In vivo pharmacodynamics of ceftobiprole against multiple bacterial pathogens in murine thigh and lung infection models

Challenges for global antibiotic regimen planning and establishing antimicrobial resistance targets: implications for the WHO Essential Medicines List and AWaRe antibiotic book dosing

SUMMARYThe World Health Organisation's 2022 AWaRe Book provides guidance for the use of 39 antibiotics to treat 35 infections in primary healthcare and hospital facilities. We review the evidence underpinning suggested dosing regimens. Few (n = 18) population pharmacokinetic studies exist for key oral AWaRe antibiotics, largely conducted in homogenous and unrepresentative populations hindering robust estimates of drug exposures. Databases of minimum inhibitory concentration distributions are limited, especially for community pathogen-antibiotic combinations. Minimum inhibitory concentration data sources are not routinely reported and lack regional diversity and community representation. Of studies defining a pharmacodynamic target for ß-lactams (n = 80), 42 (52.5%) differed from traditionally accepted 30%-50% time above minimum inhibitory concentration targets. Heterogeneity in model systems and pharmacodynamic endpoints is common, and models generally use intravenous ß-lactams. One-size-fits-all pharmacodynamic targets are used for regimen planning despite complexity in drug-pathogen-disease combinations. We present solutions to enable the development of global evidence-based antibiotic dosing guidance that provides adequate treatment in the context of the increasing prevalence of antimicrobial resistance and, moreover, minimizes the emergence of resistance.

The Application of Rat Models in Staphylococcus aureus Infections

Staphylococcus aureus (S. aureus) is a major human pathogen and can cause a wide range of diseases, including pneumonia, osteomyelitis, skin and soft tissue infections (SSTIs), endocarditis, mastitis, bacteremia, and so forth. Rats have been widely used in the field of infectious diseases due to their unique advantages, and the models of S. aureus infections have played a pivotal role in elucidating their pathogenic mechanisms and the effectiveness of therapeutic agents. This review outlined the current application of rat models in S. aureus infections and future prospects for rat models in infectious diseases caused by S. aureus.

Cefadroxil and cephalexin pharmacokinetics and pharmacodynamics in children with musculoskeletal infections

Cephalexin, a first-generation cephalosporin, is the first-line oral therapy for children with musculoskeletal infections due to methicillin-susceptible Staphylococcus aureus (MSSA). Cefadroxil, a similar first-generation cephalosporin, is an attractive alternative to cephalexin given its longer half-life. In this study, we describe the comparative pharmacokinetics (PK) and pharmacodynamics (PD) of cephalexin and cefadroxil in children with musculoskeletal infections. Children aged 6 months to 18 years with a musculoskeletal infection were enrolled in a prospective, open-label, crossover PK study and given single oral doses of cefadroxil (50-75 mg/kg up to 2,000 mg) and cephalexin (50 mg/kg up to 1,375 mg). Population PK models were developed and used for dosing simulations. Our primary PD target was the achievement of free antibiotic concentrations above the minimum inhibitory concentration (fT >MIC) for 40% of the day for MICs ≤ 4 mg/L. PK of cephalexin (n = 15) and cefadroxil (n = 14) were best described using a one-compartment, first-order absorption model, with a lag time component for cefadroxil. PK parameters were notable for cefadroxil's longer half-life (1.61 h) than cephalexin's (1.10 h). For pediatric weight bands, our primary PD target was achieved by cephalexin 25 mg/kg/dose, maximum 750 mg/dose, administered three times daily and cefadroxil 40 mg/kg/dose, maximum 1,500 mg/dose, administered twice daily. More aggressive dosing was required to achieve higher PD targets. Among children with musculoskeletal infections, oral cephalexin and cefadroxil achieved PD targets for efficacy against MSSA. Given less frequent dosing, twice-daily cefadroxil should be further considered as an alternative to cephalexin for oral step-down therapy for serious infections due to MSSA.

Safety and effectiveness of fifth generation cephalosporins for the treatment of methicillin-resistant staphylococcus aureus bloodstream infections: a narrative review exploring past, present, and future

INTRODUCTION

Methicillin-resistant Staphylococcus aureus (MRSA) bloodstream infection (BSI) is a major issue in healthcare, since it is often associated with endocarditis or deep site foci. Relevant morbidity and mortality associated with MRSA-BSIs forced the development of new antibiotic strategies; in particular, this review will focus the attention on fifth-generation cephalosporins (ceftaroline/ceftobiprole), that are the only ß-lactams active against MRSA.

