Interactions of halichondrin B and eribulin with tubulin

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.

The interaction of spongistatin 1 with tubulin

A tritiated derivative of the sponge-derived natural product spongistatin 1 was prepared, and its interactions with tubulin were examined. [3H]Spongistatin 1 was found to bind rapidly to tubulin at a single site (the low specific activity of the [3H]spongistatin 1, 0.75 Ci/mmol, prevented our defining an association rate), and the inability of spongistatin 1 to cause an aberrant assembly reaction was confirmed. Spongistatin 1 bound to tubulin very tightly, and we could detect no significant dissociation reaction from tubulin. The tubulin-[3H]spongistatin 1 complex did dissociate in 8 M urea, so there was no evidence for covalent bond formation. Apparent KD values were obtained by Scatchard analysis of binding data and by Hummel-Dreyer chromatography (3.5 and 1.1 μM, respectively). The effects of a large cohort of vinca domain drugs on the binding of [3H]spongistatin 1 to tubulin were evaluated. Compounds that did not cause aberrant assembly reactions (halichondrin B, eribulin, maytansine, and rhizoxin) caused little inhibition of [3H]spongistatin 1 binding. Little inhibition also occurred with the peptides dolastatin 15, its active pentapeptide derivative, vitilevuamide, or diazonamide A, nor with the vinca alkaloid vinblastine. Strong inhibition was observed with dolastatin 10, hemiasterlin, and cryptophycin 1, all of which cause aberrant assembly reactions that might actually mask the spongistatin 1 binding site. Spongistatin 5 was found to be a competitive inhibitor of [3H]spongistatin 1 binding, with an apparent Ki of 2.2 μM. We propose that the strong picomolar cytotoxicity of spongistatin 1 probably derives from its extremely tight binding to tubulin.

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.

Interaction of diazonamide A with tubulin

[3H]Diazonamide A ([3H]DZA), prepared from the natural product isolated from Diazona angulata, bound to tubulin in larger aberrant assembly products (>500 kDa by sizing HPLC) but not to the αβ-tubulin heterodimer. The binding reaction was rapid, but stoichiometry was low. Stoichiometry was enhanced up to 8-fold by preincubating the tubulin in the reaction mixture prior to adding the [3H]DZA. Although Mg2+ did not affect binding stoichiometry, the cation markedly increased the number of tubulin rings (diameter about 50 nm) observed by negative stain electron microscopy. Bound [3H]DZA did not dissociate from the tubulin oligomers despite extensive column chromatography but did dissociate in the presence of 8 M urea. With preincubated tubulin, a superstoichiometric amount of [3H]DZA appeared to bind to the tubulin oligomeric structures, consistent with observations that neither nonradiolabeled DZA nor DZA analogues inhibited binding of [3H]DZA to the tubulin oligomers. Only weak inhibition of binding was observed with multiple antimitotic compounds. In particular, no inhibition occurred with vinblastine, and the best inhibitors of those examined were dolastatin 10 and cryptophycin 1. We compared the aberrant assembly reaction induced by DZA to those induced by other antimitotic peptides and depsipeptides, in particular dolastatin 10, cryptophycin 1, and hemiasterlin, but the results obtained varied considerably in terms of requirements for maximal reactions, polymer morphology, and inhibitory effects observed with antimitotic compounds.

Isolation and identification of N6-isopentenyladenosine as the cytotoxic constituent of a marine sponge Oceanapia sp

N 6-Isopentenyladenosine (i6A) was isolated from a marine sponge Oceanapia sp. as the major cytotoxic constituent along with N6-isopentenyladenosine 5ʹ-monophosphate (i6AP) which was inactive. The structures of i6A and i6AP were assigned by a combination of the analysis of NMR spectroscopy and mass spectrometry. This is the first isolation of i6A and i6AP from a marine sponge.

Oncolytic Vaccinia Virus Expressing Aphrocallistes vastus Lectin as a Cancer Therapeutic Agent

Lectins display a variety of biological functions including insecticidal, antimicrobial, as well as antitumor activities. In this report, a gene encoding Aphrocallistes vastus lectin (AVL), a C-type lectin, was inserted into an oncolytic vaccinia virus vector (oncoVV) to form a recombinant virus oncoVV-AVL, which showed significant in vitro antiproliferative activity in a variety of cancer cell lines. Further investigations revealed that oncoVV-AVL replicated faster than oncoVV significantly in cancer cells. Intracellular signaling elements including NF-κB2, NIK, as well as ERK were determined to be altered by oncoVV-AVL. Virus replication upregulated by AVL was completely dependent on ERK activity. Furthermore, in vivo studies showed that oncoVV-AVL elicited significant antitumor effect in colorectal cancer and liver cancer mouse models. Our study might provide insights into a novel way of the utilization of marine lectin AVL in oncolytic viral therapies.

