Current Pharmaceutical Design - Volume 10, Issue 26, 2004

There is an urgent need for new antimycobacterial drugs, and in particular for novel agents that will shorten the duration of tuberculosis chemotherapy, or overcome drug-resistant strains of the causative organism, Mycobacterium tuberculosis. Our knowledge of the tubercle bacillus and its complex interaction with the human host has improved dramatically in recent years, particularly with the determination of its complete genome sequence. New genome-scale tools are being applied to aid in drug target identification, alongside traditional approaches aimed at understanding the basic biology of M. tuberculosis. Many potential drug targets have been identified, but very few have been validated by showing that they are essential for growth or survival of the bacterium. In this review, the landscape of potential drug targets is surveyed.

Integrated bioinformatic approaches to drug discovery exploit computational techniques to examine the flow of information from genome to structure to function. Informatics is being be used to accelerate and rationalize the process of antimycobacterial drug discovery and design, with the immediate goals to identify viable drug targets and produce a set of critically evaluated protein target models and corresponding set of probable lead compounds. Bioinformatic approaches are being successfully used for selection and prioritization of putative mycobacterial drug target genes; computational modelling and x-ray structure validation of protein targets with drug lead compounds; simulated docking and virtual screening of potential lead compounds; and lead validation and optimization using structure-activity and structurefunction relationships. Active sites can be identified, characterizing patterns of conserved residues and, where relevant, predicting catalytic residues, thus providing information to aid the design of selective and efficacious pharmacophores. In this review, we describe selected recent progress in antimycobacterial drug design, illustrating the strengths and limitations of current structural bioinformatic approaches as tools in the fight against tuberculosis.

The antimicrobial agents used in the treatment of mycobacterial infections have remained largely unchanged for several decades. Primary treatment of tuberculosis relies on four drugs, isoniazid, a rifamycin, pyrazinamide, and ethambutol (or streptomycin), and generally results in >95% cure in uncomplicated tuberculosis infection. Drug resistance greatly complicates treatment of this disease. Treatment of tuberculosis caused by multiply drug-resistant strains with “second-line” drugs remains complex, and is generally tailored to the individual patient and strain. Several of the fluoroquinolones have shown promise as second line drugs for treatment of active disease and, in combination with clarithromycin or azithromycin, ethambutol, and other agents, for treatment of Mycobacterium avium complex infection. While large clinical trials are not possible with second line drugs, clinical treatment data are available and suggest that the quinolones have various degrees of promise in treatment of these infections. Bacterial type II DNA topoisomerases, DNA gyrase and topoisomerase IV, are the targets of quinolones, and provide the genetic basis for quinolone activity in mycobacteria. Mutations in these enzymes results in resistance, and characterization of resistant mutants allows correlation of genotype with susceptibility phenotype. Structure-activity relationship studies have provided further insight into optimal use of quinolones in mycobacterial infections. Care should be taken in treating pneumonia with fluoroquinolones if there is a degree of suspicion of tuberculosis, since quinolone monotherapy may rapidly select for quinolone resistance, thereby removing that class of antibiotic from the small range of treatment options.

New macrolides, such as clarithromycin and azithromycin, are active agents to Mycobacterium avium complex (MAC). Both clarithromycin and azithromycin are well-known for the ability to improve the prognosis of AIDS patients with disseminated MAC infection. However, the administration of monotherapy with a macrolide is usually associated with the emergence of drug resistance after a few months of use. Therefore, the recommended treatment for MAC infection involved the use of at least two antibiotics, which includes a macrolide in combination with rifabutin, moxifloxacin and / or ethambutol. When used as prophylactic therapy in AIDS patients, azithromycin is more convenient (1200 mg, once a week) than clarithromycin (500 mg, twice a day). Ketolides are a semi-synthetic derivative of erythromycin A, which differs from erythromycin A by substitution of a 3- keto group for L-cladinose. Telithromycin has a carbamate group linked to an imidazolium and pyridium nucleus at C11- C12. In mice model, both telithromycin and ABT-733 were active in vivo against MAC.

Several rifamycin derivatives have been developed during the last 15 years for the treatment of mycobacterial infections. For tuberculosis, rifabutin (RFB) showed strong activity and seemed to be suitable when tuberculosis patients were also treated for their AIDS infection. Rifapentine (RPT) was evaluated in patients with or without AIDS for its intermittent use. It displayed promising activity but must be strengthened in situations, such as AIDS or patients without AIDS but with cavities. Rifalazil (RLZ) has been evaluated in mice but the dosages used were much higher than those tolerated by patients. Regarding Mycobacterium avium infections, RFB showed significant prophylactic activity in humans, RPT displayed some activity in mice and RLZ showed modest activity in mice.

Nitroaromatic antibiotics have a long and controversial history in human and veterinary medicine. This controversy lies behind the presumption of many pharmaceutical companies that nitroaromatic compounds should be filtered from the list of drug-like compounds but stands at odds with the remarkably safe clinical record of use of such compounds. In this review, we will describe the whole-cell structure-activity relationships that have been reported for antimycobacterial nitroimidazoles as well as the available in vivo data supporting efficacy with a particular emphasis on nitroimidazo[2,1-b]oxazines such as PA-824. We will also explore the unique potential of such compounds to shorten the course of tuberculosis therapy by exerting a bactericidal effect on non-replicating bacilli. We will consider the mode of action of such compounds in sensitive organisms and discuss the mechanisms by which resistance may emerge. Finally, we will review the pharmacokinetics, toxicology and laboratory and animal studies linking nitroimidazoles with carcinogenicity and mutagenicity and assess the prospects for the clinical introduction of nitroimidazoles for the treatment of tuberculosis.

