Steady-state plasma and intrapulmonary concentrations of cefepime administered in continuous infusion in critically ill patients with severe nosocomial pneumonia

Treatment of infections caused by multidrug-resistant Gram-negative bacilli: A practical approach by the Italian (SIMIT) and French (SPILF) Societies of Infectious Diseases

INTRODUCTION

The emergence of multidrug-resistant Gram-negative bacilli and the development of new antibiotics have complicated the selection of optimal regimens. International guidelines are valuable tools, but are limited by the scarcity of high-quality randomized trials in many situations.

METHODS

A panel of experts from the French and Italian Societies of Infectious Diseases aimed to address unresolved issues in clinical practice based on their experience, an updated literature review and open discussions.

RESULTS

The panel reached consensus for the following 'first choices': (i) cefepime for ventilator-acquired pneumonia due to AmpC β-lactamase-producing Enterobacterales; (ii) the β-lactam/β-lactamase inhibitor combination most active in vitro, or cefiderocol combined with fosfomycin, and aerosolized colistin or aminoglycosides, for severe pneumonia due to Pseudomonas aeruginosa resistant to ceftolozane-tazobactam; (iii) high-dose piperacillin-tazobactam (including loading dose and continuous infusion) for complicated urinary tract infections (cUTIs) caused by extended-spectrum β-lactamase-producing Enterobacterales with piperacillin-tazobactam minimum inhibitory concentration (MIC) ≤8 mg/L; (iv) high-dose cefepime for cUTIs due to AmpC β-lactamase-producing Enterobacterales other than Enterobacter spp. if cefepime MIC ≤2 mg/L; (v) ceftolozane-tazobactam or ceftazidime-avibactam plus metronidazole for intra-abdominal infections (IAIs) due to third-generation cephalosporin-resistant Enterobacterales; (vi) ceftazidime-avibactam plus aztreonam plus metronidazole for IAIs due to metallo-β-lactamase-producing Enterobacterales; (vii) ampicillin-sulbactam plus colistin for bloodstream infections (BSIs) caused by carbapenem-resistant Acinetobacter baumannii; (viii) meropenem-vaborbactam for BSIs caused by Klebsiella pneumoniae carbapenemase-producing Enterobacterales; and (ix) ceftazidime-avibactam plus fosfomycin for neurological infections caused by carbapenem-resistant P. aeruginosa.

CONCLUSIONS

These expert choices were based on the necessary balance between antimicrobial stewardship principles and the need to provide optimal treatment for individual patients in each situation.

Cefepime distribution by microdialysis in peritoneal fluid of rats with or without experimental peritonitis

The aim of this study was to investigate the penetration of cefepime into rat peritoneal fluid by microdialysis and to determine the relationship between unbound drug plasma and tissue concentration in healthy animals and in a sepsis model established through cecal ligation and puncture-induced peritonitis. Probe recovery was performed by dialysis and retrodialysis. Cefepime was administered at a dose of 110 mg/kg intravenously. Samples were collected for about 4 h, and concentrations were determined by liquid chromatography-electrospray ionization-QTOF MS. Tissue penetration was also determined. Probe recovery in vivo was 38.78% ± 3.31% and 38.83% ± 2.74% in the control and peritonitis groups, respectively. Cefepime was rapidly distributed in the peritoneal fluid in both groups. The peritoneal fluid/plasma cefepime ratio was 0.38 and 0.32 for the control and peritonitis groups, respectively. Cefepime concentrations were above the MIC of 4 mg/L for the main enterobacteria. The infection model that was used had no apparent effect on the pharmacokinetics of cefepime in rats. This was the first study to determine free cefepime concentrations in the peritoneal fluid of healthy rats and rats with experimental peritonitis.

Aminoglycosides for the Treatment of Severe Infection Due to Resistant Gram-Negative Pathogens

Aminoglycosides are a family of rapidly bactericidal antibiotics that often remain active against resistant Gram-negative bacterial infections. Over the past decade, their use in critically ill patients has been refined; however, due to their renal and cochleovestibular toxicity, their indications in the treatment of sepsis and septic shock have been gradually reduced. This article reviews the spectrum of activity, mode of action, and methods for optimizing the efficacy of aminoglycosides. We discuss the current indications for aminoglycosides, with an emphasis on multidrug-resistant Gram-negative bacteria, such as extended-spectrum β-lactamase-producing Enterobacterales, carbapenemase-producing Enterobacterales, multidrug-resistant Pseudomonas aeruginosa, and carbapenem-resistant Acinetobacter baumannii. Additionally, we review the evidence for the use of nebulized aminoglycosides.

Tissue Penetration of Antimicrobials in Intensive Care Unit Patients: A Systematic Review—Part I

The challenging severity of some infections, especially in critically ill patients, makes the diffusion of antimicrobial drugs within tissues one of the cornerstones of chemotherapy. The knowledge of how antibacterial agents penetrate tissues may come from different sources: preclinical studies in animal models, phase I–III clinical trials and post-registration studies. However, the particular physiopathology of critically ill patients may significantly alter drug pharmacokinetics. Indeed, changes in interstitial volumes (the third space) and/or in glomerular filtration ratio may influence the achievement of bactericidal concentrations in peripheral compartments, while inflammation can alter the systemic distribution of some drugs. On the contrary, other antibacterial agents may reach high and effective concentrations thanks to the increased tissue accumulation of macrophages and neutrophils. Therefore, the present review explores the tissue distribution of beta-lactams and other antimicrobials acting on the cell wall and cytoplasmic membrane of bacteria in critically ill patients. A systematic search of articles was performed according to PRISMA guidelines, and tissue/plasma penetration ratios were collected. Results showed a highly variable passage of drugs within tissues, while large interindividual variability may represent a hurdle which must be overcome to achieve therapeutic concentrations in some compartments. To solve that issue, off-label dosing regimens could represent an effective solution in particular conditions.

Clinical Pharmacokinetics and Pharmacodynamics of Cefepime

Cefepime is a broad-spectrum fourth-generation cephalosporin with activity against Gram-positive and Gram-negative pathogens. It is generally administered as an infusion over 30-60 min or as a prolonged infusion with infusion times from 3 h to continuous administration. Cefepime is widely distributed in biological fluids and tissues with an average volume of distribution of ~ 0.2 L/kg in healthy adults with normal renal function. Protein binding is relatively low (20%), and elimination is mainly renal. About 85% of the dose is excreted unchanged in the urine, with an elimination half-life of 2-2.3 h. The pharmacokinetics of cefepime is altered under certain pathophysiological conditions, resulting in high inter-individual variability in cefepime volume of distribution and clearance, which poses challenges for population dosing approaches. Consequently, therapeutic drug monitoring of cefepime may be beneficial in certain patients including those who are critically ill, have life-threatening infections, or are infected with more resistant pathogens. Cefepime is generally safe and efficacious, with a goal exposure target of 70% time of the free drug concentration over the minimum inhibitory concentration for clinical efficacy. In recent years, reports of neurotoxicity have increased, specifically in patients with impaired renal function. This review summarizes the pharmacokinetics, pharmacodynamics, and toxicodynamics of cefepime contemporarily in the setting of increasing cefepime exposures. We explore the potential benefits of extended or continuous infusions and therapeutic drug monitoring in special populations.

