Synthetic and natural coumarins as antioxidants

Coumarins, an old class of compounds, are naturally occurring benzopyrene derivatives. 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 analogs. 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 antioxidant effects were extensively examined. In this review, plant derived coumarins and their synthetic analogs will be systematically evaluated based on their plant origin, structure-activity relationship and antioxidant efficacy. Owing their diverse effects and inconclusive results from different in vitro studies, the mechanism of their action has not yet been fully understood and the correlation of effects with chemical structure is not conclusive at the moment. It is the objective of this review will be to summarize experimental data for different coumarins used as antioxidant agents, because promising data have been reported for a series of these agents. In addition, their ability to bind metal ions represents an additional means of modulating their pharmacological responses.     

Natural sesquiterpenoids as cytotoxic anticancer agents

Sesquiterpenoids are a group of naturally occurring 15-carbon isoprenoid compounds that are mainly found in higher plants, microorganism and marine life. Many of them provided encouraging leads for chemotherapeutics. In this review, the sesquiterpenoids are classified according to the ring numbering system and the functional groups presented in the core structures as acyclic, mono-, bi-, and tricyclic derivatives, and a current overview of sesquiterpenoids as potential cytotoxic anticancer agents is provided.     

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.     

Synthetic retinoids as potential antitumour agents

The retinoids, natural or synthetic derivatives of vitamin A, exhibit a variety of biological effects that may have implications in cancer chemoprevention and therapy. The chemopreventative action of classical retinoids has been ascribed to receptor-mediated modulation of a range of biological processes, including cell differentiation and proliferation. The therapeutic interest of several synthetic retinoids (also known as retinoid-related molecules) is related to their proapoptotic activity, which appears to be independent of the receptor-transactivating activity. This review focuses on the relevant features of recently reported synthetic retinoids, with particular reference to their therapeutic potential and current understanding of the mechanism of action. These agents are structurally heterogeneous compounds, including molecules closely related to natural retinoids and molecules with a retinoid-like structure (e.g., atypical retinoids) that include retinoic acid receptor (RAR) selective compounds or RAR antagonists. Promising agents of this class are adamantyl retinoids because of their apoptotic potency, in vivo antitumour activity and low toxicity. Activity of selected adamantyl retinoids (e.g., CD437, MX3350-1, ST1926) has been reported in models of various tumour types, including ovarian, breast, lung, head/neck squamous carcinoma and melanoma. The best in vivo antitumour efficacy was achieved by a protracted treatment. Orally available molecules (e.g., ST-1926) are the most suitable for this treatment schedule. Synthetic retinoids have shown synergistic interaction in combination with a variety of antitumour agents used in clinical therapy. This evidence is expected to expand the therapeutic options for clinical use of retinoids.     

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.     

Antioxidant and cytotoxic isoprenylated coumarins from Mammea americana

Antioxidant-guided fractionation of Mammea americana L. seeds resulted in the identification of three new isoprenylated coumarins, mammea B/BA hydroxycyclo F (1), mammea E/BC (2), and mammea E/BD (3). In addition, twelve known isoprenylated coumarins, mammea A/AA (4), mammea A/AA cyclo D (5), mammea A/AA cyclo F (6), mammea A/AC cyclo D (7), mammea A/AD cyclo D (8), mammea B/BA (9), mammea B/BA cyclo F (10), mammea B/BB (11), mammea B/BC (12), mammea B/BD (13), mammea E/BA (14), and mammea E/BB (15), as well as two known flavanols, (+)-catechin (16) and (-)-epicatechin (17) were identified. The fifteen isoprenylated coumarins were screened for their cytotoxicity in the SW-480, HT-29, and HCT-116 human colon cancer cell lines and antioxidant capacities in the DPPH (1,1-diphenyl-2-picrylhydrazyl) free-radical assay. Compounds 1 - 15 exhibited significant cytotoxic activities in the SW-480, HT-29, and HCT-116 human colon cancer cell lines (IC50 ranges 13.9 - 88.1, 11.2 - 85.3, and 10.7 - 76.7 microM, in the three cell lines, respectively) at concentrations comparable to 5-fluorouracil (IC50 = 53.0, 46.1, and 45.1 microM), a drug frequently used for human colon cancer treatment. Compounds 2 - 4, 9, and 11 - 15 displayed high antioxidant activity in the DPPH assay (IC50 range 86 - 135 microM), compounds 1, 5 - 8, and 10, however, had no antioxidant activity (IC50 > 200 microg/mL) in the DPPH assay. The results of these assays were used to study the structure-activity relationships for this class of compounds. In the SW-480 cell line, the three new coumarins, 1 - 3, also exhibited dose-dependent increases in sub-diploid cells by flow cytometry, indicating that they induce apoptosis.     

