Expanding the antibacterial selectivity of polyether ionophore antibiotics through diversity-focused semisynthesis

Biomimetic ion channels with subnanometer sizes for ion sieving: a mini-review

The remarkable ion selectivity of biological systems has inspired the development of artificial ion channels with Ångström-scale precision, expanding their potential applications in ion separation, energy conversion, and water purification. This mini-review systematically examines fundamental ion-sieving mechanisms operating at the subnanoscale, highlighting advanced fabrication strategies involving synthetic ion channels on lipid bilayers and solid-state ion channels. We further explore membrane material innovations spanning zero-dimensional nanopores to three-dimensional crystalline frameworks, emphasizing structure-function relationships in channel design. The discussion concludes with critical perspectives on scalability challenges and future research directions, outlining pathways toward next-generation sustainable ion sieving technologies.

Structural Basis of Sequential Enantioselective Epoxidation by a Flavin-Dependent Monooxygenase in Lasalocid A Biosynthesis

Polyether polyketides are a structurally diverse group of natural products known for their antimicrobial and antiproliferative activities. Lasalocid A is a canonical natural polyether produced by the soil bacterium Streptomyces lasalocidi. In lasalocid A biosynthesis, a polyene polyketide intermediate is converted into a bisepoxide by the flavin-dependent monooxygenase enzyme Lsd18. Remarkably, Lsd18 acts on two distinct C═C groups in the substrate molecule, forming two (R,R) epoxides. We have determined the X-ray crystal structures of Lsd18 in the substrate-free, substrate-bound, and product-bound forms. Our work has revealed that Lsd18 has an extra-large substrate-binding pocket that allows the polyene to adopt different conformations within the enzyme pocket. This feature enables Lsd18 to epoxidate both of the C═C groups. Additionally, a subpocket located near the Lsd18 active site controls stereoselectivity by dictating which face of the C═C group is placed next to the flavin. Molecular understanding of how Lsd18 transforms a polyene into a bisepoxide during lasalocid A biosynthesis lays the foundation for the production of designer polyethers for drug development.

Drug-induced cytotoxicity prediction in muscle cells, an application of the Cell Painting assay

In silico toxicity prediction offers the chance of reducing or replacing most animal testing through the integration of large experimental assay datasets with the appropriate computational approaches. The use of Cell Painting to detect various phenotypic changes induced by chemicals is emerging as a powerful technique in toxicity prediction. However, most Cell Painting approaches use cancer cells that are less relevant for many toxicological endpoints, which may limit the usefulness of this data. In this study, a myoblast cell line is used to characterize cellular responses to a panel of 30 known myotoxicants. In place of traditional structural descriptors, here each perturbation is described by a fingerprint of calculated properties, deducted from the intensity, shape, or texture of individual cells. We show that these kinds of descriptors convey information to allow the prediction of the cellular viability and fate of cells in myoblasts and differentiated myotubes of the C2C12 cell line, and the clustering of drugs by their cytotoxicity responses.

Synthetic Cation Transporters Eradicate Drug-Resistant Staphylococcus aureus, Persisters, and Biofilms

New drugs are urgently required to address the ongoing health crisis caused by methicillin-resistant Staphylococcus aureus (MRSA) infections. Added to the challenge is the difficult-to-treat persister cells and biofilm which are tolerant to the antibiotics. Here we report a new approach to these problems, describing the design and synthesis of aminoxy-acid-based dipeptides that facilitate cation transport across cell membranes to disrupt bacterial ion homeostasis. Remarkably, these synthetic cation transporters display significant antibacterial activity against MRSA, while maintaining high selectivity over mammalian cells. They also effectively eliminate bacterial persisters and reduce established biofilms. Additionally, they inhibit biofilm formation and suppress bacterial virulent protein secretion, even at subinhibitory concentrations. Their associated antibiotic effects support their in vivo efficacy in murine skin and bloodstream MRSA infection models with no observable toxicity to the host. Mode-of-action analysis indicates that these cation transporters induce cytoplasmic acidification, hyperpolarization, and calcium influx, accelerating autolysis. Given their potent activity against bacterial persisters and biofilms, synthetic cation transporters are an emergent and promising class of compounds in the fight against MRSA infections.

