Key Challenges/Gaps/Mitigations
Despite the overall robustness and effectiveness of virus retentive filters for the retention of large viruses (e.g. retroviruses) and small viruses (e.g. parvoviruses), the retention of small viruses can be highly variable and not always effective. While the newer generation small virus retentive filters appear to have less variability (Lute 2015) (Figure 2) there are still concerns with the occasional less effective clearance values. To address these concerns, the viral validation study is typically performed under challenging, or worst-case, conditions. The science seems to indicate that the target operating pressure, low pressure, a flow pause, and a buffer chase should be included during each scale down viral validation run. A pause typically occurs between the load and flush phase and is different from running at low pressure because liquid only flows through the filter during the low-pressure phase. There have been attempts to perfectly match the pressure ramp up and ramp down dynamics during viral filtration (Roush and Ma 2016), however it would be impossible to do a comprehensive experiment covering all ranges and combinations of pressure conditions and pauses that may occur during routine manufacturing. High pressure is typically not considered worst case as the limits are determined by filter structural robustness.
A variety of ways exist to set end point limit for a viral filter step. For some early model filters, flux decay due to pore plugging (Bolton, Cabatingan et al. 2005) and total virus loaded per filter area was reported to decrease virus retention (Lute, Bailey et al. 2007). Similarly, it has been reported that mass of protein per filter area (Soluk, Price et al. 2008, Chen and Chen 2015, Roush and Ma 2016, Kreil and Roush 2018) or alternatively, volume of protein solution per filter (Chen and Chen 2015, Roush and Ma 2016, Kreil and Roush 2018) may be a worst case condition. For instance, operating a low load concentration will provide the highest volumetric (liter/m2) throughput and viral loading but the lowest mass (gram/m2) throughput. Conversely, operating using a high load concentration will provide the lowest liter/m2 throughput and viral loading but the highest gram/m2 throughput, given the tendency for increased filter fouling at higher load concentrations, particularly when using scaled down filter membranes. While many ways fouling may occur and how end point limits are set, in-process controls are best described in terms of liters per square meter of filter area.
Recent and emerging applications of existing technology