Selectively high efficacy of eribulin against high-grade invasive recurrent and/or metastatic squamous cell carcinoma of the head and neck

Patients with recurrent and/or metastatic squamous cell carcinoma of the head and neck (R/M SCCHN) have a poor prognosis. Over the past decade, a major development in the first-line treatment of R/M SCCHN was the introduction of cetuximab in combination with platinum plus 5-fluorouracil chemotherapy. Currently, a promising novel treatment option in R/M SCCHN has emerged, termed immune checkpoint inhibitors. However, only a few patients presenting with R/M SCCHN have exhibited meaningful tumor regression with these agents. Therefore, novel agents are required to order improve the overall survival of patients with R/M SCCHN. Recently, we demonstrated that R/M SCCHN cells are highly sensitive to eribulin. In the present study, the effects of eribulin, paclitaxel and vinblastine were investigated in R/M SCCHN (OLC-01 and OSC-19) and locally advanced SCCHN (OSC-20) cells. Tumour-inhibitory activities of eribulin against R/M SCCHN were evaluated in orthotopic xenograft models. The data revealed that eribulin has sub-nM growth inhibitory activities in vitro against OLC-01 cells, and that it is more potent than paclitaxel and vinblastine. The reduced expression of Tubulin Beta 3 Class III (TUBB3) following treatment was correlated with a high sensitivity to eribulin. Histological analysis of OLC-01 cells in NOD-SCID mice demonstrated that they had a higher invasiveness in the tissue around the alveolar cancer when compared with the histology of OSC-19 cells, which has been reported in our previous study. Treatment with eribulin revealed marked inhibitory activities in vivo at 0.125 mg/kg against OLC-01 cells orthotopic xenografts. In conclusion, the results highlight the existence of invasive-type heterogeneity in R/M SCCHN with respect to eribulin sensitivity. Eribulin is already an approved clinical agent; therefore, the continued investigation of its preclinical antitumor attributes may contribute significantly to the future process of identifying novel uses of eribulin against R/M SCCHN.

Clinical efficacy of eribulin mesylate for the treatment of metastatic soft tissue sarcoma

INTRODUCTION

Metastatic soft tissue sarcoma, a devastating disease, has a median overall survival of only 12-18 months. Treatment options remain scarce. However, eribulin mesylate, a first-in-class halichondrin B-based microtubule dynamics inhibitor, has recently been approved for the management of patients with advanced liposarcoma. Areas covered: Based on a review of the literature between 2005 and 2017, we present a summary of eribulin mesylate's mechanism of action and the studies showing its clinical efficacy in locally advanced or metastatic sarcomas. Expert commentary: Future development includes the definition of a biomarker signature related to patient outcome with eribulin. Further investigation via controlled clinical trials is needed to identify combination regimens that can optimize the efficacy of eribulin while providing an acceptable safety profile in sarcoma patients.

Managing sarcoma: where have we come from and where are we going?

Sarcomas are a heterogeneous group of neoplasms of mesenchymal origin. Approximately 80% arise from soft tissue and 20% originate from bone. To date more than 100 sarcoma subtypes have been identified and they vary in molecular characteristics, pathology, clinical presentation and response to treatment. While sarcomas represent <1% of adult cancers, they account for approximately 21% of paediatric malignancies and thus pose some of the greatest risks of mortality and morbidity in children and young adults. Metastases occur in one-third of all patients and approximately 10-20% of sarcomas recur locally. Surgery in combination with preoperative and postoperative therapies is the primary treatment for localized sarcoma tumours and is the most promising curative possibility. Metastasized sarcomas, on the other hand, are treated primarily with single-agent or combination chemotherapy, but this rarely leads to a complete and robust response and often becomes a palliative form of treatment. The heterogeneity of sarcomas results in variable responses to current generalized treatment strategies. In light of this and the lack of curative strategies for metastatic and unresectable sarcomas, there is a need for novel subtype-specific treatment strategies. With the more recent understanding of the molecular mechanisms underlying the pathogenesis of some of these tumours, the treatment of sarcoma subtypes with targeted therapies is a rapidly evolving field. This review discusses the current management of sarcomas as well as promising new therapies that are currently underway in clinical trials.

