High Resolution Size-Exclusion Chromatography Separations of mAb Aggregates, Monomers, and Fragments using BioResolve SEC mAb Columns on UPLC, UHPLC, and HPLC Chromatography Systems

Waters BioResolve SEC mAb, 200 Å, 2.5 µm Columns are specifically optimized and quality tested for high resolution separations of monoclonal antibody (mAb) aggregates, monomers, and fragments. A mAb Size Variant Standard is used for column performance certification and to guide system and column selection for the robust separation of required mAb size variant quantification. While developed and tested for mAbs, this column can also be equally considered for the SEC separation of protein size variants ranging in molecular weight from approximately 450 kDa to 17 kDa.

The 7.8 mm I.D. columns are shown to provide maximum separation efficiency of the mAb monomer (~150 kDa) from the fragment (~100 kDa) on UPLC, UHPLC, and HPLC platforms. The 4.6 mm I.D. columns provide more economical yet comparable separation performance with accurate quantification, to that of the 7.8 mm I.D. columns, on low dispersion UPLC systems as well as for the less demanding separations of mAb aggregates from monomers on UHPLC or HPLC chromatographic platforms with high system dispersions.

Each manufactured BioResolve SEC mAb Column is performance tested with Waters NIST mAb-based size variant standard that has been supplemented with IdeS digested NISTmAb fragments. These column performance confirmatory tests are obtained using an ACQUITY UPLC H-Class Bio System. This application note will show how LC system dispersion volume affects mAb separations based on the selected SEC column I.D. and length.

Benefits

• BioResolve SEC mAb Columns are individually performance tested using the Waters mAb Size Variant Standard to help ensure out of the box performance for mAb analyses
• High resolution separations can be achieved on UPLC, UHPLC, and HPLC systems using appropriately selected I.D. and length columns
• The implementation of individual SEC column performance testing with the Waters mAb Size Variant Standard helps increase the confidence in obtaining the needed resolution for reliable mAb size variant quantification

The historical importance of aqueous size-exclusion chromatography (SEC) is well established for the relative quantification of protein aggregates and self-associated forms (high molecular weight species, HMWS) to ensure the efficacy and safety of recombinant protein-based biotherapeutic products.¹ Recently, interest in quantifying monoclonal antibody (mAb) fragments (low molecular weight species, LMWS) in non-denaturing SEC has increased. The cause of the fragmentation appears not to be enzymatically or host cell protein driven, but rather a kinetic metal ion induced hydrolytic cleavage in the upper heavy chain hinge region resulting in the generation of Fab fragment (~50 kDa), Fc with a single Fab domain (Fab/c, ~100 kDa), and low levels of Fc fragment when both Fab domains are hydrolyzed.²

The separation of HMWS (>300 kDa) from the main mAb monomer (~150 kDa) generally presents much less of a challenge compared to the separation of the Fab/c fragment, which is closer in size to the monomer and elutes at the tail of the monomer, and is typically present at low abundance (Figure 1). However, LC system improvements, higher efficiency SEC columns, and a better understanding of how LC system dispersion affects component resolution have increased the ability to obtain reliable separations of these different mAb forms.³

Each BioResolve SEC mAb Column is performance tested using a mAb Size Variant Standard on a low dispersion LC system to show its capabilities. The observed chromatographic efficiency of a separation is affected by each of the components in the chromatographic system: the injector, tubing, column, and detector. In this application note, a series of chromatograms are shown for a variety of LC systems demonstrating the relationship between column dimensions (internal diameter [I.D.] and length) and system dispersion on the observed chromatographic efficiency of a separation. The 7.8 mm I.D. x 300 mm length columns provided high resolution separations and accurate quantification of aggregates, monomers, and fragments, independent of modern LC system dispersions.

System dispersion was measured using 0.16 mg/mL caffeine diluted in 50/50 acetonitrile/water (UPLC Absorbance Test Solutions Kit, p/n: 700002642, Solution 7). The mobile phase was 50/50 acetonitrile/water (v/v) and the flow rate was 0.5mL/min. A zero dead volume union (p/n: 700002636) was used in place of the column. After a 10-min equilibration time, three blank mobile phase injection were followed by five consecutive 0.5 µL injections of caffeine using a run time of 1-2 min. The caffeine peak widths at 4.4% (5-sigma peak width) were averaged. The average was multiplied by 500 µL/min to obtain the reported 5-sigma extra-column dispersion (5s_ec) volumes. The dispersion data on the three systems (four configurations) are reported in the Table 1.

Sample Description:

Waters mAb Size Variant Standard (p/n: 186009284) contains 160 µg of stabilized and lyophilized NISTmAb RM8671 which has been supplemented with 2 µg of nonreduced IdeS digested NISTmAb fragments (F(ab')2 and (Fc/2)2). The lyophylized contents of each vial was solublizes using 70 µL of MilliQ water.

