Synthetic reactions with rare taccalonolides reveal the value of C-22,23 epoxidation for microtubule stabilizing potency

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.

A systematic review of spirostanol saponins and anticancer natural products isolated from Tacca plantaginea (Hance) Drenth

The plant genus Tacca comprises twenty species including Tacca plantaginea, essentially distributed in the Indo-China region. Medicinal preparations from the rhizomes are used traditionally to treat gastrointestinal ailments, stomach aches and inflammatory disorders. A variety of bioactive molecules have been isolated from T. plantaginea, including potent anticancer steroids such as the taccanolides which interfere with microtubules dynamic. Other efficient anticancer natural products have been isolated from the plant, in particular a series of diosgenin/yamogenin-type sapogenins including taccaoside (monodesmosidic) and taccaoside A (bidesmosidic). Taccaoside A displays marked anticancer properties through two complementary mechanisms: a direct action on cancer stem cells via HRas and Pi3K/Akt signaling and an indirect immunomodulatory action via activation of cytotoxic T cells. A similar mechanism of action has been invoked with a total saponin extract from Schizocapsa plantaginea Hance (synonym to T. plantaginea) and the saponin SSPH 1. This saponin reduced tumor growth in mice through stimulation of cytotoxic T lymphocytes. Other bioactive products have been isolated from T. plantaginea, including withanolide-type steroids (plantagiolides, chantriolides), diarylheptanoids (plantagineosides) and different saponins (diosbulbisides, lieguonins). The discussion centers around the mechanism of action of spirostanol saponins, with the objective to promote their study as immuno-active anticancer agents.

Taccalonolides: Structure, semi-synthesis, and biological activity

Microtubules are the fundamental part of the cell cytoskeleton intimately involving in cell proliferation and are superb targets in clinical cancer therapy today. Microtubule stabilizers have become one of the effectively main agents in the last decades for the treatment of diverse cancers. Taccalonolides, the highly oxygenated pentacyclic steroids isolated from the genus of Tacca, are considered a class of novel microtubule-stabilizing agents. Taccalonolides not only possess a similar microtubule-stabilizing activity as the famous drug paclitaxel but also reverse the multi-drug resistance of paclitaxel and epothilone in cellular and animal models. Taccalonolides have captured numerous attention in the field of medicinal chemistry due to their variety of structures, unique mechanism of action, and low toxicity. This review focuses on the structural diversity, semi-synthesis, modification, and pharmacological activities of taccalonolides, providing bright thoughts for the discovery of microtubule-stabilizing drugs.

Taccachatrones A-G, Highly Oxidized Steroids from the Rhizomes of Tacca chantrierii and Their Cytotoxicity Assessment

Seven highly oxidized steroids, taccachatrones A-G (1-7), together with four known taccalonolides (8-11), were characterized from the rhizomes of Tacca chantrieri. The structures of 1-7 were established on the basis of spectroscopic data analysis, while the absolute configurations were determined by single-crystal X-ray diffraction. Compounds 1-4 may be derived from taccalonolide derivatives by the degradation of three carbon atoms. Compounds 7, 8, 10, and 11 exhibited cytotoxicity to human cancer cell lines, indicating that the presence of a lactone moiety, as well as a double bond between C-22 and C-23, might play key roles in mediating their cytotoxicity.

Efficacy of a Covalent Microtubule Stabilizer in Taxane-Resistant Ovarian Cancer Models

