Recent progress in transglutaminase-mediated assembly of antibody-drug conjugates

Oriented Surface Immobilization of Antibodies Using Enzyme-Mediated Site-Specific Biotinylation for Enhanced Antigen-Binding Capacity

The effectiveness of surface-immobilized antibodies is often diminished by improper antibody orientation and limited stability, impeding the analytical performance of biosensors. Here, we report a novel enzyme-mediated strategy to biotinylate the Fc region of an anti-horseradish peroxidase (anti-HRP) antibody with site-specificity that enables oriented immobilization on a streptavidin-functionalized surface. Microbial transglutaminase (mTG) catalyzes the covalent coupling between the amine functional group on a biotin analogue (NH2-PEG4-biotin) and the side chain of a privileged glutamine residue (Q295) located on the heavy chain Fc region of IgG antibodies. For comparison, an anti-HRP antibody was biotinylated using an amine-reactive biotin analogue (NHS-PEG4-biotin) to covalently couple to lysine residues randomly located throughout the antibody. The antibody that reacted with a 40-fold excess of biotin reagent formed conjugates with a biotin-to-antibody ratio of 1.9 ± 0.3 and 5.0 ± 0.6 for the site-specific and random biotinylation strategies, respectively. Western blot analysis confirms that mTG-mediated biotinylation is restricted to the heavy chain, while lysine-targeted biotinylation is observed on both the heavy and light chains. The site-specific and randomly biotinylated antibodies were immobilized onto streptavidin-coated polystyrene 96-well plates to evaluate antigen (HRP) binding activity. The site-specific biotinylated antibody provided a 3-fold improvement in antigen binding capacity, sensitivity, and detection limit, that is attributed to the proper orientation of the antibody when immobilized through the Fc region. This chemo-enzymatic strategy is universally applicable to other antibodies for oriented antibody immobilization via site-specific linking chemistries without the need for protein engineering.

A Recognition Tag of Human Origin for Bioorthogonal Generation of Antibody-Drug Conjugates using Microbial Biotin Ligase

The use of enzymes, such as microbial transglutaminase, lipoate protein ligase A, or sortase A, for the generation of antibody-drug conjugates has proven to be a powerful tool for the site-specific payload conjugation to tumor-specific antibodies. Herein, the extension of this enzymatic toolbox by Pyrococcus horikoshii biotin ligase is reported. To this end, the therapeutic antibody trastuzumab is equipped with p67, the 67 amino acid carboxyl-terminal domain of human propionyl-CoA carboxylase α subunit, at the C-terminus of either the light or heavy chain (Trz-LC:p67 and Trz-HC:p67). Upon incubation with PhBL, the azide-bearing linker desthiobiotin azide is site-specifically coupled to the p67 domains at the antibody. Subsequent strain-promoted azide-alkyne cycloaddition with DBCO-AF488 and DBCO-Val-Cit-PAB-MMAE yielded conjugates near to full conversion. In cellular assays, these constructs exhibit single-digit nanomolar EC50 values in cellular proliferation assays on SK-BR-3 and A431 cells, where no significant difference in the performance between the two variants Trz-LC:p67-MMAE and Trz-HC:p67-MMAE is observed. On high Fc-γIIIa receptor expressing Jurkat cells, Trz-HC:p67-MMAE exhibits higher potency than Trz-LC:p67-MMAE, indicating an Fc-blocking effect of p67 when fused to the light chain.

Antifungal Activity of Novel Isoindoline-2-Yl Putrescines as Potential Autophagy-Activated Fungicide

The exacerbation of plant fungal diseases necessitates the development of new fungicides to prevent outbreaks. In this study, five novel isoindoline-2-yl putrescines (ISPs) were synthesized, and their synthetic procedures and gram-scale preparation were explored. When tested at 50 μg mL-1, ISPs did not significantly inhibit mycelial growth on agar plates. However, at 100 μg mL-1, they demonstrated remarkable in vivo efficacy in mitigating Botrytis cinerea infection, especially ISP3 showed curative and protective activities of 91.9 % and 92.6 %, respectively. Moreover, ISP3 also effectively halted lesion expansion of gray mold, Sclerotic rot, and Fusarium scabs, while inducing excessive malformed top mycelial branches of B. cinerea and Sclerotinia sclerotiorum, suppressing sclerotia formation in S. sclerotiorum, and triggering autophagic vacuolization with numerous autophagosomes in the mycelia of these fungi. Molecular docking revealed that ISP3 effectively bound to the active site of BcAtg3, forming hydrogen bonds with Ser279, Gly343, Asp370, and Asp13, along with establishing a stable salt bridge with Asp13. Furthermore, ISP3 possessed favorable ADMET properties. These findings highlight ISP3 as a promising antifungal candidate through autophagy activation.

Site-Specific Quadruple-Functionalised Antibodies

Antibody-drug conjugates (ADCs) are a growing class of chemotherapeutic agents that have yielded striking clinical successes. However, the efficacy of ADCs often suffers from issues associated with tumor heterogeneity and resistance. To overcome these problems, a new generation of ADCs comprising a single monoclonal antibody with multiple different payloads attached, termed multi-payload ADCs, have been developed. Here we deploy multiple orthogonal site-specific protein modification strategies to generate highly homogeneous multi-functionalised antibody conjugates comprising up to four different functionalities installed at four unique sites on the antibody. This work, which includes the use of a site-specific cyclopropenone (CPO)-based reagent, represents the first example of a homogeneous multi-payload ADC with a payload count greater than two, and thereby facilitates the development of the next generation of ADCs.

