Synthesis of chromone, quinolone, and benzoxazinone sulfonamide nucleosides as conformationally constrained inhibitors of adenylating enzymes required for siderophore biosynthesis

Structural Study of a New MbtI-Inhibitor Complex: Towards an Optimized Model for Structure-Based Drug Discovery

MbtI from Mycobacterium tuberculosis (Mtb) is a Mg2+-dependent salicylate synthase, belonging to the chorismate-utilizing enzyme (CUE) family. As a fundamental player in iron acquisition, MbtI promotes the survival and pathogenicity of Mtb in the infected host. Hence, it has emerged in the last decade as an innovative, potential target for the anti-virulence therapy of tuberculosis. In this context, 5-phenylfuran-2-carboxylic acids have been identified as potent MbtI inhibitors. The first co-crystal structure of MbtI in complex with a member of this class was described in 2020, showing the enzyme adopting an open configuration. Due to the high mobility of the loop adjacent to the binding pocket, large portions of the amino acid chain were not defined in the electron density map, hindering computational efforts aimed at structure-driven ligand optimization. Herein, we report a new, high-resolution co-crystal structure of MbtI with a furan-based derivative, in which the closed configuration of the enzyme allowed tracing the entirety of the active site pocket in the presence of the bound inhibitor. Moreover, we describe a new crystal structure of MbtI in open conformation and in complex with the known inhibitor methyl-AMT, suggesting that in vitro potency is not related to the observed enzyme conformation. These findings will prove fundamental to enhance the potency of this series via rational structure-based drug-design approaches.

Targeting Mycobacterium tuberculosis iron-scavenging tools: a recent update on siderophores inhibitors

Among the various bacterial infections, tuberculosis (TB) remains a life-threatening infectious disease responsible as the most significant cause of mortality and morbidity worldwide. The co-infection of human immunodeficiency virus (HIV) in association with TB burdens the healthcare system substantially. Notably, M.tb possesses defence against most antitubercular antibiotic drugs, and the efficacy of existing frontline anti-TB drugs is waning. Also, new and recurring cases of TB from resistant bacteria such as multidrug-resistant TB (MDR), extensively drug-resistant TB (XDR), and totally drug-resistant TB (TDR) strains are increasing. Hence, TB begs the scientific community to explore the new therapeutic class of compounds with their novel mechanism. M.tb requires iron from host cells to sustain, grow, and carry out several biological processes. M.tb has developed strategic methods of acquiring iron from the surrounding environment. In this communication, we discuss an overview of M.tb iron-scavenging tools. Also, we have summarized recently identified MbtA and MbtI inhibitors, which prevent M.tb from scavenging iron. These iron-scavenging tool inhibitors have the potential to be developed as anti-TB agents/drugs.

Iron Acquisition and Metabolism as a Promising Target for Antimicrobials (Bottlenecks and Opportunities): Where Do We Stand?

The emergence of multidrug-resistant (MDR) and extensively drug-resistant (XDR) infections is one of the most crucial challenges currently faced by the scientific community. Developments in the fundamental understanding of their underlying mechanisms may open new perspectives in drug discovery. In this review, we conducted a systematic literature search in PubMed, Web of Science, and Scopus, to collect information on innovative strategies to hinder iron acquisition in bacteria. In detail, we discussed the most interesting targets from iron uptake and metabolism pathways, and examined the main chemical entities that exhibit anti-infective activities by interfering with their function. The mechanism of action of each drug candidate was also reviewed, together with its pharmacodynamic, pharmacokinetic, and toxicological properties. The comprehensive knowledge of such an impactful area of research will hopefully reflect in the discovery of newer antibiotics able to effectively tackle the antimicrobial resistance issue.

