Reversed-phase liquid-chromatographic mass spectrometric N-glycan analysis of biopharmaceuticals

Recent advances in microscale separation techniques for glycome analysis

The glycomic analysis holds significant appeal due to the diverse roles that glycans and glycoconjugates play, acting as modulators and mediators in cellular interactions, cell/organism structure, drugs, energy sources, glyconanomaterials, and more. The glycomic analysis relies on liquid-phase separation technologies for molecular purification, separation, and identification. As a miniaturized form of liquid-phase separation technology, microscale separation technologies offer various advantages such as environmental friendliness, high resolution, sensitivity, fast speed, and integration capabilities. For glycan analysis, microscale separation technologies are continuously evolving to address the increasing challenges in their unique manners. This review discusses the fundamentals and applications of microscale separation technologies for glycomic analysis. It covers liquid-phase separation technologies operating at scales generally less than 100 µm, including capillary electrophoresis, nanoflow liquid chromatography, and microchip electrophoresis. We will provide a brief overview of glycomic analysis and describe new strategies in microscale separation and their applications in glycan analysis from 2014 to 2023.

Liquid Chromatography and Capillary Electrophoresis Analysis of 2AA-Labeled O-Glycans

To reveal O-glycan structures in mucins, it is necessary to release covalently bound O-glycans from the polypeptide backbone and derivatize to a form suitable for structural analysis. Various derivatization methods can now be applied in the analysis of O-glycans following the development of O-glycan release methods. Among the many derivatization methods available, we prefer to use fluorescent labeling with 2-aminobenzoic acid (anthranilic acid, 2AA). 2AA-labeled O-glycans can be detected with high sensitivity using liquid chromatography fluorescence detection (LC-FD) analysis because of the strong fluorescence. In addition, as 2AA has a carboxyl group that carries a negative charge, 2AA-labeled O-glycans can be analyzed with high sensitivity in negative ion mode mass spectrometry. Furthermore, because the negative charge of 2AA provides a driving force for electrophoresis, 2AA-labeled O-glycans can be analyzed using capillary electrophoresis (CE) and capillary affinity electrophoresis. High detection sensitivity and versatility are key advantages of the 2AA-labeling method. Here we present our preferred LC-FD and CE analytical methods for 2AA-labeled O-glycans.

Fragmentation stability and retention time-shift obtained by LC-MS/MS to distinguish sialylated N-glycan linkage isomers in therapeutic glycoproteins

Sialylated N-glycan isomers with α2-3 or α2-6 linkage(s) have distinctive roles in glycoproteins, but are difficult to distinguish. Wild-type (WT) and glycoengineered (mutant) therapeutic glycoproteins, cytotoxic T lymphocyte-associated antigen-4-immunoglobulin (CTLA4-Ig), were produced in Chinese hamster ovary cell lines; however, their linkage isomers have not been reported. In this study, N-glycans of CTLA4-Igs were released, labeled with procainamide, and analyzed by liquid chromatography-tandem mass spectrometry (MS/MS) to identify and quantify sialylated N-glycan linkage isomers. The linkage isomers were distinguished by comparison of 1) intensity of the N-acetylglucosamine ion to the sialic acid ion (Ln/Nn) using different fragmentation stability in MS/MS spectra and 2) retention time-shift for a selective m/z value in the extracted ion chromatogram. Each isomer was distinctively identified, and each quantity (>0.1%) was obtained relative to the total N-glycans (100%) for all observed ionization states. Twenty sialylated N-glycan isomers with only α2-3 linkage(s) in WT were identified, and each isomer's sum of quantities was 50.4%. Furthermore, 39 sialylated N-glycan isomers (58.8%) in mono- (3 N-glycans; 0.9%), bi- (18; 48.3%), tri- (14; 8.9%), and tetra- (4; 0.7%) antennary structures of mutant were obtained, which comprised mono- (15 N-glycans; 25.4%), di- (15; 28.4%), tri- (8; 4.8%), and tetra- (1; 0.2%) sialylation, respectively, with only α2-3 (10 N-glycans; 4.8%), both α2-3 and α2-6 (14; 18.4%), and only α2-6 (15; 35.6%) linkage(s). These results are consistent with those for α2-3 neuraminidase-treated N-glycans. This study generated a novel plot of Ln/Nn versus retention time to distinguish sialylated N-glycan linkage isomers in glycoprotein.

Technical pipeline for screening microbial communities as a function of substrate specificity through fluorescent labelling

The study of specific glycan uptake and metabolism is an effective tool in aiding with the continued unravelling of the complexities in the human gut microbiome. To this aim fluorescent labelling of glycans may provide a powerful route towards this target. Here, we successfully used the fluorescent label 2-aminobenzamide (2-AB) to monitor and study microbial degradation of labelled glycans. Both single strain and co-cultured fermentations of microbes from the common human-gut derived Bacteroides genus, are able to grow when supplemented with 2-AB labelled glycans of different monosaccharide composition, degrees of acetylation and polymerization. Utilizing a multifaceted approach that combines chromatography, mass spectrometry, microscopy and flow cytometry techniques, it is possible to better understand the metabolism of labelled glycans in both supernatants and at a single cell level. We envisage this combination of complementary techniques will help further the understanding of substrate specificity and the role it plays within microbial communities.

