Extrapolating process capacity using clogging models
From Figure 7b, reference plasma IgG (no aggregate spike) showed good
agreement between the experimental and calculated values for all four
clogging models, but this is based on experimental results from
filtration throughput up to 100 L/m2.
It is of great interest and
thought to be of importance to know whether this experimental data could
be used to predict filtration behavior for throughput beyond this range.
In order to calculate and compare the maximum practical throughput for
the filtration of plasma IgG solution without aggregate spiking shown in
Figure 6a, the theoretical filtration behavior plots calculated using
each clogging factor and clogging model for much larger filtration
volumes is shown in Figure 8. From this extrapolation, for larger
filtration volumes, the different clogging models have varying maximum
filtration volumes: about 250 L/m2 for complete
blocking model, about 350 L/m2 for standard blocking
model, about 1000 L/m2 for intermediate blocking model
and 2000 L/m2 or more for the cake filtration model.
Discussion
Based on analysis of filtration behavior and the impacts of the addition
of column chromatography on filterability, users can consider choosing
chromatography resins to improve the overall performance of their virus
filtration process. For aggregate-spiked mAb processing, the output from
mixed-mode AEX1 and mixed-mode AEX2 showed improved filterability while
normal AEX did not. Based on manufacturer information on the resins,
mixed-mode AEX1 has primary amine and butyl base, and mixed-mode AEX2
has tertiary amine and phenyl group, and as such, these mixed-mode AEX
media do not rely on the strength and weakness of an anion exchange
group and hydrophobic group. Similarly, for aggregate-spiked plasma IgG
processing, filterability was improved over reference by two resins with
a sulfate ligand, modified CEX1 with dextran sulfate and modified CEX2
with cellulose sulfate, indicating that plasma IgG filterability
improvement is due to the more effective dextran sulfate flow-through
processing. However, while both mixed-mode AEX and modified CEX column
chromatography removed larger aggregates from plasma IgG, there were
differences in removal of dimers by these two methods. Despite greater
removal by mixed-mode AEX, modified CEX showed markedly better
improvement in filterability with flux at the start of the filtration
exceeding that for the reference. This observation suggests that the
decrease in filterability of plasma IgG, which is polyclonal, is not
dependent solely on the aggregate content determined by SEC, unlike the
pattern observed for mAb solutions.
From clogging model results, we see that the clogging model that best
matches the filtration behavior for each protein filtration under
constant pressure filtration with Planova BioEX filters clearly differed
with the properties of the solution being filtered.
Solutions showing extreme
clogging, such as plasma IgG aggregate spike, follow the complete
blocking model, and solutions showing minor clogging, such as
aggregate-spiked mAb, follow the standard clogging model. The results
from all four clogging models are practically indistinguishable for
solutions that do not include any spiked substances that cause clogging,
with the cake filtration model having a slightly better fit than other
clogging models.
Although the clogging models assume simplified uniform cylindrical
pores, which may not be exactly representative of virus filters, based
on the studies and analyses presented here, applying the appropriate
clogging models to filtration behavior is an insightful way to
characterize filtration processes.
By selecting chromatography processes that are compatible with virus
filtration and that improve the filterability of the feed stream, users
can increase the capacity of their production processes. For mAb, for
example, these processes can be conducted at large scales of at least
1000 L/m2, and even larger throughput can be expected
as has already been put into practice (Lute et al., 2020). By evaluating
process filterability, determining the best fit clogging model and
utilizing it, total virus filtration process throughput can be estimated
and used for planning purposes.
As future work, application of visualization techniques will add
valuable information and contribute to better understanding of clogging
mechanisms.