Ecological footprint of ionophores in livestock production: Environmental pathways and effects

Ionophores, a class of animal antibiotics, are widely used in intensive livestock farming to enhance feed efficiency and control coccidiosis. These compounds, known for their ability to transport cations across biological membranes, are crucial in maintaining cellular homeostasis. However, their extensive use raises environmental and human health concerns. This manuscript offers a comprehensive review of ionophores in livestock production, highlighting their environmental impact and potential to contribute to antimicrobial resistance (AMR). It emphasizes the fate and transport of ionophores in various environmental matrices, providing a holistic framework for assessing ecological risks. The study calls for improved management practices like enhanced waste management through anaerobic digestion, and composting is essential. Establishing Maximum Residue Limits (MRLs) and using LC-MS/MS for residue detection will help manage exposure. Educating livestock producers and researching alternatives like probiotics can decrease reliance on ionophores to mitigate the ecological footprint of ionophores, making it a timely and relevant piece of research. Ionophores can persist in the environment, potentially contributing to AMR in gram-positive bacteria. Furthermore, their presence in manure, runoff, and agricultural soils has been documented, leading to contamination of water bodies and sediments. Ionophores pose risks to terrestrial and aquatic ecosystems, with studies revealing hazardous effects even at low concentrations. This review highlights the need for improved management practices to mitigate the environmental impacts of ionophores, particularly regarding AMR development and ecosystem disruption. Careful monitoring and sustainable use of these antibiotics are essential to reduce their ecological footprint in livestock production. PRACTITIONER POINTS: Ionophores enhance feed efficiency, but pose environmental health risks. Their persistence may lead to antimicrobial resistance in gram-positive bacteria. Ionophore contamination threatens both terrestrial and aquatic ecosystems. Monitoring and management are crucial to mitigate ionophore-related risks.

Total syntheses of cyclohelminthol I-IV reveal a new cysteine-selective covalent reactive group

Biocompatible covalent reactive groups (CRGs) play pivotal roles in several areas of chemical biology and the life sciences, including targeted covalent inhibitor design and preparation of advanced biologic drugs, such as antibody-drug conjugates. In this study, we present the discovery that the small, chlorinated polyketide natural product cyclohelminthiol II (CHM-II) acts as a new type of cysteine/thiol-targeting CRG incorporating both reversible and irreversible reactivity. We devise the first syntheses of four simple cyclohelminthols, (±)-cyclohelminthol I-IV, with selective chlorinations (at C2 and C5) and a Ni-catalyzed reductive cross coupling between an enone, a vinyl bromide and triethylsilyl chloride as the key steps. Unbiased biological profiling (cell painting) was used to discover a putative covalent mechanism for CHM-II in cells with subsequent validation experiments demonstrating mechanistic similarity to dimethyl fumarate (DMF) - a known (covalent) drug used in the treatment of multiple sclerosis. Focused biochemical experiments revealed divergent thiol-reactivity inherent to the CHM-II scaffold and through further chemical derivatization of CHM-II we applied activity-based protein profiling (ABPP)-workflows to show exclusive cysteine-labelling in cell lysate. Overall, this study provides both efficient synthetic access to the CHM-II chemotype - and neighboring chemical space - and proof-of-concept for several potential applications of this new privileged CRG-class within covalent chemical biology.

Unifying principles for the design and evaluation of natural product-inspired compound collections

Natural products play a major role in the discovery of novel bioactive compounds. In this regard, the synthesis of natural product-inspired and -derived analogues is an active field that is further developing. Several strategies and principles for the design of such compounds have been developed to streamline their access and synthesis. This perspective describes how individual strategies or their elements can be combined depending on the project goal. Illustrative examples are shown that demonstrate the blurred lines between approaches and how they can work in concert to discover new biologically active molecules. Lastly, a general set of guidelines for choosing an appropriate strategy combination for the specific purpose is presented.