Intrapulmonary Pharmacokinetic Modeling and Simulation of Cefiderocol, a Parenteral Siderophore Cephalosporin, in Patients With Pneumonia and Healthy Subjects

Cefiderocol is a siderophore cephalosporin for the treatment of infections caused by gram-negative bacteria including carbapenem-resistant strains. The aim of this study was to develop an intrapulmonary pharmacokinetic (PK) model of cefiderocol and assess the PK profile in lungs. An intrapulmonary PK model of cefiderocol was developed using the concentration data in plasma and epithelial lining fluid (ELF) from 7 patients with pneumonia requiring mechanical ventilation and 20 healthy subjects. Subsequently, the model was applied to assess the ELF exposure of 125 patients with nosocomial pneumonia. Monte Carlo simulations were performed to calculate the probability of target attainment for the percentage of time for which free ELF concentrations exceed the minimum inhibitory concentration (MIC) over the dosing interval (%fT>MIC,ELF ). The developed model adequately described ELF concentrations and suggested the delayed distribution in ELF for patients with pneumonia compared to healthy subjects. Lung penetration ratio of cefiderocol in patients with pneumonia was calculated to be 34%, which was 1.4-fold that in healthy subjects. The estimated %fT>MIC,ELF was 100% in most of patients with nosocomial pneumonia, and no PK/pharmacodynamic relationship with %fT>MIC,ELF was found for microbiological or clinical outcome. The probability of target attainment for 100% fT>MIC,ELF was ≥ 99.5% against MICs ≤2 μg/mL and ≥87.0% against MICs ≤4 μg/mL regardless of renal function. The median of simulated ELF trough concentrations at steady state was >4 μg/mL regardless of renal function. These results reveal the adequacy of cefiderocol exposure in plasma and ELF at the recommended dosing regimens adjusted on the basis of renal function in critically ill patients with pneumonia.

Treatment of ventilator-associated pneumonia due to carbapenem-resistant Gram-negative bacteria with novel agents: a contemporary, multidisciplinary ESGCIP perspective

INTRODUCTION

: In the past 15 years, treatment of VAP caused by carbapenem-resistant Gram-negative bacteria (CR-GNB) has represented an intricate challenge for clinicians.

AREAS COVERED

In this perspective article, we discuss the available clinical data about novel agents for the treatment of CR-GNB VAP, together with general PK/PD principles for the treatment of VAP, in the attempt to provide some suggestions for optimizing antimicrobial therapy of CR-GNB VAP in the daily clinical practice.

EXPERT OPINION

Recently, novel BL and BL/BLI combinations have become available that have shown potent in vitro activity against CR-GNB and have attracted much interest as novel, less toxic, and possibly more efficacious options for the treatment of CR-GNB VAP compared with previous standard of care. Besides randomized controlled trials, a good solution to enrich our knowledge on how to use these novel agents at best in the near future, while at the same time remaining adherent to current evidence-based guidelines, is to improve our collaboration to conduct larger multinational observational studies to collect sufficiently large populations treated in real life with those novel agents for which guidelines currently do not provide a recommendation (in favor or against) for certain causative organisms.

Pharmacokinetic evaluation of cefiderocol for the treatment of multidrug resistant Gram-negative infections

INTRODUCTION

Cefiderocol is a siderophore cephalosporin antibiotic and first of its kind approved by the Food and Drug Administration for the treatment of complicated urinary tract infections (cUTI) and hospital-acquired and ventilator-associated bacterial pneumonia (HABP/VABP) in patients 18 years or older caused by susceptible organisms. Cefiderocol's unique mechanism of iron chelation improves Gram-negative membrane penetration as the bacteria's iron uptake mechanism recognizes the chelated iron antibiotic and iron for entry. This also allows for the evasion of cefiderocol from cell entry-related resistance mechanisms.

AREAS COVERED

This review covers the mechanism of action, resistance mechanisms, pharmacokinetics in various patient populations, and pharmacodynamics. Relevant literature evaluating efficacy and safety are discussed.

EXPERT OPINION

Limited treatment options are available for the treatment of carbapenem-resistantorganisms. Clinical trials have demonstrated that cefiderocol is no worse than alternative treatment options for cUTIs and HABP/VABP, but more data are currently available to support the use of beta-lactam beta-lactamase inhibitor agents, where susceptible. Mortality differences demonstrated in patients with pneumonia and bloodstream infections must further be explored and logistical and practical considerations regarding susceptibility testing and use as monotherapy vs. combination therapy must be considered prior to confidently recommending cefiderocol for regular use in systemic infections.

Cefepime Dosing Requirements in Elderly Patients Attended in the Emergency Rooms

Objective

This study aimed to assess the probability of reaching an adequate pharmacokinetic/pharmacodynamic (pK/pD) index for different cefepime dosages in frail patients with bacteremia treated in the emergency room.

Methods

Simulation study based on Gram-negative bacterial strains that cause bacteremia. The probability of reaching a time above the minimum inhibitory concentration (MIC) at 50% and 100% dosing intervals (fT > 50 and fT > 80% MIC) was assessed for two different renal clearance intervals.

Results

One hundred twenty nine strains were collected, the predominant species being Escherichia coli (n = 83 [64.3%]). In patients with a ClCr of 30 mL/min, an fT > 50% MIC was reached in more than 90% of the simulations. However, a dose of at least 1 g every 12 h must be administered to reach an fT > 80% MIC. In patients with a ClCr of 30–60 mL/min, the probability of reaching an fT > 50% MIC was higher than 90% with doses of 1 g every 8 h or more, but this value was not reached in > 90% simulations for any of the doses tested in this study.

Conclusions

Standard cefepime dosing can reach an adequate PK/PD index in frail patients. Nevertheless, a high dose or extended infusion is necessary to reach an fT > 80% MIC in patients with a ClCr > 60 mL/min.

Modelled target attainment after temocillin treatment in severe pneumonia: systemic and epithelial lining fluid pharmacokinetics of continuous versus intermittent infusions

Objectives: To describe the population pharmacokinetics of temocillin administered via continuous versus intermittent infusion in critically ill patients with pneumonia. Secondary objectives included characterization of epithelial lining fluid (ELF)/plasma penetration ratios and determination of the probability of target attainment (PTA) for a range of MICs. Methods: Thirty-two mechanically ventilated patients who were treated for pneumonia with 6g of temocillin daily for in vitro sensitive pathogens were assigned either to the II (2g every 8h over 0.5h) or to the CI (6g over 24h after a loading dose of 2g) group. A population pharmacokinetic model was developed using unbound plasma and total ELF concentrations of temocillin and related Monte Carlo simulations were performed to assess PTAs. Results: The AUC_0-24 ELF/plasma penetration ratio was 0.73, at steady-state, for both modes of infusion and whatever the level of creatinine clearance. Monte Carlo simulations showed for the minimal pharmacodynamic (PD) targets of 50% T> 1X MIC (II group) and 100% T > 1X MIC (CI group), PK/PD breakpoints of 4 mg/L in plasma and 2 mg/L in ELF and 4mg/L in plasma and ELF, respectively. The breakpoint was 8 mg/L in ELF for both modes of infusion in patients with CL_CR<60mL/min. Conclusion: While CI provides better PKPD indexes, the latter remain below available recommendations for systemic infections, except in case of moderate renal impairment, thereby warranting future clinical studies in order to determine the efficacy of temocillin in severe pneumonia.

Penetration of Antibacterial Agents into Pulmonary Epithelial Lining Fluid: An Update

A comprehensive review of drug penetration into pulmonary epithelial lining fluid (ELF) was previously published in 2011. Since then, an extensive number of studies comparing plasma and ELF concentrations of antibacterial agents have been published and are summarized in this review. The majority of the studies included in this review determined ELF concentrations of antibacterial agents using bronchoscopy and bronchoalveolar lavage, and this review focuses on intrapulmonary penetration ratios determined with area under the concentration-time curve from healthy human adult studies or pharmacokinetic modeling of various antibacterial agents. If available, pharmacokinetic/pharmacodynamic parameters determined from preclinical murine infection models that evaluated ELF concentrations are also provided. There are also a limited number of recently published investigations of intrapulmonary penetration in critically ill patients with lower respiratory tract infections, where greater variability in ELF concentrations may exist. The significance of these changes may impact the intrapulmonary penetration in the setting of infection, and further studies relating ELF concentrations to clinical response are needed. Phase I drug development programs now include assessment of initial pharmacodynamic target values for pertinent organisms in animal models, followed by evaluation of antibacterial penetration into the human lung to assist in dosage selection for clinical trials in infected patients. The recent focus has been on β-lactam agents, including those in combination with β-lactamase inhibitors, particularly due to the rise of multidrug-resistant infections. This manifests as a large portion of the review focusing on cephalosporins and carbapenems, with or without β-lactamase inhibitors, in both healthy adult subjects and critically ill patients with lower respiratory tract infections. Further studies are warranted in critically ill patients with lower respiratory tract infections to evaluate the relationship between intrapulmonary penetration and clinical and microbiological outcomes. Our clinical research experience with these studies, along with this literature review, has allowed us to outline key steps in developing and evaluating dosage regimens to treat extracellular bacteria in lower respiratory tract infections.