A reductive amination-based fluorophore labelling of complex wood-derived glycans provides a proof-of-principle multi-modal platform for monitoring glycan uptake by bacteria.

Sialic Acid Derivatization of Fluorescently Labeled N-Glycans Allows Linkage Differentiation by Reversed-Phase Liquid Chromatography-Fluorescence Detection-Mass Spectrometry

Sialic acids have diverse biological roles, ranging from promoting up to preventing protein and cellular recognition in health and disease. The various functions of these monosaccharides are owed, in part, to linkage variants, and as a result, linkage-specific analysis of sialic acids is an important aspect of glycomic studies. This has been addressed by derivatization strategies using matrix-assisted laser desorption/ionization mass spectrometry (MS) or sialidase digestion arrays followed by liquid chromatography (LC)-MS. Despite this, these approaches are unable to simultaneously provide unambiguous assignment of sialic acid linkages and assess further isomeric glycan features within a single measurement. Thus, for the first time, we present the combination of procainamide fluorescent labeling with sialic acid linkage-specific derivatization via ethyl esterification and amidation for the analysis of released plasma N-glycans using reversed-phase (RP)LC-fluorescence detection (FD)-MS. As a result, α2,3- and α2,6-sialylated N-glycans, with the same mass prior to derivatization, are differentiated based on retention time, precursor mass, and fragmentation spectra, and additional sialylated isomers were also separated. Furthermore, improved glycan coverage and protocol precision were found via the novel application using a combined FD-MS quantification approach. Overall, this platform achieved unambiguous assignment of N-glycan sialic acid linkages within a single RPLC-FD-MS measurement, and by improving their retention on RPLC, this technique can be used for future investigations of released N-glycans as an additional or orthogonal method to current analytical approaches.

Analytical Similarity Assessment of Biosimilars: Global Regulatory Landscape, Recent Studies and Major Advancements in Orthogonal Platforms

Biopharmaceuticals are one of the fastest-growing sectors in the biotechnology industry. Within the umbrella of biopharmaceuticals, the biosimilar segment is expanding with currently over 200 approved biosimilars, globally. The key step towards achieving a successful biosimilar approval is to establish analytical and clinical biosimilarity with the innovator. The objective of an analytical biosimilarity study is to demonstrate a highly similar profile with respect to variations in critical quality attributes (CQAs) of the biosimilar product, and these variations must lie within the range set by the innovator. This comprises a detailed comparative structural and functional characterization using appropriate, validated analytical methods to fingerprint the molecule and helps reduce the economic burden towards regulatory requirement of extensive preclinical/clinical similarity data, thus making biotechnological drugs more affordable. In the last decade, biosimilar manufacturing and associated regulations have become more established, leading to numerous approvals. Biosimilarity assessment exercises conducted towards approval are also published more frequently in the public domain. Consequently, some technical advancements in analytical sciences have also percolated to applications in analytical biosimilarity assessment. Keeping this in mind, this review aims at providing a holistic view of progresses in biosimilar analysis and approval. In this review, we have summarized the major developments in the global regulatory landscape with respect to biosimilar approvals and also catalogued biosimilarity assessment studies for recombinant DNA products available in the public domain. We have also covered recent advancements in analytical methods, orthogonal techniques, and platforms for biosimilar characterization, since 2015. The review specifically aims to serve as a comprehensive catalog for published biosimilarity assessment studies with details on analytical platform used and critical quality attributes (CQAs) covered for multiple biotherapeutic products. Through this compilation, the emergent evolution of techniques with respect to each CQA has also been charted and discussed. Lastly, the information resource of published biosimilarity assessment studies, created during literature search is anticipated to serve as a helpful reference for biopharmaceutical scientists and biosimilar developers.

Recent advances and trends in sample preparation and chemical modification for glycan analysis

Growing significance of glycosylation in protein functions has accelerated the development of methodologies for detection, identification, and characterization of protein glycosylation. In the past decade, glycobiology research has been advanced by innovative techniques with further progression in the post-genome era. Although significant technical progress has been made in terms of analytical throughput, comprehensiveness, and sensitivity, most methods for glycosylation analysis still require laborious and time-consuming sample preparation tasks. Additionally, sample preparation methods that are focused on specific glycan(s) require an in-depth understanding of various issues in glycobiology. In this review, modern sample preparation and chemical modification methods for the structural and quantitative glycan analyses together with the challenges and advantages of recent sample preparation methods are summarized. The techniques presented herein can facilitate the exploration of biomarkers, understanding of unknown glycan functions, and development of biopharmaceuticals.

