Mild method for succinimide hydrolysis on ADCs: impact on ADC potency, stability, exposure, and efficacy

Stoichiometric and irreversible cysteine-selective protein modification using carbonylacrylic reagents

Maleimides remain the reagents of choice for the preparation of therapeutic and imaging protein conjugates despite the known instability of the resulting products that undergo thiol-exchange reactions in vivo. Here we present the rational design of carbonylacrylic reagents for chemoselective cysteine bioconjugation. These reagents undergo rapid thiol Michael-addition under biocompatible conditions in stoichiometric amounts. When using carbonylacrylic reagents equipped with PEG or fluorophore moieties, this method enables access to protein and antibody conjugates precisely modified at pre-determined sites. Importantly, the conjugates formed are resistant to degradation in plasma and are biologically functional, as demonstrated by the selective imaging and detection of apoptotic and HER2+ cells, respectively. The straightforward preparation, stoichiometric use and exquisite cysteine selectivity of the carbonylacrylic reagents combined with the stability of the products and the availability of biologically relevant cysteine-tagged proteins make this method suitable for the routine preparation of chemically defined conjugates for in vivo applications.

Carbon nanotubes allow capture of krypton, barium and lead for multichannel biological X-ray fluorescence imaging

The desire to study biology in situ has been aided by many imaging techniques. Among these, X-ray fluorescence (XRF) mapping permits observation of elemental distributions in a multichannel manner. However, XRF imaging is underused, in part, because of the difficulty in interpreting maps without an underlying cellular 'blueprint'; this could be supplied using contrast agents. Carbon nanotubes (CNTs) can be filled with a wide range of inorganic materials, and thus can be used as 'contrast agents' if biologically absent elements are encapsulated. Here we show that sealed single-walled CNTs filled with lead, barium and even krypton can be produced, and externally decorated with peptides to provide affinity for sub-cellular targets. The agents are able to highlight specific organelles in multiplexed XRF mapping, and are, in principle, a general and versatile tool for this, and other modes of biological imaging.

2-(Maleimidomethyl)-1,3-Dioxanes (MD): a Serum-Stable Self-hydrolysable Hydrophilic Alternative to Classical Maleimide Conjugation

The vast majority of antibody-drug conjugates (ADC) are prepared through amine-to-thiol conjugation. To date, N-Succinimidyl-4-(maleimidomethyl) cyclohexanecarboxylate (SMCC) has been one of the most frequently applied reagents for the preparation of ADC and other functional conjugates. However, SMCC-based conjugates suffer from limited stability in blood circulation and from a hydrophobic character of the linker, which may give rise to major pharmacokinetic implications. To address this issue, we have developed a heterobifunctional analogue of a SMCC reagent, i.e., sodium 4-(maleimidomethyl)-1,3-dioxane-5-carbonyl)oxy)-2,3,5,6- tetrafluorobenzenesulfonate (MDTF) for amine-to-thiol conjugation. By replacing the cyclohexyl ring in the SMCC structure with the 1,3-dioxane, we increased the hydrophilicity of the linker. A FRET probe based on MD linker was prepared and showed superior stability compared to the MCC linker in human plasma, as well as in a variety of aqueous buffers. A detailed investigation demonstrated an accelerated succinimide ring opening for MD linker, resulting in stabilized conjugates. Finally, the MDTF reagent was applied for the preparation of serum stable antibody-dye conjugate.

Unconjugated payload quantification and DAR characterization of antibody–drug conjugates using high-resolution MS

Aim: The application of high-resolution MS to antibody–drug conjugate (ADC) drug development may provide insight into their safety and efficacy. Quantification of unconjugated cytotoxic drug (payload) and characterization of drug-to-antibody ratio distribution were determined in plasma using orthogonal acceleration quadrupole-time-of-flight MS. Results: Unconjugated payload quantification determined by quadrupole-time-of-flight-based MRMhighresolution and triple quadrupole-based multiple reaction monitoring were comparable and achieved detection limits of 0.030 and 0.015 ng/ml, respectively. As determined by immunocapture and TOF-MS, drug-to-antibody ratio remained unchanged up to 3-weeks postdose for an ADC containing engineered glutamine linkers, but declined from four to three over 2 weeks in an ADC containing engineered cysteine linkers. Conclusion: The use of high-resolution MS in ADC drug discovery confirms its utility within the bioanalytical discipline.

Antibody–drug conjugate bioanalysis using LB-LC–MS/MS hybrid assays: strategies, methodology and correlation to ligand-binding assays

Background: Antibody–drug conjugates (ADCs) are complex drug constructs with multiple species in the heterogeneous mixture that contribute to their efficacy and toxicity. The bioanalysis of ADCs involves multiple assays and analytical platforms. Methods: A series of ligand binding and LC–MS/MS (LB-LC–MS/MS) hybrid assays, through different combinations of anti-idiotype (anti-Id), anti-payload, or generic capture reagents, and cathepsin-B or trypsin enzyme digestion, were developed and evaluated for the analysis of conjugated-payload as well as for species traditionally measured by ligand-binding assays, total-antibody and conjugated-antibody. Results & conclusion: Hybrid assays are complementary or viable alternatives to ligand-binding assay for ADC bioanalysis and PK/PD modeling. The fit-for-purpose choice of analytes, assays and platforms and an integrated strategy from Discovery to Development for ADC PK and bioanalysis are recommended.

