Oral Fosfomycin Efficacy with Variable Urinary Exposures following Single and Multiple Doses against Enterobacterales: the Importance of Heteroresistance for Growth Outcome

Rediscovering the value of fosfomycin trometamol in the era of antimicrobial resistance: A systematic review and expert opinion

The worldwide prevalence of uncomplicated lower urinary tract infections (uUTIs) caused by multidrug-resistant Escherichia coli is increasing. To address this emergency, international guidelines recommend reducing administration of fluoroquinolones, in the context of growing resistance and the long-lasting and potentially disabling side effects of these drugs. The favoured drug to replace fluoroquinolones is fosfomycin trometamol (FT), a well-known derivate of phosphonic acid with broad-spectrum activity against Gram-negative and Gram-positive bacteria, including multidrug-resistant (MDR) strains. The European Committee on Antimicrobial Susceptibility Testing (EUCAST) recently reduced the susceptibility breakpoint for E. coli from 32 mg/L to 8 mg/L regarding FT used for uUTIs. This might lead to increased appropriate use of oral fosfomycin target therapy against E. coli and other microorganisms, and may be associated with a high likelihood of success. For species such as Klebsiella spp, particularly MDR strains, the absence of clinical breakpoints might lead to reduced use of oral fosfomycin, particularly if minimum inhibitory concentration is not available. To address this issue, this review presents an overview of the preclinical evidence on the activity of FT, and a systematic review of the clinical activity of FT in uUTIs in women, and in the prevention of infectious complications after prostate biopsy. The findings indicate that the safety and microbiological and clinical effectiveness of a single oral dose of FT are similar to that for comparator regimens with longer treatment schedules in women with uUTI, and FT can be considered a viable alternative to fluoroquinolones for antimicrobial prophylaxis in prostate biopsy. These observations and a broad clinical experience support the empirical use of FT for treating uUTI and indicate that FT is a promising candidate to effectively counteract antibiotic-resistant uUTIs throughout Europe.

Plasma and Urine Pharmacokinetics of Oral Fosfomycin Tromethamine in Dogs

Fosfomycin is a broad-spectrum, bactericidal antibiotic with low toxicity. It has been used in human medicine and is a promising candidate for treating infections in veterinary medicine. Different Fosfomycin salts exhibit various degrees of bioavailability. Tromethamine salt is the most commonly used oral form due to its improved bioavailability. However, information regarding its use with dogs is limited. Therefore, this study aimed to investigate the pharmacokinetics of oral Fosfomycin tromethamine in canine plasma and urine using liquid chromatography tandem mass spectrometry (LC-MS/MS). Six healthy male beagles underwent a three-period three-treatment study: treatment 1 and 2 with single oral Fosfomycin tromethamine at 40 and 80 mg/kg (the total doses with tromethamine salt were 75 and 150 mg/kg, respectively), and treatment 3 with intravenously Fosfomycin disodium at 57 mg/kg (the total dose with disodium salt was 75 mg/kg). Dogs receiving oral Fosfomycin tromethamine at 75 and 150 mg/kg, maximal drug concentration (Cmax) in plasma produced results of 34.46 ± 12.52 and 66.40 ± 12.64 µg/mL, oral bioavailability (F) was approximately 38 and 45%, while urine Cmax was 4463.07 ± 2208.88 and 8784.93 ± 2303.46 µg/mL, respectively. No serious adverse effects were reported, except loose stool in some dogs. The tremendously high urine Fosfomycin concentrations indicate that oral Fosfomycin tromethamine is suitable as an alternative treatment for bacterial cystitis in dogs.

Fosfomycin Disk Diffusion Testing among Klebsiella pneumoniae Results in Frequent Inner Colonies and Categorical Disagreement Based on Conflicting Breakpoint Organization Recommendations

