Development of Efficient Chemistry to Generate Site-Specific Disulfide-Linked Protein– and Peptide–Payload Conjugates: Application to THIOMAB Antibody–Drug Conjugates

What influences the activity of Degrader-Antibody conjugates (DACs)

The targeted protein degradation (TPD) technology employing proteolysis-targeting chimeras (PROTACs) has been widely applied in drug chemistry and chemical biology for the treatment of cancer and other diseases. PROTACs have demonstrated significant advantages in targeting undruggable targets and overcoming drug resistance. However, despite the efficient degradation of targeted proteins achieved by PROTACs, they still face challenges related to selectivity between normal and cancer cells, as well as issues with poor membrane permeability due to their substantial molecular weight. Additionally, the noteworthy toxicity resulting from off-target effects also needs to be addressed. To solve these issues, Degrader-Antibody Conjugates (DACs) have been developed, leveraging the targeting and internalization capabilities of antibodies. In this review, we elucidates the characteristics and distinctions between DACs, and traditional Antibody-drug conjugates (ADCs). Meanwhile, we emphasizes the significance of DACs in facilitating the delivery of PROTACs and delves into the impact of various components on DAC activity. These components include antibody targets, drug-antibody ratio (DAR), linker types, PROTACs targets, PROTACs connections, and E3 ligase ligands. The review also explores the suitability of different targets (antibody targets or PROTACs targets) for DACs, providing insights to guide the design of PROTACs better suited for antibody conjugation.

The HER2-directed antibody-drug conjugate DHES0815A in advanced and/or metastatic breast cancer: preclinical characterization and phase 1 trial results

Approved antibody-drug conjugates (ADCs) for HER2-positive breast cancer include trastuzumab emtansine and trastuzumab deruxtecan. To develop a differentiated HER2 ADC, we chose an antibody that does not compete with trastuzumab or pertuzumab for binding, conjugated to a reduced potency PBD (pyrrolobenzodiazepine) dimer payload. PBDs are potent cytotoxic agents that alkylate and cross-link DNA. In our study, the PBD dimer is modified to alkylate, but not cross-link DNA. This HER2 ADC, DHES0815A, demonstrates in vivo efficacy in models of HER2-positive and HER2-low cancers and is well-tolerated in cynomolgus monkey safety studies. Mechanisms of action include induction of DNA damage and apoptosis, activity in non-dividing cells, and bystander activity. A dose-escalation study (ClinicalTrials.gov: NCT03451162) in patients with HER2-positive metastatic breast cancer, with the primary objective of evaluating the safety and tolerability of DHES0815A and secondary objectives of characterizing the pharmacokinetics, objective response rate, duration of response, and formation of anti-DHES0815A antibodies, is reported herein. Despite early signs of anti-tumor activity, patients at higher doses develop persistent, non-resolvable dermal, ocular, and pulmonary toxicities, which led to early termination of the phase 1 trial.

Saeed H, del Solar V, López-Hernández JE, Michel A, Mathew J, Lewis JS, Contel M. Shifting the Antibody-Drug Conjugate Paradigm: A Trastuzumab-Gold-Based Conjugate Demonstrates High Efficacy against Human Epidermal Growth Factor Receptor 2-Positive Breast Cancer Mouse Model

Antibody-drug conjugates (ADCs) combine the selectivity of monoclonal antibodies (mAbs) with the efficacy of chemotherapeutics to target cancers without toxicity to normal tissue. Clinically, most chemotherapeutic ADCs are based on complex organic molecules, while the conjugation of metallodrugs to mAbs has been overlooked, despite the resurgent interest in metal-based drugs as cancer chemotherapeutics. In 2019, we described the first gold ADCs containing gold-triphenylphosphane fragments as a proof of concept. The ADCs (based on the antibody trastuzumab) were selective and highly active against HER2-positive breast cancer cells. In this study, we developed site-specific ADCs (Thio-1b and Thio-2b) using the cysteine-engineered trastuzumab derivative THIOMAB antibody technology with gold(I)-containing phosphanes and a maleimide-based linker amenable to bioconjugation (1b and 2b). In addition, we developed lysine-directed ADCs with gold payloads based on phosphanes and N-heterocyclic carbenes featuring an activated ester moiety (2c and 5c) with trastuzumab (Tras-2c and Tras-5c) and another anti-HER2 antibody, pertuzumab (Per-2c and Per-5c). Both sets of ADCs demonstrated significant anticancer potency in vitro assays. Based on these results, one ADC (Tras-2c), containing the [Au(PEt3)] fragment present in FDA-approved auranofin, was selected for an in vivo antitumor efficacy study. Immunocompromised mice xenografted with the HER2-positive human cancer cell line SKBR-3 exhibited almost complete tumor reduction and low toxicity with intravenous administration of Tras-2c. With this highly selective targeting system, we demonstrated that a subnanomolar cytotoxicity profile in cells is not required for an impressive antitumor effect in a mouse xenograft model.