Understanding How the Stability of the Thiol-Maleimide Linkage Impacts the Pharmacokinetics of Lysine-Linked Antibody-Maytansinoid Conjugates

Antibody-drug conjugates (ADCs) have become a widely investigated modality for cancer therapy, in part due to the clinical findings with ado-trastuzumab emtansine (Kadcyla). Ado-trastuzumab emtansine utilizes the Ab-SMCC-DM1 format, in which the thiol-functionalized maytansinoid cytotoxic agent, DM1, is linked to the antibody (Ab) via the maleimide moiety of the heterobifunctional SMCC linker. The pharmacokinetic (PK) data for ado-trastuzumab emtansine point to a faster clearance for the ADC than for total antibody. Cytotoxic agent release in plasma has been reported with nonmaytansinoid, cysteine-linked ADCs via thiol-maleimide exchange, for example, brentuximab vedotin. For Ab-SMCC-DM1 ADCs, however, the main catabolite reported is lysine-SMCC-DM1, the expected product of intracellular antibody proteolysis. To understand these observations better, we conducted a series of studies to examine the stability of the thiol-maleimide linkage, utilizing the EGFR-targeting conjugate, J2898A-SMCC-DM1, and comparing it with a control ADC made with a noncleavable linker that lacked a thiol-maleimide adduct (J2898A-(CH2)3-DM). We employed radiolabeled ADCs to directly measure both the antibody and the ADC components in plasma. The PK properties of the conjugated antibody moiety of the two conjugates, J2898A-SMCC-DM1 and J2898A-(CH2)3-DM (each with an average of 3.0 to 3.4 maytansinoid molecules per antibody), appear to be similar to that of the unconjugated antibody. Clearance values of the intact conjugates were slightly faster than those of the Ab components. Furthermore, J2898A-SMCC-DM1 clears slightly faster than J2898A-(CH2)3-DM, suggesting that there is a fraction of maytansinoid loss from the SMCC-DM1 ADC, possibly through a thiol-maleimide dependent mechanism. Experiments on ex vivo stability confirm that some loss of maytansinoid from Ab-SMCC-DM1 conjugates can occur via thiol elimination, but at a slower rate than the corresponding rate of loss reported for thiol-maleimide links formed at thiols derived by reduction of endogenous cysteine residues in antibodies, consistent with expected differences in thiol-maleimide stability related to thiol pKa. These findings inform the design strategy for future ADCs.

Assessment of reagents for selenocysteine conjugation and the stability of selenocysteine adducts

Conventional antibody-drug conjugates (ADCs) are heterogeneous mixtures that have poor pharmacokinetic properties and decreased efficacy relative to homogenous ADCs. Furthermore, ADCs that are maleimide-based often have inadequate circulatory stability, which can result in premature drug release with consequent off-target toxicities. Selenocysteine-modified antibodies have been developed that allow site-specific antibody conjugation, yielding homogeneous ADCs. Herein, we survey several electrophilic functional groups that react with selenocystine with high efficiency. Several of these result in conjugates with stabilities that are superior to maleimide conjugates. Among these, the allenamide functional group reacts with notably high efficiency, leads to conjugates with remarkable stability, and shows exquisite selectivity for selenocysteine conjugation.

A new anti-human Fc method to capture and analyze ADCs for characterization of drug distribution and the drug-to-antibody ratio in serum from pre-clinical species

Antibody-drug conjugates (ADCs) are becoming a major class of oncology therapeutics. They combine monoclonal antibody specificity for over-expressed tumor antigens and the high cytoxicity of small molecular drugs (SMDs) and can therefore selectively kill tumor cells while minimizing toxicity to normal cells. Nevertheless, the premature deconjugation of ADCs in the circulation may trigger off target toxicity in patients. The released free drug level must be low in circulation for an extended period of time as well as the de-conjugation rate to ensure an acceptable therapeutic window. As a result, the assessment of the stability of the linker between payload and mAb in the systemic circulation is of paramount importance before entering in clinical trial. Here we report a new universal method to immunocapture and analyze by LC-MS the stability and distribution of ADCs in sera from relevant preclinical species (mouse, rat and cynomolgus monkey). Furthermore we demonstrated that this workflow can be applied to both ADCs with cleavable and non cleavable linkers. Last but not least, the results obtained in cynomolgus serum using immunoprecipitation and LC-MS analysis were cross validated using an ELISA orthogonal method. As the ligand used for immunoprecipitation is targeting the Fc part of mAb (CaptureSelect™ Human IgG-Fc PK Biotin), this protocol can be applied to analyze the stability of virtually all ADCs in sera for preclinical studies without the need to prepare specific molecular tools.

