Synergistic effects of peloruside A and laulimalide with taxoid site drugs, but not with each other, on tubulin assembly

Development of tubulin polymerization inhibitors as anticancer agents

INTRODUCTION

Microtubules are intracellular, filamentous, polymeric structures that extend throughout the cytoplasm, composed of α-tubulin and β-tubulin subunits. They regulate many cellular functions including cell polarity, cell shape, mitosis, intracellular transport, cell signaling, gene expression, cell integrity, and are associated with tumorigenesis. Inhibition of tubulin polymerization within tumor cells represents a crucial focus in the pursuit of developing anticancer treatments.

AREAS COVERED

This review focuses on the natural product and their synthetic congeners as tubulin inhibitors along with their site of interaction on tubulin. This review also covers the developed novel tubulin inhibitors and important patents focusing on the development of tubulin inhibition for cancer treatment reported from 2018 to 2023. The scientific and patent literature has been searched on PubMed, Espacenet, ScienceDirect, and Patent Guru from 2018-2023.

EXPERT OPINION

Tubulin is one of the promising targets explored extensively for drug discovery. Compounds binding in the colchicine site could be given importance because they can elude resistance mediated by the P-glycoprotein efflux pump and no colchicine site binding inhibitor is approved by FDA so far. The research on the development of antibody drug conjugates (ADCs) for tubluin polymerization inhibition could be significant strategy for cancer treatment.

Loss of function of the ALS-associated NEK1 kinase disrupts microtubule homeostasis and nuclear import

Loss-of-function variants in NIMA-related kinase 1 (NEK1) constitute a major genetic cause of amyotrophic lateral sclerosis (ALS), accounting for 2 to 3% of all cases. However, how NEK1 mutations cause motor neuron (MN) dysfunction is unknown. Using mass spectrometry analyses for NEK1 interactors and NEK1-dependent expression changes, we find functional enrichment for proteins involved in the microtubule cytoskeleton and nucleocytoplasmic transport. We show that α-tubulin and importin-β1, two key proteins involved in these processes, are phosphorylated by NEK1 in vitro. NEK1 is essential for motor control and survival in Drosophila models in vivo, while using several induced pluripotent stem cell (iPSC)-MN models, including NEK1 knockdown, kinase inhibition, and a patient mutation, we find evidence for disruptions in microtubule homeostasis and nuclear import. Notably, stabilizing microtubules with two distinct classes of drugs restored NEK1-dependent deficits in both pathways. The capacity of NEK1 to modulate these processes that are critically involved in ALS pathophysiology renders this kinase a formidable therapeutic candidate.

Preictal neuronal and vascular activity precedes the onset of childhood absence seizure: direct current potential shifts and their correlation with hemodynamic activity

SIGNIFICANCE AIMS

The neurovascular mechanisms underlying the initiation of absence seizures and their dynamics are still not well understood. The objective of this study was to better noninvasively characterize the dynamics of the neuronal and vascular network at the transition from the interictal state to the ictal state of absence seizures and back to the interictal state using a combined electroencephalography (EEG), functional near-infrared spectroscopy (fNIRS), and diffuse correlation spectroscopy (DCS) approach. The second objective was to develop hypotheses about the neuronal and vascular mechanisms that propel the networks to the 3-Hz spikes and wave discharges (SWDs) observed during absence seizures.

APPROACHES

We evaluated the simultaneous changes in electrical (neuronal) and optical dynamics [hemodynamic, with changes in (Hb) and cerebral blood flow] of 8 pediatric patients experiencing 25 typical childhood absence seizures during the transition from the interictal state to the absence seizure by simultaneously performing EEG, fNIRS, and DCS.

RESULTS

Starting from ∼ 20    s before the onset of the SWD, we observed a transient direct current potential shift that correlated with alterations in functional fNIRS and DCS measurements of the cerebral hemodynamics detecting the preictal changes.

DISCUSSION

Our noninvasive multimodal approach highlights the dynamic interactions between the neuronal and vascular compartments that take place in the neuronal network near the time of the onset of absence seizures in a very specific cerebral hemodynamic environment. These noninvasive approaches contribute to a better understanding of the electrical hemodynamic environment prior to seizure onset. Whether this may ultimately be relevant for diagnostic and therapeutic approaches requires further evaluation.

Microtubule-Interfering Drugs: Current and Future Roles in Epithelial Ovarian Cancer Treatment

Taxanes and epothilones are chemotherapeutic agents that ultimately lead to cell death through inhibition of normal microtubular function. This review summarizes the literature demonstrating their current use and potential promise as therapeutic agents in the treatment of epithelial ovarian cancer (EOC), as well as putative mechanisms of resistance. Historically, taxanes have become the standard of care in the front-line and recurrent treatment of epithelial ovarian cancer. In the past few years, epothilones (i.e., ixabepilone) have become of interest as they may retain activity in taxane-treated patients since they harbor several features that may overcome mechanisms of taxane resistance. Clinical data now support the use of ixabepilone in the treatment of platinum-resistant or refractory ovarian cancer. Clinical data strongly support the use of microtubule-interfering drugs alone or in combination in the treatment of epithelial ovarian cancer. Ongoing clinical trials will shed further light into the potential of making these drugs part of current standard practice.

