Dimercaptosuccinic acid in combination with carbapenems against isogenic strains of Escherichia coli producing or not producing a metallo-β-lactamase in vitro and in murine peritonitis

Synergistic Activity of Pep16, a Promising New Antibacterial Pseudopeptide against Multidrug-Resistant Organisms, in Combination with Colistin against Multidrug-Resistant Escherichia coli, In Vitro and in a Murine Peritonitis Model

Colistin is a drug of last resort to treat extreme drug-resistant Enterobacterales, but is limited by dose-dependent toxicity and the emergence of resistance. A recently developed antimicrobial pseudopeptide, Pep16, which acts on the cell membrane, may be synergistic with colistin and limit the emergence of resistance. We investigated Pep16 activity against Escherichia coli with varying susceptibility to colistin, in vitro and in a murine peritonitis model. Two isogenic derivatives of E. coli CFT073 (susceptible and resistant to colistin) and 2 clinical isolates (susceptible (B119) and resistant to colistin (Af31)) were used. Pep16 activity, alone and in combination with colistin, was determined in vitro (checkerboard experiments, time-kill curves, and flow cytometry to investigate membrane permeability). Toxicity and pharmacokinetic analyses of subcutaneous Pep16 were performed in mice, followed by the investigation of 10 mg/kg Pep16 + 10 mg/kg colistin (mimicking human concentrations) in a murine peritonitis model. Pep16 alone was inactive (MICs = 32-64 mg/L; no bactericidal effect). A concentration-dependent bactericidal synergy of Pep16 with colistin was evidenced on all strains, confirmed by flow cytometry. In vivo, Pep16 alone was ineffective. When Pep16 and colistin were combined, a significant decrease in bacterial counts in the spleen was evidenced, and the combination prevented the emergence of colistin-resistant mutants, compared to colistin alone. Pep16 synergizes with colistin in vitro, and the combination is more effective than colistin alone in a murine peritonitis by reducing bacterial counts and the emergence of resistance. Pep16 may optimize colistin use, by decreasing the doses needed, while limiting the emergence of colistin-resistant mutants.

Activity of the combination of colistin and fosfomycin against NDM-1-producing Escherichia coli with variable levels of susceptibility to colistin and fosfomycin in a murine model of peritonitis

BACKGROUND

Alternative treatments are needed against NDM-1-producing Escherichia coli. Colistin (COL) and fosfomycin (FOS) often remain active in vitro but selection of resistant mutants is frequent if used separately. We determined whether the combination of colistin and fosfomycin may be useful to treat infections with NDM-1-producing E. coli with varying levels of resistance.

METHODS

Isogenic derivatives of E. coli CFT073 with blaNDM-1 and variable levels of resistance to colistin and fosfomycin (CFT073-NDM1, CFT073-NDM1-COL and CFT073-NDM1-FOS, respectively) were used. The combination (colistin + fosfomycin) was tested in vitro and in a fatal peritonitis murine model. Mortality and bacterial loads were determined and resistant mutants detected.

RESULTS

Colistin MICs were 0.5, 16 and 0.5 mg/L and fosfomycin MICs were 1, 1 and 32 mg/L against CFT073-NDM1, CFT073-NDM1-COL and CFT073-NDM1-FOS, respectively. In time-kill curves, combining colistin with fosfomycin was synergistic and bactericidal against CFT073-NDM1 and CFT073-NDM1-FOS, with concentrations of 4× MIC (for both drugs), but not against CFT073-NDM1-COL (concentrations of colistin = 0.5× MIC), due to regrowth with fosfomycin-resistant mutants. Mice died less and bacterial counts were lower in spleen with the combination compared with monotherapy against all strains; the combination prevented selection of resistant mutants except for CFT073-NDM1-COL where fosfomycin-resistant mutants were found in all mice.

CONCLUSIONS

Combining colistin and fosfomycin was beneficial in vitro and in vivo against NDM-1-producing E. coli, even with strains less susceptible to colistin and fosfomycin. However, the combination failed to prevent the emergence of fosfomycin-resistant mutants against colistin-resistant strains. Combining colistin and fosfomycin constitutes an alternative for treatment of NDM-1 E. coli, except against colistin-resistant strains.

