Drug Discovery Opinion

The lifecycle of all Plasmodium species is complex and involves a round of replication in host erythrocytes. The clinical manifestations of malaria are linked to this stage in the lifecycle and are associated with rupture of the infected erythrocytes. During this growth phase, the parasite enters the erythrocyte and then releases several hundred effector proteins into the cytoplasm. These key virulence proteins provide a suitable environment for multiplication and allow the parasite to evade the host immune system. Proteins destined for export contain a conserved pentameric motif known as PEXEL and, when this is cleaved in the endoplasmic reticulum, the protein can be transported into the host cell. Two independent studies by scientists in the US and Australia have now shown that the protease responsible for cleaving the PEXEL motif is the aspartyl protease, plasmepsin V. Cleavage reveals an export signal at the amino terminus of the cargo protein which is then transported into the host cell cytoplasm, likely through a channel in the parasite’s outer membrane. Since export of the effector proteins is essential for the erythrocytic stage of the plasmodium life-cycle, drugs that block plasmepsin V should provide an effective treatment for malaria.

Both studies are published in the journal Nature (Australian study; US study).

The presence of multiple redundant and compensatory pathways controlling energy homeostasis has, so far, limited the effectiveness of anti-obesity treatments and suggests that combination therapy may be the best approach for treating the worldwide obesity epidemic. Writing in the journal Cell Metabolism, researchers at Merck have now demonstrated a role for the orphan bombesin receptor subtype 3 (BRS-3) in controlling energy balance.

Using a selective BRS-3 agonist (Bag-1) and antagonist (Bantag-1), the team have established a role for BRS-3 in the regulation of food intake, metabolic rate, and body weight. Intracerebroventricular infusion of the peptide Bantag-1 led to higher food intake and a progressive increase in adipose mass and body weight whereas oral administration of Bag-1 increased metabolic rate and reduced food intake, adipose weight, and body weight. Prolonged high levels of brain receptor occupancy by agonist increased weight loss, suggesting a lack of tachyphylaxis.

As well as suggesting a potential new target for the treatment of obesity, the discovery of selective BRS-3 agonists and antagonists will allow investigation of the mechanisms by which BRS-3 regulates energy metabolism as well as exploration of other aspects of BRS-3 biology.

Launched in 2009, the Medpedia Project aims to evolve a model for sharing and advancing knowledge in health and medicine. It provides a free online collaborative platform, allowing healthcare professionals to contribute to the growing knowledge base. As well as building a medical encyclopedia, professionals and non-professionals can share information about conditions, treatments, lifestyle choices, etc. Other parts of the Medpedia platform include Medpedia Answers for asking and answering medical and health questions; Medpedia Alerts for displaying real-time medical and health news alerts; and Medpedia News & Analysis for sharing medical news and analysis. The latter includes syndicated articles from blogs such as this one.

Now Medpedia has announced the launch of their clinical trial platform, which updates every 24 hours from data at ClinicalTrials.gov. Search results provide details on a trial’s purpose, who may participate, locations, and contact information from a database of around 80,000 registered trials. Whilst these data are accessible via other sources, including ClinicalTrials.gov itself, the new platform allows content to be “pushed” or fed automatically to appropriate contexts. Trial information can show up alongside a Medpedia article covering the same condition, in a personalized feed of someone interested in that condition, or in a patient community related to that condition.

Dr. David L. Katz MD, MPH, Director of the Prevention Research Center at Yale University School of Medicine, said:

“Clinical trials are among the most important of tools for advancing biomedical knowledge, and improving the human condition. But for this to happen, the trials must successfully recruit their participants, and awareness of the trials must be effectively disseminated. All too often, these requirements are rate limiting. In its customarily user-friendly manner, Medpedia is helping to resolve this problem with its clinical trial finder. This tool should serve doctors and patients alike, helping to get important trials done, and helping to spread the word about important findings as they come in.”