Ovarian cancer often has a poor clinical prognosis because of late detection, frequently after metastatic progression, as well as acquired resistance to taxane-based therapy. Herein, we evaluate a novel class of covalent microtubule stabilizers, the C-22,23-epoxytaccalonolides, for their efficacy against taxane-resistant ovarian cancer models in vitro and in vivo. Taccalonolide AF, which covalently binds β-tubulin through its C-22,23-epoxide moiety, demonstrates efficacy against taxane-resistant models and shows superior persistence in clonogenic assays after drug washout due to irreversible target engagement. In vivo, intraperitoneal administration of taccalonolide AF demonstrated efficacy against the taxane-resistant NCI/ADR-RES ovarian cancer model both as a flank xenograft, as well as in a disseminated orthotopic disease model representing localized metastasis. Taccalonolide-treated animals had a significant decrease in micrometastasis of NCI/ADR-RES cells to the spleen, as detected by quantitative RT-PCR, without any evidence of systemic toxicity. Together, these findings demonstrate that taccalonolide AF retains efficacy in taxane-resistant ovarian cancer models in vitro and in vivo and that its irreversible mechanism of microtubule stabilization has the unique potential for intraperitoneal treatment of locally disseminated taxane-resistant disease, which represents a significant unmet clinical need in the treatment of ovarian cancer patients.

Anti-hepatoma effect of taccalonolide A through suppression of sonic hedgehog pathway

Taccalonolide A has been reported to have anti-tumour efficiency. However, the underlying mechanism for taccalonolides A therapy of hepatocellular carcinoma (HCC) is still obscure. Cell viability was evaluated by cell counting kit-8 (CCK-8) assay. Apoptosis was determined by flow cytometry. Protein expression of B cell lymphoma (Bcl-2), Bcl-2 associated X (Bax), sonic hedgehog (Shh), Smoothened (Smo) and Gli family zinc finger 1 (Gli1) was analyzed by western blot. The expression of Shh, Smo and Gli1 mRNA was determined using quantitative real-time polymerase chain reaction (qRT-PCR). Results showed that taccalonolide A inhibited cell proliferation, induced apoptosis and cell cycle arrest at the G0/G1 phase, and improved the cytotoxicity of sorafenib in HCC cells. The expressions of Shh, Smo, Gli1 mRNA and protein were decreased after taccalonolide A treatment. More importantly, activation of the Shh pathway attenuated taccalonolide A-induced inhibition on cell viability and promotion on apoptosis and cell cycle arrest in HCC. Also, activation of the Shh pathway neutralized the effect of taccalonolide A on sorafenib cytotoxicity in HCC. We clarified that taccalonolide A suppressed cell viability facilitated apoptosis, and improved the cytotoxicity of sorafenib in HCC by inhibition of the activation of the Shh pathway, providing alternative treatments for HCC.

Taccalonolides: A Novel Class of Microtubule-Stabilizing Anticancer Agents

Simple Summary

Natural products have continued to play an important role in new drug discovery with a considerable number of marketed drugs being derived from naturally occurring compounds, particularly in the area of cancer. Taccalonolides are a new class of microtube-stabilizing agents isolated from plants of the genus Tacca demonstrating effectiveness against drug-resistant tumors in cellular and animal models. This review article highlights the discovery history of taccalonolides and their microtubule-stabilizing activities, which summarizes the naturally derived and semi-synthesized structures that have been reported so far and the advances on the mechanism of action of taccalonolides.

Abstract

Microtubule stabilizing agents, such as paclitaxel, docetaxel, and cabazitaxel have been among the most used chemotherapeutic agents in the last decades for the treatment of a wide range of cancers in the clinic. One of the concerns that limit their use in clinical practice is their intrinsic and acquired drug resistance, which is common to most anti-cancer chemotherapeutics. Taccalonolides are a new class of microtubule stabilizers isolated from the roots of a few species in the genus of Tacca. In early studies, taccalonolides demonstrated different effects on interphase and mitotic microtubules from those of paclitaxel and laulimalide suggesting a unique mechanism of action. This prompts the exploration of new taccalonolides with various functionalities through the identification of minor constituents of natural origin and semi-synthesis. The experiments on the new highly potent taccalonolides indicated that taccalonolides possessed a unique mechanism of covalently binding to the microtubule. An X-ray diffraction analysis of a crystal of taccalonolides AJ binding to tubulin indicated that the covalent binding site is at β-tubulin D226. Taccalonolides circumvent all three mechanisms of taxane drug resistance both in vitro and in vivo. To improve the activity, the structure modification through semi-synthesis was conducted and the structure-activity relationships (SARs) was analyzed based on natural and semi-synthetical taccalonolides. The C22–C23 epoxide can significantly increase the antiproliferation potency of taccalonolides due to the covalent link of C22 and the carboxylic group of D226. Great progress has been seen in the last few years in the understanding of the mechanism of this class of microtube-stabilizing agents. This review summarizes the structure diversity, structure-activity relationships (SARs), mechanism of action, and in vivo activities of taccalonolides.