Calcium‐dependent antimicrobials: Nature‐inspired materials and designs

Bacterial infection remains a major complication answering for the failures of various implantable medical devices. Tremendous extraordinary advances have been published in the design and synthesis of antimicrobial materials addressing this issue; however, the clinical translation has largely been blocked due to the challenge of balancing the efficacy and safety of these materials. Here, calcium's biochemical features, natural roles in pathogens and the immune systems, and advanced uses in infection medications are illuminated, showing calcium is a promising target for developing implantable devices with less infection tendency. The paper gives a historical overview of biomedical uses of calcium and summarizes calcium's merits in coordination, hydration, ionization, and stereochemistry for acting as a structural former or trigger in biological systems. It focuses on the involvement of calcium in pathogens' integrity, motility, and metabolism maintenance, outlining the potential antimicrobial targets for calcium. It addresses calcium's uses in the immune systems that the authors can learn from for antimicrobial synthesis. Additionally, the advances in calcium's uses in infection medications are highlighted to sketch the future directions for developing implantable antimicrobial materials. In conclusion, calcium is at the nexus of antimicrobial defense, and future works on taking advantage of calcium in antimicrobial developments are promising in clinical translation.

The Biological Activities of Polyether Ionophore Antibiotic Routiennocin is Independent of Absolute Stereochemistry

Carboxylic polyether ionophores (CPIs) are among the most prevalent agricultural antibiotics (notably in the US) and these compounds have been in use for decades. The potential to reposition CPIs beyond veterinary use, e. g. through chemical modifications to enhance their selectivity window, is an exciting challenge and opportunity, considering their general resilience towards resistance development. Given the very large societal impact of these somewhat controversial compounds, it is surprising that many aspects of their mechanisms and activities in cells remain unclear. Here, we report comparative biological activities of the CPI routiennocin and two stereoisomers, including its enantiomer. We used an efficient convergent synthesis strategy to access the compounds and conducted a broad survey of antibacterial activities against planktonic cells and biofilms as well as the compounds' effects on mammalian cells, the latter assessed both via standard cell viability assays and broad morphological profiling. Interestingly, similar bioactivity of the enantiomeric pair was observed across all assays, strongly suggesting that chiral interactions do not play a decisive role in the mode of action. Overall, our findings are consistent with a mechanistic model involving highly dynamic behaviour of CPIs in biological membranes.

An Enantioselective Aminocatalytic Cascade Reaction Affording Bioactive Hexahydroazulene Scaffolds

A novel cascade reaction initiated by an enantioselective aminocatalysed 1,3-dipolar [6+4] cycloaddition between catalytically generated trienamines and 3-oxidopyridinium betaines is presented. The [6+4] cycloadduct spontaneously undergoes an intramolecular enamine-mediated aldol, hydrolysis, and E1cb sequence, which ultimately affords a chiral hexahydroazulene framework. In this process, three new C-C bonds and three new stereocenters are formed, enabled by a formal unfolding of the pyridine moiety from the dipolar reagent. The hexahydroazulenes are formed with excellent diastereo-, regio- and periselectivity (>20 : 1), up to 96 % ee, and yields up to 52 %. Synthetic elaborations of this scaffold were performed, providing access to a variety of functionalised hydroazulene compounds, of which some were found to display biological activity in U-2OS osteosarcoma cells in cell painting assays.

Polyether ionophore resistance in a one health perspective

Antimicrobial resistance is a major threat to human health and must be approached from a One Health perspective. Use of antimicrobials in animal husbandry can lead to dissemination and persistence of resistance in human pathogens. Polyether ionophores (PIs) have antimicrobial activities and are among the most extensively used feed additives for major production animals. Recent discoveries of genetically encoded PI resistance mechanisms and co-localization of resistance mechanisms against PIs and antimicrobials used in human medicine on transferrable plasmids, have raised concerns that use of PIs as feed additives bear potential risks for human health. This review summarizes the current knowledge on PI resistance and discusses the potential consequences of PI-usage as feed additives in a One Health perspective.

