Development of Modernized Acinetobacter baumannii Susceptibility Test Interpretive Criteria for Recommended...

Acinetobacter baumannii-Acinetobacter calcoaceticus complex (referred to herein as A. baumannii) treatment guidelines contain numerous older antimicrobial agents with susceptibility test interpretive criteria (STIC, also known as susceptibility breakpoints) set using only epidemiological data. We utilized a combination of in vitro surveillance data, preclinical murine thigh and lung infection models, population pharmacokinetics, simulation, and pharmacokinetic/pharmacodynamic (PK/PD) target attainment analyses to evaluate A. baumannii STIC for four commonly recommended antimicrobials from different classes (amikacin, ceftazidime, ciprofloxacin, and minocycline). Antimicrobial in vitro surveillance data were based on 1,647 clinical A. baumannii isolates obtained from 109 centers in the United States and Europe. Among these isolates, 5 were selected for evaluation in murine infection models based on fitness and MIC variability. PK and dose-ranging studies were conducted using neutropenic murine thigh and lung infection models The MIC ranges for the 5 isolates evaluated were as follows: amikacin, 2 to 32 μg/mL; ceftazidime, 4 to 16 μg/mL; ciprofloxacin, 0.12 to 2 μg/mL; minocycline, 0.25 to 4 μg/mL. All organisms grew ≥1.5 log₁₀ CFU in both models in untreated controls. Plasma and epithelial lining fluid (ELF) pharmacokinetics for all drugs were determined in mice using liquid chromatography-tandem mass spectrometry (LC-MS/MS) methods. For each isolate, 5 dose levels of each drug were tested individually in the thigh and lung infection model. The inoculum ranged from 7.9 to 8.4 and 6.8 to 7.7 log₁₀ CFU/mL for the lung and thigh models, respectively. PK/PD targets associated with net bacterial stasis and 1- and 2-log₁₀ CFU reductions from baseline were identified for each organism/infection model using Hill-type models. Population pharmacokinetic models for each agent were identified from the literature. Using demographic variables for simulated patients with hospital-acquired or ventilator-associated bacterial pneumonia or urinary tract infections (including acute pyelonephritis) who were administered maximal dosing regimens of each agent, estimates of protein binding, and ELF penetration ratios based on data from the literature, free-drug plasma and total-drug concentration-time profiles were generated, and PK/PD indices by MIC were calculated. Percent probabilities of attaining median and randomly assigned PK/PD targets associated with the above-described endpoints were determined. Recommended susceptible breakpoints for each agent were those representing the highest MIC at which the percent probabilities of achieving PK/PD targets associated with a 1-log₁₀ CFU reduction from baseline approached or were ≥90%. The following susceptible breakpoints for A. baumannii were identified: amikacin, ≤8 μg/mL for pneumonia; ceftazidime, ≤32 and ≤8 μg/mL for pneumonia; ciprofloxacin, ≤1 μg/mL; and minocycline, ≤0.5/≤1 μg/mL which correspond to the standard and high minocycline dosing regimens of 200 mg per day and 200 mg every 12 h, respectively. Implementation of appropriate STIC will help clinicians optimally use the above-described agents and improve the likelihood of successful patient outcomes.