Antibody-receptor interactions mediate antibody-dependent cellular cytotoxicity.

Binding of antibodies to their receptors is a core component of the innate immune system. Understanding the precise interactions between antibodies and their Fc receptors has led to the engineering of novel mAb biotherapeutics with tailored biological activities. One of the most significant findings is that afucosylated monoclonal antibodies demonstrate increased affinity toward the receptor FcγRIIIa, with a commensurate increase in antibody-dependent cellular cytotoxicity. Crystal structure analysis has led to the hypothesis that afucosylation in the Fc region results in reduced steric hindrance between antibody-receptor intermolecular glycan interactions, enhancing receptor affinity; however, solution-phase data have yet to corroborate this hypothesis. In addition, recent work has shown that the fragment antigen-binding (Fab) region may directly interact with Fc receptors; however, the biological consequences of these interactions remain unclear. By probing differences in solvent accessibility between native and afucosylated immunoglobulin G1 (IgG1) using hydroxyl radical footprinting-MS, we provide the first solution-phase evidence that an IgG1 bearing an afucosylated Fc region appears to require fewer conformational changes for FcγRIIIa binding. In addition, we performed extensive molecular dynamics (MD) simulations to understand the molecular mechanism behind the effects of afucosylation. The combination of these techniques provides molecular insight into the steric hindrance from the core Fc fucose in IgG1 and corroborates previously proposed Fab-receptor interactions. Furthermore, MD-guided rational mutagenesis enabled us to demonstrate that Fab-receptor interactions directly contribute to the modulation of antibody-dependent cellular cytotoxicity activity. This work demonstrates that in addition to Fc-polypeptide and glycan-mediated interactions, the Fab provides a third component that influences IgG-Fc receptor biology.

Using differential scanning calorimetry for the development of non-reduced capillary electrophoresis sodium dodecyl sulfate methods for monoclonal antibodies.

Analysis of non-reduced and reduced monoclonal antibodies (mAbs) by capillary electrophoresis-sodium dodecyl sulfate (CE-SDS) is routinely used to detect product size variants and process-related impurities. Levels of high molecular weight (HMW) forms obtained from this method usually trend comparably to those obtained by orthogonal methods such as size-exclusion ultra-high performance liquid chromatography (SE-UHPLC). However, in the presented case study, comparison of CE-SDS data for three IgG1 mAbs (trastuzumab, mAb1, and mAb2) showed a discrepancy between amounts of observed HMW forms in mAb2 compared with its native forms determined by SE-UHPLC (~17% vs. ~0.5%, respectively). SDS chemical denaturation, as measured by differential scanning calorimetry, demonstrated that the high thermal stability of mAb2 caused an unidentified HMW peak observed by non-reduced (NR)-CE-SDS, which was the result of improper denaturing, resulting in a partially folded species. More so, this strategy enabled the rapid identification of optimal SDS concentration and temperature conditions needed for suitable denaturation for mAb2. This case study presents an alternative option for quick optimization of NR-CE-SDS methods when characterizing mAbs or other thermally stable proteins. Also, this strategy can be used to understand basic biophysical mechanisms of protein unfolding and investigate the higher-order structure imparted by specific sequences and understand how these sequences might affect the results of an analytical method such as CE-SDS.

Photodisruption of the Structurally Conserved Cys-Cys-Trp Triads Leads to Reduction-Resistant Scrambled Intrachain Disulfides in an IgG1 Monoclonal Antibody.

Photostability conditions as prescribed by ICH guidelines induced highly reduction-resistant scrambled disulfides that contribute to the population of apparent nonreducible aggregates in an IgG1 mAb. Photoinduced cross-linked species were isolated under reducing conditions using an organic phase size exclusion chromatography (OP-SEC) method, followed by O18-labeling tryptic mapping to identify cross-linked peptides. Disulfide scrambling was observed within the IgG1 structurally conserved-intrachain cysteine-cysteine-tryptophan triads (Cys-Cys-Trp), and correlated with Trp-to-kynurenine (Kyn) photodegradation within these triads. We hypothesize that intrachain disulfides protect the proximal Trp within the Cys-Cys-Trp triads from photodegradation by enabling dissipation of Trp-absorbed UV energy via electron transfer to the disulfide bond. Finally, we propose three distinct mechanisms of photochemical degradation of monoclonal antibodies mediated by Trp residues.

