Homogeneous multi-payload antibody-drug conjugates

Many systemic cancer chemotherapies comprise a combination of drugs, yet all clinically used antibody-drug conjugates (ADCs) contain a single-drug payload. These combination regimens improve treatment outcomes by producing synergistic anticancer effects and slowing the development of drug-resistant cell populations. In an attempt to replicate these regimens and improve the efficacy of targeted therapy, the field of ADCs has moved towards developing techniques that allow for multiple unique payloads to be attached to a single antibody molecule with high homogeneity. However, the methods for generating such constructs-homogeneous multi-payload ADCs-are both numerous and complex owing to the plethora of reactive functional groups that make up the surface of an antibody. Here, by summarizing and comparing the methods of both single- and multi-payload ADC generation and their key preclinical and clinical results, we provide a timely overview of this relatively new area of research. The methods discussed range from branched linker installation to the incorporation of unnatural amino acids, with a generalized comparison tool of the most promising modification strategies also provided. Finally, the successes and challenges of this rapidly growing field are critically evaluated, and from this, future areas of research and development are proposed.

Chemical Production of Cytotoxic Bispecific Antibodies Using the Ugi Multicomponent Reaction

Bispecific antibodies (bsAbs) have recently emerged as a promising platform for the treatment of several conditions, most importantly cancer. Based on the combination of two different antigen-binding motifs in a single macromolecule; bsAbs can either display the combined characteristics of their parent antibodies, or new therapeutic features, inaccessible by the sole combination of two distinct antibodies. While bsAbs are traditionally produced by molecular biology techniques, the chemical development of bsAbs holds great promises and strategies have just begun to surface. In this context, we took advantage of a chemical strategy based on the use of the Ugi reaction for the site-selective conjugation of whole antibodies and coupled the resulting conjugates in a bioorthogonal manner with Fab fragments, derived from various antibodies. We thus managed to produce five different bsAbs with 2 : 1 valency, with yields ranging from 20 % to 48 %, and showed that the affinity of the parent antibody was preserved in all bsAbs. We further demonstrated the interest of our strategy by producing two other bsAbs behaving as cytotoxic T cell engagers with IC50 values in the picomolar range in vitro.

Using atomistic solution scattering modelling to elucidate the role of the Fc glycans in human IgG4

Human immunoglobulin G (IgG) exists as four subclasses IgG1-4, each of which has two Fab subunits joined by two hinges to a Fc subunit. IgG4 has the shortest hinge with 12 residues. The Fc subunit has two glycan chains, but the importance of glycosylation is not fully understood in IgG4. Here, to evaluate the stability and structure of non-glycosylated IgG4, we performed a multidisciplinary structural study of glycosylated and deglycosylated human IgG4 A33 for comparison with our similar study of human IgG1 A33. After deglycosylation, IgG4 was found to be monomeric by analytical ultracentrifugation; its sedimentation coefficient of 6.52 S was reduced by 0.27 S in reflection of its lower mass. X-ray and neutron solution scattering showed that the overall Guinier radius of gyration RG and its cross-sectional values after deglycosylation were almost unchanged. In the P(r) distance distribution curves, the two M1 and M2 peaks that monitor the two most common distances within IgG4 were unchanged following deglycosylation. Further insight from Monte Carlo simulations for glycosylated and deglycosylated IgG4 came from 111,382 and 117,135 possible structures respectively. Their comparison to the X-ray and neutron scattering curves identified several hundred best-fit models for both forms of IgG4. Principal component analyses showed that glycosylated and deglycosylated IgG4 exhibited different conformations from each other. Within the constraint of unchanged RG and M1-M2 values, the glycosylated IgG4 models showed more restricted Fc conformations compared to deglycosylated IgG4, but no other changes. Kratky plots supported this interpretation of greater disorder upon deglycosylation, also observed in IgG1. Overall, these more variable Fc conformations may demonstrate a generalisable impact of deglycosylation on Fc structures, but with no large conformational changes in IgG4 unlike those seen in IgG1.

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.

Structural Determinants of Stimuli-Responsiveness in Amphiphilic Macromolecular Nano-assemblies

Stimuli-responsive nano-assemblies from amphiphilic macromolecules could undergo controlled structural transformations and generate diverse macroscopic phenomenon under stimuli. Due to the controllable responsiveness, they have been applied for broad material and biomedical applications, such as biologics delivery, sensing, imaging, and catalysis. Understanding the mechanisms of the assembly-disassembly processes and structural determinants behind the responsive properties is fundamentally important for designing the next generation of nano-assemblies with programmable responsiveness. In this review, we focus on structural determinants of assemblies from amphiphilic macromolecules and their macromolecular level alterations under stimuli, such as the disruption of hydrophilic-lipophilic balance (HLB), depolymerization, decrosslinking, and changes of molecular packing in assemblies, which eventually lead to a series of macroscopic phenomenon for practical purposes. Applications of stimuli-responsive nano-assemblies in delivery, sensing and imaging were also summarized based on their structural features. We expect this review could provide readers an overview of the structural considerations in the design and applications of nanoassemblies and incentivize more explorations in stimuli-responsive soft matters.

