RESEARCH HIGHLIGHTS

the β-ketoacyl-acyl carrier protein increased the binding signal and synthase FabF. To increase the chance confirmed the authors’ hypothesis of identifying a compound that that platensimycin only binds to the selectively targets FabF, the authors acyl intermediate. engineered the Gram-positive patho- The transient nature of the gen to express acyl–FabF complex also made antisense RNA against FabF. This attempts to obtain a crystal structure renders the bacteria more sensitive of platensimycin bound to its target to inhibitors of this , and so challenging. However, by substituting comparison of growth inhibition of the residue in the active the wild-type strain with that of the site of the enzyme with glutamine, antisense strain enables the identi- the side chain of which is thought fication of genuine FabF-selective to mimic a bound fatty , the inhibitors present in the fermentation authors produced a stable FabF samples. This screen identified a variant–platensimycin complex that potent antibiotic, platensimycin, was amenable to crystallographic which could otherwise have been analysis. From this they were able overlooked in a conventional to determine that platensimycin screening assay. targets the malonate-binding site Platensimycin was found to have of FabF, thereby blocking broad-spectrum activity against , and that the formation Gram-positive bacteria in in vitro of the acyl–FabF intermediate opens ANTIBACTERIAL DRUGS assays and was able to eradicate up the active site of FabF, a confor- systemic S. aureus infection in mice. mational change that is essential for Importantly, platensimycin shows platensimycin binding. New antibiotic no cross-resistance with other In recent decades, there has been classes of antibiotics, and is active a dearth of new classes of antibiotic. against bacterial infections that are Platensimycin is currently the most on the horizon? resistant to commercially available potent inhibitor of FabF, as well drugs, such as methicillin-resistant as being the only inhibitor of this S. aureus and -resistant enzyme that has broad-spectrum A return to investigating natural enterococci. activity. Fatty acid biosynthesis is so products as a source of antimicrobials To verify that the mode of action far an under-targeted pathway, and so could hold the key to tackling multi- of platensimycin was the selective it has considerable potential for drug drug resistant bacteria. Writing in Platensimycin targeting of FabF, the authors used a development. On the basis of these Nature, Wang and colleagues describe radioactive platensimycin derivative. data, and with structural information the isolation and characterization of looks to be Initially, the binding affinities of in hand, platensimycin looks to be a platensimycin, a small molecule that a promising the derivative for FabF were much promising first step towards a new represents a new class of antibiotic first step lower than anticipated, which led first-in-class antibacterial drug. and which has broad, potent activity towards a new the authors to propose that platensi- Samantha Barton against Gram-positive . mycin was targeting the transiently first-in-class A pathway yet to be fully formed (and so difficult to detect) ORIGINAL RESEARCH PAPER Wang, J. et al. exploited for antibiotic drug discov- antibacterial acyl–FabF intermediate. Indeed, Platensimycin is a selective FabF inhibitor with ery is fatty acid biosynthesis. A key preparation of a stable acyl–enzyme potent antibiotic properties. Nature 441, drug. 358–361 (2006) enzyme involved in this pathway is complex to mimic the transient form

RESEARCH HIGHLIGHTS ADVISORS ERIK DE CLERCQ F. PETER GUENGERICH MADS KROGSGAARD THOMSEN CHRISTOPHER LIPINSKI Katholieke Universiteit Leuven, Belgium Vanderbilt University Nashville, TN, USA Novo Nordisk, Bagsvaerd, Denmark Pfizer Global Research and Development, RODERICK FLOWER FRANZ HEFTI HUGO KUBINYI Groton, CT, USA William Harvey Research Institute, QMW, Rinat Neuroscience Corporation, Palo Alto, University of Heidelberg, Germany TOMI SAWYER London, UK CA, USA ROBERT LANGER Massachusetts Institute of Ariad Pharmaceuticals, Cambridge, MA, USA YOSHIJI FUJITA JOAN HELLER BROWN Technology Cambridge, MA, USA JANET WOODCOCK Clinical Proteome Center, Tokyo Medical University of California San Diego, JULIO LICINIO University of California Los Food & Drug Administration, Rockville, University CA, USA Angeles, CA, USA MD, USA

