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

A comparison of the activity, lysosomal stability, and efficacy of legumain-cleavable and cathepsin cleavable ADC linkers

1. Over the past two decades antibody-drug conjugates (ADCs) have emerged as a highly effective drug delivery technology. ADCs utilize a monoclonal antibody, a chemical linker, and a therapeutic payload to selectively deliver highly potent pharmaceutical agents to specific cell types.2. Challenges such as premature linker cleavage and clearance due to linker hydrophobicity have adversely impacted the stability and safety of ADCs. While there are various solutions to these challenges, our team has focused on replacement of hydrophobic ValCit linkers (cleaved by CatB) with Asn-containing linkers that are cleaved by lysosomal legumain.3. Legumain is abundantly present in lysosomes and is known to play a role in tumor microenvironment dynamics. Herein, we directly compare the lysosomal cleavage, cytotoxicity, plasma stability, and efficacy of a traditional cathepsin cleavable ADC to a matched Asn-containing legumain-cleavable ADC.4. We demonstrate that Asn-containing linker sequences are specifically cleaved by lysosomal legumain and that Asn-linked MMAE ADCs are broadly active against a variety of tumors, even those with low legumain expression. Finally, we show that AsnAsn-linked ADCs exhibit comparable or improved efficacy to traditional ValCit-linked ADCs. Our study paves the way for replacement of the traditional ValCit linker technology with more hydrophilic Asn-containing peptide linker sequences.

A Review of the Current FDA-Approved Antibody-Drug Conjugates: Landmark Clinical Trials and Indications

Despite considerable treatment progress, cancer remains among the leading causes of death worldwide. Antibody-drug conjugates (ADCs), a rapidly growing class of systemic therapy, show promise by combining the properties of conventional chemotherapy and targeted therapy. Antibody-drug conjugates have been shown to be more efficacious than traditional chemotherapy. To date, there are 13 ADCs approved by the United States Food and Drug Administration (FDA) for treating various hematological and solid organ cancers. There are several new promising ADCs that are being developed and are in clinical trials. This review provides an overview of the current FDA-approved ADCs, the landmark clinical trials that led to their approval, the common toxicities seen in the landmark trials, the challenges associated with ADCs, and the potential future directions.

Self-Immolative Carbamate Linkers for CD19-Budesonide Antibody-Drug Conjugates

Antibody-drug conjugates consist of potent small-molecule payloads linked to a targeting antibody. Payloads must possess a viable functional group by which a linker for conjugation can be attached. Linker-attachment options remain limited for the connection to payloads via hydroxyl groups. A releasing group based on 2-aminopyridine was developed to enable stable attachment of para-aminobenzyl carbamate (PABC) linkers to the C21-hydroxyl group of budesonide, a glucocorticoid receptor agonist. Payload release involves a cascade of two self-immolative events that are initiated by the protease-mediated cleavage of the dipeptide-PABC bond. Budesonide release rates were determined for a series of payload-linker intermediates in buffered solution at pH 7.4 and 5.4, leading to the identification of 2-aminopyridine as the preferred releasing group. Addition of a poly(ethylene glycol) group improved linker hydrophilicity, thereby providing CD19-budesonide ADCs with suitable properties. ADC23 demonstrated targeted delivery of budesonide to CD19-expressing cells and inhibited B-cell activation in mice.

Transition of average drug-to-antibody ratio of trastuzumab deruxtecan in systemic circulation in monkeys using a hybrid affinity capture liquid chromatography-tandem mass spectrometry

Trastuzumab deruxtecan (T-DXd, DS-8201a) is an antibody-drug conjugate, comprising an anti-HER2 antibody at a drug-to-antibody ratio of 7-8 with the topoisomerase I inhibitor DXd. In this study, the concentrations of antibody-conjugated DXd and total antibody were determined and observed to decrease over time following intravenous administration of T-DXd to monkeys. The drug-to-antibody ratio of T-DXd also decreased in a time-dependent manner, which reached approximately 2.5 in 21 days after administration. It was suggested that antibody-conjugated DXd of T-DXd was relatively stable in vivo compared with that of other reported antibody-drug conjugates.

Antibody-drug conjugates come of age in oncology

Antibody-drug conjugates (ADCs) combine the specificity of monoclonal antibodies with the potency of highly cytotoxic agents, potentially reducing the severity of side effects by preferentially targeting their payload to the tumour site. ADCs are being increasingly used in combination with other agents, including as first-line cancer therapies. As the technology to produce these complex therapeutics has matured, many more ADCs have been approved or are in late-phase clinical trials. The diversification of antigenic targets as well as bioactive payloads is rapidly broadening the scope of tumour indications for ADCs. Moreover, novel vector protein formats as well as warheads targeting the tumour microenvironment are expected to improve the intratumour distribution or activation of ADCs, and consequently their anticancer activity for difficult-to-treat tumour types. However, toxicity remains a key issue in the development of these agents, and better understanding and management of ADC-related toxicities will be essential for further optimization. This Review provides a broad overview of the recent advances and challenges in ADC development for cancer treatment.

