Halichondrin B and homohalichondrin B, marine natural products binding in the vinca domain of tubulin. Discovery of tubulin-based mechanism of action by analysis of differential cytotoxicity data

Bioactive natural products from Papua New Guinea marine sponges

The discovery of novel natural products for drug development relies heavily upon a rich biodiversity, of which the marine environment is an obvious example. Marine natural product research has spawned several drugs and many other candidates, some of which are the focus of current clinical trials. The sponge megadiversity of Papua New Guinea is a rich but underexplored source of bioactive natural products. Here, we review some of the many natural products derived from PNG sponges with an emphasis on those with interesting biological activity and, therefore, drug potential. Many bioactive natural products discussed here appear to be derived from non-ribosomal peptide and polyketide biosynthesis pathways, strongly suggesting a microbial origin of these compounds. With this in mind, we also explore the notion of sponge-symbiont biosynthesis of these bioactive compounds and present examples to support the working hypothesis.

Target identification for small bioactive molecules: finding the needle in the haystack

Identification and confirmation of bioactive small-molecule targets is a crucial, often decisive step both in academic and pharmaceutical research. Through the development and availability of several new experimental techniques, target identification is, in principle, feasible, and the number of successful examples steadily grows. However, a generic methodology that can successfully be applied in the majority of the cases has not yet been established. Herein we summarize current methods for target identification of small molecules, primarily for a chemistry audience but also the biological community, for example, the chemist or biologist attempting to identify the target of a given bioactive compound. We describe the most frequently employed experimental approaches for target identification and provide several representative examples illustrating the state-of-the-art. Among the techniques currently available, protein affinity isolation using suitable small-molecule probes (pulldown) and subsequent mass spectrometric analysis of the isolated proteins appears to be most powerful and most frequently applied. To provide guidance for rapid entry into the field and based on our own experience we propose a typical workflow for target identification, which centers on the application of chemical proteomics as the key step to generate hypotheses for potential target proteins.

Eribulin mesylate: a novel halichondrin B analogue for the treatment of metastatic breast cancer

PURPOSE

The pharmacology, pharmacokinetics, clinical efficacy, safety, and administration of eribulin in patients with metastatic breast cancer are reviewed.

SUMMARY

Classical chemotherapeutic agents for breast cancer have dominated treatment regimens even in the era of targeted therapy. Disease progression through these agents is often due to the development of resistance or lack of efficacy with these agents. Recently, a new nontaxane agent, eribulin mesylate, was approved for the treatment of metastatic breast cancer in patients who have received at least two prior chemotherapeutic agents. Eribulin is a member of a new class of synthetic cytotoxic agents derived from the Japanese sea sponge Halichondria okadai. Eribulin differs from other antimicrotubule agents in that it can bind to the microtubule cap and inhibit tubulin polymerization, leading to microtubule arrest. In Phase II clinical trials, eribulin demonstrated activity in extensively pretreated patients who had previously received an anthracycline, taxane, and capecitabine and had shown disease progression within the last six months of treatment. In a pivotal Phase III clinical trial of heavily pre-treated patients, patients who received eribulin versus the physician's treatment of choice showed a significant increase in overall and progression-free survival. Eribulin has a manageable adverse-effect profile, consisting mainly of neutropenia and fatigue. Eribulin has been associated with a low incidence of peripheral neuropathy.

CONCLUSION

Eribulin, a novel synthetic antimicrotubule agent that binds to the vinca domain of tubulin and inhibits the polymerization of tubulin, offers a new treatment option for metastatic breast cancer or locally advanced breast cancer.

Novel tricyclic indeno[2,1-d]pyrimidines with dual antiangiogenic and cytotoxic activities as potent antitumor agents

We designed, synthesized and evaluated 13 novel tricyclic indeno[2,1-d]pyrimidines as RTK inhibitors. These analogues were synthesized via a Dieckmann condensation of 1,2-phenylenediacetonitrile followed by cyclocondensation with guanidine carbonate to afford the 2-amino-3,9-dihydro-indeno[2,1-d]pyrimidin-4-one. Sulfonation of the 4-position followed by displacement with appropriately substituted anilines afforded the target compounds. These compounds were potent inhibitors of platelet-derived growth factor receptor β (PDGFRβ) and inhibited angiogenesis in the chicken embryo chorioallantonic membrane (CAM) assay compared to standards. In addition, compound 7 had a two digit nanomolar GI(50) against nine tumor cell lines, a submicromolar GI(50) against 29 of other tumor cell lines in the preclinical NCI 60 tumor cell line panel. Compound 7 also demonstrated significant in vivo inhibition of tumor growth and angiogenesis in a B16-F10 syngeneic mouse melanoma model.