This free resource is available now on Medpedia at www.medpedia.com/clinical-trials. All we need now is an easy way to find trial results!

Although a variety of broad-spectrum antibiotics have been developed, broad-spectrum antiviral agents have proved more difficult to identify. Effective treatments have been developed for individual viruses such as HIV, herpes viruses and influenza viruses – and vaccines have also been developed against papilloma viruses and herpes viruses – but there is a need for small molecules that are able to treat a range of viral infections and could also be used against newly emerging viruses.

Researchers led by a team at UCLA have now identified a compound, LJ-001, that can treat a range of enveloped viruses. The team screened a library of around 30,000 compounds against Nipah virus, a pathogen that was first identified in 1998 and causes severe disease in both animals and humans. Further tests showed that LJ-001 was also effective against other enveloped viruses including Ebola virus, HIV, hepatitis C virus, West Nile virus, Rift Valley fever virus, yellow fever virus and influenza A virus, but had no effect against non-enveloped viruses. The compound interacts with the viral lipid envelope and inhibits viral entry at a step after virus binding but before virus–cell fusion.

Although LJ-001 also binds to cellular membranes, the team believe that its low toxicity can be attributed to the fact that metabolically active cells are able to repair their membranes whilst static viruses are not. LJ-001showed no overt toxicity at effective anti-viral concentrations in either in vitro or in vivo studies, and pretreatment of mice with LJ-001 prevented virus-induced mortality from Ebola and Rift Valley fever viruses.

The study is published in Proceedings of the National Academy of Sciences.

Stroke continues to be a major health issue and is a significant cause of death and disability. The recent introduction of clot-dissolving therapies has had a significant impact on survival, although the narrow window of opportunity for successful treatment remains a challenge. For those surviving stroke, the period immediately following is critical for recovery of physical and cognitive abilities. There has therefore been much interest in treatments that will aid the spontaneous recovery of function observed in the first few months following a stroke.

Researchers at Carver College of Medicine and College of Public Health (Ms Acion), University of Iowa, Iowa City, have now reported results from a clinical trial with escitalopram, a selective serotonin-reuptake inhibitor (SSRI) antidepressant. The team hypothesised that treatment with antidepressants may be beneficial because of their ability to stimulate production of compounds essential for nerve growth.

In the randomised trial, 43 patients were assigned to take 5 to 10 milligrams of escitalopram daily, 45 to take placebo daily and 41 to participate in a problem-solving therapy program developed for patients with depression. After 12 weeks of treatment, patients taking escitalopram had higher scores on neuropsychological tests assessing overall cognitive function, specifically on those measuring verbal and visual memory. The beneficial effect of escitalopram on cognitive recovery was independent of its effect on depressive symptoms and was not influenced by stroke type or mechanism of ischemic stroke. In addition, escitalopram was well tolerated and the frequency of adverse effects similar to those of patients receiving placebo.

The authors of the study, published in the February issue of Archives of General Psychiatry, suggest that the utility of antidepressant therapy in post-stroke recovery warrants further investigation.

An aortic aneurysm is a bulge in the aorta, the largest blood vessel in the body, which results from weakening of the artery wall. The majority of these occur in the portion of the aorta that passes through the abdomen and are referred to as abdominal aortic aneurysms (AAA). AAA is something of a stealth disease, since it is generally asymptomatic and may only be diagnosed at a routine physical examination or following X-ray. Over time the aneurysm may expand, with an increased risk of rupturing. Unfortunately, the rapid blood loss following aneurysm rupture is frequently fatal and accounts for at least 15,000 deaths in the US annually.

The only treatment for AAA currently available is surgical intervention. Early diagnosis is followed by monitoring the size of the aneurysm until the risk of rupture exceeds the risk of surgery. However, scientists at Providence Heart + Lung Institute at St. Paul’s Hospital and the University of British Columbia (UBC) have now raised the possibility of pharmacological intervention. Using experimental models of AAA, the team have found a role for the protein-degrading enzyme Granzyme B (GMZB).