Attenuation of cytotoxic natural product DNA intercalating agents by caffeine

Many anti-tumor drugs function by intercalating into DNA. The xanthine alkaloid caffeine can also intercalate into DNA as well as form π-π molecular complexes with other planar alkaloids and anti-tumor drugs. The presence of caffeine could interfere with the intercalating anti-tumor drug by forming π-π molecular complexes with the drug, thereby blocking the planar aromatic drugs from intercalating into the DNA and ultimately lowering the toxicity of the drug to the cancer cells. The cytotoxic activities of several known DNA intercalators (berberine, camptothecin, chelerythrine, doxorubicin, ellipticine, and sanguinarine) on MCF-7 breast cancer cells, both with and without caffeine present (200 μg/mL) were determined. Significant attenuation of the cytotoxicities by caffeine was found. Computational molecular modeling studies involving the intercalating anti-tumor drugs with caffeine were also carried out using density functional theory (DFT) and the recently developed M06 functional. Relatively strong π-π interaction energies between caffeine and the intercalators were found, suggesting an "interceptor" role of caffeine protecting the DNA from intercalation.     

Synthesis and evaluation of curcumin analogues as cytotoxic agents

Seventeen curcumin analogues were prepared and evaluated for in vitro and in vivo cytotoxicity against an Ehrlich ascites carcinoma (EAC). In vitro results revealed that compounds 10, 7, and 12 were the most potent analogues against EAC respectively. However, in vivo evaluation of compound 10 proved its capability to normalize the blood picture compared with 5-fluorouracil, a well-known anticancer drug.     

Soluble tetraarylporphyrin-platinum conjugates as cytotoxic and phototoxic antitumor agents

A series of asymmetric tetraarylporphyrins was synthesized from pyrrole, para-substituted oligo- or poly(ethylene glycol) monomethyl ether benzaldehyde and from 4-hydroxybenzaldehyde etherified with diethyl bromomalonate according to the Lindsey method. After hydrolysis of the tetraarylporphyrin esters, the resulting carboxylic acid groups were used to bind platinum fragments. In comparison to analogous hematoporphyrin-platinum conjugates, the title compounds are characterized by a 30 nm bathochromic shift of their absorption bands. The antiproliferative activity of 18 platinum complexes (1, 5, and 10 microM) differing in solubility, type of the platinum fragment, and the corresponding tetraarylporphyrin ligands were studied on TCC-SUP transitional bladder cancer cells in the dark and after irradiation (lambda = 600-730 nm; 24 J/cm(2)). The most active compounds were among the tetraarylporphyrin-platinum conjugates bearing the diammine and (RR/SS)-trans-1,2-diaminocyclohexane ligands.     

Irritant and cytotoxic coumarins from Angelica glauca Edgew roots

Irritant and cytotoxic potentiality of six coumarins, isolated for the first time from the roots of Angelica glauca identified as 5,6,7-trimethoxycoumarin, 6-methoxy-7,8-methylenedioxycoumarin, bergapten, decursinol angelate, decursin, and nodakenetin, were investigated. The irritant potential was explored by open mouse ear assay, evaluating their ID(50) after acute and by IU (Irritant units) after chronic effects, while the cytotoxic capability was explored by their LC(50), using brine shrimp (Artemia salina) larvae (nauplii). All the coumarins exhibited well-defined irritancy on mouse's ears, compared with the positive controlled euphorbium reaction and cytotoxic response against brine shrimp larvae, compared with the positive control colchicine. Decursinol angelate and decursin were the most potent and persistent irritant compounds with least ID(50), whose reactions lasted for 48 h. 6-Methoxy-7,8-methylenedioxycoumarin and bergaten revealed an intermediate irritant reactions, while 5,6,7-trimethoxycoumarin and nodakenetin displayed the least irritant and least persistent reactions on mouse ears. Both decursin and decursinol angelate also appeared to be the stronger cytotoxic agents than other coumarins. 5,6,7-trimethoxycoumarin displayed an intermediate cytotoxic behaviour, while other three coumarins, i.e., 6-methoxy-7,8-methylenedioxycoumarin, bergapten, and nodakenetin, exhibited the least cytotoxic capacity against brine shrimp larvae.     