Antimitotic inhibitors

Of the agents available in the treatment of both solid and hematologic cancers, microtubule-targeted agents are among the most widely used and exploiting other mechanisms involving the microtubule and its role in mitosis is an area of continued interest. This review will focus on novel microtubule-targeted agents, both recently approved (eg, ixabepilone and eribulin) and in later-stage clinical trials, and kinase inhibitors that aim to directly inhibit the mitotic spindle, such as the aurora kinase, pololike kinase, and kinsein-spindle protein inhibitors.

Eribulin -- a review of preclinical and clinical studies

Eribulin mesylate is a non-taxane, structurally simplified, completely synthetic, halichondrin B derivative with an end poisoning, microtubule inhibitory action. Preclinical studies have demonstrated activity in various cancer cell lines and synergistic action with gemcitabine, epirubicin, trastuzumab, cisplatin, docetaxel and vinorelbine. Eribulin has recently been approved by United States Food and Drug Administration as a third line therapy for metastatic breast cancer patients, who have previously been treated with an anthracycline and a taxane. It has also advanced to phase II trials in non-small cell lung cancer, pancreatic, prostate, bladder, head and neck cancers, sarcomas and ovarian and other gynecological tumors. Combination trials with carboplatin, gemcitabine, pemetrexed, cisplatin, and erlotinib are currently ongoing. Eribulin potentially has a low incidence of peripheral neuropathy. The predominant side effects are neutropenia and fatigue, which are manageable. This article reviews the available information on eribulin with respect to its clinical pharmacology, mechanism of action, pharmacokinetics, pharmacodynamics, metabolism, preclinical studies and clinical trials.

Marine natural products: a new wave of drugs?

The largely unexplored marine world that presumably harbors the most biodiversity may be the vastest resource to discover novel 'validated' structures with novel modes of action that cover biologically relevant chemical space. Several challenges, including the supply problem and target identification, need to be met for successful drug development of these often complex molecules; however, approaches are available to overcome the hurdles. Advances in technologies such as sampling strategies, nanoscale NMR for structure determination, total chemical synthesis, fermentation and biotechnology are all crucial to the success of marine natural products as drug leads. We illustrate the high degree of innovation in the field of marine natural products, which in our view will lead to a new wave of drugs that flow into the market and pharmacies in the future.

Eribulin

Eribulin (eribulin mesylate) is a non-taxane microtubule dynamics inhibitor with tubulin-based antimitotic activity and chemotherapeutic effects. Eribulin is used in the treatment of patients with locally advanced or metastatic breast cancer who have previously been treated with at least two chemotherapeutic regimens, including an anthracycline and a taxane. In in vitro studies, eribulin displayed antiproliferative activity against human breast cancer cell lines. Regression and elimination of breast tumours were observed in human tumour xenograft models. In the randomized, open-label, multinational, phase III EMBRACE trial in patients with locally recurrent or metastatic breast cancer, median overall survival was significantly longer in patients who received intravenous eribulin (n = 508) [13.1 months] compared with that in patients who received a treatment of physician's choice (n = 254) [10.6 months; hazard ratio 0.81; 95% CI 0.66, 0.99; p = 0.041]. Prior to enrolment, study participants had received between two and five chemotherapeutic regimens, including an anthracycline and a taxane. Consistent with the findings of earlier phase I and II trials, eribulin was reported to have a manageable tolerability profile in the EMBRACE trial. Peripheral neuropathy (incidence 5%) was the most common adverse event resulting in the discontinuation of eribulin treatment. The most common grade 3/4 adverse events in the eribulin group were neutropenia, leukopenia and asthenia or fatigue.