Technical, preclinical, and clinical developments of Fc-glycan-specific antibody-drug conjugates

Antibody-drug conjugates (ADCs) have emerged as a powerful avenue in the therapeutic treatment of cancer. Site-specific antibody-drug conjugations represent the latest trend in the development of ADCs, addressing the limitations of traditional random conjugation technologies. This article summarizes the innovative development of Fc-glycan-specific ADCs (gsADCs), which utilize the conserved Fc N-glycan as the anchor point for site-specific conjugation. This approach offers significant strengths, including improved ADC homogeneity and overall hydrophilicity, enhanced pharmacokinetics and therapeutic index, and potentially reduced Fc receptor-mediated side effects. Currently dozens of gsADCs are in different preclinical and clinical development stages. Notably, JSKN003 and IBI343 have demonstrated promising results in phase 1 trials and are advancing into phase 3 studies. This review discusses the advantages of Fc-glycan-conjugation, various glycan-specific conjugation techniques, and the preclinical and clinical development of gsADCs. While challenges such as increased manufacturing cost for large-scale production need continuous innovation to overcome and there are different opinions regarding the pros and cons of reduced/diminished affinities to Fc gamma receptors, ongoing research and clinical progress underscore the potential of gsADCs to renovate ADC cancer therapy.

Evaluation of Potency and Specificity of Cryptophycin-Loaded Antibody-Drug Conjugates

An enhanced variant of the antimitotic toxin cryptophycin was conjugated to the anti-Her2 monoclonal antibody (mAb) Trastuzumab upon Michael addition. Either antibodies with freed hinge-region cysteines or THIOMAB formats with engineered cysteines in the mAbs light chain were added to a maleimide derivative of cryptophycin. These Antibody-Drug Conjugates (ADCs) showed retained binding to Her2 positive tumor cells and highly efficient cell killing in double-digit pM range on high Her2-expressing SK-BR-3 cells. Two ADCs (DAR 6, DAR 3) showed superior cell killing of the cell lines JIMT-1 and RT112 with medium receptor expression level in comparison with a DAR 6 MMAE ADC serving as reference. The observed cell cytotoxicity is target-dependent since no impact on cell viability was observed for low Her2-expressing MDA-MB468 cells. Particularly the DAR 3 ADC in THIOMAB format exhibiting desirable biophysical properties and high potency emerged as a promising candidate for further in vivo investigations.

Generation of binder-format-payload conjugate-matrices by antibody chain-exchange

The generation of antibody-drug conjugates with optimal functionality depends on many parameters. These include binder epitope, antibody format, linker composition, conjugation site(s), drug-to-antibody ratio, and conjugation method. The production of matrices that cover all possible parameters is a major challenge in identifying optimal antibody-drug conjugates. To address this bottleneck, we adapted our Format Chain Exchange technology (FORCE), originally established for bispecific antibodies, toward the generation of binder-format-payload matrices (pair-FORCE). Antibody derivatives with exchange-enabled Fc-heterodimers are combined with payload-conjugated Fc donors, and subsequent chain-exchange transfers payloads to antibody derivatives in different formats. The resulting binder-format-conjugate matrices can be generated with cytotoxic payloads, dyes, haptens, and large molecules, resulting in versatile tools for ADC screening campaigns. We show the relevance of pair-FORCE for identifying optimal HER2-targeting antibody-drug conjugates. Analysis of this matrix reveals that the notion of format-defines-function applies not only to bispecific antibodies, but also to antibody-drug conjugates.

Bioproduction Platform to Generate Functionalized Disulfide-Constrained Peptide Analogues

Disulfide-constrained peptides (DCPs) have gained increased attention as a drug modality due to their exceptional stability and combined advantages of large biologics and small molecules. Chemical synthesis, although widely used to produce DCPs, is associated with high cost, both economically and environmentally. To reduce the dependence on solid phase peptide synthesis and the negative environmental footprint associated with it, we present a highly versatile, low-cost, and environmentally friendly bioproduction platform to generate DCPs and their conjugates as well as chemically modified or isotope-labeled DCPs. Using the DCP against the E3 ubiquitin ligase Zinc and Ring Finger 3, MK1-3.6.10, as a model peptide, we have demonstrated the use of bacterial expression, combined with Ser ligation or transglutaminase-mediated XTEN ligation, to produce multivalent MK1-3.6.10 and MK1-3.6.10 with N-terminal functional groups. We have also developed a bioproduction method for the site-specific incorporation of unnatural amino acids into recombinant DCPs by the amber codon suppression system. Lastly, we produced 15N/13C-labeled MK1-3.6.10 with high yield and assessed the performance of a semiautomated resonance assignment workflow that could be used to accelerate binding studies and structural characterization of DCPs. This study provides a proof of concept to generate functionalized DCPs using bioproduction, providing a potential solution to alleviate the reliance on hazardous chemicals, reduce the cost, and expedite the timeline for DCP discovery.

LexA, an SOS response repressor, activates TGase synthesis in Streptomyces mobaraensis

Transglutaminase (EC 2.3.2.13, TGase), an enzyme that catalyzes the formation of covalent cross-links between protein or peptide molecules, plays a critical role in commercial food processing, medicine, and textiles. TGase from Streptomyces is the sole commercial enzyme preparation for cross-linking proteins. In this study, we revealed that the SOS response repressor protein LexA in Streptomyces mobaraensis not only triggers morphological development but also enhances TGase synthesis. The absence of lexA significantly diminished TGase production and sporulation. Although LexA does not bind directly to the promoter region of the TGase gene, it indirectly stimulates transcription of the tga gene, which encodes TGase. Furthermore, LexA directly enhances the expression of genes associated with protein synthesis and transcription factors, thus favorably influencing TGase synthesis at both the transcriptional and posttranscriptional levels. Moreover, LexA activates four crucial genes involved in morphological differentiation, promoting spore maturation. Overall, our findings suggest that LexA plays a dual role as a master regulator of the SOS response and a significant contributor to TGase regulation and certain aspects of secondary metabolism, offering insights into the cellular functions of LexA and facilitating the strategic engineering of TGase overproducers.