Catalyst-Free Trans-Selective Oxyiodination and Oxychlorination of Alkynes Employing N-X (Halogen) Reagents

β-halogenated enol esters and ethers are versatile building blocks in organic synthesis, which has attracted increasing attention. In this study, we report the facile trans-oxyiodination and oxychlorination of alkynes, leading to the direct construction of versatile halogenated enol esters and ethers. This transformation features an easy operation, optimal atomic economy, a strong functional group tolerance, broad substrate scope, and excellent trans-selectivity. Employing highly electrophilic bifunctional N-X (halogen) reagents was the key to achieving broad reaction generality. To our knowledge, this transformation represents the first oxyhalogenation system employing N-X (halogen) reagents as both oxylation and halogenation sources.

Polyether Ionophore Antibiotics Target Drug-Resistant Clinical Isolates, Persister Cells, and Biofilms

Polyether ionophores are complex natural products known to transport various cations across biological membranes. While several members of this family are used in agriculture (e.g., as anti-coccidiostats) and have potent antibacterial activity, they are not currently being pursued as antibiotics for human use. Polyether ionophores are typically grouped as having similar functions, despite the fact that they significantly differ in structure; for this reason, how their structure and activity are related remains unclear. To determine whether certain members of the family constitute particularly interesting springboards for in-depth investigations and future synthetic optimization, we conducted a systematic comparative study of eight different polyether ionophores for their potential as antibiotics. This includes clinical isolates from bloodstream infections and studies of the compounds' effects on bacterial biofilms and persister cells. We uncover distinct differences within the compound class and identify the compounds lasalocid, calcimycin, and nanchangmycin as having particularly interesting activity profiles for further development. IMPORTANCE Polyether ionophores are complex natural products used in agriculture as anti-coccidiostats in poultry and as growth promoters in cattle, although their precise mechanism is not understood. They are widely regarded as antimicrobials against Gram-positive bacteria and protozoa, but fear of toxicity has so far prevented their use in humans. We show that ionophores generally have very different effects on Staphylococcus aureus, both in standard assays and in more complex systems such as bacterial biofilms and persister cell populations. This will allow us to focus on the most interesting compounds for future in-depth investigations and synthetic optimizations.

Repurposing Polyether Ionophores as a New-Class of Anti-SARS-Cov-2 Agents as Adjunct Therapy

The emergence of SARS-CoV-2 and its variants have posed a significant threat to humankind in tackling the viral spread. Furthermore, currently repurposed drugs and frontline antiviral agents have failed to cure severe ongoing infections effectively. This insufficiency has fuelled research for potent and safe therapeutic agents to treat COVID-19. Nonetheless, various vaccine candidates have displayed a differential efficacy and need for repetitive dosing. The FDA-approved polyether ionophore veterinary antibiotic for treating coccidiosis has been repurposed for treating SARS-CoV-2 infection (as shown by both in vitro and in vivo studies) and other deadly human viruses. Based on selectivity index values, ionophores display therapeutic effects at sub-nanomolar concentrations and exhibit selective killing ability. They act on different viral targets (structural and non-structural proteins), host-cell components leading to SARS-CoV-2 inhibition, and their activity is further enhanced by Zn²⁺ supplementation. This review summarizes the anti-SARS-CoV-2 potential and molecular viral targets of selective ionophores like monensin, salinomycin, maduramicin, CP-80,219, nanchangmycin, narasin, X-206 and valinomycin. Ionophore combinations with Zn²⁺ are a new therapeutic strategy that warrants further investigation for possible human benefits.

Rediscovery of Tetronomycin as a Broad-Spectrum and Potent Antibiotic against Drug-Resistant Gram-Positive Bacteria

Tetronomycin (1), first isolated from a cultured broth of Streptomyces sp. by Juslen et al. in 1974, is a polycyclic polyether compound. However, the biological activity of 1 has not been thoroughly examined. In this study, we found that 1 exhibits more potent antibacterial activity than two well-known antibacterial drugs (vancomycin and linezolid) and is effective against several drug-resistant clinical isolates including methicillin-resistant Staphylococcus aureus and vancomycin-resistant Enterococci. Furthermore, we reassigned the ¹³C NMR spectra of 1 and performed a preliminary structure-activity relationship study of 1 to synthesize a chemical probe for target identification, which implied different targets based on its ionophore activity.

