Insights into Free Drug Release from Efficacious N-Acyl O-Aminophenol Duocarmycin Prodrugs

Acyclic and cyclic N-acyl O-aminophenol prodrugs of duocarmycin analogues were reported as members of a unique class of reductively cleaved prodrugs that map seamlessly onto the duocarmycin family of natural products. Although these prodrugs were explored with the expectations that they may be cleaved selectively within hypoxic tumor environments that have intrinsically higher concentrations of reducing nucleophiles, the remarkable stability of some such prodrugs suggests another mechanism of free drug release is operative. The prototype of such chemically unreactive N-acyl O-aminophenol prodrugs is 1, which proved remarkably efficacious in vivo in vertebrate tumor models; was found to lack the toxicity that is characteristic of traditional chemotherapeutic drugs as well as the free drugs in the class (e.g., myelosuppression); and displayed a preferential site (intracellular), a slow and sustained rate, and a potentially unique mechanism of free drug release. Herein, we detail studies that provide insights into this stereoselective mechanism of free drug release. Combined, the results of the studies are consistent with an exclusive protein-mediated (enantio)selective activation and free drug release from prodrug 1 by N-O bond cleavage preferentially in cancer cell lines versus cultured normal human cell lines effected by a cytosolic cysteine-based enzyme and suggest that the activating protein is one that is selectively expressed, upregulated, or preferentially activated in cancer cell lines, potentially constituting a new oncology targeted precision therapy.

Single Mutation on Trastuzumab Modulates the Stability of Antibody-Drug Conjugates Built Using Acetal-Based Linkers and Thiol-Maleimide Chemistry

Antibody-drug conjugates (ADCs) are a class of targeted therapeutics used to selectively kill cancer cells. It is important that they remain intact in the bloodstream and release their payload in the target cancer cell for maximum efficacy and minimum toxicity. The development of effective ADCs requires the study of factors that can alter the stability of these therapeutics at the atomic level. Here, we present a general strategy that combines synthesis, bioconjugation, linker technology, site-directed mutagenesis, and modeling to investigate the influence of the site and microenvironment of the trastuzumab antibody on the stability of the conjugation and linkers. Trastuzumab is widely used to produce targeted ADCs because it can target with high specificity a receptor that is overexpressed in certain breast cancer cells (HER2). We show that the chemical environment of the conjugation site of trastuzumab plays a key role in the stability of linkers featuring acid-sensitive groups such as acetals. More specifically, Lys-207, located near the reactive Cys-205 of a thiomab variant of the antibody, may act as an acid catalyst and promote the hydrolysis of acetals. Mutation of Lys-207 into an alanine or using a longer linker that separates this residue from the acetal group stabilizes the conjugates. Analogously, Lys-207 promotes the beneficial hydrolysis of the succinimide ring when maleimide reagents are used for conjugation, thus stabilizing the subsequent ADCs by impairing the undesired retro-Michael reactions. This work provides new insights for the design of novel ADCs with improved stability properties.

Targeting the immune checkpoint B7-H3 for next-generation cancer immunotherapy

Immune checkpoint inhibitors (ICIs) for programmed death-1 (PD-1) and programmed cell death-ligand 1 (PD-L1) have become preferred treatment strategies for several advanced cancers. However, response rates for these treatments are limited, which encourages the search for new ICI candidates. Recent reports have underscored significant roles of B7 homolog 3 protein (B7-H3) in tumor immunity and disease progression. While its multifaceted roles are being elucidated, B7-H3 has already entered clinical trials as a therapeutic target. In this review, we overview the recent results of clinical trials evaluating the antitumor activity and safety of B7-H3 targeting drugs. On this basis, we also discuss the challenges and opportunities arising from the application of these drugs. Finally, we point out current gaps to address in the understanding of B7-H3 function and regulation in order to fully unleash the future clinical utility of B7-H3-based therapies for the treatment of cancer.

Targeting nectin-4 by antibody-drug conjugates for the treatment of urothelial carcinoma

INTRODUCTION

Nectin-4 is a tumor-associated antigen overexpressed in urothelial carcinoma and several other malignancies. It has emerged as a compelling target for novel tumor-directed therapies, particularly as a component of antibody-drug conjugates (ADCs), a growing class of anti-cancer therapeutic agents. Development of nectin-4-directed therapies has been led by enfortumab vedotin (EV), an ADC comprised of a fully human monoclonal antibody specific for nectin-4 conjugated via a cleavable linker to the microtubule inhibitor MMAE. EV was approved in 2019 as a first-in-class agent for the treatment of urothelial carcinoma.

AREAS COVERED

This article discusses general principles relevant to ADC design and our current understanding of nectin-4 in normal physiology and malignancy, followed by a review of the development of EV as well as additional drug conjugate strategies targeting nectin-4.

EXPERT OPINION

EV offers proof-of-concept for the clinical utility of nectin-4-directed therapies and provides further support for ADCs as an important class of anti-cancer agents. Future development of nectin-4-targeted approaches will benefit from a deeper understanding of nectin-4 biology in both health and disease, as well as a detailed exploration of the mechanisms underlying therapeutic activity and resistance.