A rapid method for relative quantification of N-glycans from a therapeutic monoclonal antibody during trastuzumab biosimilar development

Glycosylation is a common post-translational modification and critical quality attribute that can modulate the efficacy of therapeutic proteins. In the production of monoclonal antibodies (mAbs), quantifying the glycoform profile is a vital characterization step. Traditional glycan analysis is time consuming and involves steps at extreme temperature or pH, which may alter glycans. Here, we describe a rapid method for glycan analysis in which glycans are released from mAb samples that are bound to protein A columns. Since host cell proteins, which may also contain glycans, were already removed, this step enables analysis of cell culture products. Glycans released from the mAb samples are then derivatized with InstantPC™ labeling agent and analyzed by HILIC-FLD-MS. To illustrate the method, the glycan profiles of six trastuzumab (Herceptin®) antibody lots and four biosimilar developmental lots were analyzed. The results derived from our novel method, which takes less than 90 min, are compared with those from a typical glycan preparation approach.

Mass Spectrometry-Based Methods for Immunoglobulin G N-Glycosylation Analysis

Mass spectrometry and its hyphenated techniques enabled by the improvements in liquid chromatography, capillary electrophoresis, novel ionization, and fragmentation modes are truly a cornerstone of robust and reliable protein glycosylation analysis. Boost in immunoglobulin G (IgG) glycan and glycopeptide profiling demands for both applied biomedical and research applications has brought many new advances in the field in terms of technical innovations, sample preparation, improved throughput, and confidence in glycan structural characterization. This chapter summarizes mass spectrometry basics, focusing on IgG and monoclonal antibody N-glycosylation analysis on several complexity levels. Different approaches, including antibody enrichment, glycan release, labeling, and glycopeptide preparation and purification, are covered and illustrated with recent breakthroughs and examples from the literature omitting excessive theoretical frameworks. Finally, selected highly popular methodologies in IgG glycoanalytics such as liquid chromatography-mass spectrometry and matrix-assisted laser desorption ionization are discussed more thoroughly yet in simple terms making this text a practical starting point either for the beginner in the field or an experienced clinician trying to make sense out of the IgG glycomic or glycoproteomic dataset.

Glycomics and glycoproteomics: Approaches to address isomeric separation of glycans and glycopeptides

Changes in the glycome of human proteins and cells are associated with the progression of multiple diseases such as Alzheimer's, diabetes mellitus, many types of cancer, and those caused by viruses. Consequently, several studies have shown essential modifications to the isomeric glycan moieties for diseases in different stages. However, the elucidation of extensive isomeric glycan profiles remains challenging because of the lack of analytical techniques with sufficient resolution power to separate all glycan and glycopeptide iso-forms. Therefore, the development of sensitive and accurate approaches for the characterization of all the isomeric forms of glycans and glycopeptides is essential to tracking the progression of pathology in glycoprotein-related diseases. This review describes the isomeric separation achievements reported in glycomics and glycoproteomics in the last decade. It focuses on the mass spectrometry-based analytical strategies, stationary phases, and derivatization techniques that have been developed to enhance the separation mechanisms in liquid chromatography systems and the detection capabilities of mass spectrometry systems.

Separation based characterization methods for the N-glycosylation analysis of prostate-specific antigen

Prostate cancer has the highest malignancy rate diagnosed in men worldwide. Albeit, the gold standard serum prostate-specific antigen (PSA) assays reduced the mortality rate of the disease, the number of false positive diagnoses steeply increased. Therefore, there is an urgent need for complementary biomarkers to enhance the specificity and selectivity of current diagnostic methods. Information about PSA glycosylation can help to fulfill this gap as alterations of its carbohydrate moieties due to cancerous transformation may represent additional markers to distinguish malignant from benign tumors. However, development of suitable methods and instrumentations to investigate the N-glycosylation profile of PSA represents a challenge. In this paper, we critically review the current bioanalytical trends and strategies in the field of PSA glycobiomarker research focusing on separation based characterization methods.

Enhancement of fucosylated N-glycan isomer separation with an ultrahigh column temperature in porous graphitic carbon liquid chromatography-mass spectrometry

Due to the heterogeneous and isomeric nature of glycans, the development of an advanced separation of distinct glycan isomers is essential for glycan research and application. In this study, we utilized porous graphite carbon (PGC) chromatography for the separation of isomeric oligosaccharides without reduction or chemical derivatization at 190 °C in a custom-built heating oven. Furthermore, the fine structures of glycan isomers could be identified by using ultrahigh temperature PGC liquid chromatography mass spectrometry (UHT-PGC-LCMS). A nonreduced hydrolyzed dextran was applied to verify the performance of UHT-PGC. When the temperature of the PGC column was increased from 25 to 190 °C, the liquid chromatography separation power of the nonreduced dextran ladder significantly increased. The advantage of the UHT-PGC column was its high peak capacity with gradient elution in 10 min at 190 °C, 6700 psi, and a 250 μL/min flow rate for native glycan analysis. Four synthetic Lewis antigen isomers were used to elucidate the separation effectiveness in UHT-PGC. Moreover, mass spectrometry-based sequencing to generate specific diagnostic ions from the four synthetic Lewis antigens was used to predict isomeric glycans based on the relative intensity ratio (RIR) of diagnostic ions. The intensities of the diagnostic ions of synthetic isomers were used to identify each isomer of the fucosylated glycan. The results clearly showed that terminal Lewis A and X residues were in the 3- and 6-arms of N-glycan, respectively.