SEC is a unique and challenging form of chromatography. Under ideal conditions (method development required) proteins migrate through the column with minimal-to-no interaction with the stationary phase. No retention, only diffusion driven separations achieved by the size hinderance provided by the specifically designed pore sizes in the porous structure of the packing. As a result, the amount of band spreading that occurs to the analyte zone as it travels from the injector to the detector will have a significant impact on SEC resolutions. An in-depth evaluation of the effect of system dispersion on the SEC analysis of mAb aggregates (HMWS) and fragments (LMWS), and the effect that extra-column dispersion has on that separation can be found in previously published application notes.^4,5

Measuring System Dispersion

When conducting band spreading experiments to evaluate system dispersion, it is important to measure the band broadening at a peak width of 4.4% of its height when SEC is our intended analysis as many of the impurities, that we intend to separate and quantify, have peak heights well below 4.4% of the peak height of the main peak (Figure 2). The effect of LC system dispersion on the 5-sigma efficiency and USP tailing factor for the main mAb peak plays a key role in achieving resolution for the ~100 kDa fragment. There is benefit in documenting these parameters during method development since these data are potential indicators of column failure or a system dispersion problem. Consequently, the data from this analysis can be incorporated in standard procedures to help ensure the column and system are fit-for-purpose prior to the analysis of valuable samples.

The Waters mAb Size Variant Standard was used to systematically investigate the effect of the various system dispersions on the chromatographic performance of each of the four columns listed below. The same columns were evaluated on each system. Columns were checked before and after all evaluations and found to maintain their performance. The individual quality report chromatogram, that is provided in the box with the shipped BioResolve SEC mAb Column, is essentially the same as that generated on the ACQUITY UPLC H-Class Bio System, in the examples below.

In these experiments, four BioResolve SEC mAb Columns (4.6 x 150 mm, 4.6 x 300 mm, 7.8 x 150 mm, and 7.8 x 300 mm) are used on three different relatively modern LC Systems:

• The lowest dispersion (UPLC) system is the ACQUITY UPLC H-Class Bio having a dispersion of 10 µL 5σ_ec for the 150 mm length column heater (CH-A) and 13 µL 5/sec for the 300 mm length column heater (CH-30A): both having active pre-heating.
• The intermediate dispersion (UHPLC) system is the ACQUITY Arc Bio having a dispersion of 30 µL 5σ_ec. For all columns the forced air convection 30-cm column heater (30-cm CH) was used.
• The largest dispersion (HPLC) system is the Alliance having a dispersion of 49 µL 5σ_ec. For all columns the forced air convection column heater box was used.

The chromatographic results of these experiments for the 7.8 mm I.D. columns are shown in Figure 3, and those for the 4.6 mm I.D. column are shown in Figure 4.

Before reviewing the chromatographic results, it is important to point out the various components in the Waters mAb Size Variant Standard. The percent composition of the lower molecular weight species in commercial mAb drug products are generally very low and variable among product batches, making the utility of such samples as standards undesirable. To address this shortcoming, Waters created a mAb Size Variant Standard. Each vial of the standard contains 160 µg of National Institute of Standards and Technology (NIST) mAb RM 8671 that has been supplemented with 2 µg of the IdeS (FabRICATOR) digested NIST mAb. Figure 5 elucidates the source of the various components in the mAb Size Variant Standard. As seen in the chromatogram (Figure 5b) the ~100 kDa Fab/c fragment endogenous to the NISTmAb elutes slightly earlier than the IdeS generated ~100 kDa F(ab’)₂ fragment. Since the order of elution is taken as a difference in hydrodynamic volume, Fab/c will be more difficult to separate from the monomer as compared to F(ab’)₂. The BioResolve SEC mAb, 7.8 x 300 mm Column was used at a lower flow rate (0.3 mL/min) in order to achieve the resolution shown for Fab/c in the NISTmAb RM 8671. More details can be found in the mAb Size Variant Standard care and use manual (p/n: 720006811EN). More information on NISTmAb RM 8671 can be found at nist.gov.⁶

As shown in Figure 3, the separation of the mAb Size Variant Standard on 7.8 mm I.D. columns, we see that the chromatograms display very similar separation performance for the HMWS, main, and fragment peaks on the 7.8 x 300 mm column run on the HPLC, UHPLC, and UPLC systems. The shorter 7.8 x 150 mm column provides, as predicted, lower resolution separations, but also shows a clearly visible degree of performance loss for LMWS1&2 on the HPLC system. HMWS is adequately separated on both 7.8 mm I.D. columns on the three LC systems.

