Natural products which interact with tubulin in the vinca domain: maytansine, rhizoxin, phomopsin A, dolastatins 10 and 15 and halichondrin B

Genome-Driven Discovery of Hygrocins in Streptomyces rapamycinicus

Ansamycins, represented by the antituberculosis drug rifamycin, are an important family of natural products. To obtain new ansamycins, Streptomyces rapamycinicus IMET 43975 harboring an ansamycin biosynthetic gene cluster was fermented in a 50 L scale, and subsequent purification work led to the isolation of five known and four new analogues, where hygrocin W (2) belongs to benzoquinonoid ansamycins, and the other three hygrocins, hygrocins X-Z (6-8), are new seco-hygrocins. The structures of ansamycins (1-8) were determined by the analysis of spectroscopic (1D/2D NMR and ECD) and MS spectrometric data. The Baeyer-Villiger enzyme which catalyzed the ester formation in the ansa-ring was confirmed through in vivo CRISPR base editing. The discovery of these compounds further enriches the structural diversity of ansamycins.

Design, synthesis and bioactivity evaluation of novel monomethyl auristatin F analogues

Monomethyl auristatin F (MMAF), a synthetic analogue of the natural compound dolastatin 10, has garnered significant attention in cancer research due to its high potency in vitro. While previous studies have focused on modifying the N-terminal extension of the amino group and the C-terminal modification of the carboxyl group, there has been limited exploration into modifying the P1 and P5 side chains. In this study, we substituted the valine residue at the P1 position with various natural or unnatural amino acids and introduced triazole functional groups at the P5 side chain. Compounds 11k and 18d exhibited excellent inhibition on tubulin. Additionally, compound 18d demonstrated enhanced cytotoxicity against HCT116 cells compared to the parent compound MMAF, suggesting its potential as a cytotoxic payload for further antibody-drug conjugates (ADCs) development.

Ten-Gram-Scale Total Synthesis of the Anticancer Drug Candidate E7130 to Supply Clinical Trials

E7130 is a novel drug candidate with an exceedingly complex chemical structure of the halichondrin class, discovered by a total synthesis approach through joint research between the Kishi group at Harvard University and Eisai. Only 18 months after completion of the initial milligram-scale synthesis, ten-gram-scale synthesis of E7130 was achieved, providing the first good manufacturing practice (GMP) batch to supply clinical trials. This paper highlights the challenges in developing ten-gram-scale synthesis from the milligram-scale synthesis.

Computational-Based Discovery of the Anti-Cancer Activities of Pyrrole-Based Compounds Targeting the Colchicine-Binding Site of Tubulin

Despite the tubulin-binding agents (TBAs) that are widely used in the clinic for cancer therapy, tumor resistance to TBAs (both inherited and acquired) significantly impairs their effectiveness, thereby decreasing overall survival (OS) and progression-free survival (PFS) rates, especially for the patients with metastatic, recurrent, and unresectable forms of the disease. Therefore, the development of novel effective drugs interfering with the microtubules' dynamic state remains a big challenge in current oncology. We report here about the novel ethyl 2-amino-1-(furan-2-carboxamido)-5-(2-aryl/tert-butyl-2-oxoethylidene)-4-oxo-4,5-dihydro-1H-pyrrole-3-carboxylates (EAPCs) exhibiting potent anti-cancer activities against the breast and lung cancer cell lines in vitro. This was due to their ability to inhibit tubulin polymerization and induce cell cycle arrest in M-phase. As an outcome, the EAPC-treated cancer cells exhibited a significant increase in apoptosis, which was evidenced by the expression of cleaved forms of PARP, caspase-3, and increased numbers of Annexin-V-positive cells. By using the in silico molecular modeling methods (e.g., induced-fit docking, binding metadynamics, and unbiased molecular dynamics), we found that EAPC-67 and -70 preferentially bind to the colchicine-binding site of tubulin. Lastly, we have shown that the EAPCs indicated above and colchicine utilizes a similar molecular mechanism to inhibit tubulin polymerization via targeting the T7 loop in the β-chain of tubulin, thereby preventing the conformational changes in the tubulin dimers required for their polymerization. Collectively, we identified the novel and potent TBAs that bind to the colchicine-binding site and disrupt the microtubule network. As a result of these events, the compounds induced a robust cell cycle arrest in M-phase and exhibited potent pro-apoptotic activities against the epithelial cancer cell lines in vitro.