Construction of homogeneous antibody-drug conjugates using site-selective protein chemistry

Systemic chemotherapy, the current standard of care for the treatment of cancer, is rarely curative and is often accompanied by debilitating side effects. Targeted drug delivery stands as an alternative to chemotherapy, with the potential to improve upon its low efficacy and systemic toxicity. Among targeted therapeutic options, antibody-drug conjugates (ADCs) have emerged as the most promising. These conjugates represent a new class of biopharmaceuticals that selectively deliver potent cytotoxic drugs to cancer cells, sparing healthy tissue throughout the body. Despite this promise, early heterogenous ADCs suffered from stability, pharmacokinetic, and efficacy issues that hindered clinical development. Recent advances in antibody engineering, linkers for drug-release, and chemical site-selective antibody conjugation have led to the creation of homogenous ADCs that have proven to be more efficacious than their heterogeneous predecessors both in vitro and in vivo. In this minireview, we focus on and discuss recent advances in chemical site-selective modification strategies for the conjugation of drugs to antibodies and the resulting potential for the development of a new generation of homogenous ADCs.

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

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

Where Did the Linker-Payload Go? A Quantitative Investigation on the Destination of the Released Linker-Payload from an Antibody-Drug Conjugate with a Maleimide Linker in Plasma

The reactive thiol of cysteine is often used for coupling maleimide-containing linker-payloads to antibodies resulting in the generation of antibody drug conjugates (ADCs). Currently, a numbers of ADCs in drug development are made by coupling a linker-payload to native or engineered cysteine residues on the antibody. An ADC conjugated via hinge-cysteines to an auristatin payload was used as a model in this study to understand the impact of the maleimide linkers on ADC stability. The payload was conjugated to trastuzumab by a protease-cleavable linker, maleimido-caproyl-valine-citruline-p-amino-benzyloxy carbonyl (mcVC-PABC). In plasma stability assays, when the ADC (Trastuzumab-mcVC-PABC-Auristatin-0101) was incubated with plasma over a 144-h time-course, a discrepancy was observed between the measured released free payload concentration and the measured loss of drug-to-antibody ratio (DAR), as measured by liquid chromatography-mass spectrometry (LC-MS). We found that an enzymatic release of payload from ADC-depleted human plasma at 144 h was able to account for almost 100% of the DAR loss. Intact protein mass analysis showed that at the 144 h time point, the mass of the major protein in ADC-depleted human plasma had an additional 1347 Da over the native albumin extracted from human plasma, exactly matching the mass of the linker-payload. In addition, protein gel electrophoresis showed that there was only one enriched protein in the 144 h ADC-depleted and antipayload immunoprecipitated plasma sample, as compared to the 0 h plasma immunoprecipitated sample, and the mass of this enriched protein was slightly heavier than the mass of serum albumin. Furthermore, the albumin adduct was also identified in 96 h and 168 h postdose in vivo cynomolgus monkey plasma. These results strongly suggest that the majority of the deconjugated mc-VC-PABC-auristatin ultimately is transferred to serum albumin, forming a long-lived albumin-linker-payload adduct. To our knowledge, this is the first report quantitatively characterizing the extent of linker-payload transfer to serum albumin and the first clear example of in vivo formation of an albumin-linker-payload adduct.

Linkers Having a Crucial Role in Antibody-Drug Conjugates

Antibody-drug conjugates (ADCs) comprised of a desirable monoclonal antibody, an active cytotoxic drug and an appropriate linker are considered to be an innovative therapeutic approach for targeted treatment of various types of tumors and cancers, enhancing the therapeutic parameter of the cytotoxic drug and reducing the possibility of systemic cytotoxicity. An appropriate linker between the antibody and the cytotoxic drug provides a specific bridge, and thus helps the antibody to selectively deliver the cytotoxic drug to tumor cells and accurately releases the cytotoxic drug at tumor sites. In addition to conjugation, the linkers maintain ADCs' stability during the preparation and storage stages of the ADCs and during the systemic circulation period. The design of linkers for ADCs is a challenge in terms of extracellular stability and intracellular release, and intracellular circumstances, such as the acid environment, the reducing environment and cathepsin, are considered as the catalysts to activate the triggers for initiating the cleavage of ADCs. This review discusses the linkers used in the clinical and marketing stages for ADCs and details the fracture modes of the linkers for the further development of ADCs.

Characterization of antibody-drug conjugates by mass spectrometry: advances and future trends

Antibody-drug conjugates (ADCs) are emerging modalities in the pharmaceutical industry. The unique target-specific binding of antibody allows targeted delivery of cytotoxic small molecules to cancer cells, and thus expands the therapeutic window. However, in-depth characterization of ADCs is complex because it involves the characterization of antibody, conjugated molecules and antibody conjugates as a whole. In this review, we describe the practical use of mass spectrometry for ADC characterization including qualitative and quantitative analysis. Technical advances, limitations and future trends will also be discussed.