Enhanced protocol for quantitative N-linked glycomics analysis using Individuality Normalization when Labeling with Isotopic Glycan Hydrazide Tags (INLIGHT)™

The analysis of N-linked glycans using liquid chromatography and mass spectrometry (LC-MS) presents significant challenges, particularly owing to their hydrophilic nature. To address these difficulties, a variety of derivatization methods have been developed to facilitate improved ionization and detection sensitivity. One such method, the Individuality Normalization when Labeling with Isotopic Glycan Hydrazide Tags (INLIGHT)™ strategy for labeling glycans, has previously been utilized in the analysis of N- and O-linked glycans in biological samples. To assess the maximum sensitivity and separability of the INLIGHT™ preparation and analysis pipeline, several critical steps were investigated. First, recombinant and nonrecombinant sources of PNGase F were compared to assess variations in the released glycans. Second, modifications in the INLIGHT™ derivatization step were evaluated including temperature optimization, solvent composition changes, reaction condition length and tag concentration. Optimization of the modified method resulted in 20-100 times greater peak areas for the detected N-linked glycans in fetuin and horseradish peroxidase compared with the standard method. Furthermore, the identification of low-abundance glycans, such as (Fuc)1(Gal)2(GlcNAc)4(Man)3(NeuAc)1 and (Gal)3(GlcNAc)5(Man)3(NeuAc)3, was possible. Finally, the optimal LC setup for the INLIGHT™ derivatized N-linked glycan analyses was found to be a C18 reverse-phase (RP) column with mobile phases typical of RPLC.

Commensal Relationship of Three Bifidobacterial Species Leads to Increase of Bifidobacterium in Vitro Fermentation of Sialylated Immunoglobulin G by Human Gut Microbiota

Sialylated immunoglobulin G (IgG) is an important immunoglobulin in breast milk, but its effect on adult gut microbiota is not yet clear due to digestion by pepsin. Based on our previous IgG protecting study, effects of sialylated IgG on gut microbiota were investigated by in vitro anaerobic fermentation in the present study. It was found that the addition of sialylated IgG could significantly promote the growth of Bifidobacterium. Meanwhile, three bifidobacterial species B. bifidum CCX 19061, Bembidion breve CCX 19041, and B. longum subsp. infantis CCX 19042 were isolated. Furthermore, B. breve CCX 19041 and B. longum subsp. infantis CCX 19042 showed co-culture growth property with B. bifidum CCX 19061 in a sialylated IgG-based medium, which was also supported by changes of free monosaccharides and N-glycan structure. These findings suggest that the increase of Bifidobacterium in vitro fermentation is attributed to the commensal relationship of the three bifidobacterial species by utilizing sugars released from sialylated IgG.

Improved Simple Sample Pretreatment Method for Quantitation of Major Human Milk Oligosaccharides Using Ultrahigh Pressure Liquid Chromatography with Fluorescence Detection

Human milk oligosaccharides (HMOs) maintain and promote infant health. Most of the current methods for HMOs quantitation require labor-intensive and time-consuming steps for sample preparation. This study presents two very simple and easy-to-operate pretreatment methods, requiring either ultrafiltration or centrifugation to separate free oligosaccharides from whole fat human milk and other milk matrix before oligosaccharides labeling for quantifying HMOs using ultrahigh pressure liquid chromatography with fluorescence detection. A single chromatography run quantified 15 sialylated and neutral HMOs with high sensitivity (with an LOD less than 8 pg for all HMOs tested: about 1 pg for 2'-fucosyllactose, 3-fucosyllactose, 4'-galactosyllactose, 3'-galactosyllactose, and 6'-galactosyllactose) and good linearity with coefficient of correlation above 0.999. Accuracy and precision were satisfactory for both pretreatment methods. Overall, the centrifugation pretreatment was efficient and reliable for samples with high levels of oligosaccharides, and the ultrafiltration pretreatment was especially suitable for samples with low oligosaccharide abundance.

