Histidine residue mediates radical-induced hinge cleavage of human IgG1

Histidine as a versatile excipient in the protein-based biopharmaceutical formulations

Adequate stabilization is essential for marketed protein-based biopharmaceutical formulations to withstand the various stresses that can be exerted during the pre- and post-manufacturing processes. Therefore, a suitable choice of excipient is a significant step in the manufacturing of such delicate products. Histidine, an essential amino acid, has been extensively used in protein-based biopharmaceutical formulations. The physicochemical properties of histidine are unique among amino acids and could afford multifaceted benefits to protein-based biopharmaceutical formulations. With a pKa of approximately 6.0 at the side chain, histidine has been primarily used as a buffering agent, especially for pH 5.5-6.5. Additionally, histidine exhibited several affirmative properties similar to those of carbohydrates (e.g., sucrose and trehalose) and could therefore be considered to be an alternative approach to established protein-based formulation strategies. The current review describes the general physicochemical properties of histidine, lists all commercial histidine-containing protein-based biopharmaceutical products, and discusses a brief outline of the existing research focused on the versatile applications of histidine, which can act as a buffering agent, stabilizer, cryo/lyo-protectant, antioxidant, viscosity reducer, and solubilizing agent. The interaction between histidine and proteins in protein-based biopharmaceutical formulations, such as the Donnan effect during diafiltration of monoclonal antibody solutions and the degradation of polysorbates in histidine buffer, has also been discussed. As the first review of histidine in protein biopharmaceuticals, it helps to deepen our understanding of the opportunities and challenges associated with histidine as an excipient for protein-based biopharmaceutical formulations.

Stability of Protein Pharmaceuticals: Recent Advances

There have been significant advances in the formulation and stabilization of proteins in the liquid state over the past years since our previous review. Our mechanistic understanding of protein-excipient interactions has increased, allowing one to develop formulations in a more rational fashion. The field has moved towards more complex and challenging formulations, such as high concentration formulations to allow for subcutaneous administration and co-formulation. While much of the published work has focused on mAbs, the principles appear to apply to any therapeutic protein, although mAbs clearly have some distinctive features. In this review, we first discuss chemical degradation reactions. This is followed by a section on physical instability issues. Then, more specific topics are addressed: instability induced by interactions with interfaces, predictive methods for physical stability and interplay between chemical and physical instability. The final parts are devoted to discussions how all the above impacts (co-)formulation strategies, in particular for high protein concentration solutions.'

Catechins prevent monoclonal antibody fragmentation during production via fed-batch culture of Chinese hamster ovary cells

Chinese hamster ovary (CHO) cells are widely used for the industrial production of therapeutic monoclonal antibodies (mAbs). To meet the increasing market demands, high productivity, and quality are required in cell culture. One of the critical attributes of mAbs, from a safety perspective, is mAb fragmentation. However, methods for preventing mAbs fragmentation in CHO cell culture are limited. In this study, we observed that the antibody fragment content increased with increasing titers in fed-batch cultures for all three cell lines expressing recombinant antibodies. Adding copper sulfate to the culture medium further increased the fragment content, suggesting the involvement of reactive oxygen species (ROS) in the fragmentation process. Though antioxidants may be helpful to scavenge ROS, several antioxidants are reported to decrease the productivity of CHO cells. Among the antioxidants examined, we observed that the addition of catechin or (-)-epigallocatechin gallate to the culture medium prevented fragmentation content by about 20% and increased viable cell density and titer by 30% and 10%, respectively. Thus, the addition of catechins or compounds of equivalent function would be beneficial for manufacturing therapeutic mAbs with a balance between high titers and good quality.

From cell factories to patients: Stability challenges in biopharmaceuticals manufacturing and administration with mitigation strategies

Active ingredients of biopharmaceuticals consist of a wide array of biomolecular structures, including those of enzymes, monoclonal antibodies, nucleic acids, and recombinant proteins. Recently, these molecules have dominated the pharmaceutical industry owing to their safety and efficacy. However, their manufacturing is hindered by high cost, inadequate batch-to-batch equivalence, inherent instability, and other quality issues. This article is an up-to-date review of the challenges encountered during different stages of biopharmaceutical production and mitigation of problems arising during their development, formulation, manufacturing, and administration. It is a broad overview discussion of stability issues encountered during product life cycle i.e., upstream processing (aggregation, solubility, host cell proteins, color change), downstream bioprocessing (aggregation, fragmentation), formulation, manufacturing, and delivery to patients.