Intrapulmonary pharmacokinetic profile of cefiderocol in mechanically ventilated patients with pneumonia

OBJECTIVES

Lung penetration of cefiderocol, a novel siderophore cephalosporin approved for treatment of nosocomial pneumonia, has previously been evaluated in healthy subjects. This study assessed the intrapulmonary pharmacokinetic profile of cefiderocol at steady state in hospitalized, mechanically ventilated pneumonia patients.

METHODS

Patients received cefiderocol 2 g (or ≤1.5 g if renally impaired), administered IV q8h as a 3 h infusion, or 2 g q6h if patients had augmented renal function (estimated CLCR > 120 mL/min). After multiple doses, each patient underwent a single bronchoalveolar lavage (BAL) procedure either at the end of the infusion or at 2 h after the end of infusion. Plasma samples were collected at 1, 3, 5 and 7 h after the start of infusion. After correcting for BAL dilution, cefiderocol concentrations in epithelial lining fluid (ELF) for each patient and the ELF/unbound plasma concentration ratio (RC, E/P) were calculated. Safety was assessed up to 7 days after the last cefiderocol dose.

RESULTS

Seven patients received cefiderocol. Geometric mean ELF concentration of cefiderocol was 7.63 mg/L at the end of infusion and 10.40 mg/L at 2 h after the end of infusion. RC, E/P was 0.212 at the end of infusion and 0.547 at 2 h after the end of infusion, suggesting delayed lung distribution. There were no adverse drug reactions.

CONCLUSIONS

The results suggest that cefiderocol penetrates the ELF in critically ill pneumonia patients with concentrations that are sufficient to treat Gram-negative bacteria with an MIC of ≤4 mg/L.

Beta-Lactams Dosing in Critically Ill Patients with Gram-Negative Bacterial Infections: A PK/PD Approach

Beta-lactam antibiotics are often the backbone of treatment for Gram-negative infections in the critically ill. Beta-lactams exhibit time-dependent killing, and their efficacy depends on the percentage of dosing interval that the concentration remains above the minimum inhibitory concentration. The Gram-negative resistance rates of pathogens are increasing in the intensive care unit (ICU), and critically ill patients often possess physiology that makes dosing more challenging. The volume of distribution is usually increased, and drug clearance is variable. Augmented renal clearance and hypermetabolic states increase the clearance of beta-lactams, while acute kidney injury reduces the clearance. To overcome the factors affecting ICU patients and decreasing susceptibilities, dosing strategies involving higher doses, and extended or continuous infusions may be required. In this review, we specifically examined pharmacokinetic models in ICU patients, to determine the desired beta-lactam regimens for clinical breakpoints of Enterobacterales and Pseudomonas aeruginosa, as determined by the European Committee on Antimicrobial Susceptibility Testing. The beta-lactams evaluated included penicillins, cephalosporins, carbapenems, and monobactams. We found that when treating less-susceptible pathogens, especially P. aeruginosa, continuous infusions are frequently needed to achieve the desired pharmacokinetic/pharmacodynamic targets. More studies are needed to determine optimal dosing strategies in the novel beta-lactams.

Evaluation of a New Culture-Based AtbFinder Test-System Employing a Novel Nutrient Medium for the Selection of Optimal Antibiotics for Critically Ill Patients with Polymicrobial Infections within 4 h

Here, we describe the validation of a new phenotypic culture-based AtbFinder method for rapid selection of antibiotics in vitro using specimens with mono- and polybacterial infections. AtbFinder, which can be applied to any type of non-blood tissue, does not require isolation of pure bacterial cultures. The method uses a novel TGV medium that allows more rapid bacterial growth of Gram-positive and Gram-negative monoisolates compared with that achieved with conventional laboratory media, demonstrating overall sensitivity, specificity, PPV, NPV values of 99.6%, 98.1%, 98.5%, and 99.4%, respectively, after 4 h. For polymicrobial infections, AtbFinder utilized a novel paradigm of the population response to antibiotics, enabling bacterial growth in the form of a mixed microbial community and selecting antibiotics targeting not only the principal pathogen, but also those bacteria that support their growth. TGV medium allowed culturing of a more diverse set of bacteria from polymicrobial biospecimens, compared with that achieved with the standard media, and enabled, within 4 h, accurate selection of the antibiotics that completely eliminated all cultivatable bacteria from clinical samples. In conclusion, the AtbFinder system may be a valuable tool in improving antibiotic selection, and enabling targeted empirical therapy and accurate antibiotic replacement, which is especially important in high-risk patients.

An evaluation of antipseudomonal dosing on the incidence of treatment failure

Introduction

Significant mortality is associated with delays in appropriate antibiotic therapy in Pseudomonas aeruginosa infections. The impact of empiric dosing on clinical outcomes has been largely unreported.

Methods

This retrospective cohort compared treatment failure in patients receiving guideline-concordant or guideline-discordant empiric therapy with cefepime, meropenem, or piperacillin/tazobactam. Patients with culture-positive P. aeruginosa between 1 July 2013 and 31 July 2019 were eligible for inclusion. Patients with cystic fibrosis, polymicrobial infection, and urinary or pulmonary colonization were excluded. The composite primary outcome was treatment failure, defined as (1) therapy modification due to resistance/perceived treatment failure, (2) increased/unchanged qSOFA, or (3) persistent fever 48 h after initiating appropriate therapy. Secondary outcomes included rate of infectious diseases consultation, all-cause inpatient mortality, mechanical ventilation requirement, and infection-related intensive care unit and hospital lengths of stay.

Results

In total, 198 patients were included: 90 guideline-concordant and 108 guideline-discordant. Baseline characteristics were balanced. Treatment failure was more common in the guideline-discordant than the guideline-concordant group (62% versus 48%; p = 0.04). This remained significant when adjusting for supratherapeutic dosing (p = 0.02). Infectious diseases consultation was higher in the guideline-discordant group (46% versus 29%, p = 0.01), while intensive care unit length of stay was longer in the guideline-concordant group (4.5 versus 3 days, p = 0.03). Additional secondary outcomes were similar.

Conclusion

Treatment failure was significantly higher in patients receiving guideline-discordant empiric antipseudomonal dosing. Guideline-directed dosing, disease states, and patient-specific factors should be assessed when considering empiric antipseudomonal dosing.

Emergence of Resistance in Klebsiella aerogenes to Piperacillin-Tazobactam and Ceftriaxone

We examined the effects of piperacillin-tazobactam (TZP) concentration and bacterial inoculum on in vitro killing and the emergence of resistance in Klebsiella aerogenes The MICs for 15 clinical respiratory isolates were determined by broth microdilution for TZP and by Etest for ceftriaxone (CRO) and cefepime (FEP). The presence of resistance in TZP-susceptible isolates (n = 10) was determined by serial passes over increasing concentrations of TZP-containing and CRO-containing agar plates. Isolates with growth on TZP 16/4-μg/ml and CRO 8-μg/ml plates (n = 5) were tested in high-inoculum (HI; 7.0 log10 CFU/ml) and low-inoculum (LI; 5.0 log10 CFU/ml) time-kill studies. Antibiotic concentrations were selected to approximate TZP 3.375 g every 8 h (q8h) via a 4-h prolonged-infusion free peak concentration (40 μg/ml [TZP40]), peak epithelial lining fluid (ELF) concentrations, and average AUC0-24 values for TZP (20 μg/ml [TZP20] and 10 μg/ml [TZP10], respectively), the ELF FEP concentration (14 μg/ml), and the average AUC0-24 CRO concentration (6 μg/ml). For HI, FEP exposure significantly reduced 24-h inocula against all comparators (P ≤ 0.05) with a reduction of 4.93 ± 0.64 log10 CFU/ml. Exposure to TZP40, TZP20, and TZP10 reduced inocula by 0.81 ± 0.43, 0.21 ± 0.18, and 0.05 ± 0.16 log10 CFU/ml, respectively. CRO-exposed isolates demonstrated an increase of 0.42 ± 0.39 log10 CFU/ml compared to the starting inocula, with four of five CRO-exposed isolates demonstrating TZP-nonsusceptibility. At LI after 24 h of exposure to TZP20 and TZP10, the starting inoculum decreased by averages of 2.24 ± 1.98 and 2.91 ± 0.50 log10 CFU/ml, respectively. TZP demonstrated significant inoculum-dependent killing, warranting dose optimization studies.

