Hybrid BisQACs: Potent Biscationic Quaternary Ammonium Compounds Merging the Structures of Two Commercial Antiseptics

Advances in the Synthesis of Biologically Active Quaternary Ammonium Compounds

This review provides a comprehensive overview of recent advancements in the design and synthesis of biologically active quaternary ammonium compounds (QACs). The covered scope extends beyond commonly reviewed antimicrobial derivatives to include synthetic agents with antifungal, anticancer, and antiviral properties. Additionally, this review highlights examples of quaternary ammonium compounds exhibiting activity against protozoa and herbicidal effects, as well as analgesic and anesthetic derivatives. The article also embraces the quaternary-ammonium-containing cholinesterase inhibitors and muscle relaxants. QACs, marked by their inherent permanent charge, also find widespread usage across diverse domains such as fabric softeners, hair conditioners, detergents, and disinfectants. The effectiveness of QACs hinges greatly on finding the right equilibrium between hydrophilicity and lipophilicity. The ideal length of the alkyl chain varies according to the unique structure of each QAC and its biological settings. It is expected that this review will provide comprehensive data for medicinal and industrial chemists to design and develop novel QAC-based products.

Antibacterial quaternary ammonium agents: Chemical diversity and biological mechanism

Bacterial infections have seriously threatened public health especially with the increasing resistance and the cliff-like decline of the number of newly approved antibacterial agents. Quaternary ammonium compounds (QACs) possess potent medicinal properties with 95 successfully marketed drugs, which also have a long history as antibacterial agents. In this review, we summarize the chemical diversity of antibacterial QACs, divided into chain-like and aromatic ring, reported over the past decade (2012 to mid-2022). Additionally, the structure-activity relationships, mainly covering hydrophobicity, charges and skeleton features, are discussed. In the cases where sufficient information is available, antibacterial mechanisms including biofilm, cell membrane, and intracellular targets are presented. It is hoped that this review will provide sufficient information for medicinal chemists to discover the new generation of antibacterial agents based on QACs.

Quaternary Phosphonium Compounds: An Examination of Non-Nitrogenous Cationic Amphiphiles That Evade Disinfectant Resistance

Quaternary ammonium compounds (QACs) serve as mainstays in the formulation of disinfectants and antiseptics. However, an over-reliance and misuse of our limited QAC arsenal has driven the development and spread of resistance to these compounds, as well as co-resistance to common antibiotics. Extensive use of these compounds throughout the COVID-19 pandemic thus raises concern for the accelerated proliferation of antimicrobial resistance and demands for next-generation antimicrobials with divergent architectures that may evade resistance. To this end, we endeavored to expand beyond canonical ammonium scaffolds and examine quaternary phosphonium compounds (QPCs). Accordingly, a synthetic and biological investigation into a library of novel QPCs unveiled biscationic QPCs to be effective antimicrobial scaffolds with improved broad-spectrum activities compared to commercial QACs. Notably, a subset of these compounds was found to be less effective against a known QAC-resistant strain of MRSA. Bioinformatic analysis revealed the unique presence of a family of small multiresistant transporter proteins, hypothesized to enable efflux-mediated resistance to QACs and QPCs. Further investigation of this resistance mechanism through efflux-pump inhibition and membrane depolarization assays illustrated the superior ability of P6P-10,10 to perturb the cell membrane and exert the observed broad-spectrum potency compared to its commercial counterparts. Collectively, this work highlights the promise of biscationic phosphonium compounds as next-generation disinfectant molecules with potent bioactivities, thereby laying the foundation for future studies into the synthesis and biological investigation of this nascent antimicrobial class.

Sustainable triazine-derived quaternary ammonium salts as antimicrobial agents

The first examples of highly efficient antimicrobial triazine-derived bis imidazolium quaternary ammonium salts (TQAS) are reported. TQAS have been prepared with an easy, atom efficient, economically sustainable strategy and tested as antimicrobial agents, reaching MIC values below 10 mg L^-1. Distinctively, TQAS have low MIC and low cytotoxicity.

