Mycobacterium tuberculosis Deficient in PdtaS Cytosolic Histidine Kinase Displays Attenuated Growth and Affords Protective Efficacy against Aerosol M. tuberculosis Infection in Mice

New control measures are urgently required to control tuberculosis (TB), as the current vaccine, Bacille Calmette-Guérin (BCG), has had a limited impact on disease spread. The identification of virulence mechanisms of Mycobacterium tuberculosis is an important strategy in vaccine design, as it permits the development of strains attenuated for growth that may have vaccine potential. In this report, we determined the role of the PdtaS response regulator in M. tuberculosis virulence and defined the vaccine potential of a pdtaS-deficient strain. Deletion of pdtaS (MtbΔpdtaS) resulted in reduced persistence of M. tuberculosis within mouse organs, which was equivalent to the persistence of the BCG vaccine in the lung and liver of infected mice. However, the generation of effector CD4+ and CD8+ T cells (CD44+CD62LloKLRG1+) was similar between wild-type M. tuberculosis and MtbΔpdtaS and greater than that elicited by BCG. Heightened immunity induced by MtbΔpdtaS compared to BCG was also observed by analysis of antigen-specific IFN-γ-secreting T cell responses induced by vaccination. MtbΔpdtaS displayed improved protection against aerosol M. tuberculosis compared to BCG, which was most apparent in the lung at 20 weeks post-infection. These results suggest that the deletion of the PdtaS response regulator warrants further appraisal as a tool to combat TB in humans.

Discovery of Anti-tubercular Analogues of Bedaquiline with Modified A-, B- and C-Ring Subunits

To date, the clinical use of the anti-tubercular therapy bedaquiline has been somewhat limited due to safety concerns. Recent investigations determined that modification of the B- and C-ring units of bedaquiline delivered new diarylquinolines (for example TBAJ-587) with potent anti-tubercular activity yet an improved safety profile due to reduced affinity for the hERG channel. Building on our recent discovery that substitution of the quinoline motif (the A-ring subunit) for C5-aryl pyridine groups within bedaquiline analogues led to retention of anti-tubercular activity, we investigated the concurrent modification of A-, B- and C-ring units within bedaquiline variants. This led to the discovery that 4-trifluoromethoxyphenyl and 4-chlorophenyl pyridyl analogues of TBAJ-587 retained relatively potent anti-tubercular activity and for the 4-chlorophenyl derivative in particular, a significant reduction in hERG inhibition relative to bedaquiline was achieved, demonstrating that modifications of the A-, B- and C-ring units within the bedaquiline structure is a viable strategy for the design of effective, yet safer (and less lipophilic) anti-tubercular compounds.

Carborane clusters increase the potency of bis-substituted cyclam derivatives against Mycobacterium tuberculosis

Bis-substituted cyclam derivatives have recently emerged as a promising new class of antibacterial agents, displaying excellent activity against drug-resistant Mycobacterium tuberculosis (Mtb) and in vivo efficacy in a zebrafish assay. Herein we report the synthesis and biological activity of new carborane derivatives within this class of antitubercular compounds. The resulting carborane-cyclam conjugates incorporating either hydrophobic closo-1,2-carborane or anionic, hydrophilic nido-7,8-carborane clusters display promising activity in an antibacterial assay employing the virulent Mtb strain H37Rv. The most active of these carborane derivatives exhibit MIC50 values of <1 μM, making them the most active compounds in this unique class of antibacterial cyclams reported to date.

Synthetic Sansanmycin Analogues as Potent Mycobacterium tuberculosis Translocase I Inhibitors

Herein, we report the design and synthesis of inhibitors of Mycobacterium tuberculosis (Mtb) phospho-MurNAc-pentapeptide translocase I (MurX), the first membrane-associated step of peptidoglycan synthesis, leveraging the privileged structure of the sansanmycin family of uridylpeptide natural products. A number of analogues bearing hydrophobic amide modifications to the pseudo-peptidic end of the natural product scaffold were generated that exhibited nanomolar inhibitory activity against Mtb MurX and potent activity against Mtb in vitro. We show that a lead analogue bearing an appended neopentylamide moiety possesses rapid antimycobacterial effects with a profile similar to the frontline tuberculosis drug isoniazid. This molecule was also capable of inhibiting Mtb growth in macrophages where mycobacteria reside in vivo and reduced mycobacterial burden in an in vivo zebrafish model of tuberculosis.

