Synthesis and Biological Evaluation of Shishijimicin A-Type Linker-Drugs and Antibody–Drug Conjugates

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.

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.

Synthesis and biological properties of maleimide-based macrocyclic lactone enediynes

Natural enediyne antibiotics are powerful DNA-cleavage agents due to the presence of the highly reactive hex-3-ene-1,5-diyne units. However, the complicated chemical structure and thermal instability make their synthesis, derivatization, and storage challenging. Heterocycle-fused enediynes, which exhibit strong antineoplastic activity, are promising analogues of natural enediynes for medicinal applications. To this end, a series of maleimide-based enediynes with macrocyclic lactone moieties were synthesized through the Sonagashira coupling reaction. Differential scanning calorimetry and electron paramagnetic resonance results showed that these macrocyclic enediynes exhibited a rather low onset temperature and the ability to generate radicals at physiological temperature. In addition, the structure-activity relationship of enediynes was analyzed by changing the ring size and the substituents on the propargyl group. Cellular experiments indicated that the diradicals produced by these enediynes efficiently cleaved DNA and disrupted the cell cycle distribution, and consequently induced tumor cell death via an apoptosis pathway at low half inhibitory concentrations. Computational studies suggested that the maleimide moiety promoted the propargyl-allenyl rearrangement of the cyclic enediyne, enabling the generation of diradical species through the Myers-Saito cyclization, and then abstracted hydrogen atoms from the H-donors.

Synthetic Antibody-Rhamnose Cluster Conjugates Show Potent Complement-Dependent Cell Killing by Recruiting Natural Antibodies

Monoclonal antibodies (mAbs) are one of the most rapidly growing drug classes used for the treatment of cancer, infectious and autoimmune diseases. Complement-dependent cytotoxicity (CDC) is one of the effector functions for antibodies to deplete target cells. We report here an efficient chemoenzymatic synthesis of structurally well-defined conjugates of a monoclonal antibody with a rhamnose- and an αGal trisaccharide-cluster to recruit natural anti-rhamnose and anti-αGal antibodies, respectively, to enhance the CDC-dependent targeted cell killing. The synthesis was achieved by using a modular antibody Fc-glycan remodeling method that includes site-specific chemoenzymatic Fc-glycan functionalization and subsequent click conjugation of synthetic rhamnose- and αGal trisaccharide-cluster to provide the respective homogeneous antibody conjugates. Cell-based assays indicated that the antibody-rhamnose cluster conjugates could mediate potent CDC activity for targeted cancer cell killing and showed much more potent efficacy than the antibody-αGal trisaccharide cluster conjugates for CDC effects.

Marine natural products

Covering: 2020This review covers the literature published in 2020 for marine natural products (MNPs), with 757 citations (747 for the period January to December 2020) referring to compounds isolated from marine microorganisms and phytoplankton, green, brown and red algae, sponges, cnidarians, bryozoans, molluscs, tunicates, echinoderms, mangroves and other intertidal plants and microorganisms. The emphasis is on new compounds (1407 in 420 papers for 2020), together with the relevant biological activities, source organisms and country of origin. Pertinent reviews, biosynthetic studies, first syntheses, and syntheses that led to the revision of structures or stereochemistries, have been included. A meta analysis of bioactivity data relating to new MNPs reported over the last five years is also presented.

A brief overview of classical natural product drug synthesis and bioactivity

This manuscript briefly overviewed the total synthesis and structure–activity relationship studies of eight classical natural products, which emphasizes the important role of total synthesis in natural product-based drug development.

Molecular hybrids: A five-year survey on structures of multiple targeted hybrids of protein kinase inhibitors for cancer therapy

Protein kinases have grown over the past few years as a crucial target for different cancer types. With the multifactorial nature of cancer, and the fast development of drug resistance for conventional chemotherapeutics, a strategy for designing multi-target agents was suggested to potentially increase drug efficacy, minimize side effects and retain the proper pharmacokinetic properties. Kinase inhibitors were used extensively in such strategy. Different kinase inhibitor agents which target EGFR, VEGFR, c-Met, CDK, PDK and other targets were merged into hybrids with conventional chemotherapeutics such as tubulin polymerization and topoisomerase inhibitors. Other hybrids were designed gathering kinase inhibitors with targeted cancer therapy such as HDAC, PARP, HSP 90 inhibitors. Nitric oxide donor molecules were also merged with kinase inhibitors for cancer therapy. The current review presents the hybrids designed in the past five years discussing their design principles, results and highlights their future perspectives.

Anthraquinone-fused enediynes: discovery, biosynthesis and development

Covering: up to the end of July, 2021Anthraquinone-fused enediynes (AFEs) are a subfamily of enediyne natural products. Dynemicin A (DYN A), the first member of the AFE family, was discovered more than thirty years ago. Subsequently, extensive studies have been reported on the mode of action and the interactions of AFEs with DNA using DYN A as a model. However, progress in the discovery, biosynthesis and clinical development of AFEs has been limited for a long time. In the past five years, four new AFEs have been discovered and significant progress has been made in the biosynthesis of AFEs, especially on the biogenesis of the anthraquinone moiety and their tailoring steps. Moreover, the streamlined total synthesis of AFEs and their analogues boosts the preparation of AFE-based linker-drugs, thus enabling the development of AFE-based antibody-drug conjugates (ADCs). This review summarizes the discovery, mechanism of action, biosynthesis, total synthesis and preclinical studies of AFEs.

Cyclohexeno[3,4]cyclodec-1,5-diyne-3-ene: A Convenient Enediyne

Enediynes are widely studied to understand their cycloaromatization and the trapping of the resulting p-dehydrobenzene diradical. However, few model substrates are known, and they are hard to synthesize and difficult to handle. Herein we report cyclohexeno[3,4]cyclodec-1,5-diyne-3-ene as a convenient model for studying the reactivity of enediynes. It can be easily synthesized from 1,2-diethynylcyclohexene and 1,4-diiodobutane. It is a solid that is stable at room temperature. In solution the p-dehydrobenzene diradical derived from its cycloaromatization can be trapped by nucleophiles. The rate-limiting step is the cyclization, which is slightly slower than that of the parent cyclodec-1,5-diyne-3-ene but faster than that of its benzo analogue, consistent with the distances between the reacting carbon atoms.

Combretastatin A4-derived payloads for antibody-drug conjugates

We describe the use of natural product combretastatin A4 (CA4) as a versatile new payload for the construction of antibody-drug conjugates (ADCs). Cetuximab conjugates consisting of CA4 derivatives were site-specially prepared by disulfide re-bridging approach using cleavable and non-cleavable linkers. These ADCs retained antigen binding and internalization efficiency and exhibited high potencies against cancer cell lines in vitro. The conjugates also demonstrated significant antitumor activities in EGFR-positive xenograft models without observed toxicities. CA4 appears to be a viable payload option for ADCs research and development.

Challenges and Opportunities to Develop Enediyne Natural Products as Payloads for Antibody-Drug Conjugates

Calicheamicin, the payload of the antibody-drug-conjugates (ADCs) gemtuzumab ozogamicin (Mylotarg^®) and inotuzumab ozogamicin (Besponsa^®), belongs to the class of enediyne natural products. Since the isolation and structural determination of the neocarzinostatin chromophore in 1985, the enediynes have attracted considerable attention for their value as DNA damaging agents in cancer chemotherapy. Due to their non-discriminatory cytotoxicity towards both cancer and healthy cells, the clinical utilization of enediyne natural products relies on conjugation to an appropriate delivery system, such as an antibody. Here we review the current landscape of enediynes as payloads of first-generation and next-generation ADCs.

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.