Abstracts


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Mechanisms of Action of Antiepileptic Drugs
Piotr Czapinski, Barbara Blaszczyk and Stanislaw J. Czuczwar

γ-Aminobutyric acid (GABA), one of the main inhibitory neurotransmitters in the brain, interacts with three types of receptors for GABA - GABAA, GABAB and GABAC. GABAA receptors, associated with binding sites for benzodiazepines and barbiturates in the form of a receptor complex, control opening of the chloride channel. When GABA binds to the receptor complex, the channel is opened and chloride anions enter the neuron, which is finally hyperpolarized. GABAB receptors are metabotropic, linked to a cascade of second messengers whilst the physiological meaning of ionotropic GABAC receptors, mainly located in the retina, is generally unknown. Novel antiepileptic drugs acting selectively through the GABA-ergic system are tiagabine and vigabatrin. The former inhibits neuronal and glial uptake of GABA whilst the latter increases the synaptic concentration of GABA by inhibition of GABA-aminotransferase. Gabapentin, designed as a precursor of GABA easily entering the brain, was shown to increase brain synaptic GABA. This antiepileptic drug also decreases influx of calcium ions into neurons via a specific subunit of voltage-dependent calcium channels. Conventional antiepileptics generally inhibit sodium currents (carbamazepine, phenobarbital, phenytoin, valproate) or enhance GABA-ergic inhibition (benzodiazepines, phenobarbital, valproate). Ethosuximide, mainly controlling absences, reduces calcium currents via T-type calcium channels. Novel antiepileptic drugs, mainly associated with an inhibition of voltage-dependent sodium channels are lamotrigine and oxcarbazepine. Since glutamate-mediated excitation is involved in the generation of seizure activity, some antiepileptics are targeting glutamatergic receptors - for instance, felbamate, phenobarbital, and topiramate. Besides, they also inhibit sodium currents. Zonisamide, apparently sharing this common mechanism, also reduces the concentration of free radicals. Novel antiepileptic drugs are better tolerated by epileptic patients and practically are devoid of important pharmacokinetic drug interactions.


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The Biology of p38 Kinase: A Central Role in Inflammation
Gary L. Schieven

The p38 kinase plays a central role in inflammation, and it has been the subject of extensive efforts in both basic research and drug discovery. This review summarizes the biology of the p38 kinase with a focus on its role in inflammation. The p38 kinase regulates the production of key inflammatory mediators, including TNFα, IL-1β, and COX- 2. In addition, p38 also acts downstream of cytokines such as TNFα, mediating some of their effects. The potential efficacy of p38 inhibitors may thus be greater than would be expected from the inhibition of the mediators alone. Inhibitors of p38 kinase are currently in development for the treatment of rheumatoid arthritis. The biological processes regulated by p38 kinase suggest a wide variety of additional potential indications.


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The Akt/PKB Family of Protein Kinases: A Review of Small Molecule Inhibitors and Progress Towards Target Validation
Stanley F. Barnett, Mark T. Bilodeau and Craig W. Lindsley

This article describes recent advances in the development and biological evaluation of small molecule inhibitors for the serine/threonine kinase Akt (PKB). Akt plays a pivotal role in cell survival and proliferation through a number of downstream effectors. Recent studies indicate that unregulated activation of the PI3K/Akt pathway is a prominent feature of many human cancers and Akt is over-expressed or activated in all major cancers. Akt is considered an attractive target for chemotherapy and it has been postulated that inhibition of Akt alone or in combination with standard cancer chemotherapeutics will reduce the apoptotic threshold and preferentially kill cancer cells. The development of specific and potent inhibitors will allow this hypothesis to be tested in animals. The majority of small molecule inhibitors in this nascent field are classic ATP-competitive inhibitors which provide little specificity. Phosphatidylinositol (PI) analogs have been reported to inhibit Akt, but these inhibitors may also have specificity problems with respect to other PH domain containing proteins and may have poor bioavailability. None of the inhibitors in these classes have been reported to have Akt isozyme specificity. Recently, novel allosteric inhibitors have been reported which are pleckstrin homology domain dependent and exhibit Akt isozyme selectivity. Inhibitors in this class may have sufficient potency and specificity to test for tumor efficacy in animal models and recently reported preliminary experiments are reviewed.