Deciphering the key heterocyclic scaffolds in targeting microtubules, kinases and carbonic anhydrases for cancer drug development

Heterocyclic scaffolds are widely utilized for drug design by taking into account the molecular structure of therapeutic targets that are related to a broad spectrum of ailments, including tumors. Such compounds display various covalent and non-covalent interactions with the specific residues of the target proteins while causing their inhibition. There is a substantial number of heterocyclic compounds approved for cancer treatment, and these compounds function by interacting with different therapeutic targets involved in tumorogenesis. In this review, we trace and emphasize the privileged heterocyclic pharmacophores that have immense potency against several essential chemotherapeutic tumor targets: microtubules, kinases and carbonic anhydrases. Potent compounds currently undergoing pre-clinical and clinical studies have also been assessed for ascertaining the effective class of chemical scaffolds that have significant therapeutic potential against multiple malignancies. In addition, we also describe briefly the role of heterocyclic compounds in various chemotherapy regimens. The optimized molecular hybridization of delineated motifs may result in the discovery of more active anticancer therapeutics and circumvent the development of resistance by specific targets in the future.

Tubulin-Based Nanotubes as Delivery Platform for Microtubule-Targeting Agents

Tubulin-based nanotubes (TNTs) to deliver microtubule-targeting agents (MTAs) for clinical oncology are reported. Three MTAs, docetaxel (DTX), laulimalide (LMD), and monomethyl auristatin E (MMAE), which attach to different binding sites in a tubulin, are loaded onto TNTs and cause structural changes in them, including shape anisotropy and tubulin layering. This drug-driven carrier transformation leads to changes in the drug-loading efficiency and stability characteristics of the carrier. TNTs coloaded with DTX and LMD efficiently deliver dual drug cargoes to cellular tubulins by the endolysosomal pathway, and results in synergistic anticancer and antiangiogenic action of the drugs in vitro. In in vivo tests, TNTs loaded with a microtubule-destabilizing agent MMAE suppress the growth of tumors with much higher efficacy than free MMAE did. This work suggests a new concept of using a drug's target protein as a carrier. The findings demonstrate that the TNTs developed here can be used universally as a delivery platform for many MTAs.

The Progress of the Anticancer Agents Related to the Microtubules Target

Anticancer drugs based on the microtubules target are potent mitotic spindle poison agents, which interact directly with the microtubules, and were classified as microtubule-stabilizing agents and microtubule-destabilizing agents. Researchers have worked tremendously towards the improvements of anticancer drugs, in terms of improving the efficacy, solubility and reducing the side effects, which brought about advancement in chemotherapy. In this review, we focused on describing the discovery, structures and functions of the microtubules as well as the progress of anticancer agents related to the microtubules, which will provide adequate references for researchers.

Mechanism-Based Rational Discovery and In Vitro Evaluation of Novel Microtubule Stabilizing Agents with Non-Taxol-Competitive Activity

Microtubules (MT) are cytoskeletal polymers of αβ-tubulin dimers that play a critical role in many cellular functions. Diverse antimitotic drugs bind to MT and disrupt their dynamics acting as MT stabilizing or destabilizing agents. The occurrence of undesired side effects and drug resistance encourages the search for novel MT binding agents with chemically diverse structures and different interaction profiles compared to known active compounds. This work reports the rational discovery of seven novel MT stabilizers using a combination of molecular modeling methods and in vitro experimental assays. Virtual screening, similarity filtering, and molecular mechanics generalized Born surface area (MM/GBSA) binding free energy refinement were employed to select seven potential candidates with high predicted affinity toward the non-taxoid site for MT stabilizers on β-tubulin. MD simulations of 150 ns on reduced MT models suggest that candidate compounds strengthen the longitudinal interactions between tubulin dimers across protofilaments, which is a primary molecular mechanism of action for known MT stabilizers. In vitro MT polymerization assays confirmed that all candidates promote MT assembly at concentrations of >50 mM and exhibit noncompetitive MT polymerization profiles when cotreating with Taxol. Preliminary HeLa cell viability assays revealed a moderate cytotoxic effect for the compounds under study at 100 μM concentration. These results support the validity of our rational discovery strategy and the use of molecular modeling methods to pursue the search and optimization of new MT targeting agents.

An overview on anti-tubulin agents for the treatment of lymphoma patients

Anti-tubulin agents constitute a large class of compounds with broad activity both in solid tumors and hematologic malignancies, due to the interference with microtubule dynamics. Since microtubules play crucial roles in the regulation of the mitotic spindles, the interference with their function usually leads to a block in cell division with arrest at the metaphase/anaphase junction of mitosis, followed to apoptosis. This explains the reason why tubulin-binding agents (TBAs) proved to be extremely active in patients with cancer. Several anti-tubulin agents are indicated in the treatment of patients with lymphomas both alone and in combination chemotherapy regimens. The article reviews the literature on classic and more recent anti-tubulin agents, providing an insight into their mechanisms of action and their use in the treatment of lymphoma.