Eribulin alone or in combination with the PLK1 inhibitor BI 6727 triggers intrinsic apoptosis in Ewing sarcoma cell lines

In this study, we investigated the molecular mechanisms of eribulin-induced cell death and its therapeutic potential in combination with the PLK1 inhibitor BI 6727 in Ewing sarcoma (ES). Here, we show that eribulin triggers cell death in a dose-dependent manner in a panel of ES cell lines. In addition, eribulin at subtoxic, low nanomolar concentrations acts in concert with BI 6727 to induce cell death and to suppress long-term clonogenic survival. Mechanistic studies reveal that eribulin monotherapy at cytotoxic concentrations and co-treatment with eribulin at subtoxic concentrations together with BI 6727 arrest cells in the M phase of the cell cycle prior to the onset of cell death. This mitotic arrest is followed by increased phosphorylation of BCL-2 and BCL-x_L as well as downregulation of MCL-1, suggesting inactivation of these antiapoptotic BCL-2 family proteins. Consistently, eribulin monotherapy and eribulin/BI 6727 co-treatment trigger activation of BAX, a key proapoptotic BCL-2 family protein, and increase proteolytic activation of caspase-9 and -3. Importantly, overexpression of BCL-2 or addition of the broad-range caspase inhibitor zVAD.fmk significantly rescue eribulin- as well as eribulin/BI 6727-induced cell death. Together, these findings demonstrate that eribulin induces cell death via the intrinsic pathway of apoptosis in ES cells, both alone at cytotoxic concentrations and in combination with BI 6727 at subtoxic concentrations. Thus, our study highlights the therapeutic potential of eribulin for the treatment of ES alone or in rational combination therapies.

HMBA is a putative HSP70 activator stimulating HEXIM1 expression that is down-regulated by estrogen

Hexamethylene bis-acetamide inducible protein 1 (HEXIM1) is identified as a novel inhibitor of estrogen stimulated breast cell growth, and it suppresses estrogen receptor-α transcriptional activity. HEXIM1 protein level has been found to be downregulated by estrogens. Recently, HEXIM1 has been found to inhibit androgen receptor transcriptional activity as well. Researchers have used Hexamethylene bis-acetamide (HMBA) for decades to stimulate HEXIM1 expression, which also inhibit estrogen stimulated breast cancer cell gene activation and androgen stimulated prostate cancer gene activation. However, the direct molecular targets of HMBA that modulate the induction of HEXIM1 expression in mammalian cells have not been identified. Based on HMBA and its more potent analog 4a1, we designed molecular probes to pull down the binding proteins of these compounds. Via proteomic approach and biological assays, we demonstrate that HMBA and 4a1 are actually heat shock protein 70 (HSP70) binders. The known HSP70 activator showed similar activity as HMBA and 4a1 to induce HEXIM1 expression, suggesting that HMBA and 4a1 might be putative HSP70 activators. Molecular target identification of HMBA and 4a1 could lead to further structural optimization of the parental compound to generate more potent derivatives to stimulate HEXIM1 expression, which could be a novel approach for hormone dependent breast cancer and prostate cancer treatment.

Eribulin targets a ch-TOG-dependent directed migration of cancer cells

Non-cytotoxic concentrations of microtubule targeting agents (MTAs) interfere with the dynamics of interphase microtubules and affect cell migration, which could impair tumor angiogenesis and metastasis. The underlying mechanisms however are still ill-defined. We previously established that directed cell migration is dependent on stabilization of microtubules at the cell leading edge, which is controlled by microtubule +end interacting proteins (+TIPs). In the present study, we found that eribulin, a recently approved MTA interacting with a new class of binding site on β-tubulin, decreased microtubule growth speed, impaired their cortical stabilization and prevented directed migration of cancer cells. These effects were reminiscent of those observed when +TIP expression or cortical localization was altered. Actually, eribulin induced a dose-dependent depletion of EB1, CLIP-170 and the tubulin polymerase ch-TOG from microtubule +ends. Interestingly, eribulin doses that disturbed ch-TOG localization without significant effect on EB1 and CLIP-170 comets, had an impact on microtubule dynamics and directed migration. Moreover, knockdown of ch-TOG led to a similar inhibition of microtubule growth speed, microtubule capture and chemotaxis. Our data suggest that eribulin binding to the tip of microtubules and subsequent loss of ch-TOG is a priming event leading to alterations in microtubule dynamics and cancer cell migration.

Arsenic trioxide induces Noxa-dependent apoptosis in rhabdomyosarcoma cells and synergizes with antimicrotubule drugs

The prognosis of metastatic or relapsed rhabdomyosarcoma (RMS) is poor, highlighting the need of new treatment options. In the present study, we evaluated the in vitro efficacy of arsenic trioxide (ATO) in RMS, a FDA-approved drug used in pediatric leukemia. Here, we report that ATO exerts antitumor activity against RMS cells both as single agent and in combination with microtubule-targeting drugs. Monotherapy with ATO reduces cell viability, triggers apoptosis and suppresses clonogenic survival of RMS cells, at least in part, by transcriptional induction of the proapoptotic BH3-only protein Noxa. siRNA-mediated knockdown of Noxa significantly rescues ATO-mediated cell death, demonstrating that Noxa is required for cell death. Also, ATO suppresses endogenous Hedgehog (Hh) signaling, as it significantly reduces Gli1 transcriptional activity and expression levels of several Hh target genes. Furthermore, we identify synergistic induction of apoptosis by ATO together with several antimicrotubule agents including vincristine (VCR), vinblastine and eribulin. The addition of the broad-range caspase inhibitor zVAD.fmk or overexpression of the antiapoptotic protein Bcl-2 significantly reduce ATO/VCR-induced cell death, indicating that the ATO/VCR combination triggers caspase-dependent apoptosis via the mitochondrial pathway. In summary, ATO exerts antitumor activity against RMS, especially in combination with antimicrotubule drugs. These findings have important implications for the development of novel therapeutic strategies for RMS.

