Current Medicinal Chemistry - Volume 13, Issue 3, 2006

Protease activated receptors (PARs) are a category of G-protein coupled receptors (GPCRs) implicated in the progression of a wide range of diseases, including thrombosis, inflammatory disorders, and proliferative diseases. Signal transduction via PARs proceeds via an unusual activation mechanism. Instead of being activated through direct interaction with an extracellular signal like most GPCRs, they are self-activated following cleavage of their extracellular N-terminus by serine proteases to generate a new receptor N-terminus that acts as an intramolecular ligand by folding back onto itself and triggering receptor activation. Short synthetic peptides corresponding to this newly exposed N-terminal tethered ligand can activate three of the four known PARs in the absence of proteases, and such PAR activating peptides (PAR-APs) have served as templates for agonist/antagonist development. In fact much of the evidence for involvement of PARs in diseases has relied upon use of PAR-APs, often of low potency and uncertain selectivity. This review summarizes current structures of PAR agonists and antagonists, the need for more selective and more potent PAR ligands that activate or antagonize this intriguing class of receptors, and outlines the background relevant to PAR activation, assay methods, and physiological properties anticipated for PAR ligands.

Urotensin-II (U-II) is a "somatostatin-like" cyclic neuropeptide which was originally isolated from goby fish urophysis, and subsequently identified in other species, including man. The interest in human U-II (hU-II) has grown enormously in the last few years, following the identification of a specific human receptor (formerly identified as the GPR14/SENR orphan receptor), now referred to as UT receptor. The U-II/UT system seems to play an important role in cardiovascular functions. hU-II vasoconstrictive potency is reported to be an order of magnitude greater than that of endothelin-1 (ET-1), which would make it the most potent mammalian vasoconstrictor identified to date. hU-II also exerts potent inotropic effects in the human heart in vitro. On the basis of its spectrum of activities, hU-II has been suggested to modulate cardiovascular homeostasis and possibly to be involved in certain cardiovascular pathologies. Central nervous effects of U-II have also been described, in particular, intracerebroventricular administration promotes anxiogenic-like behaviors in rodents. Furthermore, UT receptor overexpression has been observed in some tumor cell lines. Therefore, specific and selective UT receptor antagonists provide useful tools for investigating the (patho)physiological role(s) of the U-II/UT receptor system. In this review we aim to provide an overview of the research in the area of UT receptor antagonists as well as the progress in understanding the role of the UII/ UT system in human (patho)physiology.

Epidermal Growth Factor Receptor (EGFR) is a high priority target in anticancer drug research. Thousands of very effective EGFR inhibitors have been developed in the last decade. The known inhibitors are originated from a very diverse chemical space but - without exception - all of them act at the Adenosine TriPhosphate (ATP) binding site of the enzyme. We have collected all of the diverse inhibitor structures and the relevant biological data obtained from comparable assays and built prediction oriented Quantitative Structure- Activity Relationship (QSAR) which models the ATP binding pocket's interactive surface from the ligand side. We describe a QSAR method with automatic Variable Subset Selection (VSS) by Genetic Algorithm (GA) and goodness-of-prediction driven QSAR model building, resulting an externally validated EGFR inhibitory model built from pIC50 values of a diverse structural set of 623 EGFR inhibitors. Repeated Trainings/Evaluations (RTE) were used to obtain model fitness values and the effectiveness of VSS is amplified by using predictive ability scores of descriptors. Numerous models were generated by different methods and viable models were collected. Then, intensive RTE were applied to identify ultimate models for external validations. Finally, suitable models were validated by statistical tests. Since we use calculated molecular descriptors in the modeling, these models are suitable for virtual screening for obtaining novel potential EGFR inhibitors.

