Current Medicinal Chemistry - Volume 19, Issue 14, 2012

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On December 23rd, 1971, President Richard Nixon signed the National Cancer Act and invested more than $ 100 million “to launch an intensive campaign to find a cure for cancer”. Today, despite these considerable efforts, cancer still remains a very aggressive silent killer all over the world. Moreover, over the last decade, novel synthetic chemotherapeutic agents currently in use in the clinics did not succeed in fulfilling their expectations even though they are very cost-intensive. In parallel, there is increasing evidence for the potential of plant-derived compounds on the inhibition of different steps of tumor genesis and associated inflammatory processes, underlining the importance of these products in cancer prevention and therapy. This review summarizes the impact of selected natural compounds on the eight major alterations, known as the cancer hallmarks, and also on their two enabling characteristics that were coined by Hanahan and Weinberg earlier. Altogether these ten alterations are responsible for the progressive transition of healthy cells into neoplastic ones and their further dissemination in the body. With this review, we try to highlight molecular mechanisms by which plant extracts and their purified active components fight and overcome these pathological variations of the cell signaling pathways for the improvement of prevention and therapy. We truly believe that all diseases can be found in Nature and that Nature also provides the efficient cures.

The identification of substances that can promote the resolution of inflammation in a way that is homeostatic, modulatory, efficient, and well-tolerated by the body is of fundamental importance. Traditional medicines have long provided front-line pharmacotherapy for many millions of people worldwide. Medicinal extracts are a rich source of therapeutic leads for the pharmaceutical industry. The use of medicinal plant therapies to treat chronic illness, including rheumatoid arthritis (RA) and inflammatory bowel disease (IBD), is thus widespread and on the rise.The aim of this review is to present recent progress in clinical anti-inflammatory studies of plant extracts and compound leads such as green tea polyphenols, curcumin, resveratrol, boswellic acid, and cucurbitacins, among others, against chronic inflammatory diseases, mainly RA and IBD. In this context, the present paper also highlights the most promising experimental data on those plant extracts and pure compounds active in animal models of the aforementioned diseases.

Inflammation is a manifestation of a wide range of disorders which include; arthritis, atherosclerosis, Alzheimer's disease, inflammatory bowel syndrome, physical injury and infection amongst many others. Common treatment modalities are usually nonsteroidal anti-inflammatory drugs (NSAIDs) such as aspirin, paracetamol, indomethacin and ibuprofen as well as corticosteroids such as prednisone. These however, may be associated with a host of side effects due to non-selectivity for cyclooxygenase (COX) enzymes involved in inflammation and those with selectivity may be highly priced. Thus, there is a continuing search for safe and effective antiinflammatory molecules from natural sources. Research has confirmed that iridoids exhibit promising anti-inflammatory activity which may be beneficial in the treatment of inflammation. Iridoids are secondary metabolites present in various plants, especially in species belonging to the Apocynaceae, Lamiaceae, Loganiaceae, Rubiaceae, Scrophulariaceae and Verbenaceae families. Many of these ethnobotanicals have an illustrious history of traditional use alluding to their use to treat inflammation. Although iridoids exhibit a wide range of pharmacological activities such as cardiovascular, hepatoprotection, hypoglycaemic, antimutagenic, antispasmodic, anti-tumour, antiviral, immunomodulation and purgative effects this review will acutely focus on their anti-inflammatory properties. The paper aims to present a summary for the most prominent iridoid-containing plants for which anti-inflammatory activity has been demonstrated in vitro and / or in vivo.

Infections with protozoan parasites are a major cause of disease and mortality in many tropical countries of the world. Diseases caused by species of the genera Trypanosoma (Human African Trypanosomiasis and Chagas Disease) and Leishmania (various forms of Leishmaniasis) are among the seventeen “Neglected Tropical Diseases” (NTDs) defined as such by WHO due to the neglect of financial investment into research and development of new drugs by a large part of pharmaceutical industry and neglect of public awareness in high income countries. Another major tropical protozoan disease is malaria (caused by various Plasmodium species), which -although not mentioned currently by the WHO as a neglected disease- still represents a major problem, especially to people living under poor circumstances in tropical countries. Malaria causes by far the highest number of deaths of all protozoan infections and is often (as in this review) included in the NTDs. The mentioned diseases threaten many millions of lives world-wide and they are mostly associated with poor socioeconomic and hygienic environment. Existing therapies suffer from various shortcomings, namely, a high degree of toxicity and unwanted effects, lack of availability and/or problematic application under the life conditions of affected populations. Development of new, safe and affordable drugs is therefore an urgent need. Nature has provided an innumerable number of drugs for the treatment of many serious diseases. Among the natural sources for new bioactive chemicals, plants are still predominant. Their secondary metabolism yields an immeasurable wealth of chemical structures which has been and will continue to be a source of new drugs, directly in their native form and after optimization by synthetic medicinal chemistry. The current review, published in two parts, attempts to give an overview on the potential of such plant-derived natural products as antiprotozoal leads and/or drugs in the fight against NTDs.