Small-molecule control of antibody N-glycosylation in engineered mammalian cells

N-linked glycosylation in monoclonal antibodies (mAbs) is crucial for structural and functional properties of mAb therapeutics, including stability, pharmacokinetics, safety and clinical efficacy. The biopharmaceutical industry currently lacks tools to precisely control N-glycosylation levels during mAb production. In this study, we engineered Chinese hamster ovary cells with synthetic genetic circuits to tune N-glycosylation of a stably expressed IgG. We knocked out two key glycosyltransferase genes, α-1,6-fucosyltransferase (FUT8) and β-1,4-galactosyltransferase (β4GALT1), genomically integrated circuits expressing synthetic glycosyltransferase genes under constitutive or inducible promoters and generated antibodies with concurrently desired fucosylation (0-97%) and galactosylation (0-87%) levels. Simultaneous and independent control of FUT8 and β4GALT1 expression was achieved using orthogonal small molecule inducers. Effector function studies confirmed that glycosylation profile changes affected antibody binding to a cell surface receptor. Precise and rational modification of N-glycosylation will allow new recombinant protein therapeutics with tailored in vitro and in vivo effects for various biotechnological and biomedical applications.

Glycan analysis for protein therapeutics

Glycosylation can be a critical quality attribute for protein therapeutics due to its extensive impact on product safety and efficacy. Glycan characterization is important in the process of protein drug development, from early stage candidate selection to late stage regulatory submission. It is also an indispensable part in the evaluation of biosimilarity. This review discusses the effects of glycosylation on the stability and activity of protein therapeutics, regulatory considerations corresponding to manufacturing and structural characterization of glycosylated protein therapeutics, and focuses on mass spectrometry compatible separation methods for glycan characterization of protein therapeutics. These approaches include hydrophilic interaction liquid chromatography, reversed-phase liquid chromatography, capillary electrophoresis, porous graphitic carbon liquid chromatography and two-dimensional liquid chromatography. Advances and novelties in each separation method, as well as associated challenges and limitations, are discussed at the released glycan, glycopeptide, glycoprotein subunit and intact glycoprotein levels.

Characterization of Site-Specific N-Glycosylation

Even if a consensus sequence has been identified for a posttranslational modification, the presence of such a sequence motif only indicates the possibility, not the certainty that the modification actually occurs. Proteins can be glycosylated on certain amino acid side chains, and these modifications are designated as C-, N-, and O-glycosylation. C-mannosylation occurs on Trp residues within a relatively loosely defined consensus motif. N-glycosylated species are modified at Asn residues of Asn-Xxx-Ser/Thr/Cys sequons (where Xxx can be any amino acid except proline). N-linked oligosaccharides share a common core structure of GlcNAc₂Man₃. In addition, an enzyme, peptide N-glycosidase F (PNGase F), removes most of the common N-linked carbohydrates unaltered from proteins while hydrolyzing the originally glycosylated Asn residue to Asp. O-glycosylation occurs at Ser, Thr, and Tyr residues, usually in sequence stretches rich in hydroxy-amino acids. O-glycosylation lacks a common core structure. Mammalian proteins have been reported bearing O-linked N-acetylgalactosamine, fucose, glucose, xylose, mannose, and corresponding elongated structures, as well as N-acetylglucosamine. Chemical methods are used to liberate these oligosaccharides because no enzyme would remove all the different O-linked carbohydrates. Characterization of both N- and O-glycosylation is complicated by the fact that the same positions within a population of protein molecules may feature an array of different carbohydrate structures, or remain unmodified. This site-specific heterogeneity may vary by species and tissue, and may also be affected by physiological changes. For addressing site-specific carbohydrate heterogeneity mass spectrometry has become the method of choice. Reversed-phase HPLC directly coupled with electrospray ionization mass spectrometry (LC/ESI-MS/MS) offers the best solution. Using a mass spectrometer as online detector not only assures the analysis of every component eluting (mass mapping), but also at the same time diagnostic carbohydrate ions can be generated by collisional activation that permits the selective and specific detection of glycopeptides. In addition, ESI-compatible alternative MS/MS techniques, electron-capture and electron-transfer dissociation, aid glycopeptide identification as well as modification site assignments.

High-selectivity profiling of released and labeled N-glycans via polar-embedded reversed-phase chromatography

N-Glycosylation is the most complex post-translational modification of proteins and involved in many physiological processes and is therefore of major interest in academic research and in the biopharmaceutical industry. Reliable, robust, reproducible, and selective analysis of N-glycans is essential to understand the multitude of biological roles of N-glycosylation. So far, hydrophilic interaction liquid chromatography analysis of 2-AB or 2-AA derivatized N-glycans has been the standard method. In this work, the superiority of reversed-phase chromatography for complex N-glycosylation analysis is demonstrated. Separation of N-glycans derivatized with anthranilic acid on polar-embedded stationary alkyl phases with sub-2-μm particles results in outstanding selectivity and resolution. In combination with the highly mass spectrometry–compatible mobile phase, even very complex glycan mixtures can be separated, identified, and quantified precisely and accurately. The presented methodology can be applied broadly from basic research to analytical control and release testing of biological drug products and can be implemented in analytical laboratories with minimal effort.

