Drug Discovery Opinion

Ebola virus, named after a river in the Democratic Republic of the Congo where it was first identified, causes a severe, often fatal haemorrhagic fever. There is no vaccine against or specific treatment for Ebola virus infection, but researchers at Iowa State University have now moved a step closer to finding a treatment. Using a combination of X-ray crystallography and nuclear magnetic resonance spectroscopy, the group has succeeded in determining the structure of a key portion of the viral protein, VP35, at a resolution of 1.4 Å. The Ebola VP35 protein forms part of the viral RNA polymerase complex, acts as a viral assembly factor, and also inhibits production of host interferon.

Binding of VP35 to dsRNA correlates with suppression of interferon activity and viral virulence, and the group hopes that the new structure of the interferon inhibitory domain will allow them to design drugs that bind to VP35 and block its function. The protein forms a unique fold with two clusters of basic residues, one of which is important for dsRNA binding and inhibition of interferon production. The study is published in the January 13^th issue of PNAS.

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The increasing incidence of type I diabetes underlines the importance of securing affordable sources of insulin. The first insulins to be used therapeutically were extracted from the pancreases of pigs and cattle but, since the 1980s, most of the insulin used has the human sequence and is produced by genetically modified bacteria or yeasts. In December, SemBioSys Genetics Inc announced that it had begun a phase I//II clinical trial designed to demonstrate the bioequivalence of its SBS-1000 insulin and two commercially available insulins. The trial, involving up to 30 healthy volunteers and taking place in the UK, will compare both insulin concentrations and effects on blood glucose levels. SBS-1000 insulin is prepared from proinsulin produced by genetically modified safflower plants and has been shown to be physically, structurally and functionally indistinguishable from pharmaceutical-grade human insulin by analytical testing and in pre-clinical studies. The trial is the first in which insulin produced by plants has been administered to humans, and full results are expected to be available during the first half of 2009.

Some critics oppose the growing of genetically modified crops and believe that these pose a threat to livestock, wildlife and human health. If, however, the safflower-derived insulin is cheaper to purify – purification represents a significant proportion of the cost in manufacturing insulin by fermentation – the new technology could provide better access to insulin, especially for children in the developing world with type 1 diabetes.

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A new discovery increases the likelihood that treatments could eventually boost specific subtypes of stem cells, and promote self healing following injury or disease. In response to tissue injury or disease, progenitor cells are mobilised from bone marrow into the tissues and contribute to tissue repair and regeneration. Different subpopulations of progenitor cells are recruited depending on the type and site of disease or tissue injury. Although it is becoming apparent that specific types of progenitor cells could be used to treat a variety of diseases, there are practical and technical difficulties in harvesting, isolation, ex vivo expansion, and delivery of these cells. An alternative strategy would be to directly stimulate the mobilisation of specific populations of stem cells from the bone marrow into the circulation. Scientists at Imperial College, London, have shown that the mobilisation of progenitor cell subsets can be differentially regulated by growth factors that affect their retention in bone marrow and cell-cycle status. Treatment of mice with granulocyte colony-stimulating factor (G-CSF) followed by the CXCR4 antagonist, Mozobil™ (AMD3100), caused maximal mobilisation of hematopoietic stem cells (HPCs) and neutrophils. On the other hand, treatment with vascular endothelial growth factor (VEGF) followed by Mozobil™ maximally stimulated mobilisation of endothelial progenitor cells (EPCs) and stromal progenitor cells (SPCs). By showing that different factors and molecular mechanisms regulate the mobilisation of discrete populations of progenitor cells from the bone marrow, the study has far reaching implications for regenerative medicine. Although it is not yet clear whether such an approach would, for example, speed cardiac repair following myocardial infarction or promote bone healing after a fracture, the ability to selectively mobilise different stem cell populations will enable further research in these areas. The study is published in full in the January 9th issue of Cell Stem Cell.

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In the 1990s, it was discovered that genetically obese mice have mutations in the leptin gene, or in the gene for the leptin receptor. Leptin is a hormone, produced mainly by adipose tissue, that acts on receptors in the brain to produce a feeling of satiety and reduce appetite. Daily injections of leptin to leptin-deficient mice led to a dramatic reduction in food intake and body weight, leading to hopes that leptin treatment would have similar effects in obese people. Obese individuals, however, often have unusually high circulating leptin levels rather than suffering from a leptin deficiency, leading to the hypothesis that these people are insensitive to the effects of leptin.