Clinical Microbiology in the Intensive Care Unit: Time for Intensivists to Rejuvenate this Lost Art

We live in an era of evolving microbial infections and equally evolving drug resistance among microorganisms. In any healthcare facility, intensivists play the most pivotal role with critically ill patients under their direct care. Majority of the critically ill patients already harbor a microorganism at admission or acquire one in the form of healthcare-associated infections during their course of intensive care unit stay. It is therefore rather imperative for intensivists to possess sound knowledge in clinical microbiology. On a negative note, most clinicians have very meager and remote knowledge acquired during their undergraduate years. This knowledge is rather theoretical than applied and wanes over the years becoming nonbeneficial in intensive patient care. We, therefore, intend to explore important concepts in applied microbiology and infection control that intensivists should know and implement in their clinical practice on a day-to-day basis.

How to cite this article: Princess I, Vadala R. Clinical Microbiology in the Intensive Care Unit: Time for Intensivists to Rejuvenate this Lost Art. Indian J Crit Care Med 2021;25(5):566–574.

Intrapulmonary Pharmacokinetics of Cefepime and Enmetazobactam in Healthy Volunteers: Towards New Treatments for Nosocomial Pneumonia

Cefepime-enmetazobactam is a novel β-lactam-β-lactamase inhibitor combination with broad-spectrum antimicrobial activity against a range of multidrug-resistant Enterobacteriaceae This agent is being developed for a range of serious hospital infections. An understanding of the extent of partitioning of β-lactam-β-lactamase inhibitor combinations into the human lung is required to better understand the potential role of cefepime-enmetazobactam for the treatment of nosocomial pneumonia. A total of 20 healthy volunteers were used to study the intrapulmonary pharmacokinetics of a regimen of 2 g cefepime-1 g enmetazobactam every 8 h intravenously (2 g/1 g q8h i.v.). Each volunteer contributed multiple plasma samples and a single epithelial lining fluid (ELF) sample, obtained by bronchoalveolar lavage. Concentrations of cefepime and enmetazobactam were quantified using liquid chromatography-tandem mass spectrometry. The pharmacokinetic data were modeled using a population methodology, and Monte Carlo simulations were performed to assess the attainment of pharmacodynamic targets defined in preclinical models. The concentration-time profiles of both agents in plasma and ELF were similar. The mean ± standard deviation percentage of partitioning of total drug concentrations of cefepime and enmetazobactam between plasma and ELF was 60.59% ± 28.62% and 53.03% ± 21.05%, respectively. Using pharmacodynamic targets for cefepime of greater than the MIC and free enmetazobactam concentrations of >2 mg/liter in ELF of 20% of the dosing interval, a regimen of cefepime-enmetazobactam of 2 g/0.5 g q8h i.v. infused over 2 h resulted in a probability of target attainment of ≥90% for Enterobacteriaceae with cefepime-enmetazobactam MICs of ≤8 mg/liter. This result provides a rationale to further consider cefepime-enmetazobactam for the treatment of nosocomial pneumonia caused by multidrug-resistant Enterobacteriaceae.

Low penetrance of antibiotics in the epithelial lining fluid. The role of inhaled antibiotics in patients with bronchiectasis

Plasma drug concentrations, spectrum of antibacterial activity and minimum inhibitory concentration (MIC) had been widely considered as markers of the efficacy of antibiotics. Nonetheless, in several cases, antibiotics characterized by all these features were ineffective for the treatment of respiratory tract infections. A typical paradigm represented the case of patients with bronchiectasis who do not always benefit from antibiotics and thus experiencing increased sputum production, worse quality of life, more rapid forced expiratory volume in the first second (FEV1) decline, more frequent exacerbations and increased mortality rates, especially those with Pseudomonas aeruginosa (P. aeruginosa) chronic infection. Subsequently, penetrance of antibiotics in the epithelial lining fluid has gradually emerged as another key factor for the outcome of antibiotic treatment. Given that a plethora of antibiotics presented with poor or intermediate penetrance in the epithelial lining fluid, inhaled antibiotics targeting directly the site of infection emerged as a new option for patients with respiratory disorders including patients with bronchiectasis. This review article intends to summarize the current state of knowledge for the penetrance of antibiotics in the epithelial lining fluid and present results from clinical trials of inhaled antibiotics in patients with bronchiectasis of etiology other than cystic fibrosis.

Intrapulmonary pharmacokinetics of cefiderocol, a novel siderophore cephalosporin, in healthy adult subjects

BACKGROUND

Cefiderocol, a novel siderophore cephalosporin, has shown potent activity against Gram-negative bacteria, including MDR pathogens. Cefiderocol is under clinical investigation for the treatment of serious Gram-negative infections including nosocomial pneumonia.

OBJECTIVES

This study assessed intrapulmonary penetration after a single intravenous dose of cefiderocol (2000 mg infused over 60 min) in healthy adult males.

MATERIALS AND METHODS

Each subject underwent one bronchoscopy with bronchoalveolar lavage (BAL) to collect BAL fluid (BALF). Fifteen subjects were assigned to one of three collection timepoints (1, 2 or 4 h from start of infusion). Five additional subjects were assigned to a collection timepoint at 6 h, which was added based on concentration data between 1 and 4 h predicting measurable BALF cefiderocol concentrations at 6 h.

RESULTS

Cefiderocol concentrations in plasma, epithelial lining fluid (ELF) and alveolar macrophages (AMs) were calculated for each subject. The ELF concentration of cefiderocol was 13.8, 6.69, 2.78 and 1.38 mg/L at 1, 2, 4 and 6 h after single intravenous dosing, respectively. Over 6 h, geometric mean concentration ratios ranged from 0.0927 to 0.116 for ELF to total plasma and from 0.00496 to 0.104 for AMs to total plasma. AUC ratios of ELF and AMs to plasma were 0.101 and 0.0177 based on total drug in plasma, respectively, and 0.239 and 0.0419 based on free drug in plasma, respectively. There were no major drug-related adverse events.

CONCLUSIONS

Results of this study indicate that cefiderocol penetrates into ELF, and ELF and plasma concentrations appear to be parallel.

Gradual in vitro Evolution of Cefepime Resistance in an ST131 Escherichia coli Strain Expressing a Plasmid-Encoded CMY-2 β-Lactamase

Background

In a previous report, a clinical ST131 Escherichia coli isolate (Ec-1),producing a plasmid-encoded AmpC β-lactamase CMY-2, evolved in vivo under cefepime (FEP) treatment to the FEP-resistant Ec-2 strain expressing an extended-spectrum β-lactamase CMY-33. To compare factors responsible for in vitro and in vivo FEP resistance, we reproduced in vitro FEP resistance evolution in Ec-1.

Methods

FEP-resistant mutants were generated by subjecting Ec-1 (FEP MIC = 0.125 mg/L) to sub-inhibitory concentrations of FEP. MICs were obtained by broth microdilution or Etest. Strains were sequenced on an Illumina HiSeq platform. Transcriptional levels and plasmid copy numbers were determined by real-time PCR. Outer membrane proteins (OMPs) were extracted and separated by SDS-PAGE. Growth kinetics was evaluated by measuring OD₄₅₀.

