Synthetic and natural coumarins as cytotoxic agents

Coumarins, an old class of compounds, are naturally occurring benzopyrene derivatives. A lot of coumarins have been identified from natural sources, especially green plants. The pharmacological and biochemical properties and therapeutic applications of simple coumarins depend upon the pattern of substitution. Coumarins have attracted intense interest in recent years because of their diverse pharmacological properties. Among these properties, their cytotoxic effects were most extensively examined. In this review, their broad range of effects on the tumors as shown by various in vitro and in vivo experiments and clinical studies are discussed. Hence, these cytotoxic coumarins represent an exploitable source of new anticancer agents, which might also help addressing side-toxicity and resistance phenomena. These natural compounds have served as valuable leads for further design and synthesis of more active analogues. In this review, plant derived coumarins and their synthetic analogues were systematically evaluated based on their plant origin, structure-activity relationship and anticancer efficacy. Owing the their diverse effects and inconclusive results from different in vitro studies, the mechanism of their action is not yet fully understood and correlation of effects with chemical structures is not conclusive at the moment. It is the objective of this review to summarize experimental data for different coumarins, used as cytotoxic agents, because promising data have been reported for a series of these agents. Yet, the results from different coumarins with various tumor lines are contradictory in part. We therefore conclude that there is still a long way to go until we know which cytotoxic agent will clinically be suitable for what tumor entity for treatment. Their ability to bind metal ions represents an additional means of modulating their pharmacological responses.     

Synthetic and natural products as iron chelators

An evaluation of existing and proposed Fe chelators, both synthetic and natural products, for the treatment of Fe-overload disease must address a number of issues. There are fundamental parameters that determine the efficacy of a drug: absorption, distribution, metabolism, clearance and toxicity. However, the administration of chelator for Fe overload aims to generate Fe complexes in vivo that are able to be excreted. Hence, the chemical and pharmacological properties of the complexes formed are equally important as the chelators themselves. The redox properties of the Fe complexes formed is particularly relevant to their toxicity. If both Fe(II) and Fe(III) oxidation states of the complexes are biologically accessible, then there is potential for the auto-catalytic production of deleterious free radicals, by Fenton-type chemistry. In addition, since the burden of Fe overload disease falls predominantly on some of the poorest economies, the cost of a drug must be considered, as well as the mode of delivery. There are also possible issues with the use of naturally occurring ligands, which may form Fe complexes capable of being utilised by opportunistic bacteria. This review will concentrate on recent developments in our chemical understanding of existing chelators approved or proposed for use and will also consider some of the candidates from natural sources that have been recently proposed.     

天然香豆素的抗癌机制研究进展

已有许多研究报道天然香豆素具有抗癌活性,它的抗癌机制与其具有的多项生理调节作用有关。本文从对细胞周期的影响、诱导细胞凋亡、SAPK/JNK/p38MAP等信号通路及对GSH转硫酶/NADPH醌氧化还原酶（GST/NQO）体系的影响等方面对天然香豆素的抗癌机制进行简要综述。     

Coumarins as antioxidants

Coumarins, a well-known class of naturally occurring compounds, display a remarkable array of biochemical and pharmacological actions, some of which suggest that certain members of this group of compounds may significantly affect the function of various mammalian cellular systems. The development of coumarins as antioxidant agents has attracted much attention in recent years. Coumarins afford an opportunity for the discovery of new antioxidants with truly novel mechanisms of action. This review updates and expands the 2006 review by the same author. The review considers and incorporates the most recently published literature on coumarins as related to their antioxidant properties. A lot of coumarins have been identified from natural sources, especially green plants. These natural compounds have served as valuable leads for further design and synthesis of more active analogues. Beyond doubt, a deep understanding of the mechanisms of existing synthetic and natural coumarins will build the basis for the rational design.     

