SR – Page 2 – Drug Discovery Opinion

Asthma Treatment Could Get Tastier

Researchers at the University of Maryland School of Medicine in Baltimore have discovered functioning bitter taste receptors (TAS2Rs) on bronchial smooth muscle. Although identical to receptors on the tongue, the receptors in the bronchi are not clustered in buds and do not send signals to the brain. The lung receptors do, however, respond to substances that have a bitter taste. The team initially thought that the purpose of the lung receptors would be to reinforce the warning provided by those on the tongue against bitter substances, many of which are toxic. When the team tested such compounds on individual airway smooth muscle cells, or human and mouse airways, they found, however, that instead of causing contraction – which would lead to unpleasant feelings of chest tightness and coughing – the compounds had the opposite effect.

In laboratory tests, TAS2R agonists such as chloroquine and denatonium opened the airways more effectively than drugs currently used to treat asthma or COPD, and aerosols containing the bitter substances were also effective in a mouse model of asthma. At a cellular level, the effect of bitter compounds also provided a surprise: although the compounds cause the airway muscles to relax, they lead to an increase in intracellular Ca²⁺, which would usually be expected to cause muscle contraction.

Although the bitter tasting substances used in the current study may not be suitable for aerosol formulation, the team believe that it should be possible to discover other compounds with similar bronchodilating properties that could be formulated for delivery by an inhaler and used to treat patients with asthma and COPD.

The study is published in Nature Medicine.

FSH Receptors Outline Tumours

Researchers at Mount Sinai School of Medicine and France’s National Institute of Health and Medical Research (INSERM) have discovered a biomarker that could be applicable to a range of different cancers.

In a study of more than 1300 patients, the team found that the follicle stimulating hormone (FSH) receptor – which is normally present only at low levels in the blood vessels or the granulosa cells of the ovary and the Sertoli cells of the testis – is also expressed at higher levels in eleven different types of cancer. The receptor was absent in other normal tissues, including normal tissue from the organ bearing the tumour. Not only does the FSH receptor appear to be specific for endothelial cells in the vasculature surrounding tumour tissues, it is also present from the very early stages and is easily detectable using conventional imaging methods. In most cases, the blood vessels expressing FSH receptors were at the periphery of the tumour, in a layer about 10mm thick, making it a good target for improving cancer detection and also guiding surgery and radiation treatment. Blocking FSH receptor signalling, which stimulates angiogenesis via up-regulation of vascular endothelial growth factor, could also potentially provide a new strategy for developing anticancer drugs.

The study is published in The New England Journal of Medicine.

Targeting the Tangles

Although the relative importance of β-amyloid plaques and tau protein tangles in the progression of Alzheimer’s disease has been the subject of much debate, early emphasis was placed on the development of drugs to block production of β-amyloid. Although such compounds were shown to improve cognition in transgenic mice, unfortunately results from clinical trials have been more equivocal. Focus is now shifting to therapies that target tau pathology and, in a recent study, researchers from the University of Pennsylvania have identified a compound that reduced cognitive deficits in mutant human tau transgenic mice.

In healthy nerve cells, tau proteins interact with tubulin to stabilize axonal microtubules and promote tubulin assembly into microtubules. In Alzheimer’s disease and other ‘tauopathies’, hyperphosphorylated and misfolded tau proteins form insoluble neurofibrillary tangles that deplete levels of soluble tau and lead to destabilization of the microtubules and neuronal dysfunction. The team had previously proposed using microtubule-stabilising anti-cancer taxanes such as paclitaxel to treat tauopathies, but these do not penetrate the blood-brain barrier sufficiently well. The Penn team has now shown that once weekly treatment of tau transgenic mice with the brain-penetrant microtubule-stabilising agent, epothilone D, for three months significantly improved microtubule density and axonal integrity and also reduced cognitive deficits without notable side-effects.

The study, which is published in the Journal of Neuroscience, suggests that brain-penetrant microtubule-stabilising drugs could provide a new strategy for treating Alzheimer’s disease.

Towards a Safer, Sun-free Tan

deckchairs — Image: Flickr - Ali Harrison

Although many people believe that a suntan imparts a healthy glow, the truth is that tanning irreversibly damages the skin and can lead to skin cancer. Researchers from Massachusetts General Hospital have now shown that, in mice at least, inhibiting one of the phosphodiesterases, PDE-4D3, can activate the tanning process without exposure to harmful ultraviolet irradiation.