A Nanoparticle Platform To Evaluate Bioconjugation and Receptor-Mediated Cell Uptake Using Cross-Linked Polyion Complex Micelles Bearing Antibody Fragments

Targeted nanomedicines are a promising technology for treatment of disease; however, preparation and characterization of well-defined protein-nanoparticle systems remain challenging. Here, we describe a platform technology to prepare antibody binding fragment (Fab)-bearing nanoparticles and an accompanying real-time cell-based assay to determine their cellular uptake compared to monoclonal antibodies (mAbs) and Fabs. The nanoparticle platform was composed of core-cross-linked polyion complex (PIC) micelles prepared from azide-functionalized PEG-b-poly(amino acids), that is, azido-PEG-b-poly(l-lysine) [N3-PEG-b-PLL] and azido-PEG-b-poly(aspartic acid) [N3-PEG-b-PAsp]. These PIC micelles were 30 nm in size and contained approximately 10 polymers per construct. Fabs were derived from an antibody binding the EphA2 receptor expressed on cancer cells and further engineered to contain a reactive cysteine for site-specific attachment and a cleavable His tag for purification from cell culture expression systems. Azide-functionalized micelles and thiol-containing Fab were linked using a heterobifunctional cross-linker (FPM-PEG4-DBCO) that contained a fluorophenyl-maleimide for stable conjugation to Fabs thiols and a strained alkyne (DBCO) group for coupling to micelle azide groups. Analysis of Fab-PIC micelle conjugates by fluorescence correlation spectroscopy, size exclusion chromatography, and UV-vis absorbance determined that each nanoparticle contained 2-3 Fabs. Evaluation of cellular uptake in receptor positive cancer cells by real-time fluorescence microscopy revealed that targeted Fab-PIC micelles achieved higher cell uptake than mAbs and Fabs, demonstrating the utility of this approach to identify targeted nanoparticle constructs with unique cellular internalization properties.

Molecular Basis of Valine-Citrulline-PABC Linker Instability in Site-Specific ADCs and Its Mitigation by Linker Design

The degree of stability of antibody-drug linkers in systemic circulation, and the rate of their intracellular processing within target cancer cells are among the key factors determining the efficacy of antibody-drug conjugates (ADC) in vivo Previous studies demonstrated the susceptibility of cleavable linkers, as well as auristatin-based payloads, to enzymatic cleavage in rodent plasma. Here, we identify Carboxylesterase 1C as the enzyme responsible for the extracellular hydrolysis of valine-citrulline-p-aminocarbamate (VC-PABC)-based linkers in mouse plasma. We further show that the activity of Carboxylesterase 1C towards VC-PABC-based linkers, and consequently the stability of ADCs in mouse plasma, can be effectively modulated by small chemical modifications to the linker. While the introduced modifications can protect the VC-PABC-based linkers from extracellular cleavage, they do not significantly alter the intracellular linker processing by the lysosomal protease Cathepsin B. The distinct substrate preference of the serum Carboxylesterase 1C offers the opportunity to modulate the extracellular stability of cleavable ADCs without diminishing the intracellular payload release required for ADC efficacy. Mol Cancer Ther; 15(5); 958-70. ©2016 AACR.

Albumin-based drug delivery: harnessing nature to cure disease

The effectiveness of a drug is dependent on accumulation at the site of action at therapeutic levels, however, challenges such as rapid renal clearance, degradation or non-specific accumulation requires drug delivery enabling technologies. Albumin is a natural transport protein with multiple ligand binding sites, cellular receptor engagement, and a long circulatory half-life due to interaction with the recycling neonatal Fc receptor. Exploitation of these properties promotes albumin as an attractive candidate for half-life extension and targeted intracellular delivery of drugs attached by covalent conjugation, genetic fusions, association or ligand-mediated association. This review will give an overview of albumin-based products with focus on the natural biological properties and molecular interactions that can be harnessed for the design of a next-generation drug delivery platform.

Albumin-based drug delivery: harnessing nature to cure disease

The effectiveness of a drug is dependent on accumulation at the site of action at therapeutic levels, however, challenges such as rapid renal clearance, degradation or non-specific accumulation requires drug delivery enabling technologies. Albumin is a natural transport protein with multiple ligand binding sites, cellular receptor engagement, and a long circulatory half-life due to interaction with the recycling neonatal Fc receptor. Exploitation of these properties promotes albumin as an attractive candidate for half-life extension and targeted intracellular delivery of drugs attached by covalent conjugation, genetic fusions, association or ligand-mediated association. This review will give an overview of albumin-based products with focus on the natural biological properties and molecular interactions that can be harnessed for the design of a next-generation drug delivery platform.