Reference compounds for characterizing cellular injury in high-content cellular morphology assays

Robust, generalizable approaches to identify compounds efficiently with undesirable mechanisms of action in complex cellular assays remain elusive. Such a process would be useful for hit triage during high-throughput screening and, ultimately, predictive toxicology during drug development. Here we generate cell painting and cellular health profiles for 218 prototypical cytotoxic and nuisance compounds in U-2 OS cells in a concentration-response format. A diversity of compounds that cause cellular damage produces bioactive cell painting morphologies, including cytoskeletal poisons, genotoxins, nonspecific electrophiles, and redox-active compounds. Further, we show that lower quality lysine acetyltransferase inhibitors and nonspecific electrophiles can be distinguished from more selective counterparts. We propose that the purposeful inclusion of cytotoxic and nuisance reference compounds such as those profiled in this resource will help with assay optimization and compound prioritization in complex cellular assays like cell painting.

Nigericin is effective against multidrug resistant gram-positive bacteria, persisters, and biofilms

Multidrug-resistant (MDR) bacteria pose a significant clinical threat to human health, but the development of antibiotics cannot meet the urgent need for effective agents, especially those that can kill persisters and biofilms. Here, we reported that nigericin showed potent bactericidal activity against various clinical MDR Gram-positive bacteria, persisters and biofilms, with low frequencies of resistance development. Moreover, nigericin exhibited favorable in vivo efficacy in deep-seated mouse biofilm, murine skin and bloodstream infection models. With Staphylococcus aureus, nigericin disrupted ATP production and electron transport chain; cell death was associated with altered membrane structure and permeability. Obtaining nigericin-resistant/tolerant mutants required multiple rounds of challenge, and, cross-resistance to members of several antimicrobial classes was absent, probably due to distinct nigericin action with the GraSR two-component regulatory system. Thus, our work reveals that nigericin is a promising antibiotic candidate for the treatment of chronic or recurrent infections caused by Gram-positive bacteria.

Rational Design of Silicon-Based Zinc Ionophores

Ionophores transport ions across biological membranes and have wide-ranging applications, but a platform for their rapid development does not exist. We report a platform for developing ionophores from metal-ion chelators, which are readily available with wide-ranging affinities and specificities, and structural data that can aid rational design. Specifically, we fine-tuned the binding affinity and lipophilicity of a Zn^II -chelating ligand by introducing silyl groups proximal to the Zn^II -binding pocket, which generated ionophores that performed better than most of the currently known Zn^II ionophores. Furthermore, these silicon-based ionophores were specific for Zn^II over other metals and exhibited better antibacterial activity and less toxicity to mammalian cells than several known Zn^II ionophores, including pyrithione. These studies establish rational design principles for the rapid development of potent and specific ionophores and a new class of antibacterial agents.

Antibody Mimics as Bio-orthogonal Catalysts for Highly Selective Bacterial Recognition and Antimicrobial Therapy

Bacterial infectious diseases seriously threaten public health and life. The specific interaction between an antibody and its multivalent antigen is an attractive way to defeat infectious disease. However, due to the high expense and strict storage and applied conditions for antibodies, it is highly desirable but remains an urgent challenge for disease diagnosis and treatment to construct artificial antibodies with strong stability and binding ability and excellent selectivity. Herein, we designed and synthesized antibody-like bio-orthogonal catalysts with the ability to recognize specific bacteria and accomplish in situ drug synthesis in captured bacteria by using improved bacterial imprinting technology. On one hand, the artificial antibody possesses a matching morphology for binding pathogens, and on the other hand, it acts as a bio-orthogonal catalyst for in situ synthesis of antibacterial drugs in live bacteria. Both in vitro and in vivo experiments have demonstrated that our designed antibody can distinguish and selectively bind to specific pathogens and eliminate them on site with the activated drugs. Therefore, our work provides a strategy for designing artificial antibodies with bio-orthogonal catalytic activity and may broaden the application of bio-orthogonal chemistry.