Using transgenic mice with melanocytes in their epidermis (which normal mice do not have), the group had already shown that inducing cyclic AMP production led to significant pigmentation. Further detailed analysis of the melanin expression pathway identified PDE-4D3 as the key regulator of cyclic AMP activity in melanocytes. Topical treatment of the transgenic mice with a PDE-4 inhibitor (rolipram or Ro 20-1724) for five days caused the skin to darken appreciably and the effect could be further enhanced by co-application of forskolin, which stimulates adenylate cyclase activity.

Although PDE4 isoforms play a key role in regulating intracellular cyclic AMP levels in a variety of tissues, the researchers believe that topical application of a selective PDE-4D3 inhibitor has potential to provide safe tanning and reduce the incidence of skin cancer.

The study is published in the journal Genes & Development.

11β-HSD1 Inhibitor Improves Spatial Memory in Ageing Mice

Inhibitors of 11β-hydroxysteroid dehydrogenase type 1 (11β-HSD1), a microsomal enzyme that converts cortisone into cortisol, are being developed to treat diabetes and metabolic disorders and now, in a study supported by the Wellcome Trust, researchers at the University of Edinburgh have shown that such compounds may also help to reduce – or even reverse – age-related memory loss. Such memory loss has been linked to increased activity of 11β-HSD1 and higher levels of glucocorticoids in the hippocamus, an area of the brain associated with memory.

Ageing mice display deficits in memory and learning similar to those experienced by some elderly people and life-long partial deficiency of 11β-HSD1 prevents this decline in transgenic mice. More surprisingly, improvements in memory – as judged by performance in a Y maze – were seen in mice after only ten days treatment with a selective 11β-HSD1 inhibitor, UE1961.

The team had previously shown that a non-selective 11β-HSD1 inhibitor, carbenoxolone, improves memory in healthy elderly men and in patients with type II diabetes after only one month of treatment. They now hope to complete preclinical assessment of the new compound and begin clinical trials within a year. The study is published in the Journal of Neuroscience.

New (Host) Target for Treating HCV

Water droplet — Image: Flickr - Jack Newton

Hepatitis C virus (HCV) is one of the most important causes of chronic liver disease and infection can lead ultimately to cirrhosis and liver cancer. Current standard-of-care treatment – a combination of pegylated α-interferon and ribavirin – is unable to clear the virus in all patients and new antiviral agents designed to inhibit specific viral enzymes such as the protease, helicase and polymerase are being developed.

Researchers led by a team at the Gladstone Institute of Virology and Immunology (GIVI) have now identified a human enzyme that is also needed for viral infectivity, a discovery that may offer a new strategy for treatment. The enzyme, diacylglycerol acyltransferase 1 (DGAT1), is one of two DGAT enzymes that catalyse the final step in triglyceride synthesis. HCV infection is closely tied to lipid metabolism and the Gladstone team showed that infection and replication is severely impaired in liver cells that lack DGAT1 activity: either RNAi-mediated knockdown of DGAT1 or treatment with a DGAT1 inhibitor was effective in limiting production of infectious viral particles. The team went on to show that DGAT1 interacts with the viral nucleocapsid core protein and is required for the trafficking of the core protein to lipid droplets. Knockdown of the other enzyme involved in triglyceride synthesis, DGAT2, had no effect on viral replication.

DGAT1 inhibitors are already being developed as treatments for type II diabetes and obesity and the new study, which is published in Nature Medicine, suggests that they may also be useful for treating HCV infection.

One Step Nearer to Taxol

taxadien-5-alpha-ol Although Taxol® (paclitaxel) offers significant benefits to cancer patients, its initial isolation from the bark of the Pacific yew tree (Taxus brevifolia) raised serious ecological concerns since the trees are killed in the harvesting process. These concerns led to the development of both synthetic and semisynthetic routes to paclitaxel, but the drug is now manufactured more efficiently using plant cell cultures.

Scientists at MIT and Tufts University have now engineered a new strain of E. coli bacteria that can produce taxadiene and taxadien-5-α-ol, key precursors to paclitaxel. Although E. coli does not naturally produce taxadiene, it does produce isopentenyl pyrophosphate (IPP), a compound that is two steps back in the plant biosynthetic pathway. The team identified four bottlenecks in the eight-step E. coli biosynthesis of IPP and engineered the bacteria to produce multiple copies of the genes encoding the enzymes responsible for carrying out these four steps. To enable the bacteria to carry out the two additional steps needed to covert IPP to taxadiene, the researchers added the plant genes for the appropriate enzymes. By altering the copy numbers of genes to find the most efficient combination, the team were able to produce a strain of E. coli that produces more than 1000 times more taxadiene (ca 1g/L) than any other engineered strain. They then added an extra step towards the synthesis of paclitaxel, achieving the first microbial conversion of taxadiene to taxadien-5-α-ol.