Termination of Protofilament Elongation by Eribulin Induces Lattice Defects that Promote Microtubule Catastrophes

Microtubules are dynamic polymers built of tubulin dimers that attach in a head-to-tail fashion to form protofilaments, which further associate laterally to form a tube. Asynchronous elongation of individual protofilaments can potentially lead to an altered microtubule-end structure that promotes sudden depolymerization, termed catastrophe [1-4]. However, how the dynamics of individual protofilaments relates to overall growth persistence has remained unclear. Here, we used the microtubule targeting anti-cancer drug Eribulin [5-7] to explore the consequences of stalled protofilament elongation on microtubule growth. Using X-ray crystallography, we first revealed that Eribulin binds to a site on β-tubulin that is required for protofilament plus-end elongation. Based on the structural information, we engineered a fluorescent Eribulin molecule. We demonstrate that single Eribulin molecules specifically interact with microtubule plus ends and are sufficient to either trigger a catastrophe or induce slow and erratic microtubule growth in the presence of EB3. Interestingly, we found that Eribulin increases the frequency of EB3 comet "splitting," transient events where a slow and erratically progressing comet is followed by a faster comet. This observation possibly reflects the "healing" of a microtubule lattice. Because EB3 comet splitting was also observed in control microtubules in the absence of any drugs, we propose that Eribulin amplifies a natural pathway toward catastrophe by promoting the arrest of protofilament elongation.

Dynamic Docking of Conformationally Constrained Macrocycles: Methods and Applications

Many natural products consist of large and flexible macrocycles that engage their targets via multiple contact points. This combination of contained flexibility and large contact area often allows natural products to bind at target surfaces rather than deep pockets, making them attractive scaffolds for inhibiting protein-protein interactions and other challenging therapeutic targets. The increasing ability to manipulate such compounds either biosynthetically or via semisynthetic modification means that these compounds can now be considered as starting points for medchem campaigns rather than solely as ends. Modern medchem benefits substantially from rational improvements made on the basis of molecular docking. As such, docking methods have been enhanced in recent years to deal with the complicated binding modalities and flexible scaffolds of macrocyclic natural products and natural product-like structures. Here, we comprehensively review methods for treating and docking these large macrocyclic scaffolds and discuss some of the resulting advances in medicinal chemistry.

Complementary isonitrile-based multicomponent reactions for the synthesis of diversified cytotoxic hemiasterlin analogues

A small family of structural analogues of the antimitotic tripeptides, hemiasterlins, have been designed and synthesized as potential inhibitors of tubulin polymerization. The effectiveness of a multicomponent approach was fully demonstrated by applying complementary versions of the isocyanide-based Ugi reaction. Compounds strictly related to the lead natural products, as well as more extensively modified analogues, have been synthesized in a concise and convergent manner. In some cases, biological evaluation provided evidence for strong cytotoxic activity (six human tumor cell lines) and for potent inhibition of tubulin polymerization.

Tumour biology, metastatic sites and taxanes sensitivity as determinants of eribulin mesylate efficacy in breast cancer: results from the ERIBEX retrospective, international, multicenter study

BACKGROUND

Our retrospective, international study aimed at evaluating the activity and safety of eribulin mesylate (EM) in pretreated metastatic breast cancer (MBC) in a routine clinical setting.

METHODS

Patients treated with EM for a locally advanced or MBC between March 2011 and January 2014 were included in the study. Clinical and biological assessment of toxicity was performed at each visit. Tumour response was assessed every 3 cycles of treatment. A database was created to collect clinical, pathological and treatment data.