Synthesis and evaluation of pyridine-derived bedaquiline analogues containing modifications at the A-ring subunit

Despite promising efficacy, the clinical use of the anti-tubercular therapeutic bedaquiline has been restricted due to safety concerns. To date, limited SAR studies have focused on the quinoline ring (A-ring), and as such, we set out to explore modifications within this region in an attempt to discover new bedaquiline variants with an improved safety profile. We herein report the development of unique synthetic strategies that facilitated access to novel bedaquiline analogues leading to the discovery that anti-tubercular activity could be retained following replacement of the quinoline motif with pyridine heterocycles. This discovery is anticipated to open up multiple new avenues for exploration in the design of improved anti-tubercular therapeutics.

Advax adjuvant formulations promote protective immunity against aerosol Mycobacterium tuberculosis in the absence of deleterious inflammation and reactogenicity

The development of safe and effective adjuvants is a critical goal of vaccine development programs. In this report, we defined the immunostimulatory profile and protective effect against aerosol Mycobacterium tuberculosis infection of vaccine formulations incorporating the semi-crystalline adjuvant δ-inulin (Advax). Advax formulated with CpG oligonucleotide and the QS-21 saponin (Advax^CpQS) was the most effective combination, demonstrated by the capacity of CysVac2/Advax^CpQS to significantly reduce the bacterial burden in the lungs of M. tuberculosis-infected mice. CysVac2/Advax^CpQS protection was associated with rapid influx of neutrophils, macrophages and monocytes to the site of vaccination and the induction of antigen-specific IFN-γ⁺/IL-2⁺/TNF⁺ polyfunctional CD4⁺ T cells in the lung. When compared to the highly potent adjuvant combination of monophosphoryl lipid A and dimethyldioctadecylammonium bromide (MPL/DDA), Advax^CpQS imparted a similar level of protective efficacy yet without the profound stimulation of inflammatory cytokines and vaccination site ulceration observed with MPL/DDA. Addition of DDA to CysVac2/Advax^CpQS further improved the protective effect of the vaccine, which correlated with increased polyfunctional CD4⁺ T cells in the lung but with no increase in vaccine reactogenicity. The data demonstrate that Advax formulations can decouple protective tuberculosis immunity from reactogenicity, making them ideal candidates for human application.

Mucosal delivery of a multistage subunit vaccine promotes development of lung-resident memory T cells and affords interleukin-17-dependent protection against pulmonary tuberculosis

The development of effective vaccines against bacterial lung infections requires the induction of protective, pathogen-specific immune responses without deleterious inflammation within the pulmonary environment. Here, we made use of a polysaccharide-adjuvanted vaccine approach to elicit resident pulmonary T cells to protect against aerosol Mycobacterium tuberculosis infection. Intratracheal administration of the multistage fusion protein CysVac2 and the delta-inulin adjuvant Advax™ (formulated with a TLR9 agonist) provided superior protection against aerosol M. tuberculosis infection in mice, compared to parenteral delivery. Surprisingly, removal of the TLR9 agonist did not impact vaccine protection despite a reduction in cytokine-secreting T cell subsets, particularly CD4+IFN-γ+IL-2+TNF+ multifunctional T cells. CysVac2/Advax-mediated protection was associated with the induction of lung-resident, antigen-specific memory CD4+ T cells that expressed IL-17 and RORγT, the master transcriptional regulator of Th17 differentiation. IL-17 was identified as a key mediator of vaccine efficacy, with blocking of IL-17 during M. tuberculosis challenge reducing phagocyte influx, suppressing priming of pathogen-specific CD4+ T cells in local lymph nodes and ablating vaccine-induced protection. These findings suggest that tuberculosis vaccines such as CysVac2/Advax that are capable of eliciting Th17 lung-resident memory T cells are promising candidates for progression to human trials.