Computational-Based Discovery of the Anti-Cancer Activities of Pyrrole-Based Compounds Targeting the Colchicine-Binding Site of Tubulin

Despite the tubulin-binding agents (TBAs) that are widely used in the clinic for cancer therapy, tumor resistance to TBAs (both inherited and acquired) significantly impairs their effectiveness, thereby decreasing overall survival (OS) and progression-free survival (PFS) rates, especially for the patients with metastatic, recurrent, and unresectable forms of the disease. Therefore, the development of novel effective drugs interfering with the microtubules' dynamic state remains a big challenge in current oncology. We report here about the novel ethyl 2-amino-1-(furan-2-carboxamido)-5-(2-aryl/tert-butyl-2-oxoethylidene)-4-oxo-4,5-dihydro-1H-pyrrole-3-carboxylates (EAPCs) exhibiting potent anti-cancer activities against the breast and lung cancer cell lines in vitro. This was due to their ability to inhibit tubulin polymerization and induce cell cycle arrest in M-phase. As an outcome, the EAPC-treated cancer cells exhibited a significant increase in apoptosis, which was evidenced by the expression of cleaved forms of PARP, caspase-3, and increased numbers of Annexin-V-positive cells. By using the in silico molecular modeling methods (e.g., induced-fit docking, binding metadynamics, and unbiased molecular dynamics), we found that EAPC-67 and -70 preferentially bind to the colchicine-binding site of tubulin. Lastly, we have shown that the EAPCs indicated above and colchicine utilizes a similar molecular mechanism to inhibit tubulin polymerization via targeting the T7 loop in the β-chain of tubulin, thereby preventing the conformational changes in the tubulin dimers required for their polymerization. Collectively, we identified the novel and potent TBAs that bind to the colchicine-binding site and disrupt the microtubule network. As a result of these events, the compounds induced a robust cell cycle arrest in M-phase and exhibited potent pro-apoptotic activities against the epithelial cancer cell lines in vitro.

Distinct Mechanisms of Cytotoxicity in Novel Nitrogenous Heterocycles: Future Directions for a New Anti-Cancer Agent

Electron-rich, nitrogenous heteroaromatic compounds interact more with biological/cellular components than their non-nitrogenous counterparts. The strong intermolecular interactions with proteins, enzymes, and receptors confer significant biological and therapeutic properties to the imidazole derivatives, giving rise to a well-known and extensively used range of therapeutic drugs used for infections, inflammation, and cancer, to name a few. The current study investigates the anti-cancer properties of fourteen previously synthesized nitrogenous heterocycles, derivatives of imidazole and oxazolone, on a panel of cancer cell lines and, in addition, predicts the molecular interactions, pharmacokinetic and safety profiles of these compounds. Method: The MTT and CellTiter-Glo® assays were used to screen the imidazole and oxazolone derivatives on six cancer cell lines: HL60, MDA-MB-321, KAIMRC1, KMIRC2, MCF-10A, and HCT8. Subsequently, in vitro tubulin staining and imaging were performed, and the level of apoptosis was measured using the Promega ApoTox-Glo® triplex assay. Furthermore, several computational tools were utilized to investigate the pharmacokinetics and safety profile, including PASS Online, SEA Search, the QikProp tool, SwissADME, ProTox-II, and an in silico molecular docking study on tubulin to identify the critical molecular interactions. Results: In vitro analysis identified compounds 8 and 9 to possess the most significant potent cytotoxic activity on the HL60 and MDA-MB-231 cell lines, supported by PASS Online anti-cancer predictions with pa scores of 0.413 and 0.434, respectively. In addition, compound 9 induced caspase 3/7 dependent-apoptosis and interfered with tubulin polymerization in the MDA-MB-231 cell line, consistent with in silico docking results, identifying binding similarity to the native ligand colchicine. All the derivatives, including compounds 8 and 9, had acceptable pharmacokinetics; however, the safety profile was suboptimal for all the tested derivates except compound 4. Conclusion: The imidazole derivative compound 9 is a promising anti-cancer agent that switches on caspase-dependent apoptotic cell death and modulates microtubule function. Therefore, it could be a lead compound for further drug optimization and development.

Evolutionarily recent dual obligatory symbiosis among adelgids indicates a transition between fungus- and insect-associated lifestyles

Adelgids (Insecta: Hemiptera: Adelgidae) form a small group of insects but harbor a surprisingly diverse set of bacteriocyte-associated endosymbionts, which suggest multiple replacement and acquisition of symbionts over evolutionary time. Specific pairs of symbionts have been associated with adelgid lineages specialized on different secondary host conifers. Using a metagenomic approach, we investigated the symbiosis of the Adelges laricis/Adelges tardus species complex containing betaproteobacterial ("Candidatus Vallotia tarda") and gammaproteobacterial ("Candidatus Profftia tarda") symbionts. Genomic characteristics and metabolic pathway reconstructions revealed that Vallotia and Profftia are evolutionary young endosymbionts, which complement each other's role in essential amino acid production. Phylogenomic analyses and a high level of genomic synteny indicate an origin of the betaproteobacterial symbiont from endosymbionts of Rhizopus fungi. This evolutionary transition was accompanied with substantial loss of functions related to transcription regulation, secondary metabolite production, bacterial defense mechanisms, host infection, and manipulation. The transition from fungus to insect endosymbionts extends our current framework about evolutionary trajectories of host-associated microbes.

