Ewald T.J. van den Bremer

Therapeutic IgG4 antibodies engage in Fab-arm exchange with endogenous human IgG4 in vivo

Two humanized IgG4 antibodies, natalizumab and gemtuzumab, are approved for human use, and several others, like TGN1412, are or have been in clinical development. Although IgG4 antibodies can dynamically exchange half-molecules, Fab-arm exchange with therapeutic antibodies has not been demonstrated in humans. Here, we show that natalizumab exchanges Fab arms with endogenous human IgG4 in natalizumab-treated individuals. Gemtuzumab, in contrast, contains an IgG4 core-hinge mutation that blocks Fab-arm exchange to undetectable levels both in vitro and in a mouse model. The ability of IgG4 therapeutics to recombine with endogenous IgG4 may affect their pharmacokinetics and pharmacodynamics. Although pharmacokinetic modeling lessens concerns about undesired cross-linking under normal conditions, unpredictability remains and mutations that completely prevent Fab-arm exchange in vivo should be considered when designing therapeutic IgG4 antibodies.

Efficient generation of stable bispecific IgG1 by controlled Fab-arm exchange

The promise of bispecific antibodies (bsAbs) to yield more effective therapeutics is well recognized; however, the generation of bsAbs in a practical and cost-effective manner has been a formidable challenge. Here we present a technology for the efficient generation of bsAbs with normal IgG structures that is amenable to both antibody drug discovery and development. The process involves separate expression of two parental antibodies, each containing single matched point mutations in the CH3 domains. The parental antibodies are mixed and subjected to controlled reducing conditions in vitro that separate the antibodies into HL half-molecules and allow reassembly and reoxidation to form highly pure bsAbs. The technology is compatible with standard large-scale antibody manufacturing and ensures bsAbs with Fc-mediated effector functions and in vivo stability typical of IgG1 antibodies. Proof-of-concept studies with HER2×CD3 (T-cell recruitment) and HER2×HER2 (dual epitope targeting) bsAbs demonstrate superior in vivo activity compared with parental antibody pairs.

Modulation of antibody galactosylation through feeding of uridine, manganese chloride, and galactose.

Through process transfer and optimization for increased antibody production to 3 g/L for a GS‐CHO cell line, an undesirable drop in antibody Fc galactosylation was observed. Uridine (U), manganese chloride (M), and galactose (G), constituents involved in the intracellular galactosylation process, were evaluated in 2‐L bioreactors for their potential to specifically increase antibody galactosylation. These components were placed in the feed medium at proportionally increasing concentrations from 0 to 20× UMG, where a 1× concentration of U was 1 mM, a 1× concentration of M was 0.002 mM, and a 1× concentration of G was 5 mM. Antibody galactosylation increased rapidly from 3% at 0× UMG up to 21% at 8× UMG and then more slowly to 23% at 20× UMG. The increase was primarily due to a shift from G0F to G1F, with minimal impact on other glycoforms or product quality attributes. Cell culture performance was largely not impacted by addition of up to 20× UMG except for suppression of glucose consumption and lactate production at 16 and 20× UMG and a slight drop in antibody concentration at 20× UMG. Higher accumulation of free galactose in the medium was observed at 8× UMG and above, coincident with achieving the plateau of maximal galactosylation. A concentration of 4× UMG resulted in achieving the target of 18% galactosylation at 2‐L scale, a result that was reproduced in a 1,000‐L run. Follow‐up studies to evaluate the addition of each component individually up to 12× concentration revealed that the effect was synergistic; the combination of all three components gave a higher level of galactosylation than addition of the each effect independently. The approach was found generally useful since a second cell line responded similarly, with an increase in galactosylation from 5% to 29% from 0 to 8× UMG and no further increase or impact on culture performance up to 12× UMG. These results demonstrate a useful approach to provide exact and specific control of antibody galactosylation through manipulation of the concentrations of uridine, manganese chloride, and galactose in the cell culture medium. Biotechnol. Bioeng. 2011; 108:1591–1602.