Screening approaches and therapeutic targets: The two driving wheels of tuberculosis drug discovery

Tuberculosis (TB) is an infectious disease, infecting a quarter of world's population. Drug resistant TB further exacerbates the grim scenario of the drying TB drug discovery pipeline. The limited arsenal to fight TB presses the need for thorough efforts for identifying promising hits to combat the disease. The review highlights the efforts in the field of tuberculosis drug discovery, with an emphasis on massive drug screening campaigns for identifying novel hits against Mtb in both industry and academia. As an intracellular pathogen, mycobacteria reside in a complicated intracellular environment with multiple factors at play. Here, we outline various strategies employed in an effort to mimic the intracellular milieu for bringing the screening models closer to the actual settings. The review also focuses on the novel targets and pathways that could aid in target-based drug discovery in TB. The recent high throughput screening efforts resulting in the identification of potent hits against Mtb has been summarized in this article. There is a pressing need for effective screening strategies and approaches employing innovative tools and recent technologies; including nanotechnology, gene-editing tools such as CRISPR-cas system, host-directed bacterial killing and high content screening to augment the TB drug discovery pipeline with safer and shorter drug regimens.

Rational inhibitor design for Pseudomonas aeruginosa salicylate adenylation enzyme PchD

Pseudomonas aeruginosa is an increasingly antibiotic-resistant pathogen that causes severe lung infections, burn wound infections, and diabetic foot infections. P. aeruginosa produces the siderophore pyochelin through the use of a non-ribosomal peptide synthetase (NRPS) biosynthetic pathway. Targeting members of siderophore NRPS proteins is one avenue currently under investigation for the development of new antibiotics against antibiotic-resistant organisms. Here, the crystal structure of the pyochelin adenylation domain PchD is reported. The structure was solved to 2.11 Å when co-crystallized with the adenylation inhibitor 5'-O-(N-salicylsulfamoyl)adenosine (salicyl-AMS) and to 1.69 Å with a modified version of salicyl-AMS designed to target an active site cysteine (4-cyano-salicyl-AMS). In the structures, PchD adopts the adenylation conformation, similar to that reported for AB3403 from Acinetobacter baumannii.

The Mycobactin Biosynthesis Pathway: A Prospective Therapeutic Target in the Battle against Tuberculosis

The alarming rise in drug-resistant clinical cases of tuberculosis (TB) has necessitated the rapid development of newer chemotherapeutic agents with novel mechanisms of action. The mycobactin biosynthesis pathway, conserved only among the mycolata family of actinobacteria, a group of intracellularly surviving bacterial pathogens that includes Mycobacterium tuberculosis, generates a salicyl-capped peptide mycobactin under iron-stress conditions in host macrophages to support the iron demands of the pathogen. This in vivo essentiality makes this less explored mycobactin biosynthesis pathway a promising endogenous target for novel lead-compounds discovery. In this Perspective, we have provided an up-to-date account of drug discovery efforts targeting selected enzymes (MbtI, MbtA, MbtM, and PPTase) from the mbt gene cluster (mbtA-mbtN). Furthermore, a succinct discussion on non-specific mycobactin biosynthesis inhibitors and the Trojan horse approach adopted to impair iron metabolism in mycobacteria has also been included in this Perspective.

Synthesis of an acyl-acyl carrier protein synthetase inhibitor to study fatty acid recycling

Fatty acids are essential to most organisms and are made endogenously by the fatty acid synthase (FAS). FAS is an attractive target for antibiotics and many inhibitors are in clinical development. However, some gram-negative bacteria harbor an enzyme known as the acyl-acyl carrier protein synthetase (AasS), which allows them to scavenge fatty acids from the environment and shuttle them into FAS and ultimately lipids. The ability of AasS to recycle fatty acids may help pathogenic gram-negative bacteria circumvent FAS inhibition. We therefore set out to design and synthesize an inhibitor of AasS and test its effectiveness on an AasS enzyme from Vibrio harveyi, the most well studied AasS to date, and from Vibrio cholerae, a pathogenic model. The inhibitor C10-AMS [5′-O-(N-decanylsulfamoyl)adenosine], which mimics the tightly bound acyl-AMP reaction intermediate, was able to effectively inhibit AasS catalytic activity in vitro. Additionally, C10-AMS stopped the ability of Vibrio cholerae to recycle fatty acids from media and survive when its endogenous FAS was inhibited with cerulenin. C10-AMS can be used to study fatty acid recycling in other bacteria as more AasS enzymes continue to be annotated and provides a platform for potential antibiotic development.

