Site-Selective Antibody Conjugation with Dibromopyrazines

In recent years, antibody conjugates have evolved as state-of-the-art options for diagnostic and therapeutic applications. During site-selective antibody conjugation, incomplete rebridging of antibody chains limits the homogeneity of conjugates and calls for the development of new rebridging agents. Herein, we report a dibromopyrazine derivative optimized to reach highly homogeneous conjugates rapidly and with high conversion on rebridging of trastuzumab, even providing a feasible route for antibody modification in acidic conditions. Furthermore, coupling a fluorescent dye and a cytotoxic drug resulted in effective antibody conjugates with excellent serum stability and in vitro selectivity, demonstrating the utility of the dibromopyrazine rebridging agent to produce on-demand future antibody conjugates for diagnostic or therapeutic applications.

Recent Advances in Targeted Cancer Therapy: Are PDCs the Next Generation of ADCs?

Antibody-drug conjugates (ADCs) comprise antibodies, cytotoxic payloads, and linkers, which can integrate the advantages of antibodies and small molecule drugs to achieve targeted cancer treatment. However, ADCs also have some shortcomings, such as non-negligible drug resistance, a low therapeutic index, and payload-related toxicity. Many studies have focused on changing the composition of ADCs, and some have even further extended the concept and types of targeted conjugated drugs by replacing the targeted antibodies in ADCs with peptides, revolutionarily introducing peptide-drug conjugates (PDCs). This Perspective summarizes the current research status of ADCs and PDCs and highlights the structural innovations of ADC components. In particular, PDCs are regarded as the next generation of potential targeted drugs after ADCs, and the current challenges of PDCs are analyzed. Our aim is to offer fresh insights for the efficient design and expedited development of innovative targeted conjugated drugs.

The Nitrile Bis-Thiol Bioconjugation Reaction

Electron-poor aryl nitriles are promising reagents for bioconjugation due to their high electrophilicity and selectivity for reaction with thiols, albeit generally in a reversible manner. A transient species has previously been observed in such reactions, involving the addition of two thiols to the nitrile functional group, forming a tetrahedral amino dithioacetal (ADTA). In this work, the reaction of heteroaryl nitriles with bis-thiols is explored in an attempt to generate stable ADTAs, which could facilitate new bioconjugation protocols. By use of a 1,2-dithiol, or the incorporation of an electrophilic trap into the aryl nitrile design, the formation of stable products is achieved. The resultant "nitrile bis-thiol" (NBT) reaction is then explored in the context of protein modification, specifically to carry out antibody conjugation. By addition of these nitriles to the reduced disulfide bond of an antibody fragment, it is shown that, depending on the reagent design, cysteine-to-lysine transfer or disulfide bridged NBT products can be generated. Both represent site-selective conjugates and are shown to be stable when challenged with glutathione under physiological conditions and upon incubation in serum. Furthermore, the NBT reaction is tested in the more challenging context of a full antibody, and all four disulfide bonds are effectively modified by these new one-carbon bridging reagents. Overall, this reaction of heteroaryl-nitriles with bis-thiols is shown to be highly efficient and versatile, of tunable reversibility, and offers enticing prospects as a new addition to the toolbox of biocompatible "click"-type reactions.

Chemical technology principles for selective bioconjugation of proteins and antibodies

Proteins are multifunctional large organic compounds that constitute an essential component of a living system. Hence, control over their bioconjugation impacts science at the chemistry-biology-medicine interface. A chemical toolbox for their precision engineering can boost healthcare and open a gateway for directed or precision therapeutics. Such a chemical toolbox remained elusive for a long time due to the complexity presented by the large pool of functional groups. The precise single-site modification of a protein requires a method to address a combination of selectivity attributes. This review focuses on guiding principles that can segregate them to simplify the task for a chemical method. Such a disintegration systematically employs a multi-step chemical transformation to deconvolute the selectivity challenges. It constitutes a disintegrate (DIN) theory that offers additional control parameters for tuning precision in protein bioconjugation. This review outlines the selectivity hurdles faced by chemical methods. It elaborates on the developments in the perspective of DIN theory to demonstrate simultaneous regulation of reactivity, chemoselectivity, site-selectivity, modularity, residue specificity, and protein specificity. It discusses the progress of such methods to construct protein and antibody conjugates for biologics, including antibody-fluorophore and antibody-drug conjugates (AFCs and ADCs). It also briefs how this knowledge can assist in developing small molecule-based covalent inhibitors. In the process, it highlights an opportunity for hypothesis-driven routes to accelerate discoveries of selective methods and establish new targetome in the precision engineering of proteins and antibodies.