Recent studies indicate that discrete inner colonies (ICs) arise during fosfomycin disk diffusion (DD) testing. CLSI and EUCAST have contradicting recommendations on the interpretation of ICs; CLSI recommends considering them while EUCAST recommends ignoring them when interpreting DD results. We sought to compare the categorical agreement of DD and agar dilution (AD) MIC and to assess the implications of ICs interpretation on zone diameter readings. A convenience sample of 80 Klebsiella pneumoniae clinical isolates with varied phenotypic profiles collected from 3 United States locations was included. Susceptibility was determined in duplicate, using both organization recommendations and interpretations for Enterobacterales. Correlations between methods were calculated using EUCASTIV AD as the reference method. MIC values ranged from 1 to >256 μg/mL with an MIC50/90 of 32/256 μg/mL. Extrapolating EUCASToral and CLSI AD Escherichia coli breakpoints, 12.5% and 83.8% of isolates were susceptible, respectively, whereas 66.3% were susceptible by EUCASTIV AD-which applies to K. pneumoniae. CLSI DD measurements were 2 to 13 mm smaller than EUCAST measurements due to 66 (82.5%) isolates producing discrete ICs. Categorical agreement with EUCASTIV AD was greatest for CLSI AD (65.0%) and poorest for EUCASToral DD (6.3%). Isolates among this collection were frequently classified into different interpretive categories based on varying breakpoint organization recommendations. The more conservative oral breakpoints of EUCAST resulted in more isolates categorized as resistant despite frequent ICs. Differing zone diameter distributions and poor categorical agreement highlight issues of extrapolating E. coli breakpoints and methods to other Enterobacterales, and the clinical relevance of this issue warrants further investigation. IMPORTANCE Fosfomycin susceptibility testing recommendations are complex. Both the Clinical and Laboratory Standards Institute and the European Committee on Antimicrobial Susceptibility Testing (EUCAST) recognize agar dilution as the reference method, but they also support disk diffusion as an approved method for Escherichia coli. However, these two organizations have conflicting recommendations for the interpretation of inner colonies that arise during disk diffusion testing which can lead to varying zone diameters and interpretations despite isolates having identical MIC values. Using a collection of 80 Klebsiella pneumoniae isolates, we found that a large (82.5%) portion produced discrete inner colonies during disk diffusion and isolates were frequently classified into different interpretive categories. The more conservative breakpoints of EUCAST resulted in more isolates categorized as resistant despite frequent inner colonies. Differing zone diameter distributions and poor categorical agreement highlight issues of extrapolating E. coli breakpoints and methods to other Enterobacterales, and the clinical relevance of this issue warrants further investigation.

Antibiotic heteroresistance in ESKAPE pathogens, from bench to bedside

BACKGROUND

Heteroresistance refers to subpopulation-mediated differential antimicrobial susceptibility within a clonal bacterial population. Usually, it designates a resistant subpopulation identified within an isolate considered susceptible by classical antimicrobial susceptibility testing. Heteroresistance lacks a uniform microbiological definition for diagnostic laboratories, and its clinical impact remains unclear for most bacterial species.

OBJECTIVES

This narrative review aims to provide a practical overview on the latest developments in the field of heteroresistance for both clinical microbiologists and physicians, with a particular focus on ESKAPE pathogens.

SOURCES

A literature search was performed on Pubmed and Google with the key words heteroresistance (heterogeneity OR heterogeneous) AND antibiotic resistance. Among the 836 publications selected based on their abstracts, the most relevant for the detection, epidemiology and clinical impact of heteroresistance in ESKAPE pathogens are discussed here.

CONTENT

Heteroresistance is only clearly defined for heterogeneous vancomycin intermediate Staphylococcus aureus. We compiled a larger microbiological definition to be applicable to other bacterial species and antibiotics in the clinical context. We highlighted the key technical points of population analysis profile, which is the reference standard for detecting heteroresistance. Heteroresistance to polymyxins, β-lactams (carbapenems, cefiderocol), fosfomycin, tigecycline and aminoglycosides is frequently reported in multidrug-resistant gram-negative pathogens. Treatment failure due to heteroresistance has been described in case reports or retrospective studies, so far confirmed by meta-analyses in the case of heterogeneous vancomycin intermediate S. aureus only. Finally, to treat pandrug-resistant bacterial infections, the option of targeting susceptible subpopulations of resistant isolates using tailored antibiotic combinations is also discussed.

IMPLICATIONS

Systematic heteroresistance screening by clinical laboratories is not currently recommended. Nevertheless, we should be aware of this phenomenon, and in specific cases, such as treatment failure, heteroresistance should be tested by reference laboratories. Additional studies using standardized methods are needed to improve our understanding of heteroresistance and further assess its clinical impact.

