Infectious Disorders - Drug Targets (Formerly Current Drug Targets - Infectious Disorders) - Volume 7, Issue 2, 2007

More than one-third of the global population is infected with the tuberculosis (TB) and TB remains one of the world’s leading causes of disease and death. Each year, 8 million people become ill with TB and 2 million people die from the disease. In addition, drug resistance in the form of MDR-TB (strains resistant to isoniazid and rifampicin) and XDR-TB (strains resistant to isoniazid and rifampicin, and to a fluoroquinolone and one of the injectable TB drugs [kanamycin, amikacin or capreomycin]) is making it more difficult to deliver effective therapy leading to a cure. Although drug discovery for new antitubercular agents diminished for many years, resurgence took place over the last several years with support from governments, research institutes, and non-profit foundations. Today there is active research in identifying new targets for drug discovery, validating new targets, and discovering new lead compounds for drug development. Although there are four new agents undergoing clinical trials, most are in the early stages and the final outcome is not yet known. Thus, efforts need to continue, especially efforts to discover new agents that can dramatically shorten the time needed in therapy. This special topics issue of Infectious Disorders - Drug Targets focuses on TB drugs, drug development and drug targets. The Evolution of Extensively Drug Resistant Tuberculosis (XDR-TB): History, Status and Issues for Global Control (Goldman, Plumley and Laughon): The selection and spread of multiple drug resistant M. tuberculosis continued for decades leading to selection and spread of two operationally distinct forms, multiple drug resistant (MDR-TB) and extensively drug resistant (XDR-TB). The estimate for MDR-TB and XDR-TB cases globally for 2005 were 424,000 and 27,000 respectively, and the situation is worst in areas with high incidences of HIV infection. This situation was brought to the forefront when an outbreak of XDR-TB occurred in Tugela Ferry, KwaZulu-Natal, South Africa, in 2005 and began to spread. The evolution of XDR-TB is reviewed in terms of its causes, commentary from the research and lay communities, and future health care needs to contain the global epidemic. Programs to Facilitate Tuberculosis Drug Discovery: The Tuberculosis Antimicrobial Acquisition and Coordinating Facility (Goldman, Laughon, Reynolds, Secrist III, Maddry, Poffenberger, Kwong, and Subramaniam): The Tuberculosis Antimicrobial Acquisition and Coordinating Facility (TAACF) was founded to make comprehensive testing services available at no cost to research laboratories with an interest in discovering new TB drugs. The TAACF is managed and funded by the National Institute of Allergy and Infectious Diseases (National Institutes of Health, Bethesda, MD) as a resource to support preclinical drug discovery and development and consists of a series of government contracts supporting compound acquisition and storage, in vitro evaluation of antimycobacterial activity and cytotoxicity, development of high throughput assays, in vivo evaluation of toxicity, oral bioavailability and efficacy in animal models, specialty testing (such as activity against non-replicating persistent bacteria), and a component to assist in technology transfer for developing comprehensive promotional packages and facilitating partnerships with pharmaceutical companies for drug development. Challenges Associated with Current and Future TB Treatment (Laurenzi, Ginsberg and Spigelman): The current and future status of TB drugs is reviewed in the context of the emergence of drug resistance and the high incidence of TB-HIV co-infection. The difficulties in optimizing the use of existing drugs and challenges for development of novel, improved products for effectively treating TB are discussed. Molecular Approaches to Target Discovery: Evaluating Targets for Antituberculosis Drug Discovery Programmes (Balganesh and Furr): Issues related to the selection of valid targets for drug discovery are reviewed, including the issue of host targeting non-replicating, persistent bacteria. Factors for use in prioritizing targets are discussed along with the use of genomic information.......

Tuberculosis (TB) is a devastating disease caused by Mycobacterium tuberculosis that killed an estimated 4000-5000 person each day during 2005. Although infections with drug sensitive strains can be effectively cured with a 6 to 9 month regimen of multiple antibiotics, the inability to deliver and complete appropriate courses of therapy on a global level has led to the selection of resistant strains over the past 50 years. The selection and spread of multiple drug resistant M. tuberculosis continued for decades leading to two operationally distinct forms of the disease, multiple drug resistant (MDR-TB) and extensively drug resistant (XDR-TB). The estimate for MDR-TB and XDR-TB cases for 2005 were 424,000 and 27,000 respectively, and the situation is worst in areas with high incidences of HIV infection. The outcome was predictable based on basic microbiological principles, and the resultant and future epidemic effectively modeled mathematically. This situation was brought to the forefront when an outbreak of XDR-TB occurred in Tugela Ferry, KwaZulu- Natal, South Africa, in 2005 and began to spread. Unfortunately, we do not know the true extent of XDR-TB globally. However, all signs point to an emerging epidemic of TB infections that will be difficult, if not impossible to cure. A few new drugs are in clinical trials, but it is too early to know the final outcome; some may fail, and none will be available for several years. The situation will continue to worsen unless more resources are made available to discover and deliver better treatment options.

