Antibacterial activities of plant-derived xanthones

Asymmetric total synthesis of polycyclic xanthenes and discovery of a WalK activator active against MRSA

The development of new antibiotics continues to pose challenges, particularly considering the growing threat of multidrug-resistant Staphylococcus aureus. Structurally diverse natural products provide a promising source of antibiotics. Herein, we outline a concise approach for the collective asymmetric total synthesis of polycyclic xanthene myrtucommulone D and five related congeners. The strategy involves rapid assembly of the challenging benzopyrano[2,3-a]xanthene core, highly diastereoselective establishment of three contiguous stereocenters through a retro-hemiketalization/double Michael cascade reaction, and a Mitsunobu-mediated chiral resolution approach with high optical purity and broad substrate scope. Quantum mechanical calculations provide insight into stereoselective construction mechanism of the three contiguous stereocenters. Additionally, this work leads to the discovery of an antibacterial agent against both drug-sensitive and drug-resistant S. aureus. This compound operates through a unique mechanism that promotes bacterial autolysis by activating the two-component sensory histidine kinase WalK. Our research holds potential for future antibacterial drug development.

Isolation, derivatization, and anti-microbial evaluation of secondary metabolites from Garcinia dryobalanoides

A detailed study on secondary metabolites from the stem bark of Garcinia dryobalanoides has yielded one triterpenoid and four xanthones. Along with that, five novel rubraxanthone derivatives had been successfully synthesised via Williamson etherification with various alkyl halides. The antibacterial evaluation on crude extract, isolated secondary metabolites (1-5), and synthesised compounds (6-9) against Lactiplantibacillus plantarum, Enterobacter cloacae, Pseudomonas aeruginosa, and Serratia marcescens demonstrated moderate to active activities outlining their bacteriostatic potential. The structure-activity relationship (SAR) study conducted revealed the presence of prenyl and hydroxy groups on the xanthone attributed to good bacterial inhibition. The introduction of the alkyl chain to the hydroxy part eventually decreases the antibacterial activity of the compound which is probably due to the bulkiness that causes steric hindrances, therefore limiting the ability to bind to its target site within the bacterial cell.

Recent Advances in Amphipathic Peptidomimetics as Antimicrobial Agents to Combat Drug Resistance

The development of antimicrobial drugs with novel structures and clear mechanisms of action that are active against drug-resistant bacteria has become an urgent need of safeguarding human health due to the rise of bacterial drug resistance. The discovery of AMPs and the development of amphipathic peptidomimetics have lay the foundation for novel antimicrobial agents to combat drug resistance due to their overall strong antimicrobial activities and unique membrane-active mechanisms. To break the limitation of AMPs, researchers have invested in great endeavors through various approaches in the past years. This review summarized the recent advances including the development of antibacterial small molecule peptidomimetics and peptide-mimic cationic oligomers/polymers, as well as mechanism-of-action studies. As this exciting interdisciplinary field is continuously expanding and growing, we hope this review will benefit researchers in the rational design of novel antimicrobial peptidomimetics in the future.

Caged xanthone derivatives to promote mitochondria-mediated apoptosis in breast cancer cells

Caged xanthones represent a class of natural secondary metabolites exhibiting significant potential as antitumor agents. These compounds are characterized by their distinct cage-like structures, which offer novel and compelling frameworks for drug design. Nonetheless, there exists a dearth of research focused on the structural modification of these compounds, particularly in relation to their cage-like architectures. This study aims to address this gap by introducing an innovative synthetic method for constructing a novel caged structure that incorporates a widely employed maleimide group. Drawing upon the well-established synthetic approach for dihydroxanthones previously developed within our research group, we successfully synthesized 13 new caged xanthones using the Diels-Alder reaction. Subsequently, we evaluated their anti-proliferative activity against HepG2, A549, and MDA-MB-231 cell lines. The results revealed that compound 10i exhibited IC50 values of 15.86 µM ± 1.29, 19.27 µM ± 1.58, and 12.96 µM ± 0.09 against these cell lines, respectively. Further investigations into the mechanism of action of 10i demonstrated its ability to induce G2/M cell cycle arrest and initiate mitochondria-mediated apoptosis in breast cancer cells.

