Comprehensive characterization of monoclonal antibody by Fourier transform ion cyclotron resonance mass spectrometry

Direct glycosylation analysis of intact monoclonal antibodies combining ESI MS of glycoforms and MALDI-in source decay MS of glycan fragments

Monoclonal antibody (mAb) glycoengineering has the potential to improve the efficacy of biopharmaceuticals by fine-tuning specific biological properties. Glycosylation analysis is key to monitoring the glycoengineering process. Various mass spectrometry (MS)-based methods are available to characterize mAb glycosylation at different structural levels, but comprehensive analysis is typically time-consuming and costly. Here, we present an approach that combines conventional intact mass measurement of glycoforms by direct infusion ESI-MS with an advanced MALDI-in-source decay FT-ICR MS method for direct analysis of glycans in intact mAbs, without the need for enzymatic release and separation. Using a sodium-doped MALDI matrix, glycans were directly released as ISD fragment ions from the intact mAbs during the ionization process. Measurement of 0,2A fragment signals yielded reproducible glycan profiles that were consistent with conventional methods, yet was achieved with unprecedented speed, providing complementary information to that obtained through intact mass measurement. The methods were applied to standard and glycoengineered trastuzumab and rituximab, allowing rapid glycosylation profiling and structural analysis of glycans by tandem MS of selected ISD fragment ions. This fast approach can facilitate the early-phase development of glycoengineering processes by constraining further in-depth analyses. We envision a broader applicability in studies focused on glycosylation changes in mAbs.

In-depth characterization of a cysteine-linked ADC disitamab vedotin by multiple LC-MS analysis methods and cutting-edge imaged capillary isoelectric focusing coupled with native mass spectrometry

The characterization of cysteine-linked antibody‒drug conjugates (ADCs) can be more challenging than that of monoclonal antibodies (mAbs) and lysine-linked ADCs because the interchain disulfide bonds are reduced for payload conjugation, and the chains are noncovalently bonded to each other. Furthermore, payload conjugation and disulfide bond reduction/scrambling may introduce additional charge heterogeneity to biomolecules. This study illustrates an innovative workflow employing multiple separation techniques and tandem high-resolution mass spectrometry for comprehensive and in-depth characterization of disitamab vedotin, a recent-generation cysteine-linked ADC, including reversed-phase liquid chromatography (RPLC), ion exchange chromatography (IEX) and image capillary isoelectric focusing (icIEF). RPLC was employed for reduced chains analysis, subunit analysis and peptide mapping. IEX and icIEF were used for charge heterogeneity analysis. The innovation of the integrated methodology emphasizes the importance of cutting-edge icIEF-MS online coupling under near-native conditions to reveal the heterogeneity of disitamab vedotin.

Mass spectrometry study on SARS-CoV-2 recombinant vaccine with comprehensive separation techniques to characterize complex heterogeneity

SARS-CoV-2, the causative agent of COVID-19, has imposed a major public health threat, which needs effective therapeutics and vaccination strategies. Several potential candidate vaccines being rapidly developed are in clinical evaluation and recombinant vaccine has gained much attention thanks to its potential for greater response predictability, improved efficacy, rapid development and reduced side effects. Recombinant vaccines are designed and manufactured using bacterial, yeast cells or mammalian cells. A small piece of DNA is taken from the virus or bacterium against which we want to protect and inserted into the manufacturing cells. Due to the extremely complex heterogeneity of SARS-CoV-2 recombinant vaccine, single technology platform cannot achieve thorough and accurate characterization of such difficult proteins so integrating comprehensive technologies is essential. This study illustrates an innovative workflow employing multiple separation techniques tandem high-resolution mass spectrometry for comprehensive and in-depth characterization of SARS-CoV-2 recombinant vaccine, including ultra-high performance liquid chromatography (UHPLC), ion exchange chromatography (IEX) and imaged capillary isoelectric focusing (icIEF). The integrated methodology focuses on the importance of cutting-edge icIEF-MS online coupling and icIEF fractionation applied to revealing the heterogeneity secret of SARS-CoV-2 recombinant vaccine.

Innovations and trends in antibody repertoire analysis

Monoclonal antibodies have revolutionized the treatment of human diseases, which has made them the fastest-growing class of therapeutics, with global sales expected to reach $346.6 billion USD by 2028. Advances in antibody engineering and development have led to the creation of increasingly sophisticated antibody-based therapeutics (e.g. bispecific antibodies and chimeric antigen receptor T cells). However, approaches for antibody discovery have remained comparatively grounded in conventional yet reliable in vitro assays. Breakthrough developments in high-throughput single B-cell sequencing and immunoglobulin proteomic serology, however, have enabled the identification of high-affinity antibodies directly from endogenous B cells or circulating immunoglobulin produced in vivo. Moreover, advances in artificial intelligence offer vast potential for antibody discovery and design with large-scale repertoire datasets positioned as the optimal source of training data for such applications. We highlight advances and recent trends in how these technologies are being applied to antibody repertoire analysis.

Internal Fragments Enhance Middle-Down Mass Spectrometry Structural Characterization of Monoclonal Antibodies and Antibody-Drug Conjugates

Monoclonal antibodies (mAbs) and antibody-drug conjugates (ADCs) are important large biotherapeutics (∼150 kDa) and high structural complexity that require extensive sequence and structure characterization. Middle-down mass spectrometry (MD-MS) is an emerging technique that sequences and maps subunits larger than those released by trypsinolysis. It avoids potentially introducing artifactual modifications that may occur in bottom-up MS while achieving higher sequence coverage compared to top-down MS. However, returning complete sequence information by MD-MS is still challenging. Here, we show that assigning internal fragments in direct infusion MD-MS of a mAb and an ADC substantially improves their structural characterization. For MD-MS of the reduced NIST mAb, including internal fragments recovers nearly 100% of the sequence by accessing the middle sequence region that is inaccessible by terminal fragments. The identification of important glycosylations can also be improved after the inclusion of internal fragments. For the reduced lysine-linked IgG1-DM1 ADC, we show that considering internal fragments increases the DM1 conjugation sites coverage to 80%, comparable to the reported 83% coverage achieved by peptide mapping on the same ADC (Luo et al. Anal. Chem. 2016, 88, 695-702). This study expands our work on the application of internal fragment assignments in top-down MS of mAbs and ADCs and can be extended to other heterogeneous therapeutic molecules such as multispecifics and fusion proteins for more widespread applications.