Mass balance study of [¹⁴C]eribulin in patients with advanced solid tumors

This mass balance study investigated the metabolism and excretion of eribulin, a nontaxane microtubule dynamics inhibitor with a novel mechanism of action, in patients with advanced solid tumors. A single approximately 2 mg (approximately 80 μCi) dose of [¹⁴C]eribulin acetate was administered as a 2 to 5 min bolus injection to six patients on day 1. Blood, urine, and fecal samples were collected at specified time points on days 1 to 8 or until sample radioactivity was ≤1% of the administered dose. Mean plasma eribulin exposure (627 ng · h/ml) was comparable with that of total radioactivity (568 ng Eq · h/ml). Time-matched concentration ratios of eribulin to total radioactivity approached unity in blood and plasma, indicating that unchanged parent compound constituted almost all of the eribulin-derived radioactivity. Only minor metabolites were detected in plasma samples up to 60 min postdose, pooled across patients, each metabolite representing ≤0.6% of eribulin. Elimination half-lives for eribulin (45.6 h) and total radioactivity (42.3 h) were comparable. Eribulin-derived radioactivity excreted in feces was 81.5%, and that of unchanged eribulin was 61.9%. Renal clearance (0.301 l/h) was a minor component of total eribulin clearance (3.93 l/h). Eribulin-derived radioactivity excreted in urine (8.9%) was comparable with that of unchanged eribulin (8.1%), indicating minimal excretion of metabolite(s) in urine. Total recovery of the radioactive dose was 90.4% in urine and feces. Overall, no major metabolites of eribulin were detected in plasma. Eribulin is eliminated primarily unchanged in feces, whereas urine constitutes a minor route of elimination.

Assembly of the isoindolinone core of muironolide A by asymmetric intramolecular Diels-Alder cycloaddition

The hexahydro-1H-isoindolin-1-one core of muironolide A was prepared by asymmetric intramolecular Diels-Alder cycloaddition using a variant of the MacMillan organocatalyst which sets the C4,C5 and C11 stereocenters.

Interactions of halichondrin B and eribulin with tubulin

Compounds that modulate microtubule dynamics include highly effective anticancer drugs, leading to continuing efforts to identify new agents and improve the activity of established ones. Here, we demonstrate that [(3)H]-labeled halichondrin B (HB), a complex, sponge-derived natural product, is bound to and dissociated from tubulin rapidly at one binding site per αβ-heterodimer, with an apparent K(d) of 0.31 μM. We found no HB-induced aggregation of tubulin by high-performance liquid chromatography, even following column equilibration with HB. Binding of [(3)H]HB was competitively inhibited by a newly approved clinical agent, the truncated HB analogue eribulin (apparent K(i), 0.80 μM) and noncompetitively by dolastatin 10 and vincristine (apparent K(i)'s, 0.35 and 5.4 μM, respectively). Our earlier studies demonstrated that HB inhibits nucleotide exchange on β-tubulin, and this, together with the results presented here, indicated the HB site is located on β-tubulin. Using molecular dynamics simulations, we determined complementary conformations of HB and β-tubulin that delineated in atomic detail binding interactions of HB with only β-tubulin, with no involvement of the α-subunit in the binding interaction. Moreover, the HB model served as a template for an eribulin binding model that furthered our understanding of the properties of eribulin as a drug. Overall, these results established a mechanistic basis for the antimitotic activity of the halichondrin class of compounds.

Phase II evaluation of eribulin mesylate (E7389, NSC 707389) in patients with metastatic or recurrent squamous cell carcinoma of the head and neck: Southwest Oncology Group trial S0618

PURPOSE

Eribulin mesylate, an halichondrin B analog, binds to tubulin and microtubules and possesses broad anti-cancer activity. We conducted a multi-institutional Phase II trial to evaluate the response rate of eribulin mesylate in patients with metastatic or recurrent squamous cell carcinoma of the head and neck (SCCHN).

EXPERIMENTAL DESIGN

Forty eligible patients who had not received prior chemotherapy for metastatic or recurrent SCCHN were enrolled with the following characteristics: 29 male, 11 female; median age 61.2 years; Zubrod Performance Status of 0 (48%) and 1 (53%). Thirty-three patients (83%) had metastatic disease. Primary tumor sites included: 38% oropharynx, 30% lip/oral cavity, 15% larynx, 10% hypopharynx, 5% other/unknown and 3% nasopharynx. Patients received eribulin mesylate at 1.4 mg/m² on Days 1 and 8 of an every 21-day cycle.

RESULTS

Common Grade 3 and 4 toxicities included: lymphopenia (15%), leukocytopenia (13%), neutropenia (10%), hyponatremia (5%), fatigue (5%), diarrhea (5%) and dyspnea (5%), with one treatment-related death due to pulmonary hemorrhage. Among 40 assessable patients, two confirmed partial responses were observed, for an estimated confirmed response rate of 5% (95% confidence interval: 1-17%). The estimated median progression-free survival is 3 months (95% confidence interval: 1-3 months) and estimated median overall survival is 7 months (95% confidence interval: 5-10 months).

CONCLUSIONS

Eribulin mesylate given on Days 1 and 8 of a twenty-one day cycle in metastatic or recurrent SCCHN was well tolerated, but did not result in a clinically significant median PFS. Studies of other agents should be considered in this setting.