Site-Specific Conjugation of Native Antibody: Transglutaminase-Mediated Modification of a Conserved Glutamine While Maintaining the Primary Sequence and Core Fc Glycan via Trimming with an Endoglycosidase

A versatile chemo-enzymatic tool to site-specifically modify native (nonengineered) antibodies is using transglutaminase (TGase, E.C. 2.3.2.13). With various amines as cosubstrates, this enzyme converts the unsubstituted side chain amide of glutamine (Gln or Q) in peptides and proteins into substituted amides (i.e., conjugates). A pleasant surprise is that only a single conserved glutamine (Gln295) in the Fc region of IgG is modified by microbial TGase (mTGase, EC 2.3.2.13), thereby providing a highly specific and generally applicable conjugation method. However, prior to the transamidation (access to the glutamine residue by mTGase), the steric hindrance from the nearby conserved N-glycan (Asn297 in IgG1) must be reduced. In previous approaches, amidase (PNGase F, EC 3.5.1.52) was used to completely remove the N-glycan. However, PNGase F also converts a net neutral asparagine (Asn297) to a negatively charged aspartic acid (Asp297). This charge alteration may markedly change the structure, function, and immunogenicity of an IgG antibody. In contrast, in our new method presented herein, the N-glycan is trimmed by an endoglycosidase (EndoS2, EC 3.2.1.96), hence retaining both the core N-acetylglucosamine (GlcNAc) moiety and the neutral asparaginyl amide. The trimmed glycan also reduces or abolishes Fc receptor-mediated functions, which results in better imaging agents by decreasing nonspecific binding to other cells (e.g., immune cells). Moreover, the remaining core glycan allows further derivatization such as glycan remodeling and dual conjugation. Practical and robust, our method generates conjugates in near quantitative yields, and both enzymes are commercially available.

Catcher/Tag Toolbox: Biomolecular Click-Reactions For Protein Engineering Beyond Genetics

Manipulating protein architectures beyond genetic control has attracted widespread attention. Catcher/Tag systems enable highly specific conjugation of proteins in vivo and in vitro via an isopeptide-bond. They provide efficient, robust, and irreversible strategies for protein conjugation and are simple yet powerful tools for a variety of applications in enzyme industry, vaccines, biomaterials, and cellular applications. Here we summarize recent development of the Catcher/Tag toolbox with a particular emphasis on the design of Catcher/Tag pairs targeted for specific applications. We cover the current limitations of the Catcher/Tag systems and discuss the pH sensitivity of the reactions. Finally, we conclude some of the future directions in the development of this versatile protein conjugation method and envision that improved control over inducing the ligation reaction will further broaden the range of applications.

Trends in the Development of Antibody-Drug Conjugates for Cancer Therapy

In cancer treatment, the first-generation, cytotoxic drugs, though effective against cancer cells, also harmed healthy ones. The second-generation targeted cancer cells precisely to inhibit their growth. Enter the third-generation, consisting of immuno-oncology drugs, designed to combat drug resistance and bolster the immune system's defenses. These advanced therapies operate by obstructing the uncontrolled growth and spread of cancer cells through the body, ultimately eliminating them effectively. Within the arsenal of cancer treatment, monoclonal antibodies offer several advantages, including inducing cancer cell apoptosis, precise targeting, prolonged presence in the body, and minimal side effects. A recent development in cancer therapy is Antibody-Drug Conjugates (ADCs), initially developed in the mid-20th century. The second generation of ADCs addressed this issue through innovative antibody modification techniques, such as DAR regulation, amino acid substitutions, incorporation of non-natural amino acids, and enzymatic drug attachment. Currently, a third generation of ADCs is in development. This study presents an overview of 12 available ADCs, reviews 71 recent research papers, and analyzes 128 clinical trial reports. The overarching objective is to gain insights into the prevailing trends in ADC research and development, with a particular focus on emerging frontiers like potential targets, linkers, and drug payloads within the realm of cancer treatment.

Investigation of High Molecular Weight Size Variant Formation in Antibody-Drug Conjugates: Microbial Transglutaminase-Mediated Crosslinking

Microbial transglutaminase (mTG) has become a powerful tool for manufacturing antibody-drug conjugates (ADCs). It enables site-specific conjugation by catalyzing formation of stable isopeptide bond between glutamine (Q) side chain and primary amine. However, the downstream impact of mTG-mediated conjugation on ADC product quality, especially on high molecular weight (HMW) size variant formation has not been studied in a systematic manner. This study investigates the mechanisms underlying the formation of HMW size variants in mTG-mediated ADCs using size exclusion chromatography (SEC) and liquid chromatography-mass spectrometry (LC-MS). Our findings revealed that the mTG-mediated glutamine and lysine (K) crosslinking is the primary source of the increased level of HMW size variants in the ADCs. In the study, two monoclonal antibodies (mAbs) with glutamine engineered for site-specific conjugation were used as model systems. Based on the LC-MS analysis, a single lysine (K56) in the heavy chain (HC) was identified as the major Q-K crosslinking site in one of the two mAbs. The HC C-terminal K was observed to crosslink to the target Q in both mAbs. Quantitative correlation was established between the percentage of HMW size variants determined by SEC and the percentage of crosslinked peptides quantified by MS peptide mapping. Importantly, it was demonstrated that the level of HMW size variants in the second ADC was substantially reduced by the complete removal of HC C-terminal K before conjugation. The current work demonstrates that crosslinking and other side reactions during mTG-mediated conjugation needs to be carefully monitored and controlled to ensure process consistency and high product quality of the final ADC drug product.

