Design and application of antibody cysteine variants

Computer-Aided Site-Specific PEGylation of PET Hydrolases for Enhanced PET Degradation

The enormous accumulation of poly(ethylene terephthalate) (PET) waste has posed a serious threat to the environment and human health, and biodegradation with PET hydrolase (PETase) can be a possible solution. Herein, we propose site-specifically modifying PETase with amphiphilic polymers to improve the enzyme performance at ambient temperature. For this purpose, we devise a computer-aided strategy to prioritize the conjugation site, and polyethylene glycol (PEG) preparations of 0.55 to 10 kDa are site-specifically conjugated to PETase. The most active conjugate PETase-PEG 5k (PETase-5K) shows an increase of melting temperature (3.88 °C) and significantly improves PET degradation performance (3.5- and 3.1-fold increases at 30 and 40 °C, respectively). Experimental investigation and molecular dynamics simulations reveal that the site-specific PEGylation increases the hydrophobic solvent-accessible surface area and the binding capability to the PET surface, thickens the hydration layer, increases the intramolecular hydrogen bonding, reduces the interactions between water and the conjugated enzyme surface, and rigidifies the enzyme structure via hydrogen bonding and hydrophobic interactions between the polymer and the enzyme, thus leading to improved enzymatic performance of PETase-5K. We further validate the versatility of the site-specific PEGylation in one of the most evolved variants of PETase, FAST-PETase, by 1.8-fold improvement in PET degradation at 30 °C. The presented computer-aided site-specific conjugation strategy has opened a new avenue to enhancing PETase performance at ambient temperature, and the contribution of PEGylation to PETase unraveled in this work laid a foundation for the rational engineering of PET hydrolases.

Site-Specific Antibody Conjugation with Payloads beyond Cytotoxins

As antibody-drug conjugates have become a very important modality for cancer therapy, many site-specific conjugation approaches have been developed for generating homogenous molecules. The selective antibody coupling is achieved through antibody engineering by introducing specific amino acid or unnatural amino acid residues, peptides, and glycans. In addition to the use of synthetic cytotoxins, these novel methods have been applied for the conjugation of other payloads, including non-cytotoxic compounds, proteins/peptides, glycans, lipids, and nucleic acids. The non-cytotoxic compounds include polyethylene glycol, antibiotics, protein degraders (PROTAC and LYTAC), immunomodulating agents, enzyme inhibitors and protein ligands. Different small proteins or peptides have been selectively conjugated through unnatural amino acid using click chemistry, engineered C-terminal formylglycine for oxime or click chemistry, or specific ligation or transpeptidation with or without enzymes. Although the antibody protamine peptide fusions have been extensively used for siRNA coupling during early studies, direct conjugations through engineered cysteine or lysine residues have been demonstrated later. These site-specific antibody conjugates containing these payloads other than cytotoxic compounds can be used in proof-of-concept studies and in developing new therapeutics for unmet medical needs.

Antibody drug conjugates in gastrointestinal cancer: From lab to clinical development

The antibody-drug conjugates (ADCs) are one the fastest growing biotherapeutics in oncology and are still in their infancy in gastrointestinal (GI) cancer for clinical applications to improve patient survival. The ADC based approach is developed with tumor specific antigen, antibody carrying cytotoxic agents to precisely target and deliver chemotherapeutics at the tumor site. To date, 11 ADCs have been approved by US-FDA, and more than 80 are in the clinical development phase for different oncological indications. However, The ADCs based therapies in GI cancers are still far from having high-efficient clinical outcomes. The limited success of these ADCs and lessons learned from the past are now being used to develop a newer generation of ADC against GI cancers. In this review, we did a comprehensive assessment of the key components of ADCs, including tumor marker, antibody, cytotoxic payload, and linkage strategy, with a focus on technical improvement and some future trends in the pipeline for clinical translation. The various preclinical and clinical ADCs used in gastrointestinal malignancies, their target, composition and bioconjugation, along with preclinical and clinical outcomes, are discussed. The emphasis is also given to new generation ADCs employing novel mAb, payload, linker, and bioconjugation methods are also included.

