New Tuberculosis Therapeutics: A Growing Pipeline

Pyrido[1,2-a]benzimidazole-based agents active against tuberculosis (TB), multidrug-resistant (MDR) TB and extensively drug-resistant (XDR) TB

The struggle against tuberculosis (TB) is still far from over. TB, caused by Mycobacterium tuberculosis, is one of the deadliest infections worldwide. Co-infection with human immunodeficiency virus (HIV) and the emergence of multidrug-resistant tuberculosis (MDR-TB) and extensively drug-resistant tuberculosis (XDR-TB) strains have further increased the burden for this disease. Herein, we report the discovery of 2-(4-chlorobenzyl)-3-methyl-1-oxo-1H,5H-pyrido[1,2-a]benzimidazole-4-carbonitrile as an effective antitubercular agent and the structural modifications of this molecule that have led to analogues with improved potency and lower toxicity. A number of these derivatives were also active at sub-micromolar concentrations against resistant TB strains and devoid of apparent toxicity to Vero cells, thereby underscoring their value as novel scaffolds for the development of new anti-TB drugs.

TMC207: the first compound of a new class of potent anti-tuberculosis drugs

Disease caused by Mycobacterium tuberculosis continues as a global epidemic: over 2 billion people harbor latent TB infection, and more than 9 million new TB cases, of whom 500,000 are multidrug-resistant (MDR), and nearly 2 million deaths are estimated to occur each year. New drugs are required to shorten treatment duration of drug-sensitive TB and for the treatment of MDR-TB. TMC207 is a first-in-class diarylquinoline compound with a novel mechanism of action, the inhibition of bacterial ATP synthase, and potent activity against drug-sensitive and drug-resistant TB. It has bactericidal and sterilizing activity against M. tuberculosis and other mycobacterial species, but little activity against other bacteria. In a Phase II efficacy study conducted in patients with MDR-TB taking TMC207 plus a standard background regimen, the drug appeared to be safe and well tolerated, and showed significant efficacy after 2 months of treatment with conversion rates of sputum culture of 48% (vs 9% in the placebo group). Given the product development partnership between Tibotec and the TB Alliance, the strategies of using TMC207 in shorter first-line regimens or using it in second-line regimens for drug-resistant M. tuberculosis infections are both being pursued. No clinical data of TMC207 in TB patients with HIV coinfection have been published; drug-drug interaction studies with antiretrovirals are being conducted. Finally, the remarkable sterilizing capacity of TMC207 also makes it an attractive drug in the strategy of TB elimination. Current and future studies will determine the role of TMC207 in a shortened treatment regimen for drug-sensitive TB, a more effective and better-tolerated regimen for MDR-TB, the treatment of latent TB infection, and intermittent-TB treatment regimens.

The antipsychotic thioridazine shows promising therapeutic activity in a mouse model of multidrug-resistant tuberculosis

Multidrug- and extensively drug-resistant tuberculosis have emerged as grave threats to public health worldwide. Very few active drugs are available or likely to become available soon. To address these problems we revisited a classical observation, the applicability of phenothiazines as antimicrobial drugs. Within this pharmacological class we selected thioridazine, which is most efficacious and least toxic, when used as an antipsychotic drug. We tested thioridazine monotherapy in the Balb/c mouse model for its activity to treat both susceptible and multidrug-resistant tuberculosis by a two months daily oral administration of 32 and 70 mg/kg. In addition, we tested its additive value when combined with a standard first-line regimen for susceptible tuberculosis. Thioridazine treatment resulted in a significant reduction of colony-forming-units of the susceptible (−4.4 log CFU, p<0.05) and multidrug-resistant tuberculosis bacilli (−2.4 log CFU, p<0.009) in the lung both at one and two months after infection, compared to controls. Moreover, when thioridazine was added to a regimen containing rifampicin, isoniazid and pyrazinamide for susceptible tuberculosis, a significant synergistic effect was achieved (−6.2 vs −5.9 log CFU, p<0.01). Thioridazine may represent an effective compound for treatment of susceptible and multidrug-resistant tuberculosis. The phenothiazines and their targets represent interesting novel opportunities in the search for antituberculosis drugs.