Microtubule Targeting Agents in Disease: Classic Drugs, Novel Roles

Microtubule-targeting agents (MTAs) represent one of the most successful first-line therapies prescribed for cancer treatment. They interfere with microtubule (MT) dynamics by either stabilizing or destabilizing MTs, and in culture, they are believed to kill cells via apoptosis after eliciting mitotic arrest, among other mechanisms. This classical view of MTA therapies persisted for many years. However, the limited success of drugs specifically targeting mitotic proteins, and the slow growing rate of most human tumors forces a reevaluation of the mechanism of action of MTAs. Studies from the last decade suggest that the killing efficiency of MTAs arises from a combination of interphase and mitotic effects. Moreover, MTs have also been implicated in other therapeutically relevant activities, such as decreasing angiogenesis, blocking cell migration, reducing metastasis, and activating innate immunity to promote proinflammatory responses. Two key problems associated with MTA therapy are acquired drug resistance and systemic toxicity. Accordingly, novel and effective MTAs are being designed with an eye toward reducing toxicity without compromising efficacy or promoting resistance. Here, we will review the mechanism of action of MTAs, the signaling pathways they affect, their impact on cancer and other illnesses, and the promising new therapeutic applications of these classic drugs.

Naturally occurring heterocyclic anticancer compounds

Naturally occurring heterocyclic scaffolds are key ingredients for the development of various therapeutics employed for biomedical applications. Heterocyclic pharmacophores are widely disseminated and have been befallen in almost all categories of drugs for the alleviation of myriad ailments including diabetes, neurodegenerative, psychiatric, microbial infections, disastrous cancers etc. Countless fused heterocyclic anticancerous templates are reported to display antimetabolite, antioxidant, antiproliferative, cytostatic etc. pharmacological actions via targeting different signaling pathways (cell cycle, PI-3kinase/Akt, p53, caspase extrinsic pathway etc.), overexpressive receptors (EGRF, HER2, EGF, VEGF etc.) and physiological enzymes (topoisomerase I and II, cyclin dependent kinase etc.). A compiled description on various natural sources (plants, microbes, marine) containing anticancer agents comprising heterocyclic ring specified with presence of nitrogen (vincristine, vinblastine, indole-3-carbinol, meridianins, piperine, lamellarins etc.), oxygen (paclitaxel, halichondrin B, quercetin, myricetin, kaempferol etc.) and sulphur atoms (brugine, fucoidan, carrageenan etc.) are displayed here along with their molecular level cytotoxic action and therapeutic applications.

Mertansine Inhibits mRNA Expression and Enzyme Activities of Cytochrome P450s and Uridine 5′-Diphospho-Glucuronosyltransferases in Human Hepatocytes and Liver Microsomes

Mertansine, a tubulin inhibitor, is used as the cytotoxic component of antibody–drug conjugates (ADCs) for cancer therapy. The effects of mertansine on uridine 5′-diphospho-glucuronosyltransferase (UGT) activities in human liver microsomes and its effects on the mRNA expression of cytochrome P450s (CYPs) and UGTs in human hepatocytes were evaluated to assess the potential for drug–drug interactions (DDIs). Mertansine potently inhibited UGT1A1-catalyzed SN-38 glucuronidation, UGT1A3-catalyzed chenodeoxycholic acid 24-acyl-β-glucuronidation, and UGT1A4-catalyzed trifluoperazine N-β-d-glucuronidation, with K_i values of 13.5 µM, 4.3 µM, and 21.2 µM, respectively, but no inhibition of UGT1A6, UGT1A9, and UGT2B7 enzyme activities was observed in human liver microsomes. A 48 h treatment of mertansine (1.25–2500 nM) in human hepatocytes resulted in the dose-dependent suppression of mRNA levels of CYP1A2, CYP2B6, CYP3A4, CYP2C8, CYP2C9, CYP2C19, UGT1A1, and UGT1A9, with IC₅₀ values of 93.7 ± 109.1, 36.8 ± 18.3, 160.6 ± 167.4, 32.1 ± 14.9, 578.4 ± 452.0, 539.5 ± 233.4, 856.7 ± 781.9, and 54.1 ± 29.1 nM, respectively, and decreased the activities of CYP1A2-mediated phenacetin O-deethylase, CYP2B6-mediated bupropion hydroxylase, and CYP3A4-mediated midazolam 1′-hydroxylase. These in vitro DDI potentials of mertansine with CYP1A2, CYP2B6, CYP2C8/9/19, CYP3A4, UGT1A1, and UGT1A9 substrates suggest that it is necessary to carefully characterize the DDI potentials of ADC candidates with mertansine as a payload in the clinic.

Application of triazoles as bioisosteres and linkers in the development of microtubule targeting agents

The triazole ring system has emerged as an exciting prospect in the optimization studies of promising lead molecules in the quest for new drugs for clinical usage. Several marketed drugs possess these versatile moieties that are used in a wide range of medical indications. This stems from the unique intrinsic properties of triazoles, which impart stability to the basic pharmacophoric unit with an added advantage of being a bioisostere of different chemical functionalities. In the last decade, the use of triazoles as bioisosteres and linkers in the development of microtubule targeting agents has been extensively investigated. The present review highlights the advances in this promising area of drug discovery and development.

