Total Synthesis in Search of Potent Antibody-Drug Conjugate Payloads. From the Fundamentals to the Translational

Antibody-drug conjugates for targeted cancer therapy: Recent advances in potential payloads

Antibody-drug conjugates (ADCs) represent a promising cancer therapy modality which specifically delivers highly toxic payloads to cancer cells through antigen-specific monoclonal antibodies (mAbs). To date, 15 ADCs have been approved and more than 100 ADC candidates have advanced to clinical trials for the treatment of various cancers. Among these ADCs, microtubule-targeting and DNA-damaging agents are at the forefront of payload development. However, several challenges including toxicity and drug resistance limit the potential of this modality. To tackle these issues, multiple innovative payloads such as immunomodulators and proteolysis targeting chimeras (PROTACs) are incorporated into ADCs to enable multimodal cancer therapy. In this review, we describe the mechanism of ADCs, highlight the importance of ADC payloads and summarize recent progresses of conventional and unconventional ADC payloads, trying to provide an insight into payload diversification as a key step in future ADC development.

Natural Product Synthesis: The Endless Quest for Unreachable Perfection

Total synthesis is a field in constant progress. Its practitioners aim to develop ideal synthetic strategies to build complex molecules. As such, they are both a driving force and a showcase of the progress of organic synthesis. In this Perspective, we discuss recent notable total syntheses. The syntheses selected herein are classified according to the key strategic considerations for each approach.

Antibody-drug conjugates: Recent advances in payloads

Antibody‒drug conjugates (ADCs), which combine the advantages of monoclonal antibodies with precise targeting and payloads with efficient killing, show great clinical therapeutic value. The ADCs' payloads play a key role in determining the efficacy of ADC drugs and thus have attracted great attention in the field. An ideal ADC payload should possess sufficient toxicity, low immunogenicity, high stability, and modifiable functional groups. Common ADC payloads include tubulin inhibitors and DNA damaging agents, with tubulin inhibitors accounting for more than half of the ADC drugs in clinical development. However, due to clinical limitations of traditional ADC payloads, such as inadequate efficacy and the development of acquired drug resistance, novel highly efficient payloads with diverse targets and reduced side effects are being developed. This perspective summarizes the recent research advances of traditional and novel ADC payloads with main focuses on the structure-activity relationship studies, co-crystal structures, and designing strategies, and further discusses the future research directions of ADC payloads. This review also aims to provide valuable references and future directions for the development of novel ADC payloads that will have high efficacy, low toxicity, adequate stability, and abilities to overcome drug resistance.

Synthesis and Biological Evaluation of C(13)/C(13')-Bis(desmethyl)disorazole Z

We describe the total synthesis of the macrodiolide C(13)/C(13')-bis(desmethyl)disorazole Z through double inter-/intramolecular Stille cross-coupling of a monomeric vinyl stannane/vinyl iodide precursor to form the macrocycle. The key step in the synthesis of this precursor was a stereoselective aldol reaction of a formal Evans acetate aldol product with crotonaldehyde. As demonstrated by X-ray crystallography, the binding mode of C(13)/C(13')-bis(desmethyl)disorazole Z to tubulin is virtually identical with that of the natural product disorazole Z. Likewise, C(13)/C(13')-bis(desmethyl)disorazole Z inhibits tubulin assembly with at least the same potency as disorazole Z and it appears to be a more potent cell growth inhibitor.

Antibody-Drug Conjugates Containing Payloads from Marine Origin

Antibody-drug conjugates (ADCs) are an important class of therapeutics for the treatment of cancer. Structurally, an ADC comprises an antibody, which serves as the delivery system, a payload drug that is a potent cytotoxin that kills cancer cells, and a chemical linker that connects the payload with the antibody. Unlike conventional chemotherapy methods, an ADC couples the selective targeting and pharmacokinetic characteristics related to the antibody with the potent cytotoxicity of the payload. This results in high specificity and potency by reducing off-target toxicities in patients by limiting the exposure of healthy tissues to the cytotoxic drug. As a consequence of these outstanding features, significant research efforts have been devoted to the design, synthesis, and development of ADCs, and several ADCs have been approved for clinical use. The ADC field not only relies upon biology and biochemistry (antibody) but also upon organic chemistry (linker and payload). In the latter, total synthesis of natural and designed cytotoxic compounds, together with the development of novel synthetic strategies, have been key aspects of the consecution of clinical ADCs. In the case of payloads from marine origin, impressive structural architectures and biological properties are observed, thus making them prime targets for chemical synthesis and the development of ADCs. In this review, we explore the molecular and biological diversity of ADCs, with particular emphasis on those containing marine cytotoxic drugs as the payload.