Orthogonal Hydroxyl Functionalization of cGAMP Confers Metabolic Stability and Enables Antibody Conjugation

cGAMP is a signaling molecule produced by the cGAS-DNA complex to establish antimicrobial and antitumor immunity through STING. Whereas STING activation holds potential as a new strategy to treat cancer, cGAMP is generally considered unsuitable for in vivo use because of the rapid cleavage of its phosphodiester linkages and the limited cellular uptake under physiological conditions. Consequently, phosphorothioation and fluorination are commonly used to improve the metabolic stability and permeability of cGAMP and its synthetic analogues. We now show that methylation of the 3'-hydroxyl group of cGAMP also confers metabolic stability and that acylation of the 2'-hydroxyl group can be achieved directly and selectively to enable receptor-mediated intracellular delivery. Unlike phosphorothioation and fluorination, these modifications do not create a new stereogenic center and do not require laborious building block synthesis. As such, orthogonal hydroxyl functionalization is a simple solution to issues associated with the in vivo use of cGAMP.

Cysteine-Cysteine Cross-Conjugation of both Peptides and Proteins with a Bifunctional Hypervalent Iodine-Electrophilic Reagent

Peptide and protein bioconjugation sees ever-growing applications in the pharmaceutical sector. Novel strategies and reagents that can address the chemo- and regioselectivity issues inherent to these biomolecules, while delivering stable and functionalizable conjugates, are therefore needed. Herein, we introduce the crosslinking ethynylbenziodazolone (EBZ) reagent JW-AM-005 for the conjugation of peptides and proteins through the selective linkage of cysteine residues. This easily accessed compound gives access to peptide dimers or stapled peptides under mild and tuneable conditions. Applied to the antibody fragment of antigen binding (Fab) species, JW-AM-005 delivered rebridged proteins in a one-pot three-reaction process with high regioselectivity, outperforming the standard reagents commonly used for this transformation.

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.

oxSTEF Reagents Are Tunable and Versatile Electrophiles for Selective Disulfide-Rebridging of Native Proteins

Site-selective disulfide rebridging has emerged as a powerful strategy to modulate the structural and functional properties of proteins. Here, we introduce a novel class of electrophilic reagents, designated oxSTEF, that demonstrate excellent efficiency in disulfide rebridging via double thiol exchange. The oxSTEF reagents are prepared using an efficient synthetic sequence which may be diverted to obtain a range of derivatives allowing for tuning of reactivity or steric bulk. We demonstrate highly selective rebridging of cyclic peptides and native proteins, such as human growth hormone, and the absence of cross-reactivity with other nucleophilic amino acid residues. The oxSTEF conjugates undergo glutathione-mediated disintegration under tumor-relevant glutathione concentrations, which highlights their potential for use in targeted drug delivery. Finally, the α-dicarbonyl motif of the oxSTEF reagents enables "second phase" oxime ligation, which furthermore increases the thiol stability of the conjugates significantly.

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.

Degrader-Antibody Conjugates: Emerging New Modality

In order to deliver chemotherapeutics more efficiently, small-molecule-drug conjugates (SMDCs) and antibody-drug conjugates (ADCs) have been synthesized and explored. These conjugates not only provide selective delivery but also improve the therapeutic index of toxins. By merging this conjugate concept with target protein degradation (TPD), the degrader-antibody conjugate (DAC) field has emerged, and clinical trials have even begun in recent years. In this Perspective, we provide the concepts, applications, and recent advances in the area of DACs.

Anticancer-active 3,4-diarylthiolated maleimides synthesis via three-component radical diarylthiolation

Herein, we report an efficient and simple copper-catalyzed oxidative diarylthiolation of maleimides with sulfur powder and aryl boronic acids, in which S powder was used as a substrate and internal oxidant. The corresponding double C-S bonds coupling products were obtained in moderate to high yields under a simple catalytic system. Mechanistic studies indicated that copper-catalyzed radical thiolation of aryl boronic acids with S powder, and the resulting arylthiyl underwent radical addition with double bonds of maleimides.

A phosphine-based redox method for direct conjugation of disulfides

Technologies for cysteine disulfide detection and conjugation are pivotal to understanding protein functions and developing disulfide-derived therapeutic agents. Currently, disulfide modification requires reductive cleavage prior to functionalization, posing challenges to differentiating disulfides from free thiols. We describe herein Redox-assisted Disulfide Direct Conjugation (RDDC) as a new method to enable disulfide rebridging without cross-reacting with free thiols.

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.

