In vitro susceptibility of Mycobacterium tuberculosis to a new macrolide antibiotic: RU-28965

Recent Biochemical Advances in Antitubercular Drugs: Challenges and Future

Tuberculosis (TB) is one of the leading causes of death world-wide after AIDS. It infects around one-third of global population and approximately two million people die annually from this disease because it is a very contagious disease spread by Mycobacterium tuberculosis. The increasing number of drug-resistant strains and the failure of conventional treatments against this strain are the challenges of the coming decades. New therapeutic techniques aim to confirm cure without deterioration, to reduce deaths, contagions and the formation of drug-resistant strains. A plethora of new diagnostic tests are available to diagnose the active tuberculosis, screen latent M. tuberculosis infection, and to identify drug-resistant strains of M. tuberculosis. When effective prevention strategies do not prevail, high rates of early case detection and successive cures to control TB emergence would not be possible. In this review, we discussed the structural features of M. tuberculosis, Multi drug resistance tuberculosis (MDR-TB), extremely drug-resistant tuberculosis (XDR-TB), the mechanism of M. tuberculosis infection, the mode of action of first and second-line antitubercular drugs, the mechanism of resistance to the existing drugs, compounds in preclinical and clinical trial and drugs presently available for the treatment of tuberculosis. Moreover, the new diagnostic techniques to detect M. tuberculosis are also discussed in this review.

Novel Pharmacological Activity of Artesunate and Artemisinin: Their Potential as Anti-Tubercular Agents

Tuberculosis is a major infectious disease that globally causes the highest human mortality. From this aspect, this study was carried out to evaluate novel pharmacological activities/effects of artesunate and artemisinin causing anti-tubercular activity/effects against Mycobacterium tuberculosis (Mtb). The anti-Mtb activities/effects of artesunate and artemisinin were evaluated using different anti-Mtb indicator assays, such as the resazurin microtiter assay, the Mycobacteria Growth Indicator Tube (MGIT) 960 system assay, and the Ogawa slant medium assay, as well as in vivo tests. Artesunate showed selective anti-Mtb effects by strongly inhibiting the growth of Mtb compared to artemisinin, and consistently induced anti-Mtb activity/effects by effectively inhibiting Mtb in the MGIT 960 system and in Ogawa slant medium for 21 days with a single dose; its minimum inhibitory concentration was 300 µg/mL in in vitro testing. Furthermore, artesunate demonstrated an anti-tubercular effect/action with a daily dose of 3.5 mg/kg in an in vivo test for four weeks, which did not indicate or induce toxicity and side effects. These results demonstrate that artesunate effectively inhibits the growth and/or proliferation of Mtb through novel pharmacological activities/actions, as well as induces anti-Mtb activity. This study shows its potential as a potent candidate agent for developing new anti-tuberculosis drugs of an effective/safe next generation, and suggests novel insights into its effective use by repurposing existing drugs through new pharmacological activity/effects as one of the substantive alternatives for inhibiting tuberculosis.

Structure-activity relationships of macrolides against Mycobacterium tuberculosis

Existing 14, 15 and 16-membered macrolide antibiotics, while effective for other bacterial infections, including some mycobacteria, have not demonstrated significant efficacy in tuberculosis. Therefore an attempt was made to optimize this class for activity against Mycobacterium tuberculosis through semisyntheses and bioassay. Approximately 300 macrolides were synthesized and screened for anti-TB activity. Structural modifications on erythromycin were carried out at positions 3, 6, 9, 11, and 12 of the 14-membered lactone ring; as well as at position 4'' of cladinose and position 2' of desosamine. In general, the synthesized macrolides belong to four subclasses: 9-oxime, 11,12-carbamate, 11,12-carbazate, and 6-O-substituted derivatives. Selected compounds were assessed for mammalian cell toxicity and in some cases were further assessed for CYP3A4 inhibition, microsome stability, in vivo tolerance and efficacy. The activity of 11,12-carbamates and carbazates as well as 9-oximes is highly influenced by the nature of the substitution at these positions. For hydrophilic macrolides, lipophilic substitution may result in enhanced potency, presumably by enhanced passive permeation through the cell envelope. This strategy, however, has limitations. Removal of the C-3 cladinose generally reduces the activity. Acetylation at C-2' or 4'' maintains potency of C-9 oximes but dramatically decreases that of 11,12-substituted compounds. Further significant increases in the potency of macrolides for M. tuberculosis may require a strategy for the concurrent reduction of ribosome methylation.

Fighting tuberculosis: An old disease with new challenges

Tuberculosis (TB) caused by Mycobacterium tuberculosis remains a leading cause of mortality worldwide into 21st century. The mortality and spread of this disease has further been aggravated because of synergy of this disease with HIV. A number of anti-TB drugs are ineffective against this disease because of development of resistance strains. Internationally efforts are being made to develop new anti-tubercular agents. A number of drug targets from cell wall biosynthesis, nucleic acid biosynthesis, and many other biosynthetic pathways are being unraveled throughout the world and are being utilized for drug development. In this review, socioeconomic problems in developing countries, efforts to control this disease in different individuals, the targets (known already and newly discovered), existing anti-tubercular agents including natural products and lead molecules, and the future prospects to develop new anti-TB agents are described.