Current status and contemporary approaches to the discovery of antitumor agents from higher plants

Higher plant constituents have afforded clinically available anticancer drugs. These include both chemically unmodified small molecules and their synthetic derivatives currently used or those in clinical trials as antineoplastic agents, and an updated summary is provided. In addition, botanical dietary supplements, exemplified by mangosteen and noni constituents, are also covered as potential cancer chemotherapeutic agents. Approaches to metabolite purification, rapid dereplication, and biological evaluation including analytical hyphenated techniques, molecular networking, and advanced cellular and animal models are discussed. Further, enhanced and targeted drug delivery systems for phytochemicals, including micelles, nanoparticles and antibody drug conjugates (ADCs) are described herein.

Potential drugs used in the antibody-drug conjugate (ADC) architecture for cancer therapy

Cytotoxic small-molecule drugs have a major influence on the fate of antibody-drug conjugates (ADCs). An ideal cytotoxic agent should be highly potent, remain stable while linked to ADCs, kill the targeted tumor cell upon internalization and release from the ADCs, and maintain its activity in multidrug-resistant tumor cells. Lessons learned from successful and failed experiences in ADC development resulted in remarkable progress in the discovery and development of novel highly potent small molecules. A better understanding of such small-molecule drugs is important for development of effective ADCs. The present review discusses requirements making a payload appropriate for antitumor ADCs and focuses on the main characteristics of commonly-used cytotoxic payloads that showed acceptable results in clinical trials. In addition, the present study represents emerging trends and recent advances of payloads used in ADCs currently under clinical trials.

A landmark in drug discovery based on complex natural product synthesis

Despite their outstanding antitumour activity in mice, the limited supply from the natural sources has prevented drug discovery/development based on intact halichondrins. We achieved a total synthesis of C52-halichondrin-B amine (E7130) on a >10 g scale with >99.8% purity under GMP conditions. Interestingly, E7130 not only is a novel microtubule dynamics inhibitor but can also increase intratumoural CD31-positive endothelial cells and reduce α-SMA-positive cancer-associated fibroblasts at pharmacologically relevant compound concentrations. According to these unique effects, E7130 significantly augment the effect of antitumour treatments in mouse models and is currently in a clinical trial. Overall, our work demonstrates that a total synthesis can address the issue of limited material supply in drug discovery/development even for the cases of complex natural products.

Tubulin-binding dibenz[c,e]oxepines: Part 2. Structural variation and biological evaluation as tumour vasculature disrupting agents

5,7-Dihydro-3,9,10,11-tetramethoxybenz[c,e]oxepin-4-ol 1, prepared from a dibenzyl ether precursor via Pd-catalysed intramolecular direct arylation, possesses broad-spectrum in vitro cytotoxicity towards various tumour cell lines, and induces vascular shutdown, necrosis and growth delay in tumour xenografts in mice at sub-toxic doses. The biological properties of 1 and related compounds can be attributed to their ability to inhibit microtubule assembly at the micromolar level, by binding reversibly to the same site of the tubulin αβ-heterodimer as colchicine 2 and the allocolchinol, N-acetylcolchinol 4.

Clinical Development of Anti-mitotic Drugs in Cancer

Mitosis is one of the most fundamental processes of life by which a mammalian cell divides into two daughter cells. Mitosis has been an attractive target for anticancer therapies since fast proliferation was identified as one of the hallmarks of cancer cells. Despite efforts into developing specific inhibitors for mitotic kinases and kinesins, very few drugs have shown the efficiency of microtubule targeting-agents in cancer cells with paclitaxel being the most successful. A deeper translational research accompanying clinical trials of anti-mitotic drugs will help in identifying potent biomarkers predictive for response. Here, we review the current knowledge of mitosis targeting agents that have been tested so far in the clinics.

Structural and Biochemical Characterization of the Interaction of Tubulin with Potent Natural Analogues of Podophyllotoxin

Four natural analogues of podophyllotoxin obtained from the Mexican medicinal plant Bursera fagaroides, namely, acetyl podophyllotoxin (2), 5'-desmethoxy-β-peltatin A methyl ether (3), 7',8'-dehydro acetyl podophyllotoxin (4), and burseranin (5), have been characterized, and their interactions with tubulin have been investigated. Cytotoxic activity measurements, followed by immunofluorescence microscopy and flow cytometry studies, demonstrated that these compounds disrupt microtubule networks in cells and cause cell cycle arrest in the G2/M phase in the A549 cell line. A tubulin binding assay showed that compounds 1-4 were potent assembly inhibitors, displaying binding to the colchicine site with Kb values ranging from 11.75 to 185.0 × 10(5) M(-1). In contrast, burseranin (5) was not able to inhibit tubulin assembly. From the structural perspective, the ligand-binding epitopes of compounds 1-3 have been mapped using STD-NMR, showing that B and E rings are the major points for interaction with the protein. The obtained results indicate that the inhibition of tubulin assembly of this family of compounds is more effective when there are at least two methoxyl groups at the E ring, along with a trans configuration of the lactone ring in the aryltetralin lignan core.