Gram-scale preparation of the antibiotic lead compound salicyl-AMS, a potent inhibitor of bacterial salicylate adenylation enzymes

Salicyl-AMS (1) is a potent inhibitor of salicylate adenylation enzymes used in bacterial siderophore biosynthesis and a promising lead compound for the treatment of tuberculosis. An optimized, multigram synthesis is presented, which provides salicyl-AMS as its sodium salt (1·Na) in three synthetic steps followed by a two-step salt formation process. The synthesis proceeds in 11.6% overall yield from commercially available adenosine 2',3'-acetonide and provides highly purified material.

Synthesis and Reactivity of 6,8-Dibromo-2-ethyl-4H-benzo[d][1,3]oxazin-4-one Towards Nucleophiles and Electrophiles and Their Anticancer Activity

BACKGROUND

The genetic heterogeneity of tumor cells and the development of therapy-resistant cancer cells in addition to the high cost necessitate the continuous development of novel targeted therapies.

METHODS

In this regard, 14 novel benzoxazinone derivatives were synthesized and examined for anticancer activity against two human epithelial cancer cell lines; breast MCF-7 and liver HepG2 cells. 6,8-Dibromo-2- ethyl-4H-benzo[d][1,3]oxazin-4-one was subjected to react with nitrogen nucleophiles to afford quinazolinone derivatives and other related moieties (3-12). Benzoxazinone 2 responds to attack with oxygen nucleophile such as ethanol to give ethyl benzoate derivative 13. The reaction of benzoxazinone 2 with carbon electrophile such as benzaldehyde derivatives afforded benzoxazinone derivatives 14a and 14b.The structure of the prepared compounds was confirmed with spectroscopic tools including IR, 1H-NMR, and 13C-NMR.

RESULTS

Derivatives 3, 9, 12, 13, and 14b exhibited high antiproliferative activity and were selective against cancer cells showing no toxicity in normal fibroblasts. Derivative 3 with NH-CO group in quinazolinone ring was effective only against breast cells, while derivative 12 with NH-CO group in imidazole moiety was only effective against liver cells probably through arresting cell cycle and enabling repair mechanisms. The other derivatives (9, 13, and 14b) had broader antiproliferative activity against both cell lines. These derivatives enhance the expression of the p53 and caspases 9 and 3 to varying degrees in both cell lines. Derivative 14b caused the highest induction in the investigated genes and was the only derivative to inhibit the EGFR activity.

CONCLUSIONS

The unique features about derivative 14b could be attributed to its high lipophilicity, high carbon content, or its extended conjugation through planar aromatic system. More investigations are required to identify the lead compound(s) in animal models.

Dissecting modular synthases through inhibition: A complementary chemical and genetic approach

Modular synthases, such as fatty acid, polyketide, and non-ribosomal peptide synthases (NRPSs), are sophisticated machineries essential in both primary and secondary metabolism. Various techniques have been developed to understand their genetic background and enzymatic abilities. However, uncovering the actual biosynthetic pathways remains challenging. Herein, we demonstrate a pipeline to study an assembly line synthase by interrogating the enzymatic function of each individual enzymatic domain of BpsA, a NRPS that produces the blue 3,3'-bipyridyl pigment indigoidine. Specific inhibitors for each biosynthetic domain of BpsA were obtained or synthesized, and the enzymatic performance of BpsA upon addition of each inhibitor was monitored by pigment development in vitro and in living bacteria. The results were verified using genetic mutants to inactivate each domain. Finally, the results complemented the currently proposed biosynthetic pathway of BpsA.