Bioorthogonal site-selective conjugation of fluorescent dyes to antibodies: method and potential applications

Antibodies are immensely useful tools for biochemical research and have found application in numerous protein detection and purification methods. Moreover, monoclonal antibodies are increasingly utilised as therapeutics or, conjugated to active pharmaceutical ingredients, in targeted chemotherapy. Several reagents and protocols are reported to synthesise fluorescent antibodies for protein target detection and immunofluorescence applications. However, most of these protocols lead to non-selective conjugation, over-labelling or in the worst case antigen binding site modification. Here, we have used the antibody disulphide cleavage and re-bridging strategy to introduce bright fluorescent dyes without loss of the antibody function. The resulting fluorescent IgG1 type antibodies were shown to be effective imaging tools in western blot and direct immunofluorescence experiments.

Cyclopentadiene as a Multifunctional Reagent for Normal- and Inverse-Electron Demand Diels-Alder Bioconjugation

Optimizing the Diels-Alder (DA) reaction for aqueous coupling has resulted in practical methods to link molecules such as drugs and diagnostic agents to proteins. Both normal electron demand (NED) and inverse electron demand (IED) DA coupling schemes have been employed, but neither mechanism entails a common multipurpose reactive group. This report focuses on expanding the bioconjugation toolbox for cyclopentadiene through the identification of reactive groups that couple through NED or IED mechanisms in aqueous solution. Dienophiles and tetrazine derivatives were screened for reactivity and selectivity toward antibodies bearing cyclopentadiene amino acids to yield bioconjugates. Twelve NED dienophiles and four tetrazine-based IED substrates were identified as capable of practical biocoupling. Furthermore, tetrazine ligation to cyclopentadiene occurred at a rate of 3.3 ± 0.5 M^-1 s^-1 and was capable of bioorthogonal transformations, as evidenced by the selective protein labeling in serum. Finally, an antibody-drug conjugate (ADC)-bearing monomethyl auristatin E was prepared via tetrazine conjugation to cyclopentadiene. The resulting ADC was stable and demonstrated potent activity in vitro. These findings expand the utility of cyclopentadiene as a tool to couple entities to proteins via dual DA addition mechanisms.

Degrader-antibody conjugates

Degrader-antibody conjugates (DACs) are novel entities that combine a proteolysis targeting chimera (PROTAC) payload with a monoclonal antibody via some type of chemical linker. This review provides a current summary of the DAC field. Many general aspects associated with the creation and biological performance of traditional cytotoxic antibody-drug conjugates (ADCs) are initially presented. These characteristics are subsequently compared and contrasted with related parameters that impact DAC generation and biological activity. Several examples of DACs assembled from both the scientific and the patent literature are utilized to highlight differing strategies for DAC creation, and specific challenges associated with DAC construction are documented. Collectively, the assembled examples demonstrate that biologically-active DACs can be successfully prepared using a variety of PROTAC payloads which employ diverse E3 ligases to degrade multiple protein targets.

The use of bromopyridazinedione derivatives in chemical biology

Tools that facilitate the chemical modification of peptides and proteins are gaining an increasing amount of interest across many avenues of chemical biology as they enable a plethora of therapeutic, imaging and diagnostic applications. Cysteine residues and disulfide bonds have been highlighted as appealing targets for modification due to the highly homogenous nature of the products that can be formed through their site-selective modification. Amongst the reagents available for the site-selective modification of cysteine(s)/disulfide(s), pyridazinediones (PDs) have played a particularly important and enabling role. In this review, we outline the unique chemical features that make PDs especially well-suited to cysteine/disulfide modification on a wide variety of proteins and peptides, as well as provide context as to the problems solved (and applications enabled) by this technology.

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.

A Plug-and-Play Platform for the Formation of Trifunctional Cysteine Bioconjugates that also Offers Control over Thiol Cleavability

Linkers that enable the site-selective synthesis of chemically modified proteins are of great interest to the field of chemical biology. Homogenous bioconjugates often show advantageous pharmacokinetic profiles and consequently increased efficacy in vivo. Cysteine residues have been exploited as a route to site-selectively modify proteins, and many successfully approved therapeutics make use of cysteine directed conjugation reagents. However, commonly used linkers, including maleimide-thiol conjugates, are not stable to the low concentrations of thiol present in blood. Furthermore, only a few cysteine-targeting reagents enable the site-selective attachment of multiple functionalities: a useful tool in the fields of theranostics and therapeutic blood half-life extension. Herein, we demonstrate the application of the pyridazinedione motif to enable site-selective attachment of three functionalities to a protein bearing a single cysteine residue. Extending upon previously documented dual modification work, here we demonstrate that by exploiting a bromide leaving group as an additional reactive point on the pyridazinedione scaffold, a thiol or aniline derivative can be added to a protein, post-conjugation. Thiol cleavability appraisal of the resultant C-S and C-N linked thio-bioconjugates demonstrated C-S functionalized linkers to be cleavable and C-N functionalized linkers to be noncleavable when incubated in an excess of glutathione. The plug-and-play trifunctional platform was exemplified by attaching clinically relevant motifs: biotin, fluorescein, a polyethylene glycol chain, and a model peptide. This platform provides a rare opportunity to combine up to three functionalities on a protein in a site-selective fashion. Furthermore, by selecting the use of a thiol or an amine for functionalization, we provide unique control over linker cleavability toward thiols, allowing this novel linker to be applied in a range of physiological environments.