New Technologies Bloom Together for Bettering Cancer Drug Conjugates

Drug conjugates, including antibody-drug conjugates, are a step toward realizing Paul Ehrlich's idea from over 100 years ago of a "magic bullet" for cancer treatment. Through balancing selective targeting molecules with highly potent payloads, drug conjugates can target specific tumor microenvironments and kill tumor cells. A drug conjugate consists of three parts: a targeting agent, a linker, and a payload. In some conjugates, monoclonal antibodies act as the targeting agent, but new strategies for targeting include antibody derivatives, peptides, and even small molecules. Linkers are responsible for connecting the payload to the targeting agent. Payloads impact vital cellular processes to kill tumor cells. At present, there are 12 antibody-drug conjugates on the market for different types of cancers. Research on drug conjugates is increasing year by year to solve problems encountered in conjugate design, such as tumor heterogeneity, poor circulation, low drug loading, low tumor uptake, and heterogenous expression of target antigens. This review highlights some important preclinical research on drug conjugates in recent years. We focus on three significant areas: improvement of antibody-drug conjugates, identification of new conjugate targets, and development of new types of drug conjugates, including nanotechnology. We close by highlighting the critical barriers to clinical translation and the open questions going forward. SIGNIFICANCE STATEMENT: The development of anticancer drug conjugates is now focused in three broad areas: improvements to existing antibody drug conjugates, identification of new targets, and development of new conjugate forms. This article focuses on the exciting preclinical studies in these three areas and advances in the technology that improves preclinical development.

Site-specific construction of triptolide-based antibody-drug conjugates

Antibody-drug conjugates (ADCs) have emerging as efficient agents to target deliver cytotoxic drugs and reduce their off-target side effects. Triptolide has attracted attention to be used in ADC development. Herein, three rationally designed triptolide drug-linkers have been synthesized for use in site-specific construction of ADCs. Carbamates that were supposed to be more stable than carbonates were introduced to attach triptolide to the linkers. PEG and discrete PEG chains were incorporated to improve the hydrophilicity of drug-linkers. The ADCs were finally site-specifically prepared by conjugation of the drug-linkers to trastuzumab through disulfide re-bridging approach. The preliminary anti-tumor activities of these ADCs were evaluated and they displayed high potencies against HER2-targeted cancer in vitro and in vivo.

The promising role of antibody drug conjugate in cancer therapy: Combining targeting ability with cytotoxicity effectively

Introduction

Traditional cancer therapy has many disadvantages such as low selectivity and high toxicity of chemotherapy, as well as insufficient efficacy of targeted therapy. To enhance the cytotoxic effect and targeting ability, while reducing the toxicity of antitumor drugs, an antibody drug conjugate (ADC) was developed to deliver small molecular cytotoxic payloads directly to tumor cells by binding to specific antibodies via linkers.

Method

By reviewing published literature and the current progress of ADCs, we aimed to summarize the basic characteristics, clinical progress, and challenges of ADCs to provide a reference for clinical practice and further research.

Results

ADC is a conjugate composed of three fundamental components, including monoclonal antibodies, cytotoxic payloads, and stable linkers. The mechanisms of ADC including the classical internalization pathway, antitumor activity of antibodies, bystander effect, and non‐internalizing mechanism. With the development of new drugs and advances in technology, various ADCs have achieved clinical efficacy. To date, nine ADCs have received US Food and Drug Administration (FDA) approval in the field of hematologic tumors and solid tumors, which have become routine clinical treatments.

Conclusion

ADC has changed traditional treatment patterns for cancer patients, which enable the same treatment for pancreatic cancer patients and promote individualized precision treatment. Further exploration of indications could focus on early‐stage cancer patients and combined therapy settings. Besides, the mechanisms of drug resistance, manufacturing techniques, optimized treatment regimens, and appropriate patient selection remain the major topics.

Antibody-drug conjugates, immune-checkpoint inhibitors, and their combination in breast cancer therapeutics

INTRODUCTION

Advanced breast cancer (aBC) remains incurable and the quest for more effective systemic anticancer agents continues. Promising results have led to the FDA approval of three antibody-drug conjugates (ADCs) and two immune checkpoint inhibitors (ICIs) to date for patients with aBC.

