An efficient synthetic route for preparation of antimycobacterial wollamides and evaluation of their in vitro and in vivo efficacy

Unearthing naturally-occurring cyclic antibacterial peptides and their structural optimization strategies

Natural products with antibacterial activity are highly desired globally to combat against multidrug-resistant (MDR) bacteria. Antibacterial peptide (ABP), especially cyclic ABP (CABP), is one of the abundant classes. Most of them were isolated from microbes, demonstrating excellent bactericidal effects. With the improved proteolytic stability, CABPs are normally considered to have better druggability than linear peptides. However, most clinically-used CABP-based antibiotics, such as colistin, also face the challenges of drug resistance soon after they reached the market, urgently requiring the development of next-generation succedaneums. We present here a detail review on the novel naturally-occurring CABPs discovered in the past decade and some of them are under clinical trials, exhibiting anticipated application potential. According to their chemical structures, they were broadly classified into five groups, including (i) lactam/lactone-based CABPs, (ii) cyclic lipopeptides, (iii) glycopeptides, (iv) cyclic sulfur-rich peptides and (v) multiple-modified CABPs. Their chemical structures, antibacterial spectrums and proposed mechanisms are discussed. Moreover, engineered analogs of these novel CABPs are also summarized to preliminarily analyze their structure-activity relationship. This review aims to provide a global perspective on research and development of novel CABPs to highlight the effectiveness of derivatives design in identifying promising antibacterial agents. Further research efforts in this area are believed to play important roles in fighting against the multidrug-resistance crisis.

The Antitubercular Activities of Natural Products with Fused-Nitrogen-Containing Heterocycles

Tuberculosis (TB) is notorious as the leading cause of death worldwide due to a single infectious entity and its causative agent, Mycobacterium tuberculosis (Mtb), has been able to evolve resistance to all existing drugs in the treatment arsenal complicating disease management programs. In drug discovery efforts, natural products are important starting points in generating novel scaffolds that have evolved to specifically bind to vulnerable targets not only in pathogens such as Mtb, but also in mammalian targets associated with human diseases. Structural diversity is one of the most attractive features of natural products. This review provides a summary of fused-nitrogen-containing heterocycles found in the natural products reported in the literature that are known to have antitubercular activities. The structurally targeted natural products discussed in this review could provide a revealing insight into novel chemical aspects with novel biological functions for TB drug discovery efforts.

Wollamide Cyclic Hexapeptides Synergize with Established and New Tuberculosis Antibiotics in Targeting Mycobacterium tuberculosis

Shorter and more effective treatment regimens as well as new drugs are urgent priorities for reducing the immense global burden of tuberculosis (TB). As treatment of TB currently requires multiple antibiotics with diverse mechanisms of action, any new drug lead requires assessment of potential interactions with existing TB antibiotics. We previously described the discovery of wollamides, a new class of Streptomyces-derived cyclic hexapeptides with antimycobacterial activity. To further assess the value of the wollamide pharmacophore as an antimycobacterial lead, we determined wollamide interactions with first- and second-line TB antibiotics by determining fractional inhibitory combination index and zero interaction potency scores. In vitro two-way and multiway interaction analyses revealed that wollamide B1 synergizes with ethambutol, pretomanid, delamanid, and para-aminosalicylic acid in inhibiting the replication and promoting the killing of phylogenetically diverse clinical and reference strains of the Mycobacterium tuberculosis complex (MTBC). Wollamide B1 antimycobacterial activity was not compromised in multi- and extensively drug-resistant MTBC strains. Moreover, growth-inhibitory antimycobacterial activity of the combination of bedaquiline/pretomanid/linezolid was further enhanced by wollamide B1, and wollamide B1 did not compromise the antimycobacterial activity of the isoniazid/rifampicin/ethambutol combination. Collectively, these findings add new dimensions to the desirable characteristics of the wollamide pharmacophore as an antimycobacterial lead compound. IMPORTANCE Tuberculosis (TB) is an infectious disease that affects millions of people globally, with 1.6 million deaths annually. TB treatment requires combinations of multiple different antibiotics for many months, and toxic side effects can occur. Therefore, shorter, safer, more effective TB therapies are required, and these should ideally also be effective against drug-resistant strains of the bacteria that cause TB. This study shows that wollamide B1, a chemically optimized member of a new class of antibacterial compounds, inhibits the growth of drug-sensitive as well as multidrug-resistant Mycobacterium tuberculosis isolated from TB patients. In combination with TB antibiotics, wollamide B1 synergistically enhances the activity of several antibiotics, including complex drug combinations that are currently used for TB treatment. These new insights expand the catalogue of the desirable characteristics of wollamide B1 as an antimycobacterial lead compound that might inspire the development of improved TB treatments.

