Anticancer drugs from nature--natural products as a unique source of new microtubule-stabilizing agents

Microtubule stabilising agents and ionising radiation: multiple exploitable mechanisms for combined treatment

Combined radiochemotherapy treatment modalities are in use for many indications and therefore of high interest. Even though a combined modality in clinical use is often driven by pragmatic aspects, mechanistic preclinical-based concepts of interaction are of importance in order to translate and implement an optimal combination and scheduling of two modalities into the clinics. The use of microtubule stabilising agents is a promising strategy for anti-cancer therapy as a part of combined treatment modality with ionising radiation. Traditionally, microtubule targeting agents are classified as cytotoxic chemotherapeutics and are mostly used in a maximally tolerated dose regimen. Apart from direct cytotoxicity and similar to mechanisms of molecular targeting agents, microtubule stabilising agents interfere with multiple cellular processes, which can be exploited as part of combined treatment modalities. Recent preclinical investigations on the combination of ionising radiation and microtubule stabilising agents reveal new mechanistic interactions on the cellular and tumour level and elucidate the supra-additive tumour response observed particularly in vivo. The major focus on the mechanism of interaction was primarily based on radiosensitisation due to cell cycle arrest in the most radiosensitive G2/M-phase of the cell cycle. However, other mechanisms of interaction such as reoxygenation and direct as well as indirect endothelial damage have also been identified. In this review we summarise and allocate additive and synergistic effects induced by the combined treatment of clinically relevant microtubule stabilising agents and ionising radiation along a described radiobiological framework encompassing distinct mechanisms relevant for exploiting the combination of drugs and ionising radiation.

Cytotoxic oleane-type triterpene saponins from Glochidion eriocarpum

The anticancer activity of ten compounds from the aerial parts of Glochidion eriocarpum were evaluated on two human cancer cell lines, HL-60 and HCT-116. Compounds 1-4 displayed highly potent cytotoxic activity on the HCT-116 cancer cell line with IC(50) values ranging of 0.41∼1.16 μM. Compounds 1-4 significantly inhibited the HL-60 cell line with IC(50) values ranging of 4.51∼6.33 μM. These results suggested that the benzoyl group at the C-22 position in oleane-type triterpene saponins was essential for cytotoxicity towards tumor cells. Moreover, compounds 2 and 3 showed more potent cytotoxicity than compounds 1 and 4 against HL-60 and HCT-116 cells. With respect to the mechanism underlying cytotoxicity, compounds 1-4 increased chromatin condensation, a typical apoptotic characteristic in HL-60 and HCT-116 cells. In the mechanism of apoptosis induction, compounds 1-4 reduced Bcl-2 expression, whereas the expression of Bax was increased compared to controls in HCT-116 cells. In addition, compounds 1-4 decreased the level of procaspase-3. The cleavage of poly (ADP-ribose) polymerase (PARP), a vital substrate of effector caspase, was observed in HCT-116 cells. Furthermore, the induction of apoptosis was also accompanied by an activation of extracellular signal-regulated kinase (ERK) and p38 kinase in HCT-116 cells. These findings provide evidence demonstrating that the pro-apoptotic effects of compounds 1-4 are mediated through the activation of ERK and p38 in HCT-116 cells.

Plant immunostimulants--scientific paradigm or myth?

In traditional medicine, numerous plant preparations are used to treat inflammation both topically and systemically. Several anti-inflammatory plant extracts and a few natural product-based monosubstances have even found their way into the clinic. Unfortunately, a number of plant secondary metabolites have been shown to trigger detrimental pro-allergic immune reactions and are therefore considered to be toxic. In the phytotherapy research literature, numerous plants are also claimed to exert immunostimulatory effects. However, while the concepts of plant-derived anti-inflammatory agents and allergens are well established, the widespread notion of immunostimulatory plant natural products and their potential therapeutic use is rather obscure, often with the idea that the product is some sort of "tonic" for the immune system without actually specifying the mechanisms. In this commentary it is argued that the paradigm of oral plant immunostimulants lacks clinical evidence and may therefore be a myth, which has originated primarily from in vitro studies with plant extracts. The fact that no conclusive data on orally administered immunostimulants can be found in the scientific literature inevitably prompts us to challenge this paradigm.