State-of-the-Art Native Mass Spectrometry and Ion Mobility Methods to Monitor Homogeneous Site-Specific Antibody-Drug Conjugates Synthesis

Antibody-drug conjugates (ADCs) are biotherapeutics consisting of a tumor-targeting monoclonal antibody (mAb) linked covalently to a cytotoxic drug. Early generation ADCs were predominantly obtained through non-selective conjugation methods based on lysine and cysteine residues, resulting in heterogeneous populations with varying drug-to-antibody ratios (DAR). Site-specific conjugation is one of the current challenges in ADC development, allowing for controlled conjugation and production of homogeneous ADCs. We report here the characterization of a site-specific DAR2 ADC generated with the GlyCLICK three-step process, which involves glycan-based enzymatic remodeling and click chemistry, using state-of-the-art native mass spectrometry (nMS) methods. The conjugation process was monitored with size exclusion chromatography coupled to nMS (SEC-nMS), which offered a straightforward identification and quantification of all reaction products, providing a direct snapshot of the ADC homogeneity. Benefits of SEC-nMS were further demonstrated for forced degradation studies, for which fragments generated upon thermal stress were clearly identified, with no deconjugation of the drug linker observed for the T-GlyGLICK-DM1 ADC. Lastly, innovative ion mobility-based collision-induced unfolding (CIU) approaches were used to assess the gas-phase behavior of compounds along the conjugation process, highlighting an increased resistance of the mAb against gas-phase unfolding upon drug conjugation. Altogether, these state-of-the-art nMS methods represent innovative approaches to investigate drug loading and distribution of last generation ADCs, their evolution during the bioconjugation process and their impact on gas-phase stabilities. We envision nMS and CIU methods to improve the conformational characterization of next generation-empowered mAb-derived products such as engineered nanobodies, bispecific ADCs or immunocytokines.

R409K mutation prevents acid-induced aggregation of human IgG4

Human immunoglobulin G isotype 4 (IgG4) antibodies are suitable for use in either the antagonist or agonist format because their low effector functions prevent target cytotoxicity or unwanted cytokine secretion. However, while manufacturing therapeutic antibodies, they are exposed to low pH during purification, and IgG4 is more susceptible to low-pH-induced aggregation than IgG1. Therefore, we investigated the underlying mechanisms of IgG4 aggregation at low pH and engineered an IgG4 with enhanced stability. By swapping the constant regions of IgG1 and IgG4, we determined that the constant heavy chain (CH3) domain is critical for aggregate formation, but a core-hinge-stabilizing S228P mutation in IgG4 is insufficient for preventing aggregation. To identify the aggregation-prone amino acid, we substituted the CH3 domain of IgG4 with that of IgG1, changing IgG4 Arg409 to a Lys, thereby preventing the aggregation of the IgG4 variant as effectively as in IgG1. A stabilizing effect was also recorded with other variable-region variants. Analysis of thermal stability using differential scanning calorimetry revealed that the R409K substitution increased the Tm value of CH3, suggesting that the R409K mutation contributed to the structural strengthening of the CH3-CH3 interaction. The R409K mutation did not influence the binding to antigens/human Fcγ receptors; whereas, the concurrent S228P and R409K mutations in IgG4 suppressed Fab-arm exchange drastically and as effectively as in IgG1, in both in vitro and in vivo in mice models. Our findings suggest that the IgG4 R409K variant represents a potential therapeutic IgG for use in low-effector-activity format that exhibits increased stability.

The Effect of In Vitro Digestion on Antioxidant, ACE-Inhibitory and Antimicrobial Potentials of Traditional Serbian White-Brined Cheeses

This study deals with the effect of in vitro digestion on the functional potential of traditional Serbian white-brined cheeses. The total antioxidant capacity, reducing power and iron (II) chelating properties as well as angiotensin-converting enyzme-inhibitory (ACE-inhibitory) and antimicrobial activities of traditional Serbian white-brined cheeses before and after in vitro digestion were assayed. The traditional cheeses had different antioxidant properties as well as different ACE-inhibitory activities. In vitro digestion improved the total antioxidant capacity (8.42⁻58.56 times) and the reducing power (by 17.90⁻99.30%) of investigated cheeses, whereas their chelating ability was slightly improved or unaffected after digestion. In vitro digestion reduced the ACE-inhibitory potential of water-soluble protein fractions, and digested water-insoluble fractions were the major source of ACE-inhibitory peptides. The digestates did not exhibit any antibacterial potential, whereas they showed moderate antifungal potential toward selected micromycetes. The best antifungal potential had Svrljig ovine cheese and Homolje cow cheese. The results of this study clearly point to a significant functionality of traditional white-brined cheeses.