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

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

A comprehensive review of key factors affecting the efficacy of antibody drug conjugate

Antibody Drug Conjugate (ADC) is an emerging technology to overcome the limitations of chemotherapy by selectively targeting the cancer cells. ADC binds with an antigen, specifically over expressed on the surface of cancer cells, results decrease in bystander effect and increase in therapeutic index. The potency of an ideal ADC is entirely depending on several physicochemical factors such as site of conjugation, molecular weight, linker length, Steric hinderance, half-life, conjugation method, binding energy and so on. Inspite of the fact that there is more than 100 of ADCs are in clinical trial only 14 ADCs are approved by FDA for clinical use. However, to design an ideal ADC is still challenging and there is much more to be done. Here in this review, we have discussed the key components along with their significant role or contribution towards the efficacy of an ADC. Moreover, we also explained about the recent advancement in the conjugation method. Additionally, we spotlit the mode of action of an ADC, recent challenges, and future perspective regarding ADC. The profound knowledge regarding key components and their properties will help in the synthesis or production of different engineered ADCs. Therefore, contributes to develop an ADC with low safety concern and high therapeutic index. We hope this review will improve the understanding and encourage the practicing of research in anticancer ADCs development.

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.

Interrogating heterogeneity of cysteine-engineered antibody-drug conjugates and antibody-oligonucleotide conjugates by capillary zone electrophoresis-mass spectrometry

Production of site-specific cysteine-engineered antibody-drug conjugates (ADCs) in mammalian cells may produce developability challenges, fragments, and heterogenous molecules, leading to potential product critical quality attributes in later development stages. Liquid phase chromatography with mass spectrometry (LC-MS) is widely used to evaluate antibody impurities and drug-to-antibody ratio, but faces challenges in analysis of fragment product variants of cysteine-engineered ADCs and oligonucleotide-to-antibody ratio (OAR) species of antibody-oligonucleotide conjugates (AOCs). Here, for the first time, we report novel capillary zone electrophoresis (CZE)-MS approaches to address the challenges above. CZE analysis of six ADCs made with different parent monoclonal antibodies (mAbs) and small molecule drug-linker payloads revealed that various fragment impurities, such as half mAbs with one/two drugs, light chains with one/two drugs, light chains with C-terminal cysteine truncation, heavy chain clippings, were well resolved from the main species. However, most of these fragments were coeluted or had signal suppression during LC-MS analysis. Furthermore, the method was optimized on both ionization and separation aspects to enable the characterization of two AOCs. The method successfully achieved baseline separation and accurate quantification of their OAR species, which were also highly challenging using conventional LC-MS methods. Finally, we compared the migration time and CZE separation profiles among ADCs and their parent mAbs, and found that properties of mAbs and linker payloads significantly influenced the separation of product variants by altering their size or charge. Our study showcases the good performance and broad applicability of CZE-MS techniques for monitoring the heterogeneity of cysteine-engineered ADCs and AOCs.