Leiodermatolide, a potent antimitotic macrolide from the marine sponge Leiodermatium sp

Leiodermatolide is a structurally unique macrolide, isolated from the deep-water marine sponge Leiodermatium sp ., which exhibits potent antiproliferative activity against a range of human cancer cell lines (IC50 <10 nM) and dramatic effects on spindle formation in mitotic cells. Its unprecedented polyketide skeleton and stereochemistry were established using a combination of experimental and computational (DP4) NMR methods, and molecular modelling.

Design and synthesis of pironetin analogues with simplified structure and study of their interactions with microtubules

The preparation of a series of pironetin analogues with simplified structure is described. Their cytotoxic activity and their interactions with tubulin have been investigated. It has been found that, while less active than the parent molecule, the pironetin analogues still share the mechanism of action of the latter and compete for the same binding site to α-tubulin. Variations in the configurations of their stereocenters do not translate into relevant differences between biological activities.

Total synthesis and biological evaluation of a series of macrocyclic hybrids and analogues of the antimitotic natural products dictyostatin, discodermolide, and taxol

The design, synthesis, and biological evaluation of a series of hybrids and analogues of the microtubule-stabilizing anticancer agents dictyostatin, discodermolide, and taxol is described. A 22-membered macrolide scaffold was prepared by adapting earlier synthetic routes directed towards dictyostatin and discodermolide, taking advantage of the distinctive structural and stereochemical similarities between these two polyketide-derived marine natural products. Initial endeavors towards accessing novel discodermolide/dictyostatin hybrids led to the adoption of a late-stage diversification strategy and the construction of a small library of methyl-ether derivatives, along with the first triple hybrids bearing the side-chain of taxol or taxotere attached through an ester linkage. Biological assays of the anti-proliferative activity of these compounds in a series of human cancer cell lines, including the taxol-resistant NCI/ADR-Res cell line, allowed the proposal of various structure-activity relationships. This led to the identification of a potent macrocyclic discodermolide/dictyostatin hybrid 12 and its C9 methoxy derivative 38, accessible by an efficient total synthesis and with a similar biological profile to dictyostatin.

Design and synthesis of novel tetrahydro-2H-Pyrano[3,2-c]pyridazin-3(6H)-one derivatives as potential anticancer agents

Polyfunctional tetrahydro-2H-pyrano[3,2-c]pyridazin-3(6H)-one derivatives were synthesized and biologically evaluated as novel anticancer agents. These motifs were produced by a five-step reaction sequence in which the Achmatowicz oxidative cyclization, is the basic core for such synthesis. Compounds 15f, 16c, and 16d showed antiproliferative activity against the SK-BR-3 breast cancer cell line. Importantly, 16c and 16d showed the highest efficacy, being approximately 30-fold more potent against SK-BR-3 (IC50 0.21 and 0.15 μM, respectively) compared to other cancer cell lines tested. In addition, 16c and 16d displayed about 295 fold less toxicity against normal breast cell line MCF10A compared to SK-BR-3 breast cancer cells. These compounds form the foundation for further investigation in our continuing efforts to develop potent anticancer agents.

Anti-cancer effect of rubropunctatin against human gastric carcinoma cells BGC-823

The Monascus pigment, rubropunctatin, was extracted and purified from red mold rice (RMR) and its cytotoxic activities against human gastric adenocarcinoma BGC-823 cells were studied both in vitro and in vivo. Rubropunctatin inhibited the proliferation of BGC-823 cells with an inhibitory concentration (IC₅₀) of 12.57 μM, while it exhibited no significant toxicity to normal gastric epithelial cell GES-1 at the same concentration. Treatment of BGC-823 cells with rubropunctatin resulted in a dose- and time-dependent apoptosis, as validated by the increase in the percentage of cells in sub-G1 phase and phosphotidylserine externalization. The in vivo experimental data demonstrated that rubropunctatin could offer similar therapeutic benefits in comparison with the same dose of taxol. After five times of intravenous injection, tumor weight in BGC-823-bearing nude mice reduced 23.5% at the dose of 8 mg/kg and 37.7% at the dose of 32 mg/kg, respectively. The expressions of 30 genes related to induction of apoptosis were found up-regulated significantly. The two most expressed genes were tumor necrosis factor (TNF) and DNA-damage inducible transcript 3. TNF was considered as a major mediator of apoptosis induced by rubropunctatin. This is the first report describing the anti-proliferative effect of rubropunctatin and its apoptosis mechanism on BGC-823 cells. Rubropunctatin has potential to be developed as a new natural anti-cancer agent.