Structural Basis of Microtubule Destabilization by Potent Auristatin Anti-Mitotics

The auristatin class of microtubule destabilizers are highly potent cytotoxic agents against several cancer cell types when delivered as antibody drug conjugates. Here we describe the high resolution structures of tubulin in complex with both monomethyl auristatin E and F and unambiguously define the trans-configuration of both ligands at the Val-Dil amide bond in their tubulin bound state. Moreover, we illustrate how peptidic vinca-site agents carrying terminal carboxylate residues may exploit an observed extended hydrogen bond network with the M-loop Arg278 to greatly improve the affinity of the corresponding analogs and to maintain the M-loop in an incompatible conformation for productive lateral tubulin-tubulin contacts in microtubules. Our results highlight a potential, previously undescribed molecular mechanism by which peptidic vinca-site agents maintain unparalleled potency as microtubule-destabilizing agents.

Biosynthesis of polyketides by trans-AT polyketide synthases

This review discusses the biosynthesis of natural products that are generated bytrans-AT polyketide synthases, a family of catalytically versatile enzymes that represents one of the major group of proteins involved in the production of bioactive polyketides.

Examination of the clinical efficacy of eribulin and trastuzumab in HER2-positive recurrent breast cancer

There are limited studies reported that describe the efficacy of eribulin and trastuzumab in the treatment of recurrent breast cancer. The present study examined the therapeutic efficacy of eribulin and trastuzumab in the treatment of recurrent breast cancer. Between October 2011 and August 2013, 5 recurrent breast cancer patients who were treated with eribulin and trastuzumab were included in the study. The cancer stages in the 5 women who received this regimen were stage IIIB in 1 (20%) and stage IV in 4 (80%). The sites of recurrence were the lung in 3 patients, liver in 2, bone in 1, brain in 1, supraclavicular lymph nodes in 1, infraclavicular lymph nodes in 1 and mediastinal lymph nodes in 1. The median number of prior treatment regimens was 5 (range, 5-11). Complete response was achieved in 0 patients, 1 achieved partial response, 3 had stable disease, and 1 had progressive disease. The overall response rate was 20%, and the clinical benefit rate was 80%. Patients also reported grade 3/4 neutropenia (80.0%). However, hematological toxicity was reversible and manageable. The most common grade 3/4 nonhematological toxicities were fatigue (20.0%), peripheral neuropathy (20.0%) and appetite loss (20.0%). No patients withdrew from treatment, and favorable compliance was achieved in the study. The results indicated that eribulin and trastuzumab have the potential to be one of the drugs for treatment of recurrent breast cancer.

Pharmacodynamics (PD) and pharmacokinetics (PK) of E7389 (eribulin, halichondrin B analog) during a phase I trial in patients with advanced solid tumors: a California Cancer Consortium trial

BACKGROUND

The California Cancer Consortium completed a phase I trial of E7389 (eribulin mesylate), an analog of the marine natural product halichondrin B. This trial was to determine the pharmacodynamics, pharmacokinetics, and MTD of E7389 administered by bolus injection weekly for 3 weeks out of four.

METHODS

This trial included a rapid titration design. Real-time pharmacokinetics were utilized to guide dose escalation. Initially, single-patient cohorts were enrolled with intra- and inter-patient dose doubling. The second phase was a standard 3 + 3 dose escalation schedule. At the MTD, a cohort of patients was enrolled for target validation studies (separate manuscript). The starting dose was 0.125 mg/m(2), and doses were doubled within and between patients in the first phase. Blood and urine sampling for E7389 pharmacokinetics was performed on doses 1 and 3 of cycle 1. Levels were determined using a LC/MS/MS assay.

RESULTS

Forty patients were entered. Thirty-eight were evaluable for toxicity and 35 for response. The rapid escalation ended with a grade 3 elevation of alkaline phosphatase at 0.5 mg/m(2)/week. The second phase ended at 2.0 mg/m(2)/week with dose-limiting toxicities of grades 3 and 4 febrile neutropenia. Other toxicities included hypoglycemia, hypophosphatemia, and fatigue. The MTD was 1.4 mg/m(2)/week. Responses included four partial responses (lung cancer [2], urothelial [1], and melanoma [1]).