Estrogen Receptor-α Targeting: PROTACs, SNIPERs, Peptide-PROTACs, Antibody Conjugated PROTACs and SNIPERs

Targeting selective estrogen subtype receptors through typical medicinal chemistry approaches is based on occupancy-driven pharmacology. In occupancy-driven pharmacology, molecules are developed in order to inhibit the protein of interest (POI), and their popularity is based on their virtue of faster kinetics. However, such approaches have intrinsic flaws, such as pico-to-nanomolar range binding affinity and continuous dosage after a time interval for sustained inhibition of POI. These shortcomings were addressed by event-driven pharmacology-based approaches, which degrade the POI rather than inhibit it. One such example is PROTACs (Proteolysis targeting chimeras), which has become one of the highly successful strategies of event-driven pharmacology (pharmacology that does the degradation of POI and diminishes its functions). The selective targeting of estrogen receptor subtypes is always challenging for chemical biologists and medicinal chemists. Specifically, estrogen receptor α (ER-α) is expressed in nearly 70% of breast cancer and commonly overexpressed in ovarian, prostate, colon, and endometrial cancer. Therefore, conventional hormonal therapies are most prescribed to patients with ER + cancers. However, on prolonged use, resistance commonly developed against these therapies, which led to selective estrogen receptor degrader (SERD) becoming the first-line drug for metastatic ER + breast cancer. The SERD success shows that removing cellular ER-α is a promising approach to overcoming endocrine resistance. Depending on the mechanism of degradation of ER-α, various types of strategies of developed.

Development of a Promising 18F-Radiotracer for PET Imaging Legumain Activity In Vivo

Legumain has been found overexpressed in several cancers, which serves as an important biomarker for cancer diagnosis. In this research, a novel fluorine-18 labeled radioactive tracer [18F]SF-AAN targeting legumain was designed and synthesized for positron emission tomography (PET) imaging. Nonradioactive probe [19F]SF-AAN was obtained through chemical and solid phase peptide synthesis. After a simple one-step 18F labeling, the radiotracer [18F]SF-AAN was obtained with a high radiochemical conversion rate (>85%) and radiochemical purity (99%) as well as high molar activity (12.77 ± 0.50 MBq/nmol). The targeting specificity of [18F]SF-AAN for detecting legumain activity was investigated systematically in vitro and in vivo. In vitro cellular uptake assay showed that the uptake of [18F]SF-AAN in legumain-positive MDA-MB-468 cells was twice as much as that in legumain-negative PC-3 cells at 4 h. In vivo PET imaging revealed that the tumor uptake of [18F]SF-AAN in MDA-MB-468 tumor-bearing mice was about 2.7 times of that in PC-3 tumor-bearing mice at 10 min post injection. The experimental results indicated that [18F]SF-AAN could serve as a promising PET tracer for detecting the legumain expression sensitively and specifically, which would be beneficial for the diagnosis of legumain-related diseases.

Cell-penetrating peptides in protein mimicry and cancer therapeutics

Extensive research has been undertaken in the pursuit of anticancer therapeutics. Many anticancer drugs require specificity of delivery to cancer cells, whilst sparing healthy tissue. Cell-penetrating peptides (CPPs), now well established as facilitators of intracellular delivery, have in recent years advanced to incorporate target specificity and thus possess great potential for the targeted delivery of anticancer cargoes. Though none have yet been approved for clinical use, this novel technology has already entered clinical trials. In this review we present CPPs, discuss their classification, mechanisms of cargo internalization and highlight strategies for conjugation to anticancer moieties including their incorporation into therapeutic proteins. As the mainstay of this review, strategies to build specificity into tumor targeting CPP constructs through exploitation of the tumor microenvironment and the use of tumor homing peptides are discussed, whilst acknowledging the extensive contribution made by CPP constructs to target specific protein-protein interactions integral to intracellular signaling pathways associated with tumor cell survival and progression. Finally, antibody/antigen CPP conjugates and their potential roles in cancer immunotherapy and diagnostics are considered. In summary, this review aims to harness the potential of CPP-aided drug delivery for future cancer therapies and diagnostics whilst highlighting some of the most recent achievements in selective delivery of anticancer drugs, including cytostatic drugs, to a range of tumor cells both in vitro and in vivo.

