Antitubercular agents. Part 2: new thiolactomycin analogues active against Mycobacterium tuberculosis

Structurally modified analogues of naturally occurring antibiotic thiolactomycin, substituted at 4-position of the thiolactone ring have been prepared and evaluated for their antitubercular activity. Some of the compounds have exhibited potential activity against Mycobacterium tuberculosis.     

Salicylanilide carbamates: Antitubercular agents active against multidrug-resistant Mycobacterium tuberculosis strains

A series of 27 salicylanilide-based carbamates was prepared as a part of our ongoing search for new antituberculosis drugs. These compounds exhibited very good in vitro activity against Mycobacterium tuberculosis, Mycobacterium kansasii and Mycobacterium avium and, in particular, against five multidrug-resistant strains, with MIC values between 0.5–2 μmol/L. Moreover, they displayed moderate toxicity against intestinal cells with the selectivity index being up to 96. Furthermore, acid stability and a half-life of 43 h at pH 7.4 were shown. Thus, these novel salicylanilide derivatives are drug candidates which should be seriously consider for further screening.     

Biphenyl-based analogues of thiolactomycin, active against Mycobacterium tuberculosis mtFabH fatty acid condensing enzyme

Analogues of the antibiotic thiolactomycin, with biphenyl-based 5-substituents, were found to have excellent in vitro inhibitory activity against the recombinant Mycobacterium tuberculosis beta-ketoacyl-ACP synthase mtFabH condensing enzyme. In particular, 5-(4'-benzyloxy-biphen-4-ylmethyl)-4-hydroxy-3,5-dimethyl-5H-thiophen-2-one exhibited approximately a 4-fold increased potency against this key condensing enzyme involved in M. tuberculosis mycolic acid biosynthesis, compared to thiolactomycin.     

Acetylene-based analogues of thiolactomycin, active against Mycobacterium tuberculosis mtFabH fatty acid condensing enzyme

Analogues of the natural antibiotic thiolactomycin, with acetylene-based side chains, have the highest recorded in vitro inhibitory activity against the recombinant Mycobacterium tuberculosis beta-ketoacyl-ACP synthase mtFabH condensing enzyme. In particular, 5-[3-(4-acetyl-phenyl)-prop-2-ynyl]-4-hydroxy-3,5-dimethyl-5H-thiophen-2-one exhibited more than an 18-fold increased potency, compared to thiolactomycin, against this key condensing enzyme, involved in M. tuberculosis mycolic acid biosynthesis. Analogues of the antibiotic thiolactomycin, with acetylene-based side chains, have the highest recorded activity against cloned mtFabH condensing enzyme.     

Unique mechanism of action of the thiourea drug isoxyl on Mycobacterium tuberculosis

The thiourea isoxyl (thiocarlide; 4,4'-diisoamyloxydiphenylthiourea) is known to be an effective anti-tuberculosis drug, active against a range of multidrug-resistant strains of Mycobacterium tuberculosis and has been used clinically. Little was known of its mode of action. We now demonstrate that isoxyl results in a dose-dependent decrease in the synthesis of oleic and, consequently, tuberculostearic acid in M. tuberculosis with complete inhibition at 3 microg/ml. Synthesis of mycolic acid was also affected. The anti-bacterial effect of isoxyl was partially reversed by supplementing growth medium with oleic acid. The specificity of this inhibition pointed to a Delta9-stearoyl desaturase as the drug target. Development of a cell-free assay for Delta9-desaturase activity allowed direct demonstration of the inhibition of oleic acid synthesis by isoxyl. Interestingly, sterculic acid, a known inhibitor of Delta9-desaturases, emulated the effect of isoxyl on oleic acid synthesis but did not affect mycolic acid synthesis, demonstrating the lack of a relationship between the two effects of the drug. The three putative fatty acid desaturases in the M. tuberculosis genome, desA1, desA2, and desA3, were cloned and expressed in Mycobacterium bovis BCG. Cell-free assays and whole cell labeling demonstrated increased Delta9-desaturase activity and oleic acid synthesis only in the desA3-overexpressing strain and an increase in the minimal inhibitory concentration for isoxyl, indicating that DesA3 is the target of the drug. These results validate membrane-bound Delta9-desaturase, DesA3, as a new therapeutic target, and the thioureas as anti-tuberculosis drugs worthy of further development.     

IFNG-mediated immune responses enhance autophagy against Mycobacterium tuberculosis antigens in patients with active tuberculosis

Protective immunity against Mycobacterium tuberculosis (Mtb) requires IFNG. Besides, IFNG-mediated induction of autophagy suppresses survival of virulent Mtb in macrophage cell lines. We investigated the contribution of autophagy to the defense against Mtb antigen (Mtb-Ag) in cells from tuberculosis patients and healthy donors (HD). Patients were classified as high responders (HR) if their T cells produced significant IFNG against Mtb-Ag; and low responders (LR) when patients showed weak or no T cell responses to Mtb-Ag. The highest autophagy levels were detected in HD cells whereas the lowest quantities were observed in LR patients. Interestingly, upon Mtb-Ag stimulation, we detected a positive correlation between IFNG and MAP1LC3B-II/LC3-II levels. Actually, blockage of Mtb-Ag-induced IFNG markedly reduced autophagy in HR patients whereas addition of limited amounts of IFNG significantly increased autophagy in LR patients. Therefore, autophagy collaborates with human immune responses against Mtb in close association with specific IFNG secreted against the pathogen.     