CONCLUSIONS

E7389 was well tolerated in this trial with the major toxicity being myelosuppression. PD shows that E7389 induces significant morphologic changes (bundle formation) in the microtubules of peripheral blood mononuclear cells and tumor cells in vivo. The data suggest that lower intra-tumoral levels of β-tubulin III or higher intra-tumoral levels of MAP4 may correlate with response to E7389, while lower intra-tumoral levels of stathmin may be associated with progression. PK data reveal that E7389 exhibits a tri-exponential elimination from the plasma of patients receiving a rapid i.v. infusion. At sub-toxic doses, plasma concentrations of E7389 are maintained well above the levels required for activity in vitro for >72 h.

Antibody-drug conjugates as novel anti-cancer chemotherapeutics

Over the past couple of decades, antibody-drug conjugates (ADCs) have revolutionized the field of cancer chemotherapy. Unlike conventional treatments that damage healthy tissues upon dose escalation, ADCs utilize monoclonal antibodies (mAbs) to specifically bind tumour-associated target antigens and deliver a highly potent cytotoxic agent. The synergistic combination of mAbs conjugated to small-molecule chemotherapeutics, via a stable linker, has given rise to an extremely efficacious class of anti-cancer drugs with an already large and rapidly growing clinical pipeline. The primary objective of this paper is to review current knowledge and latest developments in the field of ADCs. Upon intravenous administration, ADCs bind to their target antigens and are internalized through receptor-mediated endocytosis. This facilitates the subsequent release of the cytotoxin, which eventually leads to apoptotic cell death of the cancer cell. The three components of ADCs (mAb, linker and cytotoxin) affect the efficacy and toxicity of the conjugate. Optimizing each one, while enhancing the functionality of the ADC as a whole, has been one of the major considerations of ADC design and development. In addition to these, the choice of clinically relevant targets and the position and number of linkages have also been the key determinants of ADC efficacy. The only marketed ADCs, brentuximab vedotin and trastuzumab emtansine (T-DM1), have demonstrated their use against both haematological and solid malignancies respectively. The success of future ADCs relies on improving target selection, increasing cytotoxin potency, developing innovative linkers and overcoming drug resistance. As more research is conducted to tackle these issues, ADCs are likely to become part of the future of targeted cancer therapeutics.

A retrospective safety and efficacy analysis of the first patients treated with eribulin for metastatic breast cancer in Stockholm, Sweden

BACKGROUNDS

Eribulin is a non-taxane, microtubule dynamics inhibitor approved for the treatment of patients with metastatic breast cancer (MBC) in Europe in March 2011.

MATERIAL AND METHODS

For the purpose of an internal quality control, all patients with MBC treated with eribulin at Karolinska University Hospital were registered in a database. Clinical data were collected retrospectively for patients that were registered by August 2012 and safety and efficacy of eribulin were evaluated. Treatment toxicity including fatigue, neurotoxicity and infection was graded according to CTCAE v4.0. Objective response to treatment was investigated using routinely performed radiological assessments. When only clinical assessments were made, the evaluation of the treating physician was used. Furthermore, the efficacy of eribulin was investigated in different tumor subtypes.

RESULTS

Forty-eight patients who received at least one cycle of eribulin were identified. Most patients were heavily pretreated with a median of 3 (range 1-7) previous chemotherapy lines prior to eribulin. Median patient age was 56 years (range 35-74). At the end of the analysis, 23 patients were alive and two were still treated with eribulin. No hypersensitivity reactions and no toxic deaths were seen. Fatigue grade 3-4 was observed in three patients (6.3%). One patient experienced grade 4 neurotoxicity. Grade 3-4 neutropenia was documented in 18.8%, and three patients were treated for a grade 3 infection. Interestingly, three individuals developed Herpes zoster reactivation. One patient responded to treatment with complete remission, while 33.3% had a partial response. 48% of all patients had a clinical benefit (objective response or stable disease for more than six months).

CONCLUSIONS

Eribulin administered outside of a clinical trial in patients with advanced breast cancer was safe and well tolerated. A clinical benefit was seen in half of the cases. No statistically significant differences in objective response or survival were observed between histopathological subgroups.

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.

Practical stereoselective synthesis of eribulin fragment toward building a hybrid macrocyclic toolbox

A practical stereoselective synthesis to obtain the substituted furan ring as the substructure of eribulin is developed. An asymmetric syn-aldol and intramolecular oxy-Michael were two key steps in our approach. The functionalized furan derivatives were then utilized further to build the 14- and 12-membered macrocyclic diversity as trans- and cis-fused (C-29 and C-30) compounds. This is the first report of building a chemical toolbox with macrocyclic small molecules having trans- or cis-fused 14- or 12-membered rings containing the substructure of eribulin and its diastereomer.

Natural antitubulin agents: importance of 3,4,5-trimethoxyphenyl fragment

Microtubules are polar cytoskeletal filaments assembled from head-to-tail and comprised of lateral associations of α/β-tubulin heterodimers that play key role in various cellular processes. Because of their vital role in mitosis and various other cellular processes, microtubules have been attractive targets for several disease conditions and especially for cancer. Antitubulin is the most successful class of antimitotic agents in cancer chemotherapeutics. The target recognition of antimitotic agents as a ligand is not much explored so far. However, 3,4,5-trimethoxyphenyl fragment has been much highlighted and discussed in such type of interactions. In this review, some of the most important naturally occurring antimitotic agents and their interactions with microtubules are discussed with a special emphasis on the role of 3,4,5-trimethoxyphenyl unit. At last, some emerging naturally occurring antimitotic agents have also been tabulated.

