Nitroarenes as Antitubercular Agents: Stereoelectronic Modulation to Mitigate Mutagenicity.

Nitroarenes are less preferred in drug discovery due to their potential to be mutagenic. However, several nitroarenes were shown to be promising antitubercular agents with specific modes of action, namely, nitroimidazoles and benzothiazinones. The nitro group in these compounds is activated through different mechanisms, both enzymatic and non-enzymatic, in mycobacteria prior to binding to the target of interest. From a whole-cell screening program, we identified a novel lead nitrobenzothiazole (BT) series that acts by inhibition of decaprenylphosphoryl-ß-d-ribose 2'-epimerase (DprE1) of Mycobacterium tuberculosis (Mtb). The lead was found to be mutagenic to start with. Our efforts to mitigate mutagenicity resulted in the identification of 6-methyl-7-nitro-5-(trifluoromethyl)-1,3-benzothiazoles (cBTs), a novel class of antitubercular agents that are non-mutagenic and exhibit an improved safety profile. The methyl group ortho to the nitro group decreases the electron affinity of the series, and is hence responsible for the non-mutagenic nature of these compounds. Additionally, the co-crystal structure of cBT in complex with Mtb DprE1 established the mode of binding. This investigation led to a new non-mutagenic antitubercular agent and demonstrates that the mutagenic nature of nitroarenes can be solved by modulation of stereoelectronic properties.

Discovery of benzothiazoles as antimycobacterial agents: Synthesis, structure-activity relationships and binding studies with Mycobacterium tuberculosis decaprenylphosphoryl-ß-D-ribose 2'-oxidase.

We report the discovery of benzothiazoles, a novel anti-mycobacterial series, identified from a whole cell based screening campaign. Benzothiazoles exert their bactericidal activity against Mycobacterium tuberculosis (Mtb) through potent inhibition of decaprenylphosphoryl-ß-d-ribose 2'-oxidase (DprE1), the key enzyme involved in arabinogalactan synthesis. Specific target linkage and mode of binding were established using co-crystallization and protein mass spectrometry studies. Most importantly, the current study provides insights on the utilization of systematic medicinal chemistry approaches to mitigate safety liabilities while improving potency during progression from an initial genotoxic hit, the benzothiazole N-oxides (BTOs) to the lead-like AMES negative, crowded benzothiazoles (cBTs). These findings offer opportunities for development of safe clinical candidates against tuberculosis. The design strategy adopted could find potential application in discovery of safe drugs in other therapy areas too.

Assays, surrogates, and alternative technologies for a TB lead identification program targeting DNA gyrase ATPase.

Mycobacterium tuberculosis (Mtb) DNA gyrase ATPase was the target of a tuberculosis drug discovery program. The low specific activity of the Mtb ATPase prompted the use of Mycobacterium smegmatis (Msm) enzyme as a surrogate for lead generation, since it had 20-fold higher activity. Addition of GyrA or DNA did not significantly increase the activity of the Msm GyrB ATPase, and an assay was developed using GyrB alone. Inhibition of the Msm ATPase correlated well with inhibition of Mtb DNA gyrase supercoiling across three chemical scaffolds, justifying its use. As the IC50 of compounds approached the enzyme concentration, surrogate assays were used to estimate potencies (e.g., the shift in thermal melt of Mtb GyrB, which correlated well with IC(50)s >10 nM). Analysis using the Morrison equation enabled determination of K(i)(app)s in the sub-nanomolar range. Surface plasmon resonance was used to confirm these IC(50)s and measure the K ds of binding, but a fragment of Mtb GyrB had to be used. Across three scaffolds, the dissociation half life, t1/2, of the inhibitor-target complex was ≤ 8 min. This toolkit of assays was developed to track the potency of enzyme inhibition and guide the chemistry for progression of compounds in a lead identification program.

2-Phenylindole and Arylsulphonamide: Novel Scaffolds Bactericidal against Mycobacterium tuberculosis.

