3.2. Column chromatographic purification of the monoclonal human
IgGs using adsorbents derived from the immobilised 40-TerPSEA
Based on the above findings, the potential of the immobilised
pyridine-based ligands (Fig. 1) in the purification of mAbs was validated,
using 40-TerPSEA as the preferred exemplar, in packed bed
chromatographic experiments. Under the chosen volume overload
and wash conditions used in these small scale laboratory experiments,
on average the losses at the breakthrough and wash stages
accounted for between 15% and 20% of the loaded mAb content.
Obviously, with design-of-experiment (DoE) optimised experimental
conditions, these losses at the load and wash stages could be
with each adsorbent were different for the three humanised IgG
mAb samples. While the humanised IgG1 mAb showed a broader
gradient elution peak (Fig. 4A), the humanised IgG2 mAb eluted
as both a lower abundance peak at a similar gradient position followed
by a later eluting, more abundant peak (Fig. 5A). SDS-PAGE
analysis of the recovered fractions (Fig. 5B) indicated that the
humanised IgG2 mAb was predominantly found in the later eluting
fractions. Thus, under these conditions, the humanised IgG2 mAb
bound more strongly than the humanised IgG1 mAb to the immobilised
40-TerPSEA ligand, a finding consistent with the batch
adsorption studies. The chromatographic profile for the humanised
IgG4 mAb was similar to the result obtained with the humanised
IgG1 mAb with a major peak followed by a lower abundance peak
of longer retention time (Fig. 6A). The majority of IgG4 was contained
in the first peak as evident from SDS-PAGE analysis (Fig. 6B).
The binding of mAbs to this new class of immobilised ligands is
a consequence of the interplay of multiple physicochemical effects,
including electrostatic and hydrophobic interactions, typical of
mixed mode adsorbents. Moreover, these results suggest the possibility
that these ligands may have the capability to bind independently
to two (or more) sites within the IgG mAbs, a possibility
that is under current investigation. Nevertheless, changes in the
buffer composition or pH are anticipated to lead to subtle variations
in selectivity for IgG mAbs of different isotype, depending
on their structural and physical attributes. For instance, the
humanised IgG2 mAb has a lower pI value (pI 7.4) than the humanised
IgG1 or IgG4 mAbs (both with pI 8.0 respectively) [28]. When
the loading buffer is set at pH 8.0, the humanised IgG2 mAb will be
more negatively charged than the humanised IgG1 or IgG4 mAb.
These differences may influence the propensity to self-associate
and form dimers/aggregates [29] and this may account for the fact
that the humanised IgG1 or IgG4 mAbs eluted as broader peaks, yet
upon SDS-PAGE analysis under non-reducing conditions, these
eluted fractions behaved as intact IgGs, i.e. as 160 kDa proteins
together with minor, lower molecular weight species generated
from the heat-induced fragmentation of these IgGs. The generation
of such lower molecular weight mAb fragments has been as noted
previously [24] and confirmed by MALDI-TOF MS analysis [19,20].