Synthetic studies on heterocyclic natural products

This article reviews past and ongoing research in the author's laboratory directed toward the synthesis of natural products displaying an azaspirocyclic framework, or incorporating a medium-ring nitrogen heterocycle. New synthetic technologies were devised in order to address the synthetic problems posed by the target molecules. Thus, efforts in the area of azaspirocyclic substances have relied on an oxidative amidation of phenols promoted by iodobenzene diacetate, whereas access to medium-ring nitrogen heterocycles has been secured by means of a ring expansion sequence that relies on the fragmentation of an aziridine triggered by a homo-Brook transposition. Details of the development of these technologies are presented, together with applications to the total synthesis of FR-901483, TAN-1251C, cylindricines, and mitomycinoids.     

Synthetic studies of heterocyclic natural products

Recent reports of the syntheses of heterocyclic natural products that are isolated from bacteria, fungi, plants and marine sources are discussed. This review emphasizes new developments in synthetic methodology and strategies for such products, and the potential biological activity of the metabolites that are described is highlighted. Concepts of conformational analysis, stereoelectronic control and reaction mechanism are all used in the development of new methodology. Despite the variety and scope of synthetic efforts, the synthesis of new organic molecules and the improved synthesis of known molecules remains a major task for organic chemists.     

Heteroaryl-substituted phenols as potential antioxidants

A series of O-heteroaryl phenols have been synthesised and structurally characterised. Photo-Fries rearrangement of these compounds represents a useful way to access the corresponding C-heteroaryl derivatives. The activity of the new phenolic compounds as radical scavengers towards the 2,2'-azino-bis (3-ethylbenzothiazoline-6-sulfonate) (ABTS(+*)) has been evaluated. 2-tert-Butyl-4-(4-phenyl-isoxazol-3-ylmethoxy)-phenol (compound 3c) showed the highest scavenger activity (IC50 value (i.e. the concentration that scavenged 50% of the radicals) 3.17 x 10(-6) M), which was one order of magnitude greater than that of the corresponding lead compound tert-butylhydroxy-anisole (BHA) (IC50 1.04 x 10(-5) M). In further experiments, compound 3c showed dose-dependent inhibition of the oxidation of linoleic acid, as well as methaemoglobin formation, promoted by the presence of the radical generator 2,2'-azobis(amidino-propane) hydrochloride (AAPH) and it was markedly more potent than BHA in these assays.     

Polyunsaturated fatty acids as antioxidants

The susceptibility of fatty acids to oxidation is thought to be directly dependent on their degree of unsaturation. However, some in vitro and in vivo studies suggest that the relation between chemical structure and susceptibility to oxidation is not as straightforward as hypothesized from theoretical viewpoints. Indeed, long chain polyunsaturated fatty acids (LC-PUFAs) might be less oxidizable than others under specific experimental conditions. We investigated the free radical-scavenging potential of PUFA and the production of reactive oxygen/nitrogen (ROS/RNS) species by human aortic endothelial cells (HAECs) supplemented with different fatty acids. Fatty acid micelles scavenged superoxide in an unsaturation-dependent manner, up to eicosapentaenoic acid, which was the most effective fatty acid. Supplementation of HAEC with polyunsaturated fatty acids of the omega 3 series resulted in lower formation of ROS, as compared with cells supplemented with saturates, monounsaturates, or polyunsaturates of the omega 6 series. This effect was maximal at concentrations of 10muM. The effects of omega 3 fatty acids on reactive species production appear to be stronger when ROS were evaluated, as a milder, albeit significant effect was observed on RNS generation. Based on in vivo data showing reduced excretion of lipid peroxidation products after omega 3 intake and our data on ROS production and direct superoxide scavenging by LC-PUFAs, notably those of the omega 3 series, we propose that this series of fatty acid might act as indirect anti- rather than pro-oxidant in vascular endothelial cells, hence diminishing inflammation and, in turn, the risk of atherosclerosis and cardiovascular disease.     