Modern natural products drug discovery and its relevance to biodiversity conservation

Natural products continue to provide a diverse and unique source of bioactive lead compounds for drug discovery, but maintaining their continued eminence as source compounds is challenging in the face of the changing face of the pharmaceutical industry and the changing nature of biodiversity prospecting brought about by the Convention on Biological Diversity. This review provides an overview of some of these challenges and suggests ways in which they can be addressed so that natural products research can remain a viable and productive route to drug discovery. Results from International Cooperative Biodiversity Groups (ICBGs) working in Madagascar, Panama, and Suriname are used as examples of what can be achieved when biodiversity conservation is linked to drug discovery.

Overcoming tumor multidrug resistance using drugs able to evade P-glycoprotein or to exploit its expression

Multidrug resistance (MDR) is a major obstacle to the effective treatment of cancer. Cellular overproduction of P-glycoprotein (P-gp), which acts as an efflux pump for various anticancer drugs (e.g. anthracyclines, Vinca alkaloids, taxanes, epipodophyllotoxins, and some of the newer antitumor drugs) is one of the more relevant mechanisms underlying MDR. P-gp belongs to the superfamily of ATP-binding cassette transporters and is encoded by the ABCB1 gene. Its overexpression in cancer cells has become a therapeutic target for circumventing MDR. As an alternative to the classical pharmacological strategy of the coadministration of pump inhibitors and cytotoxic substrates of P-gp and to other approaches applied in experimental tumor models (e.g. P-gp-targeting antibodies, ABCB1 gene silencing strategies, and transcriptional modulators) and in the clinical setting (e.g. incapsulation of P-gp substrate anticancer drugs into liposomes or nanoparticles), a more intriguing strategy for circumventing MDR is represented by the development of new anticancer drugs which are not substrates of P-gp (e.g. epothilones, second- and third-generation taxanes and other microtubule modulators, topoisomerase inhibitors). Some of these drugs have already been tested in clinical trials and, in most of cases, show relevant activity in patients previously treated with anticancer agents which are substrates of P-gp. Of these drugs, ixabepilone, an epothilone, was approved in the United States for the treatment of breast cancer patients pretreated with an anthracycline and a taxane. Another innovative approach is the use of molecules whose activity takes advantage of the overexpression of P-gp. The possibility of overcoming MDR using the latter two approaches is reviewed herein. 

Eribulin mesylate: a promising new antineoplastic agent for locally advanced or metastatic breast cancer

More than one million women worldwide are diagnosed with breast cancer every year. For those diagnosed with metastatic breast cancer, the 5-year survival rates are low as, ultimately, patients develop tumors that become refractory to treatment. In clinical trials, eribulin mesylate (E7389) – a novel nontaxane microtubule dynamics inhibitor – demonstrated efficacy in patients with various solid tumors, in particular, those with heavily pretreated metastatic breast cancer. The Eisai Metastatic Breast Cancer Study Assessing Physician’s Choice Versus E7389 (EMBRACE) study observed a significant increase in overall survival with eribulin compared with treatment of physician’s choice (median: 13.1 vs 10.6 months, respectively). Based on these results, eribulin recently received approval by the US FDA for the treatment of patients with metastatic breast cancer who have received at least two prior chemotherapeutic regimens including an anthracyline and a taxane. In addition, eribulin has a manageable tolerability profile, requires no premedication and has shorter infusion times than most other microtubule-targeted agents.

The medicinal potential of promising marine macrolides with anticancer activity

Marine natural products have become a major source of new chemical entities in the discovery of potential anticancer agents that potently suppress various molecular targets. In particular, the marine macrolides, which include an array of novel biomolecules endowed with outstanding cytotoxic and/or antiproliferative activities, are a prominent class of marine natural products that offer continued promise for breakthroughs in anticancer research. Herein we highlight some recent studies of promising marine macrolides, paying particular attention to their discovery, anticancer activities, mechanisms of action, chemical synthesis, and representative analogues.