Assessment of disulfide and hinge modifications in monoclonal antibodies

During the last years there was a substantial increase in the use of antibodies and related proteins as therapeutics. The emphasis of the pharmaceutical industry is on IgG1, IgG2, and IgG4 antibodies, which are therefore in the focus of this article. In order to ensure appropriate quality control of such biopharmaceuticals, deep understanding of their chemical degradation pathways and the resulting impact on potency, pharmacokinetics, and safety is required. Criticality of modifications may be specific for individual antibodies and has to be assessed for each molecule. However, some modifications of conserved structure elements occur in all or at least most IgGs. In these cases, criticality assessment may be applicable to related molecules or molecule formats. The relatively low dissociation energy of disulfide bonds and the high flexibility of the hinge region frequently lead to modifications and cleavages. Therefore, the hinge region and disulfide bonds require specific consideration during quality assessment of mAbs. In this review, available literature knowledge on underlying chemical reaction pathways of modifications, analytical methods for quantification and criticality are discussed. The hinge region is prone to cleavage and is involved in pathways that lead to thioether bond formation, cysteine racemization, and iso‐Asp (Asp, aspartic acid) formation. Disulfide or sulfhydryl groups were found to be prone to reductive cleavage, trisulfide formation, cysteinylation, glutathionylation, disulfide bridging to further light chains, and disulfide scrambling. With regard to potency, disulfide cleavage, hinge cleavage, disulfide bridging to further light chains, and cysteinylation were found to influence antigen binding and fragment crystallizable (Fc) effector functionalities. Renal clearance of small fragments may be faster, whereas clearance of larger fragments appears to depend on their neonatal Fc receptor (FcRn) functionality, which in turn may be impeded by disulfide bond cleavage. Certain modifications such as disulfide induced aggregation and heterodimers from different antibodies are generally regarded critical with respect to safety. However, the detection of some modifications in endogenous antibodies isolated from human blood and the possibility of in vivo repair mechanisms may reduce some safety concerns.

Role of Buffers in Protein Formulations

Buffers comprise an integral component of protein formulations. Not only do they function to regulate shifts in pH, they also can stabilize proteins by a variety of mechanisms. The ability of buffers to stabilize therapeutic proteins whether in liquid formulations, frozen solutions, or the solid state is highlighted in this review. Addition of buffers can result in increased conformational stability of proteins, whether by ligand binding or by an excluded solute mechanism. In addition, they can alter the colloidal stability of proteins and modulate interfacial damage. Buffers can also lead to destabilization of proteins, and the stability of buffers themselves is presented. Furthermore, the potential safety and toxicity issues of buffers are discussed, with a special emphasis on the influence of buffers on the perceived pain upon injection. Finally, the interaction of buffers with other excipients is examined.

Metal ion interactions with mAbs: Part 1

Fragmentation in the hinge region of an IgG1 monoclonal antibody (mAb) can affect product stability, potentially causing changes in potency and efficacy. Metals ions, such as Cu(2+), can bind to the mAb and undergo hydrolysis or oxidation, which can lead to cleavage of the molecule. To better understand the mechanism of Cu(2+)-mediated mAb fragmentation, hinge region cleavage products and their rates of formation were studied as a function of pH with and without Cu(2+). More detailed analysis of the chemical changes was investigated using model linear and cyclic peptides (with the sequence of SCDKTHTC) derived from the upper hinge region of the mAb. Cu(2+) mediated fragmentation was determined to be predominantly via a hydrolytic pathway in solution. The sites and products of hydrolytic cleavage are pH and strain dependent. In more acidic environments, rates of Cu(2+) induced hinge fragmentation are significantly slower than at higher pH. Although the degradation reaction rates between the linear and cyclic peptides are not significantly different, the products of degradation vary. mAb fragmentation can be reduced by modifying His, which is a potential metal binding site and a known ligand in other metalloproteins. These results suggest that a charge may contribute to stabilization of a specific molecular structure involved in hydrolysis, leading to the possible formation of a copper binding pocket that causes increased susceptibility of the hinge region to degradation.