A cellular activity-based screen on Mycobacterium tuberculosis (Mtb) H37Rv using a focused library from the AstraZeneca corporate collection led to the identification of 2-phenylindoles and arylsulphonamides, novel antimycobacterial scaffolds. Both the series were bactericidal in vitro and in an intracellular macrophage infection model, active against drug sensitive and drug resistant Mtb clinical isolates, and specific to mycobacteria. The scaffolds showed promising structure-activity relationships; compounds with submicromolar cellular potency were identified during the hit to lead exploration. Furthermore, compounds from both scaffolds were tested for inhibition of known target enzymes or pathways of antimycobacterial drugs including InhA, RNA polymerase, DprE1, topoisomerases, protein synthesis, and oxidative-phosphorylation. Compounds did not inhibit any of the targets suggesting the potential of a possible novel mode of action(s). Hence, both scaffolds provide the opportunity to be developed further as leads and tool compounds to uncover novel mechanisms for tuberculosis drug discovery.

UDP-N-acetylmuramic acid l-alanine ligase (MurC) inhibition in a tolC mutant Escherichia coli strain leads to cell death.

The Mur ligases play an essential role in the biosynthesis of bacterial peptidoglycan and hence are attractive antibacterial targets. A screen of the AstraZeneca compound library led to the identification of compound A, a pyrazolopyrimidine, as a potent inhibitor of Escherichia coli and Pseudomonas aeruginosa MurC. However, cellular activity against E. coli or P. aeruginosa was not observed. Compound A was active against efflux pump mutants of both strains. Experiments using an E. coli tolC mutant revealed accumulation of the MurC substrate and a decrease in the level of product upon treatment with compound A ,: indicating inhibition of MurC enzyme in these cells. Such a modulation was not observed in the E. coli wild-type cells. Further, overexpression of MurC in the E. coli tolC mutant led to an increase in the compound A MIC by ≥16-fold, establishing a correlation between MurC inhibition and cellular activity. In addition, estimation of the intracellular compound A level showed an accumulation of the compound over time in the tolC mutant strain. A significant compound A level was not detected in the wild-type E. coli strain even upon treatment with high concentrations of the compound. Therefore, the lack of MIC and absence of MurC inhibition in wild-type E. coli were possibly due to suboptimal compound concentration as a consequence of a high efflux level and/or poor permeativity of compound A.

Pyrazolopyrimidines establish MurC as a vulnerable target in Pseudomonas aeruginosa and Escherichia coli.

The bacterial peptidoglycan biosynthesis pathway provides multiple targets for antibacterials, as proven by the clinical success of ß-lactam and glycopeptide classes of antibiotics. The Mur ligases play an essential role in the biosynthesis of the peptidoglycan building block, N-acetyl-muramic acid-pentapeptide. MurC, the first of four Mur ligases, ligates l-alanine to UDP-N-acetylmuramic acid, initiating the synthesis of pentapeptide precursor. Therefore, inhibiting the MurC enzyme should result in bacterial cell death. Herein, we report a novel class of pyrazolopyrimidines with subnanomolar potency against both Escherichia coli and Pseudomonas aeruginosa MurC enzymes, which demonstrates a concomitant bactericidal activity against efflux-deficient strains. Radio-labeled precursor incorporation showed these compounds selectively inhibited peptidoglycan biosynthesis, and genetic studies confirmed the target of pyrazolopyrimidines to be MurC. In the presence of permeability enhancers such as colistin, pyrazolopyrimidines exhibited low micromolar MIC against the wild-type bacteria, thereby, indicating permeability and efflux as major challenges for this chemical series. Our studies provide biochemical and genetic evidence to support the essentiality of MurC and serve to validate the attractiveness of target for antibacterial discovery.

Simple and rapid method to determine antimycobacterial potency of compounds by using autoluminescent Mycobacterium tuberculosis.