Function-Oriented Synthesis toward Peloruside A Analogues

A convergent and rapid synthesis of original C2,C3-unsaturated, C11,C13-keto-enol macrocycles with a peloruside A skeleton has been developed. These original unsaturated macrocycles constitute valuable platforms to access peloruside A analogues with high diversity. The four-fragment strategy implemented features two aldol-type couplings with the central C12-C14 building block TES-diazoacetone and a late-stage ring-closing metathesis. Enantiopure analogue 18ab showed antiproliferative activity in the low micromolar range on NCI and MCF7 tumor cell lines.

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.

Towards modern anticancer agents that interact with tubulin

Tubulin is the primary target of an ever growing number of natural, semisynthetic and synthetic products as potential anticancer agents. The mechanisms of interaction of these molecules with tubulin are varied. These drug classes have shown to inhibit effectively several cancer types with IC₅₀ from midmicromolar to low nanomolar concentrations. However, some limiting obstacles still remain, such as the development of multidrug resistance and cytotoxicity. We have reviewed recent advances in different classes of tubulin binding agents, including colchicine site agents, Vinca alkaloids, tryprostatins, moroidin, hemiasterlin, diazonamide, taxanes, epothilones and laulimalide.

New Drugs from the Sea: Pro-Apoptotic Activity of Sponges and Algae Derived Compounds

Natural compounds derived from marine organisms exhibit a wide variety of biological activities. Over the last decades, a great interest has been focused on the anti-tumour role of sponges and algae that constitute the major source of these bioactive metabolites. A substantial number of chemically different structures from different species have demonstrated inhibition of tumour growth and progression by inducing apoptosis in several types of human cancer. The molecular mechanisms by which marine natural products activate apoptosis mainly include (1) a dysregulation of the mitochondrial pathway; (2) the activation of caspases; and/or (3) increase of death signals through transmembrane death receptors. This great variety of mechanisms of action may help to overcome the multitude of resistances exhibited by different tumour specimens. Therefore, products from marine organisms and their synthetic derivates might represent promising sources for new anticancer drugs, both as single agents or as co-adjuvants with other chemotherapeutics. This review will focus on some selected bioactive molecules from sponges and algae with pro-apoptotic potential in tumour cells.

Marine Invertebrate Natural Products that Target Microtubules

Marine natural products as secondary metabolites are a potential major source of new drugs for treating disease. In some cases, cytotoxic marine metabolites target the microtubules of the eukaryote cytoskeleton for reasons that will be discussed. This review covers the microtubule-targeting agents reported from sponges, corals, tunicates, and molluscs and the evidence that many of these secondary metabolites are produced by bacterial symbionts. The review finishes by discussing the directions for future development and production of clinically relevant amounts of these natural products and their analogues through aquaculture, chemical synthesis, and biosynthesis by bacterial symbionts.

Zampanolide Binding to Tubulin Indicates Cross-Talk of Taxane Site with Colchicine and Nucleotide Sites

The marine natural product zampanolide and analogues thereof constitute a new chemotype of taxoid site microtubule-stabilizing agents with a covalent mechanism of action. Zampanolide-ligated tubulin has the switch-activation loop (M-loop) in the assembly prone form and, thus, represents an assembly activated state of the protein. In this study, we have characterized the biochemical properties of the covalently modified, activated tubulin dimer, and we have determined the effect of zampanolide on tubulin association and the binding of tubulin ligands at other binding sites. Tubulin activation by zampanolide does not affect its longitudinal oligomerization but does alter its lateral association properties. The covalent binding of zampanolide to β-tubulin affects both the colchicine site, causing a change of the quantum yield of the bound ligand, and the exchangeable nucleotide binding site, reducing the affinity for the nucleotide. While these global effects do not change the binding affinity of 2-methoxy-5-(2,3,4-trimethoxyphenyl)-2,4,6-cycloheptatrien-1-one (MTC) (a reversible binder of the colchicine site), the binding affinity of a fluorescent analogue of GTP (Mant-GTP) at the nucleotide E-site is reduced from 12 ± 2 × 10⁵ M^-1 in the case of unmodified tubulin to 1.4 ± 0.3 × 10⁵ M^-1 in the case of the zampanolide tubulin adduct, indicating signal transmission between the taxane site and the colchicine and nucleotide sites of β-tubulin.