Quaternary Ammonium Compounds (QACs) and Ionic Liquids (ILs) as Biocides: From Simple Antiseptics to Tunable Antimicrobials

Quaternary ammonium compounds (QACs) belong to a well-known class of cationic biocides with a broad spectrum of antimicrobial activity. They are used as essential components in surfactants, personal hygiene products, cosmetics, softeners, dyes, biological dyes, antiseptics, and disinfectants. Simple but varied in their structure, QACs are divided into several subclasses: Mono-, bis-, multi-, and poly-derivatives. Since the beginning of the 20th century, a significant amount of work has been dedicated to the advancement of this class of biocides. Thus, more than 700 articles on QACs were published only in 2020, according to the modern literature. The structural variability and diverse biological activity of ionic liquids (ILs) make them highly prospective for developing new types of biocides. QACs and ILs bear a common key element in the molecular structure-quaternary positively charged nitrogen atoms within a cyclic or acyclic structural framework. The state-of-the-art research level and paramount demand in modern society recall the rapid development of a new generation of tunable antimicrobials. This review focuses on the main QACs exhibiting antimicrobial and antifungal properties, commercial products based on QACs, and the latest discoveries in QACs and ILs connected with biocide development.

The mode of antibacterial action of quaternary N-benzylimidazole salts against emerging opportunistic pathogens

Quaternary ammonium compounds (QACs) are antimicrobial agents displaying a broad spectrum of activity due to their mechanism of action targeting the bacterial membrane. The emergence of bacterial resistance to QACs, especially in times of pandemics, requires the continuous search for new and potent QACs structures. Here we report the synthesis and biological evaluation of QACs based on imidazole derivative, N-benzylimidazole. The antimicrobial activity was tested against a range of pathogenic bacteria and fungi, both ATCC and clinical isolates, showing varying activities ranging in minimal inhibitory concentrations (MICs) from as low as 7 ng/mL. The most promising compound, N-tetradecyl derivative (BnI-14), proved to be very potent against bacterial biofilms, even at sub-MIC doses, suggesting interference with the bacterial growth and/or division process. The BnI-14 treatment induces bacterial membrane disruption, as observed by fluorescence spectroscopy and atomic force microscopy and it also binds to DNA indicating that bacterial membrane might not be the only cellular target of QACs. Most importantly, BnI-14 exhibits low toxicity to healthy human cell lines, suggesting that N-benzylimidazolium-based QACs may be promising new antimicrobial agents.

Metallocene QACs: The Incorporation of Ferrocene Moieties into monoQAC and bisQAC Structures

Inspired by the incorporation of metallocene functionalities into a variety of bioactive structures, particularly antimicrobial peptides, we endeavored to broaden the structural variety of quaternary ammonium compounds (QACs) by the incorporation of the ferrocene moiety. Accordingly, 23 ferrocene-containing mono- and bisQACs were prepared in high yields and tested for activity against a variety of bacteria, including Gram-negative strains and a panel of clinically isolated MRSA strains. Ferrocene QACs were shown to be effective antiseptics with some displaying single-digit micromolar activity against all bacteria tested, demonstrating yet another step in the expansion of structural variety of antiseptic QACs.

Advancements in the Development of Non-Nitrogen-Based Amphiphilic Antiseptics to Overcome Pathogenic Bacterial Resistance

The prevalence of quaternary ammonium compounds (QACs) as common disinfecting agents for the past century has led bacteria to develop resistance to such compounds. Given the alarming increase in resistant strains, new strategies are required to combat this rise in resistance. Recent efforts to probe and combat bacterial resistance have focused on studies of multiQACs. Relatively unexplored, however, have been changes to the primary atom bearing positive charge in these antiseptics. Here we review the current state of the field of both phosphonium and sulfonium amphiphilic antiseptics, both of which hold promise as novel means to address bacterial resistance.

Metabolomics reveals differential mechanisms of toxicity of hyperbranched poly(ethyleneimine)-derived nanoparticles to the soil-borne fungus Verticillium dahliae Kleb