Rearranging the domain order of a diabody-based IgG-like bispecific antibody enhances its antitumor activity and improves its degradation resistance and pharmacokinetics

One approach to creating more beneficial therapeutic antibodies is to develop bispecific antibodies (bsAbs), particularly IgG-like formats with tetravalency, which may provide several advantages such as multivalent binding to each target antigen. Although the effects of configuration and antibody-fragment type on the function of IgG-like bsAbs have been studied, there have been only a few detailed studies of the influence of the variable fragment domain order. Here, we prepared four types of hEx3-scDb-Fc, IgG-like bsAbs, built from a single-chain hEx3-Db (humanized bispecific diabody [bsDb] that targets epidermal growth factor receptor and CD3), to investigate the influence of domain order and fusion manner on the function of a bsDb with an Fc fusion format. Higher cytotoxicities were observed with hEx3-scDb-Fcs with a variable light domain (VL)-variable heavy domain (VH) order (hEx3-scDb-Fc-LHs) compared with a VH-VL order, indicating that differences in the Fc fusion manner do not affect bsDb activity. In addition, flow cytometry suggested that the higher cytotoxicities of hEx3-scDb-Fc-LH may be attributable to structural superiority in cross-linking. Interestingly, enhanced degradation resistance and prolonged in vivo half-life were also observed with hEx3-scDb-Fc-LH. hEx3-scDb-Fc-LH and its IgG2 variant exhibited intense in vivo antitumor effects, suggesting that Fc-mediated effector functions are dispensable for effective anti-tumor activities, which may cause fewer side effects. Our results show that merely rearranging the domain order of IgG-like bsAbs can enhance not only their antitumor activity, but also their degradation resistance and in vivo half-life, and that hEx3-scDb-Fc-LHs are potent candidates for next-generation therapeutic antibodies.

Engineering upper hinge improves stability and effector function of a human IgG1

Upper hinge is vulnerable to radical attacks that result in breakage of the heavy-light chain linkage and cleavage of the hinge of an IgG1. To further explore mechanisms responsible for the radical induced hinge degradation, nine mutants were designed to determine the roles that the upper hinge Asp and His play in the radical reactions. The observation that none of these substitutions could inhibit the breakage of the heavy-light chain linkage suggests that the breakage may result from electron transfer from Cys(231) directly to the heavy-light chain linkage upon radical attacks, and implies a pathway separate from His(229)-mediated hinge cleavage. On the other hand, the substitution of His(229) with Tyr showed promising advantages over the native antibody and other substitutions in improving the stability and function of the IgG1. This substitution inhibited the hinge cleavage by 98% and suggests that the redox active nature of Tyr did not enable it to replicate the ability of His to facilitate radical induced degradation. We propose that the lower redox potential of Tyr, a residue that may be the ultimate sink for oxidizing equivalents in proteins, is responsible for the inhibition. More importantly, the substitution increased the antibody's binding to FcγRIII receptors by 2-3-fold, and improved ADCC activity by 2-fold, while maintaining a similar pharmacokinetic profile with respect to the wild type. Implications of these observations for antibody engineering and development are discussed.

Protein-excipient interactions: mechanisms and biophysical characterization applied to protein formulation development

The purpose of this review is to demonstrate the critical importance of understanding protein-excipient interactions as a key step in the rational design of formulations to stabilize and deliver protein-based therapeutic drugs and vaccines. Biophysical methods used to examine various molecular interactions between solutes and protein molecules are discussed with an emphasis on applications to pharmaceutical excipients in terms of their effects on protein stability. Key mechanisms of protein-excipient interactions such as electrostatic and cation-pi interactions, preferential hydration, dispersive forces, and hydrogen bonding are presented in the context of different physical states of the formulation such as frozen liquids, solutions, gels, freeze-dried solids and interfacial phenomenon. An overview of the different classes of pharmaceutical excipients used to formulate and stabilize protein therapeutic drugs is also presented along with the rationale for use in different dosage forms including practical pharmaceutical considerations. The utility of high throughput analytical methodologies to examine protein-excipient interactions is presented in terms of expanding formulation design space and accelerating experimental timelines.

