Process considerations for viral clearance by membrane filtration

Membrane filtration

Failure to follow proper integrity testing during manufacturing can lead to GMP non-compliance. Here are some tests to help avoid the pitfalls

by Jerold M. Martin

There is increasing application for membrane filtration systems in providing enhanced viral safety assurance in manufacturing biological and biopharmaceutical products. Filter qualification is a critical consideration in scaling up reliably from lab to production. After drug product approval, failure to follow proper filter integrity testing parameters during manufacturing can result in GMP non-compliance, which can lead to expensive cost overruns and lost productivity.

Factors that need to be addressed during large-scale filtering in a production environment are often different from those encountered at the small-scale, laboratory level; and as the science of biotechnology progresses, biopharmaceutical manufacturers need to consider a greater range of variables-from lab to production scale-to maximize opportunities for success.

Types of filter integrity tests: destructive and non-destructive.
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For viral clearance by membrane filtration, there are critical test considerations that can better enable manufacturers to meet FDA compliance and smooth GMP standards for maximum productivity.

The necessity for a “robust” purification regimen

In developing effective purification regimens, manufacturers of biopharmaceuticals need to demonstrate removal of known and potential unknown viruses, and this often involves focusing on small, non-enveloped viruses that are harder to inactivate or remove. Incorporating a “robust” purification regimen to include adsorption, inactivation and size exclusion membrane filtration is critical for ensuring viral safety.

Size of viruses associated with blood and plasma products.
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Biopharmaceutical companies must also be sure to screen all source materials for contaminants. Virus “barriers” should be implemented to include air, vent and liquid filtration, as well as inactivation. Virus detection tests for known potential contaminating viruses should be performed at each stage of production. Filters must remove viruses while still allowing transmission of desirable proteins.

Scaling down for bench-scale studies

In qualifying membrane filter systems, it is not practical for biopharmaceutical manufacturers to validate process filtration systems for viral clearance by actual in-situ viral challenge, so processes must be scaled down for bench-scale evaluation studies.

Size of viruses associated with biopharmaceuticals.
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Membrane filters are tested as small discs or assemblies and qualified for viral reduction, protein transmission and flow properties in actual process fluids. There is increasing recognition among process developers and agency reviewers of the value of using bacteriophage (viruses of bacteria) as size-based models for mammalian viruses in qualifying membrane filters for virus removal under model process conditions.

While FDA/ICH guidelines require mammalian virus removal data in final product applications, generation of bacteriophage retention data can both reduce development costs and accelerate time to filing by eliminating repetitive, costly mammalian virus challenges. Biotechnology manufacturers must also ensure that large-scale production filters can provide the same degree of viral clearance as demonstrated with small filters in bench scale trials.

Virus filter integrity testing: air flow (~BP) versus wetting fluid and test pressure.
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Crucial to meeting this requirement is the understanding of how the integrity test developed by the filter manufacturer for the process scale filter assembly is sufficiently sensitive to reject defective filters and those at the next coarser grade, as well as predict viral clearance performance of integral filters with adequate sensitivity.

Process scale filter elements should be tested by the filter manufacturer prior to release using sensitive methods, with published data showing correlation to viral clearance claims. The ease-of-use, sensitivity and reliability of the user's integrity tests for process scale filters should be considered prior to laboratory evaluations of membrane samples to ensure the selected filter is reliable when ultimately used in production.

Special air diffusion test offers advantages

The most appropriate integrity test available for process scale filters is the Forward Flow air diffusion test, which is already in wide use to quantitatively confirm the integrity of “sterilizing grade” 0.2 and 0.1 micron rated process scale filters designed for bacterial and mycoplasma removal.

It is a non-destructive, non-contaminating test that uses air or nitrogen pressure and a specified filter wetting solution to confirm that a process filter is within specification and capable of performance demonstrated by the same grade filters subjected to destructive microbial challenges. When applied to virus removal membrane filters, the test ensures the virus filter is the proper grade, in proper working condition and able to provide validated virus removal performance with adequate safety margin both before and after usage.

The Forward Flow filter test meets a high level of scrutiny under GMP compliance guidelines. In air diffusion tests, the air test pressure and the type of wetting solution applied to the filter are critical factors in determining the sensitivity and worthiness as a filter integrity test. The Pall Forward Flow test for Ultipor VF virus filters uses an air test pressure of 85 psi, an elevated pressure that increases test sensitivity to enable rejection of even most 0.1 micron rated filters. The use of a dilute alcohol as the test wetting solution further increases test sensitivity by lowering the wetting liquid surface tension.

Forward Flow test: automated upstream measurement.
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For quality assurance, an enhanced sensitivity integrity test should be performed by the filter manufacturer on membrane in production, and also on finished elements prior to shipment. The filter user can perform the same sensitive test again after use. Similar air diffusion tests conducted at lower test pressure or only with water as the wetting fluid have reduced test sensitivity, which impacts negatively on the ability to detect defects or damage that may compromise the performance of the filter. As a result, such tests cannot distinguish virus retentive filters from 0.1 µm or 0.2 µm filters, and cannot ensure that an incorrect grade or marginally defective filter will always be detected.

The Forward Flow test is designed for automation, which provides some key advantages to manufacturers. Automation improves measurement accuracy, and offers consistent testing procedures and data that are repeatable. Current automated filter integrity test instruments from Pall meet US FDA 21 CFR part II documentation requirements, minimize operator intervention and help interface with other automated systems for large volume processing.

Viral retentive membrane filtration systems have demonstrated removal of numerous bacterial and mammalian viruses, including human and murine retroviruses, SV40 virus and various hepatitis viruses and parvoviruses such as MMV and B19. New research offers promising data on retention of transmissible spongiform encephalopathy (TSE) agents-known as prions. These data suggest an even wider application for membrane filtration to enhance viral safety in manufacturing biological and biopharmaceutical products, including the ability to scale reliably and efficiently from the lab to production.

Jerold M. Martin is responsible for Pall Corporation's technical communication and industry/regulatory liaisons related to pharmaceutical and biopharmaceutical process filtration. He holds an M.Sc. in Microbiology from the University of Toronto.



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