IFNG-mediated immune responses enhance autophagy against Mycobacterium tuberculosis antigens in patients with active tuberculosis

Protective immunity against Mycobacterium tuberculosis (Mtb) requires IFNG. Besides, IFNG-mediated induction of autophagy suppresses survival of virulent Mtb in macrophage cell lines. We investigated the contribution of autophagy to the defense against Mtb antigen (Mtb-Ag) in cells from tuberculosis patients and healthy donors (HD). Patients were classified as high responders (HR) if their T cells produced significant IFNG against Mtb-Ag; and low responders (LR) when patients showed weak or no T cell responses to Mtb-Ag. The highest autophagy levels were detected in HD cells whereas the lowest quantities were observed in LR patients. Interestingly, upon Mtb-Ag stimulation, we detected a positive correlation between IFNG and MAP1LC3B-II/LC3-II levels. Actually, blockage of Mtb-Ag-induced IFNG markedly reduced autophagy in HR patients whereas addition of limited amounts of IFNG significantly increased autophagy in LR patients. Therefore, autophagy collaborates with human immune responses against Mtb in close association with specific IFNG secreted against the pathogen.     

Thiolactomycin and related analogues as novel anti-mycobacterial agents targeting KasA and KasB condensing enzymes in Mycobacterium tuberculosis

Prevention efforts and control of tuberculosis are seriously hampered by the appearance of multidrug-resistant strains of Mycobacterium tuberculosis, dictating new approaches to the treatment of the disease. Thiolactomycin (TLM) is a unique thiolactone that has been shown to exhibit anti-mycobacterial activity by specifically inhibiting fatty acid and mycolic acid biosynthesis. In this study, we present evidence that TLM targets two beta-ketoacyl-acyl-carrier protein synthases, KasA and KasB, consistent with the fact that both enzymes belong to the fatty-acid synthase type II system involved in fatty acid and mycolic acid biosynthesis. Overexpression of KasA, KasB, and KasAB in Mycobacterium bovis BCG increased in vivo and in vitro resistance against TLM. In addition, a multidrug-resistant clinical isolate was also found to be highly sensitive to TLM, indicating promise in counteracting multidrug-resistant strains of M. tuberculosis. The design and synthesis of several TLM derivatives have led to compounds more potent both in vitro against fatty acid and mycolic acid biosynthesis and in vivo against M. tuberculosis. Finally, a three-dimensional structural model of KasA has also been generated to improve understanding of the catalytic site of mycobacterial Kas proteins and to provide a more rational approach to the design of new drugs.     

Docking studies on novel analogues of 8 methoxy fluoroquinolones against GyrA mutants of Mycobacterium tuberculosis

Background

Modeling of transmembrane domains (TMDs) requires correct prediction of interfacial residues for in-silico modeling and membrane insertion studies. This implies the defining of a target sequence long enough to contain interfacial residues. However, too long sequences induce artifactual polymorphism: within tested modeling methods, the longer the target sequence, the more variable the secondary structure, as though the procedure were stopped before the end of the calculation (which may in fact be unreachable). Moreover, delimitation of these TMDs can produce variable results with sequence based two-dimensional prediction methods, especially for sequences showing polymorphism. To solve this problem, we developed a new modeling procedure using the PepLook method. We scanned the sequences by modeling peptides from the target sequence with a window of 19 residues.

Results

Using sequences whose NMR-structures are already known (GpA, EphA1 and Erb2-HER2), we first determined that the hydrophobic to hydrophilic accessible surface area ratio (ASAr) was the best criterion for delimiting the TMD sequence. The length of the helical structure and the Impala method further supported the determination of the TMD limits. This method was applied to the IL-2Rβ and IL-2Rγ TMD sequences of Homo sapiens, Rattus norvegicus, Mus musculus and Bos taurus.

Conclusions

We succeeded in reducing the variation in the TMD limits to only 2 residues and in gaining structural information.     

Evaluation of Mycobacterium smegmatis as a possible surrogate screen for selecting molecules active against multi-drug resistant Mycobacterium tuberculosis

New and better drugs are needed for tuberculosis (TB), particularly for the multi-drug resistant (MDR) disease. However, the highly infectious nature of MDR Mycobacterium tuberculosis restricts its use for large scale screening of probable drug candidates. We have evaluated the potential of a screen based on a 'fast grower' mycobacterium to shortlist compounds which could be active against MDR M. tuberculosis. Sensitivity profiles of M. smegmatis, M. phlei and M. fortuitum as well as MDR clinical isolates of M. tuberculosis were determined against anti-TB drugs isoniazid and rifampicin. Among the three fast growers, M. smegmatis was found to display a profile similar to MDR M. tuberculosis. Subsequently we evaluated the performance of M. smegmatis as a 'surrogate' screen for 120 compounds which were synthesized for anti-TB activity. Fifty of these molecules were active against M. tuberculosis H(37)Rv at a minimum inhibitory concentration (MIC) cutoff of 相似文献   