There is a real need to discover new drugs that are active on drug-resistant tuberculosis (TB), and for drugs that will shorten the time of therapy. Large pharmaceutical companies have traditionally led the quest for discovering and developing new antiinfective agents but this is not the case when it comes to diseases like tuberculosis that primarily occur in resource restricted countries. Throughout the world many research groups are actively engaged in the scientific discovery of new TB drugs. Unfortunately, most research laboratories do not have the necessary safety facilities or resources for all facets of TB drug discovery. The Tuberculosis Antimicrobial Acquisition and Coordinating Facility (TAACF) was established in order to make comprehensive testing services available at no cost to research laboratories with an interest in discovering new TB drugs. The TAACF is a consortium of contracts managed and funded by the National Institute of Allergy and Infectious Diseases (National Institutes of Health, Bethesda, MD) as a resource to support preclinical drug discovery and development. The core of the TAACF is the Southern Research Institute, Birmingham, AL, which supports compound acquisition, storage, medicinal chemistry, and high throughput assays. Other collaborating groups provide biological data on antimycobacterial activity and cytotoxicity, preliminary in vivo toxicity, oral bioavailability and efficacy in animal models, specialty testing (such as activity against non-replicating persistent bacteria), and assistance in technology transfer for developing comprehensive promotional packages and facilitating partnerships with pharmaceutical companies for drug development. The TAACF program and recent progress that has been publicly disclosed by suppliers is reviewed. There are many aspects promising of the program that will not be discussed due to confidentially.

Current tuberculosis (TB) treatment is based on a combination of drugs that were developed mostly in the central decades of the last century. Cure rates are high for drug sensitive strains of Mycobacterium tuberculosis (M. tb) when the recommended complex and lengthy treatment protocols are adhered to. However the difficulty in correctly prescribing and adhering to these protocols, the emergence of M. tb strains resistant to multiple drugs, and drug-drug interactions that interfere with optimal treatment of HIV and TB coinfected patients have generated a pressing need for improved TB therapies. Together with the ominous global burden of TB, these shortcomings of current treatment have contributed to a renewed interest in the development of improved drugs and protocols for the treatment of tuberculosis. This article highlights hurdles related to the optimized use of existing drugs and challenges related to the development of novel, improved products, focusing in particular on aspects inherent in TB drug clinical development. Concluding comments propose processes for more efficient development of new TB therapies.

Selection of appropriate targets for launching antituberculosis drug discovery programmes is challenging. This challenge is magnified by the limited repertoire of ‘validated targets’ and the paucity of clinically successful drugs. However, continued understanding of the biology of the microbe and its interaction with the host has enabled detailed evaluation of several interesting pathways and novel targets. The value of a target that is suitable for antituberculosis drug discovery needs to be defined not only in the context of its ‘essentiality’ for survival in vitro but also against a variety of properties relevant to activities in the drug discovery process, e.g.; selectivity, vulnerability, suitability for structural studies, ability to monitor inhibition in whole cells etc. It is also rarely feasible to obtain all the relevant information on the target prior to the launch of a discovery programme. Thus, there is a continuous confidencebuilding exercise on the validity of a target. Several novel approaches have enabled exploitation of the mycobacterial genome and prioritisation of putative targets; the concept of ‘sterilisation’ is now being evaluated not only through the availability of structurally diverse probe compounds but also by the ability to characterise metabolic pathways in vivo. The impact of the current knowledge base on the different facets of ‘target validation’ relevant to antituberculosis drug discovery is discussed in this article with emphasis on developing appropriate matrix systems to prioritise them. The article also discusses the influence of lead generation approaches with specific reference to antibacterial drug discovery.

Tuberculosis (TB) infects one-third of the world population. Despite 50 years of available drug treatments, TB continues to increase at a significant rate. The failure to control TB stems in part from the expense of delivering treatment to infected individuals and from complex treatment regimens. Incomplete treatment has fueled the emergence of multi-drug resistant (MDR) strains of Mycobacterium tuberculosis (Mtb). Reducing non-compliance by reducing the duration of chemotherapy will have a great impact on TB control. The development of new drugs that either kill persisting organisms, inhibit bacilli from entering the persistent phase, or convert the persistent bacilli into actively growing cells susceptible to our current drugs will have a positive effect. We are taking a multidisciplinary approach that will identify and characterize new drug targets that are essential for persistent Mtb. Targets are exposed to a battery of analyses including microarray experiments, bioinformatics, and genetic techniques to prioritize potential drug targets from Mtb for structural analysis. Our core structural genomics pipeline works with the individual laboratories to produce diffraction quality crystals of targeted proteins, and structural analysis will be completed by the individual laboratories. We also have capabilities for functional analysis and the virtual ligand screening to identify novel inhibitors for target validation. Our overarching goals are to increase the knowledge of Mtb pathogenesis using the TB research community to drive structural genomics, particularly related to persistence, develop a central repository for TB research reagents, and discover chemical inhibitors of drug targets for future development of lead compounds.