RESULTS

Two hundred and fifty-eight patients were included in the study. Median age was 59 years old. Tumours were Hormone Receptor (HR)-positive (73.3 %) HER2-positive (10.2 %), and triple negative (TN, 22.5 %). 86.4 % of the patients presented with visceral metastases, mainly in the liver (67.4 %). Median previous metastatic chemotherapies number was 4 [1-9]. Previous treatments included anthracyclines and/or taxanes (100 %) and capecitabine (90.7 %). Median number of EM cycles was 5 [1-19]. The relative dose intensity was 0.917. At the time of analysis (median follow-up of 13.9 months), 42.3 % of the patients were still alive. The objective response rate was 25.2 % (95 %CI: 20-31) with a 36.1 % clinical benefit rate (CBR). Median time to progression (TTP) and overall survival were 3.97 (95 %CI: 3.25-4.3) and 11.2 (95 %CI: 9.3-12.1) months, respectively. One- and 2-year survival rates were 45.5 and 8.5 %, respectively. In multivariate analysis, HER2 positivity (HR = 0.29), the presence of lung metastases (HR = 2.49) and primary taxanes resistance (HR = 2.36) were the only three independent CBR predictive factors, while HR positivity (HR = 0.67), the presence of lung metastases (HR = 1.52) and primary taxanes resistance (HR = 1.50) were the only three TTP independent prognostic factors. Treatment was globally well tolerated. Most common grade 3-4 toxicities were neutropenia (20.9 %), peripheral neuropathy (3.9 %), anaemia (1.6 %), liver dysfunction (0.8 %) and thrombocytopenia (0.4 %). Thirteen patients (5 %) developed febrile neutropenia.

CONCLUSION

EM is an effective new option in heavily pretreated MBC, with a favourable efficacy/safety ratio in a clinical practice setting. Our results comfort the use of this new molecule and pledge for the evaluation of EM-trastuzumab combination in this setting. Tumour biology, primary taxanes sensitivity and metastatic sites could represent useful predictive and prognostic factors.

Eribulin mesylate: mechanism of action of a unique microtubule-targeting agent

Eribulin mesylate (eribulin), an analogue of the marine natural product halichondrin B, is a microtubule-depolymerizing drug that has utility in the treatment of patients with breast cancer. Clinical trial results have demonstrated that eribulin treatment provides a survival advantage to patients with metastatic or locally advanced breast cancer previously treated with an anthracycline and a taxane. Furthermore, a pooled analysis of two pivotal phase III trials has demonstrated that eribulin also improves overall survival in several patient subgroups, including in women with HER2-negative disease and triple-negative breast cancer. This review covers the preclinical research that led to the clinical testing and approval of eribulin, as well as subsequent research that was prompted by distinct and unexpected effects of eribulin in the clinic. Initial studies with halichondrin B demonstrated unique effects on tubulin binding that resulted in distinct microtubule-dependent events and antitumor actions. Consistent with the actions of the natural product, eribulin has potent microtubule-depolymerizing activities and properties that distinguish it from other microtubule-targeting agents. Here, we review new results that further differentiate the effects of eribulin from other agents on peripheral nerves, angiogenesis, vascular remodeling, and epithelial-to-mesenchymal transition. Together, these data highlight the distinct properties of eribulin and begin to delineate the mechanisms behind the increased survival benefit provided by eribulin for patients.

The design, synthesis and biological evaluation of conformationally restricted 4-substituted-2,6-dimethylfuro[2,3-d]pyrimidines as multi-targeted receptor tyrosine kinase and microtubule inhibitors as potential antitumor agents

A series of eleven conformationally restricted, 4-substituted 2,6-dimethylfuro[2,3-d]pyrimidines was designed to explore the bioactive conformation required for dual inhibition of microtubule assembly and receptor tyrosine kinases (RTKs), and their biological activities are reported. All three rotatable single bonds in the lead compound 1 were sequentially restricted to address the role of each in SAR for microtubule and RTK inhibitory effects. Compounds 2, 3, 7 and 10 showed microtubule depolymerizing activity comparable to or better than the lead 1, some with nanomolar EC50 values. While compound 8 had no effect on microtubules, 8 and 10 both showed potent RTK inhibition with nanomolar IC50s. These compounds confirm that the bioactive conformation for RTK inhibition is different from that for tubulin inhibition. The tetrahydroquinoline analog 10 showed the most potent dual tubulin and RTK inhibitory activities (low nanomolar inhibition of EGFR, VEGFR2 and PDGFR-β). Compound 10 has highly potent activity against many NCI cancer cell lines, including several chemo-resistant cell lines, and could serve as a lead for further preclinical studies.

Selected hybrid natural products as tubulin modulators

Modulators of microtubule dynamics have received increasing attention because of their potential to stop cancer growth. Although it belongs to the category of complex protein-protein interactions (PPIs), which are generally considered difficult to modulate through small molecules, the use of microtubule is considered a well-validated target. There are a number of bioactive natural products and related compounds that are currently in use as drugs or in clinical trials as next generation anti-cancer agents. The present review article is focused on two such bioactive natural products, epothilone and halichondrin B, and covers some of the key papers published after 2005 that outline various synthetic approaches to obtain next generation structural analogs as well as the synthesis of hybrid compounds.