Synthesis and pharmacological evaluation of clubbed isopropylthiazole derived triazolothiadiazoles, triazolothiadiazines and mannich bases as potential antimicrobial and antitubercular agents

A series of novel clubbed Isopropylthiazole derivatives triazolothiadiazines 2a-g, dihydro triazolothiadiazoles 3a-g, thioxotriazoles 4a-d, triazolothiadiazole 5, arylideneamino triazolethiones 7a-h and oxadiazolethiones 11a-b were synthesized and characterized by IR, (1)H NMR, (13)C NMR, elemental and mass spectral analysis. These compounds were evaluated for their preliminary in vitro antibacterial, antifungal and antitubercular activity against Mycobacterium tuberculosis H(37)Rv strain by broth dilution assay method. All the compounds exhibited moderate to significant antibacterial and antifungal activities. Results of the antitubercular screening against M. tuberculosis H(37)Rv showed compounds 7c and 7d exhibited good antitubercular activity (MIC 4 and 8 μg/mL) respectively, when compared with first line drug such as isoniazid (0.25 μg/mL).

The emergence of extensively drug-resistant tuberculosis: a global health crisis requiring new interventions: Part II: scientific advances that may provide solutions

The need for new tuberculosis (TB) diagnostics has never been greater as TB in the HIV-infected is often sputum smear negative or extrapulmonary and may progress rapidly unless diagnosed and treated appropriately. In addition, the empirical treatment of patients with drug-resistant TB leads to the acquisition of additional resistance. Fortunately there is a robust and adequately funded developments pipeline including investigational rapid diagnostics that may replace smear, culture, and drug susceptibility testing. The dogma that drug resistance usually develops as a consequence of patient nonadherence has never been entirely plausible. Recent observations indicate that certain mutations in drug resistance genes promote the acquisition of additional resistance. Further, Mycobacterium tuberculosis (MTB) may demonstrate tolerant phenotypes due to induction of a multidrug-resistant like pump. It will be difficult to "treat our way" out of the problem of extensively drug-resistant (XDR)-TB without access to new interventions. Vaccines in development offer a distant hope. Promising new therapeutics in clinical trials may shorten the duration of treatment of TB, which will lessen the development of drug resistance or provide potent new and MTB specific agents that should be effective in treatment of XDR-TB.

In vitro interactions between new antitubercular drug candidates SQ109 and TMC207

The in vitro interactions of two new antitubercular drugs, SQ109 and TMC207, with each other and with rifampin (RIF) were evaluated. The combination of SQ109 with TMC207 (i) improved an already excellent TMC207 MIC for M. tuberculosis H37Rv by 4- to 8-fold, (ii) improved the rate of killing of bacteria over the rate of killing by each single drug, and (iii) enhanced the drug postantibiotic effect by 4 h. In no instance did we observe antagonistic activities with the combination of SQ109 and TMC207. Rifampin activates cytochrome P450 genes to reduce the area under the curve (AUC) for TMC207 in humans. The presence of RIF in three-drug combinations did not affect the synergistic activities of SQ109 and TMC207, and SQ109 also dramatically decreased the MIC of RIF. SQ109 was active by itself, and both its activity was improved by and it improved the in vitro activities of both RIF and TMC207.

Pharmacological significance of triazole scaffold

The triazole nucleus is one of the most important and well known heterocycles which is a common and integral feature of a variety of natural products and medicinal agents. Triazole nucleus is present as a core structural component in an array of drug categories such as antimicrobial, anti-inflammatory, analgesic, antiepileptic, antiviral, antineoplastic, antihypertensive, antimalarial, local anaesthetic, antianxiety, antidepressant, antihistaminic, antioxidant, antitubercular, anti-Parkinson's, antidiabetic, antiobesity and immunomodulatory agents, etc. The broad and potent activity of triazole and their derivatives has established them as pharmacologically significant scaffolds. The basic heterocyclic rings present in the various medicinal agents are 1,2,3-triazole and 1,2,4-triazole. A large volume of research has been carried out on triazole and their derivatives, which has proved the pharmacological importance of this heterocyclic nucleus. The present paper is an attempt to review the pharmacological activities reported for triazole derivatives in the current literature with an update of recent research findings on this nuclei.

Stopping tuberculosis in the 21st century: goals and strategies

The Stop TB Strategy and the Global Plan to Stop TB were launched in 2006 to achieve the tuberculosis (TB)-related Millennium Development Goals and the Stop TB Partnership targets, and to address new challenges such as that of HIV-associated TB and multi-drug-resistant TB. This paper reviews the historical and recent progress in TB control to show what has changed since the introduction of directly observed therapy (DOTS) in the mid-1990s, why we needed the new strategy and what the global agenda is today. Major progress was seen in most countries in the last two decades. Globally, the estimated rates of TB prevalence and mortality are declining, but not quickly enough to reach the 2015 Stop TB Partnership targets of halving TB prevalence and death rates compared with 1990. In 2007, it was estimated that more than one-third of TB patients were not detected or properly treated under proper conditions. Enhancing case detection, while maintaining high treatment success rates, is essential to achieve the 2015 targets. The ultimate goal of TB control is the elimination of the disease as a public health problem. The Stop TB Partnership aims at eliminating TB by 2050 by reaching a global incidence of disease of less than one case per million population. This target will not be achieved unless TB control efforts are further intensified and effective and affordable new technologies to prevent both disease and infection are developed and rapidly introduced in all countries worldwide.

