The combination of colistin and fosfomycin is synergistic against NDM-1-producing Enterobacteriaceae in in vitro pharmacokinetic/pharmacodynamic model experiments

doi:10.1016/j.ijantimicag.2015.07.019

International Journal of Antimicrobial Agents

Volume 46, Issue 5, November 2015, Pages 560-567

https://doi.org/10.1016/j.ijantimicag.2015.07.019 Get rights and content

Highlights

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We studied the interaction of colistin and fosfomycin against NDM-1-producing Enterobacteriaceae.
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We used both time–kill curve studies and in vitro pharmacodynamic model experiments.
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Combination of colistin and fosfomycin showed antimicrobial additivity and synergy.
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Combination of colistin and fosfomycin reduced the chance of emergence of resistance.
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There was no evidence of antagonism even against fosfomycin-resistant isolates.

Abstract

The chemotherapeutic options against NDM-1-producing Enterobacteriaceae infections are limited and therefore combination therapy is gaining momentum to counter the secondary resistance and potential suboptimal efficacy of monotherapy. Colistin and fosfomycin are two separate classes of antimicrobial agents that act on bacterial cells by different mechanisms. Hence, there is a potential for both synergy and antagonism. In this study, the antibacterial effects (ABEs) of colistin and fosfomycin were systematically investigated by time–kill curve studies over 48 h as well as in an in vitro pharmacokinetic model over 96 h against six well characterised strains of NDM-1-producing Enterobacteriaceae (three isolates resistant and three susceptible to fosfomycin) at a standard inoculum of 10⁶ CFU/mL. Clinically achievable free serum concentrations of colistin sulphate and fosfomycin were used. In a single-chamber in vitro model, peak/trough concentrations (C_max/C_min) and the half-life (t_1/2) for fosfomycin (250/40 mg/L and 2.7 h, respectively) and colistin sulphate (3.0/0.75 mg/L and 4 h, respectively) were used, along with a growth control. ABEs were measured by the decrease in viable bacterial counts (log kill), area under the bacterial kill curve (AUBKC) and population analysis profile (PAP). The combination of colistin and fosfomycin compared with either agent alone achieved increased bacterial killing and decreased the chance of emergence of resistance. Also, the ABEs of the combination were sustained for a longer duration and were evident both against fosfomycin-sensitive and -resistant strains. This study provides important information and support for the role of combination therapy against multidrug-resistant Gram-negative bacteria with limited therapeutic options.

Introduction

Antibiotic resistance has emerged as a major global health problem in the last decade [1]. The emergence of carbapenem-resistant Gram-negative bacteria, especially carbapenem-resistant Enterobacteriaceae, is a major concern [2]. At the same time, there is a shortage in terms of development of newer antibiotics against multidrug-resistant (MDR) bacteria [3], [4]. This has led to the re-emergence of so-called ‘older’ antibiotics such as polymyxins and fosfomycin [5], [6]. Excessive use of these agents in the treatment of carbapenem-resistant Gram-negative bacteria, especially of colistin, has inevitably led to secondary resistance and the emergence of pan-resistant bacterial isolates [7].

New Delhi metallo-β-lactamase (NDM) was first described in a clinical case as recently as in 2008, although subsequent retrospective analyses of stored cultures have identified the gene encoding bla_NDM in Enterobacteriaceae isolates from 2006 [8].

Since then, NDM carbapenemase-producing isolates have been reported from more than 40 countries around the world. Isolates carrying bla_NDM tend to carry other resistance-encoding genes and, therefore, chemotherapeutic options in the treatment of infections caused by NDM-producing isolates are very limited [9]. Combination therapy is becoming a popular strategy to combat the emerging resistance, with the additional benefits of antimicrobial synergy and breadth of antimicrobial spectrum [10]. Although antimicrobial combination therapy has been widely used in clinical practice for many decades in a variety of clinical scenarios, such as neutropenic sepsis, infective endocarditis and infections due to Pseudomonas aeruginosa, robust evidence is lacking especially when β-lactam agents are not part of the combination therapy regimen [11].

