Design, synthesis and antibacterial evaluation of amphiphilic 1,2,4-oxadiazole-based pyridinium salts

Antimicrobial resistance (AMR) is a growing global health concern that compromises the effectiveness of current antibacterial therapies and necessitates the development of new active agents. In this study, a series of 37 novel 1,2,4-oxadiazole-based pyridinium salts was designed, synthesized, and characterized. Structural variations were introduced to the positions 3- and 5- of the oxadiazole core, exploring aliphatic chains with variable length and differently substituted aromatic moieties to module lipophilicity and electronic properties. The compounds synthesized were evaluated in vitro against Gram-positive and Gram-negative reference strains, as well as against multidrug-resistant (MDR) clinical isolates. Overall, the derivatives showed a selective activity against Gram-positive bacteria, with several compounds exhibiting significant antibacterial activity. Notably, selected derivatives displayed minimum inhibitory concentration (MIC) values in the range of 2-16 μM (1-8 μg/mL), with compound 18b emerging as the most potent analogue MIC = 1 μM (0.5 μg/mL) against Staphylococcus aureus multidrug resistant strain. Structure-activity relationship (SAR) analysis revealed that antibacterial activity is strongly influenced by the nature of the aromatic substituent. Electron-donating groups, such as methoxy (-OCH3), enhanced activity, whereas electron-withdrawing substituents (i.e., -NO2 and -F) reduced potency. In contrast, amide-containing derivatives and aliphatic analogues were inactive against resistant strains. The most active compounds exhibited low cytotoxicity in HepG2 and HDnF cell lines, together with negligible haemolytic activity at concentrations up to 25 μM. Favourable selectivity indices (SI = 1.6-33.65) were observed, with compound 21b displaying the best safety profile. Overall, these findings identify 1,2,4-oxadiazole-based pyridinium salts as promising scaffolds and highlight the key role of amphiphilicity and electronic modulation in the development of effective antibacterial agents against resistant pathogens.