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LIVER DISEASE murine model of acute liver injury the phosphatidylinositol 3-kinase-Akt to demonstrate that CB1 receptors and extracellular-regulated kinase are involved in the induction of two (ERK) pathways, which are necessary A new treatment fibrogenic markers: the profibrogenic for the growth and survival of hepatic cytokine transforming growth factor myofibroblasts. Cells isolated from for liver fibrosis? (TGF)-β1 and smooth muscle α-actin. CB1 knockout mice showed decreased Administration of rimonabant reduced phosphorylation of ERK and Akt, as did Chronic liver injury in response to expression of both markers, and there rimonabant-treated wild-type hepatic alcohol or hepatitis virus B or C is was a similar reduction in fibrogenic myofibroblasts, suggesting that CB1 associated with liver fibrosis, and its This study marker expression in CB1 knockout mice, receptors on hepatic myofibroblasts may endstage, cirrhosis, is a major public opens the with or without rimonabant treatment. either be activated by an endogenous health problem. However, at present, To further assess the role of CB1 ligand or be constitutively active. there are no effective antifibrotic drugs possibility that receptors in chronic liver injury, three Overall, these results clarify the approved for human use. In a recent rimonabant mouse models were used: chronic CCl4 profibrogenic role of the CB1 receptor. paper in Nature Medicine, Teixeira-Clerc might also intoxication, chronic thioacetamide And although yet to be tested for liver and colleagues now show that use of the intoxication and bile duct ligation. fibrosis in clinical trials, this study cannabinoid CB1 receptor antagonist have potential Rimonabant lowered the fibrogenic opens the possibility that rimonabant, rimonabant could be a new therapeutic in the treat- response, independently of the agent which has been recommended to strategy for this condition. ment of liver used to induce liver injury, as shown receive marketing authorization for the The authors recently found that during by a decrease in fibrosis area, reduced treatment of obesity by the European the course of chronic hepatitis C, daily fibrosis. hepatic expression of TGF-β1, and Medicines Agency, might also have cannabis use is an independent predictor decreased number of liver fibrogenic potential in the treatment of liver fibrosis. of fibrosis progression. They therefore cells. Similar reduction of fibrogenesis Charlotte Harrison sought to determine if CB1 receptors was observed in CB1 knock-out mice. might be involved in liver fibrosis. The authors then focused on how ORIGINAL RESEARCH PAPER Teixeira-Clerc, T. Firstly, using immuno-blotting and antagonism of CB1 receptors might et al. CB1 cannabinoid receptor antagonism: a immunohistocytochemistry, Teixeira- reduce the accumulation of hepatic new strategy for the treatment of liver fibrosis. Nature Med. 12, 671–676 (2006) Clerc et al. showed that CB1 receptor myofibroblasts. Hepatic myofibroblasts FURTHER READING Lotersztajn, S. et al. Hepatic expression was upregulated in human from CB1 receptor knockout mice fibrosis: molecular mechanisms and drug targets. cirrhotic liver samples, predominantly in displayed increased apoptosis, whilst Annu. Rev. Pharmacol. Toxicol 45, 605–628 (2005) | hepatic myofibroblasts, the fibrogenic Kunos, G. et al. Cannabinoids hurt, heal in rimonabant was able to inhibit their cirrhosis. Nature Med. 12, 608–609 (2006) cells of the liver. They then used a proliferation. They also investigated

NEUROLOGICAL DISORDERS Homing in on the target of antidepressants

The mechanism of action of Neurons are continuously gener- expression of typical marker proteins most commonly used ated in the dentate gyrus region such as nestin, combined with antidepressants is of the hippocampus throughout dense packing of the cells within the poorly defined life. In animal models, this activity dentate gyrus. The authors therefore and a better increases in response to various types generated mice in which a fluorescent understanding of anti depressant therapy, including signal was localized specifically to of the effects of selective serotonin reuptake inhibitors the nucleus of these cells, producing these drugs on (SSRIs) such as fluoxetine, and poten- a labelling pattern that was easier to the brain could tially contributes to their therapeutic quantify. In combination with staining lead to new thera- effects. However, the precise stage of for various proteins that are expressed peutic approaches. the multi-step process of neurogen- at different stages of neurogenesis, the A potential link between esis at which SSRIs actually intervene authors defined six distinct sequential neurogenesis and antidepressant is unknown. A clearer understanding stages of neuronal development and treatment announced several years of the progenitor cell type targeted investigated the effects of chronic ago generated excitement in the field, by these drugs could enhance fluoxetine treatment on the prolif- but the mechanisms underlying this understanding of the pathogenesis erative activity of cells at each stage. connection have remained elusive. of depression and the development of Using this approach, the authors Now a new study by Enikolopov and improved antidepressants. showed that the cells targeted by colleagues narrows down the target Quantification and characteriza- fluoxetine to increase neurogenesis are of at least one class of antidepres- tion of different progenitor cell types ‘amplifying neural progenitors’ — the sants to a specific population of early by immunofluorescence is difficult, cells generated in the second ‘stage’ of progenitor cells. owing to the widespread, cytoplasmic neurogenesis as defined in this study.