A major obstacle in the process of discovery of drugs against Mycobacterium tuberculosis is its extremely slow growth rate and long generation time (∼20 to 24 h). Consequently, determination of MICs and minimum bactericidal concentrations (MBCs) of potential drug candidates using current methods requires 7 days (resazurin-based MIC assay [REMA]) and 1 month (CFU enumeration), respectively. We employed a synthetic luciferase operon optimized for expression in high-GC-content bacteria and adapted it for use in mycobacteria. Using luminescence-based readouts, we were able to determine the MICs and bactericidal activities of approved tuberculosis (TB) drugs, which correlated well with currently used methods. Although luminescence-based readouts have been used previously to determine the MICs and bactericidal activities of approved TB drugs, in this study we adapted this assay to carry out a pilot screen using a library of 1,114 compounds belonging to diverse chemical scaffolds. We found that MICs derived from a 3-day luminescence assay matched well with REMA-based MIC values. To determine the bactericidal potencies of compounds, a 1:10 dilution of the cultures from the MIC plate was carried out on day 7, and the bactericidal concentrations determined based on time to positivity in 2 weeks were found to be comparable with MBC values determined by the conventional CFU approach. Thus, the luminescent mycobacterium-based approach not only is very simple and inexpensive but also allowed us to generate the information in half the time required by conventional methods.

Novel N-linked aminopiperidine-based gyrase inhibitors with improved hERG and in vivo efficacy against Mycobacterium tuberculosis.

DNA gyrase is a clinically validated target for developing drugs against Mycobacterium tuberculosis (Mtb). Despite the promise of fluoroquinolones (FQs) as anti-tuberculosis drugs, the prevalence of pre-existing resistance to FQs is likely to restrict their clinical value. We describe a novel class of N-linked aminopiperidinyl alkyl quinolones and naphthyridones that kills Mtb by inhibiting the DNA gyrase activity. The mechanism of inhibition of DNA gyrase was distinct from the fluoroquinolones, as shown by their ability to inhibit the growth of fluoroquinolone-resistant Mtb. Biochemical studies demonstrated this class to exert its action via single-strand cleavage rather than double-strand cleavage, as seen with fluoroquinolones. The compounds are highly bactericidal against extracellular as well as intracellular Mtb. Lead optimization resulted in the identification of potent compounds with improved oral bioavailability and reduced cardiac ion channel liability. Compounds from this series are efficacious in various murine models of tuberculosis.

Discovery of pyrazolopyridones as a novel class of noncovalent DprE1 inhibitor with potent anti-mycobacterial activity.

A novel pyrazolopyridone class of inhibitors was identified from whole cell screening against Mycobacterium tuberculosis (Mtb). The series exhibits excellent bactericidality in vitro, resulting in a 4 log reduction in colony forming units following compound exposure. The significant modulation of minimum inhibitory concentration (MIC) against a Mtb strain overexpressing the Rv3790 gene suggested the target of pyrazolopyridones to be decaprenylphosphoryl-ß-D-ribose-2'-epimerase (DprE1). Genetic mapping of resistance mutation coupled with potent enzyme inhibition activity confirmed the molecular target. Detailed biochemical characterization revealed the series to be a noncovalent inhibitor of DprE1. Docking studies at the active site suggest that the series can be further diversified to improve the physicochemical properties without compromising the antimycobacterial activity. The pyrazolopyridone class of inhibitors offers an attractive non-nitro lead series targeting the essential and vulnerable DprE1 enzyme for the discovery of novel antimycobacterial agents to treat both drug susceptible and drug resistant strains of Mtb.

Lead optimization of 1,4-azaindoles as antimycobacterial agents.

In a previous report, we described the discovery of 1,4-azaindoles, a chemical series with excellent in vitro and in vivo antimycobacterial potency through noncovalent inhibition of decaprenylphosphoryl-ß-d-ribose-2'-epimerase (DprE1). Nevertheless, high mouse metabolic turnover and phosphodiesterase 6 (PDE6) off-target activity limited its advancement. Herein, we report lead optimization of this series, culminating in potent, metabolically stable compounds that have a robust pharmacokinetic profile without any PDE6 liability. Furthermore, we demonstrate efficacy for 1,4-azaindoles in a rat chronic TB infection model. We believe that compounds from the 1,4-azaindole series are suitable for in vivo combination and safety studies.