Advances in therapy: eribulin improves survival for metastatic breast cancer

Despite advances in cancer biology, chemotherapy remains the backbone of treatment approaches for many patients with metastatic breast cancer (MBC). Halichondrins, derived from marine sponges, have significant potential as potent antimicrotubule agents. Eribulin, with proven preclinical activity, is a synthetic halichondrin analog with novel actions on tubulin including suppression of microtubule polymerization. Phase I and II studies in MBC identified neutropenia as the dose-limiting toxicity and a maximum tolerated dose of 1.4 mg/m2 on days 1 and 8 of a 21-day cycle. An encouraging response rate of 11.5% in refractory MBC led to the launch of the phase III Eisai Metastatic Breast Cancer Study Assessing Physician's Choice versus Eribulin trial, in which heavily pretreated patients with MBC were randomized 2 : 1 to intravenous eribulin or monotherapy of the investigator's choice. Recently, it was reported that this important study of 762 patients met its primary endpoint of overall survival: eribulin was associated with an improvement in median overall survival from 10.65 months to 13.12 months (hazard ratio 0.8; 95% confidence interval 0.66-0.99) and a response rate of 12.2%. In general, the side effect profile of eribulin seems to be acceptable, as although neutropenia occurred in 45% of the patients, febrile neutropenia was rare and the incidence of neuropathy was low. These findings show that eribulin is potentially a new active agent for MBC, although results of ongoing studies are awaited to confirm the reported benefit. Future studies will investigate the potential role of eribulin in other settings, including for early breast cancer, to ascertain how to optimally incorporate this new agent into current treatment paradigms.

Pharmacological Developments Obtained from Marine Natural Products and Current Pipeline Perspective

Marine organisms represent a new extensive source for bioactive molecules. They have the potential to provide new therapeutic alternatives to treat human diseases. In this paper, we describe and discuss a variety of isolated and semisynthetic molecules obtained from marine sources. These compounds are in phase II, phase III and at the commercialization stage of new drug development. A description of the mechanism of action, dosage used and side effects are also reported. The positive results obtained from these studies have triggered the development of new studies to evaluate the prospects for utilization of marine organisms.

Eribulin induces irreversible mitotic blockade: implications of cell-based pharmacodynamics for in vivo efficacy under intermittent dosing conditions

Eribulin (E7389), a mechanistically unique microtubule inhibitor in phase III clinical trials for cancer, exhibits superior efficacy in vivo relative to the more potent compound ER-076349, a fact not explained by different pharmacokinetic properties. A cell-based pharmacodynamic explanation was suggested by observations that mitotic blockade induced by eribulin, but not ER-076349, is irreversible as measured by a flow cytometric mitotic block reversibility assay employing full dose/response treatment. Cell viability 5 days after drug washout established relationships between mitotic block reversibility and long-term cell survival. Similar results occurred in U937, Jurkat, HL-60, and HeLa cells, ruling out cell type-specific effects. Studies with other tubulin agents suggest that mitotic block reversibility is a quantifiable, compound-specific characteristic of antimitotic agents in general. Bcl-2 phosphorylation patterns parallel eribulin and ER-076349 mitotic block reversibility patterns, suggesting persistent Bcl-2 phosphorylation contributes to long-term cell-viability loss after eribulin's irreversible blockade. Drug uptake and washout/retention studies show that [3H]eribulin accumulates to lower intracellular levels than [3H]ER-076349, yet is retained longer and at higher levels. Similar findings occurred with irreversible vincristine and reversible vinblastine, pointing to persistent cellular retention as a component of irreversibility. Our results suggest that eribulin's in vivo superiority derives from its ability to induce irreversible mitotic blockade, which appears related to persistent drug retention and sustained Bcl-2 phosphorylation. More broadly, our results suggest that compound-specific reversibility characteristics of antimitotic agents contribute to interactions between cell-based pharmacodynamics and in vivo pharmacokinetics that define antitumor efficacy under intermittent dosing conditions.

Synthesis and discovery of water-soluble microtubule targeting agents that bind to the colchicine site on tubulin and circumvent Pgp mediated resistance

Two classes of molecules were designed and synthesized based on a 6-CH(3) cyclopenta[d]pyrimidine scaffold and a pyrrolo[2,3-d]pyrimidine scaffold. The pyrrolo[2,3-d]pyrimidines were synthesized by reacting ethyl 2-cyano-4,4-diethoxybutanoate and acetamidine, which in turn was chlorinated and reacted with the appropriate anilines to afford 1 and 2. The cyclopenta[d]pyrimidines were obtained from 3-methyladapic acid, followed by reaction with acetamidine to afford the cyclopenta[d]pyrimidine scaffold. Chlorination and reaction with appropriate anilines afforded (±)-3·HCl-(±)-7·HCl. Compounds 1 and (±)-3·HCl had potent antiproliferative activities in the nanomolar range. Compound (±)-3·HCl is significantly more potent than 1. Mechanistic studies showed that 1 and (±)-3·HCl cause loss of cellular microtubules, inhibit the polymerization of purified tubulin, and inhibit colchicine binding. Modeling studies show interactions of these compounds within the colchicine site. The identification of these new inhibitors that can also overcome clinically relevant mechanisms of drug resistance provides new scaffolds for colchicine site agents.