Antimicrobial Treatment Options for Difficult-to-Treat Resistant Gram-Negative Bacteria Causing Cystitis, Pyelonephritis, and Prostatitis: A Narrative Review

Urinary tract infections, including cystitis, acute pyelonephritis, and prostatitis, are among the most common diagnoses prompting antibiotic prescribing. The rise in antimicrobial resistance over the past decades has led to the increasing challenge of urinary tract infections because of multidrug-resistant and "difficult-to-treat resistance" among Gram-negative bacteria. Recent advances in pharmacotherapy and medical microbiology are modernizing how these urinary tract infections are treated. Advances in pharmacotherapy have included not only the development and approval of novel antibiotics, such as ceftazidime/avibactam, meropenem/vaborbactam, imipenem/relebactam, ceftolozane/tazobactam, cefiderocol, plazomicin, and glycylcyclines, but also the re-examination of the potential role of legacy antibiotics, including older aminoglycosides and tetracyclines. Recent advances in medical microbiology allow phenotypic and molecular mechanism of resistance testing, and thus antibiotic prescribing can be tailored to the mechanism of resistance in the infecting pathogen. Here, we provide a narrative review on the clinical and pre-clinical studies of drugs that can be used for difficult-to-treat resistant Gram-negative bacteria, with a particular focus on data relevant to the urinary tract. We also offer a pragmatic framework for antibiotic selection when encountering urinary tract infections due to difficult-to-treat resistant Gram-negative bacteria based on the organism and its mechanism of resistance.

Fosfomycin susceptibility of uropathogens at Charlotte Maxeke Johannesburg Academic Hospital

Background

Multidrug-resistant uropathogens are becoming widespread both in community and hospital setting. Safe yet effective treatments are a priority. Fosfomycin is an antibacterial that displays good activity against most bacteria causing urinary tract infections (UTIs), including multidrug-resistant bacteria. The aim of this study was to evaluate fosfomycin susceptibility for uropathogens isolated from a microbiology laboratory at a tertiary academic hospital. In addition, this was compared to the susceptibility of other oral antimicrobials.

Methods

We conducted a retrospective analysis of laboratory reports for uropathogens isolated at Charlotte Maxeke Johannesburg Academic Hospital from September 2015 to August 2017. Antimicrobial susceptibility testing of the isolates was performed using the Kirby–Bauer disk diffusion method or the Vitek^® 2 system according to the Clinical and Laboratory Standards Institute.

Results

Overall susceptibility of fosfomycin for the 4700 Enterobacteriaceae isolates was 95.7%; 95% confidence interval (CI) 95.1–96.2. The overall susceptibility for fosfomycin against the gram-positives was 98.6%. There were 37.9% multidrug-resistant Enterobacteriaceae (MDRE) isolated during the study period. Fosfomycin displayed activity against 94.4% of extended-spectrum β-lactamase (ESBL) producers and 90.7% for carbapenem-resistant Enterobacteriaceae (CRE). None of the methicillin-resistant Staphylococcus aureus and vancomycin-resistant Enterococcus isolates tested was fosfomycin resistant. The overall in vitro susceptibility was significantly higher for fosfomycin (p = 0.0001) compared to amoxicillin/clavulanic acid, cephalexin, cefuroxime, ciprofloxacin, trimethoprim/sulfamethoxazole and nitrofurantoin.

Conclusion

This study confirmed the high susceptibility of fosfomycin against UTI pathogens isolated at our institution. In an era of increasing antimicrobial resistance, fosfomycin represents a potential option for the treatment of UTIs at Charlotte Maxeke Johannesburg Academic Hospital.

Pharmacokinetic/pharmacodynamic analysis of oral fosfomycin against Enterobacterales, Pseudomonas aeruginosa and Enterococcus spp. in an in vitro bladder infection model: impact on clinical breakpoints

OBJECTIVES

Fosfomycin is an established treatment for uncomplicated urinary tract infections (UTIs), yet evidence supporting susceptibility breakpoints is limited. We examine the UTI susceptibility criteria.

METHODS

Fosfomycin susceptibility, heteroresistance and in vitro growth in a bladder infection model, after a single 3 g dose of oral fosfomycin, were bridged to human pharmacokinetics with pharmacokinetic/pharmacodynamic and Monte Carlo analyses. Data from common uropathogens (24 Escherichia coli, 20 Klebsiella pneumoniae, 4 Enterobacter cloacae, 14 Pseudomonas aeruginosa, 8 Enterococcus faecalis and 8 Enterococcus faecium) were compared and analysed to ascertain species-specific PTA.