Toxicity profile of antibody-drug conjugates in breast cancer: practical considerations

Antibody-drug conjugates (ADCs) represent a novel and evolving class of antineoplastic agents, constituted by monoclonal antibody linked to biologically active drugs, delivering cytotoxic compounds at the tumor site, reducing the likelihood of systemic exposure and toxicity. They are generally well tolerated, nevertheless some predictable adverse reactions need careful monitoring and timely approach. These include neutropenia, nausea and vomiting, alopecia, diarrhea, left ventricular dysfunction, ILD/pneumonitis. The mechanisms leading to drug-associated toxicities are summarized, and prophylaxis protocols and appropriate management strategies are proposed, based on current literature. This review aims to collect the most updated evidence on toxicities potentially occurring during breast cancer treatment with approved or under clinical investigation (advanced stage) ADCs. A focus is dedicated to monitoring protocols and clinical management, aimed at preventing and/or promptly address relevant problems, in order to avoid premature discontinuation or improper dose reduction.

2022 White Paper on Recent Issues in Bioanalysis: ICH M10 BMV Guideline & Global Harmonization; Hybrid Assays; Oligonucleotides & ADC; Non-Liquid & Rare Matrices; Regulatory Inputs (Part 1A - Recommendations on Mass Spectrometry, Chromatography and Sample Preparation, Novel Technologies, Novel Modalities, and Novel Challenges, ICH M10 BMV Guideline & Global Harmonization Part 1B - Regulatory Agencies' Inputs on Regulated Bioanalysis/BMV, Biomarkers/CDx/BAV, Immunogenicity, Gene & Cell Therapy and Vaccine)

The 16th Workshop on Recent Issues in Bioanalysis (16th WRIB) took place in Atlanta, GA, USA on September 26-30, 2022. Over 1000 professionals representing pharma/biotech companies, CROs, and multiple regulatory agencies convened to actively discuss the most current topics of interest in bioanalysis. The 16th WRIB included 3 Main Workshops and 7 Specialized Workshops that together spanned 1 week in order to allow exhaustive and thorough coverage of all major issues in bioanalysis, biomarkers, immunogenicity, gene therapy, cell therapy and vaccines. Moreover, in-depth workshops on the ICH M10 BMV final guideline (focused on this guideline training, interpretation, adoption and transition); mass spectrometry innovation (focused on novel technologies, novel modalities, and novel challenges); and flow cytometry bioanalysis (rising of the 3rd most common/important technology in bioanalytical labs) were the special features of the 16th edition. As in previous years, WRIB continued to gather a wide diversity of international, industry opinion leaders and regulatory authority experts working on both small and large molecules as well as gene, cell therapies and vaccines to facilitate sharing and discussions focused on improving quality, increasing regulatory compliance, and achieving scientific excellence on bioanalytical issues. This 2022 White Paper encompasses recommendations emerging from the extensive discussions held during the workshop and is aimed to provide the bioanalytical community with key information and practical solutions on topics and issues addressed, in an effort to enable advances in scientific excellence, improved quality and better regulatory compliance. Due to its length, the 2022 edition of this comprehensive White Paper has been divided into three parts for editorial reasons. This publication (Part 1A) covers the recommendations on Mass Spectrometry and ICH M10. Part 1B covers the Regulatory Agencies' Inputs on Bioanalysis, Biomarkers, Immunogenicity, Gene & Cell Therapy and Vaccine. Part 2 (LBA, Biomarkers/CDx and Cytometry) and Part 3 (Gene Therapy, Cell therapy, Vaccines and Biotherapeutics Immunogenicity) are published in volume 15 of Bioanalysis, issues 15 and 14 (2023), respectively.

Optimizing the safety of antibody-drug conjugates for patients with solid tumours

Over the past 5 years, improvements in the design of antibody-drug conjugates (ADCs) have enabled major advances that have reshaped the treatment of several advanced-stage solid tumours. Considering the intended rationale behind the design of ADCs, which is to achieve targeted delivery of cytotoxic molecules by linking them to antibodies targeting tumour-specific antigens, ADCs would be expected to be less toxic than conventional chemotherapy. However, most ADCs are still burdened by off-target toxicities that resemble those of the cytotoxic payload as well as on-target toxicities and other poorly understood and potentially life-threatening adverse effects. Given the rapid expansion in the clinical indications of ADCs, including use in curative settings and various combinations, extensive efforts are ongoing to improve their safety. Approaches currently being pursued include clinical trials optimizing the dose and treatment schedule, modifications of each ADC component, identification of predictive biomarkers for toxicities, and the development of innovative diagnostic tools. In this Review, we describe the determinants of the toxicities of ADCs in patients with solid tumours, highlighting key strategies that are expected to improve tolerability and enable improvements in the treatment outcomes of patients with advanced-stage and those with early stage cancers in the years to come.