Beyond the Paclitaxel and Vinca Alkaloids: Next Generation of Plant-Derived Microtubule-Targeting Agents with Potential Anticancer Activity

Plants are an important source of chemically diverse natural products that target microtubules, one of the most successful targets in cancer therapy. Colchicine, paclitaxel, and vinca alkaloids are the earliest plant-derived microtubule-targeting agents (MTAs), and paclitaxel and vinca alkaloids are currently important drugs used in the treatment of cancer. Several additional plant-derived compounds that act on microtubules with improved anticancer activity are at varying stages of development. Here, we move beyond the well-discussed paclitaxel and vinca alkaloids to present other promising plant-derived MTAs with potential for development as anticancer agents. Various biological and biochemical aspects are discussed. We hope that the review will provide guidance for further exploration and identification of more effective, novel MTAs derived from plant sources.

Elucidating target specificity of the taccalonolide covalent microtubule stabilizers employing a combinatorial chemical approach

The taccalonolide microtubule stabilizers covalently bind β-tubulin and overcome clinically relevant taxane resistance mechanisms. Evaluations of the target specificity and detailed drug-target interactions of taccalonolides, however, have been limited in part by their irreversible target engagement. In this study, we report the synthesis of fluorogenic taccalonolide probes that maintain the native biological properties of the potent taccalonolide, AJ. These carefully optimized, cell-permeable probes outperform commercial taxane-based probes and enable direct visualization of taccalonolides in both live and fixed cells with dramatic microtubule colocalization. The specificity of taccalonolide binding to β-tubulin is demonstrated by immunoblotting, which allows for determination of the relative contribution of key tubulin residues and taccalonolide moieties for drug-target interactions by activity-based protein profiling utilizing site-directed mutagenesis and computational modeling. This combinatorial approach provides a generally applicable strategy for investigating the binding specificity and molecular interactions of covalent binding drugs in a cellular environment.

Taccalonolide Microtubule Stabilizers

Microtubule stabilizers are a mainstay in the treatment of many solid cancers and continue to find utility in combination therapy with molecularly targeted anticancer agents and immunotherapeutics. However, innate and acquired resistance to microtubule stabilizers can limit their clinical efficacy. The taccalonolides are a unique class of microtubule stabilizers isolated from plants of Tacca that circumvent clinically relevant mechanisms of drug resistance. Although initial reports suggested that the microtubule-stabilizing activity of the taccalonolides was independent of direct tubulin binding, additional studies have identified that potent C-22, C-23 epoxidized taccalonolides covalently bind the Aspartate 226 residue of β-tubulin and that this interaction is critical for their microtubule-stabilizing activity. The taccalonolides have distinct properties as compared to other microtubule stabilizers with regard to their biochemical effects on tubulin structure and dynamics that promote distinct cellular phenotypes. Some taccalonolides have demonstrated in vivo antitumor efficacy in drug-resistant tumor models with exquisite potency and long-lasting antitumor efficacy as a result of their irreversible target engagement. The recent identification of a site on the taccalonolide scaffold that is amenable to modification has provided evidence of the specificity of the taccalonolide-tubulin interaction. This also affords an opportunity to further optimize the targeted delivery of the taccalonolides to further improve their anticancer efficacy and potential for clinical development.