[Search for natural substances with therapeutic activity: George R. Pettit]

This series of brief reviews covers the "life and work" of famous and iconic researchers who discovered major therapeutics from natural products: their life history, the circumstances of their discoveries, the molecules and their molecular, cellular and physiological mechanisms of action, and their biomedical applications. Dedicated to George R. Pettit, the second article reviews the life of the famous researcher, his worldwide exploration of natural products, especially of marine origin, in search of promising anticancer leads, his discovery and structural elucidation of very potent drug candidates, their synthesis and the launch of some of them on the pharmaceutical market. An extraordinary scientific career which lead George R. Pettit from exploration of Nature to state-of-the-art analytical and synthetic chemistry and from clinical trials to therapeutic successes.

New interfacial microtubule inhibitors of marine origin, PM050489/PM060184, with potent antitumor activity and a distinct mechanism

We have investigated the target and mechanism of action of a new family of cytotoxic small molecules of marine origin. PM050489 and its dechlorinated analogue PM060184 inhibit the growth of relevant cancer cell lines at subnanomolar concentrations. We found that they are highly potent microtubule inhibitors that impair mitosis with a distinct molecular mechanism. They bind with nanomolar affinity to unassembled αβ-tubulin dimers, and PM050489 binding is inhibited by known Vinca domain ligands. NMR TR-NOESY data indicated that a hydroxyl-containing analogue, PM060327, binds in an extended conformation, and STD results define its binding epitopes. Distinctly from vinblastine, these ligands only weakly induce tubulin self-association, in a manner more reminiscent of isohomohalichondrin B than of eribulin. PM050489, possibly acting like a hinge at the association interface between tubulin heterodimers, reshapes Mg(2+)-induced 42 S tubulin double rings into smaller 19 S single rings made of 7 ± 1 αβ-tubulin dimers. PM060184-resistant mutants of Aspergillus nidulans map to β-tubulin Asn100, suggesting a new binding site different from that of vinblastine at the associating β-tubulin end. Inhibition of assembly dynamics by a few ligand molecules at the microtubule plus end would explain the antitumor activity of these compounds, of which PM060184 is undergoing clinical trials.

Phylogeny drives large scale patterns in Australian marine bioactivity and provides a new chemical ecology rationale for future biodiscovery

Twenty-five years of Australian marine bioresources collecting and research by the Australian Institute of Marine Science (AIMS) has explored the breadth of latitudinally and longitudinally diverse marine habitats that comprise Australia's ocean territory. The resulting AIMS Bioresources Library and associated relational database integrate biodiversity with bioactivity data, and these resources were mined to retrospectively assess biogeographic, taxonomic and phylogenetic patterns in cytotoxic, antimicrobial, and central nervous system (CNS)-protective bioactivity. While the bioassays used were originally chosen to be indicative of pharmaceutically relevant bioactivity, the results have qualified ecological relevance regarding secondary metabolism. In general, metazoan phyla along the deuterostome phylogenetic pathway (eg to Chordata) and their ancestors (eg Porifera and Cnidaria) had higher percentages of bioactive samples in the assays examined. While taxonomy at the phylum level and higher-order phylogeny groupings helped account for observed trends, taxonomy to genus did not resolve the trends any further. In addition, the results did not identify any biogeographic bioactivity hotspots that correlated with biodiversity hotspots. We conclude with a hypothesis that high-level phylogeny, and therefore the metabolic machinery available to an organism, is a major determinant of bioactivity, while habitat diversity and ecological circumstance are possible drivers in the activation of this machinery and bioactive secondary metabolism. This study supports the strategy of targeting phyla from the deuterostome lineage (including ancestral phyla) from biodiverse marine habitats and ecological niches, in future biodiscovery, at least that which is focused on vertebrate (including human) health.

Effects of eribulin, vincristine, paclitaxel and ixabepilone on fast axonal transport and kinesin-1 driven microtubule gliding: implications for chemotherapy-induced peripheral neuropathy

Chemotherapy-induced peripheral neuropathy (CIPN) is a serious, painful and dose-limiting side effect of cancer drugs that target microtubules. The mechanisms underlying the neuronal damage are unknown, but may include disruption of fast axonal transport, an essential microtubule-based process that moves cellular components over long distances between neuronal cell bodies and nerve terminals. This idea is supported by the "dying back" pattern of degeneration observed in CIPN, and by the selective vulnerability of sensory neurons bearing the longest axonal projections. In this study, we test the hypothesis that microtubule-targeting drugs disrupt fast axonal transport using vesicle motility assays in isolated squid axoplasm and a cell-free microtubule gliding assay with defined components. We compare four clinically-used drugs, eribulin, vincristine, paclitaxel and ixabepilone. Of these, eribulin is associated with a relatively low incidence of severe neuropathy, while vincristine has a relatively high incidence. In vesicle motility assays, we found that all four drugs inhibited anterograde (conventional kinesin-dependent) fast axonal transport, with the potency being vincristine=ixabepilone>paclitaxel=eribulin. Interestingly, eribulin and paclitaxel did not inhibit retrograde (cytoplasmic dynein-dependent) fast axonal transport, in contrast to vincristine and ixabepilone. Similarly, vincristine and ixabepilone both exerted significant inhibitory effects in an in vitro microtubule gliding assay consisting of recombinant kinesin (kinesin-1) and microtubules composed of purified bovine brain tubulin, whereas paclitaxel and eribulin had negligible effects. Our results suggest that (i) inhibition of microtubule-based fast axonal transport may be a significant contributor to neurotoxicity induced by microtubule-targeting drugs, and (ii) that individual microtubule-targeting drugs affect fast axonal transport through different mechanisms.