Synthesis and Comparative In Vivo Evaluation of Site-Specifically Labeled Radioimmunoconjugates for DLL3-Targeted ImmunoPET

Delta-like ligand 3 (DLL3) is a therapeutic target for the treatment of small cell lung cancer, neuroendocrine prostate cancer, and isocitrate dehydrogenase mutant glioma. In the clinic, DLL3-targeted ⁸⁹Zr-immunoPET has the potential to aid in the assessment of disease burden and facilitate the selection of patients suitable for therapies that target the antigen. The overwhelming majority of ⁸⁹Zr-labeled radioimmunoconjugates are synthesized via the random conjugation of desferrioxamine (DFO) to lysine residues within the immunoglobulin. While this approach is admittedly facile, it can produce heterogeneous constructs with suboptimal in vitro and in vivo behavior. In an effort to circumvent these issues, we report the development and preclinical evaluation of site-specifically labeled radioimmunoconjugates for DLL3-targeted immunoPET. To this end, we modified a cysteine-engineered variant of the DLL3-targeting antibody SC16-MB1 with two thiol-reactive variants of DFO: one bearing a maleimide moiety (Mal-DFO) and the other containing a phenyloxadiazolyl methyl sulfone group (PODS-DFO). In an effort to obtain immunoconjugates with a DFO-to-antibody ratio (DAR) of 2, we explored both the reduction of the antibody with tris(2-carboxyethyl) phosphine (TCEP) as well as the use of a combination of glutathione and arginine as reducing and stabilizing agents, respectively. While exerting control over the DAR of the immunoconjugate proved cumbersome using TCEP, the use of glutathione and arginine enabled the selective reduction of the engineered cysteines and thus the formation of homogeneous immunoconjugates. A head-to-head comparison of the resulting ⁸⁹Zr-radioimmunoconjugates in mice bearing DLL3-expressing H82 xenografts revealed no significant differences in tumoral uptake and showed comparable radioactivity concentrations in most healthy nontarget organs. However, ⁸⁹Zr-DFO_PODS-^DAR2SC16-MB1 produced 30% lower uptake (3.3 ± 0.5 %ID/g) in the kidneys compared to ⁸⁹Zr-DFO_Mal-^DAR2SC16-MB1 (4.7 ± 0.5 %ID/g). In addition, H82-bearing mice injected with a ⁸⁹Zr-labeled isotype-control radioimmunoconjugate synthesized using PODS exhibited ∼40% lower radioactivity in the kidneys compared to mice administered its maleimide-based counterpart. Taken together, these results demonstrate the improved in vivo performance of the PODS-based radioimmunoconjugate and suggest that a stable, well-defined DAR2 radiopharmaceutical may be suitable for the clinical immunoPET of DLL3-expressing cancers.

Synthesis of Disulfide-Bridging Trehalose Polymers for Antibody and Fab Conjugation Using a Bis-Sulfone ATRP Initiator

Antibodies and antigen binding fragments (FABs) are widely used as therapeutics and conjugated polymers can enhance the properties of these important biomolecules. However, limitations to the selectivity and stability of current conjugation methodologies can inhibit the exploration of new antibody-polymer conjugates. Herein, we describe a new strategy for the synthesis of these conjugates that forms a stable thioether bond and can be directly incorporated into an atom transfer radical polymerization (ATRP) initiator. Specifically, a bis-sulfone alkyl bromide initiator was synthesized and utilized in the activators generated by electron transfer (AGET) ATRP of ethylene glycol methacrylate and trehalose methacrylate to form the respective polymers. The trehalose polymer was then irreversibly inserted into the disulfide bonds of Herceptin and Herceptin FAB after mild reduction to form the conjugates with quantitative conversions as verified by Western Blot and mass spectrometry after cleavage of the polymer. The binding of the Herceptin and Herceptin Fab conjugates to the receptor was investigated by indirect ELISA (enzyme-linked immunosorbent assay) and the EC50's were 0.90 and 2.74 nM, respectively, compared to Herceptin (0.26 nM) and the Fab (0.56 nM). The conjugates were subjected to heating studies at a constant 75 °C, the temperature determined in a heat ramp to be the threshold of stability for the antibody and FAB; the trehalose polymer was found to considerably increase the thermal stability of both Herceptin and Herceptin Fab. This work provides a new way to prepare polymer-antibody/Fab conjugates utilizing bis-sulfone end groups installed by atom transfer radical polymerization of the functionalized initiators and a way to stabilize these important molecules by conjugation to trehalose polymers.

