Infectious Disorders - Drug Targets (Formerly Current Drug Targets - Infectious Disorders) - Volume 6, Issue 2, 2006

The incidences of human pathogenic yeast Candida albicans and its related species acquiring resistance to antifungals have increased considerably, which poses serious problems towards its successful chemotherapy. The resistance of these pathogenic fungi is not restricted to the commonly used triazole compounds but is even encountered, though not often, with polyene derivatives as well. The efflux pump proteins belonging to ABC (ATP Binding Cassette) and MFS (Major Facilitators) super family are the most prominent contributors of multidrug resistance (MDR) in yeasts. The abundance of the drug transporters and their wider specificity suggest that these transporters may not be exclusively drug exporters in yeasts and may have other cellular functions. In this article we focus on some of the recent advances on the structure and function, evolution and transcriptional control of drug efflux proteins of Candida. A short discussion on the physiological relevance of drug transporters is also included.

Phospho-MurNAc-pentapeptide translocase (MraY, translocase I) catalyses the first step of the lipid-linked cycle of reactions of bacterial peptidoglycan biosynthesis. MraY is the target for five families of nucleoside antibacterial natural products: the tunicamycins, the mureidomycins (also pacidamycins, napsamycins), the liposidomycins, the muraymycins, and the capuramycins. Recent structure-activity studies on these families have led to the identification of active pharmacophores, and insight into their mechanisms of action. This step of peptidoglycan biosynthesis is also the target for the bacteriolytic E protein from bacteriophage X174, and for cyclic peptides of the amphomycin family which complex the undecaprenyl phosphate co-substrate. The mechanisms of enzyme inhibition by these agents are discussed, and the state of knowledge regarding the transmembrane structure, active site, and catalytic mechanism of MraY. The availability of high throughput assays and prospects of MraY as an antibacterial target are also discussed.

With almost 40 million people infected with human immunodeficiency virus (HIV), it is one of the most devastating diseases with no cure in sight. Over the past two decades, significant progress has been made to identify and validate drug targets for HIV. However, most of the 20 FDA approved drugs are targeted to two enzymes; reverse transcriptase and protease. Other drug targets derived from HIV and host factors are being validated, and novel compounds are being developed to overcome drug resistance. Recent data indicate that low and residual virus found in tissues of the lymphoid and central nervous system (CNS) is likely due to insufficient drug levels. Thus, improvement in the delivery of anti-HIV drugs to these tissues with limited drug penetration or accumulation, is equally important to maximally suppress viral replication. Novel lipid associated drugs (i.e. indinavir) targeted to the lymphoid tissue have been shown to overcome limited drug exposure in the lymph nodes and to further reduce residual virus in tissue. This review discusses viral and cellular targets that could interrupt viral replication, as well as novel and proven strategies to enhance the delivery of anti-HIV drugs to the lymphoid, CNS, and cells where low viral replication and limited drug levels exist.

Hepatitis C virus (HCV), the major etiological agent of transfusion-associated non-A, non-B hepatitis, is a severe health problem affecting up to 3% of the world population. Since its identification in 1989, enormous efforts have been made to characterize the viral cycle. However, many details regarding the virus' penetration of hepatocytes, its replication and translation, and the assembling of virions remain unknown, mostly because of a lack of an efficient culture system. This has also hampered the development of fully effective antiviral drugs. Current treatments based on the combination of interferon and ribavirin trigger a sustained virological response in only 40% of infected individuals, thus the development of alternative therapeutic strategies is a major research goal. Nucleic acid based therapeutic agents may be of some potential in hepatitis C treatment. In recent years, much effort has gone into the improvement of DNA and RNA molecules as specific gene silencing tools. This review summarizes the state of the art in the development of new HCV therapies, paying special attention to those involving antisense oligonucleotides, aptamers, ribozymes, decoys and siRNA inhibitors. The identification of potential viral targets is also discussed.

In the past 20 years, vancomycin and other glycopeptide antibiotics have been administered to patients with Streptococcal and Staphylococcal infections that were resistant to all other antibiotics or to patients who were allergic to penicillins and cephalosporins. After extensive use of vancomycin and other glycopeptide antibiotics in humans, several strains of Enterococcus have developed high-level vancomycin resistance (collectively called VRE, vancomycin-resistant Enterococcus), and this resistance phenotype has spread to other organisms. The spread of vancomycin resistance to other pathogens and, potentially, to bacterial strains on the CDC's bioterrorism watch list is a major biomedical concern. Bacteria most often become resistant to vancomycin by acquiring a transposon containing genes that encode for a number of proteins, five of which are essential for the high-level resistance phenotype. The five essential gene products are called VanR, VanS, VanH, VanA, and VanX. Previous studies have shown that the inactivation of VanX results in an organism that is sensitive to vancomycin and that VanX is an excellent inhibitor target. In this review the known inhibitors and structural and mechanistic properties of VanX will be discussed. These data will be used to offer suggestions for novel, rationally-designed or -redesigned inhibitors, which could potentially be used in combination with existing glycopeptide antibiotics as a treatment for vancomycin-resistant bacterial infections.

The resurgence of tuberculosis and the surge of multidrug-resistant clinical isolates of Mycobacterium tuberculosis have reaffirmed tuberculosis as a primary public health concern. In this review we describe some new findings on the pharmacological status of fluoroquinolones derivatives (Gatifloxacin, Moxifloxacin and Sitafloxacin), new macrolides (Clarithromycin, Azithromycin and Roxithromycin), new rifamycin derivatives (Rifapentin, Rifabutin and Rifalazil) and new oxazolidinones (Linezolid and PNU 100480). We describe also other type of agents that are being developed as antimycobacterial drugs. Some of these are under clinical investigation, while others are considered to be promising candidates for future development. Among them, nitroimidazopyrans, new ketolides, Isoxyl (ISO), pyrroles derived from BM 212, Mefloquine and Diarylquinoline R207910 are discussed. We also describe the mechanism of drug resistance in mycobacteria, as well as new potential targets.

Major advances in our understanding of malaria parasite biology have been made. Coupled with the completion of the malaria genome, this has presented exciting opportunities for target-based antimalarial drug discovery. However, the unraveling of more validated biological targets will not necessarily translate into the identification of new chemical entities that are effective against drug resistant parasites in the long term. As history has already shown, development of antiplasmodial agents aimed at a single parasite target or specialized process has failed to stem the tide of drug resistance. This review highlights recent starting points and/or approaches to antimalarial drug discovery with particular emphasis on innovative efforts, which are not necessarily based on the identification of new drug targets and attendant inhibitor design. Approaches covered include utilization of validated chemical scaffolds, bioprecursor and carrier prodrugs, double drug development and/or multi-therapeutic strategies, use of metallocenic scaffolds, the medicinal chemistry of antimalarial natural products and in silico drug design.