Targeting adenylate-forming enzymes with designed sulfonyladenosine inhibitors

Adenylate-forming enzymes are a mechanistic superfamily that are involved in diverse biochemical pathways. They catalyze ATP-dependent activation of carboxylic acid substrates as reactive acyl adenylate (acyl-AMP) intermediates and subsequent coupling to various nucleophiles to generate ester, thioester, and amide products. Inspired by natural products, acyl sulfonyladenosines (acyl-AMS) that mimic the tightly bound acyl-AMP reaction intermediates have been developed as potent inhibitors of adenylate-forming enzymes. This simple yet powerful inhibitor design platform has provided a wide range of biological probes as well as several therapeutic lead compounds. Herein, we provide an overview of the nine structural classes of adenylate-forming enzymes and examples of acyl-AMS inhibitors that have been developed for each.

Structure-Based Design, Synthesis, and Biological Evaluation of Non-Acyl Sulfamate Inhibitors of the Adenylate-Forming Enzyme MenE

N-Acyl sulfamoyladenosines (acyl-AMS) have been used extensively to inhibit adenylate-forming enzymes that are involved in a wide range of biological processes. These acyl-AMS inhibitors are nonhydrolyzable mimics of the cognate acyl adenylate intermediates that are bound tightly by adenylate-forming enzymes. However, the anionic acyl sulfamate moiety presents a pharmacological liability that may be detrimental to cell permeability and pharmacokinetic profiles. We have previously developed the acyl sulfamate OSB-AMS (1) as a potent inhibitor of the adenylate-forming enzyme MenE, an o-succinylbenzoate-CoA (OSB-CoA) synthetase that is required for bacterial menaquinone biosynthesis. Herein, we report the use of computational docking to develop novel, non-acyl sulfamate inhibitors of MenE. A m-phenyl ether-linked analogue (5) was found to be the most potent inhibitor (IC₅₀ = 8 μM; K_d = 244 nM), and its X-ray co-crystal structure was determined to characterize its binding mode in comparison to the computational prediction. This work provides a framework for the development of potent non-acyl sulfamate inhibitors of other adenylate-forming enzymes in the future.

Kinetic Analyses of the Siderophore Biosynthesis Inhibitor Salicyl-AMS and Analogues as MbtA Inhibitors and Antimycobacterial Agents

There is a paramount need for expanding the drug armamentarium to counter the growing problem of drug-resistant tuberculosis. Salicyl-AMS, an inhibitor of salicylic acid adenylation enzymes, is a first-in-class antibacterial lead compound for the development of tuberculosis drugs targeting the biosynthesis of salicylic-acid-derived siderophores. In this study, we determined the K_i of salicyl-AMS for inhibition of the salicylic acid adenylation enzyme MbtA from Mycobacterium tuberculosis (MbtA_tb), designed and synthesized two new salicyl-AMS analogues to probe structure-activity relationships (SAR), and characterized these two analogues alongside salicyl-AMS and six previously reported analogues in biochemical and cell-based studies. The biochemical studies included determination of kinetic parameters ( K_i^app, k_on^app, k_off, and t_R) and analysis of the mechanism of inhibition. For these studies, we optimized production and purification of recombinant MbtA_tb, for which K_m and k_cat values were determined, and used the enzyme in conjunction with an MbtA_tb-optimized, continuous, spectrophotometric assay for MbtA activity and inhibition. The cell-based studies provided an assessment of the antimycobacterial activity and postantibiotic effect of the nine MbtA_tb inhibitors. The antimycobacterial properties were evaluated using a strain of nonpathogenic, fast-growing Mycobacterium smegmatis that was genetically engineered for MbtA_tb-dependent susceptibility to MbtA inhibitors. This convenient model system greatly facilitated the cell-based studies by bypassing the methodological complexities associated with the use of pathogenic, slow-growing M. tuberculosis. Collectively, these studies provide new information on the mechanism of inhibition of MbtA_tb by salicyl-AMS and eight analogues, afford new SAR insights for these inhibitors, and highlight several suitable candidates for future preclinical evaluation.