Graphical abstract

Using Chemical Synthesis To Study and Apply Protein Glycosylation

Protein glycosylation is one of the most common post-translational modifications and can influence many properties of proteins. Abnormal protein glycosylation can lead to protein malfunction and serious disease. While appreciation of glycosylation's importance is growing in the scientific community, especially in recent years, a lack of homogeneous glycoproteins with well-defined glycan structures has made it difficult to understand the correlation between the structure of glycoproteins and their properties at a quantitative level. This has been a significant limitation on rational applications of glycosylation and on optimizing glycoprotein properties. Through the extraordinary efforts of chemists, it is now feasible to use chemical synthesis to produce collections of homogeneous glycoforms with systematic variations in amino acid sequence, glycosidic linkage, anomeric configuration, and glycan structure. Such a technical advance has greatly facilitated the study and application of protein glycosylation. This Perspective highlights some representative work in this research area, with the goal of inspiring and encouraging more scientists to pursue the glycosciences.

The sweet spot for biologics: recent advances in characterization of biotherapeutic glycoproteins

INTRODUCTION

Glycosylation is recognized as a Critical Quality Attribute for therapeutic glycoproteins such as monoclonal antibodies, fusion proteins and therapeutic replacement enzymes. Hence, efficient and quantitative glycan analysis techniques have been increasingly important for their discovery, development and quality control. The aim of this review is to highlight relevant and recent advances in analytical technologies for characterization of biotherapeutic glycoproteins. Areas covered: The review gives an overview of the glycosylation trends of biotherapeutics approved in 2016 and 2017 by FDA. It describes current and novel analytical technologies for characterization of therapeutic glycoproteins and is explored in the context of released glycan, glycopeptide or intact glycoprotein analysis. Ultra performance liquid chromatography, mass spectrometry and capillary electrophoresis technologies are explored in this context. Expert commentary: There is a need for the biopharmaceutical industry to incorporate novel state of the art analytical technologies into existing and new therapeutic glycoprotein workflows for safer and more efficient biotherapeutics and for the improvement of future biotherapeutic design. Additionally, at present, there is no 'gold-standard' approach to address all the regulatory requirements and as such this will involve the use of orthogonal glycoanalytical technologies with a view to gain diagnostic information about the therapeutic glycoprotein.

Building a PGC-LC-MS N-glycan retention library and elution mapping resource

Porous graphitised carbon-liquid chromatography (PGC-LC) has been proven to be a powerful technique for the analysis and characterisation of complex mixtures of isomeric and isobaric glycan structures. Here we evaluate the elution behaviour of N-glycans on PGC-LC and thereby provide the potential of using chromatographic separation properties, together with mass spectrometry (MS) fragmentation, to determine glycan structure assignments more easily. We used previously reported N-glycan structures released from the purified glycoproteins Immunoglobulin G (IgG), Immunoglobulin A (IgA), lactoferrin, α1-acid glycoprotein, Ribonuclease B (RNase B), fetuin and ovalbumin to profile their behaviour on capillary PGC-LC-MS. Over 100 glycan structures were determined by MS/MS, and together with targeted exoglycosidase digestions, created a N-glycan PGC retention library covering a full spectrum of biologically significant N-glycans from pauci mannose to sialylated tetra-antennary classes. The resultant PGC retention library ( http://www.glycostore.org/showPgc ) incorporates retention times and supporting fragmentation spectra including exoglycosidase digestion products, and provides detailed knowledge on the elution properties of N-glycans by PGC-LC. Consequently, this platform should serve as a valuable resource for facilitating the detailed analysis of the glycosylation of both purified recombinant, and complex mixtures of, glycoproteins using established workflows.

Characterization of isomeric glycan structures by LC-MS/MS

The characterization of glycosylation is critical for obtaining a comprehensive view of the regulation and functions of glycoproteins of interest. Due to the complex nature of oligosaccharides, stemming from variable compositions and linkages, and ion suppression effects, the chromatographic separation of glycans, including isomeric structures, is necessary for exhaustive characterization by MS. This review introduces the fundamental principles underlying the techniques in LC utilized by modern day glycomics researchers. Recent advances in porous graphitized carbon, reverse phase, ion exchange, and hydrophilic interaction LC utilized in conjunction with MS, for the characterization of protein glycosylation, are described with an emphasis on methods capable of resolving isomeric glycan structures.

Reversed-phase separation methods for glycan analysis

Reversed-phase chromatography is a method that is often used for glycan separation. For this, glycans are often derivatized with a hydrophobic tag to achieve retention on hydrophobic stationary phases. The separation and elution order of glycans in reversed-phase chromatography is highly dependent on the hydrophobicity of the tag and the contribution of the glycan itself to the retention. The contribution of the different monosaccharides to the retention strongly depends on the position and linkage, and isomer separation may be achieved. The influence of sialic acids and fucoses on the retention of glycans is still incompletely understood and deserves further study. Analysis of complex samples may come with incomplete separation of glycan species, thereby complicating reversed-phase chromatography with fluorescence or UV detection, whereas coupling with mass spectrometry detection allows the resolution of complex mixtures. Depending on the column properties, eluents, and run time, separation of isomeric and isobaric structures can be accomplished with reversed-phase chromatography. Alternatively, porous graphitized carbon chromatography and hydrophilic interaction liquid chromatography are also able to separate isomeric and isobaric structures, generally without the necessity of glycan labeling. Hydrophilic interaction liquid chromatography, porous graphitized carbon chromatography, and reversed-phase chromatography all serve different research purposes and thus can be used for different research questions. A great advantage of reversed-phase chromatography is its broad distribution as it is used in virtually every bioanalytical research laboratory, making it an attracting platform for glycan analysis.