Effect of natural antioxidants and apocynin on LPS-induced endotoxemia in rabbit

The objective of this study was to compare the prophylactic effects of the natural antioxidant from spinach (NAO) and apocynin, on the hepatic oxidative stress and liver damage induced by lipopolysaccharide (LPS). Male New Zealand rabbits were challenged with LPS with or without 8 days of antioxidant pretreatment. Pretreatment with NAO, but not apocynin, significantly (p < 0.05) decreased the levels of hydroperoxides and malondialdehyde (MDA) in the liver cytosolic fraction and the activity of NADPH oxidase-generated superoxide in the microsomal fraction, compared to LPS alone. The activity of glutathione peroxidase (G-POX) was significantly (p < 0.05) increased in the LPS-treated group, whereas treatment with NAO, but not apocynin, significantly (p < 0.05) decreased G-POX activity. Pretreatment with the same antioxidants had no significant effects on superoxide dismutase (SOD) activity, whereas an increased level of catalase (CAT) was obtained in all LPS-treated groups. TUNEL immunohistochemical staining in the LPS-treated animals indicated that there was no increase in apoptosis outside of necrotic foci. However, apoptotic hepatocytes were observed within areas of focal necrosis in animals exposed to LPS alone or LPS plus apocynin. Hepatocyte cell proliferation was tested by the proliferating-cell nuclear antigen (PCNA) tool, which indicated a proliferative effect in the LPS group, whereas the effect disappeared in the antioxidant-treated groups. The prophylactic effect of NAO on liver pathology and the significant decreases in lipid peroxidation products and NADPH oxidase activity suggest the use of NAO as an efficient strategy for treatment of endotoxemia.     

天然产物黄连碱的合成工艺改进

目的改进天然产物黄连碱的合成工艺。方法以2,3-二羟基苯甲醛为起始原料,经环合、缩合和闭环3步反应得到目标化合物黄连碱。其中,主要针对第二步缩合反应的纯化步骤以及第三步闭环反应的溶剂选择和纯化过程做了有效的改进工作。结果中间体和目标产物结构经1H-NMR、13C-NMR图谱数据确证,总收率可达43.0%,目标化合物经HPLC检测纯度高达98.5%。结论改进后的合成方法反应步骤少、操作简单、条件温和、产物收率及纯度较高,有利于工业化生产。     

3,6-disubstituted coumarins as mechanism-based inhibitors of thrombin and factor Xa

In this work, coumarins were screened on thrombin (THR) and factor Xa (FXa), two of the most promising targets for the development of anticoagulant drugs. This allowed us to highlight compound 30, characterized by a 2,5-dichlorophenyl ester in the 3-position and a chloromethyl moiety in the 6-position, as a very potent THR inhibitor (ki/KI= 37,000 M(-1) s(-1)). Moreover, this compound exhibits good selectivity over FXa (168-fold) and trypsin (54-fold). The mechanism of inactivation was investigated in this series and significantly differs from that previously observed with alpha-chymotrypsin. Indeed, the addition of hydrazine on the THR-inhibitor complex promotes a partial induced THR reactivation. This reactivation, confirmed by LC/MS, showed the resurgence of the native THR and a new dihydrazide complex. Docking experiments were then efficiently used to explain the trends observed in the enzymatic assays as well as to corroborate the postulated inhibition mechanism. Finally, the cell permeability of our derivatives was estimated using a computational approach.     

