In vitro activity of apramycin against multidrug-, carbapenem- and aminoglycoside-resistant Enterobacteriaceae and Acinetobacter baumannii

Abstract

Objectives: Widespread antimicrobial resistance often limits the availability of therapeutic options to only a few last-resort drugs that are themselves challenged by emerging resistance and adverse side effects. Apramycin, an aminoglycoside antibiotic, has a unique chemical structure that evades almost all resistance mechanisms including the RNA methyltransferases frequently encountered in carbapenemase-producing clinical isolates. This study evaluates the in vitro activity of apramycin against multidrug-, carbapenem- and aminoglycoside-resistant Enterobacteriaceae and Acinetobacter baumannii, and provides a rationale for its superior antibacterial activity in the presence of aminoglycoside resistance determinants.

Methods: A thorough antibacterial assessment of apramycin with 1232 clinical isolates from Europe, Asia, Africa and South America was performed by standard CLSI broth microdilution testing. WGS and susceptibility testing with an engineered panel of aminoglycoside resistance-conferring determinants were used to provide a mechanistic rationale for the breadth of apramycin activity.

Results: MIC distributions and MIC90 values demonstrated broad antibacterial activity of apramycin against Escherichia coli, Klebsiella pneumoniae, Enterobacter spp., Morganella morganii, Citrobacter freundii, Providencia spp., Proteus mirabilis, Serratia marcescens and A. baumannii. Genotypic analysis revealed the variety of aminoglycoside-modifying enzymes and rRNA methyltransferases that rendered a remarkable proportion of clinical isolates resistant to standard-of-care aminoglycosides, but not to apramycin. Screening a panel of engineered strains each with a single well-defined resistance mechanism further demonstrated a lack of cross-resistance to gentamicin, amikacin, tobramycin and plazomicin.

Conclusions: Its superior breadth of activity renders apramycin a promising drug candidate for the treatment of systemic Gram-negative infections that are resistant to treatment with other aminoglycoside antibiotics.

Figure 1.

Structural rationale for the activity of apramycin in the presence of aminoglycoside resistance determinants. The chemical structures of monosubstituted (a) versus disubstituted (b) deoxystreptamine antibiotics indicating the reactive groups that are modified by acetyltransferases (AACs), phosphotransferases (APHs) and nucleotidyltransferases (ANTs). Molecular modelling of the RMTase-catalysed N7-methylation of G1405 (red sphere) onto the crystal structure of ribosome-bound apramycin (PDB entry 4AQY) reveals no clash of the methyl group with the 4-monosubstituted 2-DOS apramycin (c). Molecular modelling of the G1405 methylation onto the crystal structure of ribosome-bound gentamicin (PDB entry 4V53) reveals considerable clash with ring III of 4,6-disubstituted 2-DOS (d). Nucleotides of the 16S rRNA are shown in pale yellow, and apramycin and gentamicin are shown in yellow and teal, respectively. The E. coli nucleotide numbering is used throughout.

Figure 2.

MIC distribution of apramycin in comparison with gentamicin and amikacin. (a) MIC distribution for Enterobacteriaceae clinical isolates of diverse geographic origin collected between 2014 and 2017 (left), and a subset of only carbapenemase-producing Enterobacteriaceae (CPE, right). (b) MIC distribution for Enterobacteriaceae and A. baumannii at the genus or species level. APR, apramycin; AMK, amikacin; GEN, gentamicin; CPE, carbapenemase-producing Enterobacteriaceae. This figure appears in colour in the online version of JAC and in black and white in the print version of JAC.