Although the 7.8 x 150 mm column in Figure 3 provides what appears to be adequate resolution for LMWS1&2 on the UPLC system, it is important to understand that most of the LMWS1&2 peak area is due to the IdeS supplemented fragment, F(ab’)₂. As previously mentioned, this fragment appears to be slightly smaller (therefore slightly better resolved) than the naturally occurring Fab/c fragment (Figure 5).

The effect of system dispersion on the 4.6 mm I.D. columns in the chromatograms in Figure 4 presents a different picture than that for the 7.8 mm I.D. columns discussed above. The LMWS1&2 are separated with the 4.6 x 300 mm column on all three systems with a noticeable loss of resolution observed as system dispersion increases. This trend is even more abrupt for the 150 mm column length. No significant change in HMWS resolution is observed for the 4.6 x 300 mm column on the different LC systems, however, a slight loss of HMWS resolution is observed for the 150 mm column length as system dispersion increases.

Resolution of Aggregates

The trends visually observed in the chromatograms of Figures 3 and 4, were quantified using Empower system suitability parameters. For aggregate resolution, the USP resolution at half height (HH) was used to assess the quality of the separations between the main monomer peak and the dimer across the three systems for these columns. The frequently proclaimed value of 1.5 for baseline resolution is not relevant for most real world chromatographic separations. A criterion adopted for acceptable resolution for real world samples is 1.75-2.0.⁷ This shift to higher values is due to effects of peak asymmetries and the dissimilar areas encountered in SEC separations.⁷ In Figure 6, for the dimer-main peak resolution values we observe that for all, but the 4.6 x 150 mm column, the resolution remains essentially the same across the three LC systems. The 26% loss in resolution for the 4.6 x 150 mm column across the three LC systems remains above acceptable levels for many aggregate analyses.

Resolution of Fragments

In order to quantitatively measure and compare the resolutions for LMWS1&2, another type of resolution metric is needed, the peak-to-valley ratio (p/v). The p/v parameter, illustrated in Figure 7, is typically used to quantify the quality of a separation between peaks that have disparate areas and are too closely eluting (partially coeluting) to obtain a resolution value from the traditionally used resolution equations. The value of p/v is used to allow quantitative assessment of very difficult separations such as those encountered for fragment analysis. An example, of the use of the p/v > 2.0 criterion for quantification of closely eluting peaks in SEC, can be found in the USP Pharmacopoeia Monograph for Insulin.⁸ The USP Insulin monograph calls for an end p/v of > 2.0 when quantifying the relative amount of high molecular weight protein (HMWP) in the insulin sample.⁸ Because USP resolution (HH) values were not available for all the column configuration/system combinations in this study, the start p/v parameter was used for these comparisons.

In Figure 8, the p/v resolution values are maintained consistently across the UPLC and UHPLC systems for all the configurations. Generally, p/v ratios are less reproducible as they increase above three (data not shown). As predicted, the resolutions on the 300 mm length columns are higher than on the 150 mm length columns. The 4.6 x 150 mm column was not included in this figure because it failed to resolve LMWS1&2 on the Alliance, and on the other systems the p/v was 1.6 and lower. As p/v resolutions increase above >1.0 the % areas for LMWS1&2 gradually decrease until they start to level off as they approach p/v > 2. This trend is visible for all the columns on the Alliance.

Main Peak

Central to all SEC mAb chromatograms is the main peak. It is apparent from the chromatograms in Figures 3 and 4 that the efficiencies for the main peak are degraded by system dispersion. As separations lose efficiency, analyte peak heights decrease. Not often discussed is the effect of a system’s dispersion on the sensitivity of the analysis due to this decrease in peak height which directly impacts the signal-to-noise ratio of the impurities that we are intending to quantify in these separations.

In Figure 9a, the correlation coefficient squared (R²) values between system dispersion volume and 5-sigma plates show increasingly stronger correlations as column volume decreases. R² values represent the percent of the variation in plates that can be explained by the difference in system dispersion volumes; meaning that 84-100% of the decrease in main peak 5-sigma plates can be explained by the increasing system dispersion volumes. The slopes of the linear correlations are steeper on the 4.6 mm I.D. columns compared to those on the 7.8 mm I.D. columns, indicating a greater impact of dispersion volume on the 4.6 mm I.D. columns. It is also noticeable that the efficiency even on the 4.6 x 300 mm column was not as high as that obtained on the 7.8 x 300 mm, suggesting that system dispersion is limiting its performance even on the UPLC system.

The dispersion volume and its tailing factor are not independent variables. However, comparing the R² values between the dispersion peak volumes (Figure 9a) and tailing factors (Figure 9b) to main peak plates suggests that it is the system dispersion tailing factor that plays a stronger role in influencing changes in the main peak plates on the 7.8 mm I.D. columns than on the 4.6 mm I.D. columns. It is not known at this time how universally applicable these observations are to all systems but does suggest that reducing tailing for the dispersion peak may lead to even better performance for 7.8 mm I.D. configurations.

