Development of Modernized Acinetobacter baumannii Susceptibility Test Interpretive Criteria for Recommended Antimicrobial Agents Using Pharmacometric Approaches.

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.

Evaluation of Oral Tebipenem as a Step-Down Therapy following Intravenous Ertapenem against Extended-Spectrum ß-Lactamase-Producing Escherichia coli in a Hollow-Fiber In Vitro Infection Model.

Tebipenem is an orally bioavailable carbapenem in development for the treatment of patients with complicated urinary tract infections. Herein, we describe the results of studies designed to evaluate tebipenem's potential as an oral (p.o.) transition therapy from intravenous (i.v.) ertapenem therapy for the most common uropathogen, Escherichia coli. These studies utilized a 7-day hollow-fiber in vitro infection model and 5 extended-spectrum ß-lactamase-producing E. coli challenge isolates. Human free-drug serum concentration-time profiles for tebipenem 600 mg p.o. every 8 h and ertapenem 1 g i.v. every 24 h were simulated in the hollow-fiber in vitro infection model. Samples were collected for bacterial density and drug concentration determination over the 7-day study period. Generally, ertapenem monotherapy resulted in a greater reduction in bacterial density than did tebipenem monotherapy. In the treatment arms in which ertapenem dosing was stopped following dosing for 1 or 3 days, immediate bacterial regrowth occurred and matched that of the growth control. Finally, in the treatment arms in which ertapenem dosing was stopped following dosing for 1 or 3 days and tebipenem dosing was initiated for the remainder of the 7-day study, the intravenous-to-oral transition regimen reduced bacterial burdens and prevented regrowth. Given that transition from intravenous to oral antibiotic therapy has been shown to reduce hospital length of stay, nosocomial infection risk, and cost, and improve patient satisfaction, these data demonstrate tebipenem's potential role as an oral transition agent from intravenous antibiotic regimens within the antibiotic stewardship paradigm.

Meropenem penetration into epithelial lining fluid in mice and humans and delineation of exposure targets.

Pseudomonas aeruginosa pneumonia remains a most-difficult-to-treat nosocomial bacterial infection. We used mathematical modeling to identify drug exposure targets for meropenem in the epithelial lining fluid (ELF) of mice with Pseudomonas pneumonia driving substantial [2 to 3 log(10) (CFU/g)] killing and which suppressed resistant subpopulation amplification. We bridged to humans to estimate the frequency with which the largest licensed meropenem dose would achieve these exposure targets. Cell kills of 2 and 3 log(10) (CFU/g) and resistant subpopulation suppression were mediated by achieving time > MIC in ELF of 32%, 50%, and 50%. Substantial variability in meropenem's ability to penetrate into ELF of both mice and humans was observed. Penetration variability and high exposure targets combined to prevent even the largest licensed meropenem dose from achieving the targets at an acceptable frequency. Even a highly potent agent such as meropenem does not adequately suppress resistant subpopulation amplification as single-agent therapy administered at maximal dose and optimal schedule. Combination chemotherapy is likely required in humans if we are to minimize resistance emergence in Pseudomonas aeruginosa pneumonia. This combination needs evaluation both in the murine pneumonia model and in humans.

Saturability of granulocyte kill of Pseudomonas aeruginosa in a murine model of pneumonia.

Outcomes for patients with dense bacterial burdens, such as ventilator-associated pneumonia (VAP) patients, are often critically influenced by the adequacy of antimicrobial chemotherapy and by the response of the immune system, particularly the granulocytes. Little information is available about the quantitation of kill of organisms over time by granulocytes. In this investigation, we examined the impact of the baseline bacterial burden on the ability of granulocytes alone (without chemotherapy) to keep the number of organisms in check or to kill them over a 24-h period. Pseudomonas aeruginosa ATCC 27853 was the study organism, and we employed a murine pneumonia model (granulocyte replete) for the study. We found that the ability of the immune system to kill P. aeruginosa was saturable. The burden at which the system was half saturated was 2.15 × 106 ± 2.66 × 106 CFU/g. Burdens greater than 107 CFU/g demonstrated net growth over 24 h. These findings suggest the need for aggressive chemotherapy early in the treatment of VAP to keep the burden from saturating the granulocytes. This should optimize the outcome for these seriously infected patients.