The chemical biology of new drugs in the development for tuberculosis

With the worldwide emergence of multidrug-resistant (MDR) and extensively drug-resistant (XDR) strains of Mycobacterium tuberculosis (Mtb), there are serious concerns about the continued ability to contain this disease. We discuss the most promising new drugs in late-stage development that might be useful in treating MDR and XDR forms of the disease. These agents have novel mechanisms of action that are not targeted by the standard drugs used presently to treat susceptible strains.

Identification, synthesis, and pharmacological evaluation of tetrahydroindazole based ligands as novel antituberculosis agents

The resurgence of tuberculosis (TB), the incidence of drug-resistant strains of Mycobacterium tuberculosis (MTB), and the coinfection between TB and HIV have led to serious infections, high mortality, and a global health threat, resulting in the urgent search for new classes of antimycobacterial agents. Herein, we report the identification of a novel class of tetrahydroindazole based compounds as potent and unique inhibitors of MTB. Compounds 6a, 6m, and 6q exhibited activity in the low micromolar range against replicating Mycobacterium tuberculosis (R-TB) phenotype, with minimum inhibitory concentrations (MICs) of 1.7, 1.9, and 1.9 muM, respectively, while showing no toxicity to Vero Ccells. Moreover, studies aimed to assess the in vitro metabolic stability of 6a and 6m in mouse liver microsomes and in vivo pharmacokinetic profiles in plasma levels gave satisfactory results. This research suggests that tetrahydroindazole based anti-TB compounds can serve as a promising lead scaffold in developing new drugs to combat tuberculosis infections.

Evaluation of standard chemotherapy in the guinea pig model of tuberculosis

The purpose of this study was 2-fold. First, we evaluated standard chemotherapy in the guinea pig model of tuberculosis to determine if this animal species could productively be used for this purpose. Second, given the similarities of the pathology of disease in guinea pigs and humans, we wished to evaluate additional parameters, including magnetic resonance imaging, microscopy, and cytokine expression and lymphocyte phenotypes, in response to an infection treated with drug therapy. This study shows that conventional rifampin-isoniazid-pyrazinamide chemotherapy significantly decreased the numbers of the highly virulent Erdman K01 strain of Mycobacterium tuberculosis, with most of the bacilli being eliminated in a month. Despite this result, bacteria could still be detected in the lungs and other tissues for at least another 3 to 4 months. Resolution of the nonnecrotic granulomas in the lungs and lymph nodes could be clearly visualized by magnetic resonance imaging at the macroscopic level. Microscopically, the majority of the pulmonary and extrapulmonary inflammation resolved spontaneously, leaving residual lesions composed of dystrophic calcification and fibrosis marking the site of necrosis of the primary lesion. Residual calcified lesions, which were also associated with pulmonary lymphangitis, contained acid-fast bacilli even following aggressive chemotherapy. The presence of intact extracellular bacilli within these lesions suggests that these could serve as the primary sites of disease reactivation. The chemotherapy reduced the level of T-cell influx into infected tissues and was accompanied by a large and sustained increase in TH1 cytokine expression. Chemotherapy also prevented the emergence in lung tissues of high levels of interleukin-10 and Foxp3-positive cells, known markers of regulatory T cells.