AREAS COVERED

The review discusses the available randomized controlled trials and real-world observational studies conducted on safety and effectiveness of ceftaroline/ceftobiprole for the treatment of MRSA-BSIs. Finally, a proposal of MRSA-BSI treatment flowchart, based on fifth-generation cephalosporins, is described.

EXPERT OPINION

The use of anti-MRSA cephalosporins is an acceptable choice either in monotherapy or combination therapy for the treatment of MRSA-BSIs due to their relevant effectiveness and safety. Particularly, their use may be advisable in combination therapy in case of severe infections (including endocarditis or persistent bacteriemia) or in monotherapy in subjects at higher risk of drugs-induced toxicity with older regimens. On the contrary, caution should be taken in case of suspected/ascertained central nervous system infections due to inconsistent data regarding penetration of these drugs in cerebrospinal fluid and brain tissues.

Population Pharmacokinetic and Pharmacodynamic Analysis for Maximizing the Effectiveness of Ceftobiprole in the Treatment of Severe Methicillin-Resistant Staphylococcal Infections

Ceftobiprole is a fifth-generation cephalosporin used for different Gram-positive bacterial infections. A population pharmacokinetic analysis was conducted in real-life clinical patients to assess the adequacy of current dosages. Population pharmacokinetics was conducted using non-linear mixed effect modeling. Monte Carlo simulations were performed to determine the probability of target attainment (PTA) of free trough or steady-state concentration over MIC (fCtrough/MIC or fCss/MIC) ≥ 1 or ≥4 associated with both the standard and intensified dosing regimens adjusted for renal function. Cumulative fraction of response (CFR) against methicillin-resistant Staphylococcus aureus (MRSA) and Staphylococcus epidermidis (MRSE) were also calculated. A total of 132 patients with 503 concentrations were included. Most of them (107/132, 81.1%) had hospital- or community-acquired pneumonia, endocarditis, and bacteremia. A three-compartment model adequately fitted ceftobiprole concentration-time data. Estimated glomerular filtration rate significantly affected drug clearance. Monte Carlo simulations showed that the optimal target of fCtrough/MIC or fCss/MIC ≥ 4 is achieved only with the use of the standard dosages administered by continuous infusion (CI) against MRSA infections in patients with preserved renal function. Intensified dosages administered by CI are needed in patients with impaired renal function and/or augmented renal clearance against MRSA and in patients with preserved renal functions against MRSE.

Neutropenic Rat Thigh Infection Model for Evaluation of the Pharmacokinetics/Pharmacodynamics of Anti-Infectives

The neutropenic mouse infection model is extensively used to characterize the pharmacokinetics/pharmacodynamics (PK/PD) of anti-infective agents. However, it is difficult to evaluate agents following intravenous (i.v.) infusions using this model. Furthermore, in many drug discovery programs, lead identification and optimization is performed in rats, and pharmacology is performed in mice. Alternative models of infection are needed for robust predictions of PK/PD in humans. The rat is an alternative model of infection which can overcome the shortcomings of the mouse model. However, the rat neutropenic thigh infection (NTI) model has not been adequately characterized for evaluation of the PK/PD of anti-infectives. The aim of this study was to characterize the PK/PD of ciprofloxacin against bacterial pathogens in a rat NTI model. We studied the PK/PD relationships of ciprofloxacin against wild-type Escherichia coli, Acinetobacter baumannii, Pseudomonas aeruginosa, and Klebsiella pneumoniae in neutropenic Wistar rats following administration of 10, 30, and 100 mg/kg as single intravenous boluses and 30- and 60-min infusions. The PK/PD of ciprofloxacin against all four pathogens was AUC/MIC dependent and independent of the duration of administration at 10, 30, and 100 mg/kg. At human-equivalent rat doses, the PK/PD targets of ciprofloxacin achieved in rats for microbiological cure were similar to those reported in human patients. The neutropenic rat thigh infection model can be used to evaluate anti-infective agents intended to be administered as infusions in the clinic, and it complements the mouse model, increasing the robustness of PK/PD predictions in humans. IMPORTANCE Many antibiotics are administered as intravenous infusions in the clinic, especially in intensive care units. Anti-infective drug discovery companies develop clinical candidates that are intended to be administered as i.v. infusions in the clinic. However, there are no well-characterized models with which they can evaluate the PK/PD of the candidates following i.v. infusions. The neutropenic rat thigh infection model reported in this study helps in evaluating anti-infective agents that are intended to be administered as i.v. infusions in the clinic. The rat model is useful for simulating the clinical conditions for i.v. infusions for treatment of infections, such as acute bacterial skin and skin structure, lung, and urinary tract infections. This model is predictive of efficacy in humans and can serve as an additional confirmatory model, along with the mouse model, for determining the proof of concept and for making robust predictions of efficacy in humans.