Infections with protozoan parasites are a major cause of disease and mortality in many tropical countries of the world. Diseases caused by species of the genera Trypanosoma (Human African Trypanosomiasis and Chagas Disease) and Leishmania (various forms of Leishmaniasis) are among the seventeen “Neglected Tropical Diseases” (NTDs) defined by the WHO. Furthermore, malaria (caused by various Plasmodium species) can be considered a neglected disease in certain countries and with regard to availability and affordability of the antimalarials. Living organisms, especially plants, provide an innumerable number of molecules with potential for the treatment of many serious diseases. The current review attempts to give an overview on the potential of such plant-derived natural products as antiprotozoal leads and/or drugs in the fight against NTDs. In part I, a general description of the diseases, the current state of therapy and need for new therapeuticals, assay methods and strategies applied in the search for new plant derived natural products against these diseases and an overview on natural products of terpenoid origin with antiprotozoal potential were given. The present part II compiles the current knowledge on natural products with antiprotozoal activity that are derived from the shikimate pathway (lignans, coumarins, caffeic acid derivatives), quinones of various structural classes, compounds formed via the polyketide pathways (flavonoids and related compounds, chromenes and related benzopyrans and benzofurans, xanthones, acetogenins from Annonaceae and polyacetylenes) as well as the diverse classes of alkaloids. In total, both parts compile the literature on almost 900 different plant-derived natural products and their activity data, taken from over 800 references. These data, as the result of enormous efforts of numerous research groups world-wide, illustrate that plant secondary metabolites represent an immensely rich source of chemical diversity with an extremely high potential to yield a wealth of lead structures towards new therapies for NTDs. Only a small percentage, however, of the roughly 200,000 plant species on earth have been studied chemically and only a small percentage of these plants or their constituents has been investigated for antiprotozoal activity. The repository of plant-derived natural products hence deserves to be investigated even more intensely than it has been up to present.

Depression is a major health problem currently recognized as a leading cause of morbidity worldwide. In the United States alone, depression affects approximately 20% of the population. With current medications suffering from major shortcomings that include slow onset of action, poor efficacy, and unwanted side effects, the search for new and improved antidepressants is ever increasing. In an effort to evade side effects, people have been resorting to popular traditional herbal medicines to relieve the symptoms of depression, and there is a need for more empirical knowledge about their use and effectiveness. This review provides an overview of the current knowledge state regarding a variety of natural plant products commonly used in depression. Herbal medicines discussed that have been used in clinical trials for the treatment of mild to moderate depression states include the popular St. John’s wort, saffron, Rhodiola, lavender, Echium, and the Chinese formula banxia houpu. In addition, new emerging herbal products that have been studied in different animal models are discussed including Polygala tenuifolia, the traditional Chinese herbal SYJN formula, gan mai da zao, and Cannabis sativa constituents. A comprehensive review of the chemical, pharmacological, and clinical aspects of each of the reviewed products is provided. Finally, recent preclinical studies reporting the antidepressant action of marine-derived natural products are discussed at the end of the review.

For many decades, the use of synthetic chemicals as drugs has been effective in the treatment of most diseases. Moreover, from ancient to modern history, many traditional plant based medicines are playing an important role in health care. Phytochemicals are natural bioactive compounds found in vegetables, fruits, medicinal plants, aromatic plants, leaves, flowers and roots which act as a defense system to combat against diseases. The phytochemicals from natural products cover a diverse range of chemical entities such as polyphenols, flavonoids, steroidal saponins, organosulphur compounds and vitamins. A number of bioactive compounds generally obtained from terrestrial plants such as isoflavones, diosgenin, resveratrol, quercetin, catechin, sulforaphane, tocotrienols and carotenoids are proven to reduce the risk of cardiovascular diseases and aid in cardioprotection which is the leading cause of death globally. The cardioprotective effects of the various phytochemicals are perhaps due to their antioxidative, antihypercholesteroemic, antiangiogenic, anti-ischemic, inhibition of platelet aggregation and anti inflammatory activities that reduce the risk of cardiovascular disorders. The multi-faceted role of the phytochemicals is mediated by its structure-function relationship and can be considered as leads for cardiovascular drug design in future. This review summarizes the findings of recent studies on selected phytochemicals as prophylactic and therapeutic agents in cardioprotection.