The paradoxical in vivo activity of β-lactams against metallo-β-lactamase-producing Enterobacterales is not restricted to carbapenems

BACKGROUND

Using murine models of infection, we previously reported the potent in vivo activity of carbapenems against MBL-producing Enterobacterales despite the observed resistance in vitro. In the current study, we examined the in vivo activity of a cefepime human-simulated regimen against MBL-producing Enterobacterales in a murine thigh infection model.

METHODS

A population of clinical isolates and isogenic engineered MBL-producing Enterobacterales transformants expressing MBLs but no detectable cefepime-hydrolysing serine β-lactamases were utilized. KPC-producing isolates were included as positive controls. Cefepime, piperacillin/tazobactam and meropenem MICs were determined using broth microdilution in conventional CAMHB and EDTA-supplemented (zinc-limited) broth. In vivo efficacy of a cefepime human-simulated regimen (2 g q8h as a 2 h infusion) was determined in the neutropenic murine thigh infection model against the test strains. Efficacy was measured as the change in log10 cfu/thigh at 24 h compared with 0 h controls.

RESULTS

MBL-producing Enterobacterales strains were found to be cefepime, piperacillin/tazobactam and meropenem non-susceptible in conventional broth. Supplementation with EDTA at a concentration of 300 mg/L resulted in multi-fold reduction in the MICs and restoration of susceptibility. In accordance with the MICs generated in zinc-limited broth, administration of a cefepime human-simulated regimen was associated with substantial bacterial reductions among mice infected with MBL-producing Enterobacterales. Absence of MIC reduction in zinc-limited broth and lack of efficacy among mice infected with KPC-producing isolates were observed.

CONCLUSIONS

For MBL-producing Enterobacterales, susceptibility testing with Mueller-Hinton broth, a zinc-rich testing medium, is flawed since it does not recapitulate the host environment, in which zinc concentrations are low.

Activity of β-Lactam Antibiotics against Metallo-β-Lactamase-Producing Enterobacterales in Animal Infection Models: a Current State of Affairs

Metallo-β-lactamases (MBLs) result in resistance to nearly all β-lactam antimicrobial agents, as determined by currently employed susceptibility testing methods. However, recently reported data demonstrate that variable and supraphysiologic zinc concentrations in conventional susceptibility testing media compared with physiologic (bioactive) zinc concentrations may be mediating discordant in vitro-in vivo MBL resistance. While treatment outcomes in patients appear suggestive of this discordance, these limited data are confounded by comorbidities and combination therapy. To that end, the goal of this review is to evaluate the extent of β-lactam activity against MBL-harboring Enterobacterales in published animal infection model studies and provide contemporary considerations to facilitate the optimization of current antimicrobials and development of novel therapeutics.

Therapeutic Options for Metallo-β-Lactamase-Producing Enterobacterales

The spread of metallo-β-lactamase (MBL)-producing Enterobacterales worldwide without the simultaneous increase in active antibiotics makes these organisms an urgent public health threat. This review summarizes recent advancements in diagnostic and treatment strategies for infections caused by MBL-producing Enterobacterales. Adequate treatment of patients infected with MBL-producing Enterobacterales relies on detection of the β-lactamase in the clinic. There are several molecular platforms that are currently available to identify clinically relevant MBLs as well as other important serine-β-lactamases. Once detected, there are several antibiotics that have historically been used for the treatment of MBL-producing Enterobacterales. Antimicrobials such as aminoglycosides, tetracyclines, fosfomycin, and polymyxins often show promising in vitro activity though clinical data are currently lacking to support their widespread use. Ceftazidime-avibactam combined with aztreonam is promising for treatment of infections caused by MBL-producing Enterobacterales and currently has the most clinical data of any available antibiotic to support its use. While cefiderocol has displayed promising activity against MBL-producing Enterobacterales in vitro and in preliminary clinical studies, further clinical studies will better shed light on its place in treatment. Lastly, there are several promising MBL inhibitors in the pipeline, which may further improve the treatment of MBL-producing Enterobacterales.