Improved characterization of trastuzumab deruxtecan with PTCR and internal fragments implemented in middle-down MS workflows

Antibody-drug conjugates (ADCs) are highly complex proteins mainly due to the structural microvariability of the mAb, along with the additional heterogeneity afforded by the bioconjugation process. Top-down (TD) and middle-down (MD) strategies allow the straightforward fragmentation of proteins to elucidate the conjugated amino acid residues. Nevertheless, these spectra are very crowded with multiple overlapping and unassigned ion fragments. Here we report on the use of dedicated software (ClipsMS) and application of proton transfer charge reduction (PTCR), to respectively expand the fragment ion search space to internal fragments and improve the separation of overlapping fragment ions for a more comprehensive characterization of a recently approved ADC, trastuzumab deruxtecan (T-DXd). Subunit fragmentation allowed between 70 and 90% of sequence coverage to be obtained. Upon addition of internal fragment assignment, the three subunits were fully sequenced, although internal fragments did not contribute significantly to the localization of the payloads. Finally, the use of PTCR after subunit fragmentation provided a moderate sequence coverage increase between 2 and 13%. The reaction efficiently decluttered the fragmentation spectra allowing increasing the number of fragment ions characteristic of the conjugation site by 1.5- to 2.5-fold. Altogether, these results show the interest in the implementation of internal fragment ion searches and more particularly the use of PTCR reactions to increase the number of signature ions to elucidate the conjugation sites and enhance the overall sequence coverage of ADCs, making this approach particularly appealing for its implementation in R&D laboratories.

Recent advances in structural mass spectrometry methods in the context of biosimilarity assessment: from sequence heterogeneities to higher order structures

Biotherapeutics and their biosimilar versions have been flourishing in the biopharmaceutical market for several years. Structural and functional characterization is needed to achieve analytical biosimilarity through the assessment of critical quality attributes as required by regulatory authorities. The role of analytical strategies, particularly mass spectrometry-based methods, is pivotal to gathering valuable information for the in-depth characterization of biotherapeutics and biosimilarity assessment. Structural mass spectrometry methods (native MS, HDX-MS, top-down MS, etc.) provide information ranging from primary sequence assessment to higher order structure evaluation. This review focuses on recent developments and applications in structural mass spectrometry for biotherapeutic and biosimilar characterization.

Imaged capillary isoelectric focusing tandem high-resolution mass spectrometry using nano electrospray ionization (ESI) for protein heterogeneity characterization

Recombinant monoclonal antibodies (mAbs) have been spurring the rapid growth of commercial biotherapeutics. During production their charge heterogeneity must be assessed as a critical quality attribute to ensure safety, efficacy, and potency. Although imaged capillary isoelectric focusing (icIEF) is a powerful tool for this process, it could be improved further with tandem high-resolution mass spectrometry (HRMS). In this work, a nano-electrospray ionization (nano-ESI) apparatus was constructed to directly couple icIEF to HRMS. The system was evaluated with the standard NISTmAb, as well as more complex mAb, bi-specific antibody, and fusion protein samples. NISTmAb concentrations as low as 0.25 mg/ml demonstrated excellent sensitivity. There were good repeatabilities at 1 mg/ml with 7.58% and 8.01% RSDs for intention time and MS intensity, respectively, and the HRMS signal showed a strong linearity (R = 0.9983) across different concentrations. Meanwhile, the fingerprinting of the complex samples illustrated the versatility and potential of icIEF-HRMS. icIEF-HRMS developed can provide a comprehensive understanding of the underlying structural modifications that impact protein charge heterogeneity. Compared to the traditional ESI, nano-ESI can significantly improve sensitivity while maintaining a reasonable repeatability and throughput. Furthermore, the interface is much easier to connect, and is compatible with many commercial HRMS instruments.

Mass spectrometry characterization of antibodies at the intact and subunit levels: from targeted to large-scale analysis

Antibodies are one of the most formidable molecular weapons available to our immune system. Their high specificity against a target (antigen) and capability of triggering different immune responses (e.g., complement system activation and antibody-dependent cell-mediated cytotoxicity) make them ideal drugs to fight many different human diseases. Currently, both monoclonal antibodies and more complex molecules based on the antibody scaffold are used as biologics. Naturally, such highly heterogeneous molecules require dedicated analytical methodologies for their accurate characterization. Mass spectrometry (MS) can define the presence and relative abundance of multiple features of antibodies, including critical quality attributes. The combination of small and large variations within a single molecule can only be determined by analyzing intact antibodies or their large (25 to 100 kDa) subunits. Hence, top-down (TD) and middle-down (MD) MS approaches have gained popularity over the last decade. In this Young Scientist Feature we discuss the evolution of TD and MD MS analysis of antibodies, including the new frontiers that go beyond biopharma applications. We will show how this field is now moving from the "quality control" analysis of a known, single antibody to the high-throughput investigation of complex antibody repertoires isolated from clinical samples, where the ultimate goal is represented by the complete gas-phase sequencing of antibody molecules without the need of any a priori knowledge.

Database Creator for Mass Analysis of Peptides and Proteins, DC-MAPP: A Standalone Tool for Simplifying Manual Analysis of Mass Spectral Data to Identify Peptide/Protein Sequences

Proteomic studies typically involve the use of different types of software for annotating experimental tandem mass spectrometric data (MS/MS) and thereby simplifying the process of peptide and protein identification. For such annotations, these softwares calculate the m/z values of the peptide/protein precursor and fragment ions, for which a database of protein sequences must be provided as an input file. The calculated m/z values are stored as another database, which the user usually cannot view. Database Creator for Mass Analysis of Peptides and Proteins (DC-MAPP) is a novel standalone software that can create custom databases for "viewing" the calculated m/z values of precursor and fragment ions, prior to the database search. It contains three modules. Peptide/Protein sequences as per user's choice can be entered as input to the first module for creating a custom database. In the second module, m/z values must be queried-in, which are searched within the custom database to identify protein/peptide sequences. The third module is suited for peptide mass fingerprinting, which can be used to analyze both ESI and MALDI mass spectral data. The feature of "viewing" the custom database can be helpful not only for better understanding the search engine processes, but also for designing multiple reaction monitoring (MRM) methods. Post-translational modifications and protein isoforms can also be analyzed. Since, DC-MAPP relies on the protein/peptide "sequences" for creating custom databases, it may not be applicable for the searches involving spectral libraries. Python language was used for implementation, and the graphical user interface was built with Page/Tcl, making this tool more user-friendly. It is freely available at https://vit.ac.in/DC-MAPP/.

Added Value of Internal Fragments for Top-Down Mass Spectrometry of Intact Monoclonal Antibodies and Antibody-Drug Conjugates

Monoclonal antibodies (mAbs) and antibody-drug conjugates (ADCs) are two of the most important therapeutic drug classes that require extensive characterization, whereas their large size and structural complexity make them challenging to characterize and demand the use of advanced analytical methods. Top-down mass spectrometry (TD-MS) is an emerging technique that minimizes sample preparation and preserves endogenous post-translational modifications (PTMs); however, TD-MS of large proteins suffers from low fragmentation efficiency, limiting the sequence and structure information that can be obtained. Here, we show that including the assignment of internal fragments in native TD-MS of an intact mAb and an ADC can improve their molecular characterization. For the NIST mAb, internal fragments can access the sequence region constrained by disulfide bonds to increase the TD-MS sequence coverage to over 75%. Important PTM information, including intrachain disulfide connectivity and N-glycosylation sites, can be revealed after including internal fragments. For a heterogeneous lysine-linked ADC, we show that assigning internal fragments improves the identification of drug conjugation sites to achieve a coverage of 58% of all putative conjugation sites. This proof-of-principle study demonstrates the potential value of including internal fragments in native TD-MS of intact mAbs and ADCs, and this analytical strategy can be extended to bottom-up and middle-down MS approaches to achieve even more comprehensive characterization of important therapeutic molecules.