Further Insight into the Interactions of the Cytotoxic Macrolides Laulimalide and Peloruside A with Their Common Binding Site

The binding site of the macrolides laulimalide and peloruside A, which is different from that of the clinically useful drugs paclitaxel/taxol and ixabepilone (tax site), is known to be between two adjacent β-tubulin units (ext site). Here, we report our study of the binding of these molecules to an α1β1/α2β2-tubulin "tetramer" model. AutoDock 4.2.6//AutoDock Vina dockings predicted that the affinities of laulimalide and peloruside A for the tax site are quite similar to those for the ext site. However, molecular dynamics (MD) simulations indicated that only when these two ligands are located at the ext site, there are contacts that help stabilize the system, favoring the β1/β2 interactions. The binding affinity of laulimalide for this site is stronger than that of peloruside A, but this is compensated for by additional β1/β2 contacts that are induced by peloruside A. MD studies also suggested that epothilones at the tax site and either laulimalide or peloruside A at the ext site cause similar stabilizing effects (mainly linking the M-loop of β1 and loop H1-B2 of β2). In a "hexamer" model (3 units of αβ-tubulin), the effects are confirmed. Metadynamics simulations of laulimalide and peloruside A, which are reported for the first time, suggest that peloruside A produces a stronger change in the M-loop, which explains the stabilization of the β1/β2 interaction.

Recent advances in microtubule-stabilizing agents

Highly dynamic mitotic spindle microtubules are superb therapeutic targets for a group of chemically diverse and clinically successful anticancer drugs. Microtubule-targeted drugs disrupt microtubule dynamics in distinct ways, and they are primarily classified into two groups: microtubule destabilizing agents (MDAs), such as vinblastine, colchicine, and combretastatin-A4, and microtubule stabilizing agents (MSAs), such as paclitaxel and epothilones. Systematic discovery and development of new MSAs have been aided by extensive research on paclitaxel, yielding a large number of promising anticancer compounds. This review focuses on the natural sources, structural features, mechanisms of action, structure-activity relationship (SAR) and chemical synthesis of MSAs. These MSAs mainly include paclitaxel, taccalonolides, epothilones, FR182877 (cyclostreptin), dictyostatin, discodermolide, eleutherobin and sarcodictyins, zampanolide, dactylolide, laulimalides, peloruside and ceratamines from natural sources, as well as small molecular microtubule stabilizers obtained via chemical synthesis. Then we discuss the application prospect and development of these anticancer compounds.

Zampanolide, a Microtubule-Stabilizing Agent, Is Active in Resistant Cancer Cells and Inhibits Cell Migration

Zampanolide, first discovered in a sponge extract in 1996 and later identified as a microtubule-stabilizing agent in 2009, is a covalent binding secondary metabolite with potent, low nanomolar activity in mammalian cells. Zampanolide was not susceptible to single amino acid mutations at the taxoid site of β-tubulin in human ovarian cancer 1A9 cells, despite evidence that it selectively binds to the taxoid site. As expected, it did not synergize with other taxoid site microtubule-stabilizing agents (paclitaxel, ixabepilone, discodermolide), but surprisingly also did not synergize in 1A9 cells with laulimalide/peloruside binding site agents either. Efforts to generate a zampanolide-resistant cell line were unsuccessful. Using a standard wound scratch assay in cell culture, it was an effective inhibitor of migration of human umbilical vein endothelial cells (HUVEC) and fibroblast cells (D551). These properties of covalent binding, the ability to inhibit cell growth in paclitaxel and epothilone resistant cells, and the ability to inhibit cell migration suggest that it would be of interest to investigate zampanolide in preclinical animal models to determine if it is effective in vivo at preventing tumor growth and metastasis.

Insights into the Distinct Mechanisms of Action of Taxane and Non-Taxane Microtubule Stabilizers from Cryo-EM Structures

A number of microtubule (MT)-stabilizing agents (MSAs) have demonstrated or predicted potential as anticancer agents, but a detailed structural basis for their mechanism of action is still lacking. We have obtained high-resolution (3.9-4.2Å) cryo-electron microscopy (cryo-EM) reconstructions of MTs stabilized by the taxane-site binders Taxol and zampanolide, and by peloruside, which targets a distinct, non-taxoid pocket on β-tubulin. We find that each molecule has unique distinct structural effects on the MT lattice structure. Peloruside acts primarily at lateral contacts and has an effect on the "seam" of heterologous interactions, enforcing a conformation more similar to that of homologous (i.e., non-seam) contacts by which it regularizes the MT lattice. In contrast, binding of either Taxol or zampanolide induces MT heterogeneity. In doubly bound MTs, peloruside overrides the heterogeneity induced by Taxol binding. Our structural analysis illustrates distinct mechanisms of these drugs for stabilizing the MT lattice and is of relevance to the possible use of combinations of MSAs to regulate MT activity and improve therapeutic potential.