Site-Specific Antibody Conjugation to Engineered Double Cysteine Residues

Site-specific antibody conjugations generate homogeneous antibody-drug conjugates with high therapeutic index. However, there are limited examples for producing the site-specific conjugates with a drug-to-antibody ratio (DAR) greater than two, especially using engineered cysteines. Based on available Fc structures, we designed and introduced free cysteine residues into various antibody CH2 and CH3 regions to explore and expand this technology. The mutants were generated using site-directed mutagenesis with good yield and properties. Conjugation efficiency and selectivity were screened using PEGylation. The top single cysteine mutants were then selected and combined as double cysteine mutants for expression and further investigation. Thirty-six out of thirty-eight double cysteine mutants display comparable expression with low aggregation similar to the wild-type antibody. PEGylation screening identified seventeen double cysteine mutants with good conjugatability and high selectivity. PEGylation was demonstrated to be a valuable and efficient approach for quickly screening mutants for high selectivity as well as conjugation efficiency. Our work demonstrated the feasibility of generating antibody conjugates with a DAR greater than 3.4 and high site-selectivity using THIOMAB^TM method. The top single or double cysteine mutants identified can potentially be applied to site-specific antibody conjugation of cytotoxin or other therapeutic agents as a next generation conjugation strategy.

Dual Site-Specific Antibody Conjugates for Sequential and Orthogonal Cargo Release

Antibody-drug conjugates utilize the antigen specificity of antibodies and the potency of chemotherapeutic and antibiotic drugs for targeted therapy. However, as cancers and bacteria evolve to resist the action of drugs, innovative controlled release methods must be engineered to deliver multidrug cocktails. In this work, we engineer lipoate-acid ligase A (LplA) acceptor peptide (LAP) tags into the constant heavy and light chain of a humanized Her2 targeted antibody, trastuzumab. These engineered LAP tags, along with the glutamine 295 (Q295) residue in the heavy chain, were used to generate orthogonally cleavable site-specific antibody conjugates via a one-pot chemoenzymatic ligation with microbial transglutaminase (mTG) and LplA. We demonstrate orthogonal cargo release from these dual-labeled antibody bioconjugates via matrix metalloproteinase-2 and cathepsin-B-mediated bond cleavage. To the best of our knowledge, this is the first demonstration of temporal control on dual-labeled antibody conjugates, and we believe this platform will allow for sequential release and cooperative drug combinations on a single antibody bioconjugate.

Implementation of an experimental and computational tool set to study protein-mAb interactions

This work focused on the development of a combined experimental and computational tool set to study protein-mAb interactions. A model protein library was first screened using cross interaction chromatography to identify proteins with the strongest retention. Fluorescence polarization was then employed to study the interactions and thermodynamics of the selected proteins-lactoferrin, pyruvate kinase, and ribonuclease B with the mAb. Binding affinities of lactoferrin and pyruvate kinase to the mAb were seen to be relatively salt insensitive in the range examined. Further, a strong entropic contribution was observed, suggesting the importance of hydrophobic interactions. On the other hand, ribonuclease B-mAb binding was seen to be enthalpically driven and salt sensitive, indicating the importance of electrostatic interactions. Protein-protein docking was then carried out and the results identified the CDR region on the mAb as an important binding site for all three proteins. The binding interfaces identified for the mAb-lactoferrin and mAb-pyruvate kinase systems were found to contain complementary hydrophobic and oppositely charged clusters on the interacting regions which were indicative of both hydrophobic and electrostatic interactions. On the other hand, the binding site on ribonuclease B was predominantly positively charged with minimal hydrophobicity. This resulted in an alignment with negatively charged clusters on the mAb, supporting the contention that these interactions were primarily electrostatic in nature. Importantly, these computational results were found to be consistent with the fluorescence polarization data and this combined approach may have utility in examining mAb-HCP interactions which can often complicate the downstream processing of biologics. © 2019 American Institute of Chemical Engineers.