Hyphenation of Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) with separation methods: The art of compromises and the possible - A review

This review provides an overview of the online hyphenation of Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FT-ICR MS) with separation methods to date. The online coupling between separation techniques (gas and liquid chromatography, capillary electrophoresis) and FT-ICR MS essentially raises questions of compromise and is not look as straightforward as hyphenation with other analyzers (QTOF-MS for instance). FT-ICR MS requires time to reach its highest resolving power and accuracy in mass measurement capabilities whereas chromatographic and electrophoretic peaks are transient. In many applications, the strengths and the weaknesses of each technique are balanced by their hyphenation. Untargeted "Omics" (e.g. proteomics, metabolomics, petroleomics, …) is one of the main areas of application for FT-ICR MS hyphenated to online separation techniques because of the complexity of the sample. FT-ICR MS achieves the required high mass measurement accuracy to determine accurate molecular formulae and resolution for isobar distinction. Meanwhile separation techniques highlight isomers and reduce the ion suppression effects extending the dynamic range. Even if the implementation of FT-ICR MS hyphenated with online separation methods is a little trickier (the art of compromise), this review shows that it provides unparalleled results to the scientific community (the art of the possible), along with raising the issue of its future in the field with the relentless technological progress.

"Lab of the Future"─Today: Fully Automated System for High-Throughput Mass Spectrometry Analysis of Biotherapeutics

Here we describe a state-of-the-art, integrated, multi-instrument automated system designed to execute methods involved in mass spectrometry characterization of biotherapeutics. The system includes liquid and microplate handling robotics and utilities, integrated LC-MS, along with data analysis software, to perform sample purification, preparation, and analysis as a seamless integrated unit. The automated process begins with tip-based purification of target proteins from expression cell-line supernatants, which is initiated once the samples are loaded onto the automated system and the metadata are retrieved from our corporate data aggregation system. Subsequently, the purified protein samples are prepared for MS, including deglycosylation and reduction steps for intact and reduced mass analysis, and proteolytic digestions, desalting, and buffer exchange via centrifugation for peptide map analysis. The prepared samples are then loaded into the LC-MS instrumentation for data acquisition. The acquired raw data are initially stored on a local area network storage system that is monitored by watcher scripts that then upload the raw MS data to a network of cloud-based servers. The raw MS data are processed with the appropriately configured analysis workflows such as database search for peptide mapping or charge deconvolution for undigested proteins. The results are verified and formatted for expert curation directly in the cloud. Finally, the curated results are appended to sample metadata in the corporate data aggregation system to accompany the biotherapeutic cell lines in subsequent processes.

Rapid Characterization of Antibodies via Automated Flow Injection Coupled with Online Microdroplet Reactions and Native-pH Mass Spectrometry

Microdroplet reactions have aroused much interest due to significant reaction acceleration (e.g., ultrafast protein digestion in microdroplets could occur in less than 1 ms). This study integrated a microdroplet protein digestion technique with automated sample flow injection and online mass spectrometry (MS) analysis, to develop a rapid and robust method for structural characterization of monoclonal antibodies (mAbs) that is essential to assess the antibody drug's safety and quality. Automated sequential aspiration and mixing of an antibody and an enzyme (IdeS or IgdE) enabled rapid analysis with high reproducibility (total analysis time: 2 min per sample; reproducibility: ∼2% coefficient of variation). Spraying the sample in ammonium acetate buffer (pH 7) using a jet stream source allowed efficient digestion of antibodies and efficient ionization of resulting antibody subunits under native-pH conditions. Importantly, it also provided a platform to directly study specific binding of an antibody and an antigen (e.g., detecting the complexes mAb/RSFV antigen and F(ab')2/RSVF in this study). Furthermore, subsequent tandem MS analysis of a resulting subunit from microdroplet digestion enabled localizing post-translational modifications on particular domains of a mAb in a rapid fashion. In combination with IdeS digestion of an antibody, additional tris(2-carboxyethyl)phosphine (TCEP) reduction and N-glycosidase F (PNGase F) deglycosylation reactions that facilitate antibody analysis could be realized in "one-pot" spraying. Interestingly, increased deglycosylation yield in microdroplets was found, simply by raising the sample temperature. We expect that our method would have a high impact for rapid characterization of monoclonal antibodies.

Imaged capillary isoelectric focusing (icIEF) tandem high resolution mass spectrometry for charged heterogeneity of protein drugs in biopharmaceutical discovery

Since the first commercial imaged capillary isoelectric focusing (icIEF) instrument was developed twenty years ago, the technology has become the gold standard of quality and manufacturing process control in the biopharmaceutical industry. This is owing to its high-resolution and high-throughput characterization of protein charge heterogeneity. In addition to a charge variant profiling, mass spectrometry (MS) analyses are also desirable to obtain an in-tact molecular weight (MW) and further identification of these charged species. While offline fractionation technologies including isoelectric focusing (IEF) and free flow electrophoresis (FFE) followed by liquid chromatography (LC)-mass spectrometry (MS) coupling have been employed for this purpose, there have been much fewer reported applications of icIEF-based MS connection and fraction collection. Factors that have impeded the development of these icIEF applications include difficulties with a direct connection to the MS interface as well as high background signal of carrier ampholytes and incompatible coated capillary cartridges. In this work, we developed a robust and flexible icIEF-MS platform which overcomes these challenges to achieve both the rapid icIEF separation and high-resolution MS (HRMS) identification of protein charged variants simultaneously. We demonstrate how this methodology proves highly-sensitive and highly reliable for the characterization of commercial monoclonal antibodies (mAbs) and antibody-drug-conjugates (ADCs). The whole workflow of icIEF-MS for protein heterogeneity is straight forward and accurate and can be performed within 45 min. Furthermore, the developed icIEF-MS configuration can flexibly switch to icIEF-based fraction collection model allowing the user to perform additional in-depth characterization such as peptide mapping by high performance liquid chromatography (HPLC) tandem mass spectrometry (LC-MS/MS).

