Resin Particle Size Distribution
The manufacturer guarantees that at least 80% of the particles that make up any lot of the cation exchange resin it releases
have diameters that fall within the range of 40–90 μm. The certificates of analysis, which accompany each lot of resin, report
the percentage of particles falling within this range. Table 3 summarizes this percentage for the four lots of resin we tested.
Table 3. Percent of particles falling within 40–90 μm
From the table, we can see that the percentage of particles within 40–90 μm diameter is variable from lot-to-lot, although
all the lots would pass the vendor's release specification. In addition, the table shows that elution volume increases as
the percentage of smaller particles, in this case those within the 40–90 μm range, decreases.6–8 On request, the manufacturer provided more detailed particle size distribution information of lots A, B, C, and D. In this
information, almost all of the particles falling outside of the 40–90 μm range are smaller than 40 μm. The percentage of these
small particles can be seen in Table 4.
Table 4. Percent of particles smaller than 40 μm
Figure 7 illustrates the fact that the resin lots containing a larger percentage of particles less than 40 μm in diameter
produce wider elution peaks.
Figure 7. Effect of resin particle size on elution volume
It is likely that smaller CEX resin particles allow for higher accessibility of the antibody to the charged sites by providing
a shorter diffusional length inside the pores. This likely increases the interaction of the antibody with the charged sites,
resulting in a wider elution peak and greater resolution (or higher selectivity) between the charged isoforms of the antibody.
Effect on the Product
It is important to understand the impact of this variable elution profile on the composition of the different charged isoforms
in the product. Additionally, it is important to assess if the lot-to-lot variability results in any differences in the performance
attributes of this cation exchange step such as yield and clearance of process and product related impurities. Analytical
results show that yield clearance of product and process related impurities were not affected by the different cation exchange
chromatography profiles obtained with the four different lots. The results are summarized in Table 5.
Table 5. Product yield and removal of impurities
The distribution of the different charged isoforms (N- and C- terminal charged isoforms) of the product was not affected either.
These results, which are almost identical for each lot tested, are shown in Table 6.
Table 6. N- and C-terminal charged isoform distribution