Structural Characterization of Cross-Linked Species in Trastuzumab Emtansine (Kadcyla).

The antibody-drug conjugate, trastuzumab emtansine (Kadcyla), is produced by attachment of the antitubulin drug, DM1, to lysine amines via a heterobifunctional linker, SMCC (succinimidyl 4-(N-maleimidomethyl)cyclohexane-1-carboxylate). Following the reaction of the N-hydroxysuccinimide activated linker with antibody lysines to produce a linker-modified intermediate (Tmab-MCC), DM1 is added to yield the desired product. In addition to the expected distribution of drug-linked forms (from 0 to 8), mass spectrometry also demonstrates the presence of a second distribution shifted by about +222 Da. This series is consistent with the presence of a population containing a bound linker without DM1 ("unconjugated linker"). Extended characterization of trastuzumab emtansine was performed using capillary isoelectic focusing, CE-SDS, peptide mapping, and LC/MS following (18)O labeling of peptide digests to identify this family of product variants. These studies demonstrate that the presence of these +222 Da species is due to an unexpected reaction of the maleimide moiety in the MCC linker with antibody lysine residues to produce cross-linked species that cannot conjugate to DM1.

A Robust Purity Method for Biotherapeutics Using New Peak Detection in an LC-MS-Based Multi-Attribute Method.

New peak detection (NPD), as part of the LC-MS-based multi-attribute method (MAM), allows for sensitive and unbiased detection of new or changing site-specific attributes between a sample and reference that is not possible with conventional UV or fluorescence detection-based methods. MAM with NPD can serve as a purity test that can establish whether a sample and the reference are similar. The broad implementation of NPD in the biopharmaceutical industry has been limited by the potential presence of false positives or artifacts, which increase the analysis time and can trigger unnecessary investigations of product quality. Our novel contributions to the success of NPD are the curation of false positives, use of the known peak list concept, pairwise analysis approach, and the development of a NPD system suitability control strategy. In this report, we also introduce a unique experimental design utilizing sequence variant co-mixes to measure NPD performance. We show that NPD has superior performance relative to conventional control system methods in the detection of an unexpected change as compared with the reference. NPD is a new frontier in purity testing that reduces subjectivity, need for analyst intervention, and potential for missing unexpected product quality changes.

Mass spectrometry-based multi-attribute method in protein therapeutics product quality monitoring and quality control.

The multi-attribute method (MAM), a liquid chromatography-mass spectrometry (LC-MS)-based peptide mapping method, has gained increased interest and applications in the biopharmaceutical industry. MAM can, in one method, provide targeted quantitation of multiple site-specific product quality attributes, as well as new peak detection. In this review, we focus on the scientific and regulatory considerations of using MAM in product quality attribute monitoring and quality control (QC) of therapeutic proteins. We highlight MAM implementation challenges and solutions with several case studies, and provide our perspective on the opportunities to use MS in QC for applications other than standard peptide mapping-based MAM.

Multi-attribute Raman spectroscopy (MARS) for monitoring product quality attributes in formulated monoclonal antibody therapeutics.

