Repurposing niclosamide for intestinal decolonization of vancomycin-resistant enterococci
Introduction
Vancomycin-resistant enterococci (VRE), particularly species of Enterococcus faecium and Enterococcus faecalis, are a leading source of healthcare-associated infections globally [1]. Patients hospitalized for extended periods are often susceptible to exposure and colonization by enterococci that can subsequently lead to bloodstream infections and urinary tract infections [2], [3]. VRE infections, in particular, are often resistant to treatment with numerous antibiotics, and thus are associated with a higher rate of mortality than infections caused by vancomycin-sensitive strains [4], [5], [6]. As highlighted in a recent study by Jiang et al, the mortality rate of patients in a Taiwanese hospital infected with VRE was more than 64%, in contrast to 39.4% for patients infected with vancomycin-sensitive enterococci [7]. Moreover, the cost to treat patients infected with VRE nearly doubled in comparison with antibiotic treatment administered to patients infected with vancomycin-sensitive enterococci [7]. Therefore, suppressing VRE colonization in patients susceptible to infection represents a promising healthcare strategy.
Decolonization of VRE from the gastrointestinal tract is critical in certain patient populations, including those undergoing solid organ (particularly liver) transplants [8] and patients in intensive care units [9], as VRE colonization has been linked to a higher risk for infection and death [9], [10]. However, decolonization of VRE is challenging. Antibiotics traditionally used to treat VRE infections are often ineffective as decolonizing agents, partly due to rapid absorption from the gastrointestinal tract (linezolid) [11], [12]. Exceptions include the clinical molecule ramoplanin and the antibiotic bacitracin which are not absorbed from the gastrointestinal tract after oral administration [13]. Ramoplanin temporarily reduced colonization of VRE in multiple studies [9], [14] and in nearly 90% of patients in a phase II clinical trial [12]. Similarly, bacitracin has been shown to be capable of temporarily reducing colonization of VRE from the gastrointestinal tract of patients in multiple reports [15], [16]. However, VRE regrowth was observed in the gastrointestinal tract of patients after cessation of treatment with both ramoplanin and bacitracin. Furthermore, resistance to bacitracin was observed in patients where this antibiotic was administered alone to decolonize VRE from the gastrointestinal tract [9]. This highlights the challenges to using agents such as bacitracin and ramoplanin alone as decolonizing agents. An alternative approach to decolonization of VRE that has been investigated recently involves the use of faecal microbiota transplantation (FMT) [17], [18]. Although this approach successfully eradicated carriage of VRE in one human patient, it took 3 months to achieve this effect [18]. A second patient died 3 months after receiving FMT due to VRE infection, which suggests that FMT alone may not be sufficient to decolonize VRE from the gastrointestinal tract of certain patient populations [18]. The limited sample size (two patients) presented in Davido et al's study suggests that further investigation is needed to evaluate if FMT can be used as an alternative to decolonizing agents to curb VRE infection. This necessitates the continued search for novel agents that can successfully decolonize VRE from the gastrointestinal tract.
Recently, the salicylanilide drug niclosamide was found to possess antibacterial activity against important Gram-positive bacterial pathogens, including vancomycin-resistant E. faecium [19], [20]. Niclosamide has been used clinically to treat tapeworm infections present in the gastrointestinal tract of humans, and has an excellent safety profile [21]. The anthelmintic drug has limited aqueous solubility, is poorly absorbed from the gastrointestinal tract, and is rapidly metabolized by the liver after reaching the systemic circulation [21]. While these features represent significant drawbacks to using niclosamide for treatment of systemic infections, these characteristics are highly advantageous for a decolonizing agent to reduce VRE carriage in the gastrointestinal tract. Thus, the main objective of this study was to examine the potential for niclosamide to be successfully repurposed to decolonize VRE from the gastrointestinal tract.
Section snippets
Media, strains and reagents
Clinical isolates of vancomycin-resistant E. faecium and E. faecalis (Table 1) were obtained from the Biodefense and Emerging Infections Research Resources Repository and American Type Culture Collection. Ampicillin (IBI Scientific, Peosta, IA, USA), closantel (Ark Pharm, Libertyville, IL, USA), gentamicin sulphate (Fisher Bioreagents, Fairlawn, NJ, USA), linezolid (Chem-Impex International Inc., Wood Dale, IL, USA), niclosamide (Cayman Chemicals, Ann Arbor, MI, USA), oxyclozanide
Susceptibility analysis of niclosamide and other salicylanilide drugs against clinical isolates of VRE
As highlighted above, Rajamuthiah et al were the first group to identify that niclosamide possesses antibacterial activity against VRE [20]. However, their study only screened niclosamide against a single strain of vancomycin-resistant E. faecium (strain E007, MIC = 0.25 µg/mL) [20]. As such, this study evaluated niclosamide against a wider panel of VRE clinical isolates (Table 1) using the broth microdilution assay. As presented in Table 2, niclosamide inhibited growth of 15 clinical isolates
Discussion
VRE infections represent a significant challenge to healthcare providers given the limited number of effective antibiotics available to clinicians. VRE is responsible for 30% of all enterococcus healthcare-associated infections (20 000 cases annually), and more than 5% of all deaths attributed to an antibiotic-resistant infection in the USA [28]. In addition to the USA, VRE infections have been identified in patients worldwide, including in Europe, Latin America and Asia [3]. Nearly 80% of all
Acknowledgments
The authors wish to thank Dr. Abdelrahman S. Mayhoub (Al-Azhar University, Cairo, Egypt) for valuable feedback regarding the structure–antibacterial activity relationship between niclosamide and other salicylanilide drugs.
Funding: This work was supported by the National Institutes of Health (Grant No. R01AI130186). Haroon Mohammad is supported by a fellowship from the Purdue Institute for Drug Discovery.
Competing interests: None declared.
Ethical approval: All animal housing and experiments were
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Haroon Mohammad and Ahmed AbdelKhalek contributed equally.