Results

The CMY-2 expressed by Ec-1 evolved to a CMY-69 (strain EC-4) by an Ala294Pro substitution after 24 passages. After 30 passages, the FEP MIC increased to 256 mg/L (strain EC-32). SDS PAGE did not reveal any lack of OMPs in the mutant strains. However, bla_CMY transcription levels were up to 14-times higher than in Ec-1, which was partially explained by mutations in the upstream region of repA resulting in a higher copy number of the bla_CMY-harboring IncI1 plasmid. All mutants showed a slight growth defect but no significant difference in relative growth rates compared to Ec-1.

Conclusion

In vitro sub-inhibitory concentrations of FEP resulted in the selection of resistance mutations altering the H-10 helix of the CMY-2 and increasing the plasmid copy number. Appropriate dosing strategies may help preventing resistance evolution during treatments.

Individualising Therapy to Minimize Bacterial Multidrug Resistance

The scourge of antibiotic resistance threatens modern healthcare delivery. A contributing factor to this significant issue may be antibiotic dosing, whereby standard antibiotic regimens are unable to suppress the emergence of antibiotic resistance. This article aims to review the role of pharmacokinetic and pharmacodynamic (PK/PD) measures for optimising antibiotic therapy to minimise resistance emergence. It also seeks to describe the utility of combination antibiotic therapy for suppression of resistance and summarise the role of biomarkers in individualising antibiotic therapy. Scientific journals indexed in PubMed and Web of Science were searched to identify relevant articles and summarise existing evidence. Studies suggest that optimising antibiotic dosing to attain defined PK/PD ratios may limit the emergence of resistance. A maximum aminoglycoside concentration to minimum inhibitory concentration (MIC) ratio of > 20, a fluoroquinolone area under the concentration-time curve to MIC ratio of > 285 and a β-lactam trough concentration of > 6 × MIC are likely required for resistance suppression. In vitro studies demonstrate a clear advantage for some antibiotic combinations. However, clinical evidence is limited, suggesting that the use of combination regimens should be assessed on an individual patient basis. Biomarkers, such as procalcitonin, may help to individualise and reduce the duration of antibiotic treatment, which may minimise antibiotic resistance emergence during therapy. Future studies should translate laboratory-based studies into clinical trials and validate the appropriate clinical PK/PD predictors required for resistance suppression in vivo. Other adjunct strategies, such as biomarker-guided therapy or the use of antibiotic combinations require further investigation.

Carbapenems vs. alternative β-lactams for the treatment of nosocomial pneumonia: A systematic review and meta-analysis

BACKGROUND

Carbapenems have shown efficacy in treating nosocomial pneumonias in clinical trials despite a reported low lung penetration compared with other β-lactams. Preserving the clinical activity of carbapenems through stewardship efforts is essential. The aim of this review was to identify any differences in outcomes potentially as a function of decreased penetration.

METHODS

PubMed and the Cochrane Library were systematically searched for clinical trials comparing carbapenems with other anti-pseudomonal β-lactams for treatment of nosocomial pneumonia through to end December 2016. Trials reporting clinical and microbiological outcomes associated with treatment were included. Pediatric studies and those with uneven comparators (e.g., carbapenem vs. combination Gram-negative therapy) were excluded. Fixed effects models were used to evaluate the impact of treatment on the odds of clinical failure, death, or microbiological failure.

RESULTS

252 unique articles were identified; five met inclusion criteria and comprised 640 patients in the carbapenem group and 634 patients in the β-lactam group. No differences in clinical failure (odds ratio [OR] 1.08, 95% confidence interval [CI] [0.81-1.44], I2=16%) or mortality (OR 0.75, CI 0.57-1.11, I2=0%) were noted between groups. Patients infected with P. aeruginosa and treated with imipenem were more likely to experience clinical failure (OR 4.21, CI 1.51-11.12, I2=44%) and to develop resistance to the study carbapenem (OR 2.86, CI 1.08-6.44, I2= 13%) than those treated with alternative β-lactams.

CONCLUSIONS

No differences in clinical outcomes were observed between carbapenems and non-carbapenem β-lactams in nosocomial pneumonias. Those infected with P. aeruginosa fared worse and were more likely to have resistance develop if they were treated with imipenem. Additional studies are warranted.

Pharmacokinetics-pharmacodynamics issues relevant for the clinical use of beta-lactam antibiotics in critically ill patients

Antimicrobials are among the most important and commonly prescribed drugs in the management of critically ill patients and beta-lactams are the most common antibiotic class used. Critically ill patient's pathophysiological factors lead to altered pharmacokinetics and pharmacodynamics of beta-lactams.A comprehensive bibliographic search in PubMed database of all English language articles published from January 2000 to December 2017 was performed, allowing the selection of articles addressing the pharmacokinetics or pharmacodynamics of beta-lactam antibiotics in critically ill patients.In critically ill patients, several factors may increase volume of distribution and enhance renal clearance, inducing high intra- and inter-patient variability in beta-lactam concentration and promoting the risk of antibiotic underdosing. The duration of infusion of beta-lactams has been shown to influence the fT > minimal inhibitory concentration and an improved beta-lactam pharmacodynamics profile may be obtained by longer exposure with more frequent dosing, extended infusions, or continuous infusions.The use of extracorporeal support techniques in the critically ill may further contribute to this problem and we recommend not reducing standard antibiotic dosage since no drug accumulation was found in the available literature and to maintain continuous or prolonged infusion, especially for the treatment of infections caused by multidrug-resistant bacteria.Prediction of outcome based on concentrations in plasma results in overestimation of antimicrobial activity at the site of infection, namely in cerebrospinal fluid and the lung. Therefore, although no studies have assessed clinical outcome, we recommend using higher than standard dosing, preferably with continuous or prolonged infusions, especially when treating less susceptible bacterial strains at these sites, as the pharmacodynamics profile may improve with no apparent increase in toxicity.A therapeutic drug monitoring-guided approach could be particularly useful in critically ill patients in whom achieving target concentrations is more difficult, such as obese patients, immunocompromised patients, those infected by highly resistant bacterial strains, patients with augmented renal clearance, and those undergoing extracorporeal support techniques.

Plasma and Intrapulmonary Concentrations of Cefepime and Zidebactam following Intravenous Administration of WCK 5222 to Healthy Adult Subjects

WCK 5222 is a combination of cefepime and the novel β-lactam enhancer zidebactam being developed for the treatment of serious Gram-negative bacterial infections. The objective of this study was to compare plasma (total), epithelial lining fluid (ELF), and alveolar macrophage (AM) concentrations of cefepime and zidebactam in healthy adult subjects. The WCK 5222 dosing regimen was 2 g cefepime/1 g zidebactam administered as a 1-h intravenous infusion every 8 h for a total of 7 doses. Subjects were assigned to one bronchoalveolar lavage (BAL) sampling time at 0.5, 1.25, 3, 6, 8, or 10 h after the seventh dose. Noncompartmental pharmacokinetic parameters were determined from serial plasma concentrations collected over 8-hour and 10-hour intervals following the first and seventh doses, respectively. Penetration ratios were calculated from the area under the plasma concentration-time curve from 0 to 8 h (AUC_0-8) for plasma, ELF, and AM using mean and median concentrations at each BAL sampling time. The plasma maximum concentration of drug (C_max) and AUC values of cefepime and zidebactam increased by 8% to 9% after the seventh versus the first dose of WCK 5222. The respective AUC_0-8 values based on mean concentrations of cefepime and zidebactam in ELF were 127.9 and 52.0 mg · h/liter, and 87.9 and 13.2 mg · h/liter in AM. The ELF to total plasma penetration ratios of cefepime and zidebactam based on mean AUC_0-8 values were 0.39 and 0.38, respectively. The AM to total plasma ratios were 0.27 and 0.10, respectively. The observed plasma, ELF, and AM concentrations of cefepime and zidebactam support studies of WCK 5222 for treatment of pneumonia caused by susceptible pathogens.

A research pathway for the study of the delivery and disposition of nebulised antibiotics: an incremental approach from in vitro to large animal models

Background

Nebulised antibiotics are frequently used for the prevention or treatment of ventilator-associated pneumonia. Many factors may influence pulmonary drug concentrations with inaccurate dosing schedules potentially leading to therapeutic failure and/or the emergence of antibiotic resistance. We describe a research pathway for studying the pharmacokinetics of a nebulised antibiotic during mechanical ventilation using in vitro methods and ovine models, using tobramycin as the study antibiotic.