Mycobacterial siderophore: A review on chemistry and biology of siderophore and its potential as a target for tuberculosis

Mycobacterium tuberculosis (MTB), the causative agent of tuberculosis is known to secrete low molecular mass compounds called siderophores especially under low iron conditions to chelate iron from host environment. Iron is essential for growth and other essential processes to sustain life of the bacterium in the host. Hence targeting siderophore is considered to be an alternative approach to prevent further virulence of bacterium into the host. This review article presents classification of siderophores, their role in transporting iron into the tubercular cell, biosynthesis of mycobactins, viability of siderophore as a therapeutic target and also focuses on overview on various approaches to target siderophore. The approaches encompass mutation effect on genes involved in siderophore recycling, synthetic as well as natural compounds that can inhibit further spread of bacterium by targeting siderophore.

Conformationally Constrained Cinnolinone Nucleoside Analogues as Siderophore Biosynthesis Inhibitors for Tuberculosis

5'-O-[N-(Salicyl)sulfamoyl]adenosine (Sal-AMS, 1) is a nucleoside antibiotic that inhibits incorporation of salicylate into siderophores required for bacterial iron acquisition and has potent activity against Mycobacterium tuberculosis (Mtb). Cinnolone analogues exemplified by 5 were designed to replace the acidic acyl-sulfamate functional group of 1 (pK_a = 3) by a more stable sulfonamide linkage (pK_a = 6.0) in an attempt to address potential metabolic liabilities and improve membrane permeability. We showed 5 potently inhibited the mycobacterial salicylate ligase MbtA (apparent K_i = 12 nM), blocked production of the salicylate-capped siderophores in whole-cell Mtb, and exhibited excellent antimycobacterial activity under iron-deficient conditions (minimum inhibitor concentration, MIC = 2.3 μM). To provide additional confirmation of the mechanism of action, we demonstrated the whole-cell activity of 5 could be fully antagonized by the addition of exogenous salicylate to the growth medium. Although the total polar surface area (tPSA) of 5 still exceeds the nominal threshold value (140 Å) typically required for oral bioavailability, we were pleasantly surprised to observe introduction of the less acidic and conformationally constrained cinnolone moiety conferred improved drug disposition properties as evidenced by the 7-fold increase in volume of distribution in Sprague-Dawley rats.

Recent Advances in Radical C-H Activation/Radical Cross-Coupling

Research and industrial interest in radical C-H activation/radical cross-coupling chemistry has continuously grown over the past few decades. These reactions offer fascinating and unconventional approaches toward connecting molecular fragments with high atom- and step-economy that are often complementary to traditional methods. Success in this area of research was made possible through the development of photocatalysis and first-row transition metal catalysis along with the use of peroxides as radical initiators. This Review provides a brief and concise overview of the current status and latest methodologies using radicals or radical cations as key intermediates produced via radical C-H activation. This Review includes radical addition, radical cascade cyclization, radical/radical cross-coupling, coupling of radicals with M-R groups, and coupling of radical cations with nucleophiles (Nu).