Colistin and fosfomycin have been used therapeutically for many decades despite limited data on their pharmacokinetics and pharmacodynamics until recently [12], [13]. Both agents are bactericidal with different mechanisms of action at separate bacterial targets, with colistin being active against the bacterial cell membrane and fosfomycin against the bacterial cell wall. Data on interaction between these two antimicrobial agents against NDM-1-producing Enterobacteriaceae are very limited.

The aim of this study was to evaluate the combination of colistin and fosfomycin against NDM-1-producing Enterobacteriaceae using a range of isolates with varying susceptibility to fosfomycin. The main focus of this study was to assess the antibacterial activity of single agents versus the combination as well as to assess the risk of emergence of secondary resistance by deploying clinically relevant dosage regimens of colistin and fosfomycin.

Section snippets

Bacterial isolates

Six well characterised NDM-1-producing Enterobacteriaceae isolates (kindly donated by Prof. T.R. Walsh, Cardiff University, Cardiff, UK) were used. All six isolates were susceptible to colistin with a minimum inhibitory concentration (MIC) of <0.5 mg/L, and three of the six isolates were resistant to fosfomycin. MICs were determined by the Clinical and Laboratory Standards Institute (CSLI) broth microdilution method [14] using Escherichia coli ATCC 29212 and P. aeruginosa ATCC 27853 as controls.

Culture media

Pharmacokinetic validation

The results of the pre-experimental PK validation studies are shown in Fig. 1. For both CST and FOF, consistent levels very close to the target values (with <5% variability) were achieved.

Pharmacokinetic/pharmacodynamic profile

PK profiles of CST and FOF in the PK/PD model are shown in Table 1.

Bacterial isolates

The identity and MIC of the isolates are shown in Table 2. All isolates were susceptible to colistin according to European Committee on Antimicrobial Susceptibility Testing (EUCAST) criteria (http://www.EUCAST.org), and three of the six

Discussion

In this study, we evaluated the interactions of CST and FOF in terms of ABE, synergy/additivity, potential antagonism and emergence of secondary resistance. The concentrations of CST and FOF were carefully chosen to reflect clinically achievable serum free drug concentrations at routine dosages.

This study shows that addition of FOF to CST resulted in additivity at most concentrations, and synergy in some. These ABEs were more prominent against FOF-sensitive strains. The combination achieved

Acknowledgments

The authors acknowledge Sharon Tomaselli for assistance in carrying out the laboratory experiments, and Prof. T.R. Walsh (Cardiff University, Cardiff, UK) for providing the six NDM-1-producing Enterobacteriaceae bacterial strains.

Funding: This research was supported by The Showering Fund (Reg. charity No. 200017 UK), which supports research in the Department of Pathology, North Bristol NHS Trust (Bristol, UK) [Grant No. SF 99].

Competing interests: None declared.

Ethical approval: Not required.

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The combination of colistin and fosfomycin is synergistic against NDM-1-producing Enterobacteriaceae in in vitro pharmacokinetic/pharmacodynamic model experiments

Highlights

Abstract

Introduction

Section snippets

Bacterial isolates

Culture media

Pharmacokinetic validation

Pharmacokinetic/pharmacodynamic profile

Bacterial isolates

Discussion

Acknowledgments

Int J Antimicrob Agents

Lancet Infect Dis

Int J Antimicrob Agents

Int J Antimicrob Agents

Int J Antimicrob Agents

Int J Antimicrob Agents

Diagn Microbiol Infect Dis

Policy package to combat antimicrobial resistance

Vital signs: carbapenem-resistant Enterobacteriaceae

MMWR Morb Mortal Wkly Rep

Challenges in anti-infective development in the era of bad bugs, no drugs: a regulatory perspective using the example of bloodstream infection as an indication

Clin Infect Dis

Colistin in the 21st century

Curr Opin Infect Dis

Characterization of a new metallo-β-lactamase gene, blaNDM-1, and a novel erythromycin esterase gene carried on a unique genetic structure in Klebsiella pneumoniae sequence type 14 from India

Antimicrob Agents Chemother

Antibiotic treatment of infections due to carbapenem-resistant Enterobacteriaceae: systematic evaluation of the available evidence

Antimicrob Agents Chemother

Characterization of a new metallo-β-lactamase gene, bla_NDM-1, and a novel erythromycin esterase gene carried on a unique genetic structure in Klebsiella pneumoniae sequence type 14 from India