Bioanalytical strategy for the characterization and bioanalysis of biologics: a global, nonregulated bioanalytical lab perspective

Over the past two decades, we have seen an increase in the complexity and diversity of biotherapeutic modalities pursued by biopharmaceutical companies. These biologics are multifaceted and susceptible to post-translational modifications and in vivo biotransformation that could impose challenges for bioanalysis. It is vital to characterize the functionality, stability and biotransformation products of these molecules to enable screening, identify potential liabilities at an early stage and devise a bioanalytical strategy. This article highlights our perspective on characterization and bioanalysis of biologics using hybrid LC-MS in our global nonregulated bioanalytical laboratories. AbbVie's suite of versatile, stage-appropriate characterization assays and quantitative bioanalytical approaches are discussed, along with guidance on their utility in answering project-specific questions to aid in decision-making.

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.

Determination of drug-to-antibody ratio of antibody-drug conjugate in biological samples using microflow-liquid chromatography/high-resolution mass spectrometry

Background: Antibody-drug conjugates (ADCs) are a promising modality for cancer treatment; however, considering their complicated nature, analytical complexity in understanding their pharmacokinetics and pharmacodynamics in the body presents a significant challenge. Results: Vorsetuzumab maleimidocaproyl valine-citrulline p-aminobenzyloxycarbonyl monomethyl auristatin E was used to develop pretreatment and analytical workflows suitable for ADCs. Monomethyl auristatin E release and drug-to-antibody ratio retention were consistent in mouse plasma but inconsistent in monkey and human plasma. Further, metabolites were species-specific. Microflow-liquid chromatography/high-resolution mass spectrometry (LC-HRMS) resulted in a 4-7-fold improvement in detection sensitivity compared with conventional flow LC-HRMS. Conclusion: Microflow-LC-HRMS can be a useful tool in understanding the complex properties of ADCs in the body from a drug metabolism and pharmacokinetics point of view.

DFT-Guided Discovery of Ethynyl-Triazolyl-Phosphinates as Modular Electrophiles for Chemoselective Cysteine Bioconjugation and Profiling

We report the density functional theory (DFT) guided discovery of ethynyl-triazolyl-phosphinates (ETPs) as a new class of electrophilic warheads for cysteine selective bioconjugation. By using Cu^I -catalysed azide alkyne cycloaddition (CuAAC) in aqueous buffer, we were able to access a variety of functional electrophilic building blocks, including proteins, from diethynyl-phosphinate. ETP-reagents were used to obtain fluorescent peptide-conjugates for receptor labelling on live cells and a stable and a biologically active antibody-drug-conjugate. Moreover, we were able to incorporate ETP-electrophiles into an azide-containing ubiquitin under native conditions and demonstrate their potential in protein-protein conjugation. Finally, we showcase the excellent cysteine-selectivity of this new class of electrophile in mass spectrometry based, proteome-wide cysteine profiling, underscoring the applicability in homogeneous bioconjugation strategies to connect two complex biomolecules.

Targeted delivery of lipid nanoparticle to lymphatic endothelial cells via anti-podoplanin antibody

Lymphatic endothelial cells (LECs) that form lymphatic vessels play a pivotal role in immune regulation. It was recently reported that LECs suppress the antigen-dependent anti-tumor immunity in cancer tissues. Thus, regulating the function of LECs is a promising strategy for cancer therapy. The objective of this study was to develop a method for the selective delivery of small interfering RNA (siRNA) to LECs. For this purpose, the siRNA was formulated into nanoparticles (LNPs) to prevent them from being degraded in body fluids and to facilitate their penetration of the cell membrane. A breakthrough technology for achieving this is ONPATTRO®, a world's first siRNA drug. Since LNPs are taken up by hepatocytes relatively well via low-density lipoprotein receptors, most of the LNP systems that have been developed so far target hepatocytes. In this study, we report on the development of a new method for the rapid and convenient method for modifying LNPs with antibodies using the CLick reaction on the Interface of the nanoParticle (CLIP). The CLIP approach was faster and more versatile than the conventional method using amide coupling. As a demonstration, we report on the LEC-targeted siRNA delivery by using antibody-modified LNPs both in vitro and in vivo. The method used for the modification of LNPs is highly promising and has the potential for expanding the LNP-based delivery of nucleic acids in the future.

Monitoring In Vivo Performances of Protein-Drug Conjugates Using Site-Selective Dual Radiolabeling and Ex Vivo Digital Imaging

In preclinical models, the development and optimization of protein-drug conjugates require accurate determination of the plasma and tissue profiles of both the protein and its conjugated drug. To this aim, we developed a bioanalytical strategy based on dual radiolabeling and ex vivo digital imaging. By combining enzymatic and chemical reactions, we obtained homogeneous dual-labeled anti-MMP-14 Fabs (antigen-binding fragments) conjugated to monomethyl auristatin E where the protein scaffold was labeled with carbon-14 (14C) and the conjugated drug with tritium (3H). These antibody-drug conjugates with either a noncleavable or a cleavable linker were then evaluated in vivo. By combining liquid scintillation counting and ex vivo dual-isotope radio-imaging, it was possible not only to monitor both components simultaneously during their circulation phase but also to quantify accurately their amount accumulated within the different organs.