Transglutaminase in Foods and Biotechnology

Stabilization and reusability of enzyme transglutaminase (TGM) are important goals for the enzymatic process since immobilizing TGM plays an important role in different technologies and industries. TGM can be used in many applications. In the food industry, it plays a role as a protein-modifying enzyme, while, in biotechnology and pharmaceutical applications, it is used in mediated bioconjugation due to its extraordinary crosslinking ability. TGMs (EC 2.3.2.13) are enzymes that catalyze the formation of a covalent bond between a free amino group of protein-bound or peptide-bound lysine, which acts as an acyl acceptor, and the γ-carboxamide group of protein-bound or peptide-bound glutamine, which acts as an acyl donor. This results in the modification of proteins through either intramolecular or intermolecular crosslinking, which improves the use of the respective proteins significantly.

The Dawn of a New Era: Targeting the "Undruggables" with Antibody-Based Therapeutics

The high selectivity and affinity of antibodies toward their antigens have made them a highly valuable tool in disease therapy, diagnosis, and basic research. A plethora of chemical and genetic approaches have been devised to make antibodies accessible to more "undruggable" targets and equipped with new functions of illustrating or regulating biological processes more precisely. In this Review, in addition to introducing how naked antibodies and various antibody conjugates (such as antibody-drug conjugates, antibody-oligonucleotide conjugates, antibody-enzyme conjugates, etc.) work in therapeutic applications, special attention has been paid to how chemistry tools have helped to optimize the therapeutic outcome (i.e., with enhanced efficacy and reduced side effects) or facilitate the multifunctionalization of antibodies, with a focus on emerging fields such as targeted protein degradation, real-time live-cell imaging, catalytic labeling or decaging with spatiotemporal control as well as the engagement of antibodies inside cells. With advances in modern chemistry and biotechnology, well-designed antibodies and their derivatives via size miniaturization or multifunctionalization together with efficient delivery systems have emerged, which have gradually improved our understanding of important biological processes and paved the way to pursue novel targets for potential treatments of various diseases.

Design of highly active substrates using molecular docking for microbial transglutaminase detection

The transglutaminase (TGase) family catalyzes a transamidation reaction between glutamine (Gln) and lysine (Lys) residues on protein substrates. Highly active substrates are important for cross-linking and modifying proteins of TGase. In the present work, high-activity substrates have been designed based on the principles of enzyme-substrate interaction, using microbial transglutaminase (mTGase) as a research model of the TGase family. Substrates with high activity were screened using a combination of molecular docking and traditional experiments. Twenty-four sets of peptide substrates all produced good catalytic activity with mTGase. FFKKAYAV as the acyl acceptor and VLQRAY as the acyl donor group had the best reaction efficiency with highly sensitive detection of 26 nM mTGase. In addition, the substrate grouping, KAYAV and AFQSAY, detected 130 nM mTGase under physiological conditions (37 °C, pH 7.4), producing 20-fold higher activity than the natural substrate, collagen. The experimental results confirmed the potential for design of high-activity substrates by a combination of molecular docking and traditional experiments under physiological conditions.

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.

Molecular crowding elicits the acceleration of enzymatic crosslinking of macromolecular substrates

Cytoplasm contains high concentrations of biomacromolecules. Protein behavior under such crowded conditions is reportedly different from that in an aqueous buffer solution, mainly owing to the effect of volume exclusion caused by the presence of macromolecules. Using a crosslinking reaction catalyzed by microbial transglutaminase (MTG) as a model, we herein systematically determined how the substrate size affects enzymatic activity in both dilute and crowded solutions of dextran. We first observed a threefold reduction in MTG-mediated crosslinking of a pair of small peptide substrates in 15 wt% dextran solution. In contrast, when proteinaceous substrates were involved, the crosslinking rates in 15 wt% dextran solutions accelerated markedly to levels comparable with the level in the absence of dextran. Our results provide new insights into the action of enzymes with regard to macromolecular substrates under crowded conditions, of which the potential utility was demonstrated by the formation of highly crosslinked protein polymers.

One-step synthesis of site-specific antibody-drug conjugates by reprograming IgG glycoengineering with LacNAc-based substrates

Glycosite-specific antibody‒drug conjugatess (gsADCs), harnessing Asn297 N-glycan of IgG Fc as the conjugation site for drug payloads, usually require multi-step glycoengineering with two or more enzymes, which limits the substrate diversification and complicates the preparation process. Herein, we report a series of novel disaccharide-based substrates, which reprogram the IgG glycoengineering to one-step synthesis of gsADCs, catalyzed by an endo-N-acetylglucosaminidase (ENGase) of Endo-S2. IgG glycoengineering via ENGases usually has two steps: deglycosylation by wild-type (WT) ENGases and transglycosylation by mutated ENGases. But in the current method, we have found that disaccharide LacNAc oxazoline can be efficiently assembled onto IgG by WT Endo-S2 without hydrolysis of the product, which enables the one-step glycoengineering directly from native antibodies. Further studies on substrate specificity revealed that this approach has excellent tolerance on various modification of 6-Gal motif of LacNAc. Within 1 h, one-step synthesis of gsADC was achieved using the LacNAc-toxin substrates including structures free of bioorthogonal groups. These gsADCs demonstrated good homogeneity, buffer stability, in vitro and in vivo anti-tumor activity. This work presents a novel strategy using LacNAc-based substrates to reprogram the multi-step IgG glycoengineering to a one-step manner for highly efficient synthesis of gsADCs.