Innovative Bioconjugation Technology for Antibody-Drug Conjugates: Proof of Concept in a CD30-Positive Lymphoma Mouse Model

To overcome stability and heterogeneity issues of antibody-drug conjugates (ADCs) produced with existing bioconjugation technologies incorporating a maleimide motif, we developed McSAF Inside, a new technology based on a trifunctionalized di(bromomethyl)pyridine scaffold. Our solution allows the conjugation of a linker-payload to previously reduced interchain cysteines of a native antibody, resulting in disulfide rebridging. This leads to highly stable and homogeneous ADCs with control over the drug-to-antibody ratio (DAR) and the linker-payload position. Using our technology, we synthesized an ADC, MF-BTX-MMAE, built from anti-CD30 antibody cAC10 (brentuximab), and compared it to Adcetris, the first line treatment against CD30-positive lymphoma, in a CD30-positive lymphoma model. MF-BTX-MMAE displayed improved DAR homogeneity, with a solid batch-to-batch reproducibility, as well as enhanced stability in thermal stress conditions or in the presence of a free thiol-containing protein, such as human serum albumin (HSA). MF-BTX-MMAE showed antigen-binding, in vitro cytotoxicity, in vivo efficacy, and tolerability similar to Adcetris. Therefore, in accordance with current regulatory expectations for the development of new ADCs, McSAF Inside technology gives access to relevant ADCs with improved characteristics and stability.

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.

The renaissance of chemically generated bispecific antibodies

Bispecific antibodies (bsAbs) target two different epitopes. These are an up-and-coming class of biologics, with two such therapeutics (emicizumab and blinatumomab) FDA approved and on the market, and many more in clinical trials. While the first reported bsAbs were constructed by chemical methods, this approach has fallen out of favour with the advent of modern genetic engineering techniques and, nowadays, the vast majority of bsAbs are produced by protein engineering. However, in recent years, relying on innovations in the fields of bioconjugation and bioorthogonal click chemistry, new chemical methods have appeared that have the potential to be competitive with protein engineering techniques and, indeed, hold some advantages. These approaches offer modularity, reproducibility and batch-to-batch consistency, as well as the integration of handles, whereby additional cargo molecules can be attached easily, e.g. to generate bispecific antibody-drug conjugates. The linker between the antibodies/antibody fragments can also be easily varied, and new formats (types, defined by structural properties or by construction methodology) can be generated rapidly. These attributes offer the potential to revolutionize the field. Here, we review chemical methods for the generation of bsAbs, showing that the newest examples of these techniques are worthy competitors to the industry-standard expression-based strategies.

Bridged Cysteine Conjugations

Preparation of antibody-drug conjugates (ADCs) with a highly homogeneous drug loading in general requires site-selective conjugation of a cytotoxic payload. Typically, functionality utilized for attachment of the payload is achieved through engineering of suitable chemical handles or by enzymatic modification of the antibody. Relatively few methods to produce ADCs with homogeneous drug loading via endogenous amino acid conjugation have been developed. Herein we describe a robust method for the conjugation of antibodies using a cysteine rebridging approach to produce ADCs with highly homogeneous drug-to-antibody ratios (DAR) at the native interchain disulfides, called ThioBridge^®. The process described relies upon an elegant cascade of addition-elimination reactions carried out under mild aqueous conditions that can be readily applied to wild-type antibodies without the need for prior modification via recombinant or enzymatic means. Using this method, conversions to a conserved DAR ADC are typically in the range of 70-95% and overall process yields of >70% are readily achieved.

Structural optimization of reversible dibromomaleimide peptide stapling

Methods to constrain peptides in a bioactive α-helical conformation for inhibition of protein-protein interactions represent an ongoing area of investigation in chemical biology. Recently, the first example of a reversible "stapling" methodology was described which exploits native cysteine or homocysteine residues spaced at the i and i + 4 positions in a peptide sequence together with the thiol selective reactivity of dibromomaleimides (a previous study). This manuscript reports on the optimization of the maleimide based constraint, focusing on the kinetics of macrocyclization and the extent to which helicity is promoted with different thiol containing amino acids. The study identified an optimal stapling combination of X 1 = L-Cys and X 5 = L-hCys in the context of the model peptide Ac-X1AAAX5-NH2, which should prove useful in implementing the dibromomaleimide stapling strategy in peptidomimetic ligand discovery programmes.

