Development of the CFD model to simulate VIA
A CFD model was initially developed for troubleshooting the excessive
product aggregation in the VI operation of the pilot lot, S100_1. The
operating condition is presented in Table III. As previously reported
(Jin et al., 2019), the lot was performed
in a SUM-100 mixer containing 66.5 L of PAE (28 g L-1IgG4-N1, pH 4.6) with a 50 rpm agitation speed and 0.1 N HCl addition at
2.62 L h-1 L-1 to achieve pH 3.6
(i.e., VIA). The acidified VIA pool was held at room temperature for 60
min for viral inactivation and followed by neutralization to pH 5.5 with
2 M Tris-base (i.e., VIN). The HMW levels of the VIN pool was 7.1%,
which exceeded the level specified for in-process pool and led to lot
rejection of the final drug substance produced. In the preliminary
troubleshooting investigation, experiments and the CFD model simulation
suggested that the localized low-pH (≤ pH 3.3) zone during VIA was the
root cause for the product aggregation.
The CFD model for VIA simulation was modified from the well-establishedk ‐ε turbulence model coupled with the species transfer model in
literature (Spann et al., 2019). Unlike
the original model that used a single bolus tracer addition to simulate
mixing time, the modified model mimicked continuous addition of 0.1 N
HCl to simulate VIA. To develop the new model, simulations were
performed to screen different volume fractions and the screening
criterion was based on the agreement between the model predictions and
the experimental pH values at the end of the VIA step (EOVIA).
The pH of the acidified PAE resulted from the amount of 0.1 N HCl added,
i.e. 0.1 N HCl mass fraction (MF0.1N HCl ) of the
acidified PAE pool as shown in Figure 2A. The data showed a linear
correlation between MF0.1N HCl and the pH of the
acidified PAE pool. Furthermore, this correlation was protein
concentration dependent. As the protein concentration increases, so does
the requirement of 0.1N HCl for a given pH adjustment. For example, it
required MF0.1N HCl of 0.078 to bring pH down
from 4.45 to 3.65 for the protein centration of 22 g
L-1, while it required MF0.1N
HCl of 0.115 to achieve the same pH adjustment for the protein
concentration of 35 g L-1.
The simulation results for screening volume fraction of outflows are
shown in Figures 2B-2D. Since the overall volume fractions was the unit
for the two outlets, for a given outflow-1 volume fraction
(VFout1 ), the outflow-2 volume fraction
(VFout2 ) equals to 1 -VFout1 . Therefore, onlyVFout1 needed to be tracked for data analysis.
The screening was first performed with simulations for lot S100_1, of
which EOVIA MF0.1N HCl was 0.1215. TheVFout1 value was initiated from 0.1, and stepwise
(0.1 per step) increased to 0.9 in the simulations. As shown in Figure
2B, there was a clear linear correlation between the simulated EOVIAMF0.1N HCl and VFout1value. The best agreement between the simulation and the experimental
EOVIA MF0.1N HCl was observed atVFout1 = 0.7 (0.1214 of simulation vs. 0.1215 of
measurement). Using VFout1 = 0.7 for lot S100_1,
the simulated MF0.1N HCl profile appeared to
overlap with the experimental profile as shown in Figure 2C.
To consolidate the CFD model with VFout1 = 0.7
for VIA simulation, the model was applied to 5 additional VI runs in 2L
or 20L Applikon reactors. The conditions for VI experiments are
presented in Table III. The experimental and simulation results are
shown in Figure 2D. The variations of the simulations and the
experimental EOVIA MF0.1N HCl values were within
10%, demonstrating that the CFD model was generally applicable to
mixers of different types and scales for VIA simulation.
In summary, the modified CFD model for VIA simulation was thek ‐ε turbulence model coupled with the species transfer
model, which facilitated continuous tracer addition and the constant
working volume during simulation using two outflows. The preferredVFout1 and VFout2 are 0.7
and 0.3, respectively.