Current Drug Targets - Volume 3, Issue 2, 2002

The recent past witnessed a decrease in the number of new antibacterial compounds approved by the regulatory agencies and an almost complete lack of molecules killing bacteria by novel mechanisms of action. The broad spectrum antimicrobial agents currently on the market carry the potential, and indeed victims, of resistance developed against them. The need for new types of antimicrobial drugs coincides with the desire of developing lead molecules that act selectively on a single strain, or perhaps on a few closely related strains. Such selectivity would exclude the likelihood of the emergence of broad-range resistance. Intracellular bacterial targets, most prevalently proteins needed for the life cycle of bacteria, carry the potential to be a resourceful target for a new family of antimicrobial compounds. Inhibition of proteinaceous functions requires stereospecificity, and a drug structurally similar to the target proteins themselves. Indeed, some antibacterial peptides show selective inhibition of intracellular targets. A few native peptides and their designed analogs appear to kill only a limited number of bacterial strains. Identification of the binding sites on the target proteins would allow the design of strain-specific antibacterial and antifungal peptides without the fear of development of common resistance to these agents.

The emergence of multidrug-resistant pathogens that has caused a serious problem in hospitals worldwide, has intensified the search for novel drugs, in order to replace or to be used in complement with the existing antibiotics. In this connection much interest has been focused on a group of antimicrobial peptides, so-called bacteriocins. These antagonising peptides, which are gene-encoded in contrast to those made by multi-enzyme-complexes, share some common physico-chemical properties, such as being small, cationic, amphiphilic and often being membrane active. However, they differ greatly from each other in their primary sequence and exhibit an impressively large inhibitory spectrum which covers almost all bacterial genera, including many important pathogens and food-spoilage bacteria. Many of these peptides are produced by lactic acid bacteria, organisms which have been used by man from ancient time in diverse fermentation processes, to improve and / or prolong self-life of many food and feed products. Numerous bacteriocins have been purified and characterised in great detail, both at biochemical and genetic levels. Still, novel bacteriocins with new properties are reported in an increasing number in recent years. In this review we will give a brief status quo of the present knowledge on bacteriocin research thus different aspects such as their diversity in nature, biochemical properties, modes of action, biosynthesis and genetics will be treated.

The adhesive interaction of tumor cells with various components of the extracellular matrix (ECM), such as laminin and fibronectin appears to play a crucial role in tumor metastasis. It has been reported that adhesive peptides, such as Tyr-Ile-Gly-Ser-Arg (YIGSR) in laminin and Arg-Gly-Asp (RGD), inhibited adhesion and invasion of various tumor cells to ECM in vitro, and exhibited inhibitory effects on pulmonary metastasis of B16-BL6 melanoma cells in mice. However, large doses of these peptides were required for significant anti-metastatic effects in vivo, probably due to their rapid degradation by various peptidases and their rapid excretion from the blood into the urine. To overcome these problems, the development of an appropriate drug delivery system (DDS) is required to improve in vivo stability and prolong plasma half-lives. Several strategies such as peptide-cyclization and D-amino acid substation have been reported to improve stability in blood by inhibiting enzymatic degradation. However, even these approaches have proven insufficient to overcome rapid renal clearance from the circulation. On the other hand, bioconjugation with water-soluble polymeric modifiers could markedly prolong the plasma half-lives by not only increasing peptidase resistance but also impeding renal excretion. In addition, it is possible to strictly control the in vivo pharmacokinetics of a peptide by introducing functional molecules with targeting or slow release capacities to the polymeric modifier. In this review we demonstrate with reference to our recent studies that bioconjugation of adhesive peptides with the appropriate polymeric modifier can enhance antimetastatic activity and may facilitate therapeutic use.

Despite the availability of the BCG vaccine and chemotherapy, tuberculosis (TB) remains a leading infectious killer worldwide. The recent rise of TB and especially the alarming increase of drug-resistant TB call for urgent need to develop new anti-TB drugs. Lengthy chemotherapy and increasing emergence of drug-resistant strains pose a significant problem for effective control. The need for a lengthy TB therapy is a consequence of the presence of persistent Mycobacterium tuberculosis, not effectively killed by current anti-TB agents. A list of new drug candidates along with proposed targets for intervention is described. Recent advances in the knowledge of the biology of the organism and the availability of the genome sequence provide a wide range of novel targets for drug design. Gene products involved in controlling vital aspects of mycobacterial metabolism, persistence, virulence and cell wall synthesis would be attractive targets. It is expected that the application of functional genomics tools, such as microarray and proteomics, in combination with modern approaches, such as structure-based drug design and combinatorial chemistry to biology-based targets, will lead to the development of new drugs that are not only active against drug-resistant TB but also can shorten the course of TB therapy.

The aspartic proteinases are a family of enzymes involved in a number of important biological processes. In animals the enzyme renin has a hypertensive action through its role in the renin-angiotensin system. The retroviral aspartic proteinases, such as the HIV proteinase, are essential for maturation of the virus particle and inhibitors have a proven therapeutic record in the treatment of AIDS. The lysosomal aspartic proteinase cathepsin D has been implicated in tumorigenesis and the stomach enzyme pepsin, which plays a major physiological role in hydrolysis of acid-denatured proteins, is responsible for much of the tissue damage in peptic ulcer disease. Since aspartic proteinases also play major roles in amyloid disease, malaria and common fungal infections such as candidiasis, inhibitors to these enzymes are much sought after as potential therapeutic agents. In all aspartic proteinases, the catalytic aspartate residues are involved in an intricate arrangement of hydrogen bonds involving a solvent molecule which is presumed to be water. The catalytic mechanism is thought to involve nucleophilic attack of the active site water molecule on the scissile bond carbonyl generating a tetrahedral gem-diol intermediate. The design of inhibitors generally involves the use of short oligopeptides containing a transition state analogue which mimic this tetrahedral intermediate. The application of structure-based drug design to members of the aspartic proteinase family is the main subject of this review.

Vaccines are one of the most cost effective methods of improving public health thereby increasing the quality of life. Prophylactic and therapeutic treatment by vaccines can prevent infectious diseases and some cancers and could also be used in the treatment of autoimmune disorders. An appreciation of this potential has resulted in a burgeoning literature which not only describes the scientific efforts being made into designing new and improved vaccines but also drives the efforts being made by public health organizations world-wide in delivering vaccines to the community. At the forefront of technologies being applied to the design of vaccines is the use of synthetic peptides the chemical technologies used to assemble peptides have made great strides over the last decade and assembly of hi-fidelity peptides which can be of high molecular weight, multimeric or even branched is now almost routine. Together with the advances in peptide technology our understanding of the molecular events that are necessary to induce immune responses has also made great strides. The central role that peptides play in immune recognition is now recognised and rules are emerging that are being applied to the construction of peptide-based vaccines that, in the right context, can induce humoral (antibody) and cellular (cytotoxic and helper T cell) immune responses. Synthetic peptides are exquisitely placed to answer questions about immune recognition and along the way to provide us with new and improved vaccines.