Advancing ADC characterization and control through integrated LC-MS workflows

Importance of the topic

Antibody-drug conjugates (ADCs) are complex biotherapeutics whose safety, efficacy and stability depend on a detailed understanding of heterogeneity arising from drug-to-antibody ratio (DAR), conjugation site distribution and post-translational modifications. High-confidence analytical workflows that preserve structural context while delivering site-resolved, quantitative characterization are essential across discovery, development and quality-control labs to support regulatory compliance and robust product comparability.

Objectives and overview of the study

The document presents an integrated suite of liquid chromatography–mass spectrometry (LC–MS) workflows designed to overcome core analytical challenges in ADC characterization. Objectives include: resolving DAR distributions at intact and subunit levels; confidently localizing payload conjugation sites in heterogeneous ADC populations; comprehensive profiling of charge variants under native conditions; accelerating reproducible hydrophobic interaction chromatography (HIC) method development for DAR determinations using AQbD principles; and implementing automated, high-throughput native LC–MS workflows for routine DAR monitoring.

Methodology

The proposed multi-level strategy combines intact native MS, subunit (middle-down) analysis, and peptide-level mapping to capture complementary structural information. Key methodological elements:

• Intact native MS to preserve higher-order structure and quantify DAR distributions directly from mass-domain deconvolution.
• Subunit analysis (e.g., IdeS digestion + reduction to produce ~25 kDa fragments) followed by targeted isolation and fragmentation to retain structural context while improving sequence coverage.
• Peptide mapping using orthogonal proteases (trypsin, AspN) and mixed fragmentation modes (HCD and EThcD) to maximize sequence coverage and localize labile modifications and conjugation sites.
• Spectral simplification strategies: MSn workflows combining EThcD (MS2) with proton transfer charge reduction (PTCR) at MS3 to reduce ion congestion and improve fragment assignment in positional isomers.
• Online 2D LC (strong cation exchange heart-cut to SEC desalting) coupled to high-resolution accurate-mass (HRAM) MS to resolve charge variants while maintaining native-like conditions for intact mass analysis.
• AQbD-driven automated DoE for HIC method optimization leveraging software-driven experimental design and Chromeleon-controlled sequences to define robust method operable design regions (MODR).
• Automated native intact LC–MS sequences with integrated deconvolution and reporting for high-throughput DAR monitoring (short ~3 min runs, batch processing up to 50 injections).

Used instrumentation

The workflows utilize a portfolio of high-performance separation and MS platforms and software including:

• Vanquish Horizon / Vanquish Flex / Vanquish Simple Switch 2D-LC UHPLC systems
• MAbPac SEC-1, MAbPac HIC-Butyl, ProPac 3R SCX, NativePac OBE-1 SEC, and Hypersil GOLD peptide columns
• Thermo Scientific Orbitrap-class mass spectrometers: Excedion Pro BioPharma, Orbitrap Ascend Tribrid (with ETD/EThcD/UVPD and PTCR capability), and Exploris 240
• Data and processing software: Chromeleon CDS, BioPharma Finder (including ReSpect deconvolution), Proteome Discoverer, Ardia Platform, Fusion QbD

Main results and discussion

The integrated multi-level LC–MS approach delivered several performance highlights across representative ADC examples (e.g., ado-trastuzumab emtansine, polatuzumab vedotin, brentuximab vedotin and ADC mimics):

• Intact native MS resolved DAR species from D0 to D8 with accurate quantitation and mass accuracy <10 ppm for major components, enabling direct mass-domain DAR profiling and glycoform discrimination after spectral deconvolution.
• Peptide mapping with combined HCD and EThcD fragmentation achieved complete sequence coverage (100% light and heavy chain coverage reported for the example ADC) and confident localization of the majority of potential conjugation sites (e.g., 43 of 46 sites localized) including peptides with multiple modifications.
• Middle-down EThcD + PTCR workflows increased identifiable fragment ions and improved sequence coverage by ~16–20%, lowering spectral congestion and enabling unambiguous mapping of positional isomers and low-abundance conjugation sites in ~25 kDa subunits.
• Online multiple-heart-cut 2D SCX→SEC→HRAM MS resolved up to ~18 distinct charge-variant peaks, and transfer/desalting preserved native state for intact mass analysis, enabling assignment of charge variants to DAR species, glycoforms and PTMs down to low-abundance levels (<0.5%).
• AQbD-driven automated DoE HIC optimization allowed rapid, reproducible identification of conditions separating DAR species across multiple cysteine-linked ADCs; model predictions matched experimental retention within tight error margins and produced MODRs with robust RT and area RSDs (<1% and <2%, respectively).
• Automated native intact LC–MS workflows processed large sequences (e.g., 50 injections) with short 3-minute runs and end-to-end deconvolution and reporting—streamlining routine DAR monitoring with minimal manual intervention.

Benefits and practical applications

The integrated workflows provide a pragmatic balance between depth and throughput: they link intact molecular measurements to subunit- and peptide-level detail, improving confidence in attribute assignment while enabling routine QC-style monitoring. Practical benefits include:

• Comprehensive structural characterization with direct DAR readout, site localization and PTM mapping in a coherent analytical framework.
• Improved confidence in conjugation-site assignments through spectral decongestion and complementary fragmentation strategies.
• Higher laboratory efficiency via automated DoE method development and CDS-integrated batch processing and reporting.
• Preservation of native-like states for charge-variant and intact-mass analyses supports meaningful assessment of non-covalent structure and aggregation behavior.

Future trends and potential applications

Emerging directions and opportunities arising from these workflows include:

• Broader adoption of MSn spectral simplification (PTCR/MS3) and complementary activation methods (UVPD, EThcD) to routinely resolve positional isomers and low-abundance modifications across diverse ADC chemistries.
• Integration of orthogonal 2D-LC architectures and automated fraction transfer to couple higher-resolution separations with native MS for even deeper variant mapping.
• Further automation and standardization (AQbD + CDS templates) to accelerate method transfer between labs and support regulatory submission packages.
• Application of these multi-level workflows to next-generation conjugates (site-specific, alternative linkers, non-Ig scaffolds) to ensure consistent quality assessment as chemistry evolves.

Conclusion

A multi-level, integrated LC–MS strategy that combines intact native MS, middle-down spectral simplification and peptide-level mapping, together with 2D separations and AQbD-driven method optimization, provides a coherent and robust approach for comprehensive ADC characterization. The described workflows improve confidence in DAR measurement, conjugation-site localization and charge-variant assignment while enabling higher throughput and reproducibility for development and routine monitoring.

Reference

No formal literature references were cited in the source document. The summarized material describes applied workflows, instrument platforms and software used to characterize representative ADCs and ADC mimics.