Bayesian prediction-based individualized dosing of anti-methicillin-resistant Staphylococcus aureus treatment: Recent advancements and prospects in therapeutic drug monitoring

As one of the efficient techniques for TDM, the population pharmacokinetic (popPK) model approach for dose individualization has been developed due to the rapidly growing innovative progress in computer technology and has recently been considered as a part of model-informed precision dosing (MIPD). Initial dose individualization and measurement followed by maximum a posteriori (MAP)-Bayesian prediction using a popPK model are the most classical and widely used approach among a class of MIPD strategies. MAP-Bayesian prediction offers the possibility of dose optimization based on measurement even before reaching a pharmacokinetically steady state, such as in an emergency, especially for infectious diseases requiring urgent antimicrobial treatment. As the pharmacokinetic processes in critically ill patients are affected and highly variable due to pathophysiological disturbances, the advantages offered by the popPK model approach make it highly recommended and required for effective and appropriate antimicrobial treatment. In this review, we focus on novel insights and beneficial aspects of the popPK model approach, especially in the treatment of infectious diseases with anti-methicillin-resistant Staphylococcus aureus agents represented by vancomycin, and discuss the recent advancements and prospects in TDM practice.

Pharmacodynamics of Flucloxacillin in a Neutropenic Murine Thigh Infection Model: A Piece of the Puzzle towards Evidence-Based Dosing

For decades, flucloxacillin has been used to treat methicillin-susceptible Staphylococcus aureus (MSSA). Little is still known about its pharmacodynamics (PD). The present study aimed to determine the pharmacokinetic (PK)/PD index and the PD-index value minimally required for efficacy. MICs of 305 MSSA isolates were measured to determine the wild-type distribution. The PD of 8 S. aureus, 1 S. pyogenes, and 1 S. agalactiae isolates were evaluated in a neutropenic murine thigh infection model. Two S. aureus isolates were used in a dose-fractionation study and a dose−response analysis was performed additionally in the in vivo model. Data were analyzed with a population PK and sigmoid maximum effect model. The end of the wild-type distribution was 1 mg/L. The percentage of time the unbound concentration was above MIC (%fT > MIC) was best correlated with efficacy. For S. aureus, median %fT > 0.25 × MIC required for 1-log reduction was 15%. The value for S. pyogenes was 10%fT > MIC and for S. agalactiae 22%fT > 0.25xMIC for a 1-log reduction. The effect of flucloxacillin reached a 2-log reduction of S. aureus at 20%fT > 0.25xMIC and also for S. pyogenes and S. agalactiae, a reduction was reached. These data may serve to optimize dosing regimens currently used in humans.

Ceftobiprole and daptomycin concentrations in valve tissue in a patient with mitralic native valve endocarditis

Ceftobiprole (CFB), especially in combination, could be a promising alternative treatment for infective endocarditis. A main determinant of clinical response to antibiotic treatment is drug concentration at the infected site. Data on CFB and Daptomycin (DPT) heart valve penetration are lacking.Here we report a clinical case of CFB and DPT treatment combination for endocarditis. Then, we measured CFB and DPT concentrations in a native infected valve to verify their pharmacokinetic penetration and relationship with pharmacodynamic microbiological markers.The isolated microorganism was a MRSA with CFB and DPT MIC < 2 mg/L and <1 mg/L, respectively. The CFB and DPT plasma concentrations were 36.2 and 14.1 mg/L, respectively and the extrapolated concentration, based on each half-life, at the operatory time were 16.4 and 19.1 mg/L for CFB and DPT, respectively; the corresponding median CFB and DPT valve concentrations were 2.26 (IQR 2.14-2.69) and 12.9 µg/g (IQR 5.69-20.9), respectively; the estimated tissue/plasma ratios for CFB and DTP were 0.14 and 0.67, respectively.The association of CFB and DPT showed a good efficacy in this single endocarditis clinical case, confirmed by plasma and tissue PK/PD data.This report shows the first data on CFB valve tissue penetration, and it needs to be confirmed in other patient valve tissues. Moreover, relative studies of correlation with clinical efficacy are needed.