Methods

In vitro studies using a laser diffractometer and a bacterial-viral filter were used to measure the effect of the type and size of tracheal tubes and antibiotic concentration on the particle size distribution of the tobramycin 400 mg (4 ml; 100 mg/ml) and 160 mg (4 ml, 40 mg/ml) aerosol and nebulised mass delivered. To compare the regional drug distribution in the lung of two routes (intravenous and nebulised) of drug administration of tobramycin 400 mg, technetium-99m-labelled tobramycin 400 mg with planar nuclear medicine imaging was used in a mechanically ventilated ovine model. To measure tobramycin concentrations by intravenous and nebulised tobramycin 400 mg (4 ml, 100 mg/ml) administration in the lung interstitial space (ISF) fluid and blood of mechanically ventilated sheep, the microdialysis technique was used over an 8-h duration.

Results

Tobramycin 100 mg/ml achieved a higher lung dose (121.3 mg) compared to 40 mg/ml (41.3 mg) solution. The imaging study with labelled tobramycin indicated that nebulised tobramycin distributed more extensively into each lung zone of the mechanically ventilated sheep than intravenous administration. A higher lung ISF peak concentration of tobramycin was observed with nebulised tobramycin (40.8 mg/l) compared to intravenous route (19.0 mg/l).

Conclusions

The research methods appear promising to describe lung pharmacokinetics for formulations intended for nebulisation during mechanical ventilation. These methods need further validation in an experimental pneumonia model to be able to contribute toward optimising dosing regimens to inform clinical trials and/or clinical use.

Application of Antibiotic Pharmacodynamics and Dosing Principles in Patients With Sepsis

Sepsis is associated with marked mortality, which may be reduced by prompt initiation of adequate, appropriate doses of antibiotic. Critically ill patients often have physiological changes that reduce blood and tissue concentrations of antibiotic and high rates of multidrug-resistant pathogens, which may affect patients' outcomes. All critical care professionals, including critical care nurses, should understand antibiotic pharmacokinetics and pharmacodynamics to ensure sound antibiotic dosing and administration strategies for optimal microbial killing and patients' outcomes. Effective pathogen eradication occurs when the dose of antibiotic reaches or maintains optimal concentrations relative to the minimum inhibitory concentration for the pathogen. Time-dependent antibiotics, such as β-lactams, can be given as extended or continuous infusions. Concentration-dependent antibiotics such as aminoglycosides are optimized by using high, once-daily dosing strategies with serum concentration monitoring. Vancomycin and fluoroquinolones are dependent on both time and concentration above the minimum inhibitory concentration.

Pharmacokinetic drug evaluation of avibactam + ceftazidime for the treatment of hospital-acquired pneumonia

INTRODUCTION

Ceftazidime-avibactam (CAZ-AVI) is a combination of a third-generation cephalosporin and a non-β-lactam, β-lactamase inhibitor, recently approved for urinary tract infections and complicated abdominal infections. Moreover, it represents a treatment option for patients with hospital acquired pneumonia (HAP), especially when caused by multidrug-resistant (MDR) bacteria. Areas covered: The review focuses on the pharmacokinetics (PK) of CAZ-AVI in HAP and on preclinical and clinical studies evaluating PK/pharmacodynamics (PD) in this field. Expert opinion: In vitro and in vivo data about PK/PD of CAZ-AVI confirm that penetration of CAZ-AVI in the epithelial lining fluid (ELF) represents approximately 30% of the plasma concentrations. Clinical studies documented that CAZ-AVI 2000 mg/500 mg every 8 h is the optimal dose regimen to achieve the PK/PD target attainment in patients with HAP. Thus, CAZ-AVI could represent an option both to treat HAP caused by Gram-negative bacilli (GNB) displaying resistance to most of the antibiotics and to reduce the use of carbapenems, limiting the onset of resistance profiles among GNB. Additional information about specific patients populations, such as critically-ill subjects or pediatric patients, are needed for a more individualized use of CAZ-AVI.

Considerations for effect site pharmacokinetics to estimate drug exposure: concentrations of antibiotics in the lung

Bronchoalveolar lavage (BAL) and microdialysis have become the most reliable and relevant methods for measuring lung concentrations of antibiotics, with the majority of BAL studies involving either healthy adult subjects or patients undergoing diagnostic bronchoscopy. Emphasis on the amount of drug that reaches the site of infection is increasingly recognized as necessary to determine whether a dose selection will translate to good clinical outcomes in the treatment of patients with pneumonia. Observed concentrations and/or parameters of exposure (e.g. area-under-the-curve) need to be incorporated with pharmacokinetic-pharmacodynamic indices so that rational dose selection can be identified for specific pathogens and types of pneumonic infection (community-acquired vs hospital-acquired bacterial pneumonia, including ventilator-associated bacterial pneumonia). Although having measured plasma or lung concentration-time data from critically ill patients to incorporate into pharmacokinetic-pharmacodynamic models is very unlikely during drug development, it is essential that altered distribution, augmented renal clearance, and renal or hepatic dysfunction should be considered. Notably, the number of published studies involving microdialysis and intrapulmonary penetration of antibiotics has been limited and mainly involve beta-lactam agents, levofloxacin, and fosfomycin. Opportunities to measure in high-resolution effect site spatial pharmacokinetics (e.g. with MALDI-MSI or PET imaging) and in vivo continuous drug concentrations (e.g. with aptamer-based probes) now exist. Going forward these studies could be incorporated into antibiotic development programs for pneumonia in order to further increase the probability of candidate success.

Pharmacokinetics and Dosing of Ceftobiprole Medocaril for the Treatment of Hospital- and Community-Acquired Pneumonia in Different Patient Populations

Hospital-acquired pneumonia (HAP) and community-acquired pneumonia (CAP) are among the most common infections treated in the hospital setting, and together they place a significant burden on healthcare systems. Successful management of HAP and CAP depends on rapid initiation of empirical antibiotic therapy with broad-spectrum antibiotics. Ceftobiprole is a new-generation, broad-spectrum cephalosporin antibiotic for the treatment of HAP (excluding ventilator-associated pneumonia) and CAP. It displays potent in vitro activity against a broad range of pathogens important in pneumonia. This review summarizes the pharmacokinetic profile of ceftobiprole, and considers the pharmacokinetic parameters and pharmacodynamics underlying the choice of dosing regimen. Ceftobiprole shows linear pharmacokinetics after single and multiple doses and is eliminated predominantly through the kidneys. Ceftobiprole is administered as a 500 mg intravenous infusion over 2 h every 8 h, and steady-state concentrations are reached on the first day of dosing. Dose adjustment is recommended for patients with moderate or severe renal impairment and for those with end-stage renal disease. Extending the infusion time of ceftobiprole to 4 h is recommended to optimize drug exposure in critically ill patients with augmented renal clearance. However, there is no need for dose adjustments based on age, sex or ethnicity, or for patients with severe obesity. Population pharmacokinetic modelling and Monte Carlo simulations were used to determine the optimal dosing regimen for ceftobiprole in special patient populations, including paediatric patients. Future studies of ceftobiprole in patients with HAP and CAP would be of interest.

The role of infection models and PK/PD modelling for optimising care of critically ill patients with severe infections

Critically ill patients with severe infections are at high risk of suboptimal antimicrobial dosing. The pharmacokinetics (PK) and pharmacodynamics (PD) of antimicrobials in these patients differ significantly from the patient groups from whose data the conventional dosing regimens were developed. Use of such regimens often results in inadequate antimicrobial concentrations at the site of infection and is associated with poor patient outcomes. In this article, we describe the potential of in vitro and in vivo infection models, clinical pharmacokinetic data and pharmacokinetic/pharmacodynamic models to guide the design of more effective antimicrobial dosing regimens. Individualised dosing, based on population PK models and patient factors (e.g. renal function and weight) known to influence antimicrobial PK, increases the probability of achieving therapeutic drug exposures while at the same time avoiding toxic concentrations. When therapeutic drug monitoring (TDM) is applied, early dose adaptation to the needs of the individual patient is possible. TDM is likely to be of particular importance for infected critically ill patients, where profound PK changes are present and prompt appropriate antibiotic therapy is crucial. In the light of the continued high mortality rates in critically ill patients with severe infections, a paradigm shift to refined dosing strategies for antimicrobials is warranted to enhance the probability of achieving drug concentrations that increase the likelihood of clinical success.