Synthesis and pharmacological evaluation of nucleoside prodrugs designed to target siderophore biosynthesis in Mycobacterium tuberculosis

The nucleoside antibiotic, 5'-O-[N-(salicyl)sulfamoyl]adenosine (1), possesses potent whole-cell activity against Mycobacterium tuberculosis (Mtb), the etiological agent of tuberculosis (TB). This compound is also active in vivo, but suffers from poor drug disposition properties that result in poor bioavailability and rapid clearance. The synthesis and evaluation of a systematic series of lipophilic ester prodrugs containing linear and α-branched alkanoyl groups from two to twelve carbons at the 3'-position of a 2'-fluorinated analog of 1 is reported with the goal to improve oral bioavailability. The prodrugs were stable in simulated gastric fluid (pH 1.2) and under physiological conditions (pH 7.4). The prodrugs were also remarkably stable in mouse, rat, and human serum (relative serum stability: human∼rat≫mouse) displaying a parabolic trend in the SAR with hydrolysis rates increasing with chain length up to eight carbons (t1/2=1.6 h for octanoyl prodrug 7 in mouse serum) and then decreasing again with higher chain lengths. The permeability of the prodrugs was also assessed in a Caco-2 cell transwell model. All of the prodrugs were found to have reduced permeation in the apical-to-basolateral direction and enhanced permeation in the basolateral-to-apical direction relative to the parent compound 2, resulting in efflux ratios 5-28 times greater than 2. Additionally, Caco-2 cells were found to hydrolyze the prodrugs with SAR mirroring the serum stability results and a preference for hydrolysis on the apical side. Taken together, these results suggest that the described prodrug strategy will lead to lower than expected oral bioavailability of 2 and highlight the contribution of intestinal esterases for prodrug hydrolysis.

Nucleoside analogs and tuberculosis: new weapons against an old enemy

Purine and pyrimidine nucleoside and nucleotide analogs have been extensively studied as anticancer and antiviral agents. In addition to this, they have recently shown great potential against Mycobacterium Tuberculosis, the causative agent of TB. TB ranks as the tenth most common cause of death in the world. The current treatment for TB infection is limited by side effects and cost of the drugs and most importantly by the development of resistance to the therapy. Therefore the development of novel drugs, capable of overcoming the drawbacks of the existing treatments, has become the focus of many research programs. In parallel to that, a tremendous effort has been made to elucidate the unique metabolism of this pathogen with the aim to identify new possible targets. This review presents the state of the art in nucleoside and nucleotide analogs in the treatment of TB. In particular, we report on the inhibitory activity of this class of compounds, both in enzymatic and whole-cell assays, providing a brief insight to which reported target these novel compounds are hitting.

Network Analysis Reveals Sex- and Antibiotic Resistance-Associated Antivirulence Targets in Clinical Uropathogens

Increasing antibiotic resistance among uropathogenic Escherichia coli (UPEC) is driving interest in therapeutic targeting of nonconserved virulence factor (VF) genes. The ability to formulate efficacious combinations of antivirulence agents requires an improved understanding of how UPEC deploy these genes. To identify clinically relevant VF combinations, we applied contemporary network analysis and biclustering algorithms to VF profiles from a large, previously characterized inpatient clinical cohort. These mathematical approaches identified four stereotypical VF combinations with distinctive relationships to antibiotic resistance and patient sex that are independent of traditional phylogenetic grouping. Targeting resistance- or sex-associated VFs based upon these contemporary mathematical approaches may facilitate individualized anti-infective therapies and identify synergistic VF combinations in bacterial pathogens.

Investigation and conformational analysis of fluorinated nucleoside antibiotics targeting siderophore biosynthesis

Antibiotic resistance represents one of the greatest threats to public health. The adenylation inhibitor 5'-O-[N-(salicyl)sulfamoyl]adenosine (SAL-AMS) is the archetype for a new class of nucleoside antibiotics that target iron acquisition in pathogenic microorganisms and is especially effective against Mycobacterium tuberculosis, the causative agent of tuberculosis. Strategic incorporation of fluorine at the 2' and 3' positions of the nucleoside was performed by direct fluorination to enhance activity and improve drug disposition properties. The resulting SAL-AMS analogues were comprehensively assessed for biochemical potency, whole-cell antitubercular activity, and in vivo pharmacokinetic parameters. Conformational analysis suggested a strong preference of fluorinated sugar rings for either a 2'-endo, 3'-exo (South), or a 3'-endo,2'-exo (North) conformation. The structure-activity relationships revealed a strong conformational bias for the C3'-endo conformation to maintain potent biochemical and whole-cell activity, whereas improved pharmacokinetic properties were associated with the C2'-endo conformation.