Bivalent EGFR-Targeting DARPin-MMAE Conjugates

Epidermal growth factor receptor (EGFR) is a validated tumor marker overexpressed in various cancers such as squamous cell carcinoma (SSC) of the head and neck and gliomas. We constructed protein-drug conjugates based on the anti-EGFR Designed Ankyrin Repeat Protein (DARPin) E01, and compared the bivalent DARPin dimer (DD1) and a DARPin-Fc (DFc) to the monomeric DARPin (DM) and the antibody derived scFv425-Fc (scFvFc) in cell culture and a mouse model. The modular conjugation system, which was successfully applied for the preparation of protein-drug and -dye conjugates, uses bio-orthogonal protein-aldehyde generation by the formylglycine-generating enzyme (FGE). The generated carbonyl moiety is addressed by a bifunctional linker with a pyrazolone for a tandem Knoevenagel reaction and an azide for strain-promoted azide-alkyne cycloaddition (SPAAC). The latter reaction with a PEGylated linker containing a dibenzocyclooctyne (DBCO) for SPAAC and monomethyl auristatin E (MMAE) as the toxin provided the stable conjugates DD1-MMAE (drug-antibody ratio, DAR = 2.0) and DFc-MMAE (DAR = 4.0) with sub-nanomolar cytotoxicity against the human squamous carcinoma derived A431 cells. In vivo imaging of Alexa Fluor 647-dye conjugates in A431-xenografted mice bearing subcutaneous tumors as the SCC model revealed unspecific binding of bivalent DARPins to the ubiquitously expressed EGFR. Tumor-targeting was verified 6 h post-injection solely for DD1 and scFvFc. The total of four administrations of 6.5 mg/kg DD1-MMAE or DFc-MMAE twice weekly did not cause any sequela in mice. MMAE conjugates showed no significant anti-tumor efficacy in vivo, but a trend towards increased necrotic areas (p = 0.2213) was observed for the DD1-MMAE (n = 5).

General and Robust Chemoenzymatic Method for Glycan-Mediated Site-Specific Labeling and Conjugation of Antibodies: Facile Synthesis of Homogeneous Antibody-Drug Conjugates

Site-specific labeling and conjugation of antibodies are highly desirable for fundamental research and for developing more efficient diagnostic and therapeutic methods. We report here a general and robust chemoenzymatic method that permits a one-pot site-specific functionalization of antibodies. A series of selectively modified disaccharide oxazoline derivatives were designed, synthesized, and evaluated as donor substrates of different endoglycosidases for antibody Fc glycan remodeling. We found that among several endoglycosidases tested, wild-type endoglycosidase from Streptococcus pyogenes of serotype M49 (Endo-S2) exhibited remarkable activity in transferring the functionalized disaccharides carrying site-selectively modified azide, biotin, or fluorescent tags to antibodies without hydrolyzing the resulting transglycosylation products. This discovery, together with the excellent Fc deglycosylation activity of Endo-S2 on recombinant antibodies, allowed direct labeling and functionalization of antibodies in a one-pot manner without the need of intermediate and enzyme separation. The site-specific introduction of varied numbers of azide groups enabled a highly efficient synthesis of homogeneous antibody-drug conjugates (ADCs) with a precise control of the drug-to-antibody ratio (DAR) ranging from 2 to 12 via a copper-free strain-promoted click reaction. Cell viability assays showed that ADCs with higher DARs were more potent in killing antigen-overexpressed cells than the ADCs with lower DARs. This new method is expected to find applications not only for antibody-drug conjugation but also for cell labeling, imaging, and diagnosis.

Lipoate-acid ligase a modification of native antibody: Synthesis and conjugation site analysis

Bioconjugation is an important chemical biology research focus, especially in the development of methods to produce pharmaceutical bioconjugates and antibody-drug conjugates (ADCs). In this report, an enzyme-catalyzed conjugation method combined with a chemical reaction was used to modify a native antibody under mild reaction conditions. Our investigation revealed that lipoic-acid ligase (LplA) modifies native IgG1 with biased site-specificity. An intact mass analysis revealed that 98.3% of IgG1 was modified by LplA and possessed at least one molecule of octanocic acid. The average number of modifications per antibody was calculated to be 4.6. Peptide mapping analysis revealed that the modified residues were K225, K249 and K363 in the Fc region, and K30, K76 and K136 in the heavy chain and K39/K42, K169, K188 and K190 in the light chain of the Fab region. Careful evaluation including solvent exposed amino acid analysis suggested that these conjugate sites were not only solvent exposed but also biased by the site-specificity of LplA. Furthermore, antibody fragment conjugation may be able to take advantage of this enzymatic approach. This feasibility study serves as a demonstration for preparing enzymatically modified antibodies with conjugation site analysis.

Microbial transglutaminase nanoflowers as an alternative nanomedicine for breast cancer theranostics

Breast cancer is the most common malignancy among women. With the aim of decreasing the toxicity of conventional breast cancer treatments, an alternative that could provide appropriate and effective drug utilization was envisioned. Thus, we contemplated and compared the in vitro effects of microbial transglutaminase nanoflowers (MTGase NFs) on breast cancer cells (MCF-7). Transglutaminase is an important regulatory enzyme acting as a site-specific cross-linker for proteins. With the versatility of MTGase facilitating the nanoflower formation by acting as molecular glue, it was demonstrated to have anti-cancer properties. The rational drug design based on a transglutaminase enzyme-assisted approach led to the uniform shape of petals in these nanoflowers, which had the capacity to act directly as an anti-cancer drug. Herein, we report the anti-cancer characteristics portrayed by enzymatic MTGase NFs, which are biocompatible in nature. This study demonstrated the prognostic and therapeutic significance of MTGase NFs as a nano-drug in breast cancer treatment. The results on MCF-7 cells showed a significantly improved in vitro therapeutic efficacy. MTGase NFs were able to exhibit inhibitory effects on cell viability (IC₅₀-8.23 μg ml⁻¹) within 24 h of dosage. To further substantiate its superior anti-proliferative role, the clonogenic potential was measured to be 62.8%, along with migratory inhibition of cells (3.76-fold change). Drastic perturbations were induced (4.61-fold increase in G0/G1 phase arrest), pointed towards apoptotic induction with a 58.9% effect. These results validated the role of MTGase NFs possessing a cytotoxic nature in mitigating breast cancer. Thus, MTGase bestows distinct functionality towards therapeutic nano-modality, i.e., nanoflowers, which shows promise in cancer treatment.

