Figure 6 illustrates the screening results with different salt conditions of a specific glycoprotein. The results show that
most resins depict a similar binding profile—elution of the glycoprotein occurs around 100–200 mM NaCl. Based on these screening
results, the best resins (with respect to binding capacity, throughput, cost price, etc.) are selected and studied in more
detail in the next step, in which scouting experiments are carried out under dynamic conditions.
In the scouting experiments, the most suitable resins are studied in greater detail in packed bed columns. The technique became
available in the second part of the nineties following the commercialization of new purification systems such as the Akta
design (GE Healthcare, Uppsala, Sweden). Scouting studies are carried out by running the columns in a series with different
buffer conditions (salt, pH, buffer, and so on). During both the screening and scouting stages, it is also possible to fine-tune
the selected ligand of the resin (e.g., selecting hydrophobic or ionic ligands).
The purification results from a hydrophobic interaction step (i.e., scouting various hydrophobic ligands) for a specific glycoprotein
are shown in Figure 7.
As observed from the elution profiles, the target protein elutes in the salt gradient depending on the structure of the ligand
coupled to the resin. With some ligands, the target protein elutes partly in the regeneration phase, together with other proteins.
Most host cell proteins remain bound to the column and elute mainly after finishing the salt gradient. After these scouting
studies, the best chromatographic resin is selected and further optimization occurs with experimental design approaches.
STATISTICAL DESIGN STUDIES
Experimental design is a statistical approach that uses various matrices; response surface and screening designs are the most
preferred methods.8 Screening designs are normally used to analyze large numbers of parameters to determine which are the most critical; at
first, only the main effects of the parameters are tested. The selected parameters are then analyzed in more detail with a
response surface design that handles fewer parameters (the most critical ones), but also determines the influence of the interactions
between parameters. Overall, experimental design is needed to increase the robustness of purification processes.
In the example below, the most important parameters—such as pH, the salt and protein concentrations in the loading phase,
and the pH and salt in the elution phase —of a specific purification step for a protein, are analyzed with a response surface
design. Figure 8 shows the effects of the main parameters (pH,conductivity, protein) in the loading and elution phases on