Alzheimer's disease (AD) has become one of the deadliest diseases for human beings with special incidence in elderly population. It is a progressive neurodegenerative disease and the most prevalent cause of dementia. The neuropathology of AD has not been fully elucidated yet, however, cholinergic hypothesis is the most accepted theory nowadays, resulting from the cholinergic deficit emerging in the brains of AD patients. Shortage of the neurotransmitters, acetylcholine and butyrylcholine has been demonstrated, and therefore, inhibition of the enzymes; acetylcholinesterase (AChE) and butyrylcholinesterase (BChE) that break down acetylcholine and butyrylcholine has become a standard approach for AD treatment. However, cholinesterase inhibitors are only effective in symptomatic treatment and have no ability to impede the disease. The pathogenesis of AD is highly complex and another hypothesis is the formation of amyloid plaques containing beta-amyloid peptide, which causes neurolesions in the brains of AD patients. Beta-amyloid peptide is generated after the sequential cleavage of amyloid precursor protein, especially by the beta- and gamma-secretase in the amyloidogenic pathway. The secretases involved in the processing of amyloid precursor protein are of particular interest and, consequently, the inhibition of secretase enzyme family of protease type has become another desired treatment strategy for AD. On the other hand, medicinal plants are attractive sources for drug research and development as they produce chemically-varying molecules with preferred biological activities. The aim of this article is to review the available data on selected inhibitors from plant secondary metabolites with emphasis on cholinesterase, prolyl endopeptidase, and secretase enzyme families as being the current treatments of AD.

Tyrosinase is a multi-copper enzyme widely distributed in different organisms, including plants & mammals, etc., which is responsible for pigmentations, undesired browning of fruits and vegetables. This is the key enzyme in the melanogenesis in human and molting process of insects. Therefore the inhibitors of the enzyme may lead to novel skin whitening agents, anti-browning substances or compounds for insect control. A large numbers of moderate to potent tyrosinase inhibitors have been reported during the last decade. From our group, we reported a number of potent inhibitors from synthetic, semi-synthetic and natural origins. The compounds are from several chemical classes, like phenolics, terpenes, steroids, chalcones, flavonoids, alkaloids, long-chain fatty acids, coumarins, sildenafil analogs, bipiperidines, biscoumarins, oxadiazole, tetraketones, etc. More recently, the crystal structure of mushroom and couple of other tyrosinases has been published and more recently the crystal structure of mushroom tyrosinase complexes with a highly potent inhibitor tropolone has been reported. Yet there is a lack of information of inhibitor-tyrosinase intermolecular interactions. To overcome such issues, some researchers started utilizing in silico tools, like molecular docking simulations, for such purposes. There are also few papers published about the successful utilization of computational tools like QSAR-based and ligand-based virtual screening to identify novel and potent inhibitors of the enzyme. In our group, we are using all possible computational tools, like ligand-based as well structure-based approaches, to identify new inhibitors. In this review, some of such examples are briefly described.

Ras proteins regulate diverse cellular pathways that are important in the growth and spread of malignancies, including cell proliferation, cell cycle regulation, cell survival, angiogenesis and cell migration. These proteins lack the conventional transmembrane or hydrophobic domain typical of membrane associated proteins. Being small and hydrophilic in nature, these proteins undergo four-stage post-translational lipid modifications viz. prenylation, AAX proteolysis, carboxymethylation and palmitoylation for membrane localization which is important for their function. Therefore, enzymes involved in these modifications viz. farnesyl transferase (FTase), geranylgeranyl transferase-I (GGTase-I), geranylgeranyl transferase-II (GGTase-II), Ras converting enzyme-1 (Rce-1) and isoprenyl cysteine methyl transferase (ICMT) are emerging as potential therapeutic targets for the discovery of newer anticancer therapeutics. Several natural products have shown modulation of these post-translational enzymes. In the present review, natural products isolated from terrestrial as well as marine sources showing ability to modulate these k-Ras post-translational targets and their promise as potential anticancer agents have been discussed. A total of 157 natural products with 141 corresponding references have been covered.

The resin of Pistacia lentiscus (L.) var. chia (Duham), an evergreen shrub belonging to the family Anacardiaceae and uniquely cultivated in southern Chios, is known as mastic. It has been used for more than 2500 years in traditional Greek medicine for treating several diseases such as gastralgia and peptic ulcers, while the actions of the gum are mentioned in the works of Herodotus, Dioscorides and Galen. Several Roman, Byzantine, Arab and European authors make extensive references to mastic’s healing properties. Modern scientific research has justified the beneficial action of mastic to gastric diseases, by revealing its in vivo and in vitro activity against Helicobacter pylori, which is considered as the main cause for gastric ulcers. Furthermore, studies of the antimicrobial, antifungal, antioxidant, hypolipidemic, anti-inflammatory, anti-Crohn and anticancer activities of mastic have characterized it as a wide-range therapeutic agent and a potential source of nature-originated treatments.