Development of a novel therapeutic nano-modality in the form of enzymatic transglutaminase nanoflowers; endowed with anti-cancerous action against breast cancers.

Factors influencing the choice of monoclonal antibodies for antibody-drug conjugates

In antibody-drug conjugates (ADCs), monoclonal antibodies (mAbs) act as carriers for a cytotoxic payload providing the therapy with targeted action against cells expressing a target cell surface antigen. An appropriate choice of mAb is crucial to developing a successful ADC for clinical development. However, problems such as immunogenicity, poor pharmacokinetic (PK) and pharmacodynamic (PD) profiles and variable drug-antibody ratios (DARs) plague ADCs. In this review, we detail recent mAb-based innovations and factors that should be considered to overcome these problems to achieve a new generation of more effective ADC therapeutics.

Current approaches for the purification of antibody-drug conjugates

In the past two decades, antibody-drug conjugates have gained increasing attention because they expand the therapeutic index when compared with that of traditional chemotherapies. Antibody-drug conjugates are highly complex structures consisting of antibodies covalently conjugated with small-molecule cytotoxic drugs. The complex structure of antibody-drug conjugates makes chemistry, manufacturing, and control difficult. In contrast to antibody production, distinct purification methods following conjugation of antibodies with drug-linkers are required for the manufacturing. For process development of antibody drug conjugates, the drug-to-antibody ratio, free drug-linkers, and aggregates are critical quality attributes that must be strictly controlled and removed by appropriate purification techniques. In this review, features of various purification methods used to purify antibody drug conjugates are described and evaluated. The future landscape of the antibody-conjugates field is also discussed briefly.

One-Pot Conversion of Free Sialoglycans to Functionalized Glycan Oxazolines and Efficient Synthesis of Homogeneous Antibody-Drug Conjugates through Site-Specific Chemoenzymatic Glycan Remodeling

Antibody-drug conjugates (ADCs) are an important class of therapeutic agents that harness the highly specific antigen targeting property of antibodies to deliver toxic drugs for targeted cell killing. Site-specific conjugation methods are highly desirable for constructing homogeneous ADCs that possess a well-defined antibody-to-drug ratio, stability, ideal pharmacological profile, and optimal therapeutic index. We report here a facile synthesis of functionalized glycan oxazolines from free sialoglycans that are key donor substrates for enzymatic Fc glycan remodeling and the application of an efficient endoglycosidase mutant (Endo-S2 D184M) for site-specific glycan transfer to construct homogeneous ADCs. We found that by a sequential use of two coupling reagents under optimized conditions, free sialoglycans could be efficiently converted to selectively functionalized glycan oxazolines carrying azide-, cyclopropene-, and norbornene-tags, respectively, in excellent yield and in a simple one-pot manner. We further demonstrated that the recently reported Endo-S2 D184 M mutant was highly efficient for Fc glycan remodeling with the selectively modified glycan oxazolines to introduce tags into an antibody, which required a significantly smaller amount of glycan oxazolines and a much shorter reaction time than that of the Endo-S D233Q-catalyzed reaction, thus minimizing the side reactions. Finally homogeneous ADCs were constructed with three different click reactions. The resulting ADCs showed excellent serum stability, and in vitro cytotoxicity assays indicated that all the three ADCs generated from the distinct click reactions possessed potent and comparable cytotoxicity for targeted cancer cell killing.

Recent advances in FRET-Based biosensors for biomedical applications

Fluorescence resonance energy transfer (FRET)-based biosensors are effective analytical tools extensively used in fields of biomedicine, pharmacology, toxicology, and food sciences. Ratiometric imaging of substantial cellular processes, molecular components, and biological interactions is widely performed by these biosensors. A variety of FRET-based biosensors have provided comprehensive insights into underlying mechanisms of pathological conditions in live cells, tissues, and organisms. Moreover, integration of FRET-based biosensors with the current bioanalytical techniques allows for accurate, rapid, and sensitive diagnosis and proposes the advanced strategies for treatment. Precise analysis of ligand-receptor interactions by FRET-based biosensors has presented a basis for determination of novel therapeutic agents. Therefore, this study was designed to review the recent developments in FRET-based biosensors and their biomedical applications. In addition, characteristics, challenges, and outlooks of these biosensors were discussed.

DogCatcher allows loop-friendly protein-protein ligation

There are many efficient ways to connect proteins at termini. However, connecting at a loop is difficult because of lower flexibility and variable environment. Here, we have developed DogCatcher, a protein that forms a spontaneous isopeptide bond with DogTag peptide. DogTag/DogCatcher was generated initially by splitting a Streptococcus pneumoniae adhesin. We optimized DogTag/DogCatcher through rational design and evolution, increasing reaction rate by 250-fold and establishing millimolar solubility of DogCatcher. When fused to a protein terminus, DogTag/DogCatcher reacts slower than SpyTag003/SpyCatcher003. However, inserted in loops of a fluorescent protein or enzyme, DogTag reacts much faster than SpyTag003. Like many membrane proteins, the ion channel TRPC5 has no surface-exposed termini. DogTag in a TRPC5 extracellular loop allowed normal calcium flux and specific covalent labeling on cells in 1 min. DogTag/DogCatcher reacts under diverse conditions, at nanomolar concentrations, and to 98% conversion. Loop-friendly ligation should expand the toolbox for creating protein architectures.