Mass Spectrometry-Based Charge Heterogeneity Characterization of Therapeutic mAbs with Imaged Capillary Isoelectric Focusing and Ion-Exchange Chromatography as Separation Techniques

Imaged capillary isoelectric focusing (icIEF) and ion-exchange chromatography (IEX) are two essential techniques that are routinely used for charge variant analysis of therapeutic monoclonal antibodies (mAbs) during their development and in quality control. These two techniques that separate mAb charge variants based on different mechanisms and IEX have been developed as front-end separation techniques for online mass spectrometry (MS) detection, which is robust for intact protein identification. Recently, an innovative, coupled icIEF-MS technology has been constructed for protein charge variant analysis in our laboratory. In this study, icIEF-MS developed and strong cation exchange (SCX)-MS were optimized for charge heterogeneity characterization of a diverse of mAbs and their results were compared based on methodological validation. It was found that icIEF-MS outperformed SCX-MS in this study by demonstrating outstanding sensitivity, low carryover effect, accurate protein identification, and higher separation resolution although SCX-MS contributed to higher analysis throughput. Ultimately, integrating our novel icIEF-HRMS analysis with the more common SCX-MS can provide a promising and comprehensive strategy for accelerating the development of complex protein therapeutics.

Fractionation and online mass spectrometry based on imaged capillary isoelectric focusing (icIEF) for characterizing charge heterogeneity of therapeutic antibody

Imaged capillary isoelectric focusing (icIEF) technology has been proved to be robust for the characterization of protein charge heterogeneity due to its high-resolution pI discrimination and high-throughput. Although high performance liquid chromatography (HPLC) tandem mass spectrometry (MS) and offline fraction collection technologies including isoelectric focusing (IEF), ion exchange chromatography (IEX) and free flow electrophoresis (FFE) have been widely utilized for protein charge variant characterization, there are a few applications of MS coupling with icIEF as a front-separation technique and related fractionation technologies for protein charge heterogeneity. However, the application of icIEF-MS has been much less frequent due to difficulties in MS interface, compatible ampholyte and coated capillary cartridge designation, ultimately impeding the breadth of icIEF applications in protein charge heterogeneity. In this study, a therapeutic monoclonal antibody (mAb-M-AT) was used for its charge variant characterization on an integrated icIEF platform with functions including analytical profiling, MS online coupling and fraction collection for charge heterogeneities. The main protein component and its four charge variants were identified using direct icIEF-MS coupling. Additionally, the two major acidic and basic charge variants were collected using preparative fractionation after the protein focused in the separation capillary. The identity of the fractions was confirmed by LC-MS at intact protein level and the results were consistent with those using icIEF-MS online coupling. The multiple operation modes of the icIEF platform described above can be rapidly and flexibly switched just by changing customized capillary separation cartridges without drastically altering instrument configuration. The whole workflow of icIEF-based profiling, fractionation and MS online coupling for protein heterogeneity is straightforward, reliable, and accurate, thus providing comprehensive solutions for in-depth protein heterogeneity characterization.

Cutting-edge mass spectrometry strategy based on imaged capillary isoelectric focusing (icIEF) technology for characterizing charge heterogeneity of monoclonal antibody

Imaging capillary isoelectric focusing (icIEF) technology has been becoming the gold criteria of monitoring monoclonal antibody (mAb) charge heterogeneity that is one of the major product-related variants in recombinant biopharmaceuticals, since the first commercial instrument developed twenty years ago. However, the protein identification in icIEF separation is just based on isoelectric point (pI) measurement of protein. Although high resolution mass spectrometry (HRMS) is currently the most powerful means of qualitative protein analysis, traditional icIEF cannot compatibly be used in conjunction with MS due to the use of less volatile reagents. In addition, protein heterogeneity characterization in depth such as peptide mapping by high performance liquid chromatography (HPLC) requires the focused protein bands to be collected as fractions after the icIEF separation, which is a great challenge in biopharmaceutical discovery. In this work, pembrolizumab was employed as targeting mAb (a highly selective anti-PD-1 humanized mAb), an integrated icIEF platform was developed including analytical profiling, MS coupling and fraction collections for charged variant preparation. Multiple operation modes can be rapidly and flexibly switched just by changing customized capillary separation cartridges without more configurations. Main component, four acidic variants (A1-A4) and three basic variants (B1-B3) were baseline separated then directly detected by icIEF-HRMS online coupling for rapid screening of intact protein heterogeneity where reliable and accurate molecular weight of protein charged variants were obtained. Next, by installing preparative capillary separation cartridge, fractions of major charge variants (A2-3 and B1-2) and main component were collected for following LC-MS peptide mapping characterization. The whole workflow of icIEF-based MS strategy for protein heterogeneity is straight forward, reliable and accurate, which provides a comprehensive and revolutionary technology for protein drug quality control (QC) monitoring, MS coupling for fingerprinting intact protein and HPLC-MS peptide mapping in depth.

Deciphering combinatorial post-translational modifications by top-down mass spectrometry

Post-translational modifications (PTMs) create vast structural and functional diversity of proteins, ultimately modulating protein function and degradation, influencing cellular signaling, and regulating transcription. The combinatorial patterns of PTMs increase the heterogeneity of proteins and further mediates their interactions. Advances in mass spectrometry-based proteomics have resulted in identification of thousands of proteins and allowed characterization of numerous types and sites of PTMs. Examination of intact proteins, termed the top-down approach, offers the potential to map protein sequences and localize multiple PTMs on each protein, providing the most comprehensive cataloging of proteoforms. This review describes some of the dividends of using mass spectrometry to analyze intact proteins and showcases innovative strategies that have enhanced the promise of top-down proteomics for exploring the impact of combinatorial PTMs in unsurpassed detail.

SR Protein Kinases Regulate the Splicing of Cardiomyopathy-Relevant Genes via Phosphorylation of the RSRSP Stretch in RBM20

(1) Background: RNA binding motif 20 (RBM20) regulates mRNA splicing specifically in muscle tissues. Missense mutations in the arginine/serine (RS) domain of RBM20 lead to abnormal gene splicing and have been linked to severe dilated cardiomyopathy (DCM) in human patients and animal models. Interestingly, many of the reported DCM-linked missense mutations in RBM20 are in a highly conserved RSRSP stretch within the RS domain. Recently, it was found that the two Ser residues within this stretch are constitutively phosphorylated, yet the identity of the kinase(s) responsible for phosphorylating these residues, as well as the function of RSRSP phosphorylation, remains unknown. (2) Methods: The ability of three known SR protein kinases (SRPK1, CLK1, and AKT2) to phosphorylate the RBM20 RSRSP stretch and regulate target gene splicing was evaluated by using both in vitro and in vivo approaches. (3) Results: We found that all three kinases phosphorylated S638 and S640 in the RSRSP stretch and regulated RBM20 target gene splicing. While SRPK1 and CLK1 were both capable of directly phosphorylating the RS domain in RBM20, whether AKT2-mediated control of the RS domain phosphorylation is direct or indirect could not be determined. (4) Conclusions: Our results indicate that SR protein kinases regulate the splicing of a cardiomyopathy-relevant gene by modulating phosphorylation of the RSRSP stretch in RBM20. These findings suggest that SR protein kinases may be potential targets for the treatment of RBM20 cardiomyopathy.