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New Approaches to the Treatment of Inflammatory Disorders Small Molecule inhibitors of p38 MAP Kinase
Christian Peifer, Gerd Wagner and Stefan Laufer

The therapy of chronic inflammatory diseases like rheumatoid arthritis (RA) and inflammatory bowel disease (IBD) has recently been enriched by the successful launch of the anti-cytokine biologicals Etanercept (tumor necrosis factor (TNF) receptor-p75 Fc fusion protein), Infliximab (chimeric anti-human TNF-α monoclonal antibody), Adalimumab (recombinant human anti-human TNF-α monoclonal antibody) and Anakinra (recombinant form of human interleukin 1β (IL-1) receptor antagonist) [1-3]. The success of these novel treatments has impressively demonstrated the clinical benefit that can be gained from therapeutic intervention in cytokine signalling, highlighting the central role of proinflammatory cytokine systems like IL-1β and TNF-α to be validated targets [4] However, all of the anti-cytokine biologicals available to date are proteins, and therefore suffering to a varying degree from the general disadvantages associated with protein drugs. Therefore, small molecular, orally active anti-cytokine agents, which target specific pathways of proinflammatory cytokines, would offer an attractive alternative to anti-cytokine biologicals. A number of molecular targets have been identified for the development of such small molecular agents but p38 mitogen-activated protein (MAP) kinase occupies a central role in the regulation of IL-1β and TNF-α signalling network at both the transcriptional and translational level [5, 6]. Since the mid-1990s, an immense number of inhibitors of p38 MAP kinase has been characterised in vitro, and to date several compounds have been advanced into clinical trials. This review will highlight the correlation between effective inhibition of p38 MAP kinase at the molecular target and cellular activity in functional assays of cytokine, particularly TNF-α and IL-1β production. SAR will be discussed regarding activity at the enzyme target, but also with regard to properties required for efficient in vitro and in vivo activity.


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From Artemisinin to New Artemisinin Antimalarials: Biosynthesis, Extraction, Old and New Derivatives, Stereochemistry and Medicinal Chemistry Requirements
Richard K. Haynes

The artemisinin derivatives, dihydroartemisinin (DHA), artesunate, atemether and arteether, are currently used for treatment of malaria in artemisinin combination therapies (ACT) with longer half-life drugs. The demand is enormous - in 2005, the estimated global demand for one such ACT alone, artemether-lumifantrine, which constitutes about 70% of all current clinically-used ACTs, is for 120 million adult treatment courses. At 0.5 gm of artemether per total dose regimen, the amount of artemisinin required is approximately 114 tons. This has placed substantial stress on total artemisinin supplies world-wide, and considerable attention is being focussed on enhancing availability of artemisinin by improvement in horticultural practice and extraction of artemisinin from Artemisia annua. Artemisinic acid, which also occurs in A. annua, can be converted into artemisinin and is the ultimate target of a biotechnological approach, which if successful, will augment artemisinin supply in the future. The conversion of artemisinin into the known artemisinin derivatives, and problems with the methods are critically reviewed. Some attention is paid to mechanistic aspects which clarify stereochemistry. The current artemisinins are by no means ideal drugs. Artesunate in particular is incompatible with basic quinolines by virtue of proton transfer, and has intrinsic chemical instability. At pH 1.2, conversion to DHA is rapid, with t1/2 26 min, and at pH 7.4, t1/2 is about 10 hours. With a pKa of 4.6, over 99% of artesunate will be ionized at pH 7.4, and thus uptake by passive diffusion from the intestinal tract will be minimal. Although a considerable effort has been vested in the search for new artemisinins, largely through functionalization of artemisinin at C-10, O-11 or at C-15 via artemisitene, or of DHA at C-10, deliberate enhancement of the 'druggability' of artemisinins by reducing lipophilicity, which at the same time will attenuate the neurotoxicity characteristic of the current derivatives, and enhance absorption, by and large has not been considered. A review of the various types of newer derivatives is given together with a consideration of medicinal chemistry aspects.