A novel natural phenyl alkene with cytotoxic activity

A novel phenyl alkene (1) was isolated from a mixture of three Florida sponges, Smenospongia aurea, Smenospongia cerebriformis, and Verongula rigida. Unlike terpenoids or amino acid derivatives, which are commonly known classes of secondary metabolites from these genera, the chemical structure of 1 showed an unprecedented linear phenyl alkene skeleton. Through comprehensive analyses of NMR and MS data, the gross structure of 1 was determined to be (E)-10-benzyl-5,7-dimethylundeca-1,5,10-trien-4-ol. The absolute configuration at C-4 was established as R by a modified Mosher's method. Based on the relative configuration between C-4 and C-7, the absolute configuration at C-7 was assigned as S. Compound 1 showed in vitro cytotoxic activity against HL-60 human leukemia cancer cells with an IC₅₀ value of 8.1 µM. Molecular docking study suggests that the structure of compound 1 matches the pharmacophore of eribulin required to display cytotoxic activity through the inhibition of microtubule activity.

Molecular simulations of drug–receptor complexes in anticancer research

Molecular modeling and computer simulation techniques have matured significantly in recent years and proved their value in the study of drug–DNA, drug–DNA–protein, drug–protein and protein–protein interactions. Evolution in this area has gone hand-in-hand with an increased availability of structural data on biological macromolecules, major advances in molecular mechanics force fields and considerable improvements in computer technologies, most significantly processing speeds, multiprocessor programming and data-storage capacity. The information derived from molecular simulations of drug–receptor complexes can be used to extract structural and energetic information that is usually beyond current experimental possibilities, provide independent accounts of experimentally observed behavior, help in the interpretation of biochemical or pharmacological results, and open new avenues for research by posing novel relevant questions that can guide the design of new experiments. As drug-screening tools, ligand- and fragment-docking platforms stand out as powerful techniques that can provide candidate molecules for hit and lead development. This review provides an overall perspective of the main methods and focuses on some selected applications to both classical and novel anticancer targets.

Eribulin: A Novel Cytotoxic Chemotherapy Agent

OBJECTIVE:

To review the chemistry, pharmacology, pharmacokinetics, safety, and efficacy of eribulin (Halaven).

DATA SOURCES:

A literature search (up to December 2011) using the terms eribulin, Halaven, ER-086526, and E7389 was performed with PubMed, Google Scholar, selected Ovid bibliography searches, and the abstract search tool from the American Society of Clinical Oncology Annual Meetings and the San Antonio Breast Cancer Symposia. Additional references from the bibliographies of these articles were also assessed.

DATA EXTRACTION:

English-language preclinical and clinical studies on the chemistry, pharmacology, pharmacokinetics, safety, and efficacy of eribulin were reviewed.

DATA SYNTHESIS:

Eribulin is a novel microtubule inhibitor with a unique mechanism of action, which involves interaction with a distinct binding site on β-tubulin leading to G2/M phase cell-cycle arrest and apoptosis. Eribulin has been approved by the Food and Drug Administration for the treatment of metastatic breast cancer in patients who have been previously treated with an anthracycline and a taxane. In a pivotal Phase 3 study conducted in patients with metastatic breast cancer, eribulin 1.4 mg/m2, administered over 2-5 minutes as an intravenous infusion on days 1 and 8 of 21-day cycles, was associated with a significantly increased median overall survival of 13.1 months compared to the median overall survival of 10.6 months in the therapy of physician's choice. Eribulin has also shown activity in Phase 2 studies in other types of cancers, such as non–small cell lung cancer, prostate cancer, urothelial cancer, soft tissue sarcomas, and platinum-susceptible ovarian, fallopian tube, or peritoneal cancers. The most severe (grade 3/4) adverse effects associated with eribulin include neutropenia, leukopenia, and peripheral neuropathy. Common toxicities include fatigue, neutropenia, alopecia, anemia, and peripheral neuropathy.

CONCLUSIONS:

Eribulin is a promising new cytotoxic chemotherapy agent due to its ability to treat cancers that are refractory or resistant to other drugs as well as its manageable toxicity profile.