Eribulin mesylate for the treatment of breast cancer

IMPORTANCE OF THE FIELD

Breast cancer is the most common cause of cancer-related death among women in the USA, and additional effective and well-tolerated chemotherapeutic agents are urgently needed. Eribulin mesylate (E7389), a synthetic analog of the marine macrolide halichondrin B, is a microtubule inhibitor with a unique tubulin binding site and mechanism of action.

AREAS COVERED IN THIS REVIEW

Based on a review of the literature between 2005 and 2010, we present a summary of eribulin and its clinical activity, specifically in metastatic breast cancer.

WHAT THE READER WILL GAIN

The mechanism of action of eribulin, preclinical data indicating antitumor activity of eribulin and data from Phase I and II clinical trials evaluating the efficacy and tolerability of eribulin are presented.

TAKE HOME MESSAGE

Based on data from Phase I and II clinical trials, we conclude that eribulin seems to have efficacy in metastatic breast cancer, even among women with heavily pretreated and taxane-resistant disease. In addition, eribulin has a manageable side-effect profile, consisting mainly of neutropenia and fatigue, and most notably a low incidence of peripheral neuropathy. With these encouraging results, additional Phase II and III studies are ongoing. Eribulin seems to be a promising new agent for the treatment of metastatic breast cancer.

Phase III trials of eribulin mesylate (E7389) in extensively pretreated patients with locally recurrent or metastatic breast cancer

Eribulin mesylate (E7389) is a nontaxane microtubule dynamics inhibitor with a novel mechanism of action. In preclinical studies, it has activity in a variety of in vivo tumor model types, including breast cancer. Following promising results from phase I and phase II studies in patients with breast cancer, 2 open-label, randomized, controlled, parallelgroup phase III studies have been initiated, and enrollment has been completed. Both study populations comprise patients with locally advanced/recurrent or metastatic disease pretreated with several chemotherapy regimens, including an anthracycline and a taxane. In Study 305, eribulin is being evaluated as late-line therapy. The primary objective is to compare overall survival (OS) between eribulin monotherapy and treatment of the physician's choice, and progression-free survival (PFS) is one of the secondary objectives. The 762 patients enrolled in Study 305 were randomized in a 2:1 ratio to receive either eribulin or treatment of the physician's choice. In Study 301, eribulin is being assessed as second-line therapy, and the primary objective is to compare eribulin and capecitabine in terms of OS and PFS. Secondary objectives include assessments of response data, duration of response, quality of life, pain intensity, analgesic consumption, and assessment of pharmacokinetic/pharmacodynamic relationships for eribulin. In Study 301, the 1102 patients enrolled were randomized to receive either eribulin or capecitabine (approximately 550 patients in each arm). Tumor assessments are carried out every 8 weeks in Study 305, and every 2 cycles (each of 3 weeks' duration) in Study 301. Safety is also assessed in both studies.

N-((1-benzyl-1H-1,2,3-triazol-4-yl)methyl)arylamide as a new scaffold that provides rapid access to antimicrotubule agents: synthesis and evaluation of antiproliferative activity against select cancer cell lines

A series of N-((1-benzyl-1H-1,2,3-triazol-4-yl)methyl)arylamides was synthesized by copper-catalyzed azide-alkyne cycloaddition (CuAAC) and afforded inhibitors of cancer cell growth. For example, compound 13e had an IC(50) of 46 nM against MCF-7 human breast tumor cells. Structure-activity relationship (SAR) studies demonstrated that (i) meta-phenoxy substitution of the N-1-benzyl group is important for antiproliferative activity and (ii) a variety of heterocyclic substitutions for the aryl group of the arylamide are tolerated. In silico COMPARE analysis of antiproliferative activity against the NCI-60 human tumor cell line panel revealed a correlation to clinically useful antimicrotubule agents such as paclitaxel and vincristine. This in silico correlation was supported by (i) in vitro inhibition of tubulin polymerization, (ii) G(2)/M-phase arrest in HeLa cells as assessed by flow cytometry, and (iii) perturbation of normal microtubule activity in HeLa cells as observed by confocal microscopy. The results demonstrate that N-((1-benzyl-1H-1,2,3-triazol-4-yl)methyl)arylamide is a readily accessible small molecule scaffold for compounds that inhibit tubulin polymerization and tumor cell growth.