Structural Determinants of the Dictyostatin Chemotype for Tubulin Binding Affinity and Antitumor Activity Against Taxane- and Epothilone-Resistant Cancer Cells

A combined biochemical, structural, and cell biology characterization of dictyostatin is described, which enables an improved understanding of the structural determinants responsible for the high-affinity binding of this anticancer agent to the taxane site in microtubules (MTs). The study reveals that this macrolide is highly optimized for MT binding and that only a few of the structural modifications featured in a library of synthetic analogues resulted in small gains in binding affinity. The high efficiency of the dictyostatin chemotype in overcoming various kinds of clinically relevant resistance mechanisms highlights its potential for therapeutic development for the treatment of drug-resistant tumors. A structural explanation is advanced to account for the synergy observed between dictyostatin and taxanes on the basis of their differential effects on the MT lattice. The X-ray crystal structure of a tubulin-dictyostatin complex and additional molecular modeling have allowed the rationalization of the structure-activity relationships for a set of synthetic dictyostatin analogues, including the highly active hybrid 12 with discodermolide. Altogether, the work reported here is anticipated to facilitate the improved design and synthesis of more efficacious dictyostatin analogues and hybrids with other MT-stabilizing agents.

2-(m-Azidobenzoyl)taxol binds differentially to distinct β-tubulin isotypes

There are seven β-tubulin isotypes present in distinct quantities in mammalian cells of different origin. Altered expression of β-tubulin isotypes has been reported in cancer cell lines resistant to microtubule stabilizing agents (MSAs) and in human tumors resistant to Taxol. To study the relative binding affinities of MSAs, tubulin from different sources, with distinct β-tubulin isotype content, were specifically photolabeled with a tritium-labeled Taxol analog, 2-(m-azidobenzoyl)taxol, alone or in the presence of MSAs. The inhibitory effects elicited by these MSAs on photolabeling were distinct for β-tubulin from different sources. To determine the exact amount of drug that binds to different β-tubulin isotypes, bovine brain tubulin was photolabeled and the isotypes resolved by high-resolution isoelectrofocusing. All bands were analyzed by mass spectrometry following cyanogen bromide digestion, and the identity and relative quantity of each β-tubulin isotype determined. It was found that compared with other β-tubulin isotypes, βIII-tubulin bound the least amount of 2-(m-azidobenzoyl)taxol. Analysis of the sequences of β-tubulin near the Taxol binding site indicated that, in addition to the M-loop that is known to be involved in drug binding, the leucine cluster region of βIII-tubulin contains a unique residue, alanine, at 218, compared with other isotypes that contain threonine. Molecular dynamic simulations indicated that the frequency of Taxol-accommodating conformations decreased dramatically in the T218A variant, compared with other β-tubulins. Our results indicate that the difference in residue 218 in βIII-tubulin may be responsible for inhibition of drug binding to this isotype, which could influence downstream cellular events.

(-)-Rhazinilam and the diphenylpyridazinone NSC 613241: Two compounds inducing the formation of morphologically similar tubulin spirals but binding apparently to two distinct sites on tubulin

The most potent microtubule assembly inhibitor of newer diphenylpyridazinone derivatives examined was NSC 613241. Because NSC 613241 and (-)-rhazinilam also induce the formation of similar 2-filament spirals, these aberrant reactions were compared. Spiral formation with both compounds was enhanced by GTP and inhibited by GDP and by 15 other inhibitors of microtubule assembly. Similarly, microtubule assembly induced by paclitaxel or laulimalide is enhanced by GTP and inhibited by GDP and assembly inhibitors, but neither [(3)H]NSC 613241 nor [(3)H](-)-rhazinilam bound to microtubules or inhibited the binding of [(3)H]paclitaxel or [(3)H]peloruside A to microtubules. Differences in the pitch of aberrant polymers were found: NSC 613241-induced and (-)-rhazinilam-induced spirals had average repeats of 85 and 79-80 nm, respectively. We found no binding of [(3)H]NSC 613241 or [(3)H](-)-rhazinilam to αβ-tubulin dimer, but both compounds were incorporated into the polymers they induced in substoichiometric reactions, with as little as 0.1-0.2 mol compound/mol of tubulin, and no cross-inhibition by NSC 613241 or (-)-rhazinilam into spirals occurred. Under reaction conditions where neither compound induced spiral formation, both compounds together synergistically induced substantial spiral formation. We conclude that (-)-rhazinilam and NSC 613241 bind to different sites on tubulin that differ from binding sites for other antitubulin agents.

Peloruside A: a lead non-taxoid-site microtubule-stabilizing agent with potential activity against cancer, neurodegeneration, and autoimmune disease

Covering: 2000 up to 2016Peloruside A, a macrocyclic secondary metabolite from a New Zealand marine sponge, Mycale hentscheli, has shown potent antiproliferative activity in cultured cancer cells as well as inhibitory effects on tumor growth in mouse models. The compound also has promising effects against cell models of neurodegenerative and autoimmune diseases. In mechanistic studies, peloruside A shares with paclitaxel (Taxol®) the ability to stabilize microtubules by binding to β-tubulin. Peloruside A, however, occupies a unique external site on β-tubulin that does not overlap the classical taxoid site that is located on the inside of the microtubule. As such, peloruside A has been of central importance in defining a new microtubule-stabilizer binding site localized on the exterior surface of the microtubule that has led to increased interest in the design of an upscaled total synthesis of the natural product and its analogues. Here, we review advances in the biochemical and biological validation of peloruside A as an attractive therapeutic candidate for the treatment of cancer, neurodegeneration, and autoimmune disease.