Identification of C-6 as a New Site for Linker Conjugation to the Taccalonolide Microtubule Stabilizers

The taccalonolides are a class of microtubule stabilizers that circumvent clinically relevant forms of drug resistance due to their unique mechanism of microtubule stabilization imparted by the covalent binding of the C-22-C-23 epoxide moiety to tubulin. A taccalonolide (8) with a fluorescein group attached with a linker at C-6 was generated, and biochemical and cell-based assays showed that it bound directly to tubulin and stabilized microtubules. This pharmacological probe has allowed, for the first time, a direct visualization of a taccalonolide binding to microtubules, verifying their cellular binding site. This C-6-modified taccalonolide showed potency comparable to the untagged compound in biochemical experiments; however, its potency was lower in cellular assays, presumably due to decreased cellular permeability. These studies provide a valuable tool to facilitate the further understanding of taccalonolide pharmacology and demonstrate that C-6 is a promising site for a linker to be added to this novel class of microtubule stabilizers for targeted drug delivery.

Current advances of tubulin inhibitors as dual acting small molecules for cancer therapy

Microtubule (MT)-targeting agents are highly successful drugs as chemotherapeutic agents, and this is attributed to their ability to target MT dynamics and interfere with critical cellular functions, including, mitosis, cell signaling, intracellular trafficking, and angiogenesis. Because MT dynamics vary in the different stages of the cell cycle, these drugs tend to be the most effective against mitotic cells. While this class of drug has proven to be effective against many cancer types, significant hurdles still exist and include overcoming aspects such as dose limited toxicities and the development of resistance. Newer generations of developed drugs attack these problems and alternative approaches such as the development of dual tubulin and kinase inhibitors are being investigated. This approach offers the potential to show increased efficacy and lower toxicities. This review covers different categories of MT-targeting agents, recent advances in dual inhibitors, and current challenges for this drug target.

Taccalonolide Microtubule Stabilizers Generated Using Semisynthesis Define the Effects of Mono Acyloxy Moieties at C-7 or C-15 and Disubstitutions at C-7 and C-25

The taccalonolides are a unique class of microtubule stabilizers isolated from Tacca spp. that have efficacy against drug-resistant tumors. Our previous studies have demonstrated that a C-15 acetoxy taccalonolide, AF, has superior in vivo antitumor efficacy compared to AJ, which bears a C-15 hydroxy group. With the goal of further improving the in vivo efficacy of this class of compounds, we semisynthesized and tested the biological activities of 28 new taccalonolides with monosubstitutions at C-7 or C-15 or disubstitutions at C-7 and C-25, covering a comprehensive range of substituents from formic acid to anthraquinone-2-carbonyl chloride. The resulting taccalonolide analogues with diverse C-7/C-15/C-25 modifications exhibited IC₅₀ values from 2.4 nM to >20 μM, allowing for extensive in vitro structure-activity evaluations. This semisynthetic strategy was unable to provide a taccalonolide with improved therapeutic window due to hydrolysis of substituents at C-7 or C-15 regardless of size or steric bulk. However, two of the most potent new taccalonolides, bearing isovalerate modifications at C-7 or C-15, demonstrated potent and highly persistent antitumor activity in a drug-resistant xenograft model when administered intratumorally. This study demonstrates that targeted delivery of the taccalonolides to the tumor could be an effective, long-lasting approach to treat drug-resistant tumors.

A Stereocontrolled Annulation of the Taccalonolide Epoxy Lactone onto the Molecular Framework of trans-Androsterone

A robust and scalable route to the taccalonolide skeleton starting from trans-androsterone is presented. The synthesis features a cyclic hydroboration carbonylation reaction, which effectively establishes the trans-hydrindane DE ring junction in a remarkable annulation reaction, as well as a Claisen rearrangement and a catalytic Ullmann-type cyclization. This work is part of a larger effort to uncover new clinical candidates from the taccalonolide class of anticancer agents through advances in chemical synthesis.