GMZB is a serine protease expressed by a variety of immune cells and is responsible for destroying unwanted tissue, such as virally-infected cells. This role is supported by the pore-forming protein, perforin, which delivers GMZB to the intracellular compartment. The UBC research has shown that GMZB, which is abundantly expressed in aneurysms from human and animal model AAA, also plays a role in the pathogenesis of AAA. Further, the experimental data suggest that this is a perforin-independent mechanism involving extracellular matrix degradation and subsequent loss of vessel wall integrity. The results suggest that an inhibitor of GMZB may provide a therapeutic option in the treatment of AAA.

The study is published in the American Journal of Pathology.

Despite a growing understanding that the conversion of normal cells into cancerous cells is accompanied by metabolic changes, it remains unclear whether many of these changes play crucial roles in malignancy and disease progression. Increased lipid synthesis by fatty acid synthase has been suggested to contribute to cancer growth through both metabolic and signalling pathways. Researchers at the Scripps Institute reasoned that increased lipid synthesis must be accompanied by a lipolytic pathway to liberate stored fatty acids and have now shown that levels of monoacylglycerol lipase (MAGL) are highly elevated in aggressive cancer cells compared with less aggressive cancer cells and that this lipase, through hydrolysis of monoacylglycerols (MAGs), controls free fatty acid (FFA) levels in cancer cells.

The resulting MAGL-FFA pathway promotes migration, survival, and in vivo tumour growth. Aggressive cancer cells thus partner lipogenesis with high lipolytic activity to generate an array of pro-tumorigenic signals that support their malignant behaviour. Treatment with the selective MAGL inhibitor, JZL184, significantly reduced FFA levels in aggressive cancer cells, a finding that contrasts with the function of MAGL in normal tissues, where the enzyme does not generally control FFA levels. Knock down of MAGL activity using shRNA probes in aggressive melanoma, ovarian and breast cancer cells reduced MAGL activity by 70%–80%, with corresponding elevations in MAGs and reductions in FFAs. shMAGL cancer cell lines showed reduced in vitro migration, invasion and survival under serum starvation conditions. Treatment with JZL184 also reduced cancer cell migration, but not survival, perhaps indicating that maximal effects on aggressiveness need sustained inhibition of MAGL. shMAGL cancer cells also showed markedly reduced tumour growth rates in subcutaneous xenograft transplantation studies performed in immune-deficient mice.

Daily treatment of mice bearing MAGL-expressing tumours with JZL184 (40 mg/kg po) produced similar impairments in tumour growth rates. Addition of palmitic or stearic acid, two principal FFAs regulated by MAGL in aggressive cancer cells, to cells with genetically or pharmacologically reduced levels of MAGL restored their migratory activity in vitro. Similarly, tumour growth was enhanced in MAGL-deficient xenografts when the mice were fed a high fat diet. Cancer cells engineered to stably over-express MAGL also showed significantly reduced MAGs and elevated FFAs, a profile that was accompanied by increased migration, invasion and survival in vitro and enhanced tumour growth in vivo.

The effects of MAGL on cancer aggressiveness were found not to be mediated by endocannabinoid signalling but are suggested instead to be, at least in part, caused by increased production of bioactive lipids such as LPA and PGE2 that act on GPCRs to promote high migratory activity.

Both in vitro and in vivo studies showed that aggressive cancer cells acquire the ability to liberate FFAs by increased expression of MAGL and that this contributes to the aggressive phenotype. Since MAGL is not required for cell survival, but instead promotes progression to a more aggressive phenotype, if shown to slow tumour progression in people, inhibitors of MAGL may have a better safety profile and offer advantages over existing treatments for cancer.

The study is published in the journal Cell.