Acyl peptidic siderophores: structures, biosyntheses and post-assembly modifications

Acyl peptidic siderophores are produced by a variety of bacteria and possess unique amphiphilic properties. Amphiphilic siderophores are generally produced in a suite where the iron(III)-binding headgroup remains constant while the fatty acid appendage varies by length and functionality. Acyl peptidic siderophores are commonly synthesized by non-ribosomal peptide synthetases; however, the method of peptide acylation during biosynthesis can vary between siderophores. Following biosynthesis, acyl siderophores can be further modified enzymatically to produce a more hydrophilic compound, which retains its ferric chelating abilities as demonstrated by pyoverdine from Pseudomonas aeruginosa and the marinobactins from certain Marinobacter species. Siderophore hydrophobicity can also be altered through photolysis of the ferric complex of certain β-hydroxyaspartic acid-containing acyl peptidic siderophores.

The tuberculosis drug discovery and development pipeline and emerging drug targets

The recent accelerated approval for use in extensively drug-resistant and multidrug-resistant-tuberculosis (MDR-TB) of two first-in-class TB drugs, bedaquiline and delamanid, has reinvigorated the TB drug discovery and development field. However, although several promising clinical development programs are ongoing to evaluate new TB drugs and regimens, the number of novel series represented is few. The global early-development pipeline is also woefully thin. To have a chance of achieving the goal of better, shorter, safer TB drug regimens with utility against drug-sensitive and drug-resistant disease, a robust and diverse global TB drug discovery pipeline is key, including innovative approaches that make use of recently acquired knowledge on the biology of TB. Fortunately, drug discovery for TB has resurged in recent years, generating compounds with varying potential for progression into developable leads. In parallel, advances have been made in understanding TB pathogenesis. It is now possible to apply the lessons learned from recent TB hit generation efforts and newly validated TB drug targets to generate the next wave of TB drug leads. Use of currently underexploited sources of chemical matter and lead-optimization strategies may also improve the efficiency of future TB drug discovery. Novel TB drug regimens with shorter treatment durations must target all subpopulations of Mycobacterium tuberculosis existing in an infection, including those responsible for the protracted TB treatment duration. This review summarizes the current TB drug development pipeline and proposes strategies for generating improved hits and leads in the discovery phase that could help achieve this goal.

Metal-free catalytic cascade to chromones: direct coupling of salicylaldehydes and activated alkynes triggered by aryloxyl radicals

The first intermolecular addition reaction of aryloxyl radicals to CC bonds was disclosed, which can afford biologically important chromonsviaa PhNMe₃I-catalyzed direct coupling of salicylaldehydes and activated internal alkynes in only one step.

Tuberculosis drug discovery and emerging targets

Current tuberculosis (TB) therapies take too long and the regimens are complex and subject to adverse effects and drug-drug interactions with concomitant medications. The emergence of drug-resistant TB strains exacerbates the situation. Drug discovery for TB has resurged in recent years, generating compounds (hits) with varying potential for progression into developable leads. In parallel, advances have been made in understanding TB pathogenesis. It is now possible to apply the lessons learned from recent TB hit generation efforts and newly validated TB drug targets to generate the next wave of TB drug leads. Use of currently underexploited sources of chemical matter and lead-optimization strategies may also improve the efficiency of future TB drug discovery. Novel TB drug regimens with shorter treatment durations must target all subpopulations of Mycobacterium tuberculosis existing in an infection, including those responsible for the protracted TB treatment duration. This review proposes strategies for generating improved hits and leads that could help achieve this goal.