Rapid release of biopharmaceutical products enables a more efficient drug manufacturing process. Multi-attribute methods that target several product quality attributes (PQAs) at one time are an essential pillar of the rapid-release strategy. The novel, high-throughput, and nondestructive multi-attribute Raman spectroscopy (MARS) method combines Raman spectroscopy, design of experiments, and multivariate data analysis (MVDA). MARS allows the measurement of multiple PQAs for formulated protein therapeutics without sample preparation from a single spectroscopic scan. Variable importance in projection analysis is used to associate the chemical and spectral basis of targeted PQAs, which assists in model interpretation and selection. This study shows the feasibility of MARS for the measurement of both protein purity-related and formulation-related PQAs; measurements of protein concentration, osmolality, and some formulation additives were achieved by a generic multiproduct model for various protein products containing the same formulation components. MARS demonstrates the potential to be a powerful methodology to improve the efficiency of biopharmaceutical development and manufacturing, as it features fast turnaround time, good robustness, less human intervention, and potential for automation.

Accurate determination of succinimide degradation products using high fidelity trypsin digestion peptide map analysis.

We report an efficient, high fidelity trypsin digestion method for peptide map analysis. This method minimizes artifacts caused by the sample preparation process, and we show its utility for the accurate determination of succinimide formation in a degraded monoclonal antibody product. A basic charge variant was detected by imaged capillary isoelectric focusing and was shown with reduced antigen binding and biological activity. Samples were reduced under denaturing conditions at pH 5.0, and digestion of the reduced protein with porcine trypsin was performed at pH 7.0 for 1 h. Following reversed phase high-performance liquid chromatography and online mass spectrometric analysis, succinimide formation was identified at Asp30 in the light chain. This result contrasts with the observation of only iso-Asp and Asp residues under conventional sample preparation conditions, which are therefore concluded to be artificially generated. The Asp30 residue is seen in the cocrystal structure model to participate in favorable charge interaction with an antigen molecule. Formation of succinimide and the resulting loss of negative charge are therefore hypothesized to be the degradation mechanism. After treatment of the degraded antibody sample to mildly alkaline pH conditions, we observed only Asp residue as the succinimide hydrolysis product and concurrent recovery of biological activity.

Optimizing Hydroxyl Radical Footprinting Analysis of Biotherapeutics Using Internal Standard Dosimetry.

Hydroxyl radical footprinting-mass spectrometry (HRF-MS) is a powerful technique for measuring protein structure by quantitating the solvent accessibility of amino acid side-chains; and when used in comparative analysis, HRF-MS data can provide detailed information on changes in protein structure. However, consistently controlling the amount of hydroxyl radical labeling of a protein requires the precise understanding of both the amount of radicals generated and half-life of the radicals in solution. The latter is particularly important for applications such as protein-protein and protein-ligand interactions, which may have different characteristics such as intrinsic reactivity and buffer components, and can cause differences in radical scavenging (herein termed "scavenging potential") between samples. To address this inherent challenge with HRF-MS analysis, we describe the comprehensive implementation of an internal standard (IS) dosimeter peptide leucine enkephalin (LeuEnk) for measuring the scavenging potential of pharmaceutically relevant proteins and formulation components. This further enabled evaluation of the critical method parameters affecting the scavenging potential of samples subjected to HRF-MS using fast photochemical oxidation of proteins. We demonstrate a direct correlation between the oxidation of the IS peptide and biotherapeutic target proteins, and show the oxidation of the IS can be used as a guide for ensuring equivalent scavenging potentials when comparing multiple samples. Establishing this strategy enables optimization of sample parameters, a system suitability approach, normalization of data, and comparison/harmonization of HRF-MS analysis across different laboratories.

Current Trends in Biotherapeutic Higher Order Structure Characterization by Irreversible Covalent Footprinting Mass Spectrometry.