Species-Specific Determinants in the IgG CH3 Domain Enable Fab-Arm Exchange by Affecting the Noncovalent CH3–CH3 Interaction Strength

A distinctive feature of human IgG4 is its ability to recombine half molecules (H chain and attached L chain) through a dynamic process termed Fab-arm exchange, which results in bispecific Abs. It is becoming evident that the process of Fab-arm exchange is conserved in several mammalian species, and thereby represents a mechanism that impacts humoral immunity more generally than previously thought. In humans, Fab-arm exchange has been attributed to the IgG4 core-hinge sequence (226-CPSCP-230) in combination with unknown determinants in the third constant H chain domain (CH3). In this study, we investigated the role of the CH3 domain in the mechanism of Fab-arm exchange, and thus identified amino acid position 409 as the critical CH3 determinant in human IgG, with R409 resulting in exchange and K409 resulting in stable IgG. Interestingly, studies with IgG from various species showed that Fab-arm exchange could not be assigned to a common CH3 domain amino acid motif. Accordingly, in rhesus monkeys (Macaca mulatta), aa 405 was identified as the CH3 determinant responsible (in combination with 226-CPACP-230). Using native mass spectrometry, we demonstrated that the ability to exchange Fab-arms correlated with the CH3–CH3 dissociation constant. Species-specific adaptations in the CH3 domain thus enable Fab-arm exchange by affecting the inter-CH3 domain interaction strength. The redistribution of Ag-binding domains between molecules may constitute a general immunological and evolutionary advantage. The current insights impact our view of humoral immunity and should furthermore be considered in the design and evaluation of Ab-based studies and therapeutics.

Quantitative Analysis of the Interaction Strength and Dynamics of Human IgG4 Half Molecules by Native Mass Spectrometry

Native mass spectrometry (MS) is a powerful technique for studying noncovalent protein-protein interactions. Here, native MS was employed to examine the noncovalent interactions involved in homodimerization of antibody half molecules (HL) in hinge-deleted human IgG4 (IgG4Δhinge). By analyzing the concentration dependence of the relative distribution of monomer HL and dimer (HL)(2) species, the apparent dissociation constant (K(D)) for this interaction was determined. In combination with site-directed mutagenesis, the relative contributions of residues at the CH3-CH3 interface to this interaction could be characterized and corresponding K(D) values quantified over a range of 10(-10)-10(-4) M. The critical importance of this noncovalent interaction in maintaining the intact dimeric structure was also proven for the full-length IgG4 backbone. Using time-resolved MS, the kinetics of the interaction could be measured, reflecting the dynamics of IgG4 HL exchange. Hence, native MS has provided a quantitative view of local structural features that define biological properties of IgG4.

Probing metal ion binding and conformational properties of the colicin E9 endonuclease by electrospray ionization time-of-flight mass spectrometry

Nano‐electrospray ionization time‐of‐flight mass spectrometry (ESI‐MS) was used to study the conformational consequences of metal ion binding to the colicin E9 endonuclease (E9 DNase) by taking advantage of the unique capability of ESI‐MS to allow simultaneous assessment of conformational heterogeneity and metal ion binding. Alterations of charge state distributions on metal ion binding/release were correlated with spectral changes observed in far‐ and near‐UV circular dichroism (CD) and intrinsic tryptophan fluorescence. In addition, hydrogen/deuterium (H/D) exchange experiments were used to probe structural integrity. The present study shows that ESI‐MS is sensitive to changes of the thermodynamic stability of E9 DNase as a result of metal ion binding/release in a manner consistent with that deduced from proteolysis and calorimetric experiments. Interestingly, acid‐induced release of the metal ion from the E9 DNase causes dramatic conformational instability associated with a loss of fixed tertiary structure, but secondary structure is retained. Furthermore, ESI‐MS enabled the direct observation of the noncovalent protein complex of E9 DNase bound to its cognate immunity protein Im9 in the presence and absence of Zn2+. Gas‐phase dissociation experiments of the deuterium‐labeled binary and ternary complexes revealed that metal ion binding, not Im9, results in a dramatic exchange protection of E9 DNase in the complex. In addition, our metal ion binding studies and gas‐phase dissociation experiments of the ternary E9 DNase‐Zn2+‐Im9 complex have provided further evidence that electrostatic interactions govern the gas phase ion stability.

In-depth qualitative and quantitative analysis of composite glycosylation profiles and other micro-heterogeneity on intact monoclonal antibodies by high-resolution native mass spectrometry using a modified Orbitrap