Graphical Abstract

Analysis and quality control of carbohydrates in therapeutic proteins with fluorescence HPLC

Conbercept is an Fc fusion protein with very complicated carbohydrate profiles which must be carefully monitored through manufacturing process. Here, we introduce an optimized fluorescence derivatization high-performance liquid chromatographic method for glycan mapping in conbercept. Compared with conventional glycan analysis method, this method has much better resolution and higher reproducibility making it excellent for product quality control.

Current landscape of protein glycosylation analysis and recent progress toward a novel paradigm of glycoscience research

This review covers the basics and some applications of methodologies for the analysis of glycoprotein glycans. Analytical techniques used for glycoprotein glycans, including liquid chromatography (LC), capillary electrophoresis (CE), mass spectrometry (MS), and high-throughput analytical methods based on microfluidics, were described to supply the essentials about biopharmaceutical and biomarker glycoproteins. We will also describe the MS analysis of glycoproteins and glycopeptides as well as the chemical and enzymatic releasing methods of glycans from glycoproteins and the chemical reactions used for the derivatization of glycans. We hope the techniques have accommodated most of the requests from glycoproteomics researchers.

Identification of Low Abundant Isomeric N-Glycan Structures in Biological Therapeutics by LC/MS

An effective LC-MS based method for online characterization of low abundant structural isomers of N-linked glycans in biological therapeutics was developed. N-linked glycans of a recombinant monoclonal antibody were released by PNGase F and labeled with 2-aminobenzamide (2-AB) fluorescent tag. The labeled glycans were analyzed by online ultraperformance liquid chromatography-hydrophilic interaction liquid chromatography (UPLC-HILIC) coupled with mass spectrometry (MS). The glycan structure was characterized by MS(n) fragmentation in negative ion mode followed by identification of the signature D ions. The assignment included monosaccharide sequence and linkage information. The developed method successfully characterized structural isomers of A1G1F (assigned as terminal sialic acid attached in the 1,6 branch at 2,3 position), and A1G1F' (assigned as terminal sialic acid attached in the 1,3 branch at 2,3 position). Moreover, using the same approach, previously unknown low abundant species were identified unambiguously. One such structural isomer at low level, terminal GlcNAc of G1F+GlcNAc, was identified to be linked at the 1,6 branch. Additionally, another low level structural isomer, previously assigned as Man8 glycan, was found to be Man7+Glc glycan as its 1,3 branch containing three mannoses and one terminal glucose. The identification was further confirmed by a purified α-1,2-endomannosidase enzyme to generate the cleavage of α-1,3 linked terminal disaccharides (Man+glucose). Using this approach, different lots or different CHO produced mAbs was thoroughly examined and found that the newly identified "Man8" (Man7+Glc) was also present in different batches and in some commercially available therapeutic mAbs.

Glycan characterization of biopharmaceuticals: Updates and perspectives

Therapeutic proteins are rapidly becoming the most promising class of pharmaceuticals on the market due to their successful treatment of a vast array of serious diseases, such as cancers and immune disorders. Therapeutic proteins are produced using recombinant DNA technology. More than 60% of therapeutic proteins are posttranslationally modified following biosynthesis by the addition of N- or O-linked glycans. Glycosylation is the most common posttranslational modifications of proteins. However, it is also the most demanding and complex posttranslational modification from the analytical point of view. Moreover, research has shown that glycosylation significantly impacts stability, half-life, mechanism of action and safety of a therapeutic protein. Considering the exponential growth of biotherapeutics, this present review of the literature (2009-2015) focuses on the characterization of protein glycosylation, which has witnessed an improvement in methodology. Furthermore, it discusses current issues in the fields of production and characterization of therapeutic proteins. This review also highlights the problem of non-standard requirements for the approval of biosimilars with regard to their glycosylation and discusses recent developments and perspectives for improved glycan characterization.

N-glycan PK Profiling Using a High Sensitivity nanoLCMS Work-Flow with Heavy Stable Isotope Labeled Internal Standard and Application to a Preclinical Study of an IgG1 Biopharmaceutical

Purpose

In this study an innovative, highly sensitive work-flow is presented that allows the analysis of a possible influence of individual glyco-variants on pharmacokinetics already during pre-clinical development. Possible effects on the pharmacokinetics caused by glyco-variants have been subject of several studies with in part contradictory results which can be related to differences in the set-up.

Methods

Using 96-well plate based affinity purification an IgG1 antibody was isolated from preclinical samples and glycans were analyzed individually by nanoLCMS. Prerequisite was a reference standard based on stable heavy isotope labeled glycans. The high sensitivity and low sample consumption enabled the integration into the preclinical development program.