BACKGROUND: Biotherapeutics, particularly monoclonal antibodies (mAbs), are a maturing class of drugs capable of treating a wide range of diseases. Therapeutic function and solutionstability are linked to the proper three-dimensional organization of the primary sequence into Higher Order Structure (HOS) as well as the timescales of protein motions (dynamics). Methods that directly monitor protein HOS and dynamics are important for mapping therapeutically relevant protein-protein interactions and assessing properly folded structures. Irreversible covalent protein footprinting Mass Spectrometry (MS) tools, such as site-specific amino acid labeling and hydroxyl radical footprinting are analytical techniques capable of monitoring the side chain solvent accessibility influenced by tertiary and quaternary structure. Here we discuss the methodology, examples of biotherapeutic applications, and the future directions of irreversible covalent protein footprinting MS in biotherapeutic research and development. CONCLUSION: Bottom-up mass spectrometry using irreversible labeling techniques provide valuable information for characterizing solution-phase protein structure. Examples range from epitope mapping and protein-ligand interactions, to probing challenging structures of membrane proteins. By paring these techniques with hydrogen-deuterium exchange, spectroscopic analysis, or static-phase structural data such as crystallography or electron microscopy, a comprehensive understanding of protein structure can be obtained.

Characterization of ELISA Antibody-Antigen Interaction using Footprinting-Mass Spectrometry and Negative Staining Transmission Electron Microscopy.

We describe epitope mapping data using multiple covalent labeling footprinting-mass spectrometry (MS) techniques coupled with negative stain transmission electron microscopy (TEM) data to analyze the antibody-antigen interactions in a sandwich enzyme-linked immunosorbant assay (ELISA). Our hydroxyl radical footprinting-MS data using fast photochemical oxidation of proteins (FPOP) indicates suppression of labeling across the antigen upon binding either of the monoclonal antibodies (mAbs) utilized in the ELISA. Combining these data with Western blot analysis enabled the identification of the putative epitopes that appeared to span regions containing N-linked glycans. An additional structural mapping technique, carboxyl group footprinting-mass spectrometry using glycine ethyl ester (GEE) labeling, was used to confirm the epitopes. Deglycosylation of the antigen resulted in loss of potency in the ELISA, supporting the FPOP and GEE labeling data by indicating N-linked glycans are necessary for antigen binding. Finally, mapping of the epitopes onto the antigen crystal structure revealed an approximate 90° relative spatial orientation, optimal for a noncompetitive binding ELISA. TEM data shows both linear and diamond antibody-antigen complexes with a similar binding orientation as predicted from the two footprinting-MS techniques. This study is the first of its kind to utilize multiple bottom-up footprinting-MS techniques and TEM visualization to characterize the monoclonal antibody-antigen binding interactions of critical reagents used in a quality control (QC) lot-release ELISA. Graphical Abstract ᅟ.

Mapping the Binding Interface of VEGF and a Monoclonal Antibody Fab-1 Fragment with Fast Photochemical Oxidation of Proteins (FPOP) and Mass Spectrometry.

We previously analyzed the Fab-1:VEGF (vascular endothelial growth factor) system described in this work, with both native top-down mass spectrometry and bottom-up mass spectrometry (carboxyl-group or GEE footprinting) techniques. This work continues bottom-up mass spectrometry analysis using a fast photochemical oxidation of proteins (FPOP) platform to map the solution binding interface of VEGF and a fragment antigen binding region of an antibody (Fab-1). In this study, we use FPOP to compare the changes in solvent accessibility by quantitating the extent of oxidative modification in the unbound versus bound states. Determining the changes in solvent accessibility enables the inference of the protein binding sites (epitope and paratopes) and a comparison to the previously published Fab-1:VEGF crystal structure, adding to the top-down and bottom-up data. Using this method, we investigated peptide-level and residue-level changes in solvent accessibility between the unbound proteins and bound complex. Mapping these data onto the Fab-1:VEGF crystal structure enabled successful characterization of both the binding region and regions of remote conformation changes. These data, coupled with our previous higher order structure (HOS) studies, demonstrate the value of a comprehensive toolbox of methods for identifying the putative epitopes and paratopes for biotherapeutic antibodies. Graphical abstract ᅟ.

Native MS and ECD Characterization of a Fab-Antigen Complex May Facilitate Crystallization for X-ray Diffraction.