Penetration of Ceftaroline into the Epithelial Lining Fluid of Healthy Adult Subjects

Ceftaroline, the active metabolite of the prodrug ceftaroline fosamil, is a cephalosporin with bactericidal activity against Gram-positive organisms, including methicillin-resistant Staphylococcus aureus (MRSA). This study aimed to (i) evaluate ceftaroline concentrations in human plasma and epithelial lining fluid (ELF) and (ii) develop a population pharmacokinetic (PK) model for plasma and ELF to be used in PK/pharmacodynamic (PD) target attainment simulations. Ceftaroline concentrations in ELF and plasma at steady state (day 4) were measured in healthy adult subjects for two dosages: 600 mg every 12 h (q12h) and 600 mg every 8 h (q8h). Both were well tolerated with no serious adverse events. The penetration of free ceftaroline into ELF, assuming 20% protein binding in plasma and no protein binding in ELF, was ≈23%. The population PK model utilized a two-compartment model for both ceftaroline fosamil and ceftaroline. Goodness-of-fit criteria revealed the model was consistent with observed data and no systematic bias remained. At 600 mg q12h and a MIC of 1 mg/liter, 98.1% of simulated patients would be expected to achieve a target free drug concentration above the MIC (fT>MIC) in plasma of 42%, and in ELF 81.7% would be expected to achieve a target fT>MIC of 17%; at 600 mg q8h, 100% were predicted to achieve an fT>MIC in plasma of 42% and 94.7% to achieve an fT>MIC of 17% in ELF. The literature and data suggest the 600 mg q12h dose is adequate for MICs of ≤1 mg/liter. There is a need for clinical data in patients with MRSA pneumonia and data to correlate PK/PD relationships in ELF with clinical outcomes.

Pharmacokinetic and Pharmacodynamic Principles of Anti-infective Dosing

PURPOSE

An understanding of the pharmacokinetic (PK) and pharmacodynamic (PD) principles that determine response to antimicrobial therapy can provide the clinician with better-informed dosing regimens. Factors influential on antibiotic disposition and clinical outcome are presented, with a focus on the primary site of infection. Techniques to better understand antibiotic PK and optimize PD are acknowledged.

METHODS

PubMed (inception-April 2016) was reviewed for relevant publications assessing antimicrobial exposures within different anatomic locations and clinical outcomes for various infection sites.

FINDINGS

A limited literature base indicates variable penetration of antibiotics to different target sites of infection, with drug solubility and extent of protein binding providing significant PK influences in addition to the major clearing pathway of the agent. PD indices derived from in vitro studies and animal models determine the optimal magnitude and frequency of dosing regimens for patients. PK/PD modeling and simulation has been shown an efficient means of assessing these PD endpoints against a variety of PK determinants, clarifying the unique effects of infection site and patient characteristics to inform the adequacy of a given antibiotic regimen.

IMPLICATIONS

Appreciation of the PK properties of an antibiotic and its PD measure of efficacy can maximize the utility of these life-saving drugs. Unfortunately, clinical data remain limited for a number of infection site-antibiotic exposure relationships. Modeling and simulation can bridge preclinical and patient data for the prescription of optimal antibiotic dosing regimens, consistent with the tenets of personalized medicine.

Pharmacokinetic Alterations of Antimicrobials in the Critically Ill

Critical illness is accompanied by multiple physiologic alterations that affect the pharmacokinetics of antimicrobials. Although the pharmacokinetics of a number of antimicrobials have been studied in critically ill individuals, an understanding of the physiological alterations in critical illness and general pharmacokinetic principles of antimicrobials is imperative for appropriate selection, dosing, and prediction of toxicity.

Cefepime free minimum concentration to minimum inhibitory concentration (fCmin/MIC) ratio predicts clinical failure in patients with Gram-negative bacterial pneumonia

Cefepime is an antibiotic commonly used in nosocomial infections. The objective of this study was to elucidate the relationship between cefepime exposure and clinical outcome in patients with Gram-negative bacterial pneumonia. A previously published population pharmacokinetic model of cefepime was validated in 12 adult patients with normal renal function by measuring plasma concentrations at steady-state. Additionally, clinical outcomes for 33 patients with Gram-negative bacterial pneumonia who received cefepime monotherapy were determined. The free minimum concentration (fCmin) to MIC ratio for each patient was determined by conditioning the validated pharmacokinetic model using patient-specific creatinine clearance (CLCr), dosing regimen and cefepime MIC of the organism isolated, and was subsequently correlated with clinical failure. Classification and regression tree (CART) analysis was used to determine the most significant drug exposure breakpoint. Mean±S.D. CLCr and cefepime Cmin in the 12 patients were 87.5±21.2mL/min and 6.2±3.8mg/L, respectively. In comparison, the Cmin predicted by the pharmacokinetic model was 5.8mg/L using a CLCr of 90mL/min. MICs of organisms ranged from 0.5mg/L to 8mg/L. Percent time free drug above MIC of 100% was achieved in 32/33 patients, but 12 patients experienced clinical failure. CART analysis determined patients with an fCmin/MIC≥2.1 had a significantly lower risk of clinical failure (OR=0.11, 95% CI 0.02-0.67; P=0.017). The fCmin/MIC ratio is a useful predictor of clinical failure in Gram-negative bacterial pneumonia. The clinical utility of fCmin/MIC in therapeutic drug monitoring should be further explored.

In vitro activity of human-simulated epithelial lining fluid exposures of ceftaroline, ceftriaxone, and vancomycin against methicillin-susceptible and -resistant Staphylococcus aureus

Staphylococcus aureus, including methicillin-susceptible (MSSA) and -resistant (MRSA) strains, is an important pathogen of bacterial pneumonia. As antibiotic concentrations at the site of infection are responsible for killing, we investigated the activity of human-simulated epithelial lining fluid (ELF) exposures of three antibiotics (ceftaroline, ceftriaxone, and vancomycin) commonly used for treatment of S. aureus pneumonia. An in vitro pharmacodynamic model was used to simulate ELF exposures of vancomycin (1 g every 12 h [q12h]), ceftaroline (600 mg q12h and q8h), and ceftriaxone (2 g q24h and q12h). Four S. aureus isolates (2 MSSA and 2 MRSA) were evaluated over 72 h with a starting inoculum of ∼ 10(6) CFU/ml. Time-kill curves were constructed, and microbiological response (change in log10 CFU/ml from 0 h and the area under the bacterial killing and regrowth curve [AUBC]) was assessed in duplicate. The change in 72-h log10 CFU/ml was largest for ceftaroline q8h (reductions of >3 log10 CFU/ml against all strains). This regimen also achieved the lowest AUBC against all organisms (P < 0.05). Vancomycin produced reliable bacterial reductions of 0.9 to 3.3 log10 CFU/ml, while the activity of ceftaroline q12h was more variable (reductions of 0.2 to 2.3 log10 CFU/ml against 3 of 4 strains). Both regimens of ceftriaxone were poorly active against MSSA tested (0.1 reduction to a 1.8-log10 CFU/ml increase). Against these S. aureus isolates, ELF exposures of ceftaroline 600 mg q8h exhibited improved antibacterial activity compared with ceftaroline 600 mg q12h and vancomycin, and therefore, this q8h regimen deserves further evaluation for the treatment of bacterial pneumonia. These data also suggest that ceftriaxone should be avoided for S. aureus pneumonia.

