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Background/Objectives: Previously, we reported that 3-O-alkyl difluoroquercetins (di-F-Q) potentiates the antimicrobial activity of aztreonam (ATM) against metallo-β-lactamase (MBL)-producing P. aeruginosa through simultaneous inhibition of MBLs and efflux pumps. However, the ATM-potentiating activity of the 3-O-alkyl di-F-Q was observed only at high and potentially toxic concentrations (32 mg/L). Methods: As both MBLs and efflux pumps reside in the periplasm of Gram-negative bacteria, their inhibitors should accumulate in the periplasmic space. However, the outer membrane porins, the major entry pathway in Gram-negative bacteria, allow the passive diffusion of hydrophilic polar molecules across the outer membrane. Thus, we reasoned that the introduction of a polar substituent at 7-OH position of 3-O-alkyl di-F-Q would enhance its periplasmic concentration to result in potentiation of ATM at lower concentrations. Results: The title compound 5 exhibited inhibitory activity against NDM-1 as well as the efflux pump of P. aeruginosa, which resulted in synergistical potentiation of ATM. A combination of ATM (8 mg/L) and 5 (8 mg/L) inhibited 80% of the ATM-resistant CPPA, while ATM alone did not show any inhibition. In addition, only 4 mg/L of 5 was needed to reduce the MIC90 of ATM four-fold in ATM-resistant CPPA (n = 15). The time–kill data further supported the effectiveness of the combined treatment of ATM with 5, and the combination of ATM (1xMIC) with 8 mg/L of 5 showed bactericidal effects in every bacterial strain tested (PA-002, blaIMP, PA-003, blaVIM, PA-014, blaGES, and PA-017, blaNDM) by reducing the bacterial loads by 5.1 log10~8.9 log10. Conclusions: The title compound 5 exhibited inhibitory activity against NDM-1 as well as the efflux pump of P. aeruginosa, and the combined inhibitory activity resulted in synergistical potentiation of ATM. It should be noted that most CPPA isolates tested were sensitized to 8 mg/L of ATM upon combination with 4~8 mg/L of 5. [ABSTRACT FROM AUTHOR]

The combination of aztreonam (ATM) and ceftazidime–avibactam (CAZ-AVI; CZA) has shown therapeutic potential against serine-β-lactamase (SBL)- and metallo-β-lactamase (MBL)-producing Enterobacterales. However, the ability of CZA to restore the antibiotic activity of ATM is severely limited in MBL-producing multidrug-resistant (MDR) Pseudomonas aeruginosa strains because of the myriad of intrinsic and acquired resistance mechanisms associated with this pathogen. We reasoned that the simultaneous inhibition of multiple targets associated with multidrug resistance mechanisms may potentiate the antibiotic activity of ATM against MBL-producing P. aeruginosa. During a search for the multitarget inhibitors through a molecular docking study, we discovered that di-F-Q, the previously reported efflux pump inhibitor of MDR P. aeruginosa, binds to the active sites of the efflux pump (MexB), as well as various β-lactamases, and these sites are open to the 3-O-position of di-F-Q. The 3-O-substituted di-F-Q derivatives were thus synthesized and showed hereto unknown multitarget MDR inhibitory activity against various ATM-hydrolyzing β-lactamases (AmpC, KPC, and New Delhi metallo-β-lactamase (NDM)) and the efflux pump of P. aeruginosa, presumably by forming additional hydrophobic contacts with the targets. The multitarget MDR inhibitor 27 effectively potentiated the antimicrobial activity of ATM and reduced the MIC of ATM more than four-fold in 19 out of 21 MBL-producing P. aeruginosa clinical strains, including the NDM-producing strains which were highly resistant to various combinations of ATM with β-lactamase inhibitors and/or efflux pump inhibitors. Our findings suggest that the simultaneous inhibition of multiple MDR targets might provide new avenues for the discovery of safe and efficient MDR reversal agents which can be used in combination with ATM against MBL-producing MDR P. aeruginosa. [ABSTRACT FROM AUTHOR]