Attachment Site Cysteine Thiol pKa Is a Key Driver for Site-Dependent Stability of THIOMAB Antibody-Drug Conjugates

The incorporation of cysteines into antibodies by mutagenesis allows for the direct conjugation of small molecules to specific sites on the antibody via disulfide bonds. The stability of the disulfide bond linkage between the small molecule and the antibody is highly dependent on the location of the engineered cysteine in either the heavy chain (HC) or the light chain (LC) of the antibody. Here, we explore the basis for this site-dependent stability. We evaluated the in vivo efficacy and pharmacokinetics of five different cysteine mutants of trastuzumab conjugated to a pyrrolobenzodiazepine (PBD) via disulfide bonds. A significant correlation was observed between disulfide stability and efficacy for the conjugates. We hypothesized that the observed site-dependent stability of the disulfide-linked conjugates could be due to differences in the attachment site cysteine thiol pK_a. We measured the cysteine thiol pK_a using isothermal titration calorimetry (ITC) and found that the variants with the highest thiol pK_a (LC K149C and HC A140C) were found to yield the conjugates with the greatest in vivo stability. Guided by homology modeling, we identified several mutations adjacent to LC K149C that reduced the cysteine thiol pK_a and, thus, decreased the in vivo stability of the disulfide-linked PBD conjugated to LC K149C. We also present results suggesting that the high thiol pK_a of LC K149C is responsible for the sustained circulation stability of LC K149C TDCs utilizing a maleimide-based linker. Taken together, our results provide evidence that the site-dependent stability of cys-engineered antibody-drug conjugates may be explained by interactions between the engineered cysteine and the local protein environment that serves to modulate the side-chain thiol pK_a. The influence of cysteine thiol pK_a on stability and efficacy offers a new parameter for the optimization of ADCs that utilize cysteine engineering.

Site Selection: a Case Study in the Identification of Optimal Cysteine Engineered Antibody Drug Conjugates

As the antibody drug conjugate (ADC) community continues to shift towards site-specific conjugation technology, there is a growing need to understand how the site of conjugation impacts the biophysical and biological properties of an ADC. In order to address this need, we prepared a carefully selected series of engineered cysteine ADCs and proceeded to systematically evaluate their potency, stability, and PK exposure. The site of conjugation did not have a significant influence on the thermal stability and in vitro cytotoxicity of the ADCs. However, we demonstrate that the rate of cathepsin-mediated linker cleavage is heavily dependent upon site and is closely correlated with ADC hydrophobicity, thus confirming other recent reports of this phenomenon. Interestingly, conjugates with high rates of cathepsin-mediated linker cleavage did not exhibit decreased plasma stability. In fact, the major source of plasma instability was shown to be retro-Michael mediated deconjugation. This process is known to be impeded by succinimide hydrolysis, and thus, we undertook a series of mutational experiments demonstrating that basic residues located nearby the site of conjugation can be a significant driver of succinimide ring opening. Finally, we show that total antibody PK exposure in rat was loosely correlated with ADC hydrophobicity. It is our hope that these observations will help the ADC community to build "design rules" that will enable more efficient prosecution of next-generation ADC discovery programs.

Strategies and challenges for the next generation of antibody-drug conjugates

Antibody-drug conjugates (ADCs) are one of the fastest growing classes of oncology therapeutics. After half a century of research, the approvals of brentuximab vedotin (in 2011) and trastuzumab emtansine (in 2013) have paved the way for ongoing clinical trials that are evaluating more than 60 further ADC candidates. The limited success of first-generation ADCs (developed in the early 2000s) informed strategies to bring second-generation ADCs to the market, which have higher levels of cytotoxic drug conjugation, lower levels of naked antibodies and more-stable linkers between the drug and the antibody. Furthermore, lessons learned during the past decade are now being used in the development of third-generation ADCs. In this Review, we discuss strategies to select the best target antigens as well as suitable cytotoxic drugs; the design of optimized linkers; the discovery of bioorthogonal conjugation chemistries; and toxicity issues. The selection and engineering of antibodies for site-specific drug conjugation, which will result in higher homogeneity and increased stability, as well as the quest for new conjugation chemistries and mechanisms of action, are priorities in ADC research.

Efficient Preparation of Site-Specific Antibody-Drug Conjugates Using Cysteine Insertion