AREAS COVERED

With the anticipated emergence of newer ADCs and ICIs for patients with several subtypes of breast cancer, and given their potential synergy, their use in combination is of clinical interest. In this article, we review the use of ADCs and ICIs in patients with breast cancer, assess the scientific rationale for their combination, and provide an overview of ongoing trials and some early efficacy and safety results of such dual therapy.

EXPERT OPINION

Improvement in the medicinal chemistry of next-generation ADCs, their rational combination with ICIs and other agents, and the development of multiparametric immune biomarkers could help to significantly improve the outlook for patients with refractory aBC.

Computational Study of the Addition of Methanethiol to 40+ Michael Acceptors as a Model for the Bioconjugation of Cysteines

A long series of Michael acceptors are studied computationally as potential alternatives to the maleimides that are used in most antibody-drug conjugates to link Cys of mAbs with cytotoxic drugs. The products of the reaction of methanethiol (CH3SH/MeSH, as a simple model of Cys) with N-methylated ethynesulfonamide, 2-ethynylpyridinium ion, propynamide, and methyl ethynephosphonamidate (that is, with HC≡C-EWG) are predicted by the M06-2X/6-311+G(d,p) method to be thermodynamically more stable, in relation to their precursors, than that of MeSH with N-methylmaleimide and, in general, with H2C═CH-EWG; calculations with AcCysOMe and tBuSH are also included. However, for the addition of the anion (MeS-), which is the reactive species, the order changes and N-methylated 2-vinylpyridinium ion, 2,3-butadienamide, and maleimide may give more easily the anionic adducts than several activated triple bonds; moreover, the calculated ΔG⧧ values increase following the order HC≡C-SO2NHMe, N-methylmaleimide, HC≡C-PO(OMe)NHMe, and HC≡C-CONHMe. In other words, MeS- is predicted to react more rapidly with maleimides than with ethynephosphonamidates and with propynamides, in agreement with the experimental results. New mechanistic details are disclosed regarding the advantageous use of some amides, especially of ethynesulfonamides, which, however, are more prone to double additions and exchange reactions.

From Plant to Patient: Thapsigargin, a Tool for Understanding Natural Product Chemistry, Total Syntheses, Biosynthesis, Taxonomy, ATPases, Cell Death, and Drug Development

Thapsigargin, the first representative of the hexaoxygenated guaianolides, was isolated 40 years ago in order to understand the skin-irritant principles of the resin of the umbelliferous plant Thapsia garganica. The pronounced cytotoxicity of thapsigargin is caused by highly selective inhibition of the intracellular sarco-endoplasmic Ca²⁺-ATPase (SERCA) situated on the membrane of the endo- or sarcoplasmic reticulum. Thapsigargin is selective to the SERCA pump and to a minor extent the secretory pathway Ca²⁺/Mn²⁺ ATPase (SPCA) pump. Thapsigargin has become a tool for investigation of the importance of SERCA in intracellular calcium homeostasis. In addition, complex formation of thapsigargin with SERCA has enabled crystallization and structure determination of calcium-free states by X-ray crystallography. These results led to descriptions of the mechanism of action and kinetic properties of SERCA and other ATPases. Inhibition of SERCA depletes Ca²⁺ from the sarco- and endoplasmic reticulum provoking the unfolded protein response, and thereby has enabled new studies on the mechanism of cell death. Development of protocols for selective transformation of thapsigargin disclosed the chemistry and facilitated total synthesis of the molecule. Conversion of trilobolide into thapsigargin offered an economically feasible sustainable source of thapsigargin, which enables a future drug production. Principles for prodrug development were used by conjugating a payload derived from thapsigargin with a hydrophilic peptide selectively cleaved by proteases in the tumor. Mipsagargin was developed in order to obtain a drug for treatment of cancer diseases characterized by the presence of prostate specific membrane antigen (PSMA) in the neovascular tissue of the tumors. Even though mipsagargin showed interesting clinical effects the results did not encourage funding and consequently the attempt to register the drug has been abandoned. In spite of this disappointing fact, the research performed to develop the drug has resulted in important scientific discoveries concerning the chemistry, biosynthesis and biochemistry of sesquiterpene lactones, the mechanism of action of ATPases including SERCA, mechanisms for cell death caused by the unfolded protein response, and the use of prodrugs for cancer-targeting cytotoxins. The presence of toxins in only some species belonging to Thapsia also led to a major revision of the taxonomy of the genus.

Phthalimide-based-SSCF3 reagent for enantioselective dithiotrifluoromethylation

A novel dithiotrifluoromethylation reagent phthN-SSCF₃ was designed and prepared for the incorporation of a SSCF₃ unit into complex molecules and the stereoselective construction of a SSCF₃-tethered quaternary carbon center.