Dry powder PA-824 aerosols for treatment of tuberculosis in guinea pigs

Novel treatments for multidrug-resistant tuberculosis (MDR-TB), extensively drug-resistant tuberculosis (XDR-TB), or latent TB are needed urgently. Recently, we reported the formulation and characterization of the nitroimidazo-oxazine PA-824 for efficient aerosol delivery as dry powder porous particles and the subsequent disposition in guinea pigs after pulmonary administration. The objective of the present study was to evaluate the effects of these PA-824 therapeutic aerosols on the extent of TB infection in the low-inoculum aerosol infection guinea pig model. Four weeks after infection by the pulmonary route, animals received daily treatment for 4 weeks of either a high or a low dose of PA-824 dry powder aerosol. Animals received PA-824 cyclodextrin/lecithin suspensions orally as positive controls, and those receiving placebo particles or no treatment were negative controls. The lungs and spleens of animals receiving the high dose of inhaled PA-824 particles exhibited a lower degree of inflammation (indicated by wet tissue weights), bacterial burden, and tissue damage (indicated by histopathology) than those of untreated or placebo animals. Treatment with oral PA-824 cyclodextrin/lecithin suspension resulted in a more significant reduction in the bacterial burden of lungs and spleen, consistent with a dose that was larger than inhaled doses (eight times the inhaled low dose and four times the inhaled high dose). However, histopathological analysis revealed that the extent of tissue damage was comparable in groups receiving the oral or either inhaled dose. The present studies indicate the potential use of PA-824 dry powder aerosols in the treatment of TB.

Drug-resistant tuberculosis: a worldwide epidemic poses a new challenge

BACKGROUND

Although the incidence of tuberculosis (TB) in Germany is now declining, the world as a whole faces the threat of a catastrophe that will also affect the industrialized nations. The main reason, aside from TB/HIV co-infection, is the increase of resistant TB strains. The situation is already serious because of the spread of multidrug-resistant TB, i.e., TB that is resistant to the two most important antituberculous drugs, and is being further aggravated by resistance to second-line drugs as well.

METHOD

Selective review of the literature.

RESULTS

There are an estimated half a million cases of multidrug-resistant TB worldwide, and so-called extensively resistant TB (XDR-TB), with additional resistance to defined second-line drugs, is now prevalent in more than 45 countries. An accurate assessment of the situation is hampered by a widespread lack of laboratory capacity and/or proper surveillance. The problem is mainly due to inappropriate treatment, which may have many causes, but is theoretically avoidable. Aside from programmatic weaknesses, a lack of diagnostic and therapeutic tools causes difficulties in many countries.

DISCUSSION

Only rapid and internationally concerted action, combined with intensified research efforts and the support of the affected nations, will be able to prevent the development of a situation that will no longer be manageable even with 21(st)-century technology.

High-throughput screening for inhibitors of Mycobacterium tuberculosis H37Rv

There is an urgent need for the discovery and development of new antitubercular agents that target new biochemical pathways and treat drug resistant forms of the disease. One approach to addressing this need is through high-throughput screening of medicinally relevant libraries against the whole bacterium in order to discover a variety of new, active scaffolds that will stimulate new biological research and drug discovery. Through the Tuberculosis Antimicrobial Acquisition and Coordinating Facility (www.taacf.org), a large, medicinally relevant chemical library was screened against M. tuberculosis strain H37Rv. The screening methods and a medicinal chemistry analysis of the results are reported herein.

Epidemiological benefits of more-effective tuberculosis vaccines, drugs, and diagnostics

The Bill and Melinda Gates Foundation supports an ambitious portfolio of novel vaccines, drug regimens, and diagnostic tools for tuberculosis (TB). We elicited the expected efficacies and improvements of the novel interventions in discussions with the foundations managing their development. Using an age-structured mathematical model of TB, we explored the potential benefits of novel interventions under development and those not yet in the portfolio, focusing on the WHO Southeast Asia region. Neonatal vaccination with the portfolio vaccine decreases TB incidence by 39% to 52% by 2050. Drug regimens that shorten treatment duration and are efficacious against drug-resistant strains reduce incidence by 10-27%. New diagnostics reduce incidence by 13-42%. A triple combination of a portfolio vaccine, drug regimen, and diagnostics reduces incidence by 71%. A short mass vaccination catch-up campaign, not yet in the portfolio, to augment the triple combination, accelerates the decrease, preventing >30% more cases by 2050 than just the triple combination. New vaccines and drug regimens targeted at the vast reservoir of latently infected people, not in the portfolio, would reduce incidence by 37% and 82%, respectively. The combination of preventive latent therapy and a 2-month drug treatment regimen reduces incidence by 94%. Novel technologies in the pipeline would achieve substantial reductions in TB incidence, but not the Stop TB Partnership target for elimination. Elimination will require new delivery strategies, such as mass vaccination campaigns, and new products targeted at latently infected people.