Antibody-Pattern Recognition Receptor Agonist Conjugates: A Promising Therapeutic Strategy for Cancer

Antibody-drug conjugates (ADCs) are composed of monoclonal antibodies linked to cytotoxic payload drugs, each of which can be diversely designed in accordance with pharmacological and clinical requirements. The use of ADCs is effective for the treatment of different diseases, including cancers, and is gaining widespread attention. To date, 12 ADCs have been approved by the U.S. Food and Drug Administration for treating cancer and improving the quality of life of patients. To expand the application of ADCs and improve their treatment efficiency, various formats have recently been manufactured, including pattern recognition receptor (PRR) agonist-based ADCs. The antibody has a unique structure that enables the specific delivery of PRR agonists to the tumor area, and this improves the therapeutic efficacy while minimizing systemic toxicity. This review briefly discusses the current landscape and future perspectives of antibody-PRR agonist conjugates for cancer therapy.

Development of a Physiologically-Based Pharmacokinetic Model for Whole-Body Disposition of MMAE Containing Antibody-Drug Conjugate in Mice

PURPOSE

To quantitate and mathematically characterize the whole-body pharmacokinetics (PK) of different ADC analytes following administration of an MMAE-conjugated ADC in tumor-bearing mice.

METHODS

The PK of different ADC analytes (total antibody, total drug, unconjugated drug) was measured following administration of an MMAE-conjugated ADC in tumor-bearing mice. The PK of ADC analytes was compared with the whole-body PK of the antibody and drug obtained following administration of these molecules alone. An ADC PBPK model was developed by linking antibody PBPK model with small-molecule PBPK model, where the drug was assumed to deconjugate in DAR-dependent manner.

RESULTS

Comparison of antibody biodistribution coefficient (ABC) values for total antibody suggests that conjugation of drug did not significantly affect the PK of antibody. Comparison of tissue:plasma AUC ratio (T/P) for the conjugated drug and total antibody suggests that in certain tissues (e.g., spleen) ADC may demonstrate higher deconjugation. It was observed that the tissue distribution profile of the drug can be altered following its conjugation to antibody. For example, MMAE distribution to the liver was found to increase while its distribution to the heart was found to decrease upon conjugation to antibody. MMAE exposure in the tumor was found to increase by ~20-fold following administration as conjugate (i.e., ADC). The PBPK model was able to a priori predict the PK of all three ADC analytes in plasma, tissues, and tumor reasonably well.

CONCLUSIONS

The ADC PBPK model developed here serves as a platform for translational and clinical investigations of MMAE containing ADCs.

Analysis of Oligonucleotide Biodistribution and Metabolization in Experimental Animals

We describe methods to follow the fate of oligonucleotides after their injection into experimental animals. The quantitation in various tissues, blood or bone marrow cells is possible by chemical ligation PCR. This method works independently of chemical modifications of the oligonucleotide and/or its conjugations to lipid or peptide moieties. Moreover, metabolization intermediates can be detected by mass spectrometry. Together with a readout assay for the biochemical or physiological effects, which will differ, depending on the particular purpose of the oligonucleotide, these methods allow for a comprehensive understanding of oligonucleotide behavior in a living organism.

Antibody-drug conjugate and free geldanamycin combination therapy enhances anti-cancer efficacy

Antibody drug-conjugates (ADCs) targeting human epidermal growth factor (HER2) are a rapidly expanding class of cancer therapeutics. Such ADCs are known to suffer from inefficient trafficking to the lysosome due to HER2 endosomal recycling, leaving most bound ADCs at the cell surface or in early endosomes. This study aims to increase the maximum cytotoxicity of ADC treatment by co-delivering a small molecule inhibitor targeting the primary chaperone of HER2, heat shock protein 90 (HSP90). We hypothesized that inhibiting HSP90 could aid ADC cytotoxicity by overcoming HER2 endosomal recycling. Flow cytometric studies tracking HER2 surface expression revealed ∼ 10 nM geldanamycin (GA) as the threshold for inhibiting HSP90 mediated HER2 recycling. Cytotoxicity studies in HER2 overexpressing cancer cell lines NCI-N87, MDA-MB-453, and SKOV3 demonstrated that co-administration of ADC alongside 100 nM GA significantly increased cytotoxicity compared to ADC alone. In all cases, baseline cytotoxicity was observed even in low HER2 expressing line MDA-MB-231 cells, indicating possible off-target effects. To mitigate this baseline cytotoxicity, a "pulse treatment" regime was adopted where cells are pre-loaded with T-DM1 or T-MMAE ADCs for 4 h, followed by a 4-hour pulse treatment with ADC and 100 nM GA to initiate trafficking of HER2 bound ADC to the lysosome. Afterwards, GA is removed, and ADC treatment is continued. GA pulse co-treatment decreased the amount of ADC required to achieve maximum cytotoxicity while minimizing baseline cytotoxicity. No such co-treatment regime featuring a pulse sequence has been explored before. Such co-treatments could offer a viable solution to increase ADC efficacy in hard to treat or resistant HER2-positive cancers.

