Synthesis, Structure Elucidation, Antibacterial Activities, and Synergistic Effects of Novel Juglone and Naphthazarin Derivatives Against Clinical Methicillin-Resistant Staphylococcus aureus Strains

Abstract

Infections caused by drug-resistant bacteria are a serious threat to human and global public health. Moreover, in recent years, very few antibiotics have been discovered and developed by pharmaceutical companies. Therefore, there is an urgent need to discover and develop new antibacterial agents to combat multidrug-resistant bacteria. In this study, two novel series of juglone/naphthazarin derivatives (43 compounds) were synthesized and evaluated for their antibacterial properties against various clinical and reference Gram-positive MSSA, clinical Gram-positive MRSA, and clinical and reference Gram-negative bacteria <i>E. coli</i> and <i>P. aeruginosa</i>. These strains are of clinical importance because they belong to ESKAPE pathogens. Compounds <b>3al</b>, <b>5ag</b>, and <b>3bg</b> showed promising activity against clinical and reference MSSA (MIC: 1-8 µg/ml) and good efficacy against clinical MRSA (MIC: 2-8 µg/ml) strains. <b>5am</b> and <b>3bm</b> demonstrated better activity on both MSSA (MIC: 0.5 µg/ml) and MRSA (MIC: 2 µg/ml) strains. Their MICs were similar to those of cloxacillin against clinical MRSA strains. The synergistic effects of active compounds <b>3al</b>, <b>5ag</b>, <b>5am</b>, <b>3bg</b>, and <b>3bm</b> were evaluated with reference antibiotics, and it was found that the antibiotic combination with <b>3bm</b> efficiently enhanced the antimicrobial activity. Compound <b>3bm</b> was found to restore the sensitivity of clinical MRSA to cloxacillin and enhanced the antibacterial activity of vancomycin when they were added together. In the presence of <b>3bm</b>, the MIC values of vancomycin and cloxacillin were lowered up to 1/16th of the original MIC with an FIC index of 0.313. Moreover, compounds <b>3al</b>, <b>5ag</b>, <b>5am</b>, <b>3bg</b>, and <b>3bm</b> did not present hemolytic activity on sheep red blood cells. <i>In silico</i> prediction of ADME profile parameter results for <b>3bm</b> is promising and encouraging for further development.