Current Medicinal Chemistry - Volume 11, Issue 20, 2004

The release of hormones is subject to a complex and finely tuned regulation system. The biosynthesis plays a key role by specifically converting the prohormone precursor into its biological active product(s). A family of mammalian proteases could be identified to be responsible for the endoproteolytic processing. These subtilisin / kexin-like prohormone convertases (PC) recognize their substrates at single or pairs of basic residues with a high substrate specificity. The so far known seven members include PC1 / 3, PC2, furin / PACE, PACE4, PC4, PC5 / 6 and PC7 / SPC7 / LPC / PC8. PC1 / 3 and PC2 are the most important enzymes for the processing of prohormones, whereas furin is the only one that causes lethality in knock-out models. Tissuespecific co-localization of the prohormone and the PC as well as distinct characteristics of both, like the secondary structures, determine the possible conversion processes. Identification of such determinants implies a great potential for the development of novel drug targets. To obtain sufficient amounts for the in vitro characterization of prohormones, chemical and recombinant synthesis methods have been developed. Application of expressed protein ligation lead to the semisynthesis of the first chemically modified analogs of a full-length proneurohormone (pro-neuropeptide Y). Structural analyses mainly on peptides of the prooxytocin / neurophysin system and on prosomatostatin highlighted the importance of flexible turn or loop structures adjacent to the cleavage site for the specific substrate-enzyme active site interaction. Prohormones and their processing show multiple functions. Therapeutic application including PC inhibitors is very promising for the treatment of disorders like cancer.

Persistent infection with the hepatitis B virus (HBV) represents a major health problem worldwide with over 350 million patients at risk of developing liver cirrhosis or hepatocellular carcinoma. HBV is a small, partially double-stranded DNA virus with four overlapping genes and a unique life cycle, creating an intracellular pool of covalently closed circular DNA molecules for persistence and an RNA template for replication via reverse transcription. Mutations occur frequently, and particular selection pressures, both endogenous (host immune clearance) and exogenous (vaccines and antivirals), readily select escape mutants. For example, HBeAg-negative chronic HBV infection with either basal core promoter or precore mutations is predominant in many parts of the world. Therapy of HBV infection with the nucleoside analogue lamivudine frequently leads to the selection of drug-resistant strains with polymerase mutations. Treatment options for chronic HBV infection include at present either interferon-alpha or the oral nucleos(t)ide analogues lamivudine or adefovir. However, these drugs have drawbacks, including possible serious side effects and low response rates in HBeAg-negative patients in the case of interferon or recurrence of viremia after cessation of therapy and development of escape mutants after a long period of treatment with nucleoside inhibitors. Recent advances of in vitro and in vivo models allow to study new antiviral strategies, including novel nucleoside analogues, nucleocapsid inhibitors or small interfering RNA. This review summarises the impact of clinically relevant mutations in the HBV genome on viral replication and drug sensitivity, the current status of therapy and promising future perspectives on novel drug regimens.

In recent years, the gaseous molecule nitric oxide (NO) has been shown to be involved in many important biological events. S-Nitrosothiols are biological substances derived endogenously from NO, and are found in a variety of tissues exhibiting NO-mimetic activity. Fundamental studies on the chemical aspects of S-nitrosothiols have become an integral part of NO research, with a view to further understanding the numerous biological functions both of NO and of S-nitrosothiols themselves. It has often been suggested that Snitrosothiols represent a means either for the storage or transport of NO, although the evidence for this is sparse. Many S-nitrosothiols have now been synthesized chemically, and at present they show the most promise as clinically useful NO-donor drugs. In this review, we examine in detail the biological functions of Snitrosothiols, as well as the chemical properties and biomedical applications of these compounds.