Phytochemicals from the Stem Bark of Calophyllum havilandii P. F. Stevens and their Biological Activities

The genus Calophyllum from the family Calophyllaceae has been extensively investigated in the past due to its rich source of bioactive phenolics such as coumarins, chromanones, and xanthones. In this study, phytochemical investigation on the stem bark of Calophyllum havilandii has afforded a new 4-propyldihydrocoumarin derivative, havilarin (1) together with calolongic acid (2), caloteysmannic acid (3), isocalolongic acid (4), euxanthone (5), and β-sitosterol (6). The chemical structure of compound 1 was elucidated and established based on detailed spectroscopic techniques, including MS, IR, UV, 1D and 2D NMR. The results of anti-bacillus study indicated that the chloroform extract showed promising activities with MIC value ranging between 0.5 to 1 μg/mL on selected bacillus strains. Besides, the plant extracts and compounds 1-4 were assessed for their cytotoxicity potential on HL-7702 cell line. All the tested plant extracts and respective chemical constituents displayed non-cytotoxic activity on HL-7702 cell line.

Xanthone Derivatives Enhance the Therapeutic Potential of Neomycin against Polymicrobial Gram-Negative Bacterial Infections

Gram-negative bacterial infections are difficult to manage as many antibiotics are ineffective owing to the presence of impermeable bacterial membranes. Polymicrobial infections pose a serious threat due to the inadequate efficacy of available antibiotics, thereby necessitating the administration of antibiotics at higher doses. Antibiotic adjuvants have emerged as a boon as they can augment the therapeutic potential of available antibiotics. However, the toxicity profile of antibiotic adjuvants is a major hurdle in clinical translation. Here, we report the design, synthesis, and biological activities of xanthone-derived molecules as potential antibiotic adjuvants. Our SAR studies witnessed that the p-dimethylamino pyridine-derivative of xanthone (X8) enhances the efficacy of neomycin (NEO) against Escherichia coli and Pseudomonas aeruginosa and causes a synergistic antimicrobial effect without any toxicity against mammalian cells. Biochemical studies suggest that the combination of X8 and NEO, apart from inhibiting protein synthesis, enhances the membrane permeability by binding to lipopolysaccharide. Notably, the combination of X8 and NEO can disrupt the monomicrobial and polymicrobial biofilms and show promising therapeutic potential against a murine wound infection model. Collectively, our results unveil the combination of X8 and NEO as a suitable adjuvant therapy for the inhibition of the Gram-negative bacterial infections.

Antibacterial activities of anthraquinones: structure-activity relationships and action mechanisms

With the increasing prevalence of untreatable infections caused by antibiotic-resistant bacteria, the discovery of new drugs from natural products has become a hot research topic. The antibacterial activity of anthraquinones widely distributed in traditional Chinese medicine has attracted much attention. Herein, the structure and activity relationships (SARs) of anthraquinones as bacteriostatic agents are reviewed and elucidated. The substituents of anthraquinone and its derivatives are closely related to their antibacterial activities. The stronger the polarity of anthraquinone substituents is, the more potent the antibacterial effects appear. The presence of hydroxyl groups is not necessary for the antibacterial activity of hydroxyanthraquinone derivatives. Substitution of di-isopentenyl groups can improve the antibacterial activity of anthraquinone derivatives. The rigid plane structure of anthraquinone lowers its water solubility and results in the reduced activity. Meanwhile, the antibacterial mechanisms of anthraquinone and its analogs are explored, mainly including biofilm formation inhibition, destruction of the cell wall, endotoxin inhibition, inhibition of nucleic acid and protein synthesis, and blockage of energy metabolism and other substances.

Antimicrobial activity of α-mangostin against Staphylococcus species from companion animals in vitro and therapeutic potential of α-mangostin in skin diseases caused by S. pseudintermedius

Antimicrobial resistance in Staphylococcus species from companion animals is becoming increasingly prevalent worldwide. S. pseudintermedius is a leading cause of skin infections in companion animals. α-mangostin (α-MG) exhibits various pharmacological activities, including antimicrobial activity against G (+) bacteria. This study investigated the antimicrobial activity of α-MG against clinical isolates of Staphylococcus species from companion animals and assessed the therapeutic potential of α-MG in skin diseases induced by S. pseudintermedius in a murine model. Furthermore, the action mechanisms of α-MG against S. pseudintermedius were investigated. α-MG exhibited antimicrobial activity against clinical isolates of five different Staphylococcus species from skin diseases of companion animals in vitro, but not G (-) bacteria. α-MG specifically interacted with the major histocompatibility complex II analogous protein (MAP) domain-containing protein located in the cytoplasmic membrane of S. pseudintermedius via hydroxyl groups at C-3 and C-6. Pretreatment of S. pseudintermedius with anti-MAP domain-containing protein polyclonal serum significantly reduced the antimicrobial activity of α-MG. The sub-minimum inhibitory concentration of α-MG differentially regulated 194 genes, especially metabolic pathway and virulence determinants, in S. pseudintermedius. α-MG in pluronic lecithin organogel significantly reduced the bacterial number, partially restored the epidermal barrier, and suppressed the expression of cytokine genes associated with pro-inflammatory, Th1, Th2, and Th17 in skin lesions induced by S. pseudintermedius in a murine model. Thus, α-MG is a potential therapeutic candidate for treating skin diseases caused by Staphylococcus species in companion animals.