Recent advances in coumarins and 1-azacoumarins as versatile biodynamic agents

Coumarins, also referred as benzopyran-2-ones, and their corresponding nitrogen counterpart, 1-azacoumarins also referred to as carbostyrils, are a family of nature-occurring lactones and lactams respectively. The plant extracts containing coumarin-related heterocycles, which were employed as herbal remedies in early days, have now been extensively studied for their biological activities. These investigations have revealed their potentials as versatile biodynamic agents. For example, coumarins with phenolic hydroxyl groups have the ability to scavenge reactive oxygen species and thus prevent the formation of 5-HETE and HHT in the arachidonic pathway of inflammation suppression. Recent in vivo studies have revealed the role of coumarins in hepatotoxicity and also in depletion of cytochrome P450. Similarly 1-azacoumarins which is part of quinoline alkaloids, are known for their diverse biological activity and recently, a 6-functionalized 1-aza coumarins are undergoing human clinical trials as an orally active anti-tumor drug in view of its farnesyl protein-inhibiting activity in the nanomolar range. Furthermore, several synthetic coumarins with a variety of pharmacophoric groups at C-3, C-4 and C-7 positions have been intensively screened for anti-microbial, anti-HIV, anti-cancer, lipid-lowering, anti-oxidant, and anti-coagulation activities. Specifically, coumarin-3-sulfonamides and carboxamides were reported to exhibit selective cytotoxicity against mammalian cancer cell lines. The C4-substituted aryloxymethyl, arylaminomethyl, and dichloroacetamidomethyl coumarins, along with the corresponding 1-azacoumarins, have been demonstrated to be potential anti-microbial and anti-inflammatory agents. To expand the structural diversity of synthetic courmarins for biological functions, attempts have also been made to attach a chloramphenicol side chain at C-3 position of courmarin. In addition, the bi- and tri-heterocyclic coumarins and 1-azacoumarins with benzofuran, furan and thiazole ring systems along with biocompatible fragments like vanillin have shown remarkable potency as anti-inflammatory agents in animal models. Photobiological studies on pyridine-fused polycyclic coumarins have highlighted their potential as thymine dimer photosensitisers and the structurally related compounds of both coumarin and carbostyrils have also been found to act via the DNA gyrase pathway in their anti-bacterial activity. Apart from the above works, the present review also addresses the potential roles of coumarins and carbostyrils as protease inhibitors, or fluorescent probes in mechanistic investigation of biochemical pathways, and their application for QSAR in theoretical studies. Though 1-Azacoumarins have received less attention as compared to coumarins in the literature, an attempt has been made to compare both the systems at various stages, so that it can spark new thoughts on synthetic methodologies, reactivity pattern and biological activities.     

Polyhydroxylated 2-styrylchromones as potent antioxidants

Four polyhydroxylated 2-styrylchromones, structurally related to flavones and cinnamic acid, have been studied. An SC derivative with OH groups only at positions 3' and 4' on the styryl moiety and another SC bearing an additional OH group at position 5 on the benzopyrone ring were more potent inhibitors of the Cu2+-induced peroxidation of LDL than the flavonoid quercetin. Fluorescence and absorption spectroscopies suggested that one LDL particle may bind 40 SC molecules. A pulse radiolysis study in pH 7 buffered micellar solutions of neutral TX100 and positively charged CTAB demonstrated that one-electron oxidation by *Br2-, *O2- and tryptophan radicals (8Trp) depends strongly on the micellar microenvironment. All SCs were readily oxidized by *O2- in CTAB micelles (rate constants: 6-18 x 10(6) M(-1) s(-1)). In TX100 micelles only the SC derivative with OH groups in position 3' and 4' reacted with *O2- (rate constant: 1.1 x 10(6) M(-1)s(-1)). In CTAB, electron transfer to *Trp radicals was observed for all SCs with rate constants > or =3.2 x 10(7) M(-1) s(-1). In TX100 micelles, this reaction occurred solely with the derivative bearing OH groups only at positions 3' and 4'.     

Molecular crosstalk between TRAIL and natural antioxidants in the treatment of cancer

The endogenous protein, tumour necrosis factor receptor apoptosis-inducing ligand (TRAIL), induces apoptosis in a wide variety of transformed and cancer cells but has little or no effect on normal cells. Therefore, TRAIL is considered to be a tumour-selective, apoptosis-inducing cytokine and a promising new candidate for cancer prevention and treatment. Some cancer cells are however resistant to TRAIL-induced apoptosis, but treatment in combination with conventional chemotherapeutic drugs or radiation generally restores TRAIL sensitivity in those cells. A novel class of molecules exhibiting synergy with TRAIL but devoid of major side effects are emerging as alternative approaches to treat resistant cancer cells, including natural antioxidants such as sulphoraphane or the flavonoids curcumin, quercetin, resveratrol, baicalein and wogonin. In this issue of the BJP, Lee et al. demonstrate that treatment of TRAIL-resistant cancer cells with wogonin restores TRAIL-induced cell death in a reactive oxygen species-dependent manner through up-regulation of p53 and Puma.