RESULTS

Glucose-6-phosphate (G6P) increased MICs of E. coli, K. pneumoniae and E. cloacae (median 2-fold dilutions 3-5), but not of P. aeruginosa and Enterococcus. Atypical E. coli lacking G6P potentiation were killed in the bladder infection model despite high MICs (32-128 mg/L). Fosfomycin heteroresistance was uncommon in E. coli (MIC > 2 mg/L) but was detected in the majority of K. pneumoniae (MIC > 1 mg/L) and P. aeruginosa (MIC >8 mg/L). For these species, baseline heteroresistance was a strong predictor for treatment failure in the model. No heteroresistance was found in Enterococcus. The fAUC/MIC targets for stasis were 1935, 3393, 9968, 2738 and 283 for typical E. coli, K. pneumoniae, E. cloacae, P. aeruginosa and E. faecalis, respectively (synthetic human urine medium alone promoted a 1 log10 kill in E. faecium). A >95% PTA for stasis was only found at MIC ≤ epidemiological cut-off (ECOFF) for E. coli (4 mg/L). For other species, PTAs were low for WT populations.

CONCLUSIONS

With the exception of E. coli, fosfomycin is a poor target for other uropathogen species. A reduction in oral fosfomycin UTI breakpoints is supported.

Activity of fosfomycin and amikacin against fosfomycin-heteroresistant Escherichia coli strains in a hollow-fiber infection model

Objectives:To evaluate human-like intravenous doses of fosfomycin (8g/Q8h) and amikacin (15mg/kg/Q24h) efficacy in monotherapy and in combination against six fosfomycin-heteroresistant Escherichia coli isolates using a hollow-fiber infection model (HFIM).Materials and methods:Six fosfomycin-heteroresistant E. coli isolates (4 with strong mutator phenotype) and the control strain E. coli ATCC 25922 were used. Mutant frequencies for rifampin (100mg/L), fosfomycin (50 and 200mg/L) and amikacin (32mg/L) were determined. Fosfomycin and amikacin MICs were assessed by agar dilution (AD), gradient strip (GSA) and broth microdilution (BMD) assays. Fosfomycin and amikacin synergies were studied by checkerboard and time-kill assays at different concentrations. Fosfomycin (8g/Q8h) and amikacin (15mg/kg/Q24h) efficacy alone and in combination were assessed using a HFIM.Results:Five isolates were resistant to fosfomycin by AD and BMD, but all susceptible by GSA. All isolates were considered susceptible to amikacin. Antibiotic combinations were synergistic in two isolates and no antagonism was detected. In time-kill assays, all isolates survived under fosfomycin at 64mg/L, although, at 307mg/L, only the normomutators and two hypermutators survived. Four isolates survived under 16mg/L amikacin and none at 45mg/L. No growth was detected under combination conditions. In HFIM, fosfomycin and amikacin monotherapies failed to sterilise bacterial cultures, however, fosfomycin and amikacin combination showed a rapid eradication.Conclusions.There may be a risk of treatment failure of fosfomycin-heteroresistant E. coli isolates using either amikacin or fosfomycin in monotherapy. These results support that the combination amikacin-fosfomycin can rapidly decrease bacterial burden and prevent the emergence of resistant subpopulations against fosfomycin-heteroresistant strains.

Antimicrobial pharmacokinetics and preclinical in vitro models to support optimized treatment approaches for uncomplicated lower urinary tract infections

INTRODUCTION

Urinary tract infections (UTIs) are extremely common. Millions of people, particularly healthy women, are affected worldwide every year. One-in-two women will have a recurrence within 12-months of an initial UTI. Inadequate treatment risks worsening infection leading to acute pyelonephritis, bacteremia and sepsis. In an era of increasing antimicrobial resistance, it is critical to provide optimized antimicrobial treatment.

AREAS COVERED

Literature was searched using PubMed and Google Scholar (up to 06/2020), examining the etiology, diagnosis and oral antimicrobial therapy for uncomplicated UTIs, with emphasis on urinary antimicrobial pharmacokinetics (PK) and the application of dynamic in vitro models for the pharmacodynamic (PD) profiling of pathogen response.