Trifunctional Dibromomaleimide Reagents Built Around A Lysine Scaffold Deliver Site-selective Dual-modality Antibody Conjugation

We describe the synthesis and application of a selection of trifunctional reagents for the dual-modality modification of native, solvent accessible disulfide bonds in trastuzumab. The reagents were developed from the dibromomaleimide (DBM) platform with two orthogonal clickable functional groups built around a lysine core. We also describe the development of an aryl diselenide additive which enables antibody disulfide reduction in 4 minutes and a rapid overall reduction-bridging-double click sequence.

Compact hydrophilic electrophiles enable highly efficacious high DAR ADCs with excellent in vivo PK profile

The recent success of antibody-drug conjugates (ADC), exemplified by seven new FDA-approvals within three years, has led to increased attention for antibody based targeted therapeutics and fueled efforts to develop new drug-linker technologies for improved next generation ADCs. We present a highly efficient phosphonamidate-based conjugation handle that combines a discrete hydrophilic PEG-substituent, an established linker-payload and a cysteine-selective electrophile in one compact building block. This reactive entity provides homogeneous ADCs with a high drug-to-antibody ratio (DAR) of 8 in a one-pot reduction and alkylation protocol from non-engineered antibodies. The compact branched PEG-architecture introduces hydrophilicity without increasing the distance between antibody and payload, allowing the generation of the first homogeneous DAR 8 ADC from VC-PAB-MMAE without increased in vivo clearance rates. This high DAR ADC exhibits excellent in vivo stability and increased antitumor activity in tumour xenograft models relative to the established FDA approved VC-PAB-MMAE ADC Adcetris, clearly showing the benefit of the phosphonamidate based building-blocks as a general tool for the efficient and stable antibody-based delivery of highly hydrophobic linker-payload systems.

Antibody-Antimicrobial Conjugates for Combating Antibiotic Resistance

As the development of new antibiotics lags far behind the emergence of drug-resistant bacteria, alternative strategies to resolve this dilemma are urgently required. Antibody-drug conjugate is a promising therapeutic platform to delivering cytotoxic payloads precisely to target cells for efficient disease treatment. Antibody-antimicrobial conjugates (AACs) have recently attracted considerable interest from researchers as they can target bacteria in the target sites and improve the effectiveness of drugs (i.e., reduced drug dosage and adverse effects), abating the upsurge of antimicrobial resistance. In this review, the selection and progress of three essential blocks that compose the AACs: antibodies, antimicrobial payloads, and linkers are discussed. The commonly used conjugation strategies and the latest applications of AACs in recent years are also summarized. The challenges and opportunities of this booming technology are also discussed at the end of this review.

Dual reactivity disulfide bridging reagents; enabling new approaches to antibody fragment bioconjugation

Disulfide bridging, also known as disulfide stapling, is a powerful strategy for the construction of site-selective protein bioconjugates. Here we describe the first examples of a new class of such reagents, containing a 'stable-labile' design. These dual-reactive reagents are designed to form a stable bond to one cysteine and a labile bond to the second; resulting in a robust attachment to the protein with one end of the bridge, whilst the other end serves as a reactive handle for subsequent bioconjugation. By incorporating thioesters into these bridges, we demonstrate that they are primed for native chemical ligation (NCL) with N-terminal cysteines; offering an alternative to the requirement for C-terminal thioesters for use in such ligations. Alternatively, the use of hydrazine as the ligating nucleophile enables a separate cargo to be attached to each cysteine residue, which are exploited to insert variably cleavable linkers. These methodologies are demonstrated on an antibody fragment, and serve to expand the scope of disulfide bridging strategies whilst offering a convenient route to the construction of multifunctional antibody fragment conjugates.

Enabling the next steps in cancer immunotherapy: from antibody-based bispecifics to multispecifics, with an evolving role for bioconjugation chemistry

In the past two decades, immunotherapy has established itself as one of the leading strategies for cancer treatment, as illustrated by the exponentially growing number of related clinical trials. This trend was, in part, prompted by the clinical success of both immune checkpoint modulation and immune cell engagement, to restore and/or stimulate the patient's immune system's ability to fight the disease. These strategies were sustained by progress in bispecific antibody production. However, despite the decisive progress made in the treatment of cancer, toxicity and resistance are still observed in some cases. In this review, we initially provide an overview of the monoclonal and bispecific antibodies developed with the objective of restoring immune system functions to treat cancer (cancer immunotherapy), through immune checkpoint modulation, immune cell engagement or a combination of both. Their production, design strategy and impact on the clinical trial landscape are also addressed. In the second part, the concept of multispecific antibody formats, notably MuTICEMs (Multispecific Targeted Immune Cell Engagers & Modulators), as a possible answer to current immunotherapy limitations is investigated. We believe it could be the next step to take for cancer immunotherapy research and expose why bioconjugation chemistry might play a key role in these future developments.