There are another fifteen to twenty steps to go to achieve a microbial synthesis of pactitaxel but, if these can be achieved, as well as producing pactitaxel, the engineered bacteria should allow access to a variety of terpenoid natural products.

The study is published in the journal Science.

Blocking Cancer Networks

spider web — Image: Flickr - Luc Viatour

The emergence of drug resistance is one of the main causes of failure in cancer treatment and is one reason that cancer drugs are often used in combination. Resistance can also arise during the use of combinations of cytotoxic agents developed by trial and error but researchers at Fox Chase Cancer Center and Georgetown University have now developed a better way of selecting drug combinations based on molecular targets.

The epidermal growth factor receptor (EGFR) is a well validated molecular target and inhibitors are already used clinically for certain types of cancer. The team used siRNA to silence 638 genes known to encode proteins involved in the EGFR signalling network and identified over 60 proteins that can rescue cells in the presence of an EGFR inhibitor. Amongst these were three proteins for which drugs are already being developed: Aurora kinase A, protein kinase C, and STAT3. Aurora kinase A inhibitors are already being evaluated clinically and a trial testing the EGFR inhibitor erlotinib with an Aurora kinase inhibitor in patients with non-small cell lung cancer is being launched. A similar network-centred approach could be used to design other combination therapies to overcome resistance mechanisms in cancer.

Interestingly, the screen did not pick out genes previously linked to resistance to EGFR inhibitors and most of the genes identified were not mutated: KRAS mutations did not appear to be needed for resistance to EGFR inhibitors although patients with KRAS mutations do not benefit from EGFR inhibitors.

The study is published in Science Signaling.

Study Casts New Light on Prozac Mechanism

Selective serotonin reuptake inhibitors (SSRIs) such as fluoxetine (Prozac®) have been used to treat depression for more than three decades but researchers from INSERM and Hoffmann-La Roche have now shed new light on their mechanism of action. SSRIs are believed to act by inhibiting uptake of serotonin into presynaptic cells, thereby increasing the amount available in the synapse to bind to postsynaptic receptors. Typically, an ‘adaptation phase’ of several weeks is needed before the antidepressant effects are fully manifest and the new study helps to explain this latency. The study identified a key role for microRNA-16 (miR-16) in regulating expression of the serotonin transporter (SERT) which is responsible for the recapture of serotonin. Under normal conditions, SERT is present in serotonergic neurons where levels of miR-16 are low, but expression is silenced in noradrenergic cells by higher levels of miR-16; a reduction of miR-16 in noradrenergic cells causes de novo SERT synthesis.

In mice, chronic treatment with fluoxetine was shown to increase levels of miR-16 in serotonergic cells, leading to reduced SERT expression. The cells also released the neurotrophic factor S100β, which decreased miR-16 in noradrenergic cells, resulting in cells with a mixed phenotype that produced both noradrenaline and serotonin and which were sensitive to fluoxetine. Treatment with fluoxetine thus increases serotonin levels both by preventing reuptake by serotonergic neurons and by stimulating production by noradrenergic neurons through reduction of miR-16.

The study is published in the journal Science.

Possible Biomarker for COX-2 Cardiovascular Risk

Die on Checkerboard — Image: Flickr - Cristian N Mormoloc

Selective COX-2 inhibitors were developed to minimise the adverse gastrointestinal effects seen with conventional NSAIDs and have provided effective pain relief for millions of arthritis patients. Long-term, high dosage use of some COX-2 inhibitors, however, was found to be associated with an increased risk of heart attacks and strokes, resulting in drug withdrawals. A clearer understanding of the mechanisms underlying the cardiovascular effects associated with COX-2 inhibitors would allow better risk/benefit assessment and could possibly lead to the development of safer inhibitors.

Researchers from the University of California, Davis and Beijing University have now shown that, in mice, oral administration of rofecoxib for 3 months leads to a more than 120-fold increase in the regulatory lipid, 20-hydroxyeicosatetraenoic acid (20-HETE) which correlates with a significantly shorter tail bleeding time. Further studies suggested that inhibition of COX-2-mediated 20-HETE degradation by rofecoxib may, at least in part, explain the increase in blood levels and shortened bleeding time and may also contribute to the cardiovascular side effects seen with rofecoxib. Although the relative importance of COX-2 in the metabolism of 20-HETE in man has not yet been determined, if it proves to be as important as in mice, blood levels of 20-HETE may be a good predictor of which patients are at higher risk of heart attack or stroke.

The study is published in the journal PNAS.