Synthesis and Structure–Activity Relationship (SAR) Studies on New 4-Aminoquinoline-Hydrazones and Isatin Hybrids as Promising Antibacterial Agents

In response to the escalating crisis of antimicrobial resistance (AMR), there is an urgent need to research and develop novel antibiotics. This study presents the synthesis and assessment of innovative 4-aminoquinoline-benzohydrazide-based molecular hybrids bearing aryl aldehydes (<b>HD1-23</b>) and substituted isatin warheads (<b>HS1-12</b>), characterized using multispectroscopic techniques with high purity confirmed by HRMS. The compounds were evaluated against a panel of clinically relevant antibacterial strains including the Gram-positive <i>Enterococcus faecium</i>, <i>Bacillus subtilis</i>, and <i>Staphylococcus aureus</i> and a Gram-negative <i>Pseudomonas aeruginosa</i> bacterial strain. Preliminary screenings revealed that several test compounds had significant antimicrobial effects, with <b>HD6</b> standing out as a promising compound. Additionally, <b>HD6</b> demonstrated impressively low minimum inhibitory concentrations (MICs) in the range of (8-128 μg/mL) against the strains <i>B. subtilis, S. aureus</i> and <i>P. aeruginosa</i>. Upon further confirmation, <b>HD6</b> not only showed bactericidal properties with low minimum bactericidal concentrations (MBCs) such as (8 μg/mL against <i>B. subtilis</i>) but also displayed a synergistic effect when combined with the standard drug ciprofloxacin (CIP), highlighted by its FICI value of (0.375) against <i>P. aeruginosa</i>, while posing low toxicity risk. Remarkably, <b>HD6</b> also inhibited a multidrug-resistant (MDR) bacterial strain, marking it as a critical addition to our antimicrobial arsenal. Computation studies were performed to investigate the possible mechanism of action of the most potent hybrid <b>HD6</b> on biofilm-causing protein (PDB ID: 7C7U). The findings suggested that <b>HD6</b> exhibits favorable binding free energy, which is supported by the MD simulation studies, presumably responsible for the bacterial growth inhibition. Overall, this study provides a suitable core for further synthetic alterations for their optimization as an antibacterial agent.