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Pathway to the Clinic: Inhibition of P38 MAP Kinase. A Review of Ten Chemotypes Selected for Development
David M. Goldstein and Tobias Gabriel

p38 mitrogen activated protein (MAP) kinase remains the most compelling therapeutic target for oral drug intervention for a wide range of autoimmune disorders based on the central role this enzyme plays in inflammatory cell signaling. Efforts to discover inhibitors of p38 suitable for clinical investigation have continued to escalate in part due to the incredible diversity of unique chemotypes reported to inhibit the enzyme. Since 1993, at least seventeen p38 inhibitors have been reported to have entered into clinical trials. Next generation inhibitors have been disclosed with improved potency for p38 and enhanced selectivity versus other protein kinases. Over the last three years, there have been multiple reports of cytokine suppression in humans following oral administration of p38 inhibitors. These results, in addition to proof of concept studies in rheumatoid patients, have established p38 inhibition as an avenue for the future management of pro-inflammatory cytokine based diseases. This review describes the discovery at Roche of novel p38 inhibitors which have advanced into clinical trials. The pharmacology of the Roche compounds is then compared with eight chemically distinct p38 inhibitors known to have entered clinical development.


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Library Design for Fragment Based Screening
Ansgar Schuffenhauer, Simon Ruedisser, Andreas Marzinzik, Wolfgang Jahnke, Paul Selzer and Edgar Jacoby

According to Hann's model of molecular complexity an increased probability of detection binding to a target protein can be expected when small, low complex molecular fragments are screened with high sensitivity instead of fullsized ligands with lower sensitivity. Analysis of the HTS summary data of Novartis and comparison with NMR screening results obtained on generic fragment libraries indicate this expectation to be true with hitrates of 0.001% - 0.151% observed in the identification of ligands with an IC50 threshold in the micromolar range in an HTS setup and hitrates above or equal to 3% observed in NMR screening of fragments with an affinity threshold in the millimolar range. It is however necessary to keep in mind that the sets of target studied were not identical for both method and the experience in NMR screening is too limited for a final conclusion. The term hitrate as used here reflects only the success rate in the observation of ligand binding event. It must not be confused with the overall success rate of fragment and high throughput screening in the lead finding process, which can be entirely different, since the steps required to follow-up a ligand binding event to a lead are different for both methods.

A survey of fragment-based lead discovery case studies given in the literature shows that in approximately half of the cases the initial hit fragment was discovered by screening a generic library, whereas in the other cases some knowledge about an initial ligands or the protein binding site has been used, whereas systematic virtual screening of fragment databases has been only rarely reported.

As comparatively high hitrates were obtained, further consideration to optimize the generic fragment screening library were directed to the chemical tractability of the fragment. As several functional groups preferred by chemists for modification and linking of the fragments are also preferentially involved in interactions between the fragments and the target protein, a set of screening fragments was derived from chemical building blocks by masking its linker group by a chemical transformation which can be later on used in the chemical follow-up of the fragment hit. For example primary amines can be masked as acetamides. If the screening fragment is active the related building block can then be used for synthesis of a follow-up library.


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Mitotic Kinesins: Prospects for Antimitotic Drug Discovery
Gustave Bergnes, Katjusa Brejc and Lisa Belmont

Kinesins, mechanochemical enzymes that utilize the energy of ATP to translocate along or destabilize microtubules, are essential for accurate completion of cell division. Recently, small moleculer inhibitors of one kinesin, kinesin spindle protein (KSP/Eg5/kinesin5), have been shown to be efficacious in pre-clinical studies, with one quinazolinone-based inhibitor advancing to Phase II clinical trials as a potential anticancer chemotherapeutic agent. This highlights the potential of KSP and other mitotic kinesins as targets for chemotherapeutic intervention. Ten other kinesins have been shown to play essential roles in cell division and thus may provide additional therapeutic opportunities. In this review, the biological roles of these proteins are described with emphasis on their importance to cell proliferation. In addition, kinesin motor domain structure and mechanism are described with particular attention given to the conformational changes that offer opportunities for chemical inhibition. Finally, a current list of KSP inhibitor classes is described in the context of their potential as clinical leads.


