7-O-Carboxylic Acid-Substituted 3-O-Alkyl Difluoroquercetin; An Aztreonam-Potentiating Agent Against Carbapenemase-Producing Pseudomonas aeruginosa Through Simultaneous Inhibition of Metallo-β-Lactamase and Efflux Pump

<b>Background/Objectives:</b> Previously, we reported that 3-<i>O</i>-alkyl difluoroquercetins (di-F-Q) potentiates the antimicrobial activity of aztreonam (ATM) against metallo-β-lactamase (MBL)-producing <i>P. aeruginosa</i> through simultaneous inhibition of MBLs and efflux pumps. However, the ATM-potentiating activity of the 3-<i>O</i>-alkyl di-F-Q was observed only at high and potentially toxic concentrations (32 mg/L). <b>Methods:</b> 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-<i>O</i>-alkyl di-F-Q would enhance its periplasmic concentration to result in potentiation of ATM at lower concentrations. <b>Results:</b> The title compound <b>5</b> exhibited inhibitory activity against NDM-1 as well as the efflux pump of <i>P. aeruginosa</i>, which resulted in synergistical potentiation of ATM. A combination of ATM (8 mg/L) and <b>5</b> (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 <b>5</b> 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 <b>5</b>, and the combination of ATM (1xMIC) with 8 mg/L of <b>5</b> showed bactericidal effects in every bacterial strain tested (PA-002, <i>bla</i>_IMP, PA-003, <i>bla</i>_VIM, PA-014, <i>bla</i>_GES, and PA-017, <i>bla</i>_NDM) by reducing the bacterial loads by 5.1 log₁₀~8.9 log₁₀. <b>Conclusions:</b> The title compound <b>5</b> exhibited inhibitory activity against NDM-1 as well as the efflux pump of <i>P. aeruginosa</i>, 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 <b>5</b>.