Irritant and cytotoxic coumarins from Angelica glauca Edgew roots

Irritant and cytotoxic potentiality of six coumarins, isolated for the first time from the roots of Angelica glauca identified as 5,6,7-trimethoxycoumarin, 6-methoxy-7,8-methylenedioxycoumarin, bergapten, decursinol angelate, decursin, and nodakenetin, were investigated. The irritant potential was explored by open mouse ear assay, evaluating their ID₅₀ after acute and by IU (Irritant units) after chronic effects, while the cytotoxic capability was explored by their LC₅₀, using brine shrimp (Artemia salina) larvae (nauplii). All the coumarins exhibited well-defined irritancy on mouse's ears, compared with the positive controlled euphorbium reaction and cytotoxic response against brine shrimp larvae, compared with the positive control colchicine. Decursinol angelate and decursin were the most potent and persistent irritant compounds with least ID₅₀, whose reactions lasted for 48 h. 6-Methoxy-7,8-methylenedioxycoumarin and bergaten revealed an intermediate irritant reactions, while 5,6,7-trimethoxycoumarin and nodakenetin displayed the least irritant and least persistent reactions on mouse ears. Both decursin and decursinol angelate also appeared to be the stronger cytotoxic agents than other coumarins. 5,6,7-trimethoxycoumarin displayed an intermediate cytotoxic behaviour, while other three coumarins, i.e., 6-methoxy-7,8-methylenedioxycoumarin, bergapten, and nodakenetin, exhibited the least cytotoxic capacity against brine shrimp larvae.     

Antiviral and cytotoxic evaluation of coumarins from Prangos ferulacea

Context: Prangos ferulacea (L.) Lindl. (Apiaceae) is a perennial plant found in the Middle-East, where it is commonly used as an antispasmodic and anti-inflammatory agent. It is a rich source of coumarins.

Objective: To purify several coumarins from P. ferulacea and to screen their cytotoxicity and anti-herpes activity.

Materials and methods: Acetone extract of roots of P. ferulacea was subjected to several chromatographic separations to render pure coumarins (1–8). Anti-herpes virus effects of 1–7 were evaluated at concentration 2.5, 5, and 10?µgmL^?1, on a confluent monolayer of Vero cells infected with 25 PFU of HSV1. Cytotoxic effects of 1 and 2 were evaluated on an A2780S cell line using the MTT assay. The cells were exposed to a series of concentrations of coumarins (0.01–2.5?mM, 37°C, 72?h).

Results: Compounds 1–8 were identified as osthole, isoimperatorin, oxypeucedanin, psoralen, oxypeucedanin hydrate, gosferol, oxypeucedanin methnolate, and pranferol. This is the first report of occurrence of 4 and 7 in this plant. Compound 1 showed a viability of 9.41%?±?2.4 at 2.5?mM on A2780S cells (IC₅₀?=?0.38?mM). The cell survival of 2 at 2.5?mM was 46.86%?±?5.5 with IC₅₀ equal to 1.1?mM.

Discussion and conclusion: Compound 1 shows cytotoxic effects on the A2780S cell line. Compound 2 is a cyclooxygenase-2 inhibitor and the A2780S cell line does not express COX-2 which may interpret the non-toxic effect of the compound on this cell line. None of the tested compounds showed an anti-HSV effect at non-toxic concentrations.     

Antineoplastic natural products and the analogues VIII synthesis of some coumarins and their cytotoxic activities on L1210 cell

Some coumarins were sythesized for the screening of their cytotoxic activities against L1210 cell. Of the coumarins sythesized, 6,7-dihydroxycoumarin (esculetin) and 7,8-dihydroxycoumain (daphnetin) as coumarins with dioxygenated A-ring, and 6-acetoxy-5,7-dimethoxycoumarin and 5,7-dimethoxy-6-hydroxycoumarin as trioxygenated ones, show considerable cytotoxic activities, ED 50 being 4.3, 8.8, 17.2 and 5.5 μg/ml in the same order as the substances. The extent of oxygenation of the A-ring and the positions of the oxygen functions eventually play an important role for the cytotoxic activity.     

Pro-oxidant natural products as anticancer agents

Cancer cells produce high levels of reactive oxygen species (ROS) that lead to a state of increased basal oxidative stress. Since this state of oxidative stress makes cancer cells vulnerable to agents that further augment ROS levels, the use of pro-oxidant agents is emerging as an exciting strategy to selectively target tumor cells. Natural products have provided a significant contribution to the development of several drugs currently used in cancer chemotherapy. Although many natural products are known to affect the redox state of the cell, most studies on these compounds have focused on their antioxidant activity instead of on their pro-oxidant properties. This article provides an overview of natural products with pro-oxidant and anticancer activities, with special focus on plant secondary metabolites, and discusses their possible use as cancer chemotherapeutic agents.     