Pyrimidine nucleotides, including UTP, UDP and UDP-glucose, are important signaling molecules which activate G protein-coupled membrane receptors (GPCRs) of the P2Y family. Four distinct pyrimidine nucleotidesensitive P2Y receptor subtypes have been cloned, P2Y2, P2Y4, P2Y6 and P2Y14. P2Y2 and P2Y4 receptors are activated by UTP (the P2Y2, and the rat but not the human P2Y4 receptor are also activated by ATP), the P2Y6 receptor is activated by UDP, and the P2Y14 receptor by UDP-glucose. Furthermore, non-P2Y GPCRs, the cysteinylleukotriene receptors (CysLT1R and CysLT2R) have been described to be activated by UDP in addition to activation by cysteinylleukotrienes. While P2Y2, P2Y4, and P2Y6 receptor activation results in stimulation of phospholipase C, the P2Y14 receptor is coupled to inhibition of adenylate cyclase. Derivatives and analogs of the physiological nucleotides UTP, UDP and ATP have been synthesized and evaluated in order to obtain enzymatically stable, subtype-selective agonists. The P2Y2 receptor agonists diuridine tetraphosphate (diquafosol) and the uracilcytosine dinucleotide denufosol are currently undergoing clinical trials for dry eye disease, retinal detachment disease, upper respiratory tract symptoms, and cystic fibrosis, respectively. The first antagonists for P2Y2 and P2Y6 receptors that appear to be selective versus other P2Y receptor subtypes have recently been described. Selective antagonists for P2Y4 and P2Y14 receptors are still lacking. Uracil nucleotide-sensitive P2Y receptor subtypes may constitute future targets for the treatment of certain cancer types, vascular diseases, inflammatory diseases, and immunomodulatory intervention. They have also been proposed to play a role in neurodegenerative diseases. This article is an updated version of "P2-Pyrimidinergic Receptors and Their Ligands" by C. E. Müller published in Curr. Pharm. Des. 2002, 8, 2353-2369.

Human Immunodeficiency Virus type 1 Reverse Transcriptase (HIV-1 RT) is one of the most important targets for treatment of Acquired Immune Deficiency Syndrome (AIDS). It catalyzes the reverse transcription of HIV-RNA into a double stranded DNA, and the knowledge of its substrate specificity and catalytic mechanism has guided the development of several inhibitors widely used on current HIV/AIDS therapy. However, mutations in HIV-1 RT structure can lead to the emergence of drug-resistant virus strains. The goal of this review is to summarize relevant structural features of HIV-1 RT and its inhibitors in such a way that this cost-effective target in the development of new antiretroviral drugs is particularly highlighted.

In healthy individuals, natural and adaptive immune responses are able to control virus entry into the host. In particular, CD8+-mediated cytotoxicity, sustained by the intervention of CD4+ cells, represents the major key event leading to virus eradication. On the other hand, viruses are able to evade from host immune response via several mechanisms, and special emphasis will be placed on hepatitis C virus and chronic Epstein-Barr infections also in view of personal data. Virokines, viroreceptors, and serpins greatly contribute to viral immune escape, and, among virokines, interleukin (IL)-10 has been object of intensive studies. Finally, all these products have been used as biopharmaceuticals, and, for instance, viral IL-10, chemokine-binding proteins, and serpins exhibit in animal models immunosuppressive, antiinflammatory, and antiatherogenic activities. As far as their use in human trials is concernded, many cautions are required in order to avoid deleterious side effects and, in particular, the purity of the product, its route and frequency of administration, as well as the immune status of the patient should be taken into serious account.

The progresses made in the field of drug design to combat tropical protozoan parasitic diseases, such as Chagas' disease, leishmaniasis, and sleeping sickness are discussed. This article is focused on different approaches based on unique aspects of parasites biochemistry and physiology, selecting the more promising molecular targets for drug design. In spite of the enormous amount of work on the above features, the chemotherapy for all of these diseases remains unsolved. It is based on old and fairly not specific drugs associated, in several cases, with long-term treatments and severe side effects. Drug resistance and different strains susceptibility are further drawbacks of the existing chemotherapy. In this review article, a thorough analysis of selected molecular targets, mainly those that are significantly different compared with the mammalian host or, even, are not present in mammals would be described in terms of their potencial usefulness for drug design. Therefore, this article covers rational approaches to the chemotherapeutic control of these parasitic infections, such as the progresses in the search for novel metabolic pathways in parasites that may be essential for parasites survival but with no counterpart in the host. Ergosterol biosynthesis is a very interesting example. There are many enzymes involved in this biosynthetic pathway such us squalene synthase, farnesylpyrophosphate synthase, and other enzymes that are able to deplete endogenous sterols will be treated in this article. The enzymes involved in trypanothione biosynthesis, glutathionyl spermidine synthetase and trypanothione synthetase do not have an equivalent in mammals, and therefore it can be predicted low toxicity for compounds that are able to produce highly selective inhibition. Trypanothione reductase (TR), glyceraldehyde-3-phosphate dehydrogenase, dihydrofolate reductase, prenyltransferases, ornithine decarboxylase, etc, will be thoroughly analyzed. The design of specific inhibitors of such metabolic activities as possible means of controlling the parasites without damaging the hosts will be presented. The recent advances in the biochemistry of pathogenic parasites including the discovery of novel organelles will be discussed.