Design, synthesis and qualitative structure-activity evaluations of novel hexahydropyrano[3,2-c][1,2]diazepin-3(4H)-one and tetrahydropyrano[3,2-b]pyrrol-2(1H)-one derivatives as anticancer agents

Polysubstituted hexahydropyrano[3,2-c][1,2]diazepin-3(4H)-one and tetrahydropyrano[3,2-b]pyrrol-2(1H)-one derivatives were synthesized and biologically evaluated as novel anticancer agents. These motifs were produced by five steps reaction sequence in which Achmatowicz oxidative cyclization, is the basic protocol for such synthesis. To understand the structure-activity relationships of the newly synthesized motifs, two traditional medicinal chemistry strategies namely, ring expansion and contraction, were followed in this article. These studies indicated that tetrahydropyrano[3,2-b]pyrrol-2(1H)-one derivatives are more selective for breast cancer cell line compared to other cell lines under investigations. Furthermore, it was found that hexahydropyrano[3,2-c][1,2]diazepin-3(4H)-one derivatives are potent anticancer agents compared to tetrahydropyrano[3,2-b]pyrrol-2(1H)-one analogs. These findings, however, form the foundation for further investigation in our continuing efforts to develop selective anticancer agents.

Total synthesis and configurational assignment of chondramide A

The first total synthesis of the cyclodepsipeptide chondramide A (2 b) is described. This depsipeptide is composed of four subunits, namely L-alanine, N-Me-D-tryptophan, 3-amino-2-methoxy-propionic acid (beta-tyrosine derivative), and a 7-hydroxy-alkenoic acid. While the configuration of the stereogenic centers in the 7-hydroxy-alkenoic acid were known, the configuration of the tyrosine derivative required clarification and turned out to be (2S,3R) or (2L,3L), respectively. The synthesis of the 3-amino-2-methoxy-3-arylpropanoic ester 20 b relied on an asymmetric dihydroxylation yielding diol ent-15 a followed by a regioselective Mitsunobu substitution leading to 3-azido-2-hydroxypropanoate 18 b. We could also show that the ester bond in the seco compound 26 b can be fashioned by a Mitsunobu esterification by using hydroxy ester (7S)-7 and the tripeptide acid 25 b. This synthesis should allow for the preparation of various analogues.

Microtubule-stabilizing drugs from marine sponges: focus on peloruside A and zampanolide

Marine sponges are an excellent source of bioactive secondary metabolites with potential therapeutic value in the treatment of diseases. One group of compounds of particular interest is the microtubule-stabilizing agents, the most well-known compound of this group being paclitaxel (Taxol), an anti-cancer compound isolated from the bark and leaves of the Pacific yew tree. This review focuses on two of the more recent additions to this important class of drugs, peloruside A and zampanolide, both isolated from marine sponges. Peloruside A was isolated from Mycale hentscheli collected in New Zealand coastal waters, and it already shows promising anti-cancer activity. Two other potent bioactive compounds with different modes of action but isolated from the same sponge, mycalamide A and pateamine, will also be discussed. The fourth compound, zampanolide, most recently isolated from the Tongan sponge Cacospongia mycofijiensis, has only recently been added to the microtubule-stabilizing group of compounds, and further work is in progress to determine its activity profile relative to peloruside A and other drugs of this class.

Epothilone analogues with benzimidazole and quinoline side chains: chemical synthesis, antiproliferative activity, and interactions with tubulin

A series of epothilone B and D analogues bearing isomeric quinoline or functionalized benzimidazole side chains has been prepared by chemical synthesis in a highly convergent manner. All analogues have been found to interact with the tubulin/microtubule system and to inhibit human cancer cell proliferation in vitro, albeit with different potencies (IC(50) values between 1 and 150 nM). The affinity of quinoline-based epothilone B and D analogues for stabilized microtubules clearly depends on the position of the N-atom in the quinoline system, while the induction of tubulin polymerization in vitro appears to be less sensitive to N-positioning. The potent inhibition of human cancer cell growth by epothilone analogues bearing functionalized benzimidazole side chains suggests that these systems might be conjugated with tumor-targeting moieties to form tumor-targeted prodrugs.