A mild TCEP-based para-azidobenzyl cleavage strategy to transform reversible cysteine thiol labelling reagents into irreversible conjugates

It has recently emerged that the succinimide linkage of a maleimide thiol addition product is fragile, which is a major issue in fields where thiol functionalisation needs to be robust. Herein we deliver a strategy that generates selective cysteine thiol labelling reagents, which are stable to hydrolysis and thiol exchange.

Methods to Design and Synthesize Antibody-Drug Conjugates (ADCs)

Antibody-drug conjugates (ADCs) have become a promising targeted therapy strategy that combines the specificity, favorable pharmacokinetics and biodistributions of antibodies with the destructive potential of highly potent drugs. One of the biggest challenges in the development of ADCs is the application of suitable linkers for conjugating drugs to antibodies. Recently, the design and synthesis of linkers are making great progress. In this review, we present the methods that are currently used to synthesize antibody-drug conjugates by using thiols, amines, alcohols, aldehydes and azides.

Cutting-edge mass spectrometry methods for the multi-level structural characterization of antibody-drug conjugates

Antibody drug conjugates (ADCs) are highly cytotoxic drugs covalently attached via conditionally stable linkers to monoclonal antibodies (mAbs) and are among the most promising next-generation empowered biologics for cancer treatment. ADCs are more complex than naked mAbs, as the heterogeneity of the conjugates adds to the inherent microvariability of the biomolecules. The development and optimization of ADCs rely on improving their analytical and bioanalytical characterization by assessing several critical quality attributes, namely the distribution and position of the drug, the amount of naked antibody, the average drug to antibody ratio, and the residual drug-linker and related product proportions. Here brentuximab vedotin (Adcetris) and trastuzumab emtansine (Kadcyla), the first and gold-standard hinge-cysteine and lysine drug conjugates, respectively, were chosen to develop new mass spectrometry (MS) methods and to improve multiple-level structural assessment protocols.

Site-selective protein-modification chemistry for basic biology and drug development

Nature has produced intricate machinery to covalently diversify the structure of proteins after their synthesis in the ribosome. In an attempt to mimic nature, chemists have developed a large set of reactions that enable post-expression modification of proteins at pre-determined sites. These reactions are now used to selectively install particular modifications on proteins for many biological and therapeutic applications. For example, they provide an opportunity to install post-translational modifications on proteins to determine their exact biological roles. Labelling of proteins in live cells with fluorescent dyes allows protein uptake and intracellular trafficking to be tracked and also enables physiological parameters to be measured optically. Through the conjugation of potent cytotoxicants to antibodies, novel anti-cancer drugs with improved efficacy and reduced side effects may be obtained. In this Perspective, we highlight the most exciting current and future applications of chemical site-selective protein modification and consider which hurdles still need to be overcome for more widespread use.

Antibody-Drug Conjugates (ADCs) Derived from Interchain Cysteine Cross-Linking Demonstrate Improved Homogeneity and Other Pharmacological Properties over Conventional Heterogeneous ADCs

Conventional antibody-drug conjugates (ADCs) are heterogeneous mixtures of chemically distinct molecules that vary in both drugs/antibody (DAR) and conjugation sites. Suboptimal properties of heterogeneous ADCs have led to new site-specific conjugation methods for improving ADC homogeneity. Most site-specific methods require extensive antibody engineering to identify optimal conjugation sites and introduce unique functional groups for conjugation with appropriately modified linkers. Alternative nonrecombinant methods have emerged in which bifunctional linkers are utilized to cross-link antibody interchain cysteines and afford ADCs containing four drugs/antibody. Although these methods have been shown to improve ADC homogeneity and stability in vitro, their effect on the pharmacological properties of ADCs in vivo is unknown. In order to determine the relative impact of interchain cysteine cross-linking on the therapeutic window and other properties of ADCs in vivo, we synthesized a derivative of the known ADC payload, MC-MMAF, that contains a bifunctional dibromomaleimide (DBM) linker instead of a conventional maleimide (MC) linker. The DBM-MMAF derivative was conjugated to trastuzumab and a novel anti-CD98 antibody to afford ADCs containing predominantly four drugs/antibody. The pharmacological properties of the resulting cross-linked ADCs were compared with analogous heterogeneous ADCs derived from conventional linkers. The results demonstrate that DBM linkers can be applied directly to native antibodies, without antibody engineering, to yield highly homogeneous ADCs via cysteine cross-linking. The resulting ADCs demonstrate improved pharmacokinetics, superior efficacy, and reduced toxicity in vivo compared to analogous conventional heterogeneous ADCs.