Native mass spectrometry (MS) and top-down electron-capture dissociation (ECD) combine as a powerful approach for characterizing large proteins and protein assemblies. Here, we report their use to study an antibody Fab (Fab-1)-VEGF complex in its near-native state. Native ESI with analysis by FTICR mass spectrometry confirms that VEGF is a dimer in solution and that its complex with Fab-1 has a binding stoichiometry of 2:2. Applying combinations of collisionally activated dissociation (CAD), ECD, and infrared multiphoton dissociation (IRMPD) allows identification of flexible regions of the complex, potentially serving as a guide for crystallization and X-ray diffraction analysis. Graphical Abstract ᅟ.

Effect of ambient light on IgG1 monoclonal antibodies during drug product processing and development.

Photostability studies are standard stress testing conducted during drug product development of various pharmaceutical compounds, including small molecules and proteins. These studies as recommended by ICH Q1B are carried out using no less than 1.2× 10(6)lux-hours in the visible region and no less than 200Wh/m(2) in UV light. However, normal drug product processing is carried out under fluorescent lamps that emit white light almost exclusively in the >400nm region with a small UV quotient. We term these as ambient or mild light conditions. We tested several IgG1 monoclonal antibodies (mAbs 1-5) under these ambient light conditions and compared them to the ICH light conditions. All the mAbs were significantly degraded under the ICH light but several mAbs (mAbs 3-5) were processed without impacting any product quality attributes under ambient or mild light conditions. Interestingly we observed site-specific Trp oxidation in mAb1, while higher aggregation and color change were observed for mAb2 under mild light conditions. The recommended ICH light conditions have a high UV component and hence may not help to rank order photosensitivity under normal protein DP processing conditions.

Mapping of Fab-1:VEGF Interface Using Carboxyl Group Footprinting Mass Spectrometry.

A proof-of-concept study was performed to demonstrate that carboxyl group footprinting, a relatively simple, bench-top method, has utility for first-pass analysis to determine epitope regions of therapeutic mAb:antigen complexes. The binding interface of vascular endothelial growth factor (VEGF) and the Fab portion of a neutralizing antibody (Fab-1) was analyzed using carboxyl group footprinting with glycine ethyl ester (GEE) labeling. Tryptic peptides involved in the binding interface between VEGF and Fab-1 were identified by determining the specific GEE-labeled residues that exhibited a reduction in the rate of labeling after complex formation. A significant reduction in the rate of GEE labeling was observed for E93 in the VEGF tryptic peptide V5, and D28 and E57 in the Fab-1 tryptic peptides HC2 and HC4, respectively. Results from the carboxyl group footprinting were compared with the binding interface identified from a previously characterized crystal structure (PDB: 1BJ1). All of these residues are located at the Fab-1:VEGF interface according to the crystal structure, demonstrating the potential utility of carboxyl group footprinting with GEE labeling for mapping epitopes. Graphical Abstract ᅟ.

Characterizing monoclonal antibody structure by carboxyl group footprinting.

Structural characterization of proteins and their antigen complexes is essential to the development of new biologic-based medicines. Amino acid-specific covalent labeling (CL) is well suited to probe such structures, especially for cases that are difficult to examine by alternative means due to size, complexity, or instability. We present here a detailed account of carboxyl group labeling (with glycine ethyl ester (GEE) tagging) applied to a glycosylated monoclonal antibody therapeutic (mAb). The experiments were optimized to preserve the structural integrity of the mAb, and experimental conditions were varied and replicated to establish the reproducibility of the technique. Homology-based models were generated and used to compare the solvent accessibility of the labeled residues, which include aspartic acid (D), glutamic acid (E), and the C-terminus (i.e., the target probes), with the experimental data in order to understand the accuracy of the approach. Data from the mAb were compared to reactivity measures of several model peptides to explain observed variations in reactivity. Attenuation of reactivity in otherwise solvent accessible probes is documented as arising from the effects of positive charge or bond formation between adjacent amine and carboxyl groups, the latter accompanied by observed water loss. A comparison of results with previously published data by Deperalta et al using hydroxyl radical footprinting showed that 55% (32/58) of target residues were GEE labeled in this study whereas the previous study reported 21% of the targets were labeled. Although the number of target residues in GEE labeling is fewer, the two approaches provide complementary information. The results highlight advantages of this approach, such as the ease of use at the bench top, the linearity of the dose response plots at high levels of labeling, reproducibility of replicate experiments (<2% variation in modification extent), the similar reactivity of the three target probes, and significant correlation of reactivity and solvent accessible surface area.