Characterization of the Time-Domain Isotopic Beat Patterns of Monoclonal Antibodies in Fourier Transform Mass Spectrometry

The time-domain transients in the Fourier transform mass spectrometry (FTMS) analysis of monoclonal antibodies (mAbs) are known to exhibit characteristic isotopic beat patterns. These patterns are defined by the isotopic distributions of all gaseous mAb ions present in the FTMS mass analyzer, originating from single or multiple charge states, and from single or multiple proteoforms. For an isolated charge state of a single proteoform, the mAb isotopic beat pattern resembles narrow splashes of signal amplitude (beats), spaced periodically in the time-domain transient, with broad (often exceeding 1 s) "valleys" between them. Here, we reinforce the importance of isotopic beat patterns for the accurate interpretation and presentation of FTMS data in the analysis of mAbs and other large biopolymers. An updated, mAb-grade version of the transient-mediated FTMS data simulation and visualization tool, FTMS Simulator is introduced and benchmarked. We then apply this tool to evaluate the charge-state dependent characteristics of isotopic beats in mAbs analyses with modern models of Orbitrap and ion cyclotron resonance (ICR) FTMS instruments, including detection of higher-order harmonics. We demonstrate the impact of the isotopic beat patterns on the analytical characteristics of the resulting mass spectra of individual and overlapping mAb proteoforms. The results reported here detail highly nonlinear dependences of resolution and signal-to-noise ratio on the time-domain transient period, absorption or magnitude mode spectra representation, and apodization functions. The provided description and the demonstrated ability to routinely conduct accurate simulations of FTMS data for large biopolymers should aid the end-users of Orbitrap and ICR FTMS instruments in the analysis of mAbs and other biopolymers, including viruses.

RBM20 phosphorylation and its role in nucleocytoplasmic transport and cardiac pathogenesis

Arginine-serine (RS) domain(s) in splicing factors are critical for protein-protein interaction in pre-mRNA splicing. Phosphorylation of RS domain is important for splicing control and nucleocytoplasmic transport in the cell. RNA-binding motif 20 (RBM20) is a splicing factor primarily expressed in the heart. A previous study using phospho-antibody against RS domain showed that RS domain can be phosphorylated. However, its actual phosphorylation sites and function have not been characterized. Using middle-down mass spectrometry, we identified 16 phosphorylation sites, two of which (S638 and S640 in rats, or S637 and S639 in mice) were located in the RSRSP stretch in the RS domain. Mutations on S638 and S640 regulated splicing, promoted nucleocytoplasmic transport and protein-RNA condensates. Phosphomimetic mutations on S638 and S640 indicated that phosphorylation was not the major cause for RBM20 nucleocytoplasmic transport and condensation in vitro. We generated a S637A knock-in (KI) mouse model (Rbm20S637A ) and observed the reduced RBM20 phosphorylation. The KI mice exhibited aberrant gene splicing, protein condensates, and a dilated cardiomyopathy (DCM)-like phenotype. Transcriptomic profiling demonstrated that KI mice had altered expression and splicing of genes involving cardiac dysfunction, protein localization, and condensation. Our in vitro data showed that phosphorylation was not a direct cause for nucleocytoplasmic transport and protein condensation. Subsequently, the in vivo results reveal that RBM20 mutations led to cardiac pathogenesis. However, the role of phosphorylation in vivo needs further investigation.

Fourier-transform ion cyclotron resonance mass spectrometry for characterizing proteoforms

Proteoforms contribute functional diversity to the proteome and aberrant proteoforms levels have been implicated in biological dysfunction and disease. Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS), with its ultrahigh mass-resolving power, mass accuracy, and versatile tandem MS capabilities, has empowered top-down, middle-down, and native MS-based approaches for characterizing proteoforms and their complexes in biological systems. Herein, we review the features which make FT-ICR MS uniquely suited for measuring proteoform mass with ultrahigh resolution and mass accuracy; obtaining in-depth proteoform sequence coverage with expansive tandem MS capabilities; and unambiguously identifying and localizing post-translational and noncovalent modifications. We highlight examples from our body of work in which we have quantified and comprehensively characterized proteoforms from cardiac and skeletal muscle to better understand conditions such as chronic heart failure, acute myocardial infarction, and sarcopenia. Structural characterization of monoclonal antibodies and their proteoforms by FT-ICR MS and emerging applications, such as native top-down FT-ICR MS and high-throughput top-down FT-ICR MS-based proteomics at 21 T, are also covered. Historically, the information gleaned from FT-ICR MS analyses have helped provide biological insights. We predict FT-ICR MS will continue to enable the study of proteoforms of increasing size from increasingly complex endogenous mixtures and facilitate the benchmarking of sensitive and specific assays for clinical diagnostics. © 2020 John Wiley & Sons Ltd. Mass Spec Rev.

Optimized C-Trap Timing of an Orbitrap 240 Mass Spectrometer for High-Throughput Screening and Native MS by IR-MALDESI

Infrared matrix-assisted laser desorption ionization (IR-MALDESI) is a hybrid mass spectrometry ionization source that combines the benefits of electrospray ionization (ESI) and matrix-assisted laser desorption ionization (MALDI) making it a great analytical tool for high-throughput screening (HTS) analyses. IR-MALDESI is coupled to an Orbitrap Exploris 240 mass spectrometer that utilizes a bent quadrupole (C-trap) to inject accumulated ions into the high-field Orbitrap mass analyzer. Here, we present a study on the optimized C-trap timing for HTS analyses by IR-MALDESI mass spectrometry. The timing between initial ion generation and the C-trap opening time was optimized to reduce unnecessary ambient ion accumulation in the mass spectrometer. The time in which the C-trap was held open, the ion accumulation time, was further optimized to maximize the accumulation of analyte ions generated using IR-MALDESI. The resulting C-trap opening scheme benefits small-molecule HTS analyses by IR-MALDESI by maximizing target ion abundances, minimizing ambient ion abundances, and minimizing the total analysis time per sample. The proposed C-trap timing scheme for HTS does not translate to large molecules; a NIST monoclonal antibody standard reference material was analyzed to demonstrate that larger analytes require longer ion accumulation times and that IR-MALDESI can measure intact antibodies in their native state.

