Pharmacodynamics of fosfomycin against ESBL- and/or carbapenemase-producing Enterobacteriaceae.

BACKGROUND: The increase in antibiotic resistance in Gram-negative bacteria and the limited therapeutic options due to the shortage of new antibiotics have increased the interest of the 'old' antibiotic fosfomycin in the treatment of infections. However, there are contradictory reports on the pharmacodynamics of and emergence of resistance to fosfomycin. METHODS: Time-kill assays were performed with 11 ESBL-positive and 3 ESBL-negative strains, exposing the bacteria to 2-fold static concentrations from 0.125× to 32× MIC. The sigmoid maximum effect (Emax) model was fitted to the time-kill curve data. Amplification of resistance over time was evaluated under various conditions of selective pressure by plating on 16× MIC plates. RESULTS: Fosfomycin was bactericidal for all strains within 8 h. Using the Emax model, no significant differences between strains were observed for the pharmacodynamic parameters. However, the large variation in Hill slope factors for Escherichia coli of 0.87 up to 4.02 indicates that the killing behaviour appears to be more time dependent for some strains but concentration dependent for others. In the fosfomycin-exposed cultures under low and high selective pressure (≥2× MIC) the median resistance proportions between the resistant and total population increased from ≤2 × 10-6 (T = 0 h) to 0.652-0.899 (T = 24 h). Resistance appeared stable after repeated subculturing. CONCLUSIONS: Killing behaviour of fosfomycin does not only differ between species but also within species and may have an impact on the design of optimal dosing regimens. Although fosfomycin was bactericidal against all strains (re)growth of resistant subpopulations occurred relatively fast. This may limit the use of fosfomycin as a single drug therapy.

Pharmacodynamics of nitrofurantoin at different pH levels against pathogens involved in urinary tract infections.

BACKGROUND: Urinary tract infections are among the most common human infections. Due to the progressive increase in ESBL-producing bacteria and the unavailability of new antibiotics, re-evaluation of 'old' antibiotics is needed. However, the pharmacodynamics of nitrofurantoin under variable pH conditions are poorly understood. We determined the pharmacodynamic properties of nitrofurantoin at different pH levels using time-kill assays. METHODS: Time-kill assays were performed at four pH levels (5.5, 6.5, 7.5 and 8.5), exposing the bacteria to 2-fold increasing concentrations from 0.125 to 32 times the MIC. Seven ESBL-positive and two ESBL-negative strains (MICs 8-32 mg/L) were used. The Δlog10 cfu/mL values at 6 and 24 h were plotted against each log10-transformed concentration and analysed with non-linear regression analysis using the sigmoid maximum effect model with variable slope. Geometric means normalized by the MIC of the EC50, stasis and 1 and 3 log10 cfu/mL kill were calculated. RESULTS: Minimum bactericidal effects differed significantly by species and pH level. At pH 5.5-6.5 bactericidal effects were observed at ≥ 0.5 × MIC for Escherichia coli and Enterobacter cloacae. At pH 8.5 only the two highest concentrations were considered bactericidal. Strong pH-dependent pharmacodynamic output parameters were observed in 6 h and especially 24 h modelling. At 24 h, pH 5.5-6.5 for E. coli and Klebsiella pneumoniae required significantly lower nitrofurantoin concentrations compared with pH 7.5 or 8.5. Although for E. cloacae similar strong decreasing trends were visible with decreasing pH, none of the tested pharmacodynamic parameters was significant. CONCLUSIONS: Nitrofurantoin bactericidal activity against Enterobacteriaceae significantly increases at lower pH levels. Bactericidal activity of nitrofurantoin may be overestimated or underestimated, which may have implications for therapy and the interpretation of clinical breakpoints.

Pharmacodynamics and differential activity of nitrofurantoin against ESBL-positive pathogens involved in urinary tract infections.