Cytotoxicity and glycan-binding properties of an 18 kDa lectin isolated from the marine sponge Halichondria okadai

A divalent cation-independent lectin-HOL-18, with cytotoxic activity against leukemia cells, was purified from a demosponge, Halichondria okadai. HOL-18 is a 72 kDa tetrameric lectin that consists of four non-covalently bonded 18 kDa subunits. Hemagglutination activity of the lectin was strongly inhibited by chitotriose (GlcNAcβ1-4GlcNAcβ1-4GlcNAc), fetuin and mucins from porcine stomach and bovine submaxillary gland. Lectin activity was stable at pH 4-12 and temperatures lower than 60 °C. Frontal affinity chromatography with 16 types of pyridylaminated oligosaccharides indicated that the lectin had an affinity for N-linked complex-type and sphingolipid-type oligosaccharides with N-acetylated hexosamines and neuramic acid at the non-reducing termini. The lectin killed Jurkat leukemia T cells and K562 erythroleukemia cells in a dose- and carbohydrate-dependent manner.

Natural products as sources of new drugs over the 30 years from 1981 to 2010

This review is an updated and expanded version of the three prior reviews that were published in this journal in 1997, 2003, and 2007. In the case of all approved therapeutic agents, the time frame has been extended to cover the 30 years from January 1, 1981, to December 31, 2010, for all diseases worldwide, and from 1950 (earliest so far identified) to December 2010 for all approved antitumor drugs worldwide. We have continued to utilize our secondary subdivision of a "natural product mimic" or "NM" to join the original primary divisions and have added a new designation, "natural product botanical" or "NB", to cover those botanical "defined mixtures" that have now been recognized as drug entities by the FDA and similar organizations. From the data presented, the utility of natural products as sources of novel structures, but not necessarily the final drug entity, is still alive and well. Thus, in the area of cancer, over the time frame from around the 1940s to date, of the 175 small molecules, 131, or 74.8%, are other than "S" (synthetic), with 85, or 48.6%, actually being either natural products or directly derived therefrom. In other areas, the influence of natural product structures is quite marked, with, as expected from prior information, the anti-infective area being dependent on natural products and their structures. Although combinatorial chemistry techniques have succeeded as methods of optimizing structures and have been used very successfully in the optimization of many recently approved agents, we are able to identify only one de novo combinatorial compound approved as a drug in this 30-year time frame. We wish to draw the attention of readers to the rapidly evolving recognition that a significant number of natural product drugs/leads are actually produced by microbes and/or microbial interactions with the "host from whence it was isolated", and therefore we consider that this area of natural product research should be expanded significantly.

Evading Pgp activity in drug-resistant cancer cells: a structural and functional study of antitubulin furan metotica compounds

Tumor resistance to antitubulin drugs resulting from P-glycoprotein (Pgp) drug-efflux activity, increased expression of the βIII tubulin isotype, and alterations in the drug-binding sites are major obstacles in cancer therapy. Consequently, novel antitubulin drugs that overcome these challenges are of substantial interest. Here, we study a novel chemotype named furan metotica that localizes to the colchicine-binding site in β-tubulin, inhibits tubulin polymerization, and is not antagonized by Pgp. To elucidate the structure-activity properties of this chiral chemotype, the enantiomers of its most potent member were separated and their absolute configurations determined by X-ray crystallography. Both isomers were active and inhibited all 60 primary cancer cell lines tested at the U.S. National Cancer Institute. They also efficiently killed drug-resistant cancer cells that overexpressed the Pgp drug-efflux pump 10(6)-fold. In vitro, the R-isomer inhibited tubulin polymerization at least 4-fold more potently than the S-isomer, whereas in human cells the difference was 30-fold. Molecular modeling showed that the two isomers bind to β-tubulin in distinct manners: the R-isomer binds in a colchicine-like mode and the S-isomer in a podophyllotoxin-like fashion. In addition, the dynamic binding trajectory and occupancy state of the R-isomer were energetically more favorable then those of the S-isomer, explaining the observed differences in biologic activities. The ability of a racemic drug to assume the binding modes of two prototypical colchicine-site binders represents a novel mechanistic basis for antitubulin activity and paves the way toward a comprehensive design of novel anticancer agents.

Antineoplastic agents. 592. Highly effective cancer cell growth inhibitory structural modifications of dolastatin 10

The dolastatin series of unique peptides, originally discovered as constituents of the sea hare Dolabella auricularia, is of increasing importance in providing biological leads, especially to new and useful anticancer drugs. Dolastatin 10 and three analogues, minor structural modifications designated auristatins, are currently in human cancer clinical trials. The present study was undertaken to explore delivery to the cancer sites by way of phosphate or quinoline modifications. The initial objectives, auristatin TP as sodium phosphate 3b (GI50 10(-2)-10(-4) μg/mL), auristatin 2-AQ (4, GI50 10(-2)-10(-3) μg/mL), and auristatin 6-AQ (5, GI50 10(-4) μg/mL), exhibited superior cancer cell growth inhibitory properties.