Stabilization of cysteine-linked antibody drug conjugates with N-aryl maleimides

Maleimides are often used to covalently attach drugs to cysteine thiols for production of antibody-drug conjugates (ADCs). However, ADCs formed with traditional N-alkyl maleimides have variable stability in the bloodstream leading to loss of drug. Here, we report that N-aryl maleimides form stable antibody conjugates under very mild conditions while also maintaining high conjugation efficiency. Thiol-maleimide coupling and ADC stabilization via thiosuccinimide hydrolysis were accelerated by addition of N-phenyl or N-fluorophenyl groups to the ring-head nitrogen. Cysteine-linked ADCs prepared with N-aryl maleimides exhibited less than 20% deconjugation in both thiol-containing buffer and serum when incubated at 37 °C over a period of 7 days, whereas the analogous ADCs prepared with N-alkyl maleimides showed 35-67% deconjugation under the same conditions. ADCs prepared with the anticancer drug N-phenyl maleimide monomethyl-auristatin-E (MMAE) maintained high cytotoxicity following long-term exposure to serum whereas the N-alkyl maleimide MMAE ADC lost potency over time. These data demonstrate that N-aryl maleimides are a convenient and flexible platform to improve the stability of ADCs through manipulation of functional groups attached to the maleimide ring-head nitrogen.

Conjugation Approach To Produce a Staphylococcus aureus Synbody with Activity in Serum

Synbodies show promise as a new class of synthetic antibiotics. Here, we explore improvements in their activity and production through conjugation chemistry. Maleimide conjugation is a widely used conjugation strategy due to its high yield, selectivity, and low cost. We used this strategy to conjugate two antibacterial peptides to produce a bivalent antibacterial peptide, called a synbody that has bactericidal activity against methicillin resistant Staphylococcus aureus (MRSA). The synbody was prepared by conjugation of a partially d-amino acid substituted synthetic antibacterial peptide to a bis-maleimide scaffold. The synbody slowly degrades in serum, but also undergoes exchange reactions with other serum proteins, such as albumin. Therefore, we hydrolyzed the thiosuccinimide ring using a mild hydrolysis protocol to produce a new synbody with similar bactericidal activity. The synbody was now resistant to exchange reactions and maintained bactericidal activity in serum for 2 h. This work demonstrates that low-cost maleimide coupling can be used to produce antibacterial peptide conjugates with activity in serum.

An emerging playbook for antibody-drug conjugates: lessons from the laboratory and clinic suggest a strategy for improving efficacy and safety

Antibody-drug conjugates (ADCs) have become de rigueur for pharmaceutical oncology drug development pipelines. There are more than 40 ADCs undergoing clinical trials and many more in preclinical development. The field has rushed to follow the initial successes of Kadcyla™ and Adcetris™, and moved forward with new targets without much pause for optimization. In some respects, the ADC space has become divided into the clinical realm-where the proven technologies continue to represent the bulk of clinical candidates with a few exceptions-and the research realm-where innovations in conjugation chemistry and linker technologies have suggested that there is much room for improvement in the conventional methods. Now, two and four years after the approvals of Kadcyla™ and Adcetris™, respectively, consensus may at last be building that these two drugs rely on rather unique target antigens that enable their success. It is becoming increasingly clear that future target antigens will require additional innovative approaches. Next-generation ADCs have begun to move out of the lab and into the clinic, where there is a pressing need for continued innovation to overcome the twin challenges of safety and efficacy.

Site-Dependent Degradation of a Non-Cleavable Auristatin-Based Linker-Payload in Rodent Plasma and Its Effect on ADC Efficacy

The efficacy of an antibody-drug conjugate (ADC) is dependent on the properties of its linker-payload which must remain stable while in systemic circulation but undergo efficient processing upon internalization into target cells. Here, we examine the stability of a non-cleavable Amino-PEG6-based linker bearing the monomethyl auristatin D (MMAD) payload site-specifically conjugated at multiple positions on an antibody. Enzymatic conjugation with transglutaminase allows us to create a stable amide linkage that remains intact across all tested conjugation sites on the antibody, and provides us with an opportunity to examine the stability of the auristatin payload itself. We report a position-dependent degradation of the C terminus of MMAD in rodent plasma that has a detrimental effect on its potency. The MMAD cleavage can be eliminated by either modifying the C terminus of the toxin, or by selection of conjugation site. Both approaches result in improved stability and potency in vitro and in vivo. Furthermore, we show that the MMAD metabolism in mouse plasma is likely mediated by a serine-based hydrolase, appears much less pronounced in rat, and was not detected in cynomolgus monkey or human plasma. Clarifying these species differences and controlling toxin degradation to optimize ADC stability in rodents is essential to make the best ADC selection from preclinical models. The data presented here demonstrate that site selection and toxin susceptibility to mouse plasma degradation are important considerations in the design of non-cleavable ADCs, and further highlight the benefits of site-specific conjugation methods.