Antibody-drug conjugates (ADCs) are a class of biopharmaceuticals that combine the specificity of antibodies with the high-potency of cytotoxic drugs. Engineering cysteine residues in the antibodies using mutagenesis is a common method to prepare site-specific ADCs. With this approach, solvent accessible amino acids in the antibody have been selected for substitution with cysteine for conjugating maleimide-bearing cytotoxic drugs, resulting in homogeneous and stable site-specific ADCs. Here we describe a cysteine engineering approach based on the insertion of cysteines before and after selected sites in the antibody, which can be used for site-specific preparation of ADCs. Cysteine-inserted antibodies have expression level and monomeric content similar to the native antibodies. Conjugation to a pyrrolobenzodiazepine dimer (SG3249) resulted in comparable efficiency of site-specific conjugation between cysteine-inserted and cysteine-substituted antibodies. Cysteine-inserted ADCs were shown to have biophysical properties, FcRn, and antigen binding affinity similar to the cysteine-substituted ADCs. These ADCs were comparable for serum stability to the ADCs prepared using cysteine-mutagenesis and had selective and potent cytotoxicity against human prostate cancer cells. Two of the cysteine-inserted variants abolish binding of the resulting ADCs to FcγRs in vitro, thereby potentially preventing non-target mediated uptake of the ADCs by cells of the innate immune system that express FcγRs, which may result in mitigating off-target toxicities. A selected cysteine-inserted ADC demonstrated potent dose-dependent anti-tumor activity in a xenograph tumor mouse model of human breast adenocarcinoma expressing the oncofetal antigen 5T4.

Mechanistic understanding of the cysteine capping modifications of antibodies enables selective chemical engineering in live mammalian cells

Protein modifications by intricate cellular machineries often redesign the structure and function of existing proteins to impact biological networks. Disulfide bond formation between cysteine (Cys) pairs is one of the most common modifications found in extracellularly-destined proteins, key to maintaining protein structure. Unpaired surface cysteines on secreted mammalian proteins are also frequently found disulfide-bonded with free Cys or glutathione (GSH) in circulation or culture, the mechanism for which remains unknown. Here we report that these so-called Cys-capping modifications take place outside mammalian cells, not in the endoplasmic reticulum (ER) where oxidoreductase-mediated protein disulfide formation occurs. Unpaired surface cysteines of extracellularly-arrived proteins such as antibodies are uncapped upon secretion before undergoing disulfide exchange with cystine or oxidized GSH in culture medium. This observation has led to a feasible way to selectively modify the nucleophilic thiol side-chain of cell-surface or extracellular proteins in live mammalian cells, by applying electrophiles with a chemical handle directly into culture medium. These findings provide potentially an effective approach for improving therapeutic conjugates and probing biological systems.

Engineering a targeted delivery platform using Centyrins

Targeted delivery of therapeutic payloads to specific tissues and cell types is an important component of modern pharmaceutical development. Antibodies or other scaffold proteins can provide the cellular address for delivering a covalently linked therapeutic via specific binding to cell-surface receptors. Optimization of the conjugation site on the targeting protein, linker chemistry and intracellular trafficking pathways can all influence the efficiency of delivery and potency of the drug candidate. In this study, we describe a comprehensive engineering experiment for an EGFR binding Centyrin, a highly stable fibronectin type III (FN3) domain, wherein all possible single-cysteine replacements were evaluated for expression, purification, conjugation efficiency, retention of target binding, biophysical properties and delivery of a cytotoxic small molecule payload. Overall, 26 of the 94 positions were identified as ideal for cysteine modification, conjugation and drug delivery. Conjugation-tolerant positions were mapped onto a crystal structure of the Centyrin, providing a structural context for interpretation of the mutagenesis experiment and providing a foundation for a Centyrin-targeted delivery platform.

Rational design of therapeutic mAbs against aggregation through protein engineering and incorporation of glycosylation motifs applied to bevacizumab

The aggregation of biotherapeutics is a major hindrance to the development of successful drug candidates; however, the propensity to aggregate is often identified too late in the development phase to permit modification to the protein's sequence. Incorporating rational design for the stability of proteins in early discovery has numerous benefits. We engineered out aggregation-prone regions on the Fab domain of a therapeutic monoclonal antibody, bevacizumab, to rationally design a biobetter drug candidate. With the purpose of stabilizing bevacizumab with respect to aggregation, 2 strategies were undertaken: single point mutations of aggregation-prone residues and engineering a glycosylation site near aggregation-prone residues to mask these residues with a carbohydrate moiety. Both of these approaches lead to comparable decreases in aggregation, with an up to 4-fold reduction in monomer loss. These single mutations and the new glycosylation pattern of the Fab domain do not modify binding to the target. Biobetters with increased stability against aggregation can therefore be generated in a rational manner, by either removing or masking the aggregation-prone region or crowding out protein-protein interactions.