4-aminoquinolone piperidine amides: noncovalent inhibitors of DprE1 with long residence time and potent antimycobacterial activity.

4-Aminoquinolone piperidine amides (AQs) were identified as a novel scaffold starting from a whole cell screen, with potent cidality on Mycobacterium tuberculosis (Mtb). Evaluation of the minimum inhibitory concentrations, followed by whole genome sequencing of mutants raised against AQs, identified decaprenylphosphoryl-ß-d-ribose 2'-epimerase (DprE1) as the primary target responsible for the antitubercular activity. Mass spectrometry and enzyme kinetic studies indicated that AQs are noncovalent, reversible inhibitors of DprE1 with slow on rates and long residence times of ∼100 min on the enzyme. In general, AQs have excellent leadlike properties and good in vitro secondary pharmacology profile. Although the scaffold started off as a single active compound with moderate potency from the whole cell screen, structure-activity relationship optimization of the scaffold led to compounds with potent DprE1 inhibition (IC50 < 10 nM) along with potent cellular activity (MIC = 60 nM) against Mtb.

Thiazolopyridone ureas as DNA gyrase B inhibitors: optimization of antitubercular activity and efficacy.

Scaffold hopping from the thiazolopyridine ureas led to thiazolopyridone ureas with potent antitubercular activity acting through inhibition of DNA GyrB ATPase activity. Structural diversity was introduced, by extension of substituents from the thiazolopyridone N-4 position, to access hydrophobic interactions in the ribose pocket of the ATP binding region of GyrB. Further optimization of hydrogen bond interactions with arginines in site-2 of GyrB active site pocket led to potent inhibition of the enzyme (IC50 2 nM) along with potent cellular activity (MIC=0.1 µM) against Mycobacterium tuberculosis (Mtb). Efficacy was demonstrated in an acute mouse model of tuberculosis on oral administration.

Optimization of pyrrolamides as mycobacterial GyrB ATPase inhibitors: structure-activity relationship and in vivo efficacy in a mouse model of tuberculosis.

Moxifloxacin has shown excellent activity against drug-sensitive as well as drug-resistant tuberculosis (TB), thus confirming DNA gyrase as a clinically validated target for discovering novel anti-TB agents. We have identified novel inhibitors in the pyrrolamide class which kill Mycobacterium tuberculosis through inhibition of ATPase activity catalyzed by the GyrB domain of DNA gyrase. A homology model of the M. tuberculosis H37Rv GyrB domain was used for deciphering the structure-activity relationship and binding interactions of inhibitors with mycobacterial GyrB enzyme. Proposed binding interactions were later confirmed through cocrystal structure studies with the Mycobacterium smegmatis GyrB ATPase domain. The most potent compound in this series inhibited supercoiling activity of DNA gyrase with a 50% inhibitory concentration (IC50) of <5 nM, an MIC of 0.03 µg/ml against M. tuberculosis H37Rv, and an MIC90 of <0.25 µg/ml against 99 drug-resistant clinical isolates of M. tuberculosis. The frequency of isolating spontaneous resistant mutants was ∼10(-6) to 10(-8), and the point mutation mapped to the M. tuberculosis GyrB domain (Ser208 Ala), thus confirming its mode of action. The best compound tested for in vivo efficacy in the mouse model showed a 1.1-log reduction in lung CFU in the acute model and a 0.7-log reduction in the chronic model. This class of GyrB inhibitors could be developed as novel anti-TB agents.

Azaindoles: noncovalent DprE1 inhibitors from scaffold morphing efforts, kill Mycobacterium tuberculosis and are efficacious in vivo.