Linker Design Impacts Antibody-Drug Conjugate Pharmacokinetics and Efficacy via Modulating the Stability and Payload Release Efficiency

The development of antibody-drug conjugates (ADCs) has significantly been advanced in the past decade given the improvement of payloads, linkers and conjugation methods. In particular, linker design plays a critical role in modulating ADC stability in the systemic circulation and payload release efficiency in the tumors, which thus affects ADC pharmacokinetic (PK), efficacy and toxicity profiles. Previously, we have investigated key linker parameters such as conjugation chemistry (e.g., maleimide vs. disulfide), linker length and linker steric hindrance and their impacts on PK and efficacy profiles. Herein, we discuss our perspectives on development of integrated strategies for linker design to achieve a balance between ADC stability and payload release efficiency for desired efficacy in antigen-expressing xenograft models. The strategies have been successfully applied to the design of site-specific THIOMAB^TM antibody-drug conjugates (TDCs) with different payloads. We also propose to conduct dose fractionation studies to gain guidance for optimal dosing regimens of ADCs in pre-clinical models.

Antibody-drug conjugates for the treatment of lymphoma: clinical advances and latest progress

Antibody-drug conjugates (ADCs) are a promising class of immunotherapies with the potential to specifically target tumor cells and ameliorate the therapeutic index of cytotoxic drugs. ADCs comprise monoclonal antibodies, cytotoxic payloads with inherent antitumor activity, and specialized linkers connecting the two. In recent years, three ADCs, brentuximab vedotin, polatuzumab vedotin, and loncastuximab tesirine, have been approved and are already establishing their place in lymphoma treatment. As the efficacy and safety of ADCs have moved in synchrony with advances in their design, a plethora of novel ADCs have garnered growing interest as treatments. In this review, we provide an overview of the essential elements of ADC strategies in lymphoma and elucidate the up-to-date progress, current challenges, and novel targets of ADCs in this rapidly evolving field.

A Simple and Efficient Method to Generate Dual Site-Specific Conjugation ADCs with Cysteine Residue and an Unnatural Amino Acid

Antibody-drug conjugates (ADCs) are complex pharmaceutical molecules that combine monoclonal antibodies with biologically active drugs through chemical linkers. ADCs are designed to specifically kill disease cells by utilizing the target specificity of antibodies and the cytotoxicity of chemical drugs. However, the traditional ADCs were only applied to a few disease targets because of some limitations such as the huge molecular weight, the uncontrollable coupling reactions, and a single mechanism of action. Here we report a simple, one-pot, successive reaction method to produce dual payload conjugates with the site-specifically engineered cysteine and p-acetyl-phenylalanine using Herceptin (trastuzumab), an anti-HER2 antibody drug widely used for breast cancer treatment, as a tool molecule. This strategy enables antibodies to conjugate with two mechanistically distinct cytotoxic drugs through different functional groups sequentially, therefore, rendering the newly designed ADCs with functional diversity and the potential to overcome drug resistance and enhance the therapeutic efficacy.

Calicheamicin Antibody-Drug Conjugates with Improved Properties

Calicheamicin antibody-drug conjugates (ADCs) are effective therapeutics for leukemias with two recently approved in the United States: Mylotarg (gemtuzumab ozogamicin) targeting CD33 for acute myeloid leukemia and Besponsa (inotuzumab ozogamicin) targeting CD22 for acute lymphocytic leukemia. Both of these calicheamicin ADCs are heterogeneous, aggregation-prone, and have a shortened half-life due to the instability of the acid-sensitive hydrazone linker in circulation. We hypothesized that we could improve upon the heterogeneity, aggregation, and circulation stability of calicheamicin ADCs by directly attaching the thiol of a reduced calicheamicin to an engineered cysteine on the antibody via a disulfide bond to generate a linkerless and traceless conjugate. We report herein that the resulting homogeneous conjugates possess minimal aggregation and display high in vivo stability with 50% of the drug remaining conjugated to the antibody after 21 days. Furthermore, these calicheamicin ADCs are highly efficacious in mouse models of both solid tumor (HER2⁺ breast cancer) and hematologic malignancies (CD22⁺ non-Hodgkin lymphoma). Safety studies in rats with this novel calicheamicin ADC revealed an increased tolerability compared with that reported for Mylotarg. Overall, we demonstrate that applying novel linker chemistry with site-specific conjugation affords an improved, next-generation calicheamicin ADC.