Compelling Cyclic Peptide Scaffolds for Antitubercular Action: An Account (2011-21) of the Natural Source

Natural cyclic peptide scaffolds are indispensable in medicinal chemistry, chemical biology, and drug discovery platforms due to their chemical diversity, structural integrity, proteolytic stability and biocompatibility. Historically, their isolation and profound understanding of target engagement have been identified as lead pharmacophore discovery. Natural cyclic peptides are the largest class of pharmacologically active scaffold, in which most show activity against drug-resistant Mycobacterium tuberculosis (Mtb). Nevertheless, eight recently discovered cyclic peptide scaffolds exhibit promising antitubercular activity among numerous naturally occurring antitubercular peptides, and they are amenable scaffolds to drug development. We examined their biological origin, scaffolds, isolations, chemical synthesis, and reasons for biological actions against Mtb. Understanding these peptide scaffold details will further allow synthetic and medicinal chemists to develop novel peptide therapeutics against tuberculosis-infected deadly diseases. This review emphasizes these cyclic peptides' in vitro and in vivo activity profiles, including their structural and chemical features.

“Upcycling” known molecules and targets for drug-resistant TB

Despite reinvigorated efforts in Tuberculosis (TB) drug discovery over the past 20 years, relatively few new drugs and candidates have emerged with clear utility against drug resistant TB. Over the same period, significant technological advances and learnings around target value have taken place. This has offered opportunities to re-assess the potential for optimization of previously discovered chemical matter against Mycobacterium tuberculosis (M.tb) and for reconsideration of clinically validated targets encumbered by drug resistance. A re-assessment of discarded compounds and programs from the “golden age of antibiotics” has yielded new scaffolds and targets against TB and uncovered classes, for example beta-lactams, with previously unappreciated utility for TB. Leveraging validated classes and targets has also met with success: booster technologies and efforts to thwart efflux have improved the potential of ethionamide and spectinomycin classes. Multiple programs to rescue high value targets while avoiding cross-resistance are making progress. These attempts to make the most of known classes, drugs and targets complement efforts to discover new chemical matter against novel targets, enhancing the chances of success of discovering effective novel regimens against drug-resistant TB.

Recent Advances in Macrocyclic Drugs and Microwave-Assisted and/or Solid-Supported Synthesis of Macrocycles

Macrocycles represent attractive candidates in organic synthesis and drug discovery. Since 2014, nineteen macrocyclic drugs, including three radiopharmaceuticals, have been approved by FDA for the treatment of bacterial and viral infections, cancer, obesity, immunosuppression, etc. As such, new synthetic methodologies and high throughput chemistry (e.g., microwave-assisted and/or solid-phase synthesis) to access various macrocycle entities have attracted great interest in this chemical space. This article serves as an update on our previous review related to macrocyclic drugs and new synthetic strategies toward macrocycles (Molecules, 2013, 18, 6230). In this work, I first reviewed recent FDA-approved macrocyclic drugs since 2014, followed by new advances in macrocycle synthesis using high throughput chemistry, including microwave-assisted and/or solid-supported macrocyclization strategies. Examples and highlights of macrocyclization include macrolactonization and macrolactamization, transition-metal catalyzed olefin ring-closure metathesis, intramolecular C–C and C–heteroatom cross-coupling, copper- or ruthenium-catalyzed azide–alkyne cycloaddition, intramolecular S_NAr or S_N2 nucleophilic substitution, condensation reaction, and multi-component reaction-mediated macrocyclization, and covering the literature since 2010.