Synthesis of Site-Specific Radiolabeled Antibodies for Radioimmunotherapy via Genetic Code Expansion

Radioimmunotherapy (RIT) delivers radioisotopes to antigen-expressing cells via monoantibodies for the imaging of lesions or medical therapy. The chelates are typically conjugated to the antibody through cysteine or lysine residues, resulting in heterogeneous chelate-to-antibody ratios and various conjugation sites. To overcome this heterogeneity, we have developed an approach for site-specific radiolabeling of antibodies by combination of genetic code expansion and click chemistry. As a proof-of-concept study, model systems including anti-CD20 antibody rituximab, positron-emitting isotope ⁶⁴Cu, and a newly synthesized bifunctional linker (4-dibenzocyclooctynol-1,4,7,10-tetraazacyclotetradecane-1,4,7,10-tetraacetic acid, DIBO-DOTA) were used. The approach consists of three steps: (1) site-specific incorporation of an azido group-bearing amino acid (NEAK) via the genetic code expansion technique at the defined sites of the antibody as a "chemical handle"; (2) site-specific and quantitative conjugation of bifunctional linkers with the antibodies under a mild condition; and (3) radiolabeling of the chelate-modified antibodies with the appropriate isotope. We used heavy-chain A122NEAK rituximab as proof-of-concept and obtained a homogeneous radioconjugate with precisely two chelates per antibody, incorporated only at the chosen sites. The conjugation did not alter the binding and pharmacokinetics of the rituximab, as indicated by in vitro assays and in vivo PET imaging. We believe our research is a good supplement to the genetic code expansion technique for the development of novel radioimmunoconjugates.

One-Step Conjugation Method for Site-Specific Antibody-Drug Conjugates through Reactive Cysteine-Engineered Antibodies

Engineered cysteine residues are particularly convenient for site-specific conjugation of antibody-drug conjugates (ADC), because no cell engineering and additives are required. Usually, unpaired cysteine residues form mixed disulfides during fermentation in Chinese hamster ovarian (CHO) cells; therefore, additional reduction and oxidization steps are required prior to conjugation. In this study, we prepared light chain (Lc)-Q124C variants in IgG and examined the conjugation efficiency. Intriguingly, Lc-Q124C exhibited high thiol reactivity and directly generated site-specific ADC without any pretreatment (named active thiol antibody: Actibody). Most of the cysteine-maleimide conjugates including Lc-Q124C showed retro-Michael reaction with cysteine 34 in albumin and were decomposed over time. In order to acquire resistance to a maleimide exchange reaction, the facile procedure for succinimide hydrolysis on anion exchange resin was employed. Hydrolyzed Lc-Q124C conjugate prepared with anion exchange procedure retained high stability in plasma. Recently, various stable linkage schemes for cysteine conjugation have been reported. The combination with direct conjugation by the use of Actibody and stable linker technology could enable the generation of stable site-specific ADC through a simple method. Actibody technology with Lc-Q124C at a less exposed position opens a new path for cysteine-based conjugation, and contributes to reducing entry barriers to the preparation and evaluation of ADC.

Chicken scFvs with an Artificial Cysteine for Site-Directed Conjugation

For the site-directed conjugation of chemicals and radioisotopes to the chicken-derived single-chain variable fragment (scFv), we investigated amino acid residues replaceable with cysteine. By replacing each amino acid of the 157 chicken variable region framework residues (FR, 82 residues on VH and 75 on VL) with cysteine, 157 artificial cysteine mutants were generated and characterized. At least 27 residues on VL and 37 on VH could be replaced with cysteine while retaining the binding activity of the original scFv. We prepared three VL (L5, L6 and L7) and two VH (H13 and H16) mutants as scFv-Ckappa fusion proteins and showed that PEG-conjugation to the sulfhydryl group of the artificial cysteine was achievable in all five mutants. Because the charge around the cysteine residue affects the in vivo stability of thiol-maleimide conjugation, we prepared 16 charge-variant artificial cysteine mutants by replacing the flanking residues of H13 with charged amino acids and determined that the binding activity was not affected in any of the mutants except one. We prepared four charge-variant H13 artificial cysteine mutants (RCK, DCE, ECD and ECE) as scFv-Ckappa fusion proteins and confirmed that the reactivity of the sulfhydryl group on cysteine is active and their binding activity is retained after the conjugation process.