Use of an in vitro pharmacodynamic model to derive a moxifloxacin regimen that optimizes kill of Yersinia pestis and prevents emergence of resistance.

Yersinia pestis, the causative agent of bubonic, septicemic, and pneumonic plague, is classified as a CDC category A bioterrorism pathogen. Streptomycin and doxycycline are the "gold standards" for the treatment of plague. However, streptomycin is not available in many countries, and Y. pestis isolates resistant to streptomycin and doxycycline occur naturally and have been generated in laboratories. Moxifloxacin is a fluoroquinolone antibiotic that demonstrates potent activity against Y. pestis in in vitro and animal infection models. However, the dose and frequency of administration of moxifloxacin that would be predicted to optimize treatment efficacy in humans while preventing the emergence of resistance are unknown. Therefore, dose range and dose fractionation studies for moxifloxacin were conducted for Y. pestis in an in vitro pharmacodynamic model in which the half-lives of moxifloxacin in human serum were simulated so as to identify the lowest drug exposure and the schedule of administration that are linked with killing of Y. pestis and with the suppression of resistance. In the dose range studies, simulated moxifloxacin regimens of ≥175 mg/day killed drug-susceptible bacteria without resistance amplification. Dose fractionation studies demonstrated that the AUC (area under the concentration-time curve)/MIC ratio predicted kill of drug-susceptible Y. pestis, while the C(max) (maximum concentration of the drug in serum)/MIC ratio was linked to resistance prevention. Monte Carlo simulations predicted that moxifloxacin at 400 mg/day would successfully treat human infection due to Y. pestis in 99.8% of subjects and would prevent resistance amplification. We conclude that in an in vitro pharmacodynamic model, the clinically prescribed moxifloxacin regimen of 400 mg/day is predicted to be highly effective for the treatment of Y. pestis infections in humans. Studies of moxifloxacin in animal models of plague are warranted.

Use of an in vitro pharmacodynamic model to derive a linezolid regimen that optimizes bacterial kill and prevents emergence of resistance in Bacillus anthracis.

Simulating the average non-protein-bound (free) human serum drug concentration-time profiles for linezolid in an in vitro pharmacodynamic model, we characterized the pharmacodynamic parameter(s) of linezolid predictive of kill and for prevention of resistance in Bacillus anthracis. In 10-day dose-ranging studies, the average exposure for > or =700 mg of linezolid given once daily (QD) resulted in >3-log CFU/ml declines in B. anthracis without resistance selection. Linezolid at < or =600 mg QD amplified for resistance. With twice-daily (q12h) dosing, linezolid at > or =500 mg q12 h was required for resistance prevention. In dose fractionation studies, killing of B. anthracis was predicted by the area under the time-concentration curve (AUC)/MIC ratio. However, resistance prevention was linked to the maximum serum drug concentration (C(max))/MIC ratio. Monte Carlo simulations predicted that linezolid at 1,100 mg QD would produce in 96.7% of human subjects a free 24-h AUC that would match or exceed the average 24-h AUC of 78.5 mg x h/liter generated by linezolid at 700 mg QD while reproducing the shape of the concentration-time profile for this pharmacodynamically optimized regimen. However, linezolid at 700 mg q12h (cumulative daily dose of 1,400 mg) would produce an exposure that would equal or exceed the average free 24-h AUC of 90 mg x h/liter generated by linezolid at 500 mg q12h in 93.8% of human subjects. In conclusion, in our in vitro studies, the QD-administered, pharmacodynamically optimized regimen for linezolid killed drug-susceptible B. anthracis and prevented resistance emergence at lower dosages than q12h regimens. The lower dosage for the pharmacodynamically optimized regimen may decrease drug toxicity. Also, the QD administration schedule may improve patient compliance.