Identification of a 2-Aminobenzimidazole Scaffold that Potentiates Gram-Positive Selective Antibiotics Against Gram-Negative Bacteria

Abstract

The development of novel therapeutic approaches is crucial in the fight against multi-drug resistant (MDR) bacteria, particularly gram-negative species. Small molecule adjuvants that enhance the activity of otherwise gram-positive selective antibiotics against gram-negative bacteria have the potential to expand current treatment options. We have previously reported adjuvants based upon a 2-aminoimidazole (2-AI) scaffold that potentiate macrolide antibiotics against several gram-negative pathogens. Herein, we report the discovery and structure-activity relationship (SAR) investigation of an additional class of macrolide adjuvants based upon a 2-aminobenzimidazole (2-ABI) scaffold. The lead compound lowers the minimum inhibitory concentration (MIC) of clarithromycin (CLR) from 512 to 2 μg/mL at 30 μM against Klebsiella pneumoniae 2146, and from 32 to 2 μg/mL at 5 μM, against Acinetobacter baumannii 5075. Preliminary investigation into the mechanism of action suggests that the compounds are binding to lipopolysaccharide (LPS) in K. pneumoniae, and modulating lipooligosaccharide (LOS) biosynthesis, assembly, or transport in A. baumannii.

Keywords: 2-aminobenzimidazole; adjuvant; gram-negative; macrolide; multi-drug resistant.

Figure 1.

Structures of previously reported macrolide adjuvants with activity against gram-negative ESKAPE pathogens.^10-13

Figure 2:

Structure of initial hit compound 6 with rationale for screening of 2ABIs (blue dashed box). Structures of hits from screen of 2ABI containing small molecules, compounds 7 and 8.

Figure 3:

BODIPY-cadaverine LPS binding assay. (A) KP15380 LPS (10 μg/mL) or (B) AB5705 LPS (10 μg/mL) and BODIPY-cadaverine (10 μM). Untreated (dark blue), 2 μg/mL COL (med blue) or increasing concentrations of compounds 27 (light blue), 29 (light green). Error bars represent standard error.

Scheme 1:

Synthesis of 2-ABI analogues 7, 8, and 10-39. Reagents and conditions (a) BrCN, H₂O/CH₃CN, reflux, 4 h.; (b) Boc₂O, DMAP, THF, 25 °C, 18 h.; (c) H₂, 5% Pd/C, MeOH, 25 °C, 16 h.; (d) ArCOCl, TEA, DCM, 25 °C, 16 h.; (e) ArCOOH, EDC, DMAP, 25 °C, 16 h.; (f): TFA, DCM, 25 °C, 3 h, then 6 M HCl, MeOH, 25 °C, 2 min.