Total Synthesis and Antimycobacterial Activity of Ohmyungsamycin A, Deoxyecumicin, and Ecumicin

The ohmyungsamycin and ecumicin natural product families are structurally related cyclic depsipeptides that display potent antimycobacterial activity. Herein the total syntheses of ohmyungsamycin A, deoxyecumicin, and ecumicin are reported, together with the direct biological comparison of members of these natural product families against Mycobacterium tuberculosis (Mtb), the etiological agent of tuberculosis (TB). The synthesis of each of the natural products employed a solid-phase strategy to assemble the linear peptide precursor, involving a key on-resin esterification and an optional on-resin dimethylation step, before a final solution-phase macrolactamization between the non-proteinogenic N-methyl-4-methoxy-l-tryptophan amino acid and a bulky N-methyl-l-valine residue. The synthetic natural products possessed potent antimycobacterial activity against Mtb with MIC₉₀ 's ranging from 110-360 nm and retained activity against Mtb in Mtb-infected macrophages. Deoxyecumicin also exhibited rapid bactericidal killing against Mtb, sterilizing cultures after 21 days.

Structure-Activity Relationships of cyclo(l-Tyrosyl-l-tyrosine) Derivatives Binding to Mycobacterium tuberculosis CYP121: Iodinated Analogues Promote Shift to High-Spin Adduct

A series of analogues of cyclo(l-tyrosyl-l-tyrosine), the substrate of the Mycobacterium tuberculosis enzyme CYP121, have been synthesized and analyzed by UV-vis and electron paramagnetic resonance spectroscopy and by X-ray crystallography. The introduction of iodine substituents onto cyclo(l-tyrosyl-l-tyrosine) results in sub-μM binding affinity for the CYP121 enzyme and a complete shift to the high-spin state of the heme Fe^III. The introduction of halogens that are able to interact with heme groups is thus a feasible approach to the development of next-generation, tight binding inhibitors of the CYP121 enzyme, in the search for novel antitubercular compounds.

Bengamides display potent activity against drug-resistant Mycobacterium tuberculosis

Mycobacterium tuberculosis infects over 10 million people annually and kills more people each year than any other human pathogen. The current tuberculosis (TB) vaccine is only partially effective in preventing infection, while current TB treatment is problematic in terms of length, complexity and patient compliance. There is an urgent need for new drugs to combat the burden of TB disease and the natural environment has re-emerged as a rich source of bioactive molecules for development of lead compounds. In this study, one species of marine sponge from the Tedania genus was found to yield samples with exceptionally potent activity against M. tuberculosis. Bioassay-guided fractionation identified bengamide B as the active component, which displayed activity in the nanomolar range against both drug-sensitive and drug-resistant M. tuberculosis. The active compound inhibited in vitro activity of M. tuberculosis MetAP1c protein, suggesting the potent inhibitory action may be due to interference with methionine aminopeptidase activity. Tedania-derived bengamide B was non-toxic against human cell lines, synergised with rifampicin for in vitro inhibition of bacterial growth and reduced intracellular replication of M. tuberculosis. Thus, bengamides isolated from Tedania sp. show significant potential as a new class of compounds for the treatment of drug-resistant M. tuberculosis.

Organometallic Conjugates of the Drug Sulfadoxine for Combatting Antimicrobial Resistance

Fourteen novel arene Ru^II , and cyclopentadienyl (Cp^x ) Rh^III and Ir^III complexes containing an N,N'-chelated pyridylimino- or quinolylimino ligand functionalized with the antimalarial drug sulfadoxine have been synthesized and characterized, including three by X-ray crystallography. The rhodium and iridium complexes exhibited potent antiplasmodial activity with IC₅₀ values of 0.10-2.0 μm in either all, or one of the three Plasmodium falciparum assays (3D7 chloroquine sensitive, Dd2 chloroquine resistant and NF54 sexual late stage gametocytes) but were only moderately active towards Trichomonas vaginalis. They were active in both the asexual blood stage and the sexual late stage gametocyte assays, whereas the clinical parent drug, sulfadoxine, was inactive. Five complexes were moderately active against Mycobacterium tuberculosis (IC₅₀ <6.3 μm), while sulfadoxine showed no antitubercular activity. An increase in the size of both the Cp^x ligand and the aromatic imino substituent increased hydrophobicity, which resulted in an increase in antiplasmodial activity.

