Application of tobramycin benzyl ether as an antibiotic adjuvant capable of sensitizing multidrug-resistant Gram-negative bacteria to rifampicin

Abstract

The emergence of aminoglycoside resistance has prompted the development of amphiphilic aminoglycoside derivatives which target bacterial membranes. Tobramycin and nebramine ether derivatives initially designed for this purpose were optimized and screened for their potential application as outer membrane (OM) permeabilizing adjuvants. Structure-activity relationship (SAR) studies revealed that the tobramycin benzyl ether was the most optimal OM permeabilizer, capable of potentiating rifampicin, novobiocin, vancomycin, minocycline, and doxycycline against Gram-negative bacteria. The innovative use of this compound as an adjuvant is highlighted by its ability to sensitize multidrug-resistant (MDR) Gram-negative bacteria to rifampicin and restore the susceptibility of MDR Escherichia coli to minocycline.

Fig. 1. Structures of tobramycin and nebramine derivatives.

Fig. 2. Synergy of rifampicin with compounds 1–10 against P. aeruginosa PAO1, E. coli ATCC 25922 and A. baumannii ATCC 17978. The checkerboard assays were performed in duplicates. Fold potentiation was determined at 8 μg mL⁻¹ of the adjuvant, with the exception of compound 2 at 1 μg mL⁻¹ against E. coli and 4 μg mL⁻¹. Synergistic interactions are characterized by ≥4-fold potentiation, indicated by a dashed line.

Fig. 3. Synergy of 8 μg mL⁻¹ of compound 1 with novobiocin, vancomycin, minocycline, and doxycycline against P. aeruginosa PAO1, E. coli ATCC 25922 and A. baumannii ATCC 17978. The checkerboard assays were performed in duplicates. Synergistic interactions are characterized by ≥4-fold potentiation, indicated by a dashed line.

Fig. 4. Potentiation of rifampicin by compound 1 against A. baumannii LAC-4. Dark colours represent higher cell density. Optical density (OD) was measured at 590 nm.

Fig. 5. Measurement of OM permeabilization induced by compound 1 with PMBN as a positive control in E. coli ATCC 25922 cells.

Fig. 6. Fold potentiation of rifampicin by 8 μg mL⁻¹ of compound 1 against P. aeruginosa PAO1, E. coli ATCC 25922 and A. baumannii ATCC 17978 in the absence and presence of 20 mM Mg²⁺ or 150 mM Ca⁺ in CAMHB. The checkerboard assays were performed in duplicates. Synergistic interactions are characterized by a potentiation of ≥4-fold, indicated by a dashed line.

Fig. 7. Fold potentiation of rifampicin at 8–11 μg mL⁻¹ of compound 1 against P. aeruginosa PAO1, E. coli ATCC 25922, and A. baumannii ATCC 17978 in the absence and presence of 10–50% FBS. The checkerboard assays were performed in duplicates. Synergistic interactions are characterized by a potentiation of ≥4-fold, indicated by a dashed line. ND: not determined.