Antibody-Drug Conjugate-Based Therapeutics: State of the Science

Antibody-drug conjugates (ADCs) are complex engineered therapeutics consisting of monoclonal antibodies, directed toward tumor-associated antigens, to which highly potent cytotoxic agents are attached using chemical linkers. This targeted drug delivery strategy couples the precision of the antibody targeting moiety with the cytocidal activity of the payload, which is generally too toxic on its own to be systemically administered. In this manner, ADCs confer a means to reduce off-target toxicities in patients by limiting the exposure of normal tissues to the payload, thus broadening the potential therapeutic window compared with traditional chemotherapy. The pace of ADC development is accelerating, with the number of investigational agents in human trials having more than tripled over the past 5 years, underscoring the enthusiasm for this transformative approach to cancer treatment. Here, we review the key structural elements of ADC design (antibody, linker, and payload), highlighting critical aspects and technological advances that have affected the clinical effectiveness of this class of biopharmaceuticals. The ADC field continues to evolve, including ongoing efforts aimed at improving target selection, developing payloads with varied mechanisms of action and increased potency, designing innovative bioconjugation strategies, as well as maximizing efficacy and tolerability in patients. An overview of the current clinical trial landscape is provided, with emphasis on the clinical experience of the four ADCs to have received regulatory approval to date, as well as additional promising candidates currently in late-stage clinical development in both solid tumor and hematological malignancies.

Potential drugs used in the antibody-drug conjugate (ADC) architecture for cancer therapy

Cytotoxic small-molecule drugs have a major influence on the fate of antibody-drug conjugates (ADCs). An ideal cytotoxic agent should be highly potent, remain stable while linked to ADCs, kill the targeted tumor cell upon internalization and release from the ADCs, and maintain its activity in multidrug-resistant tumor cells. Lessons learned from successful and failed experiences in ADC development resulted in remarkable progress in the discovery and development of novel highly potent small molecules. A better understanding of such small-molecule drugs is important for development of effective ADCs. The present review discusses requirements making a payload appropriate for antitumor ADCs and focuses on the main characteristics of commonly-used cytotoxic payloads that showed acceptable results in clinical trials. In addition, the present study represents emerging trends and recent advances of payloads used in ADCs currently under clinical trials.

The role of bracken fern illudanes in bracken fern-induced toxicities

Bracken fern is carcinogenic when fed to domestic and laboratory animals inducing bladder and ileal tumours and is currently classified as a possible human carcinogen by IARC. The carcinogenic illudane, ptaquiloside (PTQ) was isolated from bracken fern and is widely assumed to be the major bracken carcinogen. However, several other structurally similar illudanes are found in bracken fern, in some cases at higher levels than PTQ and so may contribute to the overall toxicity and carcinogenicity of bracken fern. In this review, we critically evaluate the role of illudanes in bracken fern induced toxicity and carcinogenicity, the mechanistic basis of these effects including the role of DNA damage, and the potential for human exposure in order to highlight deficiencies in the current literature. Critical gaps remain in our understanding of bracken fern induced carcinogenesis, a better understanding of these processes is essential to establish whether bracken fern is also a human carcinogen.

A Novel Depurination Methodology to Assess DNA Alkylation of Chloro-Bis-Seco-Cyclopropylbenzoindoles Allowed for Comparison of Minor-Groove Reactivity

Duocarmycins [including cyclopropyl pyrroloindole (CPI) or cyclopropyl benzoindole (CBI)] are a class of DNA minor-groove alkylators and seco-CPI/CBIs are synthetic pro-forms that can spirocyclize to CPI/CBI. Bis-CPI/CBIs are potential drug candidates because of their enhanced cytotoxicity from DNA crosslinking, but it is difficult to analyze them for structure-activity correlation because of their DNA reactivity. To study their DNA alkylation, neutral thermal hydrolysis has been frequently applied to process depurination. However, unwanted side reactions under this condition have been reported, which could lead to poor correlation of DNA alkylation data with efficacy results, especially for bis-CPI/CBIs. In this study, an acidic depurination method was developed and applied for analysis of DNA alkylation and shown to be an easier and milder method than the traditional neutral thermal hydrolysis. DNA alkylation and stability of three bis-seco-CBIs were characterized in comparison with two mono-seco-CPIs. The results suggested that: 1) The acidic depurination method was capable of capturing a more representative population, sometimes a different population, of DNA adducts as they existed on DNA compared with the heat depurination method. 2) Di-adenine adducts were captured as expected for the CBI dimers, although the major type of adduct was still mono-adenine adducts. 3) The rate of DNA alkylation, DNA adduct profile, and relative amounts of di-adduct versus mono-adduct were significantly affected by the size, and possibly lipophilicity, of the nonalkylating part of the molecules. 4) Spirocyclization and amide hydrolysis represented two major pathways of degradation. Overall, by applying acidic depurination analyses, this study has illustrated DNA adduct characteristics of novel bis-seco-CBIs with dominating mono-alkylation and provides an alternative method for evaluating DNA minor-groove alkylators. These findings provide an effective analytical tool to evaluate DNA alkylators and to study the DNA alkylation that is a disposition mechanism of these compounds.

