In vitro Apramycin Activity against multidrug-resistant Acinetobacter baumannii and Pseudomonas aeruginosa
Introduction
Acinetobacter baumannii and Pseudomonas aeruginosa are two prominent members of the ESKAPE pathogen group (Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacter species) for which emerging multidrug-resistance is of pressing concern (Calhoun et al., 2008, Rice, 2008). In addition to causing severe disease in hospitalized patients, A. baumannii and P. aeruginosa are also the most frequently isolated pathogens from combat-related injuries (Calhoun et al., 2008, Davis et al., 2005, Murray et al., 2011, Petersen et al., 2007). Unfortunately, treatment options for these pathogens are increasingly limited, and aminoglycosides in particular have become among the drugs of last resort (Michiels et al., 2016, Poole, 2005). However, clinically approved aminoglycosides have a narrow therapeutic index due to nephrotoxic and irreversible ototoxic side effects (Rybak and Ramkumar, 2007). Moreover, many Acinetobacter and Pseudomonas isolates are now also resistant to these aminoglycosides (Michiels et al., 2016, Vila et al., 1993).
Apramycin is a veterinary aminocyclitol aminoglycoside used to treat colibacillosis, salmonellosis and enteritis in farm animals (Bischoff et al., 2004, Livermore et al., 2011). Its structure, a bicyclic sugar moiety with a mono-substituted deoxystreptamine, is distinct from other aminoglycosides (Davies et al., 1965, O'Connor et al., 1976). This distinct structure may help account for two of its unique attributes. First, most aminoglycoside modifying enzymes that confer resistance to clinically-approved aminoglycosides do not inactivate apramycin (Davies and O'Connor, 1978, Miller et al., 1997, O'Connor et al., 1976, Ramirez and Tolmasky, 2010, Shaw et al., 1993). Second, apramycin appears to offer higher selectivity for bacterial over mitochrondrial ribosomes and, therefore, is presumably associated with fewer ototoxic and nephrotoxic side effects (Akiyoshi et al., 1976, Livermore et al., 2011, Matt et al., 2012, Perzynski et al., 1979). Therefore, based on these favorable characteristics, apramycin or apramycin analogues developed through future medicinal chemistry efforts may be worthy of consideration for repurposing as a human therapeutic. However, demonstration of a compelling activity spectrum against multidrug-resistant human clinical isolates is a prerequisite to justify further translational efforts.
Previous data from our lab and others have shown broad-spectrum apramycin activity against carbapenem-susceptible and -resistant Enterobacteriaceae (CRE) strains from the US and the UK (Livermore et al., 2011, Smith and Kirby, 2016a, Smith and Kirby, 2016b). However, there is sparse to no available data for contemporary human multidrug-resistant A. baumannii and P. aeruginosa isolates. Therefore, here we sought to investigate the in vitro activity spectrum of apramycin as compared to aminoglycosides approved for human clinical use in the United States. Testing was performed against a diverse strain set of multidrug-resistant (MDR), extensively drug-resistant (XDR), and pandrug-resistant (PDR) A. baumannii and P. aeruginosa clinical isolates, inclusive of strains isolated from US combat-related infections.
Section snippets
Bacterial strains and antimicrobials
Amikacin disulfate salt was from Sigma-Aldrich (St. Louis, MO, USA); apramycin sulfate and gentamicin sulfate were from Alfa Aesar (Tewksbury, MA, USA); and tobramycin sulfate was from Research Products International (Mt. Prospect, IL, USA).
Forty-four P. aeruginosa and 54 A. baumannii strains were obtained from the FDA-CDC Antimicrobial Resistance Isolate Bank (https://www.cdc.gov/drugresistance/resistance-bank/). Fifty additional A. baumannii strains, confirmed to be clonally-distinct based on
Results
The MIC50, MIC90, MIC range and percent susceptibility for tested aminoglycosides are listed in Table 1. The strain set was notable for a high degree of resistance to gentamicin tobramycin and amikacin, ranging from 57 to 95% for A. baumannii and 27–57% for P. aeruginosa. Amikacin was the most active of the aminoglycosides approved for human clinical use.
For apramycin, there are no established veterinary or clinical breakpoints for A. baumanii or P. aeruginosa. Therefore, apramycin percent
Discussion
Apramycin is currently used as an orally-dosed, non-absorbed veterinary antibiotic to treat diarrheal diseases in poultry and livestock (Livermore et al., 2011). It is also used as an injectable treatment for pneumonia in calves (Ziv et al., 1985), and mastitis in cows, goats and sheep (Ziv et al., 1995). Veterinary pharmacodynamic data are unavailable. Human studies appear not to have been performed.
As such, despite some understanding of pharmacokinetics in farm animals (Lashev et al., 1992,
Funding information
This work was supported by a Chief Academic Officer's Pilot Grant from Beth Israel Deaconess Medical Center. Anthony Kang is a recipient of the Long Term Health Education and Training Program from the US. Army Medical Department Center and School at Fort Sam Houston, TX.
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