Synthesis and SAR studies of 1,4-benzoxazine MenB inhibitors: Novel antibacterial agents against Mycobacterium tuberculosis

Menaquinone is an essential component of the electron transport chain in many pathogens and consequently enzymes in the menaquinone biosynthesis pathway are potential drug targets for the development of novel antibacterial agents. In order to identify leads that target MenB, the 1,4-dihydroxy-2-naphthoyl-CoA synthase from Mycobacterium tuberculosis, a high-throughput screen was performed. Several 1,4-benzoxazines were identified in this screen and subsequent SAR studies resulted in the discovery of compounds with excellent antibacterial activity against M. tuberculosis H37Rv with MIC values as low as 0.6 μg/ml. The 1,4-benzoxazine scaffold is thus a promising foundation for the development of antitubercular agents.     

A novel F420‐dependent anti‐oxidant mechanism protects Mycobacterium tuberculosis against oxidative stress and bactericidal agents

Mycobacterium tuberculosis (Mtb) is an aerobic bacterium that persists intracellularly in host macrophages and has evolved diverse mechanisms to combat and survive oxidative stress. Here we show a novel F₄₂₀‐dependent anti‐oxidant mechanism that protects Mtb against oxidative stress. Inactivation of the fbiC gene in Mtb results in a cofactor F₄₂₀‐deficient mutant that is hypersensitive to oxidative stress and exhibits a reduction in NADH/NAD⁺ ratios upon treatment with menadione. In agreement with the recent hypothesis on oxidative stress being an important component of the pathway resulting in cell death by bactericidal agents, F₄₂₀^? mutants are hypersensitive to mycobactericidal agents such as isoniazid, moxifloxacin and clofazimine that elevate oxidative stress. The Mtb deazaflavin‐dependent nitroreductase (Ddn) and its two homologues Rv1261c and Rv1558 encode for an F₄₂₀H₂‐dependent quinone reductase (Fqr) function leading to dihydroquinones. We hypothesize that Fqr proteins catalyse an F₄₂₀H₂‐specific obligate two‐electron reduction of endogenous quinones, thereby competing with the one‐electron reduction pathway and preventing the formation of harmful cytotoxic semiquinones, thus protecting mycobacteria against oxidative stress and bactericidal agents. These findings open up an avenue for the inhibition of the F₄₂₀ biosynthesis pathway or Fqr‐class proteins as a mechanism to potentiate the action of bactericidal agents.     

Autophagy in immune defense against Mycobacterium tuberculosis

Autophagy is a newly recognized innate and adaptive immunity defense against intracellular pathogens, in keeping with its role as a cytoplasmic maintenance pathway. Induction of autophagy by physiological, pharmacological or immunological means can eliminate intracellular Mycobacterium tuberculosis, providing one of the first examples of the immunological role of autophagy. Under normal circumstances, M. Tuberculosis survives in macrophages by inhibiting phagolysosome biogenesis. Induction of autophagy overcomes the mycobacterial phagosome maturation block, and delivers the tubercle bacilli to degradative compartments where they are eliminated.     

Anti-HIV natural product (+)-calanolide A is active against both drug-susceptible and drug-resistant strains of Mycobacterium tuberculosis

Naturally occurring anti-HIV-1 agent (+)-calanolide A was found to be active against all of the strains of Mycobacterium tuberculosis tested, including those resistant to the standard antitubercular drugs. Efficacy evaluations in macrophages revealed that (+)-calanolide A significantly inhibited intracellular replication of M. tuberculosis H37Rv at concentrations below the MIC observed in vitro. Preliminary mechanistic studies indicated that (+)-calanolide A rapidly inhibits RNA and DNA synthesis followed by an inhibition of protein synthesis. Compared with known inhibitors, this scenario is more similar to effects observed with rifampin, an inhibitor of RNA synthesis. Since (+)-calanolide A was active against a rifampin-resistant strain, it is believed that these two agents may involve different targets. (+)-Calanolide A and its related pyranocoumarins are the first class of compounds identified to possess antimycobacterial and antiretroviral activities, representing a new pharmacophore for anti-TB activity.     

Novel drug target strategies against Mycobacterium tuberculosis

The resurgence of drug resistant tuberculosis (TB) is a significant global healthcare challenge. Mycobacterium tuberculosis (MTB), TB's causative agent, evades the host immune system and drug regimes by entering prolonged periods of non-proliferation or dormancy. In infected individuals, the immune system sequesters MTB into structures called granulomas where the bacterium survives by shifting into a non-replicative state. Although still not well understood, progress has been made in characterizing the genetic program of MTB, activated by DosR (DevR) signal transduction that allows adaptation to the hypoxic, nutrient limiting granuloma microenvironment. Recent work, especially the identification genes involved in regulatory networks and the Enduring Hypoxic Response (EHR), hold promise for developing new drugs targeting dormancy phase MTB.