Focusing on the 300 mm length column, it was observed that the LMWS1&2 p/v was most strongly influenced by the tailing factors of both the main peak and the dispersion peak. The p/v values were still found to be influenced by the main peak plates (R² values of 0.88 for 4.6 mm I.D. and 0.95 for 7.8 mm I.D.) and system dispersion volume (R² values of 0.80 for 4.6 mm I.D. and 0.64 for 7.8 mm I.D.), just not as strongly as with those described in Figure 10. Since the tailing factor for the main peak is also strongly correlated with the tail factor for the dispersion peak (Figure 9b), minimization of tailing in the dispersion peak should be investigated as a means of improving the resolution for fragment analyses.

Waters BioResolve SEC mAb Columns can provide high resolution and reproducible separations of mAb HMWS, monomers, and LMWS. The 7.8 mm column I.D. configurations provide better chromatographic performance than the 4.6 mm I.D. columns on HPLC, UHPLC, and UPLC systems, and is therefore the most recommended configuration when high resolution separations, particularly, those requiring analysis of partially resolved LMWS1 (Fab/c). The 4.6 mm I.D. columns can be nearly as effective as the 7.8 mm I.D. columns on UPLC systems and for the analysis of HMWS on UHPLC and in some cases HPLC. The advantages offered by the 4.6 mm I.D. columns are reduced sample demands and mobile phase consumption, along with being more economical.

In these experiments, the LC systems used were not modified to further reduce dispersion from that of their typically configured components. For example, it is well known that reducing the length and I.D. of the connection tubing may further reduce dispersion.⁹ The “out-of-the-box” performance of BioResolve SEC mAb Columns is tested by Waters for each column using the Waters mAb Size Variant Standard on low dispersion LC systems and chromatograms should appear similar to those presented on the ACQUITY UPLC H-Class System in Figures 3 and 4. Representative quality testing results for four different batches of BioResolve SEC mAb are presented in Figure 11 and demonstrate a high degree of reproducibility.

Successful implementation of SEC in the research, development, and manufacture of mAb biotherapeutics is dependent on understanding and selecting a column that is compatible with the dispersion levels of the utilized LC system(s) to help ensure the generation of reliable data especially in a validated method. Development of separation conditions that provide robust separations while minimizing secondary interactions with both the column and LC system flow path are critical to the success of any SEC method. Finally, it is important to consider establishing and using a reliable mAb-based reference sample to help ensure column, LC system, mobile phase, and method readiness prior to the analysis of desired mAbs.

1. Hong, P.; Koza, S.; Bouvier, E. S. Size-Exclusion Chromatography for the Analysis of Protein Biotherapeutics and their Aggregates. J. Liq. Chromatogr. Relat. Technol. 2012, 35, 2923–2950.
2. Cordoba, A. J.; Shyong, B. J.; Breen, D.; Harris, R. J. Non-Enzymatic Hinge Region Fragmentation of Antibodies in Solution. J. Chromatogr., B. 2005, 818, 115–12.
3. Koza, S. M.; Chen, W. High Resolution and High Throughput Size-Exclusion Chromatography Separations of IgG Antibody Aggregates and Fragments on UHPLC and HPLC Systems with 2.5 μm BEH Particles. Waters Application Note, 720006522EN, April 2019.
4. Koza, S. M.; Reed, C. E.; Chen, W. Impact of LC System Dispersion on the Size-Exclusion Chromatography Analysis of Monoclonal IgG Antibody Aggregates and Fragments: Selecting the Optimal Column for Your Method. Waters Application Note, 720006336EN, June 2019.
5. Koza, S. M.; Reed, C. E.; Chen, W. Evaluating the Impact of LC System Dispersion on the Size-Exclusion Chromatography Analysis of Proteins. Waters Application Note, 720006337EN, June 2019.
6. NISTmAb, Humanized IgG1κ Monoclonal Antibody, Reference Material 8671: https://www-s.nist.gov/srmors/certificates/8671.pdf
7. Dolan, J. W. LC Troubleshooting: Peak Tailing and Resolution. LC•GC Europe June 2002: 1-4.
8. USP43-NF39, Official Monographs/Insulin. May 1, 2016; 4314-17.
9. Fekete, S.; Guillarme, D. Influence of Connection Tubing in Modern Size Exclusion Chromatography and Its Impact on the Characterization of mAbs. J. Pharm. Biomed. Anal. 2018, 149, 22–32.

Acknowledgements

The authors would like to thank Steve Shiner, Steven Byrd, and Justin McLaughlin for their roles in the development of the BioResolve SEC mAb Columns, and Bill Warren for his guidance and energy in support of this project.