Characterizing monoclonal antibody structure by carbodiimide/GEE footprinting.

Amino acid-specific covalent labeling is well suited to probe protein structure and macromolecular interactions, especially for macromolecules and their complexes that are difficult to examine by alternative means, due to size, complexity, or instability. Here we present a detailed account of carbodiimide-based covalent labeling (with GEE tagging) applied to a glycosylated monoclonal antibody therapeutic, which represents an important class of biologic drugs. Characterization of such proteins and their antigen complexes is essential to development of new biologic-based medicines. In this study, the experiments were optimized to preserve the structural integrity of the protein, and experimental conditions were varied and replicated to establish the reproducibility and precision of the technique. Homology-based models were generated and used to compare the solvent accessibility of the labeled residues, which include D, E, and the C-terminus, against the experimental surface accessibility data in order to understand the accuracy of the approach in providing an unbiased assessment of structure. Data from the protein were also compared to reactivity measures of several model peptides to explain sequence or structure-based variations in reactivity. The results highlight several advantages of this approach. These include: the ease of use at the bench top, the linearity of the dose response plots at high levels of labeling (indicating that the label does not significantly perturb the structure of the protein), the high reproducibility of replicate experiments (<2 % variation in modification extent), the similar reactivity of the 3 target probe residues (as suggested by analysis of model peptides), and the overall positive and significant correlation of reactivity and solvent accessible surface area (the latter values predicted by the homology modeling). Attenuation of reactivity, in otherwise solvent accessible probes, is documented as arising from the effects of positive charge or bond formation between adjacent amine and carboxyl groups, the latter accompanied by observed water loss. The results are also compared with data from hydroxyl radical-mediated oxidative footprinting on the same protein, showing that complementary information is gained from the 2 approaches, although the number of target residues in carbodiimide/GEE labeling is fewer. Overall, this approach is an accurate and precise method for assessing protein structure of biologic drugs.

Structural analysis of a therapeutic monoclonal antibody dimer by hydroxyl radical footprinting.

Hydroxyl radical footprinting is a covalent labeling strategy used to probe the conformational properties of proteins in solution. We describe the first application of this high resolution technique for characterizing the structure of a therapeutic monoclonal antibody (mAb) dimer. As monitored by size-exclusion chromatography (SEC), therapeutic mAbs typically contain small amounts of a dimer species relative to the primary monomeric form in its drug substance or drug product. To determine its structural orientation, a sample enriched in an IgG1 mAb dimer was oxidized by hydroxyl radicals generated by exposure of the aqueous solution to synchrotron X-rays in millisecond timescales. The antibody monomer that served as a control was oxidized in a similar fashion. The oxidized samples were digested with trypsin and analyzed by RP-UHPLC-MS. The footprinting data show that peptides displaying decreased rates of oxidation (i.e., regions of increased protection) in the dimer are localized in the light and heavy chains of the Fab domain. The interface region for the monomers comprising the dimer was thus inferred to be between their Fab arms, allowing us to model two possible theoretical dimer orientations: a head-to-head, single arm-bound Fab-to-Fab dimer, and a head-to-head, double arm-bound Fab (') 2-to-Fab (') 2 dimer. Lower resolution fragment-SEC analysis of the dimer and monomer samples treated with papain or FabRICATOR enzyme provided complimentary evidence to support the Fab/Fab orientation of the IgG1 dimer.