Sideritis scardica Extracts Demonstrate Neuroprotective Activity against Aβ25-35 Toxicity

Alzheimer's disease (AD) is the most prevalent neurodegenerative condition, primarily affecting seniors. Despite the significant time and money spent over the past few decades, no therapy has been developed yet. In recent years, the research has focused on ameliorating the cytotoxic amyloid beta (Aβ) peptide aggregates and the increased elevated oxidative stress, two interconnected main AD hallmarks. Medicinal plants constitute a large pool for identifying bioactive compounds or mixtures with a therapeutic effect. Sideritis scardica (SS) has been previously characterized as neuroprotective toward AD. We investigated this ability of SS by generating eight distinct solvent fractions, which were chemically characterized and assessed for their antioxidant and neuroprotective potential. The majority of the fractions were rich in phenolics and flavonoids, and all except one showed significant antioxidant activity. Additionally, four SS extracts partly rescued the viability in Aβ_25-35-treated SH-SY5Y human neuroblastoma cells, with the initial aqueous extract being the most potent and demonstrating similar activity in retinoic-acid-differentiated cells as well. These extracts were rich in neuroprotective substances, such as apigenin, myricetin-3-galactoside, and ellagic acid. Our findings indicate that specific SS mixtures can benefit the pharmaceutical industry to develop herbal drugs and functional food products that may alleviate AD.

Biosynthetic Pathways of Dimeric Natural Products Containing Bisanthraquinone and Related Xanthones

Many dimeric natural products containing bisanthraquinone and related xanthones with diverse structures and versatile bioactivities have been isolated over the years. However, the complicated biosynthetic pathways of such natural products, which have remained elusive until recently, negatively impact their mass bioproduction and biosynthetic structural modification for drug discovery. In this concept, we summarize the recent progress in gene cluster mining and biosynthetic pathway elucidation of natural products containing bisanthraquinone and related xanthones. These pioneering works may pave the way for further biosynthetic pathway elucidation and structure modification of dimeric natural products through gene and protein engineering.

Plant-Derived Xanthones against Clostridial Enteric Infections

Intestinal bacterial infections are a major threat to human and animal health. In this study, we found plant-derived antibacterial xanthones, particularly α-mangostin (AMG) from the mangosteen peel, exhibiting extraordinary activities against Clostridium perfringens. Structure-activity relationship analysis showed that prenylation modulated the activity of xanthones. The efficacy of AMG (4, 8, 20 mg/kg body weight) was also demonstrated in the broiler chicken necrotic enteritis model infected with Clostridium perfringens. In the models (n = 6 per group), feed supplementation of AMG maintained the homeostasis of the gut microbiome by reducing the colonization of clostridia and promoting the integrity of intestinal barriers via the upregulation of mucin expression. These results suggest that plant-derived xanthones may be a potential alternative to antibiotics for treating clostridial enteric infections in the clinic.

Gelatin-based nanofiber membranes loaded with curcumin and borneol as a sustainable wound dressing

Infection is a huge obstacle to wound healing. Thus, to enhance the healing of infected wounds, wound dressings that permit the dual delivery of antimicrobials and antioxidants are highly desirable. In this study, a series of gelatin-based nanofiber membranes with different curcumin contents were fabricated via solution electrospinning. The obtained membranes were characterized in terms of their morphologies, in addition to their physical, mechanical, and in vitro properties. The results showed that the membranes maintained an integrated morphology, excellent water absorption capability, satisfactory mechanical properties, and a high dissolution rate of curcumin. The addition of curcumin and borneol conferred the membranes the ability to inhibit Staphylococcus aureus and eliminate free radicals. Furthermore, cytocompatibility testing using the L929 cell line confirmed the excellent biocompatibility of the membranes. These gelatin-based nanofiber membranes loaded with curcumin and borneol can therefore be considered as promising materials for dressing wounds. Moreover, the use of biodegradable polymers and environmentally sustainable production techniques in this system render it suitable for the commercial manufacture of composite membranes.