After regulatory requirements have been addressed, it is time to look at the protein purification process, not only at the
current scale but also at the potential manufacturing scale, to ensure that virus filtration is implemented at the best location
in the process.
Parvoviruses have a diameter of ~18–26 nm, but a typical monoclonal IgG antibody has a hydrodynamic diameter of ~8–12 nm.
To achieve >4 log10 retention of the viruses and >99% recovery of the protein, parvovirus filters are required to have a very narrow pore size
distribution. They are, therefore, generally sensitive to the presence of impurities in the feed solution. Thus, optimizing
a virus filtration process involves evaluating the effect of a variety of process parameters to arrive at conditions that
will ensure a robust, consistent, economical, and scalable operation.6–9
Impact of Location in the Downstream Process Train
Typically, a normal flow virus filtration step can be implemented at any one of several points in a given downstream process.
As shown in Figure 1, for a typical monoclonal antibody process, the virus filtration step can conceivably be implemented
at three locations in the downstream purification process: following the low-pH inactivation step, following the intermediate
chromatographic operation, or following the final chromatography step. Because protein concentration, impurity concentration,
and process volumes can vary throughout the downstream process, the actual filtration requirements—including the required
filter area—are highly dependent on where in the process the virus filtration step is located.
Figure 1. Virus filters can be located at various points in a typical protein purification process. Options for locating
a virus filtration step are 1) following the low pH inactivation step 2) Following an intermediate column chromatography step
and 3) After the final column chromatography step.
Impact of Feed Concentration
Feed solution concentration can affect the virus filtration process by reducing product throughput. The level of the impact
will depend on the interaction between the filter and the components in the solution being filtered. In general, higher protein
concentrations reduce the average process flux through the virus filter. The dependence of average process flux on changes
in protein concentration is both protein specific and virus-filter specific. The effect of increasing filter capacity and
flow at lower product concentrations is offset by an increase in process volume as the product is diluted. The interplay of
these two competing effects can often result in an optimum feed concentration that minimizes the required filtration area.
Figure 2 shows the impact of the feed concentration on the filtration area needed to process the same quantity of protein
in a fixed time.
Figure 2. The effect of protein concentration on filter area needed can be significant. In this case, the optimum concentration
is between 8 and 10 g/L. The optimum concentration can vary, depending upon the protein purity and the buffer conditions.