Macrodiolide Diversification Reveals Broad Immunosuppressive Activity That Impairs the cGAS-STING Pathway

The development of new immunomodulatory agents can impact various areas of medicine. In particular, compounds with the ability to modulate innate immunological pathways hold significant unexplored potential. Herein, we report a modular synthetic approach to the macrodiolide natural product (-)-vermiculine, an agent previously shown to possess diverse biological effects, including cytotoxic and immunosuppressive activity. The synthesis allows for a high degree of flexibility in modifying the macrocyclic framework, including the formation of all possible stereoisomers. In total, 18 analogues were prepared. Two analogues with minor structural modifications showed clearly enhanced cancer cell line selectivity and reduced toxicity. Moreover, these compounds possessed broad inhibitory activity against innate immunological pathways in human PBMCs, including the DNA-sensing cGAS-STING pathway. Initial mechanistic characterization suggests a surprising impairment of the STING-TBK1 interaction.

Organocatalytic Asymmetric Multicomponent Cascade Reaction for the Synthesis of Contiguously Substituted Tetrahydronaphthols

Isobenzopyrylium ions are unique, highly reactive, aromatic intermediates which are largely unexplored in asymmetric catalysis despite their high potential synthetic utility. In this study, an organocatalytic asymmetric multicomponent cascade via dienamine catalysis, involving a cycloaddition, a nucleophilic addition, and a ring-opening reaction, is disclosed. The reaction furnishes chiral tetrahydronaphthols containing four contiguous stereocenters in good to high yield, high diastereoselectivity (up to >20:1), and excellent enantioselectivity (93-98% ee). The obtained products are important synthetic intermediates, and it is demonstrated that they can be used for the generation of frameworks such as octahydrobenzo[h]isoquinoline and [2.2.2]octane scaffolds. Furthermore, mechanistic experiments involving oxygen-18-labeling studies and density functional theory calculations provide a vivid picture of the reaction mechanism. Finally, the bioactivity of 16 representative tetrahydronaphthol compounds has been evaluated in U-2OS cancer cells with some compounds showing a unique profile and a clear morphological change.

Total Synthesis of Natural Products Containing Enamine or Enol Ether Derivatives

Enamine and enol ethers are nucleophilic functional groups that are well known to most chemists. When enamine or enol ethers are present in natural products, they are nearly exclusively found as derivatives having a direct connection to electron-withdrawing groups for stabilization, and the resulting larger entities, such as enamides or enol acylates, can be further extended or modified in the framework of natural products. The restricted conformational space that is associated with even simple enamine and enol ether derivatives can be a strong determinant of the overall molecular structure, and the more polarized derivatives can endow some natural products with electrophilic properties and thus facilitate covalent interactions with biological targets.In this Account, I describe our efforts (published since 2016) to prepare natural products from several different classes that all feature enamine or enol ether derivatives as key functionalities. Our choice of targets has been guided by a desire to illuminate unknown biological mechanisms associated with the compounds or, alternatively, to improve upon known biological activities that appear to be promising from a biomedical perspective. In the present text, however, the exclusive focus will be on the syntheses.First, I will discuss the basic properties of the functional groups and briefly present a small collection of illustrative and inspirational examples from the literature for their construction in different complex settings. Next, I will provide an overview of our work on the macrocyclic APD-CLD natural products, rakicidin A and BE-43547A₁, involving the development of an efficient macrocyclization strategy and the development of methods to construct the hallmark APD group: a modified enamide. The synthesis of the meroterpenoid strongylophorine-26 is discussed next, where we developed an oxidative quinone methoxylation to build a vinylogous ester group in the final step of the synthesis and employed FeCl₃-mediated cascade reactions for the rapid assembly of the overall scaffold to enable a short semisynthesis from isocupressic acid. An efficient core scaffold assembly was also in focus in our synthesis of the alkaloid streptazone A with the signature enaminone system being assembled through a rhodium-catalyzed Pauson-Khand reaction. Sequential, site-selective redox manipulations were developed to arrive at strepatzone A and additional members of the natural product family. Finally, I discuss our work to prepare analogs of complex polyether ionophores featuring functionalized tetronic acids as cation-binding groups. A method for the construction of a suitably protected chloromethylidene-modified tetronate is presented which enabled its installation in the full structure through a C-acylation reaction. This work exemplifies how components of abundant polyether ionophores can be recycled and used to access new structures which may possess enhanced biological activities.