Near-Infrared Light-Responsive SERS Tags Enable Positioning and Monitoring of the Drug Release of Photothermal Nanomedicines In Vivo

Understanding the in vivo behavior of photothermal nanomedicines (PTNMs) is important for drug development and evaluation. However, it is still very challenging. Herein, two key parameters, i.e., the depth of PTNMs under biological tissue and the drug release ratio of PTNMs in vivo, can be revealed by a near-infrared (NIR) light-responsive surface-enhanced Raman scattering (SERS) strategy. The fabricated PTNMs were composed of waxberry-like gold nanoparticles, model drug curcumin, and an elaborately selected NIR light-responsive Raman reporter (3,3'-diethylthiatricarbocyanine iodide, DTTC). The response mechanism of DTTC to NIR light was investigated as photodegradation. NIR light irradiation heated the gold nanoparticles, triggered the release of a model drug, and simultaneously decreased the SERS intensity of the PTNMs. In vitro experiment results revealed that the SERS intensity decrease could well reflect the depth of PTNMs with a correlation coefficient of more than 0.99. On this basis, after in situ SERS detection, the depth of PTNMs in a tumor could be revealed with satisfactory accuracy. Moreover, the decrease in the SERS intensity of PTNMs showed a highly similar trend to the increase in the drug release, suggesting that it could be used for real-time monitoring of drug release of PTNMs. This study not only opens a new avenue for the release study of many inactive fluorescent and Raman drugs of PTNMs but also provides an effective way for reporting the depth, which greatly promotes the application of PTNMs in vivo.

Recent developments in chemical conjugation strategies targeting native amino acids in proteins and their applications in antibody-drug conjugates

Many fields in chemical biology and synthetic biology require effective bioconjugation methods to achieve their desired functions and activities. Among such biomolecule conjugates, antibody-drug conjugates (ADCs) need a linker that provides a stable linkage between cytotoxic drugs and antibodies, whilst conjugating in a biologically benign, fast and selective fashion. This review focuses on how the development of novel organic synthesis can solve the problems of traditional linker technology. The review shall introduce and analyse the current developments in the modification of native amino acids on peptides or proteins and their applicability to ADC linker. Thereafter, the review shall discuss in detail each endogenous amino acid's intrinsic reactivity and selectivity aspects, and address the research effort to construct an ADC using each conjugation method.

Effect of Conjugation Site and Technique on the Stability and Pharmacokinetics of Antibody-Drug Conjugates

Appropriate selection of conjugation sites and conjugation technologies is now widely accepted as crucial for the success of antibody-drug conjugates (ADCs). Herein, we present ADCs conjugated by different conjugation methods to different conjugation positions being systematically characterized by multiple in vitro assays as well as in vivo pharmacokinetic (PK) analyses in transgenic Tg276 mice. Conjugation to cysteines, genetically introduced at positions N325, L328, S239, D265, and S442, was compared to enzymatic conjugation via microbial transglutaminase (mTG) either to C-terminal light (LC) or heavy chain (HC) recognition motifs or to endogenous position Q295 of a native antibody. All conjugations yielded homogeneous DAR 2 ADCs with similar hydrophobicity, thermal stability, human neonatal Fc receptor (huFcRn) binding, and serum stability properties, but with pronounced differences in their PK profiles. mTG-conjugated ADC variants conjugated either to Q295 or to LC recognition motifs showed superior PK behavior. Within the panel of engineered cysteine variants L328 showed a similar PK profile compared to previously described S239 but superior PK compared to S442, D265, and N325. While all positions were first tested with trastuzumab, L328 and mTG LC were further evaluated with additional antibody scaffolds derived from clinically evaluated monoclonal antibodies (mAb). Based on PK analyses, this study confirms the newly described position L328 as favorable site for cysteine conjugation, comparable to the well-established engineered cysteine position S239, and emphasizes the favorable position Q295 of native antibodies and the tagged LC antibody variant for enzymatic conjugations via mTG. In addition, hemizygous Tg276 mice are evaluated as an adequate model for ADC pharmacokinetics, facilitating the selection of suitable ADC candidates early in the drug discovery process.

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.

Assessments of the In Vitro and In Vivo Linker Stability and Catabolic Fate for the Ortho Hydroxy-Protected Aryl Sulfate Linker by Immuno-Affinity Capture Liquid Chromatography Quadrupole Time-of-Flight Mass Spectrometric Assay

Antibody-drug conjugate (ADC) linkers play an important role in determining the safety and efficacy of ADC. The Ortho Hydroxy-Protected Aryl Sulfate (OHPAS) linker is a newly developed linker in the form of a di-aryl sulfate structure consisting of phenolic payload and self-immolative group (SIG). In this study, using two bioanalytical approaches (namely "bottom-up" and "middle-up" approaches) via the liquid chromatography-quadrupole time-of-flight mass spectrometric (LC-qTOF-MS) method, in vitro and in vivo linker stability experiments were conducted for the OHPAS linker. For comparison, the valine-citrulline-p-aminobenzyloxycarbonyl (VC-PABC) linker was also evaluated under the same experimental conditions. In addition, the catabolite identification experiments at the subunit intact protein level were simultaneously performed to evaluate the catabolic fate of ADCs. As a result, the OHPAS linker was stable in the in vitro mouse/human plasma as well as in vivo pharmacokinetic studies in mice, whereas the VC-PABC linker was relatively unstable in mice in vitro and in vivo. This is because the VC-PABC linker was sensitive to a hydrolytic enzyme called carboxylesterase 1c (Ces1c) in mouse plasma. In conclusion, the OHPAS linker appears to be a good linker for ADC, and further experiments would be warranted to demonstrate the efficacy and toxicity related to the OHPAS linker.