Photoactivatable prodrug for simultaneous release of mertansine and CO along with a BODIPY derivative as a luminescent marker in mitochondria: a proof of concept for NIR image-guided cancer therapy

Controlled and efficient activation is the crucial aspect of designing an effective prodrug. Herein we demonstrate a proof of concept for a light activatable prodrug with desired organelle specificity. Mertansine, a benzoansamacrolide, is an efficient microtubule-targeting compound that binds at or near the vinblastine-binding site in the mitochondrial region to induce mitotic arrest and cell death through apoptosis. Despite its efficacy even in the nanomolar level, this has failed in stage 2 of human clinical trials owing to the lack of drug specificity and the deleterious systemic toxicity. To get around this problem, a recent trend is to develop an antibody-conjugatable maytansinoid with improved tumor/organelle-specificity and lesser systematic toxicity. Endogenous CO is recognized as a regulator of cellular function and for its obligatory role in cell apoptosis. CO blocks the proliferation of cancer cells and effector T cells, and the primary target is reported to be the mitochondria. We report herein a new mitochondria-specific prodrug conjugate (Pro-DC) that undergoes a photocleavage reaction on irradiation with a 400 nm source (1.0 mW cm⁻²) to induce a simultaneous release of the therapeutic components mertansine and CO along with a BODIPY derivative (BODIPY(PPH₃)₂) as a luminescent marker in the mitochondrial matrix. The efficacy of the process is demonstrated using MCF-7 cells and could effectively be visualized by probing the intracellular luminescence of BODIPY(PPH₃)₂. This provides a proof-of-concept for designing a prodrug for image-guided combination therapy for mainstream treatment of cancer.

Simultaneous release of two therapeutic reagents, mertansine and CO through photo-induced cleavage of a mitochondria-specific prodrug with improved drug efficacy.

Novel Auristatins with High Bystander and Cytotoxic Activities in Drug Efflux-positive Tumor Models

Auristatins, a class of clinically validated anti-tubulin agents utilized as payloads in antibody-drug conjugates, are generally classified by their membrane permeability and the extent of cytotoxic bystander activity on neighboring cells after targeted delivery. The drugs typically fall within two categories: membrane permeable monomethyl auristatin E-type molecules with high bystander activities and susceptibility to efflux pumps, or charged and less permeable monomethyl auristatin F (MMAF) analogs with low bystander activities and resistance to efflux pumps. Herein, we report the development of novel auristatins that combine the attributes of each class by having both bystander activity and cytotoxicity on multidrug-resistant (MDR⁺) cell lines. Structure-based design focused on the hydrophobic functionalization of the N-terminal N-methylvaline of the MMAF scaffold to increase cell permeability. The resulting structure-activity relationships of the new auristatins demonstrate that optimization of hydrophobicity and structure can lead to highly active free drugs and antibody-drug conjugates with in vivo bystander activities.

Perspectives from nearly five decades of total synthesis of natural products and their analogues for biology and medicine

Covering: 1970 to 2020By definition total synthesis is the art and science of making the molecules of living Nature in the laboratory, and by extension, their analogues. Although obvious, its application to the synthesis of molecules for biology and medicine was not always the purpose of total synthesis. In recent years, however, the field has acquired momentum as its power to reach higher molecular complexity and diversity is increasing, and as the demand for rare bioactive natural products and their analogues is expanding due to their recognised potential to facilitate biology and drug discovery and development. Today this component of total synthesis endeavors is considered highly desirable, and could be part of interdisciplinary academic and/or industrial partnerships, providing further inspiration and momentum to the field. In this review we provide a brief historical background of the emergence of the field of total synthesis as it relates to making molecules for biology and medicine. We then discuss specific examples of this practice from our laboratories as they developed over the years. The review ends with a conclusion and future perspectives for natural products chemistry and its applications to biology and medicine and other added-value contributions to science and society.

Exploiting azido-dichloro-triazine as a linker for regioselective incorporation of peptides through their N, O, S functional groups

In the field of bioconjugation, linker development has witnessed massive growth in recent years. 2,4,6-Trichloro-1,3,5-triazine (TCT) is a tridentate linker that can accommodate three distinct nucleophiles. Herein, the reaction of azido triazine derivatives with nucleophiles (amine, thiol and phenol) is studied. The replacement of first chlorine was performed at 0 °C while that of the last chlorine was achieved successfully at rt. As a proof of concept of this strategy with potential application in biological studies, pentapeptides (Ac-XGGFL-NH₂ where X = Lys or Tyr or Cys) were reacted with 2-azido-4,6-dichlorotriazine to replace the first and second chlorine at 0 °C and at rt, respectively. The reactivity of 2-azido-4,6-dichlorotriazine was found to be similar for the α and ε amine group present in same peptide. These findings demonstrate the applicability of 2-azido-4,6-dichlorotriazine as a linker with potential further application in bioconjugation.