DiPODS: A Reagent for Site-Specific Bioconjugation via the Irreversible Rebridging of Disulfide Linkages

Chemoselective reactions with thiols have long held promise for the site-specific bioconjugation of antibodies and antibody fragments. Yet bifunctional probes bearing monovalent maleimides-long the "gold standard" for thiol-based ligations-are hampered by two intrinsic issues: the in vivo instability of the maleimide-thiol bond and the need to permanently disrupt disulfide linkages in order to facilitate bioconjugation. Herein, we present the synthesis, characterization, and validation of DiPODS, a novel bioconjugation reagent containing a pair of oxadiazolyl methyl sulfone moieties capable of irreversibly forming covalent bonds with two thiolate groups while simultaneously rebridging disulfide linkages. The reagent was synthesized from commercially available starting materials in 8 steps, during which rotamers were encountered and investigated both experimentally and computationally. DiPODS is designed to be modular and can thus be conjugated to any payload through a pendant terminal primary amine (DiPODS-PEG₄-NH₂). Subsequently, the modification of a HER2-targeting Fab with a fluorescein-conjugated variant of DiPODS (DiPODS-PEG₄-FITC) reinforced the site-specificity of the reagent, illustrated its ability to rebridge disulfide linkages, and produced an immunoconjugate with in vitro properties superior to those of an analogous construct created using traditional stochastic bioconjugation techniques. Ultimately, we believe that this work has particularly important implications for the synthesis of immunoconjugates, specifically for ensuring that the attachment of cargoes to immunoglobulins is robust, irreversible, and biologically and structurally benign.

One-pot thiol-amine bioconjugation to maleimides: simultaneous stabilisation and dual functionalisation

Maleimide chemistry is widely used in the site-selective modification of proteins. However, hydrolysis of the resultant thiosuccinimides is required to provide robust stability to the bioconjugates. Herein, we present an alternative approach that affords simultaneous stabilisation and dual functionalisation in a one pot fashion. By consecutive conjugation of a thiol and an amine to dibromomaleimides, we show that aminothiomaleimides can be generated extremely efficiently. Furthermore, the amine serves to deactivate the electrophilicity of the maleimide, precluding further reactivity and hence generating stable conjugates. We have applied this conjugation strategy to peptides and proteins to generate stabilised trifunctional conjugates. We propose that this stabilisation-dual modification strategy could have widespread use in the generation of diverse conjugates.

Antibody-PROTAC Conjugates Enable HER2-Dependent Targeted Protein Degradation of BRD4

Targeting protein degradation with Proteolysis-Targeting Chimeras (PROTACs) is an area of great current interest in drug discovery. Nevertheless, although the high effectiveness of PROTACs against a wide variety of targets has been established, most degraders reported to date display limited intrinsic tissue selectivity and do not discriminate between cells of different types. Here, we describe a strategy for selective protein degradation in a specific cell type. We report the design and synthesis of a trastuzumab-PROTAC conjugate (Ab-PROTAC 3) in which E3 ligase-directed degrader activity is caged with an antibody linker which can be hydrolyzed following antibody-PROTAC internalization, releasing the active PROTAC and inducing catalytic protein degradation. We show that 3 selectively targets bromodomain-containing protein 4 (BRD4) for degradation only in HER2 positive breast cancer cell lines, while sparing HER2 negative cells. Using live cell confocal microscopy, we show internalization and lysosomal trafficking of the conjugate specifically in HER2 positive cells, leading to the release of active PROTAC in quantities sufficient to induce potent BRD4 degradation. These studies demonstrate proof-of-concept for tissue-specific BRD4 degradation, overcoming limitations of PROTAC selectivity, with significant potential for application to novel targets.

General dual functionalisation of biomacromolecules via a cysteine bridging strategy

Site-selective modification of peptides and proteins has resulted in the development of a host of novel tools for the study of cellular systems or the synthesis of enhanced biotherapeutics. There is a need for useful methodologies that enable site-selective modification of native peptides or proteins, which is even more prevalent when modification of the biomolecule with multiple payloads is desired. Herein, we report the development of a novel dual functional divinylpyrimidine (dfDVP) platform that enables robust and modular modification of peptides, antibody fragments and antibodies. These biomacromolecules could be easily functionalised with a range of functional payloads (e.g. fluorescent dyes, cytotoxic warheads or cell-penetrating tags). Importantly, the dual functionalised peptides and antibodies demonstrated exquisite bioactivity in a range of in vitro cellular assays, showcasing the enhanced utility of these bioactive conjugates.

Pyrocinchonimides Conjugate to Amine Groups on Proteins via Imide Transfer

Advances in bioconjugation, the ability to link biomolecules to each other, small molecules, surfaces, and more, can spur the development of advanced materials and therapeutics. We have discovered that pyrocinchonimide, the dimethylated analogue of maleimide, undergoes a surprising transformation with biomolecules. The reaction targets amines and involves an imide transfer, which has not been previously reported for bioconjugation purposes. Despite their similarity to maleimides, pyrocinchonimides do not react with free thiols. Though both lysine residues and the N-termini of proteins can receive the transferred imide, the reaction also exhibits a marked preference for certain amines that cannot solely be ascribed to solvent accessibility. This property is peculiar among amine-targeting reactions and can reduce combinatorial diversity when many available reactive amines are available, such as in the formation of antibody-drug conjugates. Unlike amides, the modification undergoes very slow reversion under high pH conditions. The reaction offers a thermodynamically controlled route to single or multiple modifications of proteins for a wide range of applications.