Divinylpyrimidine reagents generate antibody-drug conjugates with excellent in vivo efficacy and tolerability

The development of divinylpyrimidine (DVP) reagents for the synthesis of antibody-drug conjugates (ADCs) with in vivo efficacy and tolerability is reported. Detailed structural characterisation of the synthesised ADCs was first conducted followed by in vitro and in vivo evaluation of the ADCs' ability to safely and selectively eradicate target-positive tumours.

An overview of chemo- and site-selectivity aspects in the chemical conjugation of proteins

The bioconjugation of proteins-that is, the creation of a covalent link between a protein and any other molecule-has been studied for decades, partly because of the numerous applications of protein conjugates, but also due to the technical challenge it represents. Indeed, proteins possess inner physico-chemical properties-they are sensitive and polynucleophilic macromolecules-that make them complex substrates in conjugation reactions. This complexity arises from the mild conditions imposed by their sensitivity but also from selectivity issues, viz the precise control of the conjugation site on the protein. After decades of research, strategies and reagents have been developed to address two aspects of this selectivity: chemoselectivity-harnessing the reacting chemical functionality-and site-selectivity-controlling the reacting amino acid residue-most notably thanks to the participation of synthetic chemistry in this effort. This review offers an overview of these chemical bioconjugation strategies, insisting on those employing native proteins as substrates, and shows that the field is active and exciting, especially for synthetic chemists seeking new challenges.

Protein Macrocyclization for Tertiary Structure Stabilization

Proteins possess unique molecular recognition capabilities and enzymatic activities, features that are usually tied to a particular tertiary structure. To make use of proteins for biotechnological and biomedical purposes, it is often required to enforce their tertiary structure in order to ensure sufficient stability under the conditions inherent to the application of interest. The introduction of intramolecular crosslinks has proven efficient in stabilizing native protein folds. Herein, we give an overview of methods that allow the macrocyclization of expressed proteins, discussing involved reaction mechanisms and structural implications.

Leucine-rich alpha-2-glycoprotein 1 (LRG1) as a novel ADC target

Leucine-rich alpha-2-glycoprotein 1 (LRG1) is present abundantly in the microenvironment of many tumours where it contributes to vascular dysfunction, which impedes the delivery of therapeutics. In this work we demonstrate that LRG1 is predominantly a non-internalising protein. We report the development of a novel antibody-drug conjugate (ADC) comprising the anti-LRG1 hinge-stabilised IgG4 monoclonal antibody Magacizumab coupled to the anti-mitotic payload monomethyl auristatin E (MMAE) via a cleavable dipeptide linker using the site-selective disulfide rebridging dibromopyridazinedione (diBrPD) scaffold. It is demonstrated that this ADC retains binding post-modification, is stable in serum and effective in in vitro cell studies. We show that the extracellular LRG1-targeting ADC provides an increase in survival in vivo when compared against antibody alone and similar anti-tumour activity when compared against standard chemotherapy, but without undesired side-effects. LRG1 targeting through this ADC presents a novel and effective proof-of-concept en route to improving the efficacy of cancer therapeutics.

New Bifunctional Chelators Incorporating Dibromomaleimide Groups for Radiolabeling of Antibodies with Positron Emission Tomography Imaging Radioisotopes

