Approaches for Targeting the Mycobactin Biosynthesis Pathway for Novel Anti-tubercular Drug Discovery: Where We Stand

INTRODUCTION

Several decades of antitubercular drug discovery efforts have focused on novel antitubercular chemotherapies. However, recent efforts have greatly shifted towards countering extremely/multi/total drug-resistant species. Targeting the conditionally essential elements inside Mycobacterium is a relatively new approach against tuberculosis and has received lackluster attention. The siderophore, Mycobactin, is a conditionally essential molecule expressed by mycobacteria in iron-stress conditions. It helps capture the micronutrient iron, essential for the smooth functioning of cellular processes.

AREAS COVERED

The authors discuss opportunities to target the conditionally essential pathways to help develop newer drugs and prolong the shelf life of existing therapeutics, emphasizing the bottlenecks in fast-tracking antitubercular drug discovery.

EXPERT OPINION

While the lack of iron supply can cripple bacterial growth and multiplication, excess iron can cause oxidative overload. Constant up-regulation can strain the bacterial synthetic machinery, further slowing its growth. Mycobactin synthesis is tightly controlled by a genetically conserved mega enzyme family via up-regulation (HupB) or down-regulation (IdeR) based on iron availability in its microenvironment. Furthermore, the recycling of siderophores by the MmpL-MmpS4/5 orchestra provides endogenous drug targets to beat the bugs with iron-toxicity contrivance. These processes can be exploited as chinks in the armor of Mycobacterium and be used for new drug development.

Mycobactin Analogues with Excellent Pharmacokinetic Profile Demonstrate Potent Antitubercular Specific Activity and Exceptional Efflux Pump Inhibition

In this study, we have designed and synthesized pyrazoline analogues that partially mimic the structure of mycobactin, to address the requirement of novel therapeutics to tackle the emerging global challenge of antimicrobial resistance (AMR). Our investigation resulted in the identification of novel lead compounds 44 and 49 as potential mycobactin biosynthesis inhibitors against mycobacteria. Moreover, candidates efficiently eradicated intracellularly surviving mycobacteria. Thermofluorimetric analysis and molecular dynamics simulations suggested that compounds 44 and 49 bind to salicyl-AMP ligase (MbtA), a key enzyme in the mycobactin biosynthetic pathway. To the best of our knowledge, these are the first rationally designed mycobactin inhibitors to demonstrate an excellent in vivo pharmacokinetic profile. In addition, these compounds also exhibited more potent whole-cell efflux pump inhibition than known efflux pump inhibitors verapamil and chlorpromazine. Results from this study pave the way for the development of 3-(2-hydroxyphenyl)-5-(aryl)-pyrazolines as a new weapon against superbug-associated AMR challenges.

Efflux pumps in Mycobacterium tuberculosis and their inhibition to tackle antimicrobial resistance

Tuberculosis (TB), an infectious disease caused by the bacterium Mycobacterium tuberculosis, was the leading cause of mortality worldwide in 2019 due to a single infectious agent. The growing threat of strains of M. tuberculosis untreatable by modern antibiotic regimens only exacerbates this problem. In response to this continued public health emergency, research into methods of potentiating currently approved antimicrobial agents against resistant strains of M. tuberculosis is an urgent priority, and a key strategy in this regard is the design of mycobacterial efflux pump inhibitors (EPIs). This review summarises the current state of knowledge surrounding drug-related efflux pumps in M. tuberculosis and presents recent updates within the field of mycobacterial EPIs with a view to aiding the design of an effective adjunct therapy to overcome efflux-mediated resistance in TB.

Indole/isatin-containing hybrids as potential antibacterial agents

The emergence and worldwide spread of drug-resistant bacteria have already posed a serious threat to human life, creating the urgent need to develop potent and novel antibacterial drug candidates with high efficacy. Indole and isatin (indole-2,3-dione) present a wide structural and mechanistic diversity, so their derivatives possess various pharmacological properties and occupy a salient place in the development of new drugs. Indole/isatin-containing hybrids, which demonstrate a promising activity against a panel of clinically important Gram-positive and Gram-negative bacteria, are privileged scaffolds for the discovery of novel antibacterial candidates. This review, covering articles published between January 2015 and May 2020, focuses on the development and structure-activity relationship (SAR) of indole/isatin-containing hybrids with potential application for fighting bacterial infections, to facilitate further rational design of novel drug candidates.

3-(5-Nitrofuran-2-yl)prop-2-en-1-one Derivatives, with Potent Antituberculosis Activity, Inhibit A Novel Therapeutic Target, Arylamine N-acetyltransferase, in Mycobacteria

In this study, the inhibitory potential of 3-(5-nitrofuran-2-yl)prop-2-en-1-one derivatives was evaluated against a panel of bacteria, as well as mammalian cell lines to determine their therapeutic index. In addition, we investigated the mechanism of antibiotic action of the derivatives to identify their therapeutic target. We discovered compound 2 to be an extremely potent inhibitor of Mycobacterium tuberculosis H37Rv growth (MIC: 0.031 mg/L) in vitro, performing better than the currently used first-line antituberculosis drugs such as isoniazid, rifampicin, ethambutol, and pretomanid in vitro. Furthermore, compound 3 was equipotent to pretomanid against a multidrug-resistant M. tuberculosis clinical isolate. The derivatives were selective and bactericidal towards slow-growing mycobacteria. They showed low cytotoxicity towards murine RAW 264.7 and human THP-1 cell lines, with high selectivity indices. Compound 1 effectively eliminated the intracellular mycobacteria in a mycobacteria-infected macrophage model. The derivatives were assessed for their potential to inhibit mycobacterial arylamine N-acetyltransferase (NAT) and were identified as good inhibitors of recombinant mycobacterial NAT, a novel target essential for the intracellular survival of M. tuberculosis. This study provided hits for designing new potent and selective antituberculosis leads, having mycobacterial NAT inhibition as their possible endogenous mechanisms of action.

Multitargeting Compounds: A Promising Strategy to Overcome Multi-Drug Resistant Tuberculosis

Tuberculosis is still an urgent global health problem, mainly due to the spread of multi-drug resistant M. tuberculosis strains, which lead to the need of new more efficient drugs. A strategy to overcome the problem of the resistance insurgence could be the polypharmacology approach, to develop single molecules that act on different targets. Polypharmacology could have features that make it an approach more effective than the classical polypharmacy, in which different drugs with high affinity for one target are taken together. Firstly, for a compound that has multiple targets, the probability of development of resistance should be considerably reduced. Moreover, such compounds should have higher efficacy, and could show synergic effects. Lastly, the use of a single molecule should be conceivably associated with a lower risk of side effects, and problems of drug–drug interaction. Indeed, the multitargeting approach for the development of novel antitubercular drugs have gained great interest in recent years. This review article aims to provide an overview of the most recent and promising multitargeting antitubercular drug candidates.