Antibody-Mediated Delivery of Chimeric BRD4 Degraders. Part 2: Improvement of In Vitro Antiproliferation Activity and In Vivo Antitumor Efficacy

Heterobifunctional compounds that direct the ubiquitination of intracellular proteins in a targeted manner via co-opted ubiquitin ligases have enormous potential to transform the field of medicinal chemistry. These chimeric molecules, often termed proteolysis-targeting chimeras (PROTACs) in the chemical literature, enable the controlled degradation of specific proteins via their direction to the cellular proteasome. In this report, we describe the second phase of our research focused on exploring antibody-drug conjugates (ADCs), which incorporate BRD4-targeting chimeric degrader entities. We employ a new BRD4-binding fragment in the construction of the chimeric ADC payloads that is significantly more potent than the corresponding entity utilized in our initial studies. The resulting BRD4-degrader antibody conjugates exhibit potent and antigen-dependent BRD4 degradation and antiproliferation activities in cell-based experiments. Multiple ADCs bearing chimeric BRD4-degrader payloads also exhibit strong, antigen-dependent antitumor efficacy in mouse xenograft assessments that employ several different tumor models.

Antibody-Mediated Delivery of Chimeric BRD4 Degraders. Part 1: Exploration of Antibody Linker, Payload Loading, and Payload Molecular Properties

The biological and medicinal impacts of proteolysis-targeting chimeras (PROTACs) and related chimeric molecules that effect intracellular degradation of target proteins via ubiquitin ligase-mediated ubiquitination continue to grow. However, these chimeric entities are relatively large compounds that often possess molecular characteristics, which may compromise oral bioavailability, solubility, and/or in vivo pharmacokinetic properties. We therefore explored the conjugation of such molecules to monoclonal antibodies using technologies originally developed for cytotoxic payloads so as to provide alternate delivery options for these novel agents. In this report, we describe the first phase of our systematic development of antibody-drug conjugates (ADCs) derived from bromodomain-containing protein 4 (BRD4)-targeting chimeric degrader entities. We demonstrate the antigen-dependent delivery of the degrader payloads to PC3-S1 prostate cancer cells along with related impacts on MYC transcription and intracellular BRD4 levels. These experiments culminate with the identification of one degrader conjugate, which exhibits antigen-dependent antiproliferation effects in LNCaP prostate cancer cells.

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.

Synthesis of poly(disulfide)s with narrow molecular weight distributions via lactone ring-opening polymerization

We report the first example of controlled polymerization of poly(disulfide)s with narrow molecular weight distributions. 1,4,5-oxadithiepan-2-one (OTP), a disulfide-containing 7-membered ring lactone, was polymerized by using the diphenylphosphate (DPP) catalyzed lactone ring-opening polymerization method. The polymerization proceeded in a living manner, and the resulting polymers displayed very narrow polydispersity index (PDI) values below 1.1 and excellent backbone degradability responding to reducing conditions and UV irradiation.

We report the first example of controlled polymerization of poly(disulfide)s with narrow molecular weight distributions.

Steric and stereoscopic disulfide construction for cross-linkage via N-dithiophthalimides

Disulfide bonds are a significant motif in life and drug-delivery systems. In particular, steric hindrance and stereoscopic disulfide linkers are closely associated with the stability of antibody–drug conjugates, which affects the potency, selectivity, and pharmacokinetics of drugs. However, limited availability and diversity of tertiary thiols impede the construction of steric and stereoscopic disulfides for cross-linkage in biochemistry and pharmaceuticals. Through modulating the mask effect of disulfurating reagents, we develop a facile and robust strategy for construction of diverse steric and stereoscopic disulfides via N-dithiophthalimides. The practical cross-linkage of biomolecules including amino acids, saccharides, and nucleosides with different drugs and fluorescent molecules is successfully established through hindered disulfide linkers.

A series of steric and stereoscopic disulfides are constructed with N-dithiophthalimides, enabling the cross-linkage of biomolecules, drugs and fluorescent molecules.

Trends in Therapeutic Conjugates: Bench to Clinic

In recent years, therapeutic conjugates have attracted considerable attention as a new class of drug due to their unique pharmacological properties, especially from the pharmaceutical community. Their molecular structure tunability, improved targeting specificity, and therapeutic efficacy have been demonstrated in a wide range of research and clinical applications. In this topical review, we summarize selected recent advances in bioconjugation strategies for the development of therapeutic conjugates, their emerging application in clinical settings, as well as perspectives on the direction of future research.