Antibody-Drug Conjugates: Design, Formulation and Physicochemical Stability

The convergence of advanced understanding of biology with chemistry has led to a resurgence in the development of antibody-drug conjugates (ADCs), especially with two recent product approvals. Design and development of ADCs requires the synergistic combination of the monoclonal antibody, the linker and the payload. Advances in antibody science has enabled identification and generation of high affinity, highly selective, humanized or human antibodies for a given target. Novel linker technologies have been synthesized and highly potent cytotoxic drug payloads have been created. As the first generation of ADCs utilizing lysine and cysteine chemistries moves through the clinic and into commercialization, second generation ADCs involving site specific conjugation technologies are being evaluated and tested. The latter aim to be better characterized and controlled, with wider therapeutic indices as well as improved pharmacokinetic-pharmacodynamic (PK-PD) profiles. ADCs offer some interesting physicochemical properties, due to conjugation itself, and to the (often) hydrophobic payloads that must be considered during their CMC development. New analytical methodologies are required for the ADCs, supplementing those used for the antibody itself. Regulatory filings will be a combination of small molecule and biologics. The regulators have put forth some broad principles but this landscape is still evolving.

Dramatic potentiation of the antiviral activity of HIV antibodies by cholesterol conjugation

The broadly neutralizing antibodies HIV 2F5 and 4E10, which bind to overlapping epitopes in the membrane-proximal external region of the fusion protein gp41, have been proposed to use a two-step mechanism for neutralization; first, they bind and preconcentrate at the viral membrane through their long, hydrophobic CDRH3 loops, and second, they form a high affinity complex with the protein epitope. Accordingly, mutagenesis of the CDRH3 can abolish their neutralizing activity, with no change in the affinity for the peptide epitope. We show here that we can mimic this mechanism by conjugating a cholesterol group outside of the paratope of an antibody. Cholesterol-conjugated antibodies bind to lipid raft domains on the membrane, and because of this enrichment, they show increased antiviral potency. In particular, we find that cholesterol conjugation (i) rescues the antiviral activity of CDRH3-mutated 2F5, (ii) increases the antiviral activity of WT 2F5, (iii) potentiates the non-membrane-binding HIV antibody D5 10–100-fold (depending on the virus strain), and (iv) increases synergy between 2F5 and D5. Conjugation can be made at several positions, including variable and constant domains. Cholesterol conjugation therefore appears to be a general strategy to boost the potency of antiviral antibodies, and, because membrane affinity is engineered outside of the antibody paratope, it can complement affinity maturation strategies.

Facile method for the site-specific, covalent attachment of full-length IgG onto nanoparticles

Antibodies, most commonly IgGs, have been widely used as targeting ligands in research and therapeutic applications due to their wide array of targets, high specificity and proven efficacy. Many of these applications require antibodies to be conjugated onto surfaces (e.g. nanoparticles and microplates); however, most conventional bioconjugation techniques exhibit low crosslinking efficiencies, reduced functionality due to non-site-specific labeling and random surface orientation, and/or require protein engineering (e.g. cysteine handles), which can be technically challenging. To overcome these limitations, we have recombinantly expressed Protein Z, which binds the Fc region of IgG, with an UV active non-natural amino acid benzoylphenyalanine (BPA) within its binding domain. Upon exposure to long wavelength UV light, the BPA is activated and forms a covalent link between the Protein Z and the bound Fc region of IgG. This technology was combined with expressed protein ligation (EPL), which allowed for the introduction of a fluorophore and click chemistry-compatible azide group onto the C-terminus of Protein Z during the recombinant protein purification step. This enabled the crosslinked-Protein Z-IgG complexes to be efficiently and site-specifically attached to aza-dibenzocyclooctyne-modified nanoparticles, via copper-free click chemistry.

Antibody-drug conjugates for the treatment of cancer

With over 20 antibody-drug conjugates in clinical trials as well as a recently FDA-approved drug, it is clear that this is becoming an important and viable approach for selectively delivering highly cytotoxic agents to tumor cells while sparing normal tissue. This review discusses the critical aspects for this approach with an emphasis on the properties of the linker between the antibody and the cytotoxic payload that are required for an effective antibody-drug conjugate. Different linkers are illustrated with attention focused on (i) the specifics of attachment to the antibody, (ii) the polarity of the linker, (iii) the trigger on the linker that initiates cleavage from the drug, and (iv) the self-immolative spacer that liberates the active payload. Future directions in the field are proposed.

