Virus filtration is a robust technique which is a vital part of the overall viral clearance strategy (inactivation and removal). Filtration’s size exclusion mechanism complements other inactivation or removal techniques by targeting the physical dimensions of the virus to enable a high degree of virus safety through a method orthogonal to inactivation or adsorption.
Flow pausing and/or filtration pressure interruption during virus filtration has shown to increase the risk of virus passage. In this application note we discuss risk causes and mitigation strategy for our Pegasus™ Prime virus filters.
Start/stop process interruption risk factors for virus filtration
Process interruption sometimes occurs in clinical or commercial scale manufacturing due to risk factors including:
Common risk factors
- Feed vessel switching
- When you finish the product filtration from one feed vessel and then move on to a buffer for product recovery.
- Where you have multiple product storage bags or tanks, which you need to switch mid run.
Less likely risk factors
- Power outages
- Mechanical failures
- Scheduling of personnel / shift breaks
How process interruptions lead to loss of virus removal performance in virus filters
The loss of virus removal performance in some small virus retentive filters when the process is interrupted is primarily caused by diffusion. While size-exclusion is the primary retention mechanism for virus retentive filters, since the viruses are not permanently adsorbed, there is always a small chance that viruses can diffuse from fully retentive areas of the membrane matrix to areas along a lower-retention flow path. During normal processing the convective flow rate is much greater than the diffusion rate of viral particles and therefore minimizes this effect. However, where the convective flow is reduced or eliminated by pressure interruption, a small number of viruses may diffuse to a position where they could pass through the membrane.
Although multiple factors which relate to diffusion can be considered (temperature, viscosity, virus load), the key risk factors that exacerbate this phenomenon are:
- Duration of pressure interruption
- Magnitude of pressure interruption
Pegasus™ Prime virus filter pressure interruption study
To assess the impact of pressure interruption on Pegasus™ Prime virus filters we conducted a study with two process interruptions. IgG in PBS was spiked with bacteriophage PP7 and filtered at 2.1 bar (30 psi/0.21 MPa). Data represented in Figure 1 are the mean values from triplicate tests. LRV (log reduction value) x-axis error bars represent the size of the aliquots taken for the data point shown.
All aliquot virus concentrations were below the limit of quantitation therefore there is no observed difference in LRV before and after each 1 h process stop.
Fig 1. Pegasus™ Prime virus filter pressure interruption study showing no impact on retention of two 1 h pauses. All filters had PP7 TR > 7.4 log at the end of the challenge.
Recommended virus filter risk mitigation strategy
We recommend always attempting to minimize or avoid pressure interruption during process development and process scale. Even if the impact of pressure interruptions is not quantifiable for a particular virus filter, the theoretical risk exists and interruptions represent a worst-case scenario. Automated feed vessel switching using process-scale virus filtration systems (see Fig 2) can significantly reduce the impact or prevent the occurrence of pressure interruption. Other mitigation measures include:
- Ensuring pressure interruption steps are included as part of virus filtration validation studies.
- Implementing process control of pressure interruptions and pressure differentials.
- Recording pressure differentials in batch documentation.
- Preventing or mitigating the possibility of power outages.
- Mitigating the probability of equipment failure (e.g., pumps or valves) through regular maintenance schedules.
- Scheduling personnel to monitor and control your process.
Although no virus filter can claim to be completely immune to this effect, choose a virus removal membrane proven to be highly robust to process interruptions.
Fig 2. Differential pressure control of Allegro™ Connect virus filtration system during vessel switching. This demonstrates a smooth transition to the buffer flush phase, avoiding deviations in the critical pressure differential processing parameter.
Conclusion
The data in this application note supports the successful operation of Pegasus™ Prime virus removal filters with two separate pressure interruptions of 1 h. The pore structure and pore size distribution of Pegasus™ Prime virus removal membrane from the macro- to the nanoscale allows excellent throughput performance while still restricting diffusion of captured viruses.
To summarize:
Step 1 Evaluate the possible risks in your process-scale virus filtration for pressure interruption.
Step 2 Mitigate or eliminate the pressure interruption risks.
Step 3 Based on the process control limits implemented in Step 2, a risk-based approach should be taken to define pressure interruptions during virus validation.
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