In vivo biotransformations of antibody–drug conjugates

The selective delivery of potent pharmacologically active compounds to target tissue or cells by antibody–drug conjugates makes this immuno-conjugate a promising modality for the treatment of cancers. A thorough understanding of the structural integrity of the linker, the payload and the conjugation site during biological exposure is critical throughout the process of novel linker-payload design and optimization of PK profile. This understanding is a key aspect of the effort to maximize efficacy while minimizing toxicity in preclinical testing and to ensure the translation to the clinical setting. The complexity of this bioconjugate modality is a source of significant challenge for analytical interrogation and analysis in vivo. Therefore, we report herein a survey of various types of biotransformation events that have been elucidated in recent years.

A platform for efficient, thiol-stable conjugation to albumin's native single accessible cysteine

Herein we report the use of bromomaleimides for the construction of stable albumin conjugates via conjugation to its native, single accessible, cysteine followed by hydrolysis. Advantages over the classical maleimide approach are highlighted in terms of quantitative hydrolysis and absence of undesirable retro-Michael deconjugation.

Chemoenzymatic Conjugation of Toxic Payloads to the Globally Conserved N-Glycan of Native mAbs Provides Homogeneous and Highly Efficacious Antibody-Drug Conjugates

A robust, generally applicable, nongenetic technology is presented to convert monoclonal antibodies into stable and homogeneous ADCs. Starting from a native (nonengineered) mAb, a chemoenzymatic protocol allows for the highly controlled attachment of any given payload to the N-glycan residing at asparagine-297, based on a two-stage process: first, enzymatic remodeling (trimming and tagging with azide), followed by ligation of the payload based on copper-free click chemistry. The technology, termed GlycoConnect, is applicable to any IgG isotype irrespective of glycosylation profile. Application to trastuzumab and maytansine, both components of the marketed ADC Kadcyla, demonstrate a favorable in vitro and in vivo efficacy for GlycoConnect ADC. Moreover, the superiority of the native glycan as attachment site was demonstrated by in vivo comparison to a range of trastuzumab-based glycosylation mutants. A side-by-side comparison of the copper-free click probes bicyclononyne (BCN) and a dibenzoannulated cyclooctyne (DBCO) showed a surprising difference in conjugation efficiency in favor of BCN, which could be even further enhanced by introduction of electron-withdrawing fluoride substitutions onto the azide. The resulting mAb-conjugates were in all cases found to be highly stable, which in combination with the demonstrated efficacy warrants ADCs with a superior therapeutic index.

In Vitro and In Vivo Evaluation of Cysteine Rebridged Trastuzumab-MMAE Antibody Drug Conjugates with Defined Drug-to-Antibody Ratios

The conjugation of monomethyl auristatin E (MMAE) to trastuzumab using a reduction bis-alkylation approach that is capable of rebridging reduced (native) antibody interchain disulfide bonds has been previously shown to produce a homogeneous and stable conjugate with a drug-to-antibody ratio (DAR) of 4 as the major product. Here, we further investigate the potency of the DAR 4 conjugates prepared by bis-alkylation by comparing to lower drug loaded variants to maleimide linker based conjugates possessing typical mixed DAR profiles. Serum stability, HER2 receptor binding, internalization, in vitro potency, and in vivo efficacy were all evaluated. Greater stability compared with maleimide conjugation was observed with no significant decrease in receptor/FcRn binding. A clear dose-response was obtained based on drug loading (DAR) with the DAR 4 conjugate showing the highest potency in vitro and a much higher efficacy in vivo compared with the lower DAR conjugates. Finally, the DAR 4 conjugate demonstrated superior efficacy compared to trastuzumab-DM1 (T-DM1, Kadcyla), as evaluated in a low HER2 expressing JIMT-1 xenograft model.

Exploiting protected maleimides to modify oligonucleotides, peptides and peptide nucleic acids

This manuscript reviews the possibilities offered by 2,5-dimethylfuran-protected maleimides. Suitably derivatized building blocks incorporating the exo Diels-Alder cycloadduct can be introduced at any position of oligonucleotides, peptide nucleic acids, peptides and peptoids, making use of standard solid-phase procedures. Maleimide deprotection takes place upon heating, which can be followed by either Michael-type or Diels-Alder click conjugation reactions. However, the one-pot procedure in which maleimide deprotection and conjugation are simultaneously carried out provides the target conjugate more quickly and, more importantly, in better yield. This procedure is compatible with conjugates involving oligonucleotides, peptides and peptide nucleic acids. A variety of cyclic peptides and oligonucleotides can be obtained from peptide and oligonucleotide precursors incorporating protected maleimides and thiols.

Emerging formats for next-generation antibody drug conjugates

INTRODUCTION

Antibody drug conjugates now make up a significant fraction of biopharma's oncology pipeline due to great advances in the understanding of the three key components and how they should be optimised together. With this clinical success comes innovation to produce new enabling technologies that can deliver more effective antibody-drug conjugates (ADCs) with a larger therapeutic index.