Positron Emission Tomography (PET) imaging with antibody-based contrast agents frequently uses the radioisotopes [64Cu]Cu2+ and [89Zr]Zr4+. The macrobicyclic chelator commonly known as sarcophagine (sar) is ideal for labeling receptor-targeted biomolecules with [64Cu]Cu2+. The siderophore chelator, desferrioxamine-B (dfo), has been widely used to incorporate [89Zr]Zr4+ into antibodies. Here, we describe new bifunctional chelators of sar and dfo: these chelators have been functionalized with dibromomaleimides (dbm), that enable site-specific and highly stable attachment of molecular cargoes to reduced, solvent-accessible, interstrand native disulfide groups. The new sar-dbm and dfo-dbm derivatives can be easily conjugated with the IgG antibody trastuzumab via reaction with reduced interstrand disulfide groups to give site-specifically modified dithiomaleamic acid (dtm) conjugates, sar-dtm-trastuzumab and dfo-dtm-trastuzumab, in which interstrand disulfides are rebridged covalently with a small molecule linker. Both sar- and dfo-dtm-trastuzumab conjugates have been radiolabeled with [64Cu]Cu2+ and [89Zr]Zr4+, respectively, in near quantitative radiochemical yield (>99%). Serum stability studies, in vivo PET imaging, and biodistribution analyses using these radiolabeled immunoconjugates demonstrate that both [64Cu]Cu-sar-dtm-trastuzumab and [89Zr]Zr-dfo-dtm-trastuzumab possess high stability in biological milieu. Dibromomaleimide technology can be easily applied to enable stable, site-specific attachment of radiolabeled chelators, such as sar and dfo, to native interstrand disulfide regions of antibodies, enabling tracking of antibodies with PET imaging.

Rapid and robust cysteine bioconjugation with vinylheteroarenes

Methods for residue-selective and stable modification of canonical amino acids enable the installation of distinct functionality which can aid in the interrogation of biological processes or the generation of new therapeutic modalities. Herein, we report an extensive investigation of reactivity and stability profiles for a series of vinylheteroarene motifs. Studies on small molecule and protein substrates identified an optimum vinylheteroarene scaffold for selective cysteine modification. Utilisation of this lead linker to modify a number of protein substrates with various functionalities, including the synthesis of a homogeneous, stable and biologically active antibody-drug conjugate (ADC) was then achieved. The reagent was also efficient in labelling proteome-wide cysteines in cell lysates. The efficiency and selectivity of these reagents as well as the stability of the products makes them suitable for the generation of biotherapeutics or studies in chemical biology.

The Chemistry Behind ADCs

Combining the selective targeting of tumor cells through antigen-directed recognition and potent cell-killing by cytotoxic payloads, antibody-drug conjugates (ADCs) have emerged in recent years as an efficient therapeutic approach for the treatment of various cancers. Besides a number of approved drugs already on the market, there is a formidable follow-up of ADC candidates in clinical development. While selection of the appropriate antibody (A) and drug payload (D) is dictated by the pharmacology of the targeted disease, one has a broader choice of the conjugating linker (C). In the present paper, we review the chemistry of ADCs with a particular emphasis on the medicinal chemistry perspective, focusing on the chemical methods that enable the efficient assembly of the ADC from its three components and the controlled release of the drug payload.

Silicon-based fluorescent platforms for copper(ii) detection in water

The potential of silicon-based fluorescent platforms for the detection of trace toxic metal ions was investigated in an aqueous environment. To this aim, silicon chips were first functionalized with amino groups, and fluorescein organic dyes, used as sensing molecules, were then covalently linked to the surface via formation of thiourea groups. The obtained hybrid heterostructures exhibited high sensitivity and selectivity towards copper(ii), a limit of detection compatible with the recommended upper limits for copper in drinking water, and good reversibility using a standard metal-chelating agent. The fluorophore-analyte interaction mechanism at the basis of the reported fluorescence quenching, as well as the potential of performance improvement, were also studied. The herein presented sensing architecture allows, in principle, tailoring of the selectivity towards other metal ions by proper fluorophore selection, and provides a favorable outlook for integration of fluorescent chemosensors with silicon photonics technology.

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.

Current strategies for the discovery and bioconjugation of smaller, targetable drug conjugates tailored for solid tumor therapy

Introduction: Antibody-Drug Conjugates (ADCs) have undergone a recent resurgence with 5 product approvals over the last 2 years but for those close to the field, it's been repeated cycles of setbacks and new innovations. A new wave of innovation is in the type of format used to deliver the cytotoxic payloads, with smaller bio-molecules being designed to have more optimal penetration and elimination properties tailored for solid tumors.Areas covered: In this review, the authors cover many of the recently described smaller-format drug conjugates (including formats such as diabodies, Fabs, scFvs, domain antibodies) with an emphasis on the types of conjugation technologies used to attach the chemical linker-payload.Expert opinion: Smaller formats are highly influenced by the structure of the linker-payload, arguably more-so than larger ADCs, so careful consideration is needed where solublising and pharmacokinetic modulation is required. High-quality conjugates are being developed with in vivo tumor efficacy and tolerability properties competitive with ADCs and with a few formats already in clinical development, we expect the pipeline to expand and to reach the market.