First Identification of Boronic Species as Novel Potential Inhibitors of the Staphylococcus aureus NorA Efflux Pump

Abstract

Overexpression of efflux pumps is an important mechanism of bacterial resistance that results in the extrusion of antimicrobial agents outside the bacterial cell. Inhibition of such pumps appears to be a promising strategy that could restore the potency of existing antibiotics. The NorA efflux pump of Staphylococcus aureus confers resistance to a wide range of unrelated substrates, such as hydrophilic fluoroquinolones, leading to a multidrug-resistance phenotype. In this work, approximately 150 heterocyclic boronic species were evaluated for their activity against susceptible and resistant strains of S. aureus. Twenty-four hit compounds, although inactive when tested alone, were found to potentiate ciprofloxacin activity by a 4-fold increase at concentrations ranging from 0.5 to 8 μg/mL against S. aureus 1199B, which overexpresses NorA. Boron-free analogues showed no biological activity, thus revealing that the boron atom is crucial for biological activity. This work describes the first reported efflux pump inhibitory activity of boronic acid derivatives.

Searching for Novel Inhibitors of the S. aureus NorA Efflux Pump: Synthesis and Biological Evaluation of the 3‐Phenyl‐1,4‐benzothiazine Analogues

“…The lack of the crystal structure for NorA and the structural heterogeneity of known NorA EPIs– prompted us to pursue a classical medicinal chemistry approach entailing modifications of the benzothiazine nucleus aimed at the substitution of the sulfur atom with the goals being to: 1) resolve the chemical stability issues, 2) retain the EPI activity, and 3) contribute to better delineate the SAR of this class of NorA EPIs. Thus, the sulfur atom at position 1 of the 2 H ‐benzo[1,4]thiazine scaffold was substituted by oxygen (‐ O ‐), nitrogen (‐NH‐), or a methylene unit (‐CH 2 ‐) to obtain the corresponding 3‐phenyl‐2 H ‐benzo[1,4]oxazine ( 1 ), 2‐phenyl‐1,2,3,4‐tetrahydroquinoxaline ( 2 ), and 2‐phenyl‐1,2,3,4‐tetrahydroquinoline ( 3 ) nuclei, respectively.…”

Efflux pump inhibitors of clinically relevant multidrug resistant bacteria

“…As a consequence, there can be no structure‐based approach to the design of inhibitors of this transporter . Several EPIs have been reported to target NorA . However, these compounds have not entered the clinic because of the unwanted side effects observed, especially following long‐term administration.…”

“…However, they were still less effective than the positive control, reserpine . In addition, boronic acids are also considered as potent β‐lactamase inhibitors and possess a variety of other biological effects . Thus, selectivity for efflux pumps should be addressed in the future development of this class of EPIs.…”

“…84 Several EPIs have been reported to target NorA. [119][120][121][122][123] However, these compounds have not entered the clinic because of the unwanted side effects observed, especially following long-term administration. The large number of known inhibitors and their broad structural diversity reflect the low substrate specificity of NorA, similar to that shown for the mammalian multidrug transporter P-glycoprotein.…”

Novel cajaninstilbene acid derivatives as antibacterial agents

“…The active compounds (5b, 5c, 5j and 5k) were further examined for cytotoxicity in mouse macrophage cell lines (RAW 264.7) at concentration gradients [20]. The cells were cultured in DMEM medium supplemented with 10% fetal bovine serum and 2 mM Lglutamine and 100 units/mL penicillin/streptomycin.…”