Pediatric TB: issues related to current and future treatment options

Pediatric TB continues to be a neglected disease in many endemic areas where limited resources restrict the focus of treatment to only the most infectious TB cases. However, recognition that children contribute to a significant proportion of the global TB disease burden and suffer severe TB-related morbidity and mortality is growing. The WHO published guidelines on the management of pediatric TB in 2006 and child-friendly drug formulations have been made available to deserving low-income nations via the Global Drug Fund since 2008. Increased awareness and improved drug availability re-emphasized the considerable programmatic barriers that remain and the difficulty of establishing an accurate diagnosis in resource-limited settings. This article provides an overview of current treatment practices, factors that influence the provision of effective TB therapy to children in endemic areas and potential future advances. It includes a brief summary of the relevant literature and presents the authors’ personal perspectives on issues related to the treatment of pediatric TB.

The DosR regulon of M. tuberculosis and antibacterial tolerance

Adaptation of Mycobacterium tuberculosis to an anaerobic dormant state that is tolerant to several antibacterials is mediated largely by a set of highly expressed genes controlled by DosR. A DosR mutant was constructed to investigate whether the DosR regulon is involved in antibacterial tolerance. We demonstrate that induction of the regulon is not required for drug tolerance either in vivo during a mouse infection or in vitro during anaerobic dormancy. Thus, drug tolerance observed in these models is due to other mechanisms such as the bacilli simply being in a non-replicating or low metabolic state. Our data also demonstrate that the DosR regulon is not essential for virulence during chronic murine infection. However, decreased lung pathology was observed in the DosR mutant. We also show that the DosR regulon genes are more highly conserved in environmental mycobacteria, than in pathogenic mycobacteria lacking a latent phase or environmental reservoir. It is possible that the DosR regulon could contribute to drug tolerance in human infections; however, it is not the only mechanism and not the primary mechanism for tolerance during a mouse infection. These data suggest that the regulon evolved not for pathogenesis or drug tolerance but for adaptation to anaerobic conditions in the environment and has been adapted by M. tuberculosis for survival during latent infection.

Synthesis and in-vitro antimycobacterial evaluation of 1-(cyclopropyl/2,4-difluorophenyl/tert-butyl)-1,4-dihydro- 8-methyl-6-nitro-4-oxo-7-(substituted secondary amino)quinoline-3-carboxylic acids

Fifty one newer 1-(cyclopropyl/2,4-difluorophenyl/tert-butyl)-1,4-dihydro-8-methyl-6-nitro-4-oxo-7-(substituted secondary amino)quinoline-3-carboxylic acids were synthesized from 1,3-dichloro-2-methylbenzene and evaluated for in-vitro antimycobacterial activities against Mycobacterium tuberculosis H37Rv (MTB), multi-drug resistant Mycobacterium tuberculosis (MDR-TB), and Mycobacterium smegmatis (MC(2)). Among the synthesized compounds, 1-cyclopropyl-1,4-dihydro-7-(3,4-dihydro-6,7-dimethoxyisoquinolin-2(1H)-yl)-8-methyl-6-nitro-4-oxoquinoline-3-carboxylic acid 9p was found to be the most active compound in vitro with a MIC value of 0.39 microM against MTB. Against MDR-TB, compound 7-(2-carboxy-5,6-dihydroimidazo[1,2-a]pyrazin-7(8H)-yl)-1-cyclopropyl-1,4-dihydro-8-methyl-6-nitro-4-oxoquinoline-3-carboxylic acid 9n was found to be the most active with a MIC value of 0.09 microM.

Design of novel antituberculosis compounds using graph-theoretical and substructural approaches

The increasing resistance of Mycobacterium tuberculosis to the existing drugs has alarmed the worldwide scientific community. In an attempt to overcome this problem, two models for the design and prediction of new antituberculosis agents were obtained. The first used a mixed approach, containing descriptors based on fragments and the topological substructural molecular design approach (TOPS-MODE) descriptors. The other model used a combination of two-dimensional (2D) and three-dimensional (3D) descriptors. A data set of 167 compounds with great structural variability, 72 of them antituberculosis agents and 95 compounds belonging to other pharmaceutical categories, was analyzed. The first model showed sensitivity, specificity, and accuracy values above 80% and the second one showed values higher than 75% for these statistical indices. Subsequently, 12 structures of imidazoles not included in this study were designed, taking into account the two models. In both cases accuracy was 100%, showing that the methodology in silico developed by us is promising for the rational design of antituberculosis drugs.

Treatment of tuberculosis in children

The treatment of children with TB is influenced by a number of factors specific to both the bacterium and the child. We review the variables impacting the selection of individual medications; indications, pharmacology, dosing and side effects for first- and second-line agents; adjunctive therapy; and special cases, including treatment of TB in HIV-infected children and multidrug-resistant TB. Finally, evolving trends in TB therapy, such as the impact of HIV and multidrug-resistant TB on future therapeutics, emerging or re-emerging medication options, shorter-course regimens and immunomodulation, are discussed.