A new tubulin-binding site and pharmacophore for microtubule-destabilizing anticancer drugs

The recent success of antibody-drug conjugates (ADCs) in the treatment of cancer has led to a revived interest in microtubule-destabilizing agents. Here, we determined the high-resolution crystal structure of the complex between tubulin and maytansine, which is part of an ADC that is approved by the US Food and Drug Administration (FDA) for the treatment of advanced breast cancer. We found that the drug binds to a site on β-tubulin that is distinct from the vinca domain and that blocks the formation of longitudinal tubulin interactions in microtubules. We also solved crystal structures of tubulin in complex with both a variant of rhizoxin and the phase 1 drug PM060184. Consistent with biochemical and mutagenesis data, we found that the two compounds bound to the same site as maytansine and that the structures revealed a common pharmacophore for the three ligands. Our results delineate a distinct molecular mechanism of action for the inhibition of microtubule assembly by clinically relevant agents. They further provide a structural basis for the rational design of potent microtubule-destabilizing agents, thus opening opportunities for the development of next-generation ADCs for the treatment of cancer.

Preparation of new alkyne-modified ansamitocins by mutasynthesis

The preparation of alkyne-modified ansamitocins by mutasynthetic supplementation of Actinosynnema pretiosum mutants with alkyne-substituted aminobenzoic acids is described. This modification paved the way to introduce a thiol linker by Huisgen-type cycloaddition which can principally be utilized to create tumor targeting conjugates. In bioactivity tests, only those new ansamitocin derivatives showed strong antiproliferative activity that bear an ester side chain at C-3.

Ansamitocin P3 depolymerizes microtubules and induces apoptosis by binding to tubulin at the vinblastine site

Maytansinoid conjugates are currently under different phases of clinical trials and have been showing promising activity for various types of cancers. In this study, we have elucidated the mechanism of action of ansamitocin P3, a structural analogue of maytansine for its anticancer activity. Ansamitocin P3 potently inhibited the proliferation of MCF-7, HeLa, EMT-6/AR1 and MDA-MB-231 cells in culture with a half-maximal inhibitory concentration of 20±3, 50±0.5, 140±17, and 150±1.1 pM, respectively. Ansamitocin P3 strongly depolymerized both interphase and mitotic microtubules and perturbed chromosome segregation at its proliferation inhibitory concentration range. Treatment of ansamitocin P3 activated spindle checkpoint surveillance proteins, Mad2 and BubR1 and blocked the cells in mitotic phase of the cell cycle. Subsequently, cells underwent apoptosis via p53 mediated apoptotic pathway. Further, ansamitocin P3 was found to bind to purified tubulin in vitro with a dissociation constant (K_d) of 1.3±0.7 µM. The binding of ansamitocin P3 induced conformational changes in tubulin. A docking analysis suggested that ansamitocin P3 may bind partially to vinblastine binding site on tubulin in two different positions. The analysis indicated that the binding of ansamitocin P3 to tubulin is stabilized by hydrogen bonds. In addition, weak interactions such as halogen-oxygen interactions may also contribute to the binding of ansamitocin P3 to tubulin. The study provided a significant insight in understanding the antiproliferative mechanism of action of ansamitocin P3.

From micrograms to grams: scale-up synthesis of eribulin mesylate

The synthesis of eribulin mesylate from microgram to multi-gram scale is described in this Highlight. Key coupling reactions include formation of the C30a to C1 carbon-carbon bond and macrocyclic ring closure through an intramolecular Nozaki-Hiyama-Kishi reaction.

Eribulin for advanced breast cancer: a drug evaluation

Eribulin is a synthetic microtubule dynamics inhibitor that was developed from a marine natural product halichondrin B. It exhibited in vitro and in vivo activities against a wide number of malignancies. A number of advanced phase trials showed improved survival following eribulin treatment in pretreated advanced breast cancer patients. This review provides an overview of the background to the therapeutic use of eribulin in oncology, including its pharmacology, pharmacokinetics, clinical efficacy, safety, and potential economic factors.

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.

Monoclonal antibody-based therapies in cancer: advances and challenges

Conventional anticancer therapeutics often suffer from lack of specificity, resulting in toxicities to normal healthy tissues and poor therapeutic index. Antibody-mediated delivery of anticancer drugs or toxins to tumor cells through tumor selective or overexpressed antigens is progressively being recognized as an effective strategy for increasing the therapeutic index of anticancer drugs. In this review we focus on three therapeutic modalities in the field of antibody-mediated targeting, including antibody-drug conjugates (ADCs), immunotoxins (ITs) and immunoliposomes (ILs). Design considerations for development of each of the above therapeutic modalities are discussed. Furthermore, an overview of ADCs, ITs or ILs approved for use in clinical oncology and those currently in clinical development is provided. Challenges encountered by the field of antibody-based targeting are discussed and concepts around development of the next generation of antibody therapeutics are presented.