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Spontaneous transamidation at internal sites harnessing a DogTag/DogCatcher pair

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DogCatcher is designed and bred for high solubility and rapid reaction

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Within protein loops DogTag can clamp on its partner faster than SpyTag003

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Fast and faithful fluorescent labeling of an ion channel at the cell surface via DogTag

Approaching transglutaminase from Streptomyces bacteria over three decades

Transglutaminases are protein cross-linking and protein-modifying enzymes that have attracted considerable interest due to their causal involvement in various diseases and versatility in industrial applications. In particular, microbial transglutaminases (MTG) from Streptomyces bacteria have managed in recent years to evolve from simple food additives to specialized enzymes for the site-directed modification of therapeutic proteins. The review summarizes relevant studies from the beginning dealing with the occurrence, production, structure, catalysis, and substrate molecules of MTG enzymes. It also addresses biotechnological procedures with MTG from S. mobaraensis (SmMTG) as the most prominent representative in focus. Reassessment of the available data revealed unexpected insights into catalysis of SmMTG and other transglutaminases, suggesting selection of glutamine donor proteins by subsites at the front vestibule and the existence of distinct lysine pockets. Flexibility of the SmMTG-accessible glutamine donor substrate regions seems to be more important than the glutamine environment. Nevertheless, residues in close vicinity to glutamines also determine interaction with the SmMTG subsites. The apparent lack of subsites for lysine donor proteins suggests self-assembly of the substrate proteins prior to enzymatic cross-linking. The study of natural substrate proteins, especially their mutual interaction, is proposed to further illuminate catalysis of SmMTG. To this end, structure and function of the characterized substrate proteins from S. mobaraensis are discussed in conclusion.

Enzymatic Methods for the Site-Specific Radiolabeling of Targeting Proteins

Site-specific conjugation of proteins is currently required to produce homogenous derivatives for medicine applications. Proteins derivatized at specific positions of the polypeptide chain can actually show higher stability, superior pharmacokinetics, and activity in vivo, as compared with conjugates modified at heterogeneous sites. Moreover, they can be better characterized regarding the composition of the derivatization sites as well as the conformational and activity properties. To this aim, several site-specific derivatization approaches have been developed. Among these, enzymes are powerful tools that efficiently allow the generation of homogenous protein-drug conjugates under physiological conditions, thus preserving their native structure and activity. This review will summarize the progress made over the last decade on the use of enzymatic-based methodologies for the production of site-specific labeled immunoconjugates of interest for nuclear medicine. Enzymes used in this field, including microbial transglutaminase, sortase, galactosyltransferase, and lipoic acid ligase, will be overviewed and their recent applications in the radiopharmaceutical field will be described. Since nuclear medicine can benefit greatly from the production of homogenous derivatives, we hope that this review will aid the use of enzymes for the development of better radio-conjugates for diagnostic and therapeutic purposes.

Site-selective modification strategies in antibody-drug conjugates

Antibody-drug conjugates (ADCs) harness the highly specific targeting capabilities of an antibody to deliver a cytotoxic payload to specific cell types. They have garnered widespread interest in drug discovery, particularly in oncology, as discrimination between healthy and malignant tissues or cells can be achieved. Nine ADCs have received approval from the US Food and Drug Administration and more than 80 others are currently undergoing clinical investigations for a range of solid tumours and haematological malignancies. Extensive research over the past decade has highlighted the critical nature of the linkage strategy adopted to attach the payload to the antibody. Whilst early generation ADCs were primarily synthesised as heterogeneous mixtures, these were found to have sub-optimal pharmacokinetics, stability, tolerability and/or efficacy. Efforts have now shifted towards generating homogeneous constructs with precise drug loading and predetermined, controlled sites of attachment. Homogeneous ADCs have repeatedly demonstrated superior overall pharmacological profiles compared to their heterogeneous counterparts. A wide range of methods have been developed in the pursuit of homogeneity, comprising chemical or enzymatic methods or a combination thereof to afford precise modification of specific amino acid or sugar residues. In this review, we discuss advances in chemical and enzymatic methods for site-specific antibody modification that result in the generation of homogeneous ADCs.

Site-specific conjugation of native antibody

Traditionally, non-specific chemical conjugations, such as acylation of amines on lysine or alkylation of thiols on cysteines, are widely used; however, they have several shortcomings. First, the lack of site-specificity results in heterogeneous products and irreproducible processes. Second, potential modifications near the complementarity-determining region may reduce binding affinity and specificity. Conversely, site-specific methods produce well-defined and more homogenous antibody conjugates, ensuring developability and clinical applications. Moreover, several recent side-by-side comparisons of site-specific and stochastic methods have demonstrated that site-specific approaches are more likely to achieve their desired properties and functions, such as increased plasma stability, less variability in dose-dependent studies (particularly at low concentrations), enhanced binding efficiency, as well as increased tumor uptake. Herein, we review several standard and practical site-specific bioconjugation methods for native antibodies, i.e., those without recombinant engineering. First, chemo-enzymatic techniques, namely transglutaminase (TGase)-mediated transamidation of a conserved glutamine residue and glycan remodeling of a conserved asparagine N-glycan (GlyCLICK), both in the Fc region. Second, chemical approaches such as selective reduction of disulfides (ThioBridge) and N-terminal amine modifications. Furthermore, we list site-specific antibody–drug conjugates in clinical trials along with the future perspectives of these site-specific methods.