Extending the capabilities of intact-mass analyses to monoclonal immunoglobulins of the E-isotype (IgE)

Mass spectrometry (MS) has become an indispensable tool in structural characterization and quality control of monoclonal antibodies (mAbs). Intact-mass analysis is a particularly attractive option that provides a powerful and cost-effective means to not only confirm the structural integrity of the protein, but also probe its interactions with therapeutic targets. To a certain extent, this success can be attributed to relatively modest glycosylation levels exhibited by IgG molecules, which limits their structural heterogeneity and enables straightforward mass measurements at the intact molecule level. The recent surge of interest in expanding the repertoire of mAbs to include other classes of immunoglobulins places a premium on efforts to adapt the IgG-tailored experimental strategies to other classes of antibodies, but their dramatically higher levels of glycosylation may create insurmountable obstacles. The monoclonal murine IgE antibody explored in this work provides a challenging model system, as its glycosylation level exceeds that of conventional IgG mAbs by a factor of nine. The commercial sample, which included various IgE fragments, yields a poorly resolved ionic signal in intact-mass measurements, from which little useful information can be extracted. However, coupling MS measurements with the limited charge reduction of select polycationic species in the gas phase gives rise to well-defined charge ladders, from which both ionic masses and charges can be readily determined. The measurements reveal significant variation of the extent of glycosylation within intact IgE molecules, as well as the presence of low-molecular weight impurities in the commercial IgE sample. Furthermore, incubation of the monoclonal IgE with its antigen (ovalbumin) gives rise to the formation of complexes with varying stoichiometries, which can also be uniquely identified using a combination of native MS, limited charge reduction in the gas phase and data fitting procedures. This work demonstrates that following appropriate modifications, intact-mass analysis measurements can be successfully applied to mAbs beyond the IgG isotype, providing a wealth of information not only on the mass distribution of the intact IgE molecules, but also their large-scale conformational integrity, the integrity of their covalent structure, and their interactions with antigens.

Size Exclusion Chromatography Strategies and MASH Explorer for Large Proteoform Characterization

Top-down mass spectrometry (MS)-based analysis of larger proteoforms (>50 kDa) is typically challenging due to an exponential decay in the signal-to-noise ratio with increasing protein molecular weight (MW) and coelution with low-MW proteoforms. Size exclusion chromatography (SEC) fractionates proteins based on their size, separating larger proteoforms from those of smaller size in the proteome. In this protocol, we initially describe the use of SEC to fractionate high-MW proteoforms from low-MW proteoforms. Subsequently, the SEC fractions containing the proteoforms of interest are subjected to reverse-phase liquid chromatography (RPLC) coupled online with high-resolution MS. Finally, proteoforms are characterized using MASH Explorer, a user-friendly software environment for in-depth proteoform characterization.

Mimicking LysC Proteolysis by 'Arginine-Modification-cum-Trypsin digestion': Comparison of Bottom-Up & Middle-Down Proteomic Approaches by ESI-QTOF-MS

BACKGROUND

Middle-down (MD) proteomics is an emerging approach for reliable identification of post- translational modifications and isoforms, as this approach focuses on proteolytic peptides containing > 25 - 30 amino acid residues (a.a.r.), which are longer than typical tryptic peptides. Such longer peptides can be obtained by AspN, GluC, LysC proteases. Additionally, some special proteases were developed specifically to effect MD approach, e.g., OmpT, Sap9, etc. However, these proteases are expensive. Herein we report a cost-effective strategy, 'arginine modification-cum trypsin digestion', which can produce longer tryptic peptides resembling LysC peptides derived from proteins.

OBJECTIVE

To obtain proteolytic peptides that resemble LysC peptides, by using 'trypsin', which is an less expensive protease.

METHODS

This strategy is based on the simple principle that trypsin cannot act at the C-termini of those arginines in proteins, whose sidechain guanidine groups are modified by 1,2-cyclohexanedione or phenylglyoxal.

RESULTS

As a proof of concept, we demonstrate this strategy on four models: β-casein (bovine), β- lactoglobulin (bovine), ovalbumin (chick) and transferrin (human), by electrospray ionization-mass spectrometry (ESI-MS) involving hybrid quadrupole time-of-flight. From the ESI-MS of these models, we obtained several arginine modified tryptic peptides, whose lengths are in the range, 30 - 60 a.a.r. The collision-induced dissociation MS/MS characteristics of some of the arginine modified longer tryptic peptides are compared with the unmodified standard tryptic peptides.

CONCLUSION

The strategy followed in this proof-of-concept study, not only helps in obtaining longer tryptic peptides that mimic LysC proteolytic peptides, but also facilitates in enhancing the probability of missed cleavages by the trypsin. Hence, this method aids in evading the possibility of obtaining very short peptides that are < 5 - 10 a.a.r. Therefore, this is indeed an cost-effective alternative/substitute for LysC proteolysis and in turn, for those MD proteomic studies that utilize LysC. Additionally, this methodology can be fruitful for mass spectrometry based de novo protein and peptide sequencing.

Structural O-Glycoform Heterogeneity of the SARS-CoV-2 Spike Protein Receptor-Binding Domain Revealed by Top-Down Mass Spectrometry

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) utilizes an extensively glycosylated surface spike (S) protein to mediate host cell entry, and the S protein glycosylation plays key roles in altering the viral binding/function and infectivity. However, the molecular structures and glycan heterogeneity of the new O-glycans found on the S protein regional-binding domain (S-RBD) remain cryptic because of the challenges in intact glycoform analysis by conventional bottom-up glycoproteomic approaches. Here, we report the complete structural elucidation of intact O-glycan proteoforms through a hybrid native and denaturing top-down mass spectrometry (MS) approach employing both trapped ion mobility spectrometry (TIMS) quadrupole time-of-flight and ultrahigh-resolution Fourier transform ion cyclotron resonance (FTICR)-MS. Native top-down TIMS-MS/MS separates the protein conformers of the S-RBD to reveal their gas-phase structural heterogeneity, and top-down FTICR-MS/MS provides in-depth glycoform analysis for unambiguous identification of the glycan structures and their glycosites. A total of eight O-glycoforms and their relative molecular abundance are structurally elucidated for the first time. These findings demonstrate that this hybrid top-down MS approach can provide a high-resolution proteoform-resolved mapping of diverse O-glycoforms of the S glycoprotein, which lays a strong molecular foundation to uncover the functional roles of their O-glycans. This proteoform-resolved approach can be applied to reveal the structural O-glycoform heterogeneity of emergent SARS-CoV-2 S-RBD variants as well as other O-glycoproteins in general.

Protein Analysis by Electrospray-Orbital Frequency Analyzer with Charge Detection Mass Spectrometry Algorithm

Orbital frequency analyzer (OFA) is one of the electrostatic ion trap mass analysers for FTMS, which offers ultrahigh mass resolution and high ion charge capacity. In order to analyze multiply charged proteins and other large biological particles by means of charge detection mass spectrometry, a data processing algorithm was created to suit the image charge signal of nonharmonic waveform nature. The algorithm is capable of detecting collisions between ions and residual gas molecules, to determine lifetime of ions, and to evaluate the charge and mass values for ions having lifetime above a threshold. With the filtering of the lifetime and charge value, the chemical noise from small molecules and protein fragments can be eliminated in the reconstructed spectrum, facilitating measurement of protein content at a very low concentration, down to tens of nanomolars. The standard deviation of charge measurement is between 1.1 to 1.8 e for ions with oscillation lifetimes from 1 to 0.4 s, and this in turn determines the CDMS spectrum mass peak width. It has been found that the lower voltage setting of the OFA results in a larger population of ions surviving for longer times and thus produces narrower mass peak width. While OFA is able to run multiplexed CDMS without ion motion interference, coexisting ions of the same or very close m/z can cause interference between their image charge signals, which increases the error in charge determination.