Hopes that leptin could be used to treat obesity have now been revived by scientists at the Children’s Hospital in Boston who have found a way to increase leptin sensitivity in mice. They showed that obesity increases endoplasmic reticulum (ER) stress and initiates the unfolded protein response in the hypothalamus, leading to inhibition of leptin receptor signalling. The resulting leptin resistance, which led to increased obesity, was reversed by the known chemical chaperones, 4-phenyl butyric acid (PBA), and tauroursodeoxycholic acid (TUDCA) which improve ER function and decrease the accumulation and aggregation of misfolded proteins. The study is published in the January 7th issue of the journal Cell Metabolism.

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The majority (75-80%) of breast cancers are hormone-sensitive and their growth is stimulated by the hormones estrogen and progesterone. In premenopausal women, most estrogen is produced by the ovaries, and selective estrogen receptor modulators such as tamoxifen are used to block the cancer-promoting properties of estrogen. In post-menopausal women, however, estrogens are produced largely by the action of aromatase on androgens produced by the adrenal glands, and reversible (anastrazole and letrazole) and irreversible (exemestane) inhibitors of aromatase have become widely used as treatments in these women. Now Dr Debashis Ghosh’s group at the Hauptman-Woodward Medical Research Institute have solved the structure of the aromatase cytochrome P450 enzyme from human placenta at 2.9Å resolution. The work is published in the 8 January 2009 issue of the journal Nature.

Unlike the active sites of many microsomal P450s that metabolise drugs and other xenobiotics, the aromatase, which is anchored in the membrane of the endoplasmic reticulum, has an androgen-specific cleft that forms hydrophobic and polar interactions with the substrate, androstenedione. The group hope that the new structural information will pave the way to improved aromatase inhibitors for the treatment of breast cancer. The group has previously solved the structures of two other enzymes involved in estrogen biosynthesis, estrone sulfatase (2003) and 17β-hydroxysteroid dehydrogenase type 1 (1996).

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Our sense of taste is critical for evaluating which substances are good to eat, and its importance is recognised in the many metaphors that surround the four tastes traditionally recognised in the West: a sweet melody, the souring of a relationship, bitter enemies, and salt of the earth. In 1908, a Japanese scientist, Kikunae Ikeda, recognised a fifth taste which he called umami, meaning “deliciousness”. Umami is a pleasant savoury taste that results from the detection of glutamic acid or glutamates which occur in protein-rich foods such as meat, cheese, and seafood. A truncated form of the metabotropic glutamate receptor, mGluR4 (taste mGluR4), and a heterodimeric (T1R1/T1R3) taste receptor complex have both been linked to the sensation of umami.

The umami taste triggered by L-glutamate is known to be dramatically enhanced by 5’ ribonucleotides, such as 5′-inosine monophosphate and guanosine 5′-monophosphate, and a new study describes the mechanism underlying this synergy. Scientists from Senomyx and BioPredict used chimeric T1R receptors, site-directed mutagenesis and molecular modelling to devise a cooperative ligand-binding model in which L-glutamate binds close to the hinge region of the ‘Venus flytrap’ (VFT) domain of T1R1, and the 5′ ribonucleotide binds to an adjacent site to further stabilize the closed conformation.

Allosteric modulators are attractive as drug candidates, especially since these might be expected to afford greater subtype selectivity in the case of closely related receptors that share the same ligand, such as mGluRs. Positive allosteric modulators, which have an effect only when the natural ligand is present in vivo, also have the potential to reduce side effects that could occur with constitutively active agonists. Identification of the L-glutamate and 5′-inosine monophosphate binding sites on the VFT domain of T1R1 may facilitate the development of allosteric modulators of other family C GPCRs as well as the development of new flavour enhancers.

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Amongst its many functions in the brain, dopamine plays an important role in pleasure systems, including those governing motivation and reward. Naturally enjoyable experiences such as food and sex cause dopamine release. Some drugs, such as cocaine and amphetamine, also lead to dopamine release, either directly or indirectly. A small study from researchers at Vanderbilt University has now shown that the people with ‘novelty-seeking’ personality traits process dopamine differently from more risk-averse individuals. The study, in 34 healthy men and women, found that thrill-seeking was inversely linked to the density of dopamine auto-receptors, receptors on dopamine-producing cells that switch off production when dopamine levels rise. Fewer auto-receptors mean higher levels of the neurotransmitter which may in turn trigger a bigger reward from new and risky experiences. Having fewer auto-receptors may also make giving up pleasurable experiences more difficult, and New Year’s resolutions that bit harder to keep.