Microtubule-stabilizing agents: New drug discovery and cancer therapy

Microtubule-stabilizing agents (MSAs) have been highly successful in the treatment of cancer in the past 20years. To date, three classes of MSAs have entered the clinical trial stage or have been approved for clinical anticancer chemotherapy, and more than 10 classes of novel structural MSAs have been derived from natural resources. The microtubule typically contains two MSA-binding sites: the taxoid site and the laulimalide/peloruside site. All defined MSAs are known to bind at either of these sites, with subtle but significant differences. MSAs with different binding sites may produce a synergistic effect. Although having been extensively applied in the clinical setting, paclitaxel and other approved MSAs still pose many challenges such as multidrug resistance, low bioavailability, poor solubility, high toxicity, and low passage through the blood-brain barrier. A variety of studies focus on the structure-activity relationship in order to improve the pharmaceutical properties of these agents. Here, the mechanisms of action, advancements in pharmacological research, and clinical developments of defined MSAs during the past decade are discussed. The latest discovered MSAs are also briefly introduced in this review. The increasing number of natural MSAs indicates the potential discovery of more novel, natural MSAs with different structural bases, which will further promote the development of anticancer chemotherapy.

Stabilizing versus destabilizing the microtubules: a double-edge sword for an effective cancer treatment option?

Microtubules are dynamic and structural cellular components involved in several cell functions, including cell shape, motility, and intracellular trafficking. In proliferating cells, they are essential components in the division process through the formation of the mitotic spindle. As a result of these functions, tubulin and microtubules are targets for anticancer agents. Microtubule-targeting agents can be divided into two groups: microtubule-stabilizing, and microtubule-destabilizing agents. The former bind to the tubulin polymer and stabilize microtubules, while the latter bind to the tubulin dimers and destabilize microtubules. Alteration of tubulin-microtubule equilibrium determines the disruption of the mitotic spindle, halting the cell cycle at the metaphase-anaphase transition and, eventually, resulting in cell death. Clinical application of earlier microtubule inhibitors, however, unfortunately showed several limits, such as neurological and bone marrow toxicity and the emergence of drug-resistant tumor cells. Here we review several natural and synthetic microtubule-targeting agents, which showed antitumor activity and increased efficacy in comparison to traditional drugs in various preclinical and clinical studies. Cryptophycins, combretastatins, ombrabulin, soblidotin, D-24851, epothilones and discodermolide were used in clinical trials. Some of them showed antiangiogenic and antivascular activity and others showed the ability to overcome multidrug resistance, supporting their possible use in chemotherapy.

A Survey of Marine Natural Compounds and Their Derivatives with Anti-cancer Activity Reported in 2012

Although considerable effort and progress has been made in the search for new anticancer drugs and treatments in the last several decades, cancer remains a major public health problem and one of the major causes of death worldwide. Many sources, including plants, animals, and minerals, are of interest in cancer research because of the possibility of identifying novel molecular therapeutics. Moreover, structure-activity-relationship (SAR) investigations have become a common way to develop naturally derived or semi-synthetic molecular analogues with improved efficacy and decreased toxicity. In 2012, approximately 138 molecules from marine sources, including isolated compounds and their associated analogues, were shown to be promising anticancer drugs. Among these, 62% are novel compounds. In this report, we review the marine compounds identified in 2012 that may serve as novel anticancer drugs.

The Unique Binding Mode of Laulimalide to Two Tubulin Protofilaments

Laulimalide, a cancer chemotherapeutic in preclinical development, has a unique binding site located on two adjacent β-tubulin units between tubulin protofilaments of a microtubule. Our extended protein model more accurately mimics the microtubule environment, and together with a 135 ns molecular dynamics simulation, identifies a new binding mode for laulimalide, which differs from the modes presented in work using smaller protein models. The new laulimalide-residue interactions that are computationally revealed explain the contacts observed via independent mass shift perturbation experiments. The inclusion of explicit solvent shows that many laulimalide-tubulin interactions are water mediated. The new contacts between the drug and the microtubule structure not only improve our understanding of laulimalide binding but also will be essential for efficient derivatization and optimization of this prospective cancer chemotherapy agent. Observed changes in secondary protein structure implicate the S7-H9 loop (M-loop) and H1'-S2 loop in the mechanism by which laulimalide stabilizes microtubules to exert its cytotoxic effects.