Gymnochromes E and F, cytotoxic phenanthroperylenequinones from a deep-water crinoid, Holopus rangii

Bioactivity-guided fractionation of metabolites from the crinoid Holopus rangii led to the discovery of two new phenanthroperylenequinone derivatives, gymnochromes E (1) and F (2). Gymnochrome E showed cytotoxic activity toward the NCI/ADR-Res with an IC(50) of 3.5 microM. It also inhibited histone deacetylase-1 with an IC(50) of 3.3 microM. Gymnochrome F was a moderate inhibitor of myeloid cell leukemia sequence 1 (MCL-1) binding to Bak. Two anthraquinone metabolites, emodic acid (4) and its new bromo derivative (5), were also isolated from the crinoid and show remarkable similarity to the phenanthroperylenequinone core, suggesting that these metabolites share the same polyketide biosynthetic pathway.

Eribulin binds at microtubule ends to a single site on tubulin to suppress dynamic instability

Eribulin mesylate (E7389), a synthetic analogue of the marine natural product halichondrin B, is in phase III clinical trials for the treatment of cancer. Eribulin targets microtubules, suppressing dynamic instability at microtubule plus ends through an inhibition of microtubule growth with little or no effect on shortening [Jordan, M. A., et al. (2005) Mol. Cancer Ther. 4, 1086-1095]. Using [(3)H]eribulin, we found that eribulin binds soluble tubulin at a single site; however, this binding is complex with an overall K(d) of 46 microM, but also showing a real or apparent very high affinity (K(d) = 0.4 microM) for a subset of 25% of the tubulin. Eribulin also binds microtubules with a maximum stoichiometry of 14.7 +/- 1.3 molecules per microtubule (K(d) = 3.5 microM), strongly suggesting the presence of a relatively high-affinity binding site at microtubule ends. At 100 nM, the concentration that inhibits microtubule plus end growth by 50%, we found that one molecule of eribulin is bound per two microtubules, indicating that the binding of a single eribulin molecule at a microtubule end can potently inhibit its growth. Eribulin does not suppress dynamic instability at microtubule minus ends. Preincubation of microtubules with 2 or 4 microM vinblastine induced additional lower-affinity eribulin binding sites, most likely at splayed microtubule ends. Overall, our results indicate that eribulin binds with high affinity to microtubule plus ends and thereby suppresses dynamic instability.

Microtubule targeting agents: from biophysics to proteomics

This review explores various aspects of the interaction between microtubule targeting agents and tubulin, including binding site, affinity, and drug resistance. Starting with the basics of tubulin polymerization and microtubule targeting agent binding, we then highlight how the three-dimensional structures of drug-tubulin complexes obtained on stabilized tubulin are seeded by precise biological and biophysical data. New avenues opened by thermodynamics analysis, high throughput screening, and proteomics for the molecular pharmacology of these drugs are presented. The amount of data generated by biophysical, proteomic and cellular techniques shed more light onto the microtubule-tubulin equilibrium and tubulin-drug interaction. Combining these approaches provides new insight into the mechanism of action of known microtubule interacting agents and rapid in-depth characterization of next generation molecules targeting the interaction between microtubules and associated modulators of their dynamics. This will facilitate the design of improved and/or alternative chemotherapies targeting the microtubule cytoskeleton.

Iron complexation to oxygen rich marine natural products: a computational study

The natural products kahalalide F, halichondrin B, and discodermolide are relatively large structures that were originally harvested from marine organisms. They are oxygen rich structures that, to varying degrees, should have the ability to bind iron (II or III) by Fe-O and/or Fe-N bonds. In this semi empirical study, the binding of these natural products to iron (II) is studied and the aqueous stability factor (ASF) is used to determine which bonding configuration is most stable. The energy, the complex charge (+1), the average Fe-O (or Fe-N) bond distances and the dipole moments are used to calculate the ASF. The ASF provides insight to which complex will be the most stable and water soluble, important for a medicinal application. The ability of a molecule with a more than six oxygen and/or nitrogen atoms to bind iron (hexavalent, octahedral) by shifting which six atoms (O/N) are bound to the iron qualifies it as a polarity adaptive molecule.

Impact of marine drugs on animal reproductive processes

The discovery and description of bioactive substances from natural sources has been a research topic for the last 50 years. In this respect, marine animals have been used to extract many new compounds exerting different actions. Reproduction is a complex process whose main steps are the production and maturation of gametes, their activation, the fertilisation and the beginning of development. In the literature it has been shown that many substances extracted from marine organisms may have profound influence on the reproductive behaviour, function and reproductive strategies and survival of species. However, despite the central importance of reproduction and thus the maintenance of species, there are still few studies on how reproductive mechanisms are impacted by marine bioactive drugs. At present, studies in either marine and terrestrial animals have been particularly important in identifying what specific fine reproductive mechanisms are affected by marine-derived substances. In this review we describe the main steps of the biology of reproduction and the impact of substances from marine environment and organisms on the reproductive processes.