We report 1,4-azaindoles as a new inhibitor class that kills Mycobacterium tuberculosis in vitro and demonstrates efficacy in mouse tuberculosis models. The series emerged from scaffold morphing efforts and was demonstrated to noncovalently inhibit decaprenylphosphoryl-ß-D-ribose2'-epimerase (DprE1). With "drug-like" properties and no expectation of pre-existing resistance in the clinic, this chemical class has the potential to be developed as a therapy for drug-sensitive and drug-resistant tuberculosis.

Thiazolopyridine ureas as novel antitubercular agents acting through inhibition of DNA Gyrase B.

A pharmacophore-based search led to the identification of thiazolopyridine ureas as a novel scaffold with antitubercular activity acting through inhibition of DNA Gyrase B (GyrB) ATPase. Evaluation of the binding mode of thiazolopyridines in a Mycobacterium tuberculosis (Mtb) GyrB homology model prompted exploration of the side chains at the thiazolopyridine ring C-5 position to access the ribose/solvent pocket. Potent compounds with GyrB IC50 ≤ 1 nM and Mtb MIC ≤ 0.1 µM were obtained with certain combinations of side chains at the C-5 position and heterocycles at the C-6 position of the thiazolopyridine core. Substitutions at C-5 also enabled optimization of the physicochemical properties. Representative compounds were cocrystallized with Streptococcus pneumoniae (Spn) ParE; these confirmed the binding modes predicted by the homology model. The target link to GyrB was confirmed by genetic mapping of the mutations conferring resistance to thiazolopyridine ureas. The compounds are bactericidal in vitro and efficacious in vivo in an acute murine model of tuberculosis.

Aminopyrazinamides: novel and specific GyrB inhibitors that kill replicating and nonreplicating Mycobacterium tuberculosis.

Aminopyrazinamides originated from a high throughput screen targeting the Mycobacterium smegmatis (Msm) GyrB ATPase. This series displays chemical tractability, robust structure-activity relationship, and potent antitubercular activity. The crystal structure of Msm GyrB in complex with one of the aminopyrazinamides revealed promising attributes of specificity against other broad spectrum pathogens and selectivity against eukaryotic kinases due to novel interactions at hydrophobic pocket, unlike other known GyrB inhibitors. The aminopyrazinamides display excellent mycobacterial kill under in vitro, intracellular, and hypoxic conditions.

A high-throughput screening assay for simultaneous selection of inhibitors of Mycobacterium tuberculosis 1-deoxy-D-xylulose-5-phosphate synthase (Dxs) or 1-deoxy-D-xylulose 5-phosphate reductoisomerase (Dxr).

1-deoxy-D-xylulose 5-phosphate reductoisomerase (Dxr) is involved in the synthesis of isoprenoids by the methylerythritol phosphate pathway. Dxr is essential in Mycobacterium tuberculosis (Mtu), absent in humans and amenable to structure-aided design. To further assess the druggability of the enzyme, the energetics of binding of fosmidomycin to Mtu Dxr was studied by isothermal calorimetry. Binding was enhanced by nicotinamide adenine dinucleotide phosphate hydrogen (NADPH) and driven by enthalpy (ΔH -10.2 kcal/mol, ΔS 1.1 cal mol(-1)K(-1)). This suggests the possibility of finding novel inhibitors that bind enthalpically, making Dxr an attractive target. The cost of the Dxr substrate, 1-deoxy-D-xylulose-5-phosphate, for high-throughput screening (HTS) is prohibitive. Hence, an HTS assay that couples Dxr to the upstream enzyme 1-deoxy-D-xylulose-5-phosphate synthase (Dxs), also a valid target, was developed. A high concentration of NADPH was used to bias it to detect Dxr inhibitors that bind like fosmidomycin. The assay Z' was 0.75. It was equally sensitive to inhibitors of Dxs and Dxr, that is, fosmidomycin and fluropyruvate inhibited it with IC(50)s similar to that in the individual enzyme assays (79 vs 54 nM for fosmidomycin). To distinguish inhibitors of Dxs from Dxr, individual enzyme assays and a microplate thermofluor binding assay were developed. The assay simultaneously screens two targets and is cost-effective.