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SEPSIS Unpicking the pathogenesis of sepsis

Severe sepsis, which results from the body’s levels of several inflammatory and response to infection, remains a leading cause coagulation mediators indicated that VEGF of death and disability, despite decades of sensitizes endothelial cells to TNF. intensive research and billions of dollars Finally, to investigate whether VEGF of investment in the development of potential inhibition might be used therapeutically, mice therapies. Aird and colleagues, reporting in the were treated with soluble VEGFR1 after the onset Journal of Experimental Medicine, now show that of sepsis, which resulted in a marked vascular endothelial growth factor (VEGF) seems improvement in cardiac physiology and survival. to have a key role in the pathogenesis of sepsis, It therefore seems that further studies to and could represent a promising new target for determine both the diagnostic/prognostic value therapeutic intervention. of VEGF and the therapeutic potential of anti- A major challenge in drug development for VEGF strategies in sepsis are warranted. Such sepsis is that the complex interplay between studies might be initiated relatively rapidly, as mediators of the inflammatory and coagulation several anti-VEGF agents have been approved for pathways that are involved in the disorder is cancer treatment: the anti-VEGF antibody poorly understood. Recently, however, studies bevacizumab, and the small-molecule VEGFR have indicated an association between severe kinase inhibitors sunitinib and sorafenib. sepsis and elevated levels of VEGF, a key promoter Peter Kirkpatrick of endothelial permeability and proliferation that also seems to have pro-inflammatory and ORIGINAL RESEARCH PAPER Yano, K. et al. Vascular endothelial growth factor is an important determinant of sepsis morbidity and procoagulant effects. Aird and colleagues mortality. J. Exp. Med. 203, 1447–1458 (2006) therefore set out to test the idea that VEGF has a FURTHER READING Buras, J. A., Holzmann, B. & Sitkovsky, M. Animal models of sepsis: setting the stage. Nature Rev. Drug Discov. pathogenic role in sepsis. 4, 854–865 (2005) First, the authors assayed plasma levels of VEGF and the related placental growth factor (PlGF) in mouse and human models of infection, and found that sepsis is associated with increased expression and circulating levels of VEGF and It will be vital to determine whether PlGF. Peak levels occurred later than those of these cells are a common target for early-response cytokines such as tumour-necrosis other types of antidepressant, and factor-α (TNFα), interleukin-1 (IL-1) and IL-6. it remains to be confirmed whether VEGF mediates its effects through binding to these effects will translate to the two transmembrane receptor tyrosine kinases, human condition. Nevertheless, this VEGFR1 (also known as Flt-1) and VEGFR2 (also study sheds light on the cells targeted known as Flk-1), whereas PlGF binds to VEGFR1. by SSRIs in the brain and should A naturally occurring soluble form of VEGFR1 is direct further studies to define the also known to bind to VEGF and PlGF, and thereby precise molecular targets of these block their interaction with cell-surface receptors. drugs. Furthermore, this model So to investigate the potential for therapeutic allows different early progenitor intervention, and the roles of these various types to be more clearly defined, proteins, Aird et al. assessed the effects of several which could contribute to a better anti-VEGF strategies on sepsis pathogenesis. understanding of both hippocampal In the first strategy, adenovirus-mediated neurogenesis and the pathological overexpression of a soluble form of VEGFR1 in effects of depression. mouse models of sepsis attenuated the rise in Katherine Whalley free VEGF and PlGF levels, and blocked the adverse effects of endotoxaemia on cardiac ORIGINAL RESEARCH PAPER Encinas, J. M. et al. function, vascular permeability and mortality. Fluoxetine targets early progenitor cells in the adult brain. Proc. Natl Acad. Sci. USA 103, A second strategy, involving pre-treatment with 8233–8238 (2006) anti-VEGFR1 or anti-VEGFR2 antibodies, FURTHER READING Duman, R. S. et al. Regulation revealed that anti-VEGFR2 antibodies, but not of adult neurogenesis by antidepressant treatment. Neuropsychopharmacology 25, 836–844 (2001) | anti-VEGFR1 antibodies, reduced mortality, Santarelli, L. et al. Requirement of hippocampal which suggests that it is VEGF, and not PlGF, neurogenesis for the behavioural effects of that is a crucial mediator of the sepsis antidepressants. Science 301, 805–809 (2003) phenotype. In addition, assessing the expression

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IN BRIEF

DRUG DELIVERY Efficient gene delivery to pancreatic islets with ultrasonic microbubble destruction technology. Chen, S. et al. Proc. Natl Acad. Sci. USA 103, 8469–8474 (2006) One strategy against diabetes is to deliver genes to pancreatic islets that enhance insulin secretion, but most approaches to do this result in variable expression, require invasive techniques or generate an inflammatory reaction. Chen and colleagues report a novel technique that uses gas-filled phospholipid microbubbles containing plasmid DNA, which are then infused into rats and destroyed within the pancreatic microcirculation using ultrasound. Using this method, it was possible to deliver SCREENING a plasmid containing the rat insulin 1 promoter (RIP) fused to either human insulin or human hexokinase, both of which were able to decrease blood glucose levels. Dirty drugs’ secrets uncovered The biggest fear in pharmaceutical R&D is arguably that of the unknown — the PARKINSON’S DISEASE effects of an investigational drug that you simply can’t predict. But help could

