Many parvovirus filters exhibit decay in virus LRV with filter plugging.11,12 Furthermore, impurities present in virus preparations can result in premature plugging of some virus retentive filters.13 These points should be considered when designing virus retention studies in order to obtain the desired throughput and the
desired virus reduction. When the desired throughput or LRV cannot be obtained because of issues with the protein solution
or virus spike quality, it may be useful to discuss alternative spiking methods with the appropriate regulatory agencies.
A sample decision tree for designing a virus retention qualification study is shown in Figure 4.
Figure 4. Decision tree for designing a virus retention qualification study
VIRUS FILTER–RELATED CONSIDERATIONS
After the virus clearance step has been optimized and virus retention studies completed, an implementation strategy is required
for robust process operation. After determining the filter capacity (L/m2) required for a process during process simulation, process scale-up, and virus retention studies, the filter area required
for processing a given batch volume can be calculated. Various filter configurations are made available by manufacturers to
facilitate large-scale implementation. When multiple filter modules are required to process a given batch volume, the modules
may be installed in parallel within a multi-round housing.
A typical sequence of operations in a virus filtration process includes the following steps. (Some of these will be discussed
later in more detail.)
- Filter installation, flushing, and water permeability measurement
- Sterilization and sanitization
- Pre-use integrity testing
- Buffer pre-conditioning and permeability measurement
- Processing and product recovery
- Post-production integrity testing
Sanitization and Sterilization
In a typical downstream purification process, virus clearance filters are used downstream of a chromatography column and upstream
of an ultrafiltration/diafiltration step, neither of which is considered an aseptic operation. However, there appears to be
an industry trend to sanitize or sterilize the virus filter to reduce the bioburden. Some virus filters are available pre-sterilized
and, therefore, will eliminate the sanitization step. It is important to ensure that the filters are compatible with a sanitization
or sterilization method that is likely to be implemented at manufacturing scale. Furthermore, it is important to ensure that
process steps used during large-scale processing are also carried out during the scaled-down virus retention studies.
To ensure that virus clearance is consistent with results obtained during virus retention studies, it is recommended that
filter integrity be checked both pre- and post-use.14,15 To facilitate this, filter manufacturers have developed a variety of destructive and non-destructive physical integrity tests
that are related to virus retention. Ultimately, the objectives of properly designed physical integrity testing are threefold:
- To confirm that the virus removal filter is properly installed
- To confirm that the filter is free from gross defects and damage
- To confirm that the filter removes viruses consistent with both manufacturer's specifications and end-user virus retention
To satisfy these requirements, a series of tests is needed to confirm filter integrity. Some of these tests, typically carried
out by the end-user, are better suited for confirming proper installation and for confirming the absence of gross defects.
Other tests, generally performed by the filter manufacturer, may be better suited for detecting subtle changes in filter pore
size distribution. A more detailed summary of the various tests can be found in Parenteral Drug Association Technical Report