Roles have been suggested for brain-derived neurotrophic factor (BDNF) – which helps to support neurons and also stimulates and controls neurogenesis – in preventing or treating degenerative diseases such amyotrophic lateral sclerosis, Parkinson’s disease, and Alzheimer’s disease. The use of BDNF itself in therapy is limited by a poor pharmokinetic profile including rapid metabolism and poor CNS penetration. BDNF elicits at least some of its effects through binding to the high affinity tyrosine kinase receptor B, TrkB, and investigators at Emory University School of Medicine have now identified a small, high-affinity molecule that can also activate signalling through TrkB.

7,8-Dihydroxyflavone was shown to protect wild-type, but not TrkB-deficient, neurons from apoptosis. Following intraperitoneal administration, the compound was also found to activate TrkB in the brain and to be protective in animal models of seizure, stroke and Parkinson’s disease. The compound was also found to have low toxicity on chronic dosing. Although favonoids such as 7,8-dihydroxyflavone occur in a wide range of foodstuffs, levels obtained from a normal diet are believed to be insufficient for a sustained effect.

The study is published in the online early edition of PNAS.

Scientists at Vanderbilt University have previously used zebrafish embryos to identify compounds that interfere with signalling pathways involved in early development – pathways that also play a role in many disease processes. One of these compounds, dorsomorphin, was shown to block bone morphogenetic protein (BMP) signalling, a pathway that is involved in bone and cartilage formation and that has also been linked to anaemia and inflammatory responses. Subsequent studies showed that dorsomorphin also blocked the vascular endothelial growth factor (VEGF) type-2 receptor and disrupted angiogenesis.

To identify more selective compounds, the team turned again to zebrafish embryos. It was quickly discovered that the two effects could be separated, with some compounds only affecting patterning and some only affecting angiogenesis. The former were shown to be potent and selective inhibitors of BMP signalling and the latter to be selective VEGF inhibitors. As well as identifying a VEGF inhibitor that outperformed a compound that had entered phase III clinical trials, the team also discovered a BMP inhibitor, DMH1, which exclusively targets the BMP pathway. Using zebrafish embryos for structure-activity analyses allows selectivity and bioavailability to be assessed at the same time as efficacy and the team believe that zebrafish provide an attractive complementary platform for drug discovery. The potential of small molecule signalling inhibitors is often limited by off-target activities and zebrafish provide a very good model for assessing selectivity since compounds that hit multiple targets are toxic to the embryos.

The study is published in ACS Chemical Biology.

Ischaemic stroke is a leading cause of adult disability and death. Glutamate plays an essential role in neural development, excitatory synaptic transmission, and plasticity but, during a stroke, glutamate accumulates at synapses, resulting in neuronal death. Excessive influx of Ca²⁺ ions through N-methyl-D-aspartate (NMDA) glutamate receptors is a major contributor to cell death and brain damage following ischaemic stroke.

So far, directly targeting glutamate receptors has not proved to be an effective way of treating stroke but scientists at the University of Central Florida and Louisiana State University have discovered that uncoupling a kinase from the NR2B subunit of the NMDA receptor blocks damaging Ca²⁺ influx through the receptor channels and protects neurons against the harmful effects of ischemia. Death-associated protein kinase 1 (DAPK1) is recruited into the NMDA receptor NR2B protein complex during ischaemia and phosphorylates NR2B at Ser-1303, enhancing the NR1/NR2B channel conductance. Genetic deletion of DAPK1 or administration of the peptide, NR2BCT – which blocks the interaction of DAPK1 with the NR2B subunit – protected mice from the damaging effects of cerebral ischaemia. NR2BCT did not affect the catalytic activity of DAPK-1 or the normal physiological functioning of NMDA receptors and the authors hope that, as well as providing new insights into the mechanisms of stroke damage, their discovery will provide a new target for the treatment of stroke which could show advantage over NMDA antagonists.

The study is published in the January 22^nd issue of Cell.