Microdroplet Ultrafast Reactions Speed Antibody Characterization

Recently, microdroplet reactions have aroused much interest because the microdroplet provides a unique medium where organic reactions could be accelerated by a factor of 10³ or more. However, microdroplet reactions of proteins have been rarely studied. We report the occurrence of multiple-step reactions of a large protein, specifically, the digestion, reduction, and deglycosylation of an intact antibody, which can take place in microseconds with high reaction yields in aqueous microdroplets at room temperature. As a result, fast structural characterization of a monoclonal antibody, essential for assessing its quality as a therapeutic drug, can be enabled. We found that the IgG1 antibody can be digested completely by the IdeS protease in aqueous microdroplets in 250 microseconds, a 7.5 million-fold improvement in speed in comparison to traditional digestion in bulk solution (>30 min). Strikingly, inclusion of the reductant tris(2-carboxyethyl)phosphine in the spray solution caused simultaneous antibody digestion and disulfide bond reduction. Digested and reduced antibody fragments were either collected or analyzed online by mass spectrometry. Further addition of PNGase F glycosylase into the spray solution led to antibody deglycosylation, thereby producing reduced and deglycosylated fragments of analytical importance. In addition, glycated fragments of IgG1 derived from glucose modification were identified rapidly with this ultrafast digestion/reduction technique. We suggest that microdroplets can serve as powerful microreactors for both exploring large-molecule reactions and speeding their structural analyses.

Fractionated charge variants of biosimilars: A review of separation methods, structural and functional analysis

The similarity between originator and biosimilar monoclonal antibody candidates are rigorously assessed based on primary, secondary, tertiary, quaternary structures, and biological functions. Minor differences in such parameters may alter target-binding, potency, efficacy, or half-life of the molecule. The charge heterogeneity analysis is a prerequisite for all biotherapeutics. Monoclonal antibodies are prone to enzymatic or non-enzymatic structural modifications during or after the production processes, leading to the formation of fragments or aggregates, various glycoforms, oxidized, deamidated, and other degraded residues, reduced Fab region binding activity or altered FcR binding activity. Therefore, the charge variant profiles of the monoclonal antibodies must be regularly and thoroughly evaluated. Comparative structural and functional analysis of physically separated or fractioned charged variants of monoclonal antibodies has gained significant attention in the last few years. The fraction-based charge variant analysis has proved very useful for the biosimilar candidates comprising of unexpected charge isoforms. In this report, the key methods for the physical separation of monoclonal antibody charge variants, structural and functional analyses by liquid chromatography-mass spectrometry, and surface plasmon resonance techniques were reviewed.

Rapid Analysis of Reduced Antibody Drug Conjugate by Online LC-MS/MS with Fourier Transform Ion Cyclotron Resonance Mass Spectrometry

Antibody drug conjugates (ADCs), which harness the high targeting specificity of monoclonal antibodies (mAb) with the potency of small molecule therapeutics, are one of the fastest growing pharmaceutical classes. Nevertheless, ADC conjugation techniques and processes may introduce intrinsic heterogeneity including primary sequence variants, varied drug-to-antibody ratio (DAR) species, and drug positional isomers, which must be monitored to ensure the safety and efficacy of ADCs. Liquid chromatography coupled to mass spectrometry (LC-MS) is a powerful tool for characterization of ADCs. However, the conventional bottom-up MS analysis workflows require an enzymatic digestion step which can be time consuming and may introduce artifactual modifications. Herein, we develop an online LC-MS/MS method for rapid analysis of reduced ADCs without digestion, enabling determination of DAR, characterization of the primary sequence, and localization of the drug conjugation site of the ADC using high-resolution Fourier transform ion cyclotron resonance (FTICR) MS. Specifically, a model cysteine-linked ADC was reduced to generate six unique subunits: light chain (Lc) without drug (Lc0), Lc with 1 drug (Lc1), heavy chain (Hc) without drug (Hc0), and Hc with 1-3 drugs (Hc1-3, respectively). A concurrent reduction strategy is applied to assess ADC subunits in both the partially reduced (intrachain disulfide bonds remain intact) and fully reduced (all disulfide bonds are cleaved) forms. The entire procedure including the sample preparation and LC-MS/MS takes less than 55 min, enabling rapid multiattribute analysis of ADCs.

Recent progress in the molecular imaging of therapeutic monoclonal antibodies

Therapeutic monoclonal antibodies have become one of the central components of the healthcare system and continuous efforts are made to bring innovative antibody therapeutics to patients in need. It is equally critical to acquire sufficient knowledge of their molecular structure and biological functions to ensure the efficacy and safety by incorporating new detection approaches since new challenges like individual differences and resistance are presented. Conventional techniques for determining antibody disposition including plasma drug concentration measurements using LC-MS or ELISA, and tissue distribution using immunohistochemistry and immunofluorescence are now complemented with molecular imaging modalities like positron emission tomography and near-infrared fluorescence imaging to obtain more dynamic information, while methods for characterization of antibody's interaction with the target antigen as well as visualization of its cellular and intercellular behavior are still under development. Recent progress in detecting therapeutic antibodies, in particular, the development of methods suitable for illustrating the molecular dynamics, is described here.

Top-down proteomics: challenges, innovations, and applications in basic and clinical research

Introduction- A better understanding of the underlying molecular mechanism of diseases is critical for developing more effective diagnostic tools and therapeutics toward precision medicine. However, many challenges remain to unravel the complex nature of diseases. Areas covered- Changes in protein isoform expression and post-translation modifications (PTMs) have gained recognition for their role in underlying disease mechanisms. Top-down mass spectrometry (MS)-based proteomics is increasingly recognized as an important method for the comprehensive characterization of proteoforms that arise from alternative splicing events and/or PTMs for basic and clinical research. Here, we review the challenges, technological innovations, and recent studies that utilize top-down proteomics to elucidate changes in the proteome with an emphasis on its use to study heart diseases. Expert opinion- Proteoform-resolved information can substantially contribute to the understanding of the molecular mechanisms underlying various diseases and for the identification of novel proteoform targets for better therapeutic development . Despite the challenges of sequencing intact proteins, top-down proteomics has enabled a wealth of information regarding protein isoform switching and changes in PTMs. Continuous developments in sample preparation, intact protein separation, and instrumentation for top-down MS have broadened its capabilities to characterize proteoforms from a range of samples on an increasingly global scale.