There is a consensus on the urge for the discovery and assessment of alternative, improved sources of bioactivity that could be developed as plant protection products (PPPs), in order to combat issues that the agrochemical sector is facing. Based on the recent advances in nanotechnology, nanoparticles seem to have a great potential towards the development of the next generation nano-PPPs used as active ingredients (a.i.) per se or as nanocarriers in their formulation. Nonetheless, information on their mode(s)-of-action (MoA) and mechanisms of toxicity is yet largely unknown, representing a bottleneck in their further assessment and development. Therefore, we have undertaken the task to assess the fungitoxicity of hyperbranched poly(ethyleneimine) (HPEI), quaternized hyperbranched poly(ethyleneimine) (QPEI), and guanidinylated hyperbranched poly(ethyleneimine) (GPEI) nanoparticles to the soil-born plant pathogenic fungus Verticillium dahliae Kleb, and dissect their effects on its metabolism applying GC/EI/MS metabolomics. Results revealed that functionalization of HPEI nanoparticles with guanidinium end groups (GPEI) increases their toxicity to V. dahliae, while functionalization with quaternary ammonium end groups (QPEI) decreases it. The treatments with the nanoparticles affected the chemical homeostasis of the fungus, altering substantially its amino acid pool, energy production, and fatty acid content, causing additionally oxidative and osmotic stresses. To the best of our knowledge, this is the first report on the comparative toxicity of HPEI, QPEI, and GPEI to filamentous fungi applying metabolomics. The findings could be exploited in the study of the quantitative structure-activity relationship (QSAR) of HPEI-derived nanoparticles and their further development as nano-PPPs.

Further Investigations into Rigidity-Activity Relationships in BisQAC Amphiphilic Antiseptics

Thirty-six biscationic quaternary ammonium compounds were efficiently synthesized in one step to examine the effect of molecular geometry of two-carbon linkers on antimicrobial activity. The synthesized compounds showed strong antimicrobial activity against a panel of both Gram-positive and Gram-negative bacteria, including methicillin-resistant Staphylococcus aureus (MRSA). While the linker geometry showed only a modest correlation with antimicrobial activity, several of the synthesized bisQACs are promising potential antiseptics due to good antimicrobial activity (MIC≤2 μM) and their higher therapeutic indices compared to previously reported QACs.

More QACs, more questions: Recent advances in structure activity relationships and hurdles in understanding resistance mechanisms

Quaternary ammonium compounds (QACs) are a class of antimicrobials that have been around for over a century; nevertheless, they have found continued renewal in the structures to which they can be appended. Ranging from antimicrobial polymers to adding novel modes of action to existing antibiotics, QACs have found ongoing use due to their potent properties. However, resistance against QACs has begun to emerge, and the mechanism of resistance is still only partially understood. In this review, we aim to summarize the current state of the field and what is known about the mechanisms of resistance so that the QACs of the future can be designed to be evermore efficacious and utilized to unearth the remaining mysteries that surround bacteria's resistance to them.

An Investigation into Rigidity-Activity Relationships in BisQAC Amphiphilic Antiseptics

Twenty-one mono- and biscationic quaternary ammonium amphiphiles (monoQACs and bisQACs) were rapidly prepared in order to investigate the effects of rigidity of a diamine core structure on antiseptic activity. As anticipated, the bioactivity against a panel of six bacteria including methicillin-resistant Staphylococcus aureus (MRSA) strains was strong for bisQAC structures, and is clearly correlated with the length of non-polar side chains. Modest advantages were noted for amide-containing side chains, as compared with straight-chained alkyl substituents. Surprisingly, antiseptics with more rigidly disposed side chains, such as those in DABCO-12,12, showed the highest level of antimicrobial activity, with single-digit MIC values or better against the entire bacterial panel, including sub-micromolar activity against an MRSA strain.

New Generation of N-Chloramine/QAC Composite Biocides: Efficient Antimicrobial Agents To Target Antibiotic-Resistant Bacteria in the Presence of Organic Load

We previously reported that covalently joining an amide-based N-chloramine with a quaternary ammonium compound (QAC) can yield a new composite biocide with faster inactivation of various bacteria. Importantly, the composite biocide was found to reduce the risk for potential bacterial resistance associated with QAC. However, similar to other N-chloramines and QACs, this high-performance composite biocide becomes less potent against pathogenic bacteria in the presence of high protein fluids. In this study, we substituted the amide-based N-chloramine moiety in the previously reported composite biocide with a secondary amine-based N-chloramine to improve the biocidal efficacy in biological fluids. The N-Cl bond in the synthesized tetramethylpiperidine-based composite biocides is more stable in a high protein medium (HPM) than that in the hydantoin (amide)-based composite biocides. The composite biocide, 2-[4-(1-chloro-2,2,6,6-tetramethyl-piperidin-4-yloxymethyl)-[1,2,3]triazol-1-yl]-ethyl-dodecyl-dimethyl-ammonium chloride (6a), showed the best antibacterial activity in both phosphate-buffered saline and HPM among various composite biocides and benzyldodecyldimethylammonium chloride used in this study.