge test. The definition of what is close to the limit should come from the filter manufacturer. According to my experience this might differ from supplier to supplier.
4. Is the PUPSIT setup also available for much smaller filters?
In case you want a single-use assembly I would contact the filter supplier. Usually they can customize the setup based on the filter you need.
5. Does the Annex 1 address the minimum environmental condition (cleanroom class) for the positioning of the sterile filter?
I can't find any environmental conditions for sterile filtration. However, the position should be justified. Guidelines about positions are given based on the filter conditions. For example, if the filter is sterilized in a closed system (e.g. SIP) then the rules for closed systems apply. Hence, integrity must be maintained from sterilization until end of the batch if located outside of grade A. This is similar also for single-use system.
6. Does the Annex 1 address the required distance/proximity of the sterile filter to the point of filling?
No. It doesn't specify a distance. However, depending on how you interpret chapter 8.80, the additional filter (depending on the point of view this could be seen as the redundant filter?!) should be as close to point of fill as possible. On the other hand, you should justify the location of your filter which could also be related to the number of aseptic connections instead of distance. In the chapter 4.28 "Cleanroom and clean air equipment qualification", it is mentioned that "[…] Critical processing locations should be determined by documented risk assessment and knowledge of the process and operations to be performed in the area […]".
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7. Wouldn't the likelihood of an event, where flaw masking combined with filter damage during sterilization occurs, result even in a much lower dimension than looking at both processes separately? Are there any data in the industry how realistic/likely this scenario is?
The PDA has established studies with regard to bubble point test whether a masking effect of a damaged filter happens due to fouling. Only in rare cases with extreme fouling such masking effects have been confirmed. Details can be found in the publications.
In summary
Sterile filtration and PUPSIT are integral components of a robust aseptic production strategy. While the new EU GMP Annex 1 defines clear requirements, there is still room for scientifically sound, risk-based decisions – especially for clinical batches or small volumes. It is important that every decision is documented in a comprehensible manner, justified in relation to the process, and embedded in the contamination control strategy (CCS). This is the only way to ensure long-term regulatory acceptance and product safety.
For further Q&As on sterile filtration and PUPSIT, please also see the parts 1 and 2 in previous issues of the GMP Journal.
About the Author
Matthias Schaar has been working for Novartis in Switzerland since 2007. He began building up his knowledge in microbiological quality assurance and quality control. Currently, his main focus is on supporting the validation team and routine manufacturing in the context of sterile filter validation and its application.
