Process performance analysis
After the low-pH-treated protein A eluate of Mab-T was neutralized to pH 7.25 and clarified, it was directly applied into
1 mL STIC Nano at a loading capacity of 0.5 g/mL-STIC in flowthrough mode. As expected, 94% of Mab-T was successfully recovered
in the flowthrough/wash pool (see Figure 5). The residual HCP was reduced to 10 ppm while aggregates were reduced to 1.02%.
Furthermore, levels of residual DNA and leached protein A in the STIC purified mAb-T met the requirement for drug substance.
We also examined whether residual impurities were efficiently removed through the STIC membrane adsorber for Mab-D, Mab-K
and Mab-S (see Figure 5). The residual HCP was reduced to less than 10 ppm. The clearance of residual DNA and leached protein
A were sufficient to meet product specifications. The STIC thus served as a polishing step for Mab-D, Mab-K and Mab-S with
acceptable process performance and product quality.
Salt tolerant nature of STIC membrane adsorber
In traditional ion exchange chromatography, the interaction strength of proteins with resin ligands is controlled by solution
pH and NaCl concentration. At a given pH, the interaction decreases dramatically with increasing NaCl concentration. Thus,
both product recovery and impurity clearance are functions of pH and NaCl concentration, which were evaluated as the critical
operating parameters during the condition screening and optimization in this study. As shown in Figure 1, at a given pH condition,
HCP clearance did not vary significantly with NaCl concentration, which highlighted the unique salt tolerant nature of the
STIC membrane adsorber. A consistent process performance has been achieved in the tested conditions of pH and NaCl concentration
(see Figures 2 and 5), indicating a wide design space for the STIC polishing step.
It should be noted that both Mab-D and Mab-S have solubility issues in the current Q equilibration buffer (conductivity <
5 mS/cm), which posed challenges when the current purification production process and related operating conditions were applied.
A low process yield was observed at both pre-Q column TFF and Q column polishing steps. When the solution ionic strength was
increased to overcome the solubility issue, the separation efficiency of the Q column chromatography diminished appreciably.
In both cases, STIC provided a viable alternative to the Q polishing step, able to remove trace amount of impurities from
antibodies at a higher ionic strength buffer condition. This strategy can be applied to other antibodies, exhibiting similar
solubility issues as Mab-D and Mab-S.