Antitubercular Bis-Substituted Cyclam Derivatives: Structure-Activity Relationships and in Vivo Studies

We recently reported the discovery of nontoxic cyclam-derived compounds that are active against drug-resistant Mycobacterium tuberculosis. In this paper we report exploration of the structure-activity relationship for this class of compounds, identifying several simpler compounds with comparable activity. The most promising compound identified, possessing significantly improved water solubility, displayed high levels of bacterial clearance in an in vivo zebrafish embryo model, suggesting this compound series has promise for in vivo treatment of tuberculosis.

Total Synthesis of Ecumicin

The first total synthesis of the potent anti-mycobacterial cyclic depsipeptide natural product ecumicin is described. Synthesis was achieved via a solid-phase strategy, incorporating the synthetic non-proteinogenic amino acids N-methyl-4-methoxy-l-tryptophan and threo-β-hydroxy-l-phenylalanine into the growing linear peptide chain. The synthesis employed key on-resin esterification and dimethylation steps as well as a final macrolactamization between the unusual N-methyl-4-methoxy-l-tryptophan unit and a bulky N-methyl-l-valine residue. The synthetic natural product possessed potent antimycobacterial activity against the virulent H37Rv strain of Mycobacterium tuberculosis (MIC₉₀ = 312 nM).

Broad activity of diphenyleneiodonium analogues against Mycobacterium tuberculosis, malaria parasites and bacterial pathogens

In this study, a structure-activity relationship (SAR) compound series based on the NDH-2 inhibitor diphenyleneiodonium (DPI) was synthesised. Compounds were evaluated primarily for in vitro efficacy against Gram-positive and Gram-negative bacteria, commonly responsible for nosocomial and community acquired infections. In addition, we also assessed the activity of these compounds against Mycobacterium tuberculosis (Tuberculosis) and Plasmodium spp. (Malaria). This led to the discovery of highly potent compounds active against bacterial pathogens and malaria parasites in the low nanomolar range, several of which were significantly less toxic to mammalian cells.

Delta inulin-based adjuvants promote the generation of polyfunctional CD4+ T cell responses and protection against Mycobacterium tuberculosis infection

There is an urgent need for the rational design of safe and effective vaccines to protect against chronic bacterial pathogens such as Mycobacterium tuberculosis. Advax™ is a novel adjuvant based on delta inulin microparticles that enhances immunity with a minimal inflammatory profile and has entered human trials to protect against viral pathogens. In this report we determined if Advax displays broad applicability against important human pathogens by assessing protective immunity against infection with M. tuberculosis. The fusion protein CysVac2, comprising the M. tuberculosis antigens Ag85B (Rv1886c) and CysD (Rv1285) formulated with Advax provided significant protection in the lungs of M. tuberculosis-infected mice. Protection was associated with the generation of CysVac2-specific multifunctional CD4+ T cells (IFN-γ+TNF+IL-2+). Addition to Advax of the TLR9 agonist, CpG oligonucleotide (AdvaxCpG), improved both the immunogenicity and protective efficacy of CysVac2. Immunisation with CysVac2/AdvaxCpG resulted in heightened release of the chemoattractants, CXCL1, CCL3, and TNF, and rapid influx of monocytes and neutrophils to the site of vaccination, with pronounced early priming of CysVac2-specific CD4+ T cells. As delta inulin adjuvants have shown an excellent safety and tolerability profile in humans, CysVac2/AdvaxCpG is a strong candidate for further preclinical evaluation for progression to human trials.

Modulation of Roquin Function in Myeloid Cells Reduces Mycobacterium tuberculosis-Induced Inflammation