Bifunctional Duocarmycin Analogues as Inhibitors of Protein Tyrosine Kinases

Bifunctional duocarmycin analogues are highly cytotoxic compounds that have been shown to be irreversible aldehyde dehydrogenase 1 inhibitors. Interestingly, cells with low aldehyde dehydrogenase 1 expression are also sensitive to bifunctional duocarmycin analogues, suggesting the existence of another target. Through in silico approaches, including principal component analysis, structure-similarity search, and docking calculations, protein tyrosine kinases, and especially the vascular endothelial growth factor receptor 2 (VEGFR-2), were predicted as targets of bifunctional duocarmycin analogues. Biochemical validation was performed in vitro, confirming the in silico results. Structural optimization was performed to mainly target VEGFR-2, but not aldehyde dehydrogenase 1. The optimized bifunctional duocarmycin analogue was synthesized. In vitro assays revealed this bifunctional duocarmycin analogue as a strong inhibitor of VEGFR-2, with low residual aldehyde dehydrogenase 1 activity. Altogether, studies revealed bifunctional duocarmycin analogues as a new class of naturally derived compounds that express a very high cytotoxicity to cancer cells overexpressing aldehyde dehydrogenase 1 as well as VEGFR-2.

Antibody-drug conjugates as novel anti-cancer chemotherapeutics

Over the past couple of decades, antibody-drug conjugates (ADCs) have revolutionized the field of cancer chemotherapy. Unlike conventional treatments that damage healthy tissues upon dose escalation, ADCs utilize monoclonal antibodies (mAbs) to specifically bind tumour-associated target antigens and deliver a highly potent cytotoxic agent. The synergistic combination of mAbs conjugated to small-molecule chemotherapeutics, via a stable linker, has given rise to an extremely efficacious class of anti-cancer drugs with an already large and rapidly growing clinical pipeline. The primary objective of this paper is to review current knowledge and latest developments in the field of ADCs. Upon intravenous administration, ADCs bind to their target antigens and are internalized through receptor-mediated endocytosis. This facilitates the subsequent release of the cytotoxin, which eventually leads to apoptotic cell death of the cancer cell. The three components of ADCs (mAb, linker and cytotoxin) affect the efficacy and toxicity of the conjugate. Optimizing each one, while enhancing the functionality of the ADC as a whole, has been one of the major considerations of ADC design and development. In addition to these, the choice of clinically relevant targets and the position and number of linkages have also been the key determinants of ADC efficacy. The only marketed ADCs, brentuximab vedotin and trastuzumab emtansine (T-DM1), have demonstrated their use against both haematological and solid malignancies respectively. The success of future ADCs relies on improving target selection, increasing cytotoxin potency, developing innovative linkers and overcoming drug resistance. As more research is conducted to tackle these issues, ADCs are likely to become part of the future of targeted cancer therapeutics.

Aldehyde dehydrogenases in cancer: an opportunity for biomarker and drug development?

Aldehyde dehydrogenases (ALDHs) belong to a superfamily of 19 isozymes that are known to participate in many physiologically important biosynthetic processes including detoxification of specific endogenous and exogenous aldehyde substrates. The high expression levels of an emerging number of ALDHs in various cancer tissues suggest that these enzymes have pivotal roles in cancer cell survival and progression. Mapping out the heterogeneity of tumours and their cancer stem cell (CSC) component will be key to successful design of strategies involving therapeutics that are targeted against specific ALDH isozymes. This review summarises recent progress in ALDH-focused cancer research and discovery of small-molecule-based inhibitors.

Preparation and properties of clickable amino analogues of the duocarmycins: factors that affect the efficiency of their fluorescent labelling of DNA

Herein we report the synthesis of three DNA-alkylating amino analogues of the duocarmycins that carry an alkyne functional group suitable for copper-catalysed click chemistry. The alkyne-containing substituents are connected via a side chain position which projects away from the minor groove, and have only a small effect on DNA alkylation and cytotoxicity. The efficiency of click reactions with fluorophore azides was studied using alkylated ctDNA by analysing the adenine adducts produced after thermal depurination. Click reactions "on DNA" were sensitive to steric effects (tether length to the alkyne) and, surprisingly, to the nature of the fluorophore azide. With the best combination of click partners and reagents, adducts could be detected in the nuclei of treated cells by microscopy or flow cytometry, provided that an appropriate detergent (Triton X-100 and not Tween 20) was used for permeabilisation. The method is sensitive enough to detect adducts at physiologically relevant concentrations, and could have application in the development of nitro analogues of the duocarmycins as hypoxia-activated anticancer prodrugs.