Synthesis and Evaluation of Antimycobacterial and Antiplasmodial Activities of Hirsutellide A and Its Analogues

Hirsutellide A is nature-derived cyclic hexadepsipeptide with reported antimycobacterial and antiplasmodial activities. To verify its structure, hirsutellide A was synthesized following a solution-phase peptide synthesis approach. A detailed analysis of the ¹H and ¹³C NMR spectra of the synthesized compound revealed structural variation from what had been originally assigned for hirsutellide A, despite the use of identical building blocks. This variation occurred at the two allo-Ile moieties. To investigate the structure-activity relationship, the depsipeptide and peptide analogues of hirsutellide A were prepared and tested for antimycobacterial and antiplasmodial activities. The compounds displayed antiplasmodial potency against Plasmodium falciparum 3D7 while showing weak or no activity against Mycobacterium tuberculosis H37Rv. The drug-likeness of the series was assessed through in vitro absorption, distribution, metabolism, and excretion (ADME) profiling, revealing systematic differences between the pharmacokinetic properties of cyclic hexapeptides and hexadepsipeptides.

Solid-Phase Synthesis of Wollamide Cyclohexapeptide Analogs

Mycobacterium tuberculosis (Mtb) is a bacterial pathogen that causes a potentially serious infectious disease called tuberculosis (TB). Cyclohexapeptide wollamides A and B were recently isolated from Streptomyces nov. sp. (MST-115088) and subsequently reported to show excellent in vitro antituberculosis activity with minimum inhibitory concentration (MIC) of 1.56 μg/mL against Mtb (H37Rv) and favorable selectivity profile. This chapter describes the detailed synthesis of antitubercular wollamide analogs using solid-phase synthesis of linear hexapeptide precursors, followed by solution-phase HBTU-mediated macrocyclization and global side chain deprotection.

New potential drug leads against MDR-MTB: A short review

Multidrug resistant Mycobacterium tuberculosis (MDR-MTB) infections have created a critical health problem globally. The appalling rise in drug resistance to all the current therapeutics has triggered the need for identifying new antimycobacterial agents effective against multidrug-resistant Mycobacterium tuberculosis. Structurally unique chemical entities with new mode of action will be required to combat this pressing issue. This review gives an overview of the structures and outlines on various aspects of in vitro pharmacological activities of new antimycobacterial agents, mechanism of action and brief structure activity relationships in the perspective of drug discovery and development. This review also summarizes on recent reports of new antimycobacterial agents.

On-Resin Macrocyclization of Peptides Using Vinyl Sulfonamides as a Thiol-Michael "Click" Acceptor

Macrocyclization of linear peptides imparts improved stability to enzymatic degradation and increases potency of function. Many successful macrocyclization of peptides both in solution and on-resin have been achieved but are limited in scope as they lack selectivity, require long reaction times, or necessitate heat. To overcome these drawbacks a robust and facile strategy was developed employing thiol-Michael click chemistry via an N-methyl vinyl sulfonamide. We demonstrate its balance of reactivity and high stability through FTIR model kinetic studies, reaching 88% conversion over 30 min, and NMR stability studies, revealing no apparent degradation over an 8 day period in basic conditions. Using a commercially available reagent, 2-chloroethane sulfonyl chloride, the cell adhesion peptide, RGDS, was functionalized and macrocyclized on-resin with a relative efficiency of over 95%. The simplistic nature of this process demonstrates the effectiveness of vinyl sulfonamides as a thiol-Michael click acceptor and its applicability to many other bioconjugation applications.