Many non-cardiovascular drugs of common clinical use cause, as an unwanted accessory property, the prolongation of the cardiac repolarisation process, due to the block of the HERG (Human Ether-a-go-go Related Gene) potassium channel, responsible for the repolarising IKr current. This delayed cardiac repolarisation process can be often unmasked by a prolongation of the QT interval of the ECG. In these conditions, premature action potentials can generate morphologically anomalous after-polarisations, and trigger a dangerous kind of polymorphic ventricular tachyarrhythmia, known as torsade de pointes, which can evolve in ventricular fibrillation and death. The risk associated with the torsadogenic cardiotoxicity of drugs, which prolong the QT interval has been the topic of documents produced by many health authorities, giving important issues about the preclinical and clinical evaluation of cardiac safety. Besides, public and private research laboratories developed several experimental in vitro or in vivo strategies, aimed to an early recognition of the influence of a drug (or of a drugcandidate) on the HERG channel and / or on the cardiac repolarisation process. Also the identification of a possible pharmacophore model, common in all or at least in numerous torsadogenic drugs, could represent a first step for the development of useful in silico approaches, allowing a preliminary indication about the potential torsadogenic property of a given molecule. In this work, we described the electrophysiological basis of torsade de pointes and listed several pharmacological classes of torsadogenic drugs. Among them, we focused our attention on antipsychotics, with an accurate overview on the experimental and clinical reports about their torsadogenic properties. Moreover, a common structural feature exhibited by these drugs, despite of their remarkable chemical differences, is evidenced by a computational approach and is indicated as a possible “facilitating” requirement for their torsadogenic properties. Together with other remarks, coming from different computational studies, the individuation of a satisfactory “toxicophore” model could be greatly useful, for the theoretical prediction of torsadogenic properties of a given chemical moiety and for the design of new drugs devoid of such an undesired and potentially lethal side-effect.

Diabetes Mellitus is by far one of the most propagated chronic diseases, affecting 150 million people worldwide. This affliction is caused by a malfunction of pancreatic endocrine cells, which provokes a failure in the insulin release and glucose homeostasis. Plasma membrane KATP channels have a key role in the stimulus-secretion coupling of pancreatic β-cells. Consequently, many investigations have developed efficient drugs for the treatment of diabetes, such as sulphonylureas, which specifically close KATP channels leading to an enhanced insulin secretion. Recent studies show that, in addition to its well-known plasma membrane location, sulphonylurea receptors and sulphonylurea-sensitive KATP channels are also present in various intracellular sites including secretory granules, mitochondria, endoplasmic reticulum and more recently, the nucleus. What roles do they play in these organelles? Intracellular KATP channels and sulphonylurea receptors, which operate in conjunction with classical pathways, can provide specific signaling circuits to establish direct links between extracellular signals and different cell functions, such as secretion or gene expression. The study of these intracellular channels provides novel perspectives in the signal transduction of the pancreatic β-cell, and may offer clues for the development of new strategies in diabetes therapy. In this review we will address this topic with special emphasis on the biophysical basis and functional implications in the pancreatic β-cell.

Glucose transporters, or membrane proteins, which incorporate glucose into the cell, can be divided into two groups: the facilitative type glucose transporter (GLUT), and the sodium / glucose cotransporter (SGLT). Among the GLUT family isoforms, GLUT4 is particularly important for maintaining glucose metabolism homeostasis since it is involved in insulin or exercise-induced glucose transport into muscle and adipose tissues via movement from intracellular sites to the plasma membrane in response to stimulation. Thus, agents which induce GLUT4 translocation or improve insulin sensitivity, involved in this insulininduced step, hold the promise of being potent anti-diabetic drugs. On the other hand, SGLT is expressed specifically in the intestines and kidneys. Oral administration of a SGLT inhibitor, T-1095, lowers the blood glucose concentration via excretion of glucose in the urine, due to suppression of renal SGLT function. In addition to this direct blood glucose lowering effect, T-1095 has been shown to restore impaired insulin secretion from pancreatic β-cells, as well as to improve insulin resistance in muscle and liver. Thus, this SGLT inhibitor is regarded as a novel and promising agent for the treatment of diabetes mellitus.

Reversible phosphorylation of Tyr residues in proteins plays a central role in the transduction of signals. For both SH2 domains and for protein tyrosine phosphatases (PTPs) the phosphate group of phosphotyrosine (pTyr) of peptides provides a key affinity element, but its highly charged nature and its hydrolytic lability render it unsuitable in inhibitor design. The research in the recent years has been addressed to find pTyr bioisosters devoid of the phenylphosphate moiety and more potent inhibitors with less peptidic character. Several derivatives were prepared as pTyr bioisosters, and their activity appears to depend on the nature of the substrate, peptidic or low-molecular weight compounds, in which they are placed. In the field of PTPs, the research was mainly focused on new and selective PTP1B inhibitors, possibly useful in the treatment of Type 2 diabetes. The discovery of non-peptidic low molecular weight compounds able to inhibit PTP1B, by means of docking procedures and HTS screening, and the presence of secondary binding sites on PTP1B afforded new potent and selective inhibitors; several leads devoid of negative charges were also found. To date, however, few compounds have been tested In vivo and found to show a significant activity in diabetic mouse models. Other neutral compounds, mainly quinones, were found to inhibit CD45 and Cdc25. Several papers have appeared in recent years on the discovery of new Grb2, Src, Syk, and Lck SH2 domains binding antagonists. In this field very good inhibitors derived from high affinity peptides were found, with less peptidic character and with a reduced number of negative charges; however the presence of some negative charges, especially the one present on the pTyr bioisoster moiety, seems to be indispensable. As regards Grb2, Src and Lck SH2 domains, rigidification of the starting high affinity binding peptides afforded derivatives with improved affinity; cellular activity was achieved by modification of the side chains of these inhibitors.