Microtubule-stabilizing agents as potential therapeutics for neurodegenerative disease

Microtubules (MTs), cytoskeletal elements found in all mammalian cells, play a significant role in cell structure and in cell division. They are especially critical in the proper functioning of post-mitotic central nervous system neurons, where MTs serve as the structures on which key cellular constituents are trafficked in axonal projections. MTs are stabilized in axons by the MT-associated protein tau, and in several neurodegenerative diseases, including Alzheimer's disease, frontotemporal lobar degeneration, and Parkinson's disease, tau function appears to be compromised due to the protein dissociating from MTs and depositing into insoluble inclusions referred to as neurofibrillary tangles. This loss of tau function is believed to result in alterations of MT structure and function, resulting in aberrant axonal transport that likely contributes to the neurodegenerative process. There is also evidence of axonal transport deficiencies in other neurodegenerative diseases, including amyotrophic lateral sclerosis and Huntington's disease, which may result, at least in part, from MT alterations. Accordingly, a possible therapeutic strategy for such neurodegenerative conditions is to treat with MT-stabilizing agents, such as those that have been used in the treatment of cancer. Here, we review evidence of axonal transport and MT deficiencies in a number of neurodegenerative diseases, and summarize the various classes of known MT-stabilizing agents. Finally, we highlight the growing evidence that small molecule MT-stabilizing agents provide benefit in animal models of neurodegenerative disease and discuss the desired features of such molecules for the treatment of these central nervous system disorders.

Recent progress with microtubule stabilizers: new compounds, binding modes and cellular activities

Nature has yielded numerous classes of chemically distinct microtubule stabilizers. Several of these, including paclitaxel (Taxol) and docetaxel (Taxotere), are important drugs used in the treatment of cancer. New microtubule stabilizers and novel formulations of these agents continue to provide advances in cancer therapy. In this review we cover recent progress in the chemistry and biology of these diverse microtubule stabilizers focusing on the wide range of organisms that produce these compounds, their mechanisms of inhibiting microtubule-dependent processes, mechanisms of drug resistance, and their interactions with tubulin including their distinct binding sites and modes. A new potential role for microtubule stabilizers in neurodegenerative diseases is reviewed.

Structural basis of microtubule stabilization by laulimalide and peloruside A

Laulimalide and peloruside A are microtubule-stabilizing agents (MSAs), the mechanism of action on microtubules of which is poorly defined. Here, using X-ray crystallography it is shown that laulimalide and peloruside A bind to a unique non-taxane site on β-tubulin and use their respective macrolide core structures to interact with a second tubulin dimer across protofilaments. At the same time, they allosterically stabilize the taxane-site M-loop that establishes lateral tubulin contacts in microtubules. Structures of ternary complexes of tubulin with laulimalide/peloruside A and epothilone A are also solved, and a crosstalk between the laulimalide/peloruside and taxane sites via the M-loop of β-tubulin is found. Together, the data define the mechanism of action of laulimalide and peloruside A on tubulin and microtubules. The data further provide a structural framework for understanding the synergy observed between two classes of MSAs in tubulin assembly and the inhibition of cancer cell growth.

Imaging preictal hemodynamic changes in neocortical epilepsy

OBJECT

The ability to predict seizure occurrence is extremely important to trigger abortive therapies and to warn patients and their caregivers. Optical imaging of hemodynamic parameters such as blood flow, blood volume, and tissue and hemoglobin oxygenation has already been shown to successfully localize epileptic events with high spatial and temporal resolution. The ability to actually predict seizure occurrence using hemodynamic parameters is less well explored.

METHODS

In this article, the authors critically review data from the literature on neocortical epilepsy and optical imaging, and they discuss the preictal hemodynamic changes and their application in neurosurgery.

RESULTS

Recent optical mapping studies have demonstrated preictal hemodynamic changes in both human and animal neocortex.

CONCLUSIONS

Optical measurements of blood flow and oxygenation may become increasingly important for predicting and localizing epileptic events. The ability to successfully predict ictal onsets may be useful to trigger closed-loop abortive therapies.

Tubulin inhibitors: a patent review

INTRODUCTION

Microtubules play an important role in several cellular processes, particularly in the formation of the mitotic spindle during the process of mitosis. These highly dynamic mitotic-spindle microtubules have become a successful target of cancer therapy. Microtubule-targeting agents, such as vinca alkaloids and taxanes, were used in clinic over 50 years. In past decades, development of new antimicrotubule agents that possess different structure and binding sites of tubulin has shown potent activity against the proliferation of various cancer cells, as well as in multidrug-resistant cancers. Interestingly, many of these agents represent an attractive ability that targeting the tumor blood vessels results in tumor vascular disruption. Therefore, exploring new agents and strategies may provide more effective therapeutic options in the related treatment of cancer.

AREAS COVERED

In past few years, there are many chemical compounds that successfully interferes the microtubules and display antitumor effect. In these, published compounds supply the fresh minds in modification of present drugs and new insights into the development of tubulin inhibitors.

EXPERT OPINION

This article arranges the microtubule-targeting agents that have published in patent in recent years. It may help in the investigation of new tubulin binding site and development of novel drug candidate in the future.