Bis(vinylsulfonyl)piperazines as efficient linkers for highly homogeneous antibody-drug conjugates

Disulfide re-bridging strategy has demonstrated significant advantages in the construction of homogeneous antibody drug conjugates (ADCs). However, a major issue that disulfide scrambling at the hinge region of antibody leads to the formation of "half-antibody" has appeared for many re-bridging linkers. We present bis(vinylsulfonyl)piperazines (BVP) as efficient linkers to selectively re-bridge disulfides at the antigen-binding fragment (Fab) regions and produce highly homogeneous conjugates with a loading of two drugs without disulfide scrambling. We also found that optically active (S)-configuration linkers led to more sufficient conjugation compared with (R)-configuration. The BVP-linked ADCs demonstrated superior efficacy and antigen-selectivity in vitro cytotoxicity.

Optimised approach to albumin-drug conjugates using monobromomaleimide-C-2 linkers

Conjugation of therapeutics to human serum albumin (HSA) using bromomaleimides represents a promising platform for half-life extension. We show here that the Cys-34 crevice substantially reduces the rate of serum stabilising maleimide hydrolysis in these conjugates, necessitating reagent optimisation. This improved reagent design is applied to the construction of an HSA-paclitaxel conjugate, preventing drug loss during maleimide hydrolysis.

Cysteine-to-lysine transfer antibody fragment conjugation

The modification of lysine residues with acylating agents has represented a ubiquitous approach to the construction of antibody conjugates, with the resulting amide bonds being robustly stable and clinically validated. However, the conjugates are highly heterogeneous, due to the presence of numerous lysines on the surface of the protein, and greater control of the sites of conjugation are keenly sought. Here we present a novel approach to achieve the targeted modification of lysines distal to an antibody fragment's binding site, using a disulfide bond as a temporary 'hook' to deliver the acylating agent. This cysteine-to-lysine transfer (CLT) methodology offers greatly improved homogeneity of lysine conjugates, whilst retaining the advantages offered by the formation of amide linkages.

A general approach for the site-selective modification of native proteins, enabling the generation of stable and functional antibody-drug conjugates

Antibody-drug conjugates (ADCs) are a class of targeted therapeutics that utilize the specificity of antibodies to selectively deliver highly potent cytotoxins to target cells. Although recent years have witnessed significant interest in ADCs, problems remain with the standard linkage chemistries used for cytotoxin-antibody bioconjugation. These typically (1) generate unstable constructs, which may lead to premature cytotoxin release, (2) often give a wide variance in drug-antibody ratios (DAR) and (3) have poor control of attachment location on the antibody, resulting in a variable pharmacokinetic profile. Herein, we report a novel divinylpyrimidine (DVP) linker platform for selective bioconjugation via covalent re-bridging of reduced disulfide bonds on native antibodies. Model studies using the non-engineered trastuzumab antibody validate the utility of this linker platform for the generic generation of highly plasma-stable and functional antibody constructs that incorporate variable biologically relevant payloads (including cytotoxins) in an efficient and site-selective manner with precise control over DAR. DVP linkers were also used to efficiently re-bridge both monomeric and dimeric protein systems, demonstrating their potential utility for general protein modification, protein stabilisation or the development of other protein-conjugate therapeutics.

Site-selective protein conjugation at histidine

Site-selective conjugation generally requires both (i) molecular engineering of the protein of interest to introduce a conjugation site at a defined location and (ii) a site-specific conjugation technology. Three N-terminal interferon α2-a (IFN) variants with truncated histidine tags were prepared and conjugation was examined using a bis-alkylation reagent, PEG(10kDa)-mono-sulfone 3. A histidine tag comprised of two histidines separated by a glycine (His2-tag) underwent PEGylation. Two more IFN variants were then prepared with the His2-tag engineered at different locations in IFN. Another IFN variant was prepared with the His-tag introduced in an α-helix, and required three contiguous histidines to ensure that two histidine residues in the correct conformation would be available for conjugation. Since histidine is a natural amino acid, routine methods of site-directed mutagenesis were used to generate the IFN variants from E. coli in soluble form at titres comparable to native IFN. PEGylation conversions ranged from 28-39%. A single step purification process gave essentially the pure PEG-IFN variant (>97% by RP-HPLC) in high recovery with isolated yields ranging from 21-33%. The level of retained bioactivity was strongly dependent on the site of PEG conjugation. The highest biological activity of 74% was retained for the PEG10-106(HGHG)-IFN variant which is unprecedented for a PEGylated IFN. The His2-tag at 106(HGHG)-IFN is engineered at the flexible loop most distant from IFN interaction with its dimeric receptor. The biological activity for the PEG10-5(HGH)-IFN variant was determined to be 17% which is comparable to other PEGylated IFN conjugates achieved at or near the N-terminus that have been previously described. The lowest retained activity (10%) was reported for PEG10-120(HHH)-IFN which was prepared as a negative control targeting a IFN site thought to be involved in receptor binding. The presence of two histidines as a His2-tag to generate a site-selective target for bis-alkylating PEGylation is a feasible approach for achieving site-selective PEGylation. The use of a His2-tag to strategically engineer a conjugation site in a protein location can result in maximising the retention of the biological activity following protein modification.