Bifunctional Reagents for Formylglycine Conjugation: Pitfalls and Breakthroughs

Formylglycine-generating enzymes specifically oxidize cysteine within the consensus sequence CxPxR to Cα -formylglycine (FGly). This noncanonical electrophilic amino acid can subsequently be addressed selectively by bioorthogonal hydrazino-iso-Pictet-Spengler (HIPS) or Knoevenagel ligation to attach payloads like fluorophores or drugs to proteins to obtain a defined payload-to-protein ratio. However, the disadvantages of these conjugation techniques include the need for a large excess of conjugation building block, comparably low reaction rates and limited stability of FGly-containing proteins. Therefore, functionalized clickable HIPS and tandem Knoevenagel building blocks were synthesized, conjugated to small proteins (DARPins) and subsequently linked to strained alkyne-containing payloads for protein labeling. This procedure allowed the selective bioconjugation of one or two DBCO-carrying payloads with nearly stoichiometric amounts at low concentrations. Furthermore, an azide-modified tandem Knoevenagel building block enabled the synthesis of branched PEG linkers and the conjugation of two fluorophores, resulting in an improved signal-to-noise ratio in live-cell fluorescence-imaging experiments targeting the EGF receptor.

Transforming nanobodies into high-precision tools for protein function analysis

Single-domain antibodies, derived from camelid heavy antibodies (nanobodies) or shark variable new antigen receptors, have attracted increasing attention in recent years due to their extremely versatile nature and the opportunities they offer for downstream modification. Discovered more than three decades ago, these 120-amino acid (∼15-kDa) antibody fragments are known to bind their target with high specificity and affinity. Key features of nanobodies that make them very attractive include their single-domain nature, small size, and affordable high-level expression in prokaryotes, and their cDNAs are routinely obtained in the process of their isolation. This facilitates and stimulates new experimental approaches. Hence, it allows researchers to formulate new answers to complex biomedical questions. Through elementary PCR-based technologies and chemical modification strategies, their primary structure can be altered almost at leisure while retaining their specificity and biological activity, transforming them into highly tailored tools that meet the increasing demands of current-day biomedical research. In this review, various aspects of camelid nanobodies are expounded, including intracellular delivery in recombinant format for manipulation of, i.e., cytoplasmic targets, their derivatization to improve nanobody orientation as a capturing device, approaches to reversibly bind their target, their potential as protein-silencing devices in cells, the development of strategies to transfer nanobodies through the blood-brain barrier and their application in CAR-T experimentation. We also discuss some of their disadvantages and conclude with future prospects.

An overview of process development for antibody-drug conjugates produced by chemical conjugation technology

Introduction: We discuss chemical conjugation strategies for antibody-drug conjugates (ADCs) from an industrial perspective and compare three promising chemical conjugation technologies to produce site-specific ADCs.Areas covered: Currently, nine ADCs are commercially approved and all are produced by chemical conjugation technology. However, seven of these ADCs contain a relatively broad drug distribution, potentially limiting their therapeutic indices. In 2019, the first site-specific ADC was launched on the market by Daiichi-Sankyo. This achievement, and an analysis of clinical trials over the last decade, indicates that current industrial interest in the ADC field is shifting toward site-specific conjugation technologies. From an industrial point of view, we aim to provide guidance regarding established conjugation methodologies that have already been applied to scale-up stages. With an emphasis on highly productive, scalable, and synthetic process robustness, conjugation methodologies for ADC production is discussed herein.Expert opinion: All three chemical conjugation technologies described in this review have various advantages and disadvantages, therefore drug developers can utilize these depending on their biological and/or protein targets. The future landscape of the ADC field is also discussed.

Glutamine-walking: Creating reactive substrates for transglutaminase-mediated protein labeling

Chemically modified proteins are increasingly being tested and approved as therapeutic products. Batch-to-batch homogeneity is crucial to ensure safety and quality of therapeutic products. Highly selective protein modification may be achieved using enzymatic routes. Microbial transglutaminase (mTG) is a robust, easy to use and well-established enzyme that is used at a very large scale in the food industry such that its efficacy and its safety for human consumption are well established. In the context of therapeutic protein modification, mTG should crosslink one or more glutamines on the target protein with an aminated moiety such as a solubilizer, a tracer or a cytotoxic moiety. mTG has the advantage of being unreactive toward the majority of surface-exposed glutamines on most proteins, reducing sample heterogeneity. The caveat is that there may be no reactive glutamine on the target protein, or else a reactive glutamine may be found in a location where its modification compromises function of the target protein. Here we describe the glutamine-walk (Gln-walk), a straightforward method to create a glutamine-substrate site that is reactive to mTG in a target protein. Iterative substitution of single amino acids to a glutamine is followed by facile identification of reactivity with mTG, where covalent labeling of the target with an aminated fluorophore allows visualization of the most reactive modified targets. The approach is empirical; knowledge of the target protein structure and functional regions facilitates application of the method.

Recombinant production of active microbial transglutaminase in E. coli by using self-cleavable zymogen with mutated propeptide

Microbial transglutaminase from Streptomyces mobaraensis (MTG) has been widely used in food industry and also in research and medical applications, since it can site-specifically modify proteins by the cross-linking reaction of glutamine residue and the primary amino group. The recombinant expression system of MTG in E. coli provides better accessibility for the researchers and thus can promote further utilization of MTG. Herein, we report production of active and soluble MTG in E. coli by using a chimeric protein of tobacco etch virus (TEV) protease and MTG zymogen. A chimera of TEV protease and MTG zymogen with native propeptide resulted in active MTG contaminated with cleaved propeptide due to the strong interaction between the propeptide and catalytic domain of MTG. Introduction of mutations of K9R and Y11A to the propeptide facilitated dissociation of the cleaved propeptide from the catalytic domain of MTG and active MTG without any contamination of the propeptide was obtained. The specific activity of the active MTG was 22.7 ± 2.6 U/mg. The successful expression and purification of active MTG by using the chimera protein of TEV protease and MTG zymogen with mutations in the propeptide can advance the use of MTG and the researches using MTG mediated cross-linking reactions.