Making epothilones fluoresce: design, synthesis, and biological characterization of a fluorescent N12-aza-epothilone (azathilone)

A green fluorescent 12-aza-epothilone (azathilone) derivative has been prepared through the attachment of the 4-nitro-2,1,3-benzoxadiazole (NBD) fluorophore to the 12-nitrogen atom of the azamacrolide core structure. While less potent than natural epothilones or different N12-acylated azathilone derivatives, NBD-azathilone (3) promotes tubulin assembly, inhibits cancer cell proliferation in vitro and arrests the cell cycle at the G2/M transition. Most significantly, the binding of 3 to cellular microtubules (MTs) could be directly visualized by confocal fluorescence microscopy. Based on competition binding experiments with laulimalide-stabilized MTs in vitro, the N12-Boc substituted azathilone 1, Epo A, and NBD-azathilone (3) all interact with the same tubulin-binding site. Computational studies provided a structural model of the complexes between beta-tubulin and 1 or 3, respectively, in which the NBD moiety of 3 or the BOC moiety of 1 directly and specifically contribute to MT binding. Collectively, these data demonstrate that the cellular effects of 3 and, by inference, also of other azathilones are the result of their interactions with the cellular MT network.

Antitumor effects of ursolic acid in a mouse model of postmenopausal breast cancer

Over the past two decades, bioactive natural compounds have been shown to be a plausible adjunct to the treatment of breast cancer, the second leading cause of cancer death among American women. This study was designed to investigate the effects of ursolic acid (UA), a pentacyclic triterpene found in many foods and herbs, in a model of postmenopausal breast cancer. Ovariectomized C57BL/6 mice (n = 40) were randomized to receive control diet (AIN-93G) or diet supplemented with UA at 1 of 3 doses (wt/wt): 0.05%, 0.10%, or 0.25% (≈54, 106, or 266 mg/kg body weight/day, respectively). After 3 wk, syngeneic MMTV-Wnt-1 mammary tumor cells were injected in the mammary fat pad, and mice continued on their respective diets for 5 more wk. All UA doses decreased tumor cell proliferation, as assessed by Ki67 immunostaining; nevertheless, UA at 0.10% was most effective in inhibiting tumor take and decreasing tumor final tumor size. Modulation of Akt/mTOR signaling and induction of apoptosis appeared to mediate these effects on tumor growth. UA potently disrupted cell cycle progression and induced necrosis in a clonal MMTV-Wnt-1 mammary tumor cell line in vitro. This study supports the potential of UA as an antitumorigenic agent.

Total synthesis of a library of designed hybrids of the microtubule-stabilising anticancer agents taxol, discodermolide and dictyostatin

A hybrid library of the marine natural products dictyostatin and discodermolide, incorporating the taxol or taxotere side chains, were synthesised; preliminary biological evaluation in the PANC-1 cancer cell line revealed significant antiproliferative activity, demonstrating that a macrolide scaffold is an effective surrogate for the baccatin core of taxol.

Natural product scaffolds as leads to drugs

Numerous ‘scaffolds’ that have been identified in natural product structures have led to very significant numbers of approved drugs and drug candidates for a multiplicity of diseases over the years. In this mini-review, we discuss the base scaffolds (chemical skeletons) that we feel have produced very significant numbers of agents as drugs or drug leads and, in a number of cases, compounds that can be used as chemical synthons or that present activities in biological areas that were not obvious from their earlier history.

Anticancer drug discovery in the future: an evolutionary perspective

Identification of agents that are pharmacologically active against human cancer has depended largely on the screening of natural products and their analogs. Many anticancer drugs have been discovered fortuitously through random investigation of organisms; indeed, serendipity remains important in anticancer drug discovery. Although it is broadly accepted that cancers comprise an evolutionary microcosm, this idea has not been advanced to understand and control carcinogenic progression. Here, we address anticancer drug discovery from an evolutionary perspective and present a series of case studies that demonstrate that the rate of anticancer drug discovery can be increased greatly by targeted screening of natural compounds from ancient species.