The treatment of viral diseases remains one of the major challenges to modern medicine. During the past two decades there has been increased recognition of the consequences of serious viral illnesses that are not controlled by vaccination. These illnesses include human immunodeficiency virus, human herpes viruses, and viruses that cause hepatitis. There are now eight pathogens recognized in the herpes virus family that cause infections in humans. Infections by the herpes viruses are opportunistic and often life-threatening, leading to significant morbidity and mortality in the increasing number of chronically immune compromised individuals such as AIDS patients, cancer patients and transplant recipients on immunosuppressive therapy. Nearly all individuals with AIDS are infected with one or more of the herpes viruses. Antiviral therapy with guanosine nucleoside analogs acyclovir and ganciclovir has had a major impact on diseases caused by herpes simplex virus type-1 and type-2 (HSV-1, HSV-2), Varicella zoster virus (VZV), and human cytomegalovirus (HCMV) but development of resistant virus strains and the absence of any effective treatment for other members of the herpes family provide a stimulus for increased search of new agents effective against various herpes viruses. Pyrimidine nucleosides have taken up an important role in the therapy of virus infection. Significant progress in the study of anti-herpes nucleosides has been made by the advent of 5-substituted pyrimidine nucleosides such as 5-iodo-, 5-ethyl-, 5-(2-chloroethyl)-, and (E)-5-(2-bromovinyl)- derivatives of 2'-deoxyuridine. These are highly specific inhibitors of HSV-1, HSV-2, and / or VZV infections. However, Epstein Barr virus (EBV) and HCMV are much less sensitive to these agents. In 5-substituted pyrimidine nucleosides the nature of substituents, particularly at the C-5 position, has been found to be an important determinant of anti-herpes activity. Structural requirements at the C-2 carbon of the 5-substituent of pyrimidine nucleosides have been well established for anti-herpes activity. However, there is little qualitative or mechanistic knowledge of the derivatives with substitution at the C-1 carbon of the 5-substituent of pyrimidine nucleosides. During the last few years of our research, we have investigated a variety of C-1 functionalized substituents at the 5-position of the pyrimidine nucleosides to determine their usefulness as antiviral (herpes) agents. In the 5-(1-substituted) group of pyrimidine nucleosides, we demonstrated that novel substituents present at the C-1 carbon of the 5-side chain of the pyrimidine nucleosides are important determinants of potent and broad spectrum antiviral (herpes) activity including EBV and HCMV. In this article the work on design, synthesis and structure activity relationships of several 5-[(1-substituted) alkyl (or vinyl)] pyrimidine nucleoside derivatives as potential inhibitors of herpes viruses is reviewed.

Advanced glycation endproducts (AGE) are a class of compounds resulting from glycation and oxidation of proteins, lipids or nucleic acids. Glycation is the non-enzymatic addition or insertion of saccharide derivatives to these molecules. This leads to the formation of intermediary Schiff bases and Amadori products and finally to irreversible AGE. This classical view has been modified in recent years with recognition of the importance of oxidative and carbonyl stress in endogenous AGE formation. AGE may also have exogenous sources, in certain foods and tobacco smoke. A whole class of specific and non-specific receptors binding AGE has been characterized. Apart from cross-linking of proteins by AGE, the effects of receptor stimulation contribute to the development of chronic complications of conditions like diabetes mellitus, renal failure, and atherosclerosis. Possible interventions to reduce the effects of AGE accumulation include AGE formation inhibitors or breakers, or receptor blockers, but possibly also dietary interventions. Some of the problems with current assay or diagnostic techniques, and several unresolved issues on the role of AGE in disease will be discussed. Our review will focus on the clinical and pharmaceutical implications of these developments.