RGS4-dependent attenuation of M4 autoreceptor be at hand. A paper in Nature Chemical Biology by MacDonald et al. describes a function in striatal cholinergic interneurons following screen that can predict ‘hidden phenotypes’ caused by a variety of drugs. dopamine depletion. The strategy uses protein-fragment complementation assays (PCA) in which Ding, J. et al. Nature Neurosci. 9, 832–842 (2006) proteins are fused with complementary fragments of a reporter enzyme that only The motor symptoms associated with Parkinson’s disease result fluoresce when the two proteins are brought into proximity. The authors from reduced striatal dopamine levels, which causes striatal constructed PCAs for known protein complexes involved in key cellular acetylcholine to rise. This was previously thought to be caused processes and measured the change in fluorescence in the presence of different by a loss of interneuronal regulation by inhibitory D2 dopamine drugs. After extracting the fluorescent signal using algorithms designed to receptors. Now, Ding et al. challenge that hypothesis with the measure an increase, decrease or change in localization of the signal, the results finding that the increase in acetylcholine is mediated by the from each assay were tabulated and subjected to hierarchical clustering to M4 muscarinic autoreceptor. They show that this adaptation results from the selective upregulation of RGS4 — a GTPase reveal relationships between drug structure and function. accelerator — which attenuates autoreceptor signalling. The authors screened 107 drugs representing six therapeutic indications at different time points in 49 PCAs. As expected, the activities of the drugs could be ANTIINFECTIVES clustered according to structure and target class, but the assays also picked up Coexpression of virulence and fosfomycin unexpected activities. To investigate these previously hidden phenotypes further, susceptibility in Listeria: molecular basis of an the authors studied a ‘supercluster’ of structurally unrelated drugs that all antimicrobial in vitro–in vivo paradox. affected growth-regulation pathways. The observation that some of these drugs were antiproliferative led the authors to propose that antiproliferative activity Scortti, M. et al. Nature Med. 12, 515–517 (2006) could be a hidden phenotype for the other drugs within that cluster. Indeed, Resistance to an antibiotic in in vitro susceptibility tests can sometimes be at odds with a successful treatment outcome in four drugs with no known antiproliferative activity — fenofibrate, niclosamide, the clinic — known as the in vitro–in vivo paradox. Scortti et al. cinnarizine and the antidepressant sertraline — blocked the proliferation of a provide the first mechanistic explanation of this paradox. They human prostate carcinoma cell line as well as four other human cancer cell lines. identified a virulence factor in Listeria monocytogenes, Hpt, that Having established that compounds that affect common pathways cause is expressed in vivo and is responsible for uptake of fosfomycin similar phenotypes, the authors attempted to evaluate the predictive power into the bacterial cell. The absence of Hpt in vitro explains the of the PCAs for the antiproliferative phenotype. By characterizing the resistance of L. monocytogenes to fosfomycin in susceptibility tests antiproliferative activity of all 107 drugs and dividing them into two and highlights the need for in vivo confirmation of in vitro data. antiproliferative or non-antiproliferative subsets they were able to identify a KINASE INHIBITORS subset of 25 PCAs that were robustly predictive of antiproliferative activity. Although proliferation is a phenotype that involves several pathways, the Selective kinase inhibition by exploiting differential power of this technique lies in its capacity to detect subtle drug-induced pathway sensitivity. changes in protein activity that might otherwise be overlooked. Equally, Kung, C. et al. Chem. Biol. 13, 399–407 (2006) because the assays are selected to report drug effects on specific pathways, Selective kinase inhibition is a desirable property of many drug it is possible to test hypotheses about how the drugs might affect specific or candidates but can be challenging to achieve for kinases within multiple cellular processes. In addition to its potential for predicting toxicity, the same subfamily. Kung et al. provide evidence to show that this screen could predict beneficial off-target effects that could broaden a kinases differ in their intrinsic sensitivity to inhibitors. They used drug’s indications or improve its superiority over other marketed therapeutics. two cyclin dependent kinases — CDK1 and PHO85 — which have distinct functions and found that an oxindole inhibitor is more Joanna Owens selective for the PHO85 than the CDK1 pathway. This suggests ORIGINAL RESEARCH PAPER MacDonald, M. L. et al. Identifying off-target effects and hidden phenotypes that hypersensitive kinases might exist that could be promising of drugs in human cells. Nature Chem. Biol. 2, 329–337 (2006) therapeutic targets for selective modulation of kinase pathways.

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