Small Molecule Potentiation of Gram-Positive Selective Antibiotics against Acinetobacter baumannii

Abstract

In 2016, the World Health Organization deemed antibiotic resistance one of the biggest threats to global health, food security, and development. The need for new methods to combat infections caused by antibiotic resistant pathogens will require a variety of approaches to identifying effective new therapeutic strategies. One approach is the identification of small molecule adjuvants that potentiate the activity of antibiotics of demonstrated utility, whose efficacy is abated by resistance, both acquired and intrinsic. To this end, we have identified compounds that enhance the efficacy of antibiotics normally ineffective against Gram-negative pathogens because of the outer membrane permeability barrier. We identified two adjuvant compounds that dramatically enhance sensitivity of Acinetobacter baumannii to macrolide and glycopeptide antibiotics, with reductions in minimum inhibitory concentrations as high as 256-fold, and we observed activity across a variety of clinical isolates. Mode of action studies indicate that these adjuvants likely work by modulating lipopolysaccharide synthesis or assembly. The adjuvants were active in vivo in a Galleria mellonella infection model, indicating potential for use in mammalian infections.

Keywords: macrolide antibiotics; antibiotic adjuvant; antibiotic resistance; glycopeptide antibiotics; lipopolysaccharide.

Figure 1.

Structures of compounds identified from initial screen for potentiation of macrolide activity against AB5075.

Figure 2.

Kaplan—Meier curve showing treatment of A. baumannii infection in G. mellonella model using combination therapy of compound 1 and clarithromycin. Results are an average of seven trials each containing ten larvae. Blue, A. baumannii only; Red, Clarithromycin at 25 mg/kg; Green, compound 1 at 100 mg/kg; Purple, Clarithromycin and compound 1 combination; Orange, Rifampin at 30 mg/kg.

Figure 3.

Structure of inactive analogue 3.

Figure 4.

Relationship of reduction in clarithromycin MIC and increase in membrane permeability between pentamidine and compounds 1 and 3.

Figure 5.

Qpantification of LPS fatty acid content in untreated vs compound 1 treated AB5075.

Figure 6.

LPS gel of AB5075 samples grown either in the absence or in the presence of compound 1. All lanes contain 20 μg of sample. A: protein ladder, B: AB5075 treated for 16 h with compound 1, C: 16 h treatment replicate, D: untreated control, E: untreated control replicate.