Engineering a therapeutic IgG molecule to address cysteinylation, aggregation and enhance thermal stability and expression

Antibodies can undergo a variety of covalent and non-covalent degradation reactions that have adverse effects on efficacy, safety, manufacture and storage. We had identified an antibody to Angiopoietin 2 (Ang2 mAb) that neutralizes Ang2 binding to its receptor in vitro and inhibits tumor growth in vivo. Despite favorable pharmacological activity, the Ang2 mAb preparations were heterogeneous, aggregated rapidly and were poorly expressed. Here, we report the engineering of the antibody variable and constant domains to generate an antibody with reduced propensity to aggregate, enhanced homogeneity, 11°C elevated T(m), 26-fold improved level of expression and retained activity. The engineered molecule, MEDI-3617, is now compatible with the large scale material supply required for clinical trials and is currently being evaluated in Phase 1 in cancer patients. This is the first report to describe the stability engineering of a therapeutic antibody addressing non canonical cysteine residues and the design strategy reported here is generally applicable to other therapeutic antibodies and proteins.

7th annual European Antibody Congress 2011: November 29-December 1, 2011, Geneva, Switzerland

The 7th European Antibody Congress (EAC), organized by Terrapin Ltd., was again held in Geneva, Switzerland, following on the tradition established with the 4th EAC. The 2011 version of the EAC was attended by nearly 250 delegates who learned of the latest advances and trends in the global development of antibody-based therapeutics. The first day focused on advances in understanding structure-function relationships, choosing the best format, glycoengineering biobetter antibodies, improving the efficacy and drugability of mAbs and epitope mapping. On the second day, the discovery of novel targets for mAb therapy, clinical pipeline updates, use of antibody combinations to address resistance, generation and identification of mAbs against new targets and biosimilar mAb development were discussed. Antibody-drug conjugates, domain antibodies and new scaffolds and bispecific antibodies were the topics of the third day. In total, nearly 50 speakers provided updates of programs related to antibody research and development on-going in the academic, government and commercial sectors.

Developability index: a rapid in silico tool for the screening of antibody aggregation propensity

Determining the aggregation propensity of protein-based biotherapeutics is an important step in the drug development process. Typically, a great deal of data collected over a large period of time is needed to estimate the aggregation propensity of biotherapeutics. Thus, candidates cannot be screened early on for aggregation propensity, but early screening is desirable to help streamline drug development. Here, we present a simple molecular computational method to predict the aggregation propensity via hydrophobic interactions, thought to be the most common mechanism of aggregation, and electrostatic interactions. This method uses a new quantity termed Developability Index. It is a function of an antibody's net charge, calculated on the full-length antibody structure, and the spatial aggregation propensity, calculated on the complementarity-determining region structure. Its accuracy is due to the molecular level details and the incorporation of the tertiary structure of the antibody. It is particularly applicable to antibodies or other proteins for which structures are available or could be determined accurately using homology modeling. Applications include the selection of molecules in the discovery or early development process, selection of mutants for stability, and estimation of resources needed for development of a given biomolecule.

Dynamic fluctuations of protein-carbohydrate interactions promote protein aggregation

Protein-carbohydrate interactions are important for glycoprotein structure and function. Antibodies of the IgG class, with increasing significance as therapeutics, are glycosylated at a conserved site in the constant Fc region. We hypothesized that disruption of protein-carbohydrate interactions in the glycosylated domain of antibodies leads to the exposure of aggregation-prone motifs. Aggregation is one of the main problems in protein-based therapeutics because of immunogenicity concerns and decreased efficacy. To explore the significance of intramolecular interactions between aromatic amino acids and carbohydrates in the IgG glycosylated domain, we utilized computer simulations, fluorescence analysis, and site-directed mutagenesis. We find that the surface exposure of one aromatic amino acid increases due to dynamic fluctuations. Moreover, protein-carbohydrate interactions decrease upon stress, while protein-protein and carbohydrate-carbohydrate interactions increase. Substitution of the carbohydrate-interacting aromatic amino acids with non-aromatic residues leads to a significantly lower stability than wild type, and to compromised binding to Fc receptors. Our results support a mechanism for antibody aggregation via decreased protein-carbohydrate interactions, leading to the exposure of aggregation-prone regions, and to aggregation.