EXPERT OPINION

The majority of antimicrobial agents included in international guidelines were developed decades ago without well-described dose-response relationships. Microbiology laboratories still apply standard diagnostic methodology that has essentially remained unchanged for decades. Furthermore, it is uncertain how relevant standard in vitro susceptibility is for predicting antimicrobial efficacy in urine. In order to optimize UTI treatments, clinicians must exploit the urine-specific PK of antimicrobial agents. Dynamic in vitro models are valuable tools to examine the PK/PD and urodynamic variables associated with UTIs, while informing uropathogen susceptibility reporting, optimized dosing schedules, clinical trials and treatment guidelines.

Treatment options for K. pneumoniae, P. aeruginosa and A. baumannii co-resistant to carbapenems, aminoglycosides, polymyxins and tigecycline: an approach based on the mechanisms of resistance to carbapenems

The management of carbapenem-resistant infections is often based on polymyxins, tigecycline, aminoglycosides and their combinations. However, in a recent systematic review, we found that Gram-negative bacteria (GNB) co-resistant to carbapanems, aminoglycosides, polymyxins and tigecycline (CAPT-resistant) are increasingly being reported worldwide. Clinical data to guide the treatment of CAPT-resistant GNB are scarce and based exclusively on few case reports and small case series, but seem to indicate that appropriate (in vitro active) antimicrobial regimens, including newer antibiotics and synergistic combinations, may be associated with lower mortality. In this review, we consolidate the available literature to inform clinicians dealing with CAPT-resistant GNB about treatment options by considering the mechanisms of resistance to carbapenems. In combination with rapid diagnostic methods that allow fast detection of carbapenemase production, the approach proposed in this review may guide a timely and targeted treatment of patients with infections by CAPT-resistant GNB. Specifically, we focus on the three most problematic species, namely Klebsiella pneumoniae, Pseudomonas aeruginosa and Acinetobacter baumannii. Several treatment options are currently available for CAPT-resistant K. pneumonia. Newer β-lactam-β-lactamase combinations, including the combination of ceftazidime/avibactam with aztreonam against metallo-β-lactamase-producing isolates, appear to be more effective compared to combinations of older agents. Options for P. aeruginosa (especially metallo-β-lactamase-producing strains) and A. baumannii remain limited. Synergistic combination of older agents (e.g., polymyxin- or fosfomycin-based synergistic combinations) may represent a last resort option, but their use against CAPT-resistant GNB requires further study.

Oral Fosfomycin Treatment for Enterococcal Urinary Tract Infections in a Dynamic In Vitro Model

There are limited treatment options for enterococcal urinary tract infections, especially vancomycin-resistant Enterococcus (VRE). Oral fosfomycin is a potential option, although limited data are available guiding dosing and susceptibility. We undertook pharmacodynamic profiling of fosfomycin against E. faecalis and E. faecium isolates using a dynamic in vitro bladder infection model. Eighty-four isolates underwent fosfomycin agar dilution susceptibility testing (E. faecalis MIC_50/90 32/64 μg/ml; E. faecium MIC_50/90 64/128 μg/ml). Sixteen isolates (including E. faecalis ATCC 29212 and E. faecium ATCC 35667) were chosen to reflect the MIC range and tested in the bladder infection model with synthetic human urine (SHU). Under drug-free conditions, E. faecium demonstrated greater growth restriction in SHU compared to E. faecalis (E. faecium maximal growth 5.8 ± 0.6 log₁₀ CFU/ml; E. faecalis 8.0 ± 1.0 log₁₀ CFU/ml). Isolates were exposed to high and low fosfomycin urinary concentrations after a single dose, and after two doses given over two days with low urinary concentration exposure. Simulated concentrations closely matched the target (bias 2.3%). E. faecalis isolates required greater fosfomycin exposure for 3 log₁₀ kill from the starting inoculum compared with E. faecium The ƒAUC_0-72/MIC and ƒ%T > MIC_0-72 for E. faecalis were 672 and 70%, compared to 216 and 51% for E. faecium, respectively. There was no rise in fosfomycin MIC postexposure. Two doses of fosfomycin with low urinary concentrations resulted in equivalent growth inhibition to a single dose with high urinary concentrations. With this urinary exposure, fosfomycin was effective in promoting suppression of regrowth (>3 log₁₀ kill) in the majority of isolates.