Results

The data of an IgG1 biopharmaceutical from a preclinical rabbit study showed that some N-glycoforms have a different PK profile compared with the average of all molecule variants as determined by ELISA. IgG1 high mannose glycoforms M5 and M6 were removed from circulation at a higher rate.

Conclusion

The results of the preclinical study demonstrated the applicability of the developed innovative workflow. The PK profile of glyco-variants could be determined individually. It was concluded that M6 was converted by mannosidases in circulation to M5 which in turn was selectively cleared by mannose receptor binding which is in-line with previously published results. Therefore the developed technology delivers reliable results and can be applied for PK profiling of other mAbs and other types of biopharmaceuticals.

Electronic supplementary material

The online version of this article (doi:10.1007/s11095-015-1724-0) contains supplementary material, which is available to authorized users.

Quantitative isomer-specific N-glycan fingerprinting using isotope coded labeling and high performance liquid chromatography-electrospray ionization-mass spectrometry with graphitic carbon stationary phase

Glycan reductive isotope labeling (GRIL) using (12)C6-/(13)C6-aniline as labeling reagent is reported with the aim of quantitative N-glycan fingerprinting. Porous graphitized carbon (PGC) as stationary phase in capillary scale HPLC coupled to electrospray mass spectrometry with time of flight analyzer was applied for the determination of labeled N-glycans released from glycoproteins. The main benefit of using stable isotope-coding in the context of comparative glycomics lies in the improved accuracy and precision of the quantitative analysis in combined samples and in the potential of correcting for structure-dependent incomplete enzymatic release of oligosaccharides when comparing identical target proteins. The method was validated with respect to mobile phase parameters, reproducibility, accuracy, linearity and limit of detection/quantification (LOD/LOQ) using test glycoproteins. It is shown that the developed method is capable of determining relative amounts of N-glycans (including isomers) comparing two samples in one single HPLC-MS run. The analytical potential and usefulness of GRIL in combination with PGC-ESI-TOF-MS is demonstrated comparing glycosylation in human monoclonal antibodies produced in Chinese hamster ovary cells (CHO) and hybridoma cell lines.

Small scale affinity purification and high sensitivity reversed phase nanoLC-MS N-glycan characterization of mAbs and fusion proteins

N-glycosylation is a complex post-translational modification with potential effects on the efficacy and safety of therapeutic proteins and known influence on the effector function of biopharmaceutical monoclonal antibodies (mAbs). Comprehensive characterization of N-glycosylation is therefore important in biopharmaceutical development. In early development, e.g. during pool or clone selection, however, only minute protein amounts of multiple samples are available for analytics. High sensitivity and high throughput methods are thus needed. An approach based on 96-well plate sample preparation and nanoLC-MS of 2- anthranilic acid or 2-aminobenzoic acid (AA) labeled N-glycans for the characterization of biopharmaceuticals in early development is reported here. With this approach, 192 samples can be processed simultaneously from complex matrices (e.g., cell culture supernatant) to purified 2-AA glycans, which are then analyzed by reversed phase nanoLC-MS. Attomolar sensitivity has been achieved by use of nanoelectrospray ionization, resulting in detailed glycan maps of mAbs and fusion proteins that are exemplarily shown in this work. Reproducibility, robustness and linearity of the approach are demonstrated, making use in a routine manner during pool or clone selection possible. Other potential fields of application, such as glycan biomarker discovery from serum samples, are also presented.

Single tag for total carbohydrate analysis

Anthranilic acid (2-aminobenzoic acid, 2-AA) has the remarkable property of reacting rapidly with every type of reducing carbohydrate. Reactivity of 2-AA with carbohydrates in aqueous solutions surpasses all other tags reported to date. This unique capability is attributed to the strategically located -COOH which accelerates Schiff base formation. Monosaccharides, oligosaccharides (N-, O-, and lipid linked and glycans in secretory fluids), glycosaminoglycans, and polysaccharides can be easily labeled with 2-AA. With 2-AA, labeling is simple in aqueous solutions containing proteins, peptides, buffer salts, and other ingredients (e.g., PNGase F, glycosidase, and transferase reaction mixtures). In contrast, other tags require relatively pure glycans for labeling in anhydrous dimethyl sulfoxide-acetic acid medium. Acidic conditions are known to cause desialylation, thus requiring a great deal of attention to sample preparation. Simpler labeling is achieved with 2-AA within 30-60 min in mild acetate-borate buffered solution. 2-AA provides the highest sensitivity and resolution in chromatographic methods for carbohydrate analysis in a simple manner. Additionally, 2-AA is uniquely qualified for quantitative analysis by mass spectrometry in the negative mode. Analyses of 2-AA-labeled carbohydrates by electrophoresis and other techniques have been reported. Examples cited here demonstrate that 2-AA is the universal tag for total carbohydrate analysis.