Dry powder nitroimidazopyran antibiotic PA-824 aerosol for inhalation

We formulated PA-824, a nitroimidazopyran with promise for the treatment of tuberculosis, for efficient aerosol delivery to the lungs in a dry powder porous particle form. The objectives of this study were to prepare and characterize a particulate form of PA-824, assess the stability of this aerosol formulation under different environmental conditions, and determine the pharmacokinetic parameters for the powder after pulmonary administration. The drug was spray dried into porous particles containing a high drug load and possessing desirable aerosol properties for efficient deposition in the lungs. The physical, aerodynamic, and chemical properties of the dry powder were stable at room temperature for 6 months and under refrigerated conditions for at least 1 year. Pharmacokinetic parameters were determined in guinea pigs after the pulmonary administration of the PA-824 powder formulation at three doses (20, 40, and 60 mg/kg of body weight) and compared to those after the intravenous (20 mg/kg) and oral (40 mg/kg) delivery of the drug. Oral and inhaled delivery of PA-824 achieved equivalent systemic delivery at the same body dose within the first 12 h of dosing. However, animals dosed by the pulmonary route showed drug loads that remained locally in the lungs for 32 h postexposure, whereas those given the drug orally cleared the drug more rapidly. Therefore, we expect from these pharmacokinetic data that pulmonary delivery may achieve the same efficacy as oral delivery at the same body dose, with a potential improvement in efficacy related to pulmonary infection. This may translate into the ability to deliver lower body doses of this drug for the treatment of tuberculosis by aerosol.

Doomsday postponed? Preventing and reversing epidemics of drug-resistant tuberculosis

Extensively drug-resistant (XDR) strains of Mycobacterium tuberculosis, now known to be present in 50 countries, heighten the threat posed by untreatable and fatal human tuberculosis (TB). To combat epidemics of drug-resistant TB, it is vital to understand why some resistant strains have greater reproductive fitness--a greater propensity to spread--than drug-susceptible strains. If public health malpractice has been a more important determinant of reproductive success than genetic mechanisms, then improved diagnosis and treatment could keep the frequency of resistant strains among TB cases low in any population. Recent data suggest that national TB control programmes that use existing drugs efficiently can postpone and even reverse epidemics of multidrug-resistant TB, although the effect of such programmes on XDR strains remains largely unknown.

Multidrug-resistant and extensively drug-resistant tuberculosis: a review

PURPOSE OF REVIEW

The spread of multidrug-resistant tuberculosis (MDR-TB) and extensively drug-resistant TB (XDR-TB) is a major medical and public health concern for the world. These two forms of highly drug-resistant TB threaten to make TB into an untreatable and highly fatal disease, particularly in resource-poor countries with a high prevalence of AIDS. The focus of this review is to highlight the current extent of the problem.

RECENT FINDINGS

There is a great variability in clinical outcomes for MDR-TB, in part due to differences in the definitions of outcome measures and retrospective nature of the studies. Outcomes for XDR-TB are uniformly worse than those for MDR-TB.

SUMMARY

A multifaceted approach is needed to prevent a more widespread epidemic of MDR-TB and XDR-TB. Rapid diagnostic assays to detect highly drug-resistant TB are essential in preventing delays in treatment of MDR-TB and XDR-TB and curbing their spread. Development of new drugs to effectively treat all forms of TB in a shorter period of time is urgently needed.

Low hanging fruit in infectious disease drug development

Cost estimates for developing new molecular entities (NME) are reaching non-sustainable levels and coupled with increasing regulatory requirements and oversight have led many pharmaceutical sponsors to divest their anti-microbial development portfolios [Projan SJ: Why is big Pharma getting out of anti-bacterial drug discovery?Curr Opin Microbiol 2003, 6:427-430] [Spellberg B, Powers JH, Brass EP, Miller LG, Edwards JE, Jr: Trends in antimicrobial drug development: implications for the future.Clin Infect Dis 2004, 38:1279-1286]. Operational issues such as study planning and execution are significant contributors to the overall cost of drug development that can benefit from the leveraging of pre-randomization data in an evidence-based approach to protocol development, site selection and patient recruitment. For non-NME products there is even greater benefit from available data resources since these data may permit smaller and shorter study programs. There are now many available open source intelligence (OSINT) resources that are being integrated into drug development programs, permitting an evidence-based or 'operational epidemiology' approach to study planning and execution.