Pharmacokinetic factors in the modern drug treatment of tuberculosis

Tuberculosis is increasing in prevalence throughout the world, particularly in sub-Saharan Africa, Asia and Latin America. This resurgence can partly be attributed to increasing poverty, particularly in developing countries, and the human immunodeficiency virus (HIV) pandemic. However, there is also increasing concern at the development of multidrug-resistant tuberculosis caused by the misuse of the agents available. The modern treatment of patients with tuberculosis should start, in most cases, with 4 first-line agents in order to minimise the risk of drug resistance developing. A6-month drug regimen is usually satisfactory for pulmonary and nonpulmonary tuberculosis, although not for patients with tuberculous meningitis, in whom a longer course of treatment is required. Coinfection with HIV may produce an atypical clinical and radiological presentation, but the treatment regimen is essentially similar to other situations. Several of the first-line agents, in particular rifampicin (rifampin) and isoniazid, are likely to cause clinically significant drug interactions and/or toxicity, particularly in patients with HIV infection. Consideration of the pharmacodynamic and pharmacokinetic interactions between the host, the mycobacterium and the drug may contribute to the development of pharmacokinetically optimised regimens that make best use of the existing range of antituberculosis drugs. However, such idealised regimens need to be tested in prospective clinical trials. The use of therapeutic drug monitoring in selected groups of patients may improve outcomes, avoid drug toxicity and reduce the development of multidrug-resistant tuberculosis. The management of multidrug-resistant tuberculosis requires a high level of clinical expertise and such patients should start on at least 5 drugs to which the organism is thought to be susceptible. Up to 50% of patients with tuberculosis may not adhere to their drug regimen, resulting in persisting infectiousness, relapse or the development of drug resistance. Directly observed treatment with antituberculosis drugs, combined with a serious commitment to tuberculosis control, is required if we are to combat this increasing epidemic.

Molecular action of anti-mycobacterial agents

In terms of the paradigms for antibacterial action presented in the introduction, there is good evidence that broad spectrum agents exert their anti-mycobacterial activity by interaction with classical targets occurring in a wide range of organisms including the mycobacteria. This is supported either by direct evidence (e.g., inhibition by rifampicin of mycobacterial RNA polymerase), or indirectly by the characterization of drug-resistant mycobacteria where mutations conferring resistance have been mapped to target sites homologous to those found in other bacteria (fluoroquinolones, macrolides, rifampicin, streptomycin). On the other hand, although the mode of action of some of the agents with an anti-mycobacterial spectrum is not fully understood, it is evident that the restricted spectrum is likely to arise from the possession of unique targets, or specific pro-drug conversion systems, or to a combination of both mechanisms. In several cases the narrow spectrum of the agents can be attributed to inhibition of molecular targets involved in the biosynthesis of the mycobacterial cell envelope that contains many unique polymers. The recent re-emergence of tuberculosis as an important human pathogen has led to improved methods for exploring the structure, biochemistry and genetics of the mycobacteria. These technical advances can now be used to gain a better understanding of the molecular basis of drug action in mycobacteria.

Drug treatment of tuberculosis--1992

The impact of the acquired immunodeficiency syndrome (AIDS) pandemic has made tuberculosis an increasing worldwide problem, and the effectiveness of modern chemotherapy has been blunted by the high incidence of primary drug resistance, especially in developing countries. The prospect of finding new and highly effective drugs similar to isoniazid or rifampicin is dim, yet the maximum benefits from the existing drugs which are highly effective have not been received. A 6-month regimen of isoniazid plus rifampicin, supplemented by pyrazinamide during the first 2 months, for treatment of uncomplicated tuberculosis is highly effective and the regimen of choice. Ethambutol should be added if the risk of isoniazid resistance is increased. A regimen of isoniazid, rifampicin, pyrazinamide and streptomycin for 4 months provides effective defence against smear-negative pulmonary tuberculosis. Re-treatment of multiple drug-resistant tuberculosis remains a difficult therapeutic problem. At least 3 drugs that the patient has never previously received, and that are effective according to laboratory susceptibility testing, must be used. Preventive therapy against tuberculosis is accomplished with isoniazid for 6 to 12 months, although rifampicin plus isoniazid for 3 months has been used in the United Kingdom with success. In a mouse model, rifampicin plus pyrazinamide for 2 months is more effective than isoniazid for 6 months as preventive treatment. Patient noncompliance with medication remains the biggest problem in tuberculosis control, and is a complex issue. It can only be resolved by multiple approaches. Intermittent directly observed short course chemotherapy is a major, but not the only, possible solution.