Macromolecular interaction of halichondrin B analogues eribulin (E7389) and ER-076349 with tubulin by analytical ultracentrifugation

Halichondrin B is an antimitotic drug that inhibits microtubule assembly. To understand the molecular details of its interaction with tubulin, we investigated the binding of two halichondrin B analogues, eribulin (previously, ER-086526, E7389) and ER-076349, to tubulin by quantitative analytical ultracentrifugation. Eribulin is currently undergoing phase III clinical trials for cancer; ER-076349 is a closely related analogue with C.35 hydroxyl instead of C.35 primary amine [Towle, M. J., et al. (2001) Cancer Res. 61, 1013]. Below the critical concentration for microtubule assembly and in the presence of GDP, tubulin undergoes weak self-association into short curved oligomers. Eribulin inhibits this oligomer formation 4-6-fold, while ER-076349 slightly stimulates oligomer formation by 2-fold. This is in contrast to vinblastine which strongly stimulates large spiral polymers by 1000-fold under these same conditions. Vinblastine-induced spiral formation is strongly inhibited by both eribulin and ER-076349. Colchicine binding to the intradimer interface has no significant effect on small oligomer formation or the inhibitory activity of eribulin on this process. These results suggest that halichondrin B analogues bind to the interdimer interface or to the beta-subunit alone, disrupt polymer stability, and compete with vinblastine-induced spiral formation. Stathmin is known to form a tight 1:2 complex with tubulin. Eribulin strongly inhibits formation of the 1:2 stathmin-tubulin complex (>3.3 kcal/mol), while ER-076349 weakens formation of the 1:2 complex by approximately 1.9 kcal/mol. These results suggest that eribulin is a global inhibitor of tubulin polymer formation, disrupting tubulin-tubulin contacts at the interdimer interface. ER-076349 also perturbs tubulin-tubulin contacts, but in a more polymer specific manner, reflecting adaptability of the interdimer interface to drug and polymer polymorphism. These results suggest halichondrin B analogues exhibit unique tubulin-based activities that may underlie the clinical utility of these compounds.

Eribulin: rediscovering tubulin as an anticancer target

Eribulin mesylate (E7389) is a synthetic analog of the marine macrolide halichondrin B, which acts as a novel microtubule modulator with a distinct mechanism of action. Two eribulin mesylate phase 1 studies exploring weekly and 3-weekly schedules are reported in this issue. These trials show linear pharmacokinetics, a toxicity profile consisting in neutropenia and fatigue, and early hints of antitumor activity. In this commentary we give a brief historical perspective of the halichondrins and put into context eribulin mesylate as a novel tubulin modulator.

Phase I study of eribulin mesylate administered once every 21 days in patients with advanced solid tumors

PURPOSE

To evaluate the maximum tolerated dose (MTD), dose-limiting toxicities (DLT), and pharmacokinetics of eribulin mesylate (E7389), a halichondrin B analogue, administered every 21 days in patients with advanced solid tumors.

EXPERIMENTAL DESIGN

Eribulin mesylate was given as a 1-hour infusion every 21 days at doses of 0.25, 0.5, 1, 2, 2.8, and 4 mg/m(2). The MTD was identified using an accelerated titration design. The pharmacokinetics of eribulin were evaluated in the plasma and urine with the first dose.

RESULTS

Twenty-one patients were enrolled. At 4 mg/m(2), three patients experienced a DLT of febrile neutropenia on day 7. The dose level was reduced to 2.8 mg/m(2) where two of three patients experienced dose-limiting febrile neutropenia. Six additional patients were enrolled at 2 mg/m(2) (seven patients in total received this dose) and one of these patients experienced a neutropenic DLT. The MTD of eribulin mesylate was therefore 2 mg/m(2). Nonhematologic toxicities included alopecia, fatigue, anorexia, and nausea. Pharmacokinetic analysis showed linear kinetics for eribulin over the dose range studied and a terminal half-life of 2 days. The plasma-concentration-time profile exhibited a rapid distribution phase followed by a slow elimination phase. Drug clearance was nonrenal. One patient with non-small cell lung cancer achieved an unconfirmed partial response and 12 patients had stable disease.

CONCLUSIONS

Eribulin mesylate administered as a 1-hour infusion every 21 days has a manageable toxicity profile at 2 mg/m(2), with further dose escalation limited by neutropenia.