Site-specific conjugation of native antibody

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

Analytical Methods for the Detection and Quantification of ADCs in Biological Matrices

Understanding pharmacokinetics and biodistribution of antibody–drug conjugates (ADCs) is a one of the critical steps enabling their successful development and optimization. Their complex structure combining large and small molecule characteristics brought out multiple bioanalytical methods to decipher the behavior and fate of both components in vivo. In this respect, these methods must provide insights into different key elements including half-life and blood stability of the construct, premature release of the drug, whole-body biodistribution, and amount of the drug accumulated within the targeted pathological tissues, all of them being directly related to efficacy and safety of the ADC. In this review, we will focus on the main strategies enabling to quantify and characterize ADCs in biological matrices and discuss their associated technical challenges and current limitations.

An overview of antibody-drug conjugates in oncological practice

Antibody–drug conjugates (ADCs) are designed to manipulate the toxic efficacy of specific chemotherapeutic compounds, employing the high affinity of antibody-mediated delivery so as to drive them selectively to target cancer cells. These immunoconjugates encompass the general tendency towards precision medicine and avert the systemic toxicities of conventional chemotherapy, accomplishing an improved therapeutic index. Cumulative experience acquired from first-generation ADCs offers new perspectives to these promising therapeutic modalities for various hematological and solid cancers and propels their clinical development in a faster-than-ever pace, as indicated by the approval of four novel ADCs during the last year. This paper aims to provide an up-to-date overview of the eight ADCs approved by the US Food and Drug Administration and their current indications in oncological practice. Starting from their bio-pharmaceutical background, we track their clinical evolution, with an emphasis on the pivotal trials that led to their commercial release. Late-stage studies examining these eight ADCs in other-than-approved settings as well as the investigation of potential new candidates are also reviewed. In the close future, more data are expected to expand ADCs’ oncological utility and to further reshape their role in cancer therapeutics.

Current LC-MS-based strategies for characterization and quantification of antibody-drug conjugates

The past few years have witnessed enormous progresses in the development of antibody-drug conjugates (ADCs). Consequently, comprehensive analysis of ADCs in biological systems is critical in supporting discovery, development and evaluation of these agents. Liquid chromatography-mass spectrometry (LC-MS) has emerged as a promising and versatile tool for ADC analysis across a wide range of scenarios, owing to its multiplexing ability, rapid method development, as well as the capability of analyzing a variety of targets ranging from small-molecule payloads to the intact protein with a high, molecular resolution. However, despite this tremendous potential, challenges persist due to the high complexity in both the ADC molecules and the related biological systems. This review summarizes the up-to-date LC-MS-based strategies in ADC analysis and discusses the challenges and opportunities in this rapidly-evolving field.

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.

Bispecific Antibodies and Antibody-Drug Conjugates for Cancer Therapy: Technological Considerations

The ability of monoclonal antibodies to specifically bind a target antigen and neutralize or stimulate its activity is the basis for the rapid growth and development of the therapeutic antibody field. In recent years, traditional immunoglobulin antibodies have been further engineered for better efficacy and safety, and technological developments in the field enabled the design and production of engineered antibodies capable of mediating therapeutic functions hitherto unattainable by conventional antibody formats. Representative of this newer generation of therapeutic antibody formats are bispecific antibodies and antibody–drug conjugates, each with several approved drugs and dozens more in the clinical development phase. In this review, the technological principles and challenges of bispecific antibodies and antibody–drug conjugates are discussed, with emphasis on clinically validated formats but also including recent developments in the fields, many of which are expected to significantly augment the current therapeutic arsenal against cancer and other diseases with unmet medical needs.

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

Antibody-drug conjugates (ADCs) are hybrid molecules intended to overcome the drawbacks of conventional small molecule chemotherapy and therapeutic antibodies by merging beneficial characteristics of both molecule classes to develop more efficient and patient-friendly options for cancer treatment. During the last decades a versatile bioconjugation toolbox that comprises numerous chemical and enzymatic technologies have been developed to covalently attach a cytotoxic cargo to a tumor-targeting antibody. Microbial transglutaminase (mTG) that catalyzes isopeptide bond formation between proteinaceous or peptidic glutamines and lysines, provides many favorable properties that are beneficial for the manufacturing of these conjugates. However, to efficiently utilize the enzyme for the constructions of ADCs, different drawbacks had to be overcome that originate from the enzyme's insufficiently understood substrate specificity. Within this review, pioneering methodologies, recent achievements and remaining limitations of mTG-assisted assembly of ADCs will be highlighted.

