Extensive LC-Top-Down MS Sequence Confirmation of the NISTmAb Reference Material 8671

Importance of the topic

Monoclonal antibodies (mAbs) require thorough verification of their primary sequence, post-translational modifications and terminal integrity to ensure safety and efficacy. Traditional bottom-up workflows are time-consuming and prone to artefacts. The combination of middle-up fragmentation with top-down MALDI-TOF sequencing offers a rapid, high-coverage alternative for detailed characterization of therapeutic antibodies in research and QC labs.

Objectives and Study Overview

This study aimed to apply an LC-MALDI-TOF/TOF top-down sequencing workflow to the NISTmAb Reference Material 8671. Specific goals included:

• Confirmation of the heavy chain fragments (Fd and Fc/2) and the light chain (LC) primary sequences.
• Localization of N- and C-terminal modifications such as pyro-glutamination, glycosylation variants (G0F, G1F, G2F) and C-terminal lysine clipping.
• Assessment of sequence coverage, complementarity determining region (CDR) validation and overall method robustness.

Methodology and Instrumentation

The workflow combined IdeS digestion, nanoLC fractionation, MALDI sample preparation under inert atmosphere and MALDI-ISD top-down sequencing:

1. Sample Preparation
   
   • Reduction and IdeS digestion of NISTmAb in guanidine chloride with DTT at 50°C.
   • Injection of 4 pmol digest onto a nanoElute UHPLC with a Fortis C4 column.
2. Automated Fraction Collection and Spotting
   
   • spotOn fraction collector deposited 192 fractions (15 s each) onto MTP AnchorChip 384 targets.
   • Continuous nitrogen flow in an inert gas mask minimized oxidation artifacts.
   • sDHB matrix and bovine ubiquitin calibrant were co-deposited automatically.
3. MALDI-TOF/TOF Top-Down Sequencing
   
   • rapiflex TOF/TOF instrument acquired in-source decay (ISD) spectra across the 800–10 000 Da range.
   • AutoXecute laser raster targeted crystal rims for optimal signal.
   • Data processed in flexAnalysis and BioPharma Compass 3.0 for sequence coverage and sequence validation percentage (SVP) calculation.

Key Results and Discussion

LC fractionation yielded discrete peaks for Fc/2 (Rt 31.5 min), LC (33.5 min) and Fd (35.5 min). Top-down MALDI-ISD sequencing delivered:

• Light Chain (LC): 73.2 % sequence coverage, 85.4 % SVP; all three CDRs confirmed.
• Fd Fragment: 61 % coverage, 72 % SVP; detection of N-terminal pyro-glutamination, two of three CDRs validated.
• Fc/2 Fragment: Differentiation of G0F, G1F and G2F glycoforms; confirmation of C-terminal lysine clipping; up to 75 % SVP for the predominant G1F variant.

The data demonstrate high confidence in sequence identity and terminal modification assignments. Minor gaps in coverage highlight areas for further optimization of ion fragmentation and data acquisition parameters.

Benefits and Practical Applications

This integrated LC-MALDI-TDS approach offers:

• Rapid, artifact-minimized confirmation of large protein fragments without multiple enzyme digests.
• High throughput sample preparation and automated spotting compatible with routine QC workflows.
• Direct localization of critical modifications (glycosylation, pyro-glutamination, clipping) relevant for mAb quality control.
• Potential for de novo sequencing of unknown or variant antibodies.

Future Trends and Potential Applications

Emerging developments include:

• Enhanced fragmentation strategies to improve coverage beyond 90 % SVP.
• Automation of data analysis pipelines for streamlined reporting in regulated environments.
• Extension to other biotherapeutics, multi-domain antibodies and fusion proteins.
• Integration with orthogonal techniques (native MS, ion mobility) for deeper structural insights and variant quantification.

Conclusion

The presented LC-MALDI-TOF/TOF top-down sequencing workflow enables reliable, high-coverage confirmation of mAb subunits and critical modifications. Its simplicity, speed and data quality position it as a valuable tool for biopharmaceutical research, development and QC laboratories.

Reference

[1] Resemann A et al. Full validation of therapeutic antibody sequences by middle-up mass measurements and middle-down protein sequencing. MAbs; 8(2):318–330.
[2] Consortium for Top-Down Proteomics monoclonal antibody project (in preparation).
[3] Resemann A et al. Top-down de novo protein sequencing of a 13.6 kDa camelid antibody by MALDI-TOF/TOF. Anal Chem. 2010;82(8):3283–3292.