Tubulin Inhibitor-Based Antibody-Drug Conjugates for Cancer Therapy

Antibody-drug conjugates (ADCs) are a class of highly potent biopharmaceutical drugs generated by conjugating cytotoxic drugs with specific monoclonal antibodies through appropriate linkers. Specific antibodies used to guide potent warheads to tumor tissues can effectively reduce undesired side effects of the cytotoxic drugs. An in-depth understanding of antibodies, linkers, conjugation strategies, cytotoxic drugs, and their molecular targets has led to the successful development of several approved ADCs. These ADCs are powerful therapeutics for cancer treatment, enabling wider therapeutic windows, improved pharmacokinetic/pharmacodynamic properties, and enhanced efficacy. Since tubulin inhibitors are one of the most successful cytotoxic drugs in the ADC armamentarium, this review focuses on the progress in tubulin inhibitor-based ADCs, as well as lessons learned from the unsuccessful ADCs containing tubulin inhibitors. This review should be helpful to facilitate future development of new generations of tubulin inhibitor-based ADCs for cancer therapy.

Mechanism of microtubule stabilization by taccalonolide AJ

As a major component of the cytoskeleton, microtubules consist of αβ-tubulin heterodimers and have been recognized as attractive targets for cancer chemotherapy. Microtubule-stabilizing agents (MSAs) promote polymerization of tubulin and stabilize the polymer, preventing depolymerization. The molecular mechanisms by which MSAs stabilize microtubules remain elusive. Here we report a 2.05 Å crystal structure of tubulin complexed with taccalonolide AJ, a newly identified taxane-site MSA. Taccalonolide AJ covalently binds to β-tubulin D226. On AJ binding, the M-loop undergoes a conformational shift to facilitate tubulin polymerization. In this tubulin–AJ complex, the E-site of tubulin is occupied by GTP rather than GDP. Biochemical analyses confirm that AJ inhibits the hydrolysis of the E-site GTP. Thus, we propose that the β-tubulin E-site is locked into a GTP-preferred status by AJ binding. Our results provide experimental evidence for the connection between MSA binding and tubulin nucleotide state, and will help design new MSAs to overcome taxane resistance.

Microtubule-stabilizing agents (MSAs) promote the polymerization of tubulin and are of great interest as anticancer drugs. Here the authors present the crystal structure of the MSA taccalonolide AJ bound to tubulin and give insights into its mode of action, which might help in the design of novel MSAs.

Pharmacokinetic Analysis and in Vivo Antitumor Efficacy of Taccalonolides AF and AJ

The taccalonolides are microtubule stabilizers that covalently bind tubulin and circumvent clinically relevant forms of resistance to other drugs of this class. Efforts are under way to identify a taccalonolide with optimal properties for clinical development. The structurally similar taccalonolides AF and AJ have comparable microtubule-stabilizing activities in vitro, but taccalonolide AF has excellent in vivo antitumor efficacy when administered systemically, while taccalonolide AJ does not elicit this activity even at maximum tolerated dose. The hypothesis that pharmacokinetic differences underlie the differential efficacies of taccalonolides AF and AJ was tested. The effects of serum on their in vivo potency, metabolism by human liver microsomes and in vivo pharmacokinetic properties were evaluated. Taccalonolides AF and AJ were found to have elimination half-lives of 44 and 8.1 min, respectively. Furthermore, taccalonolide AJ was found to have excellent and highly persistent antitumor efficacy when administered directly to the tumor, suggesting that the lack of antitumor efficacy seen with systemic administration of AJ is likely due to its short half-life in vivo. These results help define why some, but not all, taccalonolides inhibit the growth of tumors at systemically tolerable doses and prompt studies to further improve their pharmacokinetic profile and antitumor efficacy.

B2(OH)4-mediated one-pot synthesis of tetrahydroquinoxalines from 2-amino(nitro)anilines and 1,2-dicarbonyl compounds in water

A practical one-pot synthesis of tetrahydroquinoxalines from readily available 2-amino(nitro)anilines and 1,2-dicarbonyl compounds mediated by diboronic acid with water as both a solvent and a hydrogen donor under metal-free conditions has been discovered.