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Eicosanoids in Inflammation: Biosynthesis, Pharmacology, and Therapeutic Frontiers
Subhash P. Khanapure, David S. Garvey, David R. Janero and L. Gordon Letts

In mammalian cells, eicosanoid biosynthesis is usually initiated by the activation of phospholipase A2 and the release of arachidonic acid (AA) from membrane phospholipids. The AA is subsequently transformed by cyclooxygenase (COX) and lipoxygenase (LO) pathways to prostaglandins, thromboxane and leukotrienes collectively termed eicosanoids. Eicosanoid production is considerably increased during inflammation. Both COX and LO pathways are of particular clinical relevance. The COX pathway is the major target for non-steroidal anti-inflammatory drugs (NSAIDs), the most popular medications used to treat pain, fever and inflammation. Although their anti-inflammatory effects are well known, their long-term use is associated with gastrointestinal (GI) complications such as ulceration. In 1991, it was discovered that COX exists in two distinct isozymes, COX-1 and COX-2, of which COX-2 is primarily expressed at sites of inflammation and produces pro-inflammatory eicosanoids. For this reason, COX-2 selective inhibitors (COXIBs) have been developed recently as anti-inflammatory agents to minimize the risk of GI toxicity. Recently, some COX-2 selective inhibitors have shown adverse cardiovascular side effects, resulting in the withdrawal of rofecoxib and valdecoxib from the market. Selective inhibition of COX-2 without reducing COX-1-mediated thromboxane production could alter the balance between prostacyclin and thromboxane and promote a prothrombotic state, thereby explaining the observed COX- 2 cardiovascular risk. In this review, we describe mechanisms for the production of pro-inflammatory eicosanoid mediators contributing to inflammation and summarize promising options for the prevention of inflammatory mediator formation and the therapeutic inhibition of pain and inflammation.


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Predicting Antimicrobial Drugs and Targets with the MARCH-INSIDE Approach
Humberto Gonzalez-Diaz, Francisco Prado-Prado and Florencio M. Ubeira

The method MARCH-INSIDE (MARkovian CHemicals IN SIlico DEsign) is a simple but efficient computational approach to the study of Quantitative Structure-Activity Relationships (QSAR) in Medicinal Chemistry. The method uses the theory of Markov Chains to generate parameters that numerically describe the chemical structure of drugs and drug targets. This approach generates two principal types of parameters Stochastic Topological Indices (sto-TIs) and stochastic 3D-Topographic Indices (sto-TPGIs). The use of these parameters allows the rapid collection, annotation, retrieval, comparison and mining of molecular and macromolecular chemical structures within large databases. In the work described here, we review and comment on the several applications of MARCH-INSIDE to the Medicinal Chemistry of Antimicrobial agents as well as their molecular targets. First we revised the use of classic sto-TIs to predict antiparasite compounds for the treatment of Fascioliasis. Next, we revised the use of chiral sto-TIs (sto-CTIs) to predict new antibacterial, antiviral and anti-coccidial compounds. After that, we review multi-target sto-TIs (mt-sto-TIs), which unifying QSAR models predicting antifungal, antibacterial, or anti-parasite drugs with multiple targets (microbial species). We also discussed the uses of mt-sto-TIs to assemble drug-drug similarity Complex Networks of antimicrobial compounds based on molecular structure. Last, we review the use of MARCH-INSIDE to generate macromolecular TIs and TPGIs for proteins or RNA targets for antimicrobial drugs.