Assessing ADC Plasma Stability by LC-MS Methods

Plasma stability of ADCs can have a profound impact on ADC efficacy and safety. LC-MS methods enable the detection and characterization of ADC to evaluate its stability in plasma. Here we describe a procedure and LC-MS method for assessing ADC plasma stability.

Ethynylphosphonamidates for the Rapid and Cysteine-Selective Generation of Efficacious Antibody-Drug Conjugates

Requirements for novel bioconjugation reactions for the synthesis of antibody–drug conjugates (ADCs) are exceptionally high, since conjugation selectivity as well as the stability and hydrophobicity of linkers and payloads drastically influence the performance and safety profile of the final product. We report Cys‐selective ethynylphosphonamidates as new reagents for the rapid generation of efficacious ADCs from native non‐engineered monoclonal antibodies through a simple one‐pot reduction and alkylation. Ethynylphosphonamidates can be easily substituted with hydrophilic residues, giving rise to electrophilic labeling reagents with tunable solubility properties. We demonstrate that ethynylphosphonamidate‐linked ADCs have excellent properties for next‐generation antibody therapeutics in terms of serum stability and in vivo antitumor activity.

Thiolation of Q295: Site-Specific Conjugation of Hydrophobic Payloads without the Need for Genetic Engineering

Site-specific conjugation technology frequently relies on antibody engineering to incorporate rare or non-natural amino acids into the primary sequence of the protein. However, when the primary sequence is unknown or when antibody engineering is not feasible, there are very limited options for site-specific protein modification. We have developed a transglutaminase-mediated conjugation that incorporates a thiol at a "privileged" location on deglycosylated antibodies (Q295). Perhaps surprisingly, this conjugation employs a reported transglutaminase inhibitor, cystamine, as the key enzyme substrate. The chemical incorporation of a thiol at the Q295 site allows for the site-specific attachment of a plethora of commonly used and commercially available payloads via maleimide chemistry. Herein, we demonstrate the utility of this method by comparing the conjugatability, plasma stability, and in vitro potency of these site-specific antibody-drug conjugates (ADCs) with analogous endogenous cysteine conjugates. Cytotoxic ADCs prepared using this methodology are shown to exhibit comparable in vitro efficacy to stochastic cysteine conjugates while displaying dramatically improved plasma stability and conjugatability. In particular, we note that this technique appears to be useful for the incorporation of highly hydrophobic linker payloads without the addition of PEG modifiers. We postulate a possible mechanism for this feature by probing the local environment of the Q295 site with two fluorescent probes that are known to be sensitive to the local hydrophobic environment. In summary, we describe a highly practical method for the site-specific conjugation of genetically nonengineered antibodies, which results in plasma-stable ADCs with low intrinsic hydrophobicity. We believe that this technology will find broad utility in the ADC community.

Site-Specific Bioconjugation and Multi-Bioorthogonal Labeling via Rapid Formation of a Boron-Nitrogen Heterocycle

Precise control of covalent bond formation in the presence of multiple functional groups is pertinent in the development of many next-generation bioconjugates and materials. Strategies derived from bioorthogonal chemistries are contributing greatly in that regard; however, the gain of chemoselectivity is often compromised by the slow rates of many of these existing chemistries. Recent work on a variation of the classical aldehyde/ketone condensation based on ortho-carbonylphenylboronic acids has uncovered markedly accelerated rates compared to those of the simple carbonyl counterparts. The products of these reactions are distinct, often in the form of boron-nitrogen heterocycles. In particular, we have shown that 2-formylphenylboronic acid (2fPBA), when coupled with an α-amino-hydrazide, produces a unique zwitterionic and stable 2,3,1-benzodiazaborine derivative. In this work, we apply this chemistry to generate chemically defined and functional bioconjugates, herein illustrated with immunoconjugates. We show that an antibody and a fluorophore (as payload) equipped with the relevant reactive handles undergo rapid conjugation at near-stoichiometric ratios, displaying a reaction half-life of only ∼5 min with 2 equiv of the linker payload. Importantly, the reaction can be extended to multicomponent labeling by partnering with the popular strain-promoted azide-alkyne cycloaddition and tetrazine- trans-cyclooctene (Tz-TCO) ligation. The mutual orthogonality to both of these chemistries allows simultaneous triple bioorthogonal conjugations, a rare feat thus far that will widen the scope of various multilabeling applications. Further collaboration with the Tz-TCO reaction enables rapid one-pot synthesis of a site-specific dual-payload antibody conjugate. Altogether, we envision that the 2fPBA-α-amino-hydrazide ligation will facilitate efficient assembly of diverse bioconjugates and materials, enabling access to more complex modalities via partnership with other orthogonal chemistries.