Damaging inflammation is a hallmark of Mycobacterium tuberculosis infection, and understanding how this is regulated is important for the development of new therapies to limit excessive inflammation. The E3 ubiquitin ligase, Roquin, is involved in immune regulation; however, its role in immunity to M. tuberculosis is unknown. To address this, we infected mice with a point mutation in Roquin1/Rc3h1 (sanroque). Aerosol-infected sanroque mice showed enhanced control of M. tuberculosis infection associated with delayed bacterial dissemination and upregulated TNF production in the lungs after 2 wk. However, this early control of infection was not maintained, and by 8 wk postinfection sanroque mice demonstrated an increased bacterial burden and dysregulated inflammation in the lungs. As the inflammation in the lungs of the sanroque mice could have been influenced by emerging autoimmune conditions that are characteristic of the mice aging, the function of Roquin was examined in immune cell subsets in the absence of autoimmune complications. M. bovis bacillus Calmette-Guérin-primed sanroque T cells transferred into Rag1^-/- mice provided equivalent protection in the spleen and liver. Interestingly, the transfer of mycobacteria-specific (P25 CD4⁺ TCR transgenic) wild-type spleen cells into sanroqueRag1^-/- mice actually led to enhanced protection with reduced bacterial load, decreased chemokine expression, and reduced inflammation in the lungs compared with transfers into Rag1^-/- mice expressing intact Roquin. These studies suggest that modulation of Roquin in myeloid cells may reduce both inflammation and bacterial growth during the chronic phase of M. tuberculosis infection.

Synthesis of Norfijimycin A with Activity against Mycobacterium tuberculosis

The total synthesis of norfijimycin A, a simplified analogue of the marine natural product fijimycin A, is described. Fijimycin A is a cyclic depsipeptide that has been shown to possess activity against methicillin-resistant Staphylococcus aureus. The natural product contains a rare N,β-dimethyl leucine unit with unknown stereochemistry at the β-carbon. To evaluate the importance of the β-methyl group for antimicrobial activity, we introduced N-methyl leucine into the natural product scaffold. The resulting norfijimycin A was shown to possess significant activity against Mycobacterium tuberculosis, the etiological agent of tuberculosis.

Inhalation of Respirable Crystalline Rifapentine Particles Induces Pulmonary Inflammation

Rifapentine is an anti-tuberculosis (anti-TB) drug with a prolonged half-life, but oral delivery results in low concentrations in the lungs because of its high binding (98%) to plasma proteins. We have shown that inhalation of crystalline rifapentine overcomes the limitations of oral delivery by significantly enhancing and prolonging the drug concentration in the lungs. The delivery of crystalline particles to the lungs may promote inflammation. This in vivo study characterizes the inflammatory response caused by pulmonary deposition of the rifapentine particles. The rifapentine powder was delivered to BALB/c mice by intratracheal insufflation at a dose of 20 mg/kg. The inflammatory response in the lungs and bronchoalveolar lavage (BAL) was examined at 12 h, 24 h, and 7 days post-treatment by flow cytometry and histopathology. At 12 and 24 h post-treatment, there was a significant influx of neutrophils into the lungs, and this returned to normal by day 7. A significant recruitment of macrophages occurred in the BAL at 24 h. Consistent with these findings, histopathological analysis demonstrated pulmonary vascular congestion and significant macrophage recruitment at 12 and 24 h post-treatment. In conclusion, the pulmonary delivery of crystalline rifapentine caused a transient neutrophil-associated inflammatory response in the lungs that resolved over 7 days. This observation may limit pulmonary delivery of rifapentine to once a week at a dose of 20 mg/kg or less. The effectiveness of weekly dosing with inhalable rifapentine will be assessed in murine Mycobacterium tuberculosis infection.

Mycobacterium tuberculosis components expressed during chronic infection of the lung contribute to long-term control of pulmonary tuberculosis in mice

Tuberculosis (TB) remains a major cause of mortality and morbidity worldwide, yet current control strategies, including the existing BCG vaccine, have had little impact on disease control. The tubercle bacillus modifies protein expression to adapt to chronic infection of the host, and this can potentially be exploited to develop novel therapeutics. We identified the gene encoding the first step of the Mycobacterium tuberculosis sulphur assimilation pathway, cysD, as highly induced during chronic infection in the mouse lung, suggesting therapies based on CysD could be used to target infection. Vaccination with the composite vaccine CysVac2, a fusion of CysD and the immunogenic Ag85B of M. tuberculosis, resulted in the generation of multifunctional CD4+ T cells (interferon (IFN)-γ+TNF+IL-2+IL-17+) in the lung both pre- and post-aerosol challenge with M. tuberculosis. CysVac2 conferred significant protection against pulmonary M. tuberculosis challenge and was particularly effective at controlling late-stage infection, a property not shared by BCG. CysVac2 delivered as a booster following BCG vaccination afforded greater protection against M. tuberculosis challenge than BCG alone. The antigenic components of CysVac2 were conserved amongst M. tuberculosis strains, and protective efficacy afforded by CysVac2 was observed across varying murine MHC haplotypes. Strikingly, administration of CysVac2 to mice previously infected with M. tuberculosis reduced bacterial load and immunopathology in the lung compared with BCG-vaccinated mice. These results indicate that CysVac2 warrants further investigation to assess its potential to control pulmonary TB in humans.

