Thiazole-Based Heterocyclic Derivative 2-(4-chlorophenyl)-4-(1H-pyrazol-3-yl)thiazole as Inhibitor of Bacterial Efflux Pumps in Multidrug-Resistant Gram-Negative Pathogens

The growing levels of multidrug-resistant (MDR) Gram-negative pathogens are a severe threat to human health. Among the circumvention mechanisms that are pronounced, the resistance-nodulation-division (RND) superfamily of efflux pumps represents the AcrAB-TolC axis, which is the most potent axis of antibiotic expulsion. Efflux pump inhibitors (EPIs) have become appealing supplements which can re-sensitize recalcitrant bacterial isolates to older antimicrobials. This study aimed to synthesize and characterize a novel thiazole-pyrazole hybrid (H1),  2-(4-chlorophenyl)-4-(1H-pyrazol-3-yl)thiazole, and evaluate its potential as an efflux pump inhibitor against MDR Escherichia coli and Pseudomonas aeruginosa. Compound H1 was made using a Hantzsch condensation and characterized with H NMR, ¹³C NMR, FTIR, and ESI-MS. Antibacterial potency was measured using the method of broth micro-dilution as per to CLSI recommendations. Efflux inhibition was determined by ethidium -bromide accumulation assays, assessing the increase of fluorescence during 30 min and the synergistic efflux inhibition with the fluoroquinolones was quantified by checkerboard analysis and indices of the fractional inhibitory concentration (FICI) were obtained. Even though H1 did not exhibited any intrinsic bactericidal activity (MIC > 256 µg/mL ) it produced a 2.3 fold increase of EtBr retention at 50 mg/mL which was similar to the standard inhibitor PAβN. Simultaneous use with levofloxacin or ciprofloxacin reduced MICs by eightfold in MDR isolates (128→16 μg/mL and 256→32 μg/mL, respectively) and the FICI values ranged between 0.312 and 0.406 indicating strong synergy. Our data place the thiazole-pyrazole scaffold as a promising lead to future development of EPI, and there is the potential to revitalise the therapeutic applications of currently existing antibiotics to multidrug-resistant gram negative infections.