C-2 phenyl replacements to obtain potent quinoline-based Staphylococcus aureus NorA inhibitors

Abstract

NorA is the most studied efflux pump of Staphylococcus aureus and is responsible for high level resistance towards fluoroquinolone drugs. Although along the years many NorA efflux pump inhibitors (EPIs) have been reported, poor information is available about structure-activity relationship (SAR) around their nuclei and reliability of data supported by robust assays proving NorA inhibition. In this regard, we focussed efforts on the 2-phenylquinoline as a promising chemotype to develop potent NorA EPIs. Herein, we report SAR studies about the introduction of different aryl moieties on the quinoline C-2 position. The new derivative 37a showed an improved EPI activity (16-fold) with respect to the starting hit 1. Moreover, compound 37a exhibited a high potential in time-kill curves when combined with ciprofloxacin against SA-1199B (norA+). Also, 37a exhibited poor non-specific effect on bacterial membrane polarisation and showed an improvement in terms of "selectivity index" in comparison to 1.

Keywords: Antimicrobial resistance breakers; NorA; Staphylococcus aureus; antimicrobial resistance; efflux pump inhibitors.

Figure 1.

Known SAR around the 2-phenylquinoline scaffold and new designed compounds.

Scheme 1.

(i) SOCl₂, 60 °C, 30 min; (ii) Et₃N, dry THF, rt, 90 min-6 h, 50–99%; (iii) NaOH, dry dioxane, 110 °C, 2–8 h, 57–100%; (iv) chloroalkylamine hydrochloride, K₂CO₃, dry DMF, 80 °C, 2–12 h, 17–69%.

Figure 2.

Chequerboard assays of compounds 30a, 30b, 35b, 36b, 37a and reference compounds 1 and 2 in combination with CPX against SA-1199B.

Figure 3.

Time-kill curves of CPX and combination of compound 37a with different concentrations of CPX against SA-1199B.

Figure 4.

Haemolysis assays of compounds 37a and starting hit 1.

Figure 5.

Membrane polarisation assays of compounds 1 and 37a against SA-1199B at three different concentrations (1, 5 and 10 µg/mL) using the BacLight Bacterial Membrane Potential Kit. CCCP was used as positive control at 1 µg/mL (5 µM). % of membrane polarisation was calculated from the red/green fluorescence ratio by comparing bacterial cells in the presence of compounds with untreated cells.

Figure 6.

Predicted reactivity for compounds 1 (A), 2 (B) and 37a (C). The atoms are colour-coded based on their predicted reactivity (red: high reactivity; blue: low reactivity). The blue sphere highlights the most probable site of metabolism.