Synthetic iminosugar derivatives as new potential immunosuppressive agents

Several iminosugar derivatives were synthesized, and their effects on the secretion of IL-4 and IFN-gamma from the mouse splenocytes were examined. The effects on membrane expression of other T cell-associated molecules (CD3, CD4, CD8) and B cell-associated molecules (CD19) were also investigated. The experimental data demonstrated that synthetic iminosugars hold potential as immunosuppressive agents.     

Synthesis of berberine-piperazine conjugates as potential antioxidant and cytotoxic agents

Piperazine derivatives bearing different electron-withdrawing and electron-donating functional groups were linked to the well-known isoquinoline alkaloid derivative, berberine via efficient organic transformations. The entire target berberine-based analogues were examined for their in vitro antioxidant potency using 2,2-diphenyl-1-picrylhydrazyl and 2,2′-azino-bis-3-ethylbenzthiazoline-6-sulphonic acid bioassays, and anticancer activities using sulforhodamine B assay against HeLa and CaSki cervical cancer cell lines in addition to the cytotoxicity using Madin-Darby canine kidney non-cancer cell lines and, ascorbic acid and berberine used as a control for antioxidant and anticancer activities, respectively. Bioassay results revealed that newer compounds were more active against CaSki and HeLa cell lines with therapeutic indices better than that of parent berberine and showed tolerable cytotoxicity to the normal cells. A final analogue 5a with 4-methylpiperazine substituent indicated most significant anticancer potency with a therapeutic index of 58.53 (HeLa) and 48.76 (CaSki), followed by those bearing meta-chloropiperazine rings with a therapeutic index of 41.83 (HeLa) and 47.35 (CaSki), respectively.In addition, newly synthesized analogues exerted a significant radical scavenging activity against 2,2′-azino-bis-3-ethylbenzthiazoline-6-sulphonic acid cation with IC₅₀ values of 8.917?µg/mL, and were good to moderate scavengers of 2,2-diphenyl-1-picrylhydrazyl radical with IC₅₀ values of 25.40?µg/mL. Synthesized compound was characterized using several techniques, fourier transform infrared spectroscopy, ¹H nuclear magnetic resonance, ¹³C nuclear magnetic resonance, mass spectroscopy and elemental (CHN) analyses.     

Sulfonylpiperazines based on a flavone as antioxidant and cytotoxic agents

Chrysin‐based sulfonylpiperazines 7a‐k were synthesized and investigated for their in vitro free radical scavenging potential as well as cytotoxic efficacies against selected cancer cell lines. Cytotoxicity of the new compounds toward noncancer cells was confirmed using the SRB assay against Madin–Darby Canine Kidney cells. Reaction of piperazine with different substituted benzenesulfonyl chlorides in triethylamine furnished sulfonylpiperazines ( 3a‐k ), which were then allowed to react with 7‐(4‐bromobutoxy)‐5‐hydroxy‐2‐phenyl‐4H‐chromen‐4‐one ( 6 ) prepared reacting chrysin with 1,4‐dibromobutane to give the final derivatives 7a‐k . The results concluded that chrysin‐sulfonylpiperazines exerted better antioxidant and anticancer efficacies than previously studied chrysin‐piperazine precursors. For example, compounds 7h, 7j , and 7k with 4‐OCF₃, 4‐OCH₃, and 2,4‐diOCH₃ groups exhibited the best antioxidant potential against 2,2‐diphenyl‐1‐picrylhydrazyl (DPPH) and 2,2′‐azino‐bis‐3‐ethylbenzothiazoline‐6‐sulfonic acid (ABTS) radicals. Moreover, halogenated analogues ( 7b, 7c, 7g , and 7h ) demonstrated promising anticancer potential against SK‐OV3, HeLa, and HT‐29 cell lines, whereas those bearing a methoxy functional group ( 7j and 7k ) had beneficial effects against the cell lines A‐549 and HT‐29. Thus, it can be confirmed from the bioassay results that the overall structural design as well as proper substitution is crucial to deliver the anticipated biological effects. Spectroscopic techniques such as FT‐IR, ¹H NMR, ¹³C NMR, mass and elemental analysis (CHN) were carried out to confirm the final structures.