Streamlined Symmetrical Total Synthesis of Disorazole B1 and Design, Synthesis, and Biological Investigation of Disorazole Analogues

Taking advantage of the C₂-symmetry of the antitumor naturally occurring disorazole B₁ molecule, a symmetrical total synthesis was devised with a monomeric advanced intermediate as the key building block, whose three-step conversion to the natural product allowed for an expeditious entry to this family of compounds. Application of the developed synthetic strategies and methods provided a series of designed analogues of disorazole B₁, whose biological evaluation led to the identification of a number of potent antitumor agents and the first structure-activity relationships (SARs) within this class of compounds. Specifically, the substitutions of the epoxide units and lactone moieties with cyclopropyl and lactam structural motifs, respectively, were found to be tolerable for biological activities and beneficial with regard to chemical stability.

Tubulysin Synthesis Featuring Stereoselective Catalysis and Highly Convergent Multicomponent Assembly

A concise and modular total synthesis of the highly potent N14-desacetoxytubulysin H (1) has been accomplished in 18 steps in an overall yield of up to 30%. Our work highlights the complexity-augmenting and route-shortening power of diastereoselective multicomponent reaction (MCR) as well as the role of bulky ligands to perfectly control both the regioselective and diastereoselective synthesis of tubuphenylalanine in just two steps. The total synthesis not only provides an operationally simple and step economy but will also stimulate major advances in the development of new tubulysin analogues.

Cellular Delivery of Bioorthogonal Pretargeting Therapeutics in PSMA-Positive Prostate Cancer

Prostate cancer is primarily fatal after it becomes metastatic and castration-resistant despite novel combined hormonal and chemotherapeutic regimens. Hence, new therapeutic concepts and drug delivery strategies are urgently needed for the eradication of this devastating disease. Here we report the highly specific, in situ click chemistry driven pretargeted delivery of cytotoxic drug carriers to PSMA(+) prostate cancer cells. Anti-PSMA 5D3 mAb and its F(ab')₂ fragments were functionalized with trans-cyclooctene (TCO), labeled with a fluorophore, and used as pretargeting components. Human serum albumin (ALB) was loaded with the DM1 antitubulin agent, functionalized with PEGylated tetrazine (PEG₄-Tz), labeled with a fluorophore, and used as the drug delivery component. The internalization kinetics of components and the therapeutic efficacy of the pretargeted click therapy were studied in PSMA(+) PC3-PIP and PSMA(-) PC3-Flu control cells. The F(ab')₂ fragments were internalized faster than 5D3 mAb in PSMA(+) PC3-PIP cells. In the two-component pretargeted imaging study, both components were colocalized in a perinuclear location of the cytoplasm of PC3-PIP cells. Better colocalization was achieved when 5D3 mAb was used as the pretargeting component. Consecutively, the in vitro cell viability study shows a significantly higher therapeutic effect of click therapy in PC3-PIP cells when 5D3 mAb was used for pretargeting, compared to its F(ab')₂ derivative. 5D3 mAb has a longer lifetime on the cell surface, when compared to its F(ab')₂ analogue, enabling efficient cross-linking with the drug delivery component and increased efficacy. Pretargeting and drug delivery components were cross-linked via multiple bioorthogonal click chemistry reactions on the surface of PSMA(+) PC cells forming nanoclusters, which undergo fast cellular internalization and intracellular transport to perinuclear locations.

Yangpumicins F and G, Enediyne Congeners from Micromonospora yangpuensis DSM 45577

Enediyne natural products are among the most cytotoxic small molecules and thus excellent payload candidates for the development of antibody-drug conjugates (ADCs). Here we report the isolation and structural elucidation of two new 10-membered anthraquinone-fused enediynes, yangpumicins (YPM) F (6) and G (7), together with five known congeners, YPM A-E (1-5), from Micromonospora yangpuensis DSM 45577. YPM F (6) and G (7) showed strong cytotoxicity against the tested human cancer cell lines, as well as activity against several Gram-positive and Gram-negative pathogens. The 1,2-diols in 6 and 7 promise to enable new linker chemistry for the development of YPM-based ADCs.