AREAS COVERED

There are many reviews that discuss the various strategies for ADCs design but the last 5 years or so have witnessed the emergence of a number of different antibody formats compete with the standard whole immunoglobulin. Using published research, patent applications and conference disclosures, the authors review the many antibody and antibody-like formats, discussing innovations in protein engineering and how these new formats impact on the conjugation strategy and ultimately the performance. The alternative chemistries that are now available offer new linkages, stability profiles, drug:antibody ratio, pharmacokinetics and efficacy. The different sizes being considered promise to address issues, such as tumour penetration, circulatory half-life and side-effects.

EXPERT OPINION

ADCs are at the beginning of the next stage in their evolution and as these newer formats are developed and examined in the clinic, we will discover if the predicted features have a clinical benefit. From the commercial activity, it is envisaged that smaller or fragment-based ADCs will expand oncological applications.

Effect of attachment site on stability of cleavable antibody drug conjugates

The systemic stability of the antibody-drug linker is crucial for delivery of an intact antibody-drug conjugate (ADC) to target-expressing tumors. Linkers stable in circulation but readily processed in the target cell are necessary for both safety and potency of the delivered conjugate. Here, we report a range of stabilities for an auristatin-based payload site-specifically attached through a cleavable valine-citrulline-p-aminobenzylcarbamate (VC-PABC) linker across various sites on an antibody. We demonstrate that the conjugation site plays an important role in determining VC-PABC linker stability in mouse plasma, and that the stability of the linker positively correlates with ADC cytotoxic potency both in vitro and in vivo. Furthermore, we show that the VC-PABC cleavage in mouse plasma is not mediated by Cathepsin B, the protease thought to be primarily responsible for linker processing in the lysosomal degradation pathway. Although the VC-PABC cleavage is not detected in primate plasma in vitro, linker stabilization in the mouse is an essential prerequisite for designing successful efficacy and safety studies in rodents during preclinical stages of ADC programs. The divergence of linker metabolism in mouse plasma and its intracellular cleavage offers an opportunity for linker optimization in the circulation without compromising its efficient payload release in the target cell.

Tumor cells chronically treated with a trastuzumab-maytansinoid antibody-drug conjugate develop varied resistance mechanisms but respond to alternate treatments

Antibody-drug conjugates (ADC) are emerging as clinically effective therapy. We hypothesized that cancers treated with ADCs would acquire resistance mechanisms unique to immunoconjugate therapy and that changing ADC components may overcome resistance. Breast cancer cell lines were exposed to multiple cycles of anti-Her2 trastuzumab-maytansinoid ADC (TM-ADC) at IC80 concentrations followed by recovery. The resistant cells, 361-TM and JIMT1-TM, were characterized by cytotoxicity, proteomic, transcriptional, and other profiling. Approximately 250-fold resistance to TM-ADC developed in 361-TM cells, and cross-resistance was observed to other non-cleavable-linked ADCs. Strikingly, these 361-TM cells retained sensitivity to ADCs containing cleavable mcValCitPABC-linked auristatins. In JIMT1-TM cells, 16-fold resistance to TM-ADC developed, with cross-resistance to other trastuzumab-ADCs. Both 361-TM and JIMT1-TM cells showed minimal resistance to unconjugated mertansine (DM1) and other chemotherapeutics. Proteomics and immunoblots detected increased ABCC1 (MRP1) drug efflux protein in 361-TM cells, and decreased Her2 (ErbB2) in JIMT1-TM cells. Proteomics also showed alterations in various pathways upon chronic exposure to the drug in both cell models. Tumors derived from 361-TM cells grew in mice and were refractory to TM-ADC compared with parental cells. Hence, acquired resistance to trastuzumab-maytansinoid ADC was generated in cultured cancer cells by chronic drug treatment, and either increased ABCC1 protein or reduced Her2 antigen were primary mediators of resistance. These ADC-resistant cell models retain sensitivity to other ADCs or standard-of-care chemotherapeutics, suggesting that alternate therapies may overcome acquired ADC resistance. Mol Cancer Ther; 14(4); 952-63. ©2015 AACR.

CBTF: new amine-to-thiol coupling reagent for preparation of antibody conjugates with increased plasma stability

Amine-to-thiol coupling is the most common route for the preparation of antibody-drug conjugates (ADC). It is usually achieved by using heterobifunctional reagents possessing an activated ester at one end and a maleimide group at the other. However, maleimide-based conjugates were recently revealed to have limited stability in blood circulation, which can compromise therapeutic efficacy of the conjugate. To address this issue, we have developed a heterobifunctional reagent, sodium 4-((4-(cyanoethynyl)benzoyl)oxy)-2,3,5,6-tetrafluorobenzenesulfonate (CBTF), for amine-to-thiol coupling. It comprises a recently described 3-arylpropionitrile (APN) function in replacement of maleimide and allows for the preparation of remarkably stable conjugates. A series of antibody-dye conjugates have been prepared using this reagent and shown superior stability in human blood plasma compared to maleimide-derived conjugates.