The identification of new antibacterial targets is urgently needed to address multidrug resistant and latent tuberculosis infection. Sulfur metabolic pathways are essential for survival and the expression of virulence in many pathogenic bacteria, including Mycobacterium tuberculosis. In addition, microbial sulfur metabolic pathways are largely absent in humans and therefore, represent unique targets for therapeutic intervention. In this review, we summarize our current understanding of the enzymes associated with the production of sulfated and reduced sulfur-containing metabolites in Mycobacteria. Small molecule inhibitors of these catalysts represent valuable chemical tools that can be used to investigate the role of sulfur metabolism throughout the Mycobacterial lifecycle and may also represent new leads for drug development. In this light, we also summarize recent progress in the development of inhibitors of sulfur metabolism enzymes.

Bacterial DNA gyrase is an important target of antibacterial agents, including fluoroquinolones. In most bacterial species, fluoroquinolones inhibit DNA gyrase and topoisomerase IV and cause bacterial cell death. Other naturally occurring bacterial DNA gyrase inhibitors, such as novobiocin, are also known to be effective as antibacterial agents. DNA gyrase is an ATP-dependent enzyme that acts by creating a transient double-stranded DNA break. It is unique in catalyzing the negative supercoiling of DNA and is essential for efficient DNA replication, transcription, and recombination. DNA gyrase is a tetrameric A2B2 protein. The A subunit carries the breakage-reunion active site, whereas the B subunit promotes ATP hydrolysis. The M. tuberculosis genome analysis has identified a gyrB-gyrA contig in which gyrA and gyrB encode the A and B subunits, respectively. There is no evidence that M. tuberculosis has homologs of the topoisomerase IV, parC and parE genes, which are present in most other bacteria. Newer fluoroquinolones, including moxifloxacin and gatifloxacin, exhibit potent activity against M. tuberculosis, and show potential to shorten the duration for TB treatment. Resistance to fluoroquinolones remains uncommon in clinical isolates of M. tuberculosis. M. tuberculosis DNA gyrase is thus a validated target for anti-tubercular drug discovery. Inhibitors of this enzyme are also active against non-replicating mycobacteria, which might be important for the eradication of persistent organisms. A novel inhibitor of M. tuberculosis DNA gyrase would be effective against multi-drug resistant (MDR)-TB, and it could also be effective against fluoroquinolone-resistant M. tuberculosis.

Mycobacterium tuberculosis (Mtb) remains the deadliest bacterial pathogen worldwide, causing an estimated 1.7 million deaths in 2004 among an infected population of approximately 2 billion people, according to the World Health Organization (WHO). Therapeutic options are limited to a few drugs that are becoming increasingly ineffective. Multidrug-resistant (MDR) Mtb strains are prevalent globally, fueled by inadequate patient compliance of drug intake. Recently, a high incidence of extensively drug-resistant (XDR) strains resistant to all currently used drugs was reported among patients with the human immunodeficiency virus (HIV) in KwaZulu Natal, South Africa [1]. The high mortality rate and short survival time of patients with XDR Mtb was especially alarming. The emergence of XDR mycobacteria emphasizes the urgent need for the identification of novel targets and development of new drugs. New potential drug targets exist in the Mtb respiratory chain. Certain classes of drugs have long been shown to exert significant tuberculocidal activity, such as the phenothiazines [2, 3]. Phenothiazines inhibit one of the key enzymes of the respiratory chain; type II NADH:menaquinone oxidoreductase or NDH-2 [4]. The effectiveness of this class of drugs against Mtb justifies further research into the respiratory chain, with the aim of elucidating its physiologic roles in in vitro and in vivo survival, and discovering new (sub)classes of drugs that can safely serve as inhibitors for clinical use. In this chapter, we critically review the recent advances in this field, with particular emphasis on NDH-2, and underscore the kinds of research further needed for drug development.

Mycobacteria have a unique cell wall, which is rich in drug targets. The cell wall core consists of a peptidoglycan layer, a mycolic acid layer, and an arabinogalactan polysaccharide connecting them. The detailed structure of the cell wall core is largely, although not completely, understood and will be presented. The biosynthetic pathways of all three components reveal significant drug targets that are the basis of present drugs and/or have potential for new drugs. These pathways will be reviewed and include enzymes involved in polyisoprene biosynthesis, soluble arabinogalactan precursor production, arabinogalactan polymerization, fatty acid synthesis, mycolate maturation, and soluble peptidoglycan precursor formation. Information relevant to targeting all these enzymes will be presented in tabular form. Selected enzymes will then be discussed in more detail. It is thus hoped this chapter will aid in the selection of targets for new drugs to combat tuberculosis.