Extensively drug-resistant tuberculosis

Extensively drug-resistant tuberculosis (XDR-TB) is defined as Mycobacterium tuberculosis infection that is resistant to isoniazid, rifampin, any fluoroquinolone, and any injectable drug (amynoglicosides or polypetides). Although initially described in South Africa, it has emerged as a global threat, and cases have been reported from several countries, including the United States. XDR-TB has emerged mainly as a consequence of previous inadequate or poorly administered treatment, from failure of the public health infrastructure. As the diagnosis of this condition requires antibiotic susceptibility confirmation, a broad network of reference laboratories and the development of faster and more accurate tests for the identification of active cases of tuberculosis are urgently required. The treatment of XDR-TB is challenging and requires the use of multiple second-line drugs and, potentially, surgery. Infection control measures do not differ from those used for susceptible cases but may require more stringent application.

Distinct specificities of Mycobacterium tuberculosis and mammalian proteasomes for N-acetyl tripeptide substrates

The proteasome of Mycobacterium tuberculosis (Mtb) is a validated and drug-treatable target for therapeutics. To lay ground-work for developing peptide-based inhibitors with a useful degree of selectivity for the Mtb proteasome over those of the host, we used a library of 5,920 N-acetyl tripeptide-aminomethylcoumarins to contrast the substrate preferences of the recombinant Mtb proteasome wild type and open gate mutant, the Rhodococcus erythropolis proteasome, and the bovine proteasome with activator PA28. The Mtb proteasome was distinctive in strictly preferring P1 = tryptophan, particularly in combination with P3 = glycine, proline, lysine or arginine. Screening results were validated with Michalis-Menten kinetic analyses of 21 oligopeptide aminomethyl-coumarin substrates. Bortezomib, a proteasome inhibitor in clinical use, and 17 analogs varying only at P1 were used to examine the differential impact of inhibitors on human and Mtb proteasomes. The results with the inhibitor panel confirmed those with the substrate panel in demonstrating differential preferences of Mtb and mammalian proteasomes at the P1 amino acid. Changing P1 in bortezomib from Leu to m-CF(3)-Phe led to a 220-fold increase in IC(50) against the human proteasome, whereas changing a P1 Ala to m-F-Phe decreased the IC(50) 400-fold against the Mtb proteasome. The change of a P1 Ala to m-Cl-Phe led to an 8000-fold shift in inhibitory potency in favor of the Mtb proteasome, resulting in 8-fold selectivity. Combinations of preferred amino acids at different sites may thus improve the species selectivity of peptide-based inhibitors that target the Mtb proteasome.

Characterization of active site structure in CYP121. A cytochrome P450 essential for viability of Mycobacterium tuberculosis H37Rv

Mycobacterium tuberculosis (Mtb) cytochrome P450 gene CYP121 is shown to be essential for viability of the bacterium in vitro by gene knock-out with complementation. Production of CYP121 protein in Mtb cells is demonstrated. Minimum inhibitory concentration values for azole drugs against Mtb H37Rv were determined, the rank order of which correlated well with Kd values for their binding to CYP121. Solution-state spectroscopic, kinetic, and thermodynamic studies and crystal structure determination for a series of CYP121 active site mutants provide further insights into structure and biophysical features of the enzyme. Pro346 was shown to control heme cofactor conformation, whereas Arg386 is a critical determinant of heme potential, with an unprecedented 280-mV increase in heme iron redox potential in a R386L mutant. A homologous Mtb redox partner system was reconstituted and transported electrons faster to CYP121 R386L than to wild type CYP121. Heme potential was not perturbed in a F338H mutant, suggesting that a proposed P450 superfamily-wide role for the phylogenetically conserved phenylalanine in heme thermodynamic regulation is unlikely. Collectively, data point to an important cellular role for CYP121 and highlight its potential as a novel Mtb drug target.

1,5-Diphenylpyrrole derivatives as antimycobacterial agents. Probing the influence on antimycobacterial activity of lipophilic substituents at the phenyl rings

Synthesis and biological evaluation of new derivatives of 1,5-bis(4-chlorophenyl)-2-methyl-3-(4-methylpiperazin-1-yl)methyl-1H-pyrrole (BM 212, 16) are reported. Variously substituted phenyl rings with different substitution pattern and lipophilicity were added to the pyrrole nucleus to evaluate their influence on the activity toward Mycobacterium tuberculosis (MTB) and atypical mycobacteria. The most active derivatives showed activity between 0.125-0.5 microg/mL (better than 16 and streptomycin) and protection index (64-256) higher than 16 (4) and similar to isoniazid and streptomycin (128).