The Chemical Methods of Disulfide Bond Formation and Their Applications to Drug Conjugates

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The disulfide bond possesses unique chemical and biophysical properties which distinguish it as one of the key structural elements of bioactive proteins and peptides, important drugs and other materials. The chemo-selective synthesis of these structures and the exploration of their function have been of longstanding interest to the chemistry community. The past decades have witnessed significant progress in both areas. This review will summarize the historically established and recently developed chemical methods in disulfide bond formation. The discussion will also be extended to the use of the disulfide linkers in small molecules, and peptide- and protein-drug conjugates. It is hoped that the combined overview of the fundamental chemistries and applications to drug discovery will inspire creative thinking and stimulate future novel uses of these versatile chemistries.

Disulfide based prodrugs for cancer therapy

Advances in the tumor microenvironment have facilitated the development of novel anticancer drugs and delivery vehicles for improved therapeutic efficacy and decreased side effects. Disulfide bonds with unique chemical and biophysical properties can be used as cleavable linkers for the delivery of chemotherapeutic drugs. Accordingly, small molecule-, peptide-, polymer- and protein-based multifunctional prodrugs bearing cleavable disulfide bonds are well accepted in clinical settings. Herein, we first briefly introduce a number of prodrugs and divide them into three categories, namely, disulfide-containing small molecule conjugates, disulfide-containing cytotoxic agent–targeted fluorescent agent conjugates, and disulfide-containing cytotoxic agent–macromolecule conjugates. Then, we discuss the complex redox environment and the underlying mechanism of free drug release from disulfide based prodrugs in in vivo settings. Based on these insights, we analyze the impact of electronics, steric hindrance and substituent position of the disulfide linker on the extracellular stability and intracellular cleavage rate of disulfide containing prodrugs. Current challenges and future opportunities for the disulfide linker are provided at the end.

This review summarizes the progress in disulfide linker technology to balance extracellular stability and intracellular cleavage for optimized disulfide-containing prodrugs.

Development of a Cysteine-Conjugatable Disulfide FRET Probe: Influence of Charge on Linker Cleavage and Payload Trafficking for an Anti-HER2 Antibody Conjugate

Disulfide-linked bioconjugates allow the delivery of pharmacologically active or other cargo to specific tissues in a redox-sensitive fashion. However, an understanding of the kinetics, subcellular distribution, and mechanism of disulfide cleavage in such bioconjugates is generally lacking. Here, we report a modular disulfide-linked TAMRA-BODIPY based FRET probe that can be readily synthesized, modified, and conjugated to a cysteine-containing biomolecule to enable real-time monitoring of disulfide cleavage during receptor-mediated endocytosis in cells. We demonstrate the utility of this probe to study disulfide reduction during HER2 receptor-mediated uptake of a Cys-engineered anti-HER2 THIOMAB antibody. We found that introduction of positive, but not negative, charges in the probe improved retention of the BODIPY catabolite. This permitted the observation of significant disulfide cleavage in endosomes or lysosomes on par with proteolytic cleavage of a similarly charged valine-citrulline peptide-based probe. In general, the FRET probe we describe should enable real-time cellular monitoring of disulfide cleavage in other targeted delivery systems for mechanistic or diagnostic applications. Furthermore, modifications to the released BODIPY moiety permit evaluation of physicochemical properties that govern lysosomal egress or retention, which may have implications for the development of next-generation antibody-drug conjugates.

Antibody-mediated delivery of chimeric protein degraders which target estrogen receptor alpha (ERα)

Chimeric molecules which effect intracellular degradation of target proteins via E3 ligase-mediated ubiquitination (e.g., PROTACs) are currently of high interest in medicinal chemistry. However, these entities are relatively large compounds that often possess molecular characteristics which may compromise oral bioavailability, solubility, and/or in vivo pharmacokinetic properties. Accordingly, we explored whether conjugation of chimeric degraders to monoclonal antibodies using technologies originally developed for cytotoxic payloads might provide alternate delivery options for these novel agents. In this report we describe the construction of several degrader-antibody conjugates comprised of two distinct ERα-targeting degrader entities and three independent ADC linker modalities. We subsequently demonstrate the antigen-dependent delivery to MCF7-neo/HER2 cells of the degrader payloads that are incorporated into these conjugates. We also provide evidence for efficient intracellular degrader release from one of the employed linkers. In addition, preliminary data are described which suggest that reasonably favorable in vivo stability properties are associated with the linkers utilized to construct the degrader conjugates.