Breaking the light and heavy chain linkage of human immunoglobulin G1 (IgG1) by radical reactions

We report that the production of hydrogen peroxide by radical chain reductions of molecular oxygen into water in buffers leads to hinge degradation of a human IgG1 under thermal incubation conditions. The production of the hydrogen peroxide can be accelerated by superoxide dismutase or redox active metal ions or inhibited by free radical scavengers. The hydrogen peroxide production rate correlates well with the hinge cleavage. In addition to radical reaction mechanisms described previously, new degradation pathways and products were observed. These products were determined to be generated via radical reactions initiated by electron transfer and addition to the interchain disulfide bond between Cys(215) of the light chain and Cys(225) of the heavy chain. Decomposition of the resulting disulfide bond radical anion breaks the C-S bond at the side chain of Cys, converting it into dehydroalanine and generating a sulfur radical adduct at its counterpart. The hydrolysis of the unsaturated dehydropeptides removes Cys and yields an amide at the C terminus of the new fragment. Meanwhile, the competition between the carbonyl (-C(α)ONH-) and the side chain of Cys allows an electron transfer to the α carbon, forming a new intermediate radical species (-(·)C(α)(O(-))NH-) at Cys(225). Dissociative deamidation occurs along the N-C(α) bond, resulting in backbone cleavage. Given that hydrogen peroxide is a commonly observed product of thermal stress and plays a role in mediating the unique degradation of an IgG1, strategies for improving stability of human antibody therapeutics are discussed.

Fragmentation of monoclonal antibodies

Fragmentation is a degradation pathway ubiquitously observed in proteins despite the remarkable stability of peptide bond; proteins differ only by how much and where cleavage occurs. The goal of this review is to summarize reports regarding the non-enzymatic fragmentation of the peptide backbone of monoclonal antibodies (mAbs). The sites in the polypeptide chain susceptible to fragmentation are determined by a multitude of factors. Insights are provided on the intimate chemical mechanisms that can make some bonds prone to cleavage due to the presence of specific side-chains. In addition to primary structure, the secondary, tertiary and quaternary structures have a significant impact in modulating the distribution of cleavage sites by altering local flexibility, accessibility to solvent or bringing in close proximity side chains that are remote in sequence. This review focuses on cleavage sites observed in the constant regions of mAbs, with special emphasis on hinge fragmentation. The mechanisms responsible for backbone cleavage are strongly dependent on pH and can be catalyzed by metals or radicals. The distribution of cleavage sites are different under acidic compared to basic conditions, with fragmentation rates exhibiting a minimum in the pH range 5 to 6; therefore, the overall fragmentation pattern observed for a mAb is a complex result of structural and solvent conditions. A critical review of the techniques used to monitor fragmentation is also presented; usually a compromise has to be made between a highly sensitive method with good fragment separation and the capability to identify the cleavage site. The effect of fragmentation on the function of a mAb must be evaluated on a case-by-case basis depending on whether cleavage sites are observed in the variable or constant regions, and on the mechanism of action of the molecule.

Insights into the functionality of the putative residues involved in enterocin AS-48 maturation

AS-48 is a 70-residue, α-helical, cationic bacteriocin produced by Enterococcus faecalis and is very singular in its circular structure and its broad antibacterial spectrum. The AS-48 preprotein consists of an N-terminal signal peptide (SP) (35 residues) followed by a proprotein moiety that undergoes posttranslational modifications to yield the mature and active circular protein. For the study of the specificity of the region of AS-48 that is responsible for maturation, three single mutants have been generated by site-directed mutagenesis in the as-48A structural gene. The substitutions were made just in the residues that are thought to constitute a recognition site for the SP cleavage enzyme (His-1, Met1) and in those involved in circularization (Met1, Trp70). Each derivative was expressed in the enterococcal JH2-2 strain containing the necessary native biosynthetic machinery for enterocin production. The importance of these derivatives in AS-48 processing has been evaluated on the basis of the production and structural characterization of the corresponding derivatives. Notably, only two of them (Trp70Ala and Met1Ala derivatives) could be purified in different forms and amounts and are characterized for their bactericidal activity and secondary structure. We could not detect any production of AS-48 in JH2-2(pAM401-81(His-1Ile)) by using the conventional chromatographic techniques, despite the high efficiency of the culture conditions applied to produce this enterocin. Our results underline the different important roles of the mutated residues in (i) the elimination of the SP, (ii) the production levels and antibacterial activity of the mature proteins, and (iii) protein circularization. Moreover, our findings suggest that His-1 is critically involved in cleavage site recognition, its substitution being responsible for the blockage of processing, thereby hampering the production of the specific protein in the cellular culture supernatant.