Measuring drug distribution in the critically ill patient

Critically ill patients often present with a combination of disease states and comorbid conditions that progress over a clinical course. This can manifest in physiological changes, such as fluid shifts, alterations in protein binding, and acid-base balance issues, which may in turn alter a drug's distribution, potentially towards or away from its site of action. It's vital that these factors are examined for drugs used in critical illness in varying disease states, acute and chronic in nature. Several methods have been used to study the variations in target site penetration, but few provide a feasible option to reliably measure active drug concentrations at the site of action over time. This review examines these techniques, their merits and shortcomings, generally and as they relate to use in critically ill.

Comparison of intrapulmonary and systemic pharmacokinetics of colistin methanesulfonate (CMS) and colistin after aerosol delivery and intravenous administration of CMS in critically ill patients

Colistin is an old antibiotic that has recently gained a considerable renewal of interest for the treatment of pulmonary infections due to multidrug-resistant Gram-negative bacteria. Nebulization seems to be a promising form of administration, but colistin is administered as an inactive prodrug, colistin methanesulfonate (CMS); however, differences between the intrapulmonary concentrations of the active moiety as a function of the route of administration in critically ill patients have not been precisely documented. In this study, CMS and colistin concentrations were measured on two separate occasions within the plasma and epithelial lining fluid (ELF) of critically ill patients (n = 12) who had received 2 million international units (MIU) of CMS by aerosol delivery and then intravenous administration. The pharmacokinetic analysis was conducted using a population approach and completed by pharmacokinetic-pharmacodynamic (PK-PD) modeling and simulations. The ELF colistin concentrations varied considerably (9.53 to 1,137 mg/liter), but they were much higher than those in plasma (0.15 to 0.73 mg/liter) after aerosol delivery but not after intravenous administration of CMS. Following CMS aerosol delivery, typically, 9% of the CMS dose reached the ELF, and only 1.4% was presystemically converted into colistin. PK-PD analysis concluded that there was much higher antimicrobial efficacy after CMS aerosol delivery than after intravenous administration. These new data seem to support the use of aerosol delivery of CMS for the treatment of pulmonary infections in critical care patients.

Community-acquired pneumonia: identification and evaluation of nonresponders

Community acquired pneumonia (CAP) is a relevant public health problem, constituting an important cause of morbidity and mortality. It accounts for a significant number of adult hospital admissions and a large number of those patients ultimately die, especially the population who needed mechanical ventilation or vasopressor support. Thus, early identification of CAP patients and its rapid and appropriate treatment are important features with impact on hospital resource consumption and overall mortality. Although CAP diagnosis may sometimes be straightforward, the diagnostic criteria commonly used are highly sensitive but largely unspecific. Biomarkers and microbiological documentation may be useful but have important limitations. Evaluation of clinical response is also critical especially to identify patients who fail to respond to initial treatment since these patients have a high risk of in-hospital death. However, the criteria of definition of non-response in CAP are largely empirical and frequently markedly diverse between different studies. In this review, we aim to identify criteria defining nonresponse in CAP and the pitfalls associated with this diagnosis. We also aim to overview the main causes of treatment failure especially in severe CAP and the possible strategies to identify and reassess non-responders trying to change the dismal prognosis associated with this condition.

The lack of specificity of tracheal aspirates in the diagnosis of pulmonary infection in intubated children

OBJECTIVES

Ventilator-associated pneumonia is the first or second most commonly diagnosed nosocomial infection in the PICU. Centers for Disease Control diagnostic criteria include clinical signs or symptoms in conjunction with a "positive" tracheal aspirate, defined as more than 10 colony-forming units/mL of bacteria on quantitative culture and/or more than 25 polymorphonuclear neutrophils per low-power field on Gram stain. We hypothesized that tracheal aspirate cultures and Gram stains would not correlate with clinical signs and symptoms and would therefore not distinguish between colonization and infection.

DESIGN

Prospective observational study.

SETTING

PICU in an academic tertiary care center.

PATIENTS

Children intubated more than 48 hours.

INTERVENTIONS

Sequential tracheal aspirate quantitative cultures and Gram stains in conjunction with daily collection of concordant clinical signs and symptoms.

MEASUREMENTS AND MAIN RESULTS

Time since intubation correlated strongly (p < 0.001) with the proportion of positive (> 10 colony-forming units/mL) tracheal aspirate quantitative cultures, but Centers for Disease Control-defined clinical signs or symptoms of ventilator-associated pneumonia, either singly or in combination, did not. Use of in-line suction catheters versus new, sterile catheters to obtain tracheal aspirates was associated with significantly greater proportion of positive tracheal aspirate bacterial cultures (p < 0.001). Most subjects had more than 25 polymorphonuclear neutrophils per low-power field on Gram stain; polymorphonuclear neutrophils on Gram stain correlated with positive bacterial culture (p = 0.04). Seventy-seven percent of the bacterial isolates detected in positive quantitative cultures were "pathogens." Antibiotic use at the time tracheal aspirates were obtained was associated with a lower frequency of positive quantitative cultures only with antibiotic regimens that included cefepime.

CONCLUSIONS

Positive bacterial cultures of tracheal aspirates increase rapidly after intubation and usually include bacteria considered to be pathogens. Tracheal aspirate cultures and Gram stains do not appear to distinguish between infection and colonization. Antibiotic regimens that include cefepime decrease the frequency of positive cultures, but the significance of this is unclear.

Individualised antibiotic dosing for patients who are critically ill: challenges and potential solutions

Infections in critically ill patients are associated with persistently poor clinical outcomes. These patients have severely altered and variable antibiotic pharmacokinetics and are infected by less susceptible pathogens. Antibiotic dosing that does not account for these features is likely to result in suboptimum outcomes. In this Review, we explore the challenges related to patients and pathogens that contribute to inadequate antibiotic dosing and discuss how to implement a process for individualised antibiotic therapy that increases the accuracy of dosing and optimises care for critically ill patients. To improve antibiotic dosing, any physiological changes in patients that could alter antibiotic concentrations should first be established; such changes include altered fluid status, changes in serum albumin concentrations and renal and hepatic function, and microvascular failure. Second, antibiotic susceptibility of pathogens should be confirmed with microbiological techniques. Data for bacterial susceptibility could then be combined with measured data for antibiotic concentrations (when available) in clinical dosing software, which uses pharmacokinetic/pharmacodynamic derived models from critically ill patients to predict accurately the dosing needs for individual patients. Individualisation of dosing could optimise antibiotic exposure and maximise effectiveness.

Continuous infusion of antibiotics in the critically ill: The new holy grail for beta-lactams and vancomycin?

The alarming global rise of antimicrobial resistance combined with the lack of new antimicrobial agents has led to a renewed interest in optimization of our current antibiotics. Continuous infusion (CI) of time-dependent antibiotics has certain theoretical advantages toward efficacy based on pharmacokinetic/pharmacodynamic principles. We reviewed the available clinical studies concerning continuous infusion of beta-lactam antibiotics and vancomycin in critically ill patients. We conclude that CI of beta-lactam antibiotics is not necessarily more advantageous for all patients. Continuous infusion is only likely to have clinical benefits in subpopulations of patients where intermittent infusion is unable to achieve an adequate time above the minimal inhibitory concentration (T > MIC). For example, in patients with infections caused by organisms with elevated MICs, patients with altered pharmacokinetics (such as the critically ill) and possibly also immunocompromised patients. For vancomycin CI can be chosen, not always for better clinical efficacy, but because it is practical, cheaper, associated with less AUC_24h (area under the curve >24 h)-variability, and easier to monitor.

In vitro activities of clindamycin, imipenem, metronidazole, and piperacillin-tazobactam against susceptible and resistant isolates of Bacteroides fragilis evaluated by kill kinetics

The aim of the present study was to investigate the activities of clindamycin, imipenem, metronidazole, and piperacillin-tazobactam against 12 Bacteroides fragilis isolates (resistant and susceptible strains) by kill kinetics over 24 h. In contrast to the other antimicrobial agents, clindamycin did not affect strains with MICs of >8.0 μg/ml. For those strains with MICs of ≤ 8.0 μg/ml, all employed antibiotics except clindamycin showed nearly bactericidal activity. Metronidazole proved to be the most active antimicrobial agent.