Purpose: To evaluate the administration regimen of ceftazidime/avibactam (CZA) for bloodstream infections caused by Enterobacteriaceae and Pseudomonas aeruginosa .
Methods: The minimal inhibitory concentrations (MICs) of CZA against Enterobacteriaceae  and P. aeruginosa  isolated from blood cultures at member hospitals in BRICS (Blood Bacterial Resistant Investigation Collaborative System) in 2019 were determined by broth micro-dilution methodology. A 10,000-patient Monte Carlo simulation (MCS) was used to calculate the probability of target attainment (PTA) and cumulative fraction of response (CFR) for different CZA dosage regimens to evaluate their efficacies and optimize the best initial dosage regimen.
Results: Altogether, 6487 Enterobacteriaceae  and P. aeruginosa  strains were isolated from the blood cultures. The overall CZA resistance rate was 2.31%, of which the Enterobacteriaceae  and P. aeruginosa  rates were 1.57% and 14.29%, respectively. The MCS showed that the greater the MIC value, the worse the therapeutic effect. When the CZA MIC was ≤ 8 mg/L, the standard dose (2.5g iv q8h) achieved 90% PTA in the subset of patients with creatinine clearance (CrCl) values from 51 to 120 mL/min. Although the high-dose regimen (3.75g iv q8h) achieved 90% PTA in patients with CrCl values from 121 to 190 mL/min, implementing the low-dose regimen (1.25g iv q8h) was also effective for patients in the 51– 89 mL/min CrCl range. Generally, the high-dose regimen (3.75g iv q8h) reached 90% CFR against all of the strains. Conversely, in patients with CrCl values of 121– 190 mL/min, the standard dose (2.5g iv q8h) failed to reach 90% CFR against some Enterobacteriaceae  members and P. aeruginosa . When the dose was reduced to the low-dose regimen (1.25g iv q8h), no patients reached 90% CFR against some Enterobacteriaceae  members and P. aeruginosa.
Conclusion: CZA has good antibacterial activity against Enterobacteriaceae  and P. aeruginosa  in bloodstream infections. Clinicians could make individualized treatment regimens in accordance with the sensitivity of the strains and the level of renal function in their patients to best predict the drug-related clinical responses.
Keywords: Gram-negative bacteria, extended-spectrum β-lactamase, dosage regimens, pharmacokinetics, pharmacodynamics, minimum inhibitory concentration