Disulfide-Based Self-Immolative Linkers and Functional Bioconjugates for Biological Applications

It is of vital importance to reversibly mask and selectively activate bioactive agents for advanced therapeutic and diagnostic purposes, aiming to efficiently suppress background interferences and attenuate systemic toxicity. This strategy has been involved in diverse applications spanning from chemical/biological sensors and diagnostics to drug delivery nanocarriers. Among these, redox-responsive disulfide linkages have been extensively utilized by taking advantage of extracellular and intracellular glutathione (GSH) gradients. However, direct conjugation of cleavable triggers to bioactive agents through disulfide bonds suffers from bulky steric hindrance and limited choice of trigger-drug combinations. Fortunately, the emergence of disulfide self-immolative linkers (DSILs) provides a general and robust strategy to not only mask various bioactive agents through the formation of dynamic disulfide linkages but also make it possible to be selectively activated upon disulfide cleavage in the reductive cytoplasmic milieu. In this review, recent developments in DSILs are focused with special attention on emerging chemical design strategies and functional applications in the biomedical field.

Tetravalent biepitopic targeting enables intrinsic antibody agonism of tumor necrosis factor receptor superfamily members

Agonism of members of the tumor necrosis factor receptor superfamily (TNFRSF) with monoclonal antibodies is of high therapeutic interest due to their role in immune regulation and cell proliferation. A major hurdle for pharmacologic activation of this receptor class is the requirement for high-order clustering, a mechanism that imposes a reliance in vivo on Fc receptor-mediated crosslinking. This extrinsic dependence represents a potential limitation of virtually the entire pipeline of agonist TNFRSF antibody drugs, of which none have thus far been approved or reached late-stage clinical trials. We show that tetravalent biepitopic targeting enables robust intrinsic antibody agonism for two members of this family, OX40 and DR5, that is superior to extrinsically crosslinked native parental antibodies. Tetravalent biepitopic anti-OX40 engagement co-stimulated OX40^low cells, obviated the requirement for CD28 co-signal for T cell activation, and enabled superior pharmacodynamic activity relative to native IgG in a murine vaccination model. This work establishes a proof of concept for an engineering approach that addresses a major gap for the therapeutic activation of this important receptor class.

Stabilizing p-Dithiobenzyl Urethane Linkers without Rate-Limiting Self-Immolation for Traceless Drug Release

Exploiting the redox sensitivity of disulfide bonds is a prevalent strategy in targeted prodrug designs. In contrast to aliphatic disulfides, p-thiobenzyl-based disulfides have rarely been used for prodrug designs, given their intrinsic instability caused by the low pK_a of aromatic thiols. Here, we examined the interplay between steric hindrance and the low-pK_a effect on thiol-disulfide exchange reactions and uncovered a new thiol-disulfide exchange process for the self-immolation of p-thiobenzyl-based disulfides. We observed a central leaving group shifting effect in the α,α-dimethyl-substituted p-dithiobenzyl urethane linkers (DMTB linkers), which leads to increased disulfide stability by more than two orders of magnitude, an extent that is significantly greater than that observed with typical aliphatic disulfides. In particular, the DMTB linkers display not only high stability, but also rapid self-immolation kinetics due to the low pK_a of the aromatic thiol, which can be used as a general and robust linkage between targeting reagents and cytotoxic drugs for targeted prodrug designs. The unique and promising stability characteristics of the present DMTB linker will likely inspire the development of novel targeted prodrugs to achieve traceless release of drugs into cells.