Tetrahdroxysqualene from Rhus taitensis shows antimycobacterial activity against Mycobacterium tuberculosis

Tuberculosis has become a major health problem, in particular with the emergence of extremely drug resistant tuberculosis (XDRTB). In our search for new therapeutic leads against TB, we isolated a new triterpene (1) from the plant Rhus taitensis collected in Papua New Guinea. Tetrahydroxysqualene (1) was isolated using bioassay-guided fractionation of the methanolic extract of R. taitensis leaves and twigs. The structure of tetrahydroxysqualene (1) was elucidated on the basis of HRESIMS and 1D and 2D NMR spectra. Tetrahydroxysqualene (1) exhibited antituberculosis activity with an MIC of 10.0 microg/mL, while showing only modest cytotoxicity.

A microbiological assessment of novel nitrofuranylamides as anti-tuberculosis agents

OBJECTIVES

Nitrofuranylamides (NFAs) are nitroaromatic compounds that have recently been discovered and have potent anti-tuberculosis (TB) activity. A foundational study was performed to evaluate whether this class of agents possesses microbiological properties suitable for future antimycobacterial therapy.

METHODS

Five representative compounds of the NFA series were evaluated by standard microbiological assays to determine MICs, MBCs, activity against anaerobic non-replicating persistent Mycobacterium tuberculosis, post-antibiotic effects (PAEs), antibiotic synergy and the basis for resistance.

RESULTS

The antimicrobial activity of these compounds was restricted to bacteria of the M. tuberculosis complex, and all compounds were highly active against drug-susceptible and -resistant strains of M. tuberculosis, with MICs 0.0004-0.05 mg/L. Moreover, no antagonism was observed with front-line anti-TB drugs. Activity was also retained against dormant bacilli in two in vitro low-oxygen models for M. tuberculosis persistence. A long PAE was observed, which was comparable to that of rifampicin, but superior to isoniazid and ethambutol. Spontaneous NFA-resistant mutants arose at a frequency of 10(-5)-10(-7), comparable to that for isoniazid (10(-5)-10(-6)). Some of these mutants exhibited cross-resistance to one or both of the nitroimidazoles PA-824 and OPC-67683. Cross-resistance was associated with inactivation of the reduced F(420)-deazaflavin cofactor pathway and not with inactivation of the Rv3547, the nitroreductase for PA-824 and OPC-67683.

CONCLUSIONS

Based on these studies, NFAs have many useful antimycobacterial properties applicable to TB chemotherapy and probably possess a unique mode of action that results in good activity against active and dormant M. tuberculosis. Therefore, the further development of lead compounds in this series is warranted.

Scaling up programmatic management of drug-resistant tuberculosis: a prioritized research agenda

Frank Cobelens and colleagues outline key research questions that need to be addressed to maximize the impact of programmatic management of drug-resistant tuberculosis.

Impact of HIV on novel therapies for tuberculosis control

OBJECTIVE AND DESIGN

The increased risk for tuberculosis in HIV-infected people has fueled a worldwide resurgence of tuberculosis. A major hindrance to controlling tuberculosis is the long treatment duration, leading to default, jeopardizing cure, and generating drug resistance. We investigated how tuberculosis is impacted by reducing treatment duration alone or combined with enhanced case detection and/or cure under different HIV prevalence levels.

METHODS

Our model includes HIV stages I-IV and was calibrated to long-term tuberculosis and HIV data from Kenya. Benefits were assessed in terms of absolute and relative reductions in new tuberculosis cases and deaths.

RESULTS

Compared with present-day strategies, at 3-20% HIV prevalence we attain a 6-20% decrease in incidence and mortality in 25 years when reducing treatment duration alone; benefits exceed 300% when combined with increased detection and cure. Benefits vary substantially according to HIV status and prevalence. Challenges arise because in absolute terms the number of infected people and deaths increases dramatically with increasing HIV prevalence, and because the relative efficacy of tuberculosis control policies displays a nonlinear pattern whereby they become less effective on a per capita basis at HIV prevalence levels greater than 15%. Benefits of reducing treatment duration may even be reversed at extreme HIV prevalence levels. Benefits of increasing cure versus detection increase as HIV prevalence increases.

CONCLUSION

Reducing tuberculosis treatment duration, alone or in combination with other control strategies, can provide enormous benefits at high HIV prevalence. Tuberculosis control policies need to account for HIV levels because the efficacy of different interventions varies substantially with HIV prevalence.