Comparison of the clinical efficacy between tigecycline plus extended-infusion imipenem and sulbactam plus imipenem against ventilator-associated pneumonia with pneumonic extensively drug-resistant Acinetobacter baumannii bacteremia, and correlation of clinical efficacy with in vitro synergy tests

Background/Purpose: To compare the clinical efficacy between salvage antimicrobial regimen consisting of tigecycline plus extended-infusion imipenem/cilastatin (TIC) and regimen of sulbactam plus imipenem/cilastatin (SIC) for patients with ventilator-associated pneumonia and pneumonic bacteremia due to extensively drug-resistant (XDR) Acinetobacter baumannii (Ab) isolates, and determine the correlation of results of invitro tigecycline-imipenem synergy test with clinical efficacy. Methods: The comparative survey was conducted at a medical center in Taiwan in 2013. Patients comprising the TIC group (n=28) received tigecycline plus extended-infusion imipenem/cilastatin following unresponsiveness to 3-day sulbactam-imipenem/cilastatin therapy, and those in the SIC group (n=56) received sulbactam-imipenem/cilastatin throughout the course. Univariate and multivariate analyses were applied to explore 30-day case-fatality independent predictors. Additionally, the checkerboard test and time-kill analysis were performed for the bloodstream XDR-Ab isolates from patients in the TIC group, and molecular characterization was done for the bloodstream XDR-Ab strains of all patients. Results: We found that the TIC scheme has a significant benefit on improving patients' survival status (the mortality rate of TIC and SIC group patients was 14.3% and 64.3%, respectively), corresponding well with invitro synergy or additivity results by the checkerboard test. Twenty TIC group cases had monomicrobial XDR-Ab cultured from tracheal aspirates after 10 days of tigecycline-imipenem/cilastatin therapy, but none developed subsequent pneumonia. However, breakthrough primary Burkholderia cepacia (n=3) and Pseudomonas aeruginosa (n=1) bacteremias were attributed to four TIC case fatalities. Shock, SIC regimen usage, and development of breakthrough bacteremia were independent predictors of 30-day in-hospital mortality. Conclusion: Although the TIC regimen showed good efficacy, its value regarding managing XDR-Ab ventilator-associated pneumonia bacteremia needs further evaluation.