Interlaboratory Study for Characterizing Monoclonal Antibodies by Top-Down and Middle-Down Mass Spectrometry

The Consortium for Top-Down Proteomics (www.topdownproteomics.org) launched the present study to assess the current state of top-down mass spectrometry (TD MS) and middle-down mass spectrometry (MD MS) for characterizing monoclonal antibody (mAb) primary structures, including their modifications. To meet the needs of the rapidly growing therapeutic antibody market, it is important to develop analytical strategies to characterize the heterogeneity of a therapeutic product's primary structure accurately and reproducibly. The major objective of the present study is to determine whether current TD/MD MS technologies and protocols can add value to the more commonly employed bottom-up (BU) approaches with regard to confirming protein integrity, sequencing variable domains, avoiding artifacts, and revealing modifications and their locations. We also aim to gather information on the common TD/MD MS methods and practices in the field. A panel of three mAbs was selected and centrally provided to 20 laboratories worldwide for the analysis: Sigma mAb standard (SiLuLite), NIST mAb standard, and the therapeutic mAb Herceptin (trastuzumab). Various MS instrument platforms and ion dissociation techniques were employed. The present study confirms that TD/MD MS tools are available in laboratories worldwide and provide complementary information to the BU approach that can be crucial for comprehensive mAb characterization. The current limitations, as well as possible solutions to overcome them, are also outlined. A primary limitation revealed by the results of the present study is that the expert knowledge in both experiment and data analysis is indispensable to practice TD/MD MS.

Direct measurement of light and heavy antibody chains using ion mobility and middle-down mass spectrometry

The analysis of monoclonal antibodies (mAbs) by a middle-down mass spectrometry (MS) approach is a growing field that attracts the attention of many researchers and biopharmaceutical companies. Usually, liquid fractionation techniques are used to separate mAbs polypeptides chains before MS analysis. Gas-phase fractionation techniques such as high-field asymmetric waveform ion mobility spectrometry (FAIMS) can replace liquid-based separations and reduce both analysis time and cost. Here, we present a rapid FAIMS tandem MS method capable of characterizing the polypeptide sequence of mAbs light and heavy chains in an unprecedented, easy, and fast fashion. This new method uses commercially available instruments and takes ~24 min, which is 40-60% faster than regular liquid chromatography-MS/MS analysis, to acquire fragmentation data using different dissociation methods.

Susceptibility of protein therapeutics to spontaneous chemical modifications by oxidation, cyclization, and elimination reactions

Peptides and proteins are preponderantly emerging in the drug market, as shown by the increasing number of biopharmaceutics already approved or under development. Biomolecules like recombinant monoclonal antibodies have high therapeutic efficacy and offer a valuable alternative to small-molecule drugs. However, due to their complex three-dimensional structure and the presence of many functional groups, the occurrence of spontaneous conformational and chemical changes is much higher for peptides and proteins than for small molecules. The characterization of biotherapeutics with modern and sophisticated analytical methods has revealed the presence of contaminants that mainly arise from oxidation- and elimination-prone amino-acid side chains. This review focuses on protein chemical modifications that may take place during storage due to (1) oxidation (methionine, cysteine, histidine, tyrosine, tryptophan, and phenylalanine), (2) intra- and inter-residue cyclization (aspartic and glutamic acid, asparagine, glutamine, N-terminal dipeptidyl motifs), and (3) β-elimination (serine, threonine, cysteine, cystine) reactions. It also includes some examples of the impact of such modifications on protein structure and function.

Middle-Down Multi-Attribute Analysis of Antibody-Drug Conjugates with Electron Transfer Dissociation

Antibody-drug conjugates (ADCs) are designed to combine the target specificity of monoclonal antibodies and potent cytotoxin drugs to achieve better therapeutic outcomes. Comprehensive evaluation of the quality attributes of ADCs is critical for drug development but remains challenging due to heterogeneity of the construct. Currently, peptide mapping with reversed-phase liquid chromatography (RPLC) coupled to mass spectrometry (MS) is the predominant approach to characterize ADCs. However, it is suboptimal for sequence characterization and quantification of ADCs because it lacks a comprehensive view of coexisting variants and suffers from varying ionization effects of drug-conjugated peptides compared to unconjugated counterparts. Here, we present the first middle-down RPLC-MS analysis of both cysteine (Adcetris; BV) and lysine (Kadcyla; T-DM1) conjugated ADCs at the subunit level (∼25 kDa) with electron transfer dissociation (ETD). We successfully achieved high-resolution separation of subunit isomers arising from different drug conjugation and subsequently localized the conjugation sites. Moreover, we obtained a comprehensive overview of the microvariants associated with each subunits and characterized them such as oxidized variants with different sites. Furthermore, we observed relatively high levels of conjugation near complementarity-determining regions (CDRs) from the heavy chain but no drug conjugation near CDRs of light chain (Lc) from lysine conjugated T-DM1. Based on the extracted ion chromatograms, we accurately measured average drug to antibody ratio (DAR) values and relative occupancy of drug-conjugated subunits. Overall, the middle-down MS approach enables the evaluation of multiple quality attributes including DAR, positional isomers, conjugation sites, occupancy, and microvariants, which potentially opens up a new avenue to characterize ADCs.

Middle-down approach: a choice to sequence and characterize proteins/proteomes by mass spectrometry

Owing to rapid growth in the elucidation of genome sequences of various organisms, deducing proteome sequences has become imperative, in order to have an improved understanding of biological processes. Since the traditional Edman method was unsuitable for high-throughput sequencing and also for N-terminus modified proteins, mass spectrometry (MS) based methods, mainly based on soft ionization modes: electrospray ionization and matrix-assisted laser desorption/ionization, began to gain significance. MS based methods were adaptable for high-throughput studies and applicable for sequencing N-terminus blocked proteins/peptides too. Consequently, over the last decade a new discipline called 'proteomics' has emerged, which encompasses the attributes necessary for high-throughput identification of proteins. 'Proteomics' may also be regarded as an offshoot of the classic field, 'biochemistry'. Many protein sequencing and proteomic investigations were successfully accomplished through MS dependent sequence elucidation of 'short proteolytic peptides (typically: 7-20 amino acid residues), which is called the 'shotgun' or 'bottom-up (BU)' approach. While the BU approach continues as a workhorse for proteomics/protein sequencing, attempts to sequence intact proteins without proteolysis, called the 'top-down (TD)' approach started, due to ambiguities in the BU approach, e.g., protein inference problem, identification of proteoforms and the discovery of posttranslational modifications (PTMs). The high-throughput TD approach (TD proteomics) is yet in its infancy. Nevertheless, TD characterization of purified intact proteins has been useful for detecting PTMs. With the hope to overcome the pitfalls of BU and TD strategies, another concept called the 'middle-down (MD)' approach was put forward. Similar to BU, the MD approach also involves proteolysis, but in a restricted manner, to produce 'longer' proteolytic peptides than the ones usually obtained in BU studies, thereby providing better sequence coverage. In this regard, special proteases (OmpT, Sap9, IdeS) have been used, which can cleave proteins to produce longer proteolytic peptides. By reviewing ample evidences currently existing in the literature that is predominantly on PTM characterization of histones and antibodies, herein we highlight salient features of the MD approach. Consequently, we are inclined to claim that the MD concept might have widespread applications in future for various research areas, such as clinical, biopharmaceuticals (including PTM analysis) and even for general/routine characterization of proteins including therapeutic proteins, but not just limited to analysis of histones or antibodies.