Synthesis and biological evaluation of indole-based 2-Aryl-2,3-epoxy-1,4-naphthoquinones as methicillin-resistant staphylococcus aureus (MRSA) inhibitors

Abstract

Background: Methicillin-resistant Staphylococcus aureus (MRSA) is one of the most successful human pathogens responsible for causing a wide range of infections ranging from Toxic shock syndrome to boils and endocarditis. MRSA has demonstrated extreme ability to resist antibiotics, thus necessitating a continuous search for new scaffolds active against it. Fosfomycin, a clinically utilized antibiotic for the treatment of MRSA infections, is known to act through covalent modification of biological thiols with its epoxide functionality. 2,3 epoxy-1,4-naphthoquinones are found to be constituents of numerous natural products. These compounds are found to be reactive with cellular thiols and generate reactive oxygen species (ROS), which can act to induce oxidative stress. Thiols, such as cysteine and glutathione are important for the maintenance of redox homeostasis in cells and hence, depletion of thiols could induce cellular stress and trigger cell death. Thus, such compounds might have therapeutic potential against MRSA infections.Methods & Materials: We synthesized a library of indole-based 2,3 epoxy-1,4-naphthoquinones, varying the substitution on the epoxide carbons, and studied their reactivity with thiols, such as cysteine and glutathione. The synthesis of these compounds was undertaken in five steps from commercially available indole-3-carboxaldehyde.Results: These compounds showed potent activity against drug-resistant clinical isolates of S. aureus including VRSA with MIC ranging from 0.06-0.12 mg/L. A good correlation was found between thiol-mediated decomposition profiles and MIC values of these compounds. The selectivity index of these compounds was >200, thus indicating specificity for bacterial cells and were found to be non-haemolytic against human RBCs. These compounds exhibited concentration dependent bactericidal activity with a ?5 log killing at 24 h in comparison to drug free control. These compounds exhibited a potent post-antibiotic effect when compared to Vancomycin. In order to elucidate their mechanism of action, resistant mutants with a MIC of 64 mg/L were generated (frequency ?10-7) and are being characterized at the molecular level to decipher mechanism of action of these compounds.Conclusion: A series of 2-Aryl indole-based 2,3-epoxy-1,4-naphthoquinones have been synthesized with potent anti-MRSA activity. These compounds potentially deplete thiols, thus enhancing ROS in bacteria, which might help in overcoming drug resistance.