Nontoxic Metal-Cyclam Complexes, a New Class of Compounds with Potency against Drug-Resistant Mycobacterium tuberculosis

Tuberculosis (TB) accounted for 1.5 million deaths in 2014, and new classes of anti-TB drugs are required. We report a class of functionalized 1,8-disubstituted cyclam derivatives that display low micromolar activity against pathogenic mycobacteria. These compounds inhibit intracellular growth of Mycobacterium tuberculosis, are nontoxic to human cell lines, and are active against multidrug-resistant M. tuberculosis strains, indicating a distinct mode of action. These compounds warrant further appraisal as novel agents to control TB in humans.

Microparticles from mycobacteria-infected macrophages promote inflammation and cellular migration

Mycobacterium tuberculosis infection is characterized by a strong inflammatory response whereby a few infected macrophages within the granuloma induce sustained cellular accumulation. The mechanisms coordinating this response are poorly characterized. We hypothesized that microparticles (MPs), which are submicron, plasma membrane-derived vesicles released by cells under both physiological and pathological conditions, are involved in this process. Aerosol infection of mice with M. tuberculosis increased CD45(+) MPs in the blood after 4 wk of infection, and in vitro infection of human and murine macrophages with mycobacteria enhanced MP release. MPs derived from mycobacteria-infected macrophages were proinflammatory, and when injected into uninfected mice they induced significant neutrophil, macrophage, and dendritic cell recruitment to the injection site. When incubated with naive macrophages, these MPs enhanced proinflammatory cytokine and chemokine release, and they aided in the disruption of the integrity of a respiratory epithelial cell monolayer, providing a mechanism for the egress of cells to the site of M. tuberculosis infection in the lung. In addition, MPs colocalized with the endocytic recycling marker Rab11a within macrophages, and this association increased when the MPs were isolated from mycobacteria-infected cells. M. tuberculosis-derived MPs also carried mycobacterial Ag and were able to activate M. tuberculosis-specific CD4(+) T cells in vivo and in vitro in a dendritic cell-dependent manner. Collectively, these data identify an unrecognized role for MPs in host response against M. tuberculosis by promoting inflammation, intercellular communication, and cell migration.

M. tuberculosis induces potent activation of IDO-1, but this is not essential for the immunological control of infection

Indoleamine 2,3-dioxygenesae-1 (IDO-1) catalyses the initial, rate-limiting step in tryptophan metabolism, thereby regulating tryptophan availability and the formation of downstream metabolites, including picolinic and quinolinic acid. We found that Mycobacterium tuberculosis infection induced marked upregulation of IDO-1 expression in both human and murine macrophages in vitro and in the lungs of mice following aerosol challenge with M. tuberculosis. The absence of IDO-1 in dendritic cells enhanced the activation of mycobacteria-specific T cells in vitro. Interestingly, IDO-1-deficiency during M. tuberculosis infection in mice was not associated with altered mycobacteria-specific T cell responses in vivo. The bacterial burden of infected organs, pulmonary inflammatory responses, and survival were also comparable in M. tuberculosis-infected IDO-1 deficient and wild type animals. Tryptophan is metabolised into either picolinic acid or quinolinic acid, but only picolinic acid inhibited the growth of M. tuberculosis in vitro. By contrast macrophages infected with pathogenic mycobacteria, produced quinolinic, rather than picolinic acid, which did not reduce M. tuberculosis growth in vitro. Therefore, although M. tuberculosis induces robust expression of IDO-1 and activation of tryptophan metabolism, IDO-1-deficiency fails to impact on the immune control and the outcome of the infection in the mouse model of tuberculosis.