dAvibactam is a new non-␤-lactam ␤-lactamase inhibitor that shows promising restoration of ceftazidime activity against microorganisms producing Ambler class A extended-spectrum ␤-lactamases (ESBLs) and carbapenemases such as KPCs, class C ␤-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.A ll over the world, health care professionals are struggling with the problem of antibiotic resistance, and extended-spectrum ␤-lactamase (ESBL)-and/or carbapenemase-producing microorganisms especially form a global threat (1-7). Apart from antibiotic stewardship, vaccines, and hygiene measurements, the development of new classes of antibiotics is of life-saving importance. Another, previously successful approach to overcome resistance is to combine a clinically proven ␤-lactam antibiotic with an inhibitor of the ␤-lactamases that confer resistance to it.AstraZeneca and Actavis (formerly Forest-Cerexa) are developing the combination of ceftazidime with avibactam, a new non-␤-lactam ␤-lactamase inhibitor that forms a hydrolytically stable linkage with serine-based ␤-lactamases to overcome resistance (8, 9). The combination showed in vitro activity against Ambler class A ESBLs, KPC class A enzymes, class C (AmpC) enzymes, and some class D enzymes. Studies in vitro have shown that ceftazidime MIC values against resistant strains were reduced drastically in the presence of this inhibitor, causing the strains to become susceptible to ceftazidime (9, 10). In dose-respo...

Ceftolozane is a new broad-spectrum cephalosporin and is combined with tazobactam to broaden the activity of ceftolozane against strains producing extended-spectrum beta-lactamases (ESBLs). We determined the pharmacodynamics (PD) of the combination in the neutropenic mouse thigh model to determine the optimal exposure of tazobactam. Treatment of CD-1 neutropenic mice was started 2 h after infection with ceftolozane every 2 h (q2h) alone or in combination with tazobactam at different dosing frequencies for 24 h, and the number of CFU in the thighs was determined before and after treatment. The maximum effect model was fit to the dose-response and the pharmacokinetic/PD index (PDI)-response to determine the PDI values for ceftolozane alone and ceftolozane in combination with tazobactam resulting in a static effect and a 1-log kill. The effect of tazobactam was dependent on the percentage of time that the free drug concentration remained above the concentration threshold (percent fT ϾC T ), whereby dosing q2h was more efficacious than dosing every 8 h (q8h), reducing the tazobactam daily dose by a factor 6.9 to 59.0 (n ‫؍‬ 3 strains) to obtain a static effect. Using R 2 as an indicator of the best fit of the percent fT ϾC T -response relationships, the concentration threshold best correlating with the response varied from 0.5 to 2 mg/liter, depending on the strain. A similar result was obtained when the q2h and q8h regimens were analyzed. For all isolates tested, the mean fT ϾC T for 0.5 mg/ liter tazobactam was 28.2% (range, 17.5 to 45.8%) and 44.4% (range, 26.6 to 54.7%) for a static effect and a 1-log kill, respectively, at ceftolozane exposures that produced a ceftolozane concentration of 4 mg/liter (a concentration greater than the MIC) for 33.9 to 63.3% of a 24-h period under steady-state pharmacokinetic conditions. The main PDI that correlated with the effect of tazobactam was the fT ϾC T achieved with a C T of 0.5 mg/liter tazobactam. Resistance to beta-lactam antibiotics and the rapid spread thereof are major concerns and are likely to remain so. In particular, infections caused by beta-lactamase-producing bacteria present a therapeutic dilemma when the beta-lactam antibiotic alone is ineffective (1-4). Ceftolozane is a novel cephalosporin that has activity against Pseudomonas aeruginosa and other Gramnegative bacteria as well as certain Gram-positive bacteria. However, the drug is susceptible to degradation by extended-spectrum beta-lactamases (ESBLs). In vitro studies have shown that elevated MICs of ceftolozane against resistant strains of the Enterobacteriaceae or Pseudomonas strains were restored to the levels for susceptible strains in the presence of the beta-lactamase inhibitor tazobactam (5, 6). This increase of drug efficacy after addition of tazobactam has been confirmed in a murine thigh infection model (7). However, the optimal tazobactam dosing regimen for the treatment of infections caused by ESBL producers still needs to be verified.Little is known about the antimicrobial effect of ceftoloz...

C eftazidime (CAZ) is a potent ␤-lactam antibiotic against Gram-negative bacteria in particular (1). However, since more and more Gram-negative bacteria have emerged that carry extended-spectrum ␤-lactamases (ESBLs) (2, 3) and class C ␤-lactamases (4), resistance has led to difficulty in identifying ␤-lactam therapies that would minimize the risk of resistance-related failure (5). Moreover, Klebsiella pneumoniae carbapenemase (KPC) and OXA-48 carbapenemase are narrowing treatment options against Gram-negative bacteria even further (6-8). For this reason, alternatives have been sought. The use of ␤-lactamase inhibitors seems to be a reasonable approach, and combinations consisting of a ␤-lactam agent and a ␤-lactamase inhibitor, such as piperacillintazobactam and amoxicillin-clavulanic acid, are widely used. AstraZeneca and Actavis (formerly Forest-Cerexa) are developing a combination of ceftazidime (CAZ) with avibactam, a new promising ␤-lactamase inhibitor, to overcome resistance caused by ␤-lactamases (9, 10). This combination has an extended spectrum of activity and is active against Ambler class A extended-spectrum ␤-lactamases (ESBLs), KPC class A enzymes, class C (AmpC) enzymes, and some class D enzymes (11)(12)(13)(14)(15). In vitro studies have shown that the drug MIC values for resistant clinical isolates, including Pseudomonas aeruginosa, were drastically reduced in the presence of this inhibitor and that the isolates thereby became susceptible to ceftazidime (11,(16)(17)(18)(19). Activity of the inhibitor in vivo has been shown as well (see, e.g., reference 13).As pneumonia is one of the leading causes of death in humans (20) and treatments using several new compounds have failed in patients with lower respiratory tract infections (21,22), it is important to understand the mechanism of activity, including the pharmacokinetic/pharmacodynamic (PK/PD) properties of the drugs used for this indication. For the combination ceftazidimeavibactam, concentrations of ceftazidime and avibactam in the lungs relative to each other might be different from the relative concentrations in plasma and therefore might result in bacterial responses in lung infection that are different from those in infections in other tissues.In the present study, we determined the pharmacokinetics of ceftazidime and avibactam and concentration-time profiles of the two compounds relative to each other in plasma and epithelial lining fluid (ELF) of infected neutropenic mice. Both thigh infection and lung infection models were used, to determine whether different kinds of infections would have different impacts on the pharmacokinetic profiles of each compound. The pharmacokinetic parameter estimates and the penetration of the two com-

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.