Here, we describe a fast, easy-to-use, and sensitive method to profile in-depth structural micro-heterogeneity, including intricate N-glycosylation profiles, of monoclonal antibodies at the native intact protein level by means of mass spectrometry using a recently introduced modified Orbitrap Exactive Plus mass spectrometer. We demonstrate the versatility of our method to probe structural micro-heterogeneity by describing the analysis of three types of molecules: (1) a non-covalently bound IgG4 hinge deleted full-antibody in equilibrium with its half-antibody, (2) IgG4 mutants exhibiting highly complex glycosylation profiles, and (3) antibody-drug conjugates. Using the modified instrument, we obtain baseline separation and accurate mass determination of all different proteoforms that may be induced, for example, by glycosylation, drug loading and partial peptide backbone-truncation. We show that our method can handle highly complex glycosylation profiles, identifying more than 20 different glycoforms per monoclonal antibody preparation and more than 30 proteoforms on a single highly purified antibody. In analyzing antibody-drug conjugates, our method also easily identifies and quantifies more than 15 structurally different proteoforms that may result from the collective differences in drug loading and glycosylation. The method presented here will aid in the comprehensive analytical and functional characterization of protein micro-heterogeneity, which is crucial for successful development and manufacturing of therapeutic antibodies

Tandem Native Mass-Spectrometry on Antibody-Drug Conjugates and Submillion Da Antibody-Antigen Protein Assemblies on an Orbitrap EMR Equipped with a High-Mass Quadrupole Mass Selector.

Native mass spectrometry is emerging as a powerful tool for the characterization of intact antibodies and antibody-based therapeutics. Here, we demonstrate new possibilities provided by the implementation of a high mass quadrupole mass selector on the recently introduced Orbitrap Exactive EMR mass spectrometer. This configuration allows precursor ion selection, and thus tandem mass spectrometry experiments, even on analytes with masses in the hundreds of kilodaltons. We apply tandem mass spectrometry to localize the drug molecules in the therapeutic antibody-drug conjugate brentuximab vedotin, which displays a heterogeneous drug load. Our tandem MS data reveal that drug conjugation takes place nonhomogeneously to cysteine residues both on the light and heavy chains. Next, we analyzed how many antigens bind to IgG hexamers, based on a recently described antibody mutant IgG1-RGY that forms hexamers and activates complement in solution. The fully saturated IgG1-RGY-antigen complexes displayed a stoichiometry of IgG:CD38 of 6:12, possessing a molecular weight of about 1.26 MDa and demonstrating that IgG assembly does not hamper antigen binding. Through tandem MS experiments, we retrieve information about the spatial arrangement and stoichiometry of the subunits within this complex. These examples underscore the potential of this further modified Orbitrap-EMR instrument especially for the in-depth characterization by native tandem mass spectrometry of antibodies and antibody-based constructs.

Mutation of Y407 in the CH3 domain dramatically alters glycosylation and structure of human IgG.

Antibody engineering is increasingly being used to influence the properties of monoclonal antibodies to improve their biotherapeutic potential. One important aspect of this is the modulation of glycosylation as a strategy to improve efficacy. Here, we describe mutations of Y407 in the CH3 domain of IgG1 and IgG4 that significantly increase sialylation, galactosylation, and branching of the N-linked glycans in the CH2 domain. These mutations also promote the formation of monomeric assemblies (one heavy-light chain pair). Hydrogen-deuterium exchange mass spectrometry was used to probe conformational changes in IgG1-Y407E, revealing, as expected, a more exposed CH3–CH3 dimerization interface. Additionally, allosteric structural effects in the CH2 domain and in the CH2–CH3 interface were identified, providing a possible explanation for the dramatic change in glycosylation. Thus, the mutation of Y407 in the CH3 domain remarkably affects both antibody conformation and glycosylation, which not only alters our understanding of antibody structure, but also reveals possibilities for obtaining recombinant IgG with glycosylation tailored for clinical applications.

Controlled Fab-arm exchange for the generation of stable bispecific IgG1

The generation of bispecific antibodies (bsAbs) with natural IgG architecture in a practical and efficient manner has been a longstanding challenge. Here we describe controlled Fab-arm exchange (cFAE), which is an easy-to-use method to generate bispecific IgG1 (bsIgG1). The protocol involves the following: (i) separate expression of two parental IgG1s containing single matching point mutations in the CH3 domain; (ii) mixing of parental IgG1s under permissive redox conditions in vitro to enable recombination of half-molecules; (iii) removal of the reductant to allow reoxidation of interchain disulfide bonds; and (iv) analysis of exchange efficiency and final product using chromatography-based or mass spectrometry (MS)–based methods. The protocol generates bsAbs with regular IgG architecture, characteristics and quality attributes both at bench scale (micrograms to milligrams) and at a mini-bioreactor scale (milligrams to grams) that is designed to model large-scale manufacturing (kilograms). Starting from good-quality purified proteins, exchange efficiencies of ≥95% can routinely be obtained within 2–3 d (including quality control).