The study is published in the Journal of Neuroscience.

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One of the earliest fluorine-containing drugs was the antineoplastic agent, 5-fluorouracil, which was first synthesised in the 1950s. Since then, medicinal chemists have continued to add fluorine atoms to molecules and around 20% of modern drugs, including atorvastatin (Lipitor™), lansoprazole (Prevacid™) and fluoxetine (Prozac™), contain fluorine atoms. Fluorine substitution has evolved as a preferred strategy in the drug discovery process for a number of reasons. Quite often, fluorine is introduced to improve metabolic stability by blocking metabolically labile sites. Replacing hydrogen with fluorine can also have a dramatic effect on biological activity by changing lipophilicity and basicity. Incorporation of one or more fluorine atoms may also have a significant effect on conformation and, increasingly, is being used as a way to enhance binding affinity for the target protein. Despite the beneficial effects of incorporating fluorine atoms and the large number of medicines that contain fluorine, selective fluorination reactions are rarely straightforward and often employ expensive, dangerous and difficult-to-use reagents.

Although bacteria are a rich source of compounds that can form starting points for drug discovery, very few bacteria are able to produce molecules containing fluorine atoms – a notable exception is the soil bacterium, Streptomyces cattleya. Now researchers at the University of St Andrews have recreated the process by which S. cattleya synthesises fluorinated molecules. The bacterium produces a fluorinase which was first isolated by the group in 2002. This enzyme is unusual in that it can catalyse the formation of a C–F bond from inorganic fluoride. In the present study, the fluorinase, together with four other enzymes from the bacterium, was over-expressed and the resulting enzyme cocktail shown to be capable of synthesising a fluorinated amino acid, 4-fluoro threonine, from the mineral source, fluoride. The process is environmentally friendly, operates in water, and does not have particular waste or toxicology hazards. The paper is published in full in the journal Chemistry & Biology.

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Luxuriant eyelashes are considered to be highly attractive by most cultures – kohl has been used since the Bronze Age to protect and enhance lashes. More recently, mascara and eyelash extensions have become almost universal beauty aids. Now Allergan has announced that the US Food and Drug Administration (FDA) has approved the use of Latisse™ to enhance eyelashes. Bimatoprost, the active ingredient of Latisse™ was first approved in 2001 to lower intraocular pressure in people with chronic glaucoma or ocular hypertension. Patients treated with bimatoprost experienced eyelash growth as a side effect of the treatment. Bimatoprost is a prostaglandin analogue which is believed to bind to prostaglandin receptors in the dermal papilla and outer root sheath of the eyelash. Although the precise mechanism of action is unknown, prostaglandin receptors are thought to play a role in the development and regrowth of the hair follicle by increasing the percentage of hairs in the anagen (active growth) phase, as well as increasing the duration of the anagen phase. Following daily application of Latisse™, users can expect longer, fuller and darker eyelashes in as little as eight weeks, with full results in sixteen weeks. To maintain the effect, application must be continued – if discontinued the eyelashes will gradually return to pre-treatment appearance over a period of weeks to months (the average length of the eyelash hair cycle).

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At this time of year many smokers will be trying to quit, and a recent study suggests that some will find it harder than others. Most smokers believe that smoking improves concentration and, undoubtedly, withdrawal symptoms cause feelings of irritability and agitation that make concentration difficult. When nicotine levels fall, smokers experience withdrawal symptoms which can only be alleviated by another cigarette. Scientists at the Abramson Cancer Center and Department of Psychiatry in the University of Pennsylvania School of Medicine had previously shown that smokers with a polymorphism in the catechol-O-methyltransferase (COMT) gene that results in a methionine to valine substitution suffer more from concentration problems associated with nicotine withdrawal. COMT is one of a number of enzymes that degrade catecholamines such as dopamine, and individuals with the valine COMT variant degrade dopamine at a faster rate than those with the methionine variant. The group has now used functional magnetic resonance imaging to examine brain function in smokers both during periods of abstinence and normal smoking. During the brain scans, the subjects were asked to hold in their minds a series of complex geometrical figures. The results showed that smokers with the valine variant suffered greater deficits in working memory and brain function when they had refrained from smoking for 14 or more hours, and this group also reported more severe withdrawal symptoms. The findings are published in the journal Molecular Psychiatry.

Smokers with the valine COMT variant are less responsive to existing therapies for smoking cessation and the study suggests that this group could be helped to quit by COMT inhibitors.

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