Evaluation of the post-antibiotic effect in vivo for the combination of a ß-lactam antibiotic and a ß-lactamase inhibitor: ceftazidime-avibactam in neutropenic mouse thigh and lung infections.

The post-antibiotic effect (PAE) of ceftazidime-avibactam in vivo was evaluated using models of thigh- and lung-infection with Pseudomonas aeruginosa in neutropenic mice. In thigh-infected mice, the PAE was negative (-2.18 to -0.11 h) for three of four strains: caused by a 'burst' of rapid bacterial growth after the drug concentrations had fallen below their pre-specified target values. With lung infection, PAE was positive, and longer for target drug concentrations in ELF (>2 h) than plasma (1.69-1.88 h). The time to the start of regrowth was quantified as a new parameter, PAER, which was positive (0.35-1.00 h) in both thigh- and lung-infected mice. In the context that measurements of the PAE of ß-lactam/ß-lactamase inhibitor combinations in vivo have not previously been reported, it is noted that the negative values were consistent with previous measurements of the PAE of ceftazidime-avibactam in vitro and of ceftazidime alone in vivo.

Pharmacodynamics of Ceftazidime and Avibactam in Neutropenic Mice with Thigh or Lung Infection.

Avibactam is a new non-ß-lactam ß-lactamase inhibitor that shows promising restoration of ceftazidime activity against microorganisms producing Ambler class A extended-spectrum ß-lactamases (ESBLs) and carbapenemases such as KPCs, class C ß-lactamases (AmpC), and some class D enzymes. To determine optimal dosing combinations of ceftazidime-avibactam for treating infections with ceftazidime-resistant Pseudomonas aeruginosa, pharmacodynamic responses were explored in murine neutropenic thigh and lung infection models. Exposure-response relationships for ceftazidime monotherapy were determined first. Subsequently, the efficacy of adding avibactam every 2 h (q2h) or q8h to a fixed q2h dose of ceftazidime was determined in lung infection for two strains. Dosing avibactam q2h was significantly more efficacious, reducing the avibactam daily dose for static effect by factors of 2.7 and 10.1, whereas the mean percentage of the dosing interval that free drug concentrations remain above the threshold concentration of 1 mg/liter (%fT>C(T) 1 mg/liter) yielding bacteriostasis was similar for both regimens, with mean values of 21.6 (q2h) and 18.5 (q8h). Dose fractionation studies of avibactam in both the thigh and lung models indicated that the effect of avibactam correlated well with %fT>C(T) 1 mg/liter. This parameter of avibactam was further explored for four P. aeruginosa strains in the lung model and six in the thigh model. Parameter estimates of %fT>C(T) 1 mg/liter for avibactam ranged from 0 to 21.4% in the lung model and from 14.1 to 62.5% in the thigh model to achieve stasis. In conclusion, addition of avibactam enhanced the effect of ceftazidime, which was more pronounced at frequent dosing and well related with %fT>C(T) 1 mg/liter. The thigh model appeared more stringent, with higher values, ranging up to 62.5% fT>C(T) 1 mg/liter, required for a static effect.

Pharmacokinetics and penetration of ceftazidime and avibactam into epithelial lining fluid in thigh- and lung-infected mice.

Ceftazidime and the ß-lactamase inhibitor avibactam constitute a new, potentially highly active combination in the battle against extended-spectrum-ß-lactamase (ESBL)-producing bacteria. To determine possible clinical use, it is important to know the pharmacokinetic profiles of the compounds related to each other in plasma and the different compartments of infection in experimentally infected animals and in humans. We used a neutropenic murine thigh infection model and lung infection model to study pharmacokinetics in plasma and epithelial lining fluid (ELF). Mice were infected with ca. 10(6) CFU of Pseudomonas aeruginosa intramuscularly into the thigh or intranasally to cause pneumonia and were given 8 different (single) subcutaneous doses of ceftazidime and avibactam in various combined concentrations, ranging from 1 to 128 mg/kg of body weight in 2-fold increases. Concomitant samples of serum and bronchoalveolar lavage fluid were taken at up to 12 time points until 6 h after administration. Pharmacokinetics of both compounds were linear and dose proportional in plasma and ELF and were independent of the infection type, with estimated half-lives (standard deviations [SD]) in plasma of ceftazidime of 0.28 (0.02) h and of avibactam of 0.24 (0.04) h and volumes of distribution of 0.80 (0.14) and 1.18 (0.34) liters/kg. The ELF-plasma (area under the concentration-time curve [AUC]) ratios (standard errors [SE]) were 0.24 (0.03) for total ceftazidime and 0.27 (0.03) for unbound ceftazidime; for avibactam, the ratios were 0.20 (0.02) and 0.22 (0.02), respectively. No pharmacokinetic interaction between ceftazidime and avibactam was observed. Ceftazidime and avibactam showed linear plasma pharmacokinetics that were independent of the dose combinations used or the infection site in mice. Assuming pharmacokinetic similarity in humans, this indicates that similar dose ratios of ceftazidime and avibactam could be used for different types and sites of infection.

In vitro activity of ceftazidime-avibactam combination in in vitro checkerboard assays.

To evaluate the in vitro effects of the combination of ceftazidime and avibactam on the MICs of both compounds, checkerboard assays were performed for 81 clinical strains, including 55 Enterobacteriaceae strains (32 Klebsiella pneumoniae, 19 Escherichia coli, 1 Citrobacter freundii, and 3 Enterobacter cloacae) and 26 strains of Pseudomonas aeruginosa, all with known resistance mechanisms such as extended-spectrum ß-lactamases (ESBLs) and carbapenemases, phenotypically or molecularly determined. Phenotypically ceftazidime-resistant strains (n=69) were analyzed in more detail. For the Enterobacteriaceae strains, a concentration-dependent effect of avibactam was found for most strains with a maximum effect of avibactam at a concentration of 4 mg/liter, which decreased all ceftazidime MICs to ≤4 mg/liter. Avibactam alone also showed antibacterial activity (the MIC50 and MIC90 being 8 and 16 mg/liter, respectively). For the ceftazidime-resistant P. aeruginosa strains, considerable inhibition of ß-lactamases by avibactam was acquired at a concentration of 4 mg/liter, which decreased all ceftazidime MICs except one to ≤8 mg/liter (the CLSI and EUCAST susceptible breakpoint). Increasing the concentration of avibactam further decreased the MICs, resulting in a maximum effect for most strains at 8 to 16 mg/liter. In summary, for most strains, the tested addition of avibactam of 4 mg/liter restored the antibacterial activity of ceftazidime to a level comparable to that of wild-type strains, indicating full inhibition, and strains became susceptible according to the EUCAST and CLSI criteria. Based on these in vitro data, avibactam is a promising inhibitor of different ß-lactamases, including ESBLs and carbapenemases.