BACKGROUND: Although nitrofurantoin has been used for >60 years for the treatment of uncomplicated urinary tract infections, its pharmacodynamic properties are not fully explored. Use is increasing because of increasing resistance to other antimicrobials due to ESBLs. METHODS: We tested nine ESBL+ and two ESBL- strains in time-kill assays. Bactericidal activity and regrowth were assessed for all species and concentrations. Early-phase pharmacodynamics was analysed with a sigmoidal Emax model and the maximal killing rate, slope and EC50/MIC ratio were determined for each species. RESULTS: A bactericidal effect was found at ≥2× MIC for Enterobacter cloacae after 4-8 h, for Klebsiella pneumoniae after 8-10 h and for Escherichia coli after 12-16 h. Overall, no killing was observed at low sub-MIC concentrations, whereas regrowth was found at 0.5-1× MIC after a short decline in cfu. The lowest maximal killing rates were observed for E. coli (0.21 ±â€Š0.05 h(-1)), followed by K. pneumoniae (0.37 ±â€Š0.09 h(-1)) and E. cloacae (0.87 ±â€Š0.01 h(-1)). Surprisingly, the Hill slopes for these three species were significantly different (10.45 ±â€Š9.37, 2.68 ±â€Š0.64 and 1.01 ±â€Š0.06, respectively), indicating a strong concentration-dependent early-phase antibacterial activity against E. cloacae. EC50/MIC ratios were significantly lower for E. coli (0.24 ±â€Š0.08 mg/L) and K. pneumoniae (0.27 ±â€Š0.03 mg/L) as compared with E. cloacae (0.77 ±â€Š0.18 mg/L). CONCLUSIONS: Nitrofurantoin was bactericidal against all species, demonstrating an unusual differential pattern of activity with concentration-dependent-type killing behaviour against E. cloacae and time-dependent killing behaviour against E. coli, which may have significant consequences on species-dependent dosing regimens. The results also demonstrate that the pharmacodynamic properties of some drugs cannot be generalized within a family, here the Enterobacteriaceae.

In Vitro Activity of Ceftolozane Alone and in Combination with Tazobactam against Extended-Spectrum-ß-Lactamase-Harboring Enterobacteriaceae.

Ceftolozane, formally CXA-101, is a new antipseudomonal cephalosporin that is also active in vitro against Enterobacteriaceae but is vulnerable to extended-spectrum ß-lactamases (ESBLs). The addition of tazobactam is intended to broaden coverage to most ESBL-producing Escherichia coli and Klebsiella pneumonia as well as other Enterobacteriaceae. The in vitro activities of ceftolozane-tazobactam combinations against 67 clinically and molecularly characterized ESBL-producing isolates were examined by checkerboard MIC testing to evaluate their potential clinical feasibility and to assess the optimal tazobactam concentrations to be used in MIC determinations of ceftolozane. Isolates included those from E. coli (n = 32), K. pneumoniae (n = 19), Enterobacter cloacae (n = 15), and Citrobacter freundii (n = 1). Checkerboard experiments were performed to study interactions over the range of 0.008 to 64 mg/liter ceftolozane and 0.063 to 32 mg/liter tazobactam using 2-fold-dilution series. The MIC50 and MIC90 of ceftolozane alone for all isolates were 16 and ≥64 mg/liter, respectively. Increasing concentrations of tazobactam resulted in decreasing MICs of ceftolozane. The 50th and 90th percentile concentrations of tazobactam required to reduce the MIC of ceftolozane to 8 mg/liter for all organisms in this ESBL collection were 0.5 and 4 mg/liter, respectively. For E. coli, K. pneumoniae, and E. cloacae, these values were 0.5 and 2, 1 and 16, and 0.5 and 4 mg/liter, respectively. When combined with a fixed amount of 4 mg/liter tazobactam (current CLSI concentration used for susceptibility testing), 90% of the isolates would have an MIC of ≤4 mg/liter. The combination ceftolozane-tazobactam is a promising alternative option for treating infections due to ESBL-harboring isolates.