Macrocyclisation and functionalisation of unprotected peptides via divinyltriazine cysteine stapling

We report a novel divinyltriazine linker for the stapling of two cysteine residues to form macrocyclic peptides from their unprotected linear counterparts. The stapling reaction occurred rapidly under mild conditions on a range of unprotected peptide sequences. The resulting constrained peptides displayed greater stability in a serum stability assay when compared to their linear counterparts.

Application of Next-Generation Maleimides (NGMs) to Site-Selective Antibody Conjugation

Site-selective antibody conjugation is widely recognized as a key strategy for the optimum construction of antibody-drug conjugates (ADCs). Achieving such bioconjugation directly onto native antibodies would represent the ideal solution, as it would afford greatly improved homogeneity whilst avoiding the need for genetic engineering, and even allow the repurposing of existing antibodies "off-the shelf." Here we describe a protocol for the use of next-generation maleimides (NGMs) for the selective modification of the four interchain disulfide bonds present in a typical IgG1 antibody format. These reagents retain the efficiency of classical maleimides whilst serving to rebridge each reduced disulfide bond, affording one attachment per disulfide. The approach is simple, uses readily available reagents, and generates robustly stable conjugates which are ideal for in vitro or in vivo applications. In addition to use in the construction of ADCs these reagents can also be used to develop antibody conjugates for imaging, bispecifics, and broadly for use across biology and medicine.

Homogeneous antibody-drug conjugates via site-selective disulfide bridging

Antibody-drug conjugates (ADCs) constructed using site-selective labelling methodologies are likely to dominate the next generation of these targeted therapeutics. To this end, disulfide bridging has emerged as a leading strategy as it allows the production of highly homogeneous ADCs without the need for antibody engineering. It consists of targeting reduced interchain disulfide bonds with reagents which reconnect the resultant pairs of cysteine residues, whilst simultaneously attaching drugs. The 3 main reagent classes which have been exemplified for the construction of ADCs by disulfide bridging will be discussed in this review; bissulfones, next generation maleimides and pyridazinediones, along with others in development.

Minireview: Addressing the retro-Michael instability of maleimide bioconjugates

Bioconjugation, the modification of biological macromolecules such as proteins, is an up and coming area in the field of chemical biology. Antibody-drug conjugates (ADCs), combining the antigen-selectivity of natural antibodies with the cytotoxic potency of small molecule drugs, are a powerful therapeutic technology. Four such constructs are currently on the market for cancer therapy. However, the conjugation methodology employed in these therapeutics is far from ideal. Herein we provide an overview on methods that attempt to increase the safety and efficacy of ADCs via "self-hydrolysing maleimides" or by improving the stability of maleimide-conjugates by other means. We find that some very promising reagents have been reported, however the mechanism by which each of these reagents acts is not clear, thus limiting rational design for some strategies.

Bioconjugation with Maleimides: A Useful Tool for Chemical Biology

Maleimide chemistry stands out in the bioconjugation toolbox by virtue of its synthetic accessibility, excellent reactivity, and practicability. The second-generation of clinically approved antibody-drug conjugates (ADC) and much of the current ADC pipeline in clinical trials contain the maleimide linkage. However, thiosuccinimide linkages are now known to be less robust than once thought, and ergo, are correlated with suboptimal pharmacodynamics, pharmacokinetics, and safety profiles in some ADC constructs. Rational design of novel generations of maleimides and maleimide-type reagents have been reported to address the shortcomings of classical maleimides, allowing for the formation of robust bioconjugate linkages. This review highlights the main strategies for rational reagent design that have allowed irreversible bioconjugations in cysteines, reversible labelling strategies and disulfide re-bridging.

Site-specific chelator-antibody conjugation for PET and SPECT imaging with radiometals

Antibodies and their derivatives radiolabelled with positron- and gamma-emitting radiometals enable sensitive and quantitative molecular Positron Emission Tomography (PET) and Single Photon Emission Computed Tomography (SPECT) imaging of antibody distribution in vivo. Chelators that are covalently attached to antibodies allow radiolabelling with metallic PET and SPECT radioisotopes. Conventional strategies for chelator-protein conjugation generate heterogeneous mixtures of bioconjugates that can exhibit reduced affinity for their receptor targets, and undesirable biodistribution and pharmacokinetics. Recent advances in bioconjugation technology enable site-specific modification to generate well-defined constructs with superior properties. Herein we survey existing site-specific chelator-protein conjugation methods. These include chelator attachment to cysteines/disulfide bonds or the glycan region of the antibody, enzyme-mediated chelator conjugation, and incorporation of sequences of amino acids that chelate the radiometal. Such technology will allow better use of PET and SPECT imaging in the development of antibody-based therapies.