Current Medicinal Chemistry - Anti-Infective Agents - Volume 3, Issue 4, 2004

βlactamases are bacterial enzymes highly involved in resistance to βlactam antibiotics. They have demonstrated to be structurally very flexible. Amongst them, Ambler's class A enzymes are widely spread and has revealed an unbelievable plasticity of their structure including their active site. From the ancestral plasmidmediated βlactamases: TEM-1, TEM-2 and SHV-1, a large number of Extended-Spectrum- (ESBL) and Inhibitor- Resistant- (IRBL) βlactamases have been identified. Surprisingly few narrow-spectrum variant enzymes were also identified. By the end of 2003, more than 120 TEM- and more than 50 SHV-mutant enzymes were reported from clinical isolates. They differ from the parental enzymes by a rather small number of amino acid substitutions located at a large number of possible locations. Some of these substitutions are critical for modification of the catalytic properties and have been often well explored, mostly by directed mutagenesis: the “major substitutions”, whereas others seem to be poorly related with these properties: the “minor substitutions”. The possible role of these substitutions is discussed in function of their location in the crystal structures of some of these enzymes.

Staphylococci are important pathogens of nosocomial infections and foreign body associated infections. Although anti-microbial agents have played important roles in modern medicine and are well developed, antibiotic resistance of microorganisms has increased rapidly, shortening the useful lifetimes of currently available agents. Taking into account that the first clinical isolate of vancomycin resistant Staphylococcus aureus (VRSA) was reported in the United States in 2002, it is urgent that new anti-microbial agents should be developed to deal with this circumstance. In this paper, some novel approaches to develop anti-staphylococcal agents will be reviewed.

The aminocoumarin antibiotics novobiocin, clorobiocin and coumermycin A1 are potent inhibitors of DNA gyrase. They compete with ATP for binding to the B subunit of this enzyme and inhibit the ATP-dependent DNA supercoiling catalysed by gyrase. Cloning, sequencing and analysis of the biosynthetic gene clusters for novobiocin, clorobiocin and coumermycin A1 revealed that the structural differences and similarities of these antibiotics are perfectly reflected by the genetic organisation of the clusters. In the last four years, the function of most of the biosynthetic genes for these antibiotics could be identified by gene inactivation experiments as well as by heterologous expression in Escherichia coli or Streptomyces lividans as His6-tag or GST-tag fusion proteins. Using the information from the biosynthetic gene clusters, more than 50 new aminocoumarin derivatives have been produced by combinatorial biosynthesis, by mutasynthesis experiments or by chemoenzymatic synthesis. The biological activities of the new aminocoumarines were evaluated against E. coli gyrase and various bacterial strains. The results showed that the acyl moieties at 3''-OH as well as the methyl group at 4''-OH of noviose are essential for the biological activity; the methyl group and the chlorine atom at 8'-position of the coumarin ring also play an important role. Aminocoumarins with a pyrrole moiety at 2''-OH instead of 3''-OH showed reduced biological activity. Ring A of novobiocin and clorobiocin is also involved in the interaction with gyrase.

Invasive mycoses are important causes of morbidity and mortality among neutropenic and other immunocompromised patients. Candidiasis and aspergillosis are the most frequent mycoses but many other opportunistic fungal infections are emerging. The lack of appropriate diagnostic tools for an early diagnosis often implies that many patients receive empirical antifungal therapy or that therapy is started when the disease is in an advanced stage with only a remote possibility for cure. Moreover, there is a relative shortage of antifungal agents for treating these life-threatening mycoses. Nowadays, a formidable technical challenge is faced by the pharmaceutical industry in the discovery of new antifungal targets and the development of effective antifungal drugs. The number of new antifungal agents at different stages of the development pipeline is high in a clear contrast to the fact that during more than four decades, amphotericin B has been nearly the unique available therapy for invasive mycoses. This review will summarise the advances in the field of new antifungal agents, including those agents in the development process, with particular emphasis on antifungal agents currently used in the therapy of superficial and invasive fungal infections, and the increasing recognition of the importance of fungal biofilms in the outcome of treatment.

The substantial increase in the incidence of fungal infections during the past two decades has been suggested as the major cause of morbidity and mortality among immunocompromised patients. Aggressive immunosuppression, HIV-1 infection, cancer, and organ transplantation have opened the door to pathogenic fungal organisms. Disseminated candidiasis and pulmonary aspergillosis caused by emerging opportunistic fungi are the most common of the serious mycoses. Reliability of radioisotope-based measurements and specificity of immune reactions, radioimmune tests, like radioimmunoassay (RIA) and indirect radioimmunometric assay (IRMA) make the applications of radiotechniques among the most promising methods in the antifungal drug research and diagnostic tools. The following applications are discussed in this article: mechanisms of fungal physiology and biochemistry, transporter systems, evaluation of mycoses, radiolabeled metabolites, antifungal drug efficacy and discovery, and antifungal drug metabolism and pharmacokinetics in animal models, therapeutic drug monitoring and in-vitro diagnosis.

Sialic acids are characterized as ubiquitous sugar carboxylic acid with 9-carbon atoms as a backbone, often involved in important cell surface communications and infection processes. Modification of the functional groups attached on the sialic acid may lead to changes in steric and / or electronic properties of the molecule, which can be used to probe space and charge requirements of sialic acid-recognizing proteins. Among the diverse arrays of compounds related to sialic acid family, 2-deoxy-2, 3-didehydro sialic acids have been known as an inhibitor of sialidase from both bacterial and viral sources, occupying its important position in the modern medicinal chemistry. These core structures are also incorporated in the design of sialyltransferase inhibitors based on the transition state of the sialyl donor nucleotide. Focusing on our recent progress, this article reviews the basis and recent progress of the synthesis of 2-deoxy-2, 3- didehydro sialic acid analogs and their biological evaluation against various sialidases