Tubulin-interactive natural products as anticancer agents

This review provides an overview of the discovery, structures, and biological activities of anticancer natural products that act by inhibiting or promoting the assembly of tubulin to microtubules. The emphasis is on providing recent information on those compounds in clinical use or in advanced clinical trials. The vinca alkaloids, the combretastatins, NPI-2358, the halichondrin B analogue eribulin, dolastatin 10, noscapine, hemiasterlin, and rhizoxin are discussed as tubulin polymerization inhibitors, while the taxanes and the epothilones are the major classes of tubulin polymerization promoters presented, with brief treatments of discodermolide, eleutherobin, and laulimalide. The challenges and future directions of tubulin-interactive natural products-based drug discovery programs are also discussed briefly.

How scientific is the science in ethnopharmacology? Historical perspectives and epistemological problems

This commentary is based on a general concern regarding the low level of self-criticism (-evaluation) in the interpretation of molecular pharmacological data published in ethnopharmacology-related journals. Reports on potentially new lead structures or pharmacological effects of medicinal plant extracts are mushrooming. At the same time, nonsense in bioassays is an increasing phenomenon in herbal medicine research. Only because a dataset is reproducible does not imply that it is meaningful. Currently, there are thousands of claims of pharmacological effects of medicinal plants and natural products. It is argued that claims to knowledge in ethnopharmacology, as in the exact sciences, should be rationally criticized if they have empirical content as it is the case with biochemical and pharmacological analyses. Here the major problem is the misemployment of the concentration-effect paradigm and the overinterpretation of data obtained in vitro. Given the almost exponential increase of scientific papers published it may be the moment to adapt to a falsificationist methodology.

Isolation, biology and chemistry of the disorazoles: new anti-cancer macrodiolides

Covering: 1994 to 2008. The disorazoles comprise a family of 29 closely related macrocyclic polyketides isolated in 1994 from the fermentation broth of the gliding myxobacterium Sorangium cellulosum. Disorazoles A1, E and C1 have shown exceptional biological activites toward inhibiting the proliferation of human cancer cell lines in picomolar and nanomolar concentrations through the disruption of microtubule polymerization. This review gives a brief introduction describing the biosynthesis and the significance of the disorazoles as a new class of microtubulin disruptors. Another portion of the review focuses on the biology of the disorazoles, specifically disorazole A1 and C1, and their antiproliferative efficacy against animal and human tumor cell lines, as well as the available SAR data. The majority of the discussion addresses synthetic efforts, including partial syntheses of various disorazoles and a summary of the total synthesis of disorazole C1.

De(side chain) model of epothilone: bioconformer interconversions DFT study

Using ab initio methods, we have studied conformations of the de(sidechain)de(dioxy)difluoroepothilone model to quantify the effect of stability change between the exo and endo conformers of the epoxy ring. The DFT minimization of the macrolactone ring reveals four low energy conformers, although MP2 predicted five stable structures. The model tested with DFT hybride functional (B3LYP/6-31+G(d,p)) exhibits the global minimum for one of the exo forms (C), experimentally observed in the solid state, but unexpectedly with the MP2 electron correlation method for the virtual endo form (W). Using the QST3 technique, several pathways were found for the conversion of the low energy conformers to the other low energy exo representatives, as well as within the endo analog subset. The potential energy relationships obtained for several exo forms suggest a high conformational mobility between three, experimentally observed, conformers. The high rotational barrier, however, excludes direct equilibrium with experimental EC-derived endo form S. The highest calculated transition state for the conversion of the most stable exo M interligand to the endo S form is approximately a 28 kcal/mol above the energy of the former. The two-step interconversion of the exo H conformer to the endo S requires at least 28 kcal/mol. Surprisingly, we found that the transition state energy of the H form to the virtual endo W has the acceptable value of about 9 kcal/mol and the next energy barrier for free interconversion of endo W to endo S is 13 kcal/mol.

Total synthesis of novel dictyostatin analogs and hybrids as microtubule-stabilizing anticancer agents

Structural modification of the dictyostatin macrolide template through adaptation of our total synthesis has led to the identification of a number of potent analogs of this novel microtubule-stabilizing agent. A common synthetic strategy was exploited, employing a (Z)-selective Still-Gennari olefination between various advanced C11-C26 aldehyde and C4-C10 (or C1-C10) β-ketophosphonate intermediates. In vitro evaluation of the growth inhibitory activity of these analogs against both Taxol-sensitive and -resistant human cancer cell lines has provided a foundation for structure-activity relationship (SAR) studies to help define the pharmacophore region.