Hanxun Zou

Rapid bactericidal action of alpha-mangostin against MRSA as an outcome of membrane targeting.

The emergence of methicillin-resistant Staphylococcus aureus (MRSA) has created the need for better therapeutic options. In this study, five natural xanthones were extracted and purified from the fruit hull of Garcinia mangostana and their antimicrobial properties were investigated. α-Mangostin was identified as the most potent among them against Gram-positive pathogens (MIC=0.78-1.56 μg/mL) which included two MRSA isolates. α-Mangostin also exhibited rapid in vitro bactericidal activity (3-log reduction within 5 min). In a multistep (20 passage) resistance selection study using a MRSA isolated from the eye, no resistance against α-mangostin in the strains tested was observed. Biophysical studies using fluorescence probes for membrane potential and permeability, calcein encapsulated large unilamellar vesicles and scanning electron microscopy showed that α-mangostin rapidly disrupted the integrity of the cytoplasmic membrane leading to loss of intracellular components in a concentration-dependent manner. Molecular dynamic simulations revealed that isoprenyl groups were important to reduce the free energy for the burial of the hydrophobic phenyl ring of α-mangostin into the lipid bilayer of the membrane resulting in membrane breakdown and increased permeability. Thus, we suggest that direct interactions of α-mangostin with the bacterial membrane are responsible for the rapid concentration-dependent membrane disruption and bactericidal action.

Amino acid modified xanthone derivatives: novel, highly promising membrane-active antimicrobials for multidrug-resistant Gram-positive bacterial infections.

Antibiotic resistance is a critical global health care crisis requiring urgent action to develop more effective antibiotics. Utilizing the hydrophobic scaffold of xanthone, we identified three components that mimicked the action of an antimicrobial cationic peptide to produce membrane-targeting antimicrobials. Compounds 5c and 6, which contain a hydrophobic xanthone core, lipophilic chains, and cationic amino acids, displayed very promising antimicrobial activity against multidrug-resistant Gram-positive bacteria, including MRSA and VRE, rapid time-kill, avoidance of antibiotic resistance, and low toxicity. The bacterial membrane selectivity of these molecules was comparable to that of several membrane-targeting antibiotics in clinical trials. 5c and 6 were effective in a mouse model of corneal infection by S. aureus and MRSA. Evidence is presented indicating that 5c and 6 target the negatively charged bacterial membrane via a combination of electrostatic and hydrophobic interactions. These results suggest that 5c and 6 have significant promise for combating life-threatening infections.

Nonpeptidic Amphiphilic Xanthone Derivatives: Structure–Activity Relationship and Membrane-Targeting Properties

We recently reported the bioinspired synthesis of a highly potent nonpeptidic xanthone, 2c (AM-0016), with potent antibacterial activity against MRSA. Herein, we report a thorough structure-activity relationship (SAR) analysis of a series of nonpeptidic amphiphilic xanthone derivatives in an attempt to identify more potent compounds with lower hemolytic activity and greater membrane selectivity. Forty-six amphiphilic xanthone derivatives were analyzed in this study and structurally classified into four groups based on spacer length, cationic moieties, lipophilic chains, and triarm functionalization. We evaluated and explored the effects of the structures on their membrane-targeting properties. The SAR analysis successfully identified 3a with potent MICs (1.56-3.125 μ/mL) and lower hemolytic activity (80.2 μg/mL for 3a versus 19.7 μg/mL for 2c). Compound 3a displayed a membrane selectivity of 25.7-50.4. Thus, 3a with improved HC50 value and promising selectivity could be used as a lead compound for further structural optimization for the treatment of MRSA infection.

A novel fragment based strategy for membrane active antimicrobials against MRSA

Membrane active antimicrobials are a promising new generation of antibiotics that hold the potential to avert antibiotic resistance. However, poor understanding of the action mechanism and the lack of general design principles have impeded their development. Here we extend the concept of fragment based drug design and propose a pharmacophore model based on first principles for the design of membrane active antimicrobials against Gram positive pathogens. Elaborating on a natural xanthone-based hydrophobic scaffold, two derivatives of the pharmacophore model are proposed, and these demonstrate excellent antimicrobial activity. Rigorous molecular dynamics simulations combined with biophysical experiments suggest a three-step mechanism of action (absorption-translocation-disruption) which allows us to identify key factors for the practical optimization of each fragment of the pharmacophore. Moreover, the model matches the structures of several membrane active antimicrobials which are currently in clinical trials. Our model provides a novel and rational approach for the design of bactericidal molecules that target the bacterial membrane.

Membrane-targeting AM-0016 kills mycobacterial persisters and shows low propensity for resistance development

AIM To test the hypothesis that targeting the cytoplasmic membrane may be an effective way to kill persister mycobacteria and delay the emergence of resistance. METHODS In vitro activity of AM-0016, a novel xanthone-based antibacterial, was assessed against growing and persister tubercle bacilli. Resistance mutation frequencies were determined. Biochemical membrane and electron microscopic analyses were carried out. RESULTS AM-0016 rapidly sterilized growing tubercle bacillus cultures and displayed strong bactericidal activity against persister bacteria. Spontaneous resistance mutation frequency was lower than 10(-8). Exposure to AM-0016 resulted in rapid collapse of the membrane potential. Imaging revealed deformation of the cell envelope. CONCLUSION Targeting the cytoplasmic membrane may be an attractive approach to eliminate persister mycobacteria and slow down the emergence of genetic drug resistance.

Symmetrically Substituted Xanthone Amphiphiles Combat Gram-Positive Bacterial Resistance with Enhanced Membrane Selectivity

This is the first report of the design of a new series of symmetric xanthone derivatives that mimic antimicrobial peptides using a total synthesis approach. This novel design is advantageous because of its low cost, synthetic simplicity and versatility, and easy tuning of amphiphilicity by controlling the incorporated cationic and hydrophobic moieties. Two water-soluble optimized compounds, 6 and 18, showed potent activities against Gram-positive bacteria, including MRSA and VRE (MICs = 0.78-6.25 μg/mL) with a rapid bactericidal effect, low toxicity, and no emergence of drug resistance. Both compounds demonstrated enhanced membrane selectivity that was higher than those of most membrane-active antimicrobials in clinical trials or previous reports. The compounds appear to kill bacteria by disrupting their membranes. Significantly, 6 was effective in vivo using a mouse model of corneal infection. These results provide compelling evidence that these compounds have therapeutic potential as novel antimicrobials for multidrug-resistant Gram-positive infections.