ADME Considerations and Bioanalytical Strategies for Pharmacokinetic Assessments of Antibody-Drug Conjugates

Antibody-drug conjugates (ADCs) are a unique class of biotherapeutics of inherent heterogeneity and correspondingly complex absorption, distribution, metabolism, and excretion (ADME) properties. Herein, we consider the contribution of various components of ADCs such as various classes of warheads, linkers, and conjugation strategies on ADME of ADCs. Understanding the metabolism and disposition of ADCs and interpreting exposure-efficacy and exposure-safety relationships of ADCs in the context of their various catabolites is critical for design and subsequent development of a clinically successful ADCs. Sophisticated bioanalytical assays are required for the assessments of intact ADC, total antibody, released warhead and relevant metabolites. Both ligand-binding assays (LBA) and hybrid LBA-liquid chromatography coupled with tandem mass spectrometry (LBA-LC-MS/MS) methods have been employed to assess pharmacokinetics (PK) of ADCs. Future advances in bioanalytical techniques will need to address the rising complexity of this biotherapeutic modality as more innovative conjugation strategies, antibody scaffolds and novel classes of warheads are employed for the next generation of ADCs. This review reflects our considerations on ADME of ADCs and provides a perspective on the current bioanalytical strategies for pharmacokinetic assessments of ADCs.

Synthesis and evaluation of novel dolastatin 10 derivatives for versatile conjugations

Dolastatin 10 (1) is a highly potent cytotoxic microtubule inhibitor (cytotoxicity IC₅₀ < 5.0 nM) and several of its analogs have recently been used as payloads in antibody drug conjugates. Herein, we describe the design and synthesis of a series of novel dolastatin 10 analogs useful as payloads for conjugated drugs. We explored analogs containing functional groups at the thiazole moiety at the C-terminal of dolastatin 10. The functional groups included amines, alcohols, and thiols, which are representative structures used in known conjugated drugs. These novel analogs showed excellent potency in a tumor cell proliferation assay, and thus this series of dolastatin 10 analogs is suitable as versatile payloads in conjugated drugs. Insights into the structure-activity relationships of the analogs are also discussed.

High-Throughput Cysteine Scanning To Identify Stable Antibody Conjugation Sites for Maleimide- and Disulfide-Based Linkers

THIOMAB antibody technology utilizes cysteine residues engineered onto an antibody to allow for site-specific conjugation. The technology has enabled the exploration of different attachment sites on the antibody in combination with small molecules, peptides, or proteins to yield antibody conjugates with unique properties. As reported previously ( Shen , B. Q. , et al. ( 2012 ) Nat. Biotechnol. 30 , 184 - 189 ; Pillow , T. H. , et al. ( 2017 ) Chem. Sci. 8 , 366 - 370 ), the specific location of the site of conjugation on an antibody can impact the stability of the linkage to the engineered cysteine for both thio-succinimide and disulfide bonds. High stability of the linkage is usually desired to maximize the delivery of the cargo to the intended target. In the current study, cysteines were individually substituted into every position of the anti-HER2 antibody (trastuzumab), and the stabilities of drug conjugations at those sites were evaluated. We screened a total of 648 THIOMAB antibody-drug conjugates, each generated from a trastuzamab prepared by sequentially mutating non-cysteine amino acids in the light and heavy chains to cysteine. Each THIOMAB antibody variant was conjugated to either maleimidocaproyl-valine-citrulline-p-aminobenzyloxycarbonyl-monomethyl auristatin E (MC-vc-PAB-MMAE) or pyridyl disulfide monomethyl auristatin E (PDS-MMAE) using a high-throughput, on-bead conjugation and purification method. Greater than 50% of the THIOMAB antibody variants were successfully conjugated to both MMAE derivatives with a drug to antibody ratio (DAR) of >0.5 and <50% aggregation. The relative in vitro plasma stabilities for approximately 750 conjugates were assessed using enzyme-linked immunosorbent assays, and stable sites were confirmed with affinity-capture LC/MS-based detection methods. Highly stable conjugation sites for the two types of MMAE derivatives were identified on both the heavy and light chains. Although the stabilities of maleimide conjugates were shown to be greater than those of the disulfide conjugates, many sites were identified that were stable for both. Furthermore, in vitro stabilities of selected stable sites translated across different cytotoxic payloads and different target antibodies as well as to in vivo stability.