Mycobacteria are intracellular pathogens that invade and reside inside macrophages. There has been a rapid resurgence in infections caused by the genus mycobacteria. Chemotherapy of mycobacterial infections is prolonged, hepatotoxic and very often inadequate in achieving optimal drug concentrations inside the cells. Recent advances in controlled delivery systems for drugs such as liposomes have sparked a renewed interest in their potential application for the treatment of mycobacterial infections. The versatility of liposomes in incorporation of hydrophilic / hydrophobic components, non-toxic nature, biodegradability, biocompatibility and property of sustained release makes them attractive candidates for the delivery of antitubercular drugs. Liposome research in the area of mycobacterial diseases has evolved and matured through several phases; from the laboratory to the clinics. This review, thus focuses on the use of liposomes for the treatment of various types of mycobacterial diseases.

The purpose of this review article is to examine the various studies that have evaluated microspheres for delivery of antimycobacterial drugs. Some of the studies strictly involve the development and evaluation of microspheres for use in antimycobacterial drug delivery, whereas others actually use drug-loaded microspheres to treat mycobacterial infections in cell lines and small animals. Although there is a potential to use microspheres to treat a variety of mycobacterial infections, it appears that most of the studies so far have focused on the etiological agent of tuberculosis, Mycobacterium tuberculosis. As a result, the infectious studies presented here all entail the treatment of that mycobacterial agent. This review will address the following aspects that are important if microspheres are to be considered an acceptable therapeutic tool: 1) in vitro release characteristics, 2) delivery, release and efficacy in macrophages, 3) effectiveness in infected small animal models, 4) safe and combined use with other antimycobacterial agents, and 5) reduced toxicity. It is hoped that once all of these parameters are evaluated, a conclusion regarding the benefit of microsphere technology in the treatment of mycobacterial diseases can be reached.

Sterilizing drugs kill Mycobacterium tuberculosis that persists during chemotherapy. Predictive models should mimic the conditions causing persistence in the lesions of cavitary disease, and should grade current anti-tuberculosis drugs according to their sterilizing activity determined in clinical trials. Models should start with old, stationary cultures grown micro-aerophilically. In these, persistent bacilli occur in different populations in which there is no appreciable cell division. Population 1. Grows in liquid culture medium but not on solid medium. Killed by rifampicin. Population 2. Grows on solid culture medium. Killed by rifampicin. Population 3. Grows in liquid medium but not on solid medium. Tolerant of rifampicin. Population 4. Bacilli from Cornell model mice, after treatment with pyrazinamide and isoniazid, cannot grow in liquid or on solid culture medium. Some of these populations are incorporated in models which start with 100-day liquid medium cultures. In model 1 (population 2) the new drug is added and colony counted after 7 days incubation. In models 2 and 3, 100 mg / L rifampicin is added to the 100-day culture when the bacilli lose their ability to grow on solid culture medium (population 3). After re-suspension in rifampicin-free liquid medium for 7 days, the bacilli recover growth on solid medium, when a colony count is done. The new drug is added during incubation with rifampicin in model 3 and at the start of recovery in drug-free medium in model 2. Models 1 and 3 grade isoniazid, rifampicin and pyrazinamide according to their sterilizing activity determined by clinical trials.

In order to cope with the worldwide increase in the prevalence of multidrug-resistant tuberculosis and Mycobacterium avium complex (MAC) infections, a number of new antimycobacterial drugs have been or are being synthesized and developed. Development of new protocols for chemotherapy of refractory mycobacterioses is also sharing promise. In this context, one promising strategy is to devise regimens to treat patients with refractory mycobacterioses using ordinary antimycobacterial agents in combination with appropriate immunomodulators. This article deals with the following matters: an outline of the host immune response to mycobacterial pathogens, particularly in terms of mobilization of the cytokine network in response to mycobacterial infection, and adjunctive immunotherapy using (1) recombinant immunomodulating cytokines, (especially Th-1 and Th-1-like cytokines such as IFN-γ, IL-2, IL-12, IL-18 and GM-CSF), (2) inhibitors of immunosuppressive cytokines (TGF-β) and some proinflammatory tissue-damaging cytokines (TNF-α), and (3) immunomodulatory agents such as ATP and its analogs, imidazoquinoline, diethyldithiocarbamate, poloxamer, dibenzopyran, galactosylceramide, nonsteroidal anti-inflammatory drugs, Chinese traditional medicines, levamisole, synthesized mycobacterial oligoDNA, DNA vaccine expressing mycobacterial HSP65 or IL-12, and heat-killed Mycobacterium vaccae. Although adjunctive immunotherapy is fairly efficacious in treating intractable mycobacterioses, it still features serious problems and dilemmas, such as high cost, occasionally severe side effects, and, in many cases, only modest efficacy in potentiating host defense mechanisms against mycobacterial infections, primarily because of the induction of macrophage-deactivating cytokines during the course of long-term administration of adjunctive agents.