Pharmacokinetic and Immunological Considerations for Expanding the Therapeutic Window of Next-Generation Antibody-Drug Conjugates

Antibody-drug conjugate (ADC) development has evolved greatly over the last 3 decades, including the Food and Drug Administration (FDA) approval of several new drugs. However, translating ADCs from the design stage and preclinical promise to clinical success has been a major hurdle for the field, particularly for solid tumors. The challenge in clinical development can be attributed to the difficulty in connecting the design of these multifaceted agents with the impact on clinical efficacy, especially with the accelerated development of 'next-generation' ADCs containing a variety of innovative biophysical developments. Given their complex nature, there is an urgent need to integrate holistic ADC characterization approaches. This includes comprehensive in vivo assessment of systemic, intratumoral and cellular pharmacokinetics, pharmacodynamics, toxicodynamics, and interactions with the immune system, with the aim of optimizing the ADC therapeutic window. Pharmacokinetic/pharmacodynamic factors influencing the ADC therapeutic window include (1) selecting optimal target and ADC components for prolonged and stable plasma circulation to increase tumoral uptake with minimal non-specific systemic toxicity, (2) balancing homogeneous intratumoral distribution with efficient cellular uptake, and (3) translating improved ADC potency to better clinical efficacy. Balancing beneficial immunological effects such as Fc-mediated and payload-mediated immune cell activation against harmful immunogenic/toxic effects is also an emerging concern for ADCs. Here, we review practical considerations for tracking ADC efficacy and toxicity, as aided by high-resolution biomolecular and immunological tools, quantitative pharmacology, and mathematical models, all of which can elucidate the relative contributions of the multitude of interactions governing the ADC therapeutic window.

Antibody-Drug Conjugates for the Therapy of Thoracic Malignancies

Antibody-drug conjugates (ADCs) are a novel class of therapeutic agents incorporating both target-specific monoclonal antibodies and cytotoxic small molecules via a chemical linker. They were first introduced into the clinic for the treatment of advanced hematologic malignancies. The only approved ADC for solid tumors targets erb-b2 receptor tyrosine kinase (HER2), a validated antigen in breast cancer. Many ADCs are under active investigation for various types of solid tumors. In this article, we review the literature from several perspectives including the design, pharmacology, and mechanism-based toxicities of antibody-drug conjugates. We then discuss ADCs currently in clinical development for thoracic malignancies.

Antibody-drug conjugates (ADCs) for cancer therapy: Strategies, challenges, and successes

Targeted delivery of therapeutic molecules into cancer cells is considered as a promising strategy to tackle cancer. Antibody-drug conjugates (ADCs), in which a monoclonal antibody (mAb) is conjugated to biologically active drugs through chemical linkers, have emerged as a promising class of anticancer treatment agents, being one of the fastest growing fields in cancer therapy. The failure of early ADCs led researchers to explore strategies to develop more effective and improved ADCs with lower levels of unconjugated mAbs and more-stable linkers between the drug and the antibody, which show improved pharmacokinetic properties, therapeutic indexes, and safety profiles. Such improvements resulted in the US Food and Drug Administration approvals of brentuximab vedotin, trastuzumab emtansine, and, more recently, inotuzumab ozogamicin. In addition, recent clinical outcomes have sparked additional interest, which leads to the dramatically increased number of ADCs in clinical development. The present review explores ADCs, their main characteristics, and new research developments, as well as discusses strategies for the selection of the most appropriate target antigens, mAbs, cytotoxic drugs, linkers, and conjugation chemistries.

Exploring the Orthogonal Chemoselectivity of 2,4,6-Trichloro-1,3,5-Triazine (TCT) as a Trifunctional Linker With Different Nucleophiles: Rules of the Game

The study involves exploring the three orthogonal sites for aromatic nucleophilic substitution in cyanuric chloride (TCT). The preferential order of incorporation of different nucleophiles (such as alcohol, thiol, and amine) was addressed both experimentally and theoretically. The preferential order for incorporating nucleophiles in TCT was found to be alcohol > thiol > amine.

Review of approaches and examples for monitoring biotransformation in protein and peptide therapeutics by MS

Biotherapeutic drugs have emerged in quantity in pharmaceutical pipelines, and increasingly diverse biomolecules are progressed through preclinical and clinical development. As purification, separation, mass spectrometer detection and data processing capabilities improve, there is opportunity to monitor drug concentration by traditional ligand-binding assay or MS measurement and to monitor metabolism, catabolism or other biomolecular mass variants present in circulation. This review highlights approaches and examples of monitoring biotransformation of biotherapeutics by MS as these techniques are poised to add value to drug development in years to come. The increased use of such approaches, and the successful quantitation of biotherapeutic structural modifications, will provide insightful data for the benefit of both researchers and patients.

Perspectives on potentiating immunocapture-LC-MS for the bioanalysis of biotherapeutics and biomarkers

The integration of ligand-binding assay and LC-MS/MS (immunocapture-LC-MS) has unleashed the combined advantages of both powerful techniques for addressing the ever increasing bioanalytical challenges for biotherapeutics and biomarker assays. The highly specific, selective and sensitive characteristics of the immunocapture-LC-MS-based assays have enabled the determination of biotherapeutics and biomarkers in biomatrices with ease of method development, less requirements on key reagents as well as structural specificity for endogenous and engineered biomolecules. In addition, the versatile immunocapture-LC-MS technology has expanded into many challenging areas to enhance mechanistic studies of drug interactions with their targets. This paper intends to summarize our perspectives on enhancing the use of immunocapture-LC-MS in drug discovery and development for emerging new modalities.