Synthesis and evaluation of novel anti-proliferative pyrroloazepinone and indoloazepinone oximes derived from the marine natural product hymenialdisine

The tetrahydroazepinone pharmacophore is a component of many interesting compounds, including several marine natural products, with anti-cancer properties. The synthesis and biological evaluation of a novel series of pyrroloazepinone and indoloazepinone oximes is reported. These compounds showed promising growth inhibition activity against four human cancer cell lines but did not significantly inhibit the cell cycle regulator cyclin dependent kinase 2. The most active compounds in this series displayed improved anti-proliferative activity over the related synthetic indoloazepine kenpaullone. The structure activity relationships exhibited by the azepinone pharmacophore suggests several novel lead compounds for anti-cancer drug discovery.

Unprecedented deoxygenation at C-7 of the ansamitocin core during mutasynthetic biotransformations

We describe the unprecedented formation of six ansamitocin derivatives that are deoxygenated at C-7 of the ansamitocin core, obtained during fermentation experiments by employing a variety of Actinosynnema pretiosum mutants and mutasynthetic approaches. We suggest that the formation of these derivatives is based on elimination at C-7/C-8 followed by reduction(s) of the intermediate enone. In bioactivity tests, only ansamitocin derivatives bearing an ester side chain at C-3 showed strong antiproliferative activity.

Photochemical oxazole-nitrile conversion downstream of rhizoxin biosynthesis and its impact on antimitotic activity

Through metabolic profiling of mutants and wild type of the endofungal bacterium Burkholderia rhizoxinica two novel rhizoxin derivatives with unusual nitrile substitutions were discovered. The nitrile groups result from a photochemical oxidative cleavage of the oxazolyl moiety. In vitro studies revealed that the photooxidation by singlet oxygen also takes place in the absence of a photosensitizer, and that also a thiazolyl-substituted rhizoxin analogue undergoes the same transformation. The resulting nitriles have antimitotic properties but are significantly less active than the parent compounds. These results highlight the impact of photoreactions onto the antiproliferative agent and encourage the introduction of bioisosteric groups that render the compound less susceptible towards photooxidation.

Potential clinical applications of halichondrins in breast cancer and other neoplasms

Halichondrin B is a large polyether macrolide found in a rare Japanese sponge, Halichondria okadai and has been shown to have anticancer activity. Eribulin mesylate is a completely synthetic analog of halichondrin B with a unique mechanism of action relative to other antimicrotubule agents. This new agent has demonstrated activity in preclinical studies, and it is being developed for the treatment of different tumor types. Eribulin has been approved by the United States Food and Drug Administration and the European Medicines Agency as late-line therapy for metastatic breast cancer patients previously treated with an anthracycline and a taxane. It has demonstrated superiority over other treatments in overall survival (OS) (hazard ratio: 0.81, P = 0.041), leading to its regulatory approbation for clinical practice use. Median OS for the eribulin-treated group was 13.1 months versus 10.6 months in the physician's treatment-of-choice group. Eribulin demonstrated a manageable toxicity profile. Most common adverse events associated with treatment were mild neutropenia and fatigue, mainly of grade 1 or 2. In contrast to other antimicrotubule agents, eribulin has a relatively low incidence of peripheral neuropathy and alopecia. Eribulin has been extensively studied in breast cancer and is currently being developed for treatment of other cancer types. Eribulin has demonstrated activity in Phase II trials in non-small cell lung cancer, pancreatic cancer, urothelial tract cancer, and sarcomas. Further studies in these cancers are ongoing. This article reviews pharmacology, mechanism of action, pharmacokinetics and efficacy of eribulin in breast cancer and other neoplasms.

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.

Eribulin mesylate as a microtubule inhibitor for treatment of patients with metastatic breast cancer

Metastatic breast cancer (MBC) remains an incurable disease, with the goals of care aimed at maximizing the patient's duration and quality of life. Treatment options for MBC have become more efficacious and numerous. In addition to endocrine and chemotherapy agents, a number of targeted agents, including trastuzumab and bevacizumab, have further enhanced the landscape of therapeutic options. Eribulin mesylate (E7389) is a nontaxane microtubule dynamics inhibitor, and a structurally simplified synthetic analog of the natural marine product, halichondrin B, with a novel mechanism of action that has shown antitumor activity in pretreated MBC. Eribulin has shown a manageable tolerability profile in Phase I-II clinical trials and an improvement in overall survival compared with treatment of physician's choice, without relevant toxicities in a recently published Phase III trial. This review will focus on eribulin as a new active agent for MBC and its role in the management of breast disease.