Docking and molecular dynamics studies of the binding between Peloruside A and tubulin

The molecular docking, MD simulation and binding free energy calculation were performed to explore the probable binding modes between PLA and tubulin. Through docking study, three possible binding sites for PLA were speculated as follows: the taxane site, the alternative site and a new site in α-tubulin. Then, 12.0 ns MD simulations show that these binding modes predicted by docking have been changed more or less, whereas the MD simulations offer more reliable binding details. The MM-PBSA binding free-energy calculations reasonably identify that the taxane site is the most favorable binding site of PLA and the alternative site is the secondary one, which can be used to explain some experimental facts. These studies theoretically resolve the priority of binding sites for PLA and offer the reliable binding modes between PLA and tubulin, and thus help to understanding the action mechanism for this kind of inhibitor.

Microtubule-stabilizing agents delay the onset of EAE through inhibition of migration

We have shown previously that microtubule-stabilizing agents (MSA), a class of anti-proliferative compounds, can delay disease onset and reduce cumulative disease in an experimental model of multiple sclerosis, experimental autoimmune encephalomyelitis (EAE). To explore how MSA could alter EAE disease processes, we compared the effect of administering MSA before or after peak antigen-specific proliferation and found that treatment before proliferation completely inhibited antigen-specific responses in the spleen; whereas administration of an MSA such as paclitaxel or docetaxel after peak proliferation did not. Despite the presence of antigen-specific responses in mice treated at the later time point, both treatment periods resulted in similar protection against EAE, suggesting that the protective effect of MSA in EAE could not be solely attributed to anti-proliferative activity. Instead, using in vivo migration assays, it was shown that MSA inhibit immune cell infiltration into the central nervous system (CNS). Furthermore, we found that the efficacy of an MSA could be enhanced by administering low doses of two different MSA together, such as peloruside A and ixabepilone, indicating that these MSA synergize in vivo to suppress disease. Taken together, these data suggest that MSA can suppress EAE by at least two distinct mechanisms of action--prevention of proliferation and inhibition of migration into the CNS. Finally, we have shown that a combination treatment with synergizing MSA may provide enhanced protection at lower therapeutic doses.

Taccalonolide binding to tubulin imparts microtubule stability and potent in vivo activity

The taccalonolides are highly acetylated steroids that stabilize cellular microtubules and overcome multiple mechanisms of taxane resistance. Recently, two potent taccalonolides, AF and AJ, were identified that bind to tubulin directly and enhance microtubule polymerization. Extensive studies were conducted to characterize these new taccalonolides. AF and AJ caused aberrant mitotic spindles and bundling of interphase microtubules that differed from the effects of either paclitaxel or laulimalide. AJ also distinctly affected microtubule polymerization in that it enhanced the rate and extent of polymerization in the absence of any noticeable effect on microtubule nucleation. In addition, the resulting microtubules were found to be profoundly cold stable. These data, along with studies showing synergistic antiproliferative effects between AJ and either paclitaxel or laulimalide, suggest a distinct binding site. Direct binding studies demonstrated that AJ could not be displaced from microtubules by paclitaxel, laulimalide, or denaturing conditions, suggesting irreversible binding of AJ to microtubules. Mass spectrometry confirmed a covalent interaction of AJ with a peptide of β-tubulin containing the cyclostreptin-binding sites. Importantly, AJ imparts strong inter-protofilament stability in a manner different from other microtubule stabilizers that covalently bind to tubulin, consistent with the distinct effects of the taccalonolides as compared with other stabilizers. AF was found to be a potent and effective antitumor agent that caused tumor regression in the MDA-MB-231 breast cancer xenograft model. The antitumor efficacy of some taccalonolides, which stabilize microtubules in a manner different from other microtubule stabilizers, provides the impetus to explore the therapeutic potential of this site.

Laulimalide induces dose-dependent modulation of microtubule behaviour in the C. elegans embryo

Laulimalide is a microtubule-binding drug that was originally isolated from marine sponges. High concentrations of laulimalide stabilize microtubules and inhibit cell division similarly to paclitaxel; however, there are important differences with respect to the nature of the specific cellular defects between these two drugs and their binding sites on the microtubule. In this study, we used Caenorhabditis elegans embryos to investigate the acute effects of laulimalide on microtubules in vivo, with a direct comparison to paclitaxel. We observed surprising dose-dependent effects for laulimalide, whereby microtubules were stabilized at concentrations above 100 nM, but destabilized at concentrations between 50 and 100 nM. Despite this behaviour at low concentrations, laulimalide acted synergistically with paclitaxel to stabilize microtubules when both drugs were used at sub-effective concentrations, consistent with observations of synergistic interactions between these two drugs in other systems. Our results indicate that laulimalide induces a concentration-dependent, biphasic change in microtubule polymer dynamics in the C. elegans embryo.

Macrolide-based microtubule-stabilizing agents - chemistry and structure-activity relationships

This article provides an overview on the chemistry and structure-activity relationships of macrolide-based microtubule-stabilizing agents. The primary focus will be on the total synthesis or examples thereof, but a brief summary of the current state of knowledge on the structure-activity relationships of epothilones, laulimalide, dictyostatin, and peloruside A will also be given. This macrolide class of compounds, over the last decade, has become the subject of growing interest due to their ability to inhibit human cancer cell proliferation through a taxol-like mechanism of action.