Pharmacodynamic Thresholds for Beta-Lactam Antibiotics: A Story of Mouse Versus Man

Beta-lactams remain a critical member of our antibiotic armamentarium and are among the most commonly prescribed antibiotic classes in the inpatient setting. For these agents, the percentage of time that the free concentration remains above the minimum inhibitory concentration (%fT > MIC) of the pathogen has been shown to be the best predictor of antibacterial killing effects. However, debate remains about the quantity of fT > MIC exposure needed for successful clinical response. While pre-clinical animal based studies, such as the neutropenic thigh infection model, have been widely used to support dosing regimen selection for clinical development and susceptibility breakpoint evaluation, pharmacodynamic based studies in human patients are used validate exposures needed in the clinic and for guidance during therapeutic drug monitoring (TDM). For the majority of studied beta-lactams, pre-clinical animal studies routinely demonstrated the fT > MIC should exceed approximately 40–70% fT > MIC to achieve 1 log reductions in colony forming units. In contrast, clinical studies tend to suggest higher exposures may be needed, but tremendous variability exists study to study. Herein, we will review and critique pre-clinical versus human-based pharmacodynamic studies aimed at determining beta-lactam exposure thresholds, so as to determine which targets may be best suited for optimal dosage selection, TDM, and for susceptibility breakpoint determination. Based on our review of murine and clinical literature on beta-lactam pharmacodynamic thresholds, murine based targets specific to each antibiotic are most useful during dosage regimen development and susceptibility breakpoint assessment, while a range of exposures between 50 and 100% fT > MIC are reasonable to define the beta-lactam TDM therapeutic window for most infections.

Pharmacokinetics and Safety of Ceftobiprole in Pediatric Patients

BACKGROUND

Ceftobiprole, the active moiety of the prodrug ceftobiprole medocaril, is an advanced-generation, broad-spectrum, intravenous cephalosporin, which is currently approved for the treatment of adults with hospital-acquired or community-acquired pneumonia.

METHODS

Noncompartmental pharmacokinetics and safety were analyzed from 2 recently completed pediatric studies, a single-dose, phase 1 study in neonates and infants up to 3 months of age (7.5 mg/kg) and a phase 3 study in patients 3 months to 17 years of age with pneumonia (10-20 mg/kg with a maximum of 500 mg per dose every 8 hours for up to 14 days).

RESULTS

Total ceftobiprole plasma concentrations peaked at the end of infusion. Half life (median ranging from 1.9 to 2.9 hours) and overall exposure (median AUC ranging from 66.6 to 173 μg•h/mL) were similar to those in adults (mean ± SD, 3.3 ± 0.3 hours and 102 ± 11.9 μg•h/mL, respectively). Calculated free-ceftobiprole concentrations in the single-dose study remained above a minimum inhibitory concentration (MIC) of 4 mg/L (fT > MIC of 4 mg/L) for a mean of 5.29 hours after dosing. In the pneumonia study, mean fT > MIC of 4 mg/L was ≥5.28 hours in all dose groups. Ceftobiprole was well tolerated in both studies.

CONCLUSIONS

Pharmacokinetic parameters of ceftobiprole characterized in the pediatric population were within the range of those observed in adults. In the pneumonia study, the lowest percentage of the dosing interval with fT > MIC of 4 mg/L was 50.8%, which suggests that pharmacokinetic-pharmacodynamic target attainment can be sufficient in pediatric patients. Ceftobiprole was well tolerated.

Optimizing Ceftobiprole Dosage in Pediatric Patients: A Model-Based Approach

Ceftobiprole is an advanced-generation cephalosporin for intravenous administration with activity against Gram-positive and Gram-negative organisms. A population pharmacokinetic (PK) model characterizing the disposition of ceftobiprole in plasma using data from patients in three pediatric studies was developed. Model-based simulations were subsequently performed to assist in dose optimization for the treatment of pediatric patients with hospital-acquired or community-acquired pneumonia. The population PK data set comprised 518 ceftobiprole plasma concentrations from 107 patients from 0 (birth) to 17 years of age. Ceftobiprole PK was well described by a three-compartment model with linear elimination. Ceftobiprole clearance was modeled as a function of glomerular filtration rate; other PK parameters were scaled to body weight. The final population PK model provided a robust and reliable description of the PK of ceftobiprole in the pediatric study population. Model-based simulations using the final model suggested that a ceftobiprole dose of 15 mg/kg of body weight infused over 2 h and administered every 12 h in neonates and infants <3 months of age or every 8 h in older pediatric patients would result in a ceftobiprole exposure consistent with that in adults and good pharmacokinetic-pharmacodynamic target attainment. The dose should be reduced to 10 mg/kg every 12 h in neonates and infants <3 months of age who weigh <4 kg to avoid high exposures. Extended intervals and reduced doses may be required for pediatric patients older than 3 months of age with renal impairment.

Pharmacokinetics, pharmacodynamics, and safety of single- and multiple-dose intravenous ceftobiprole in healthy Chinese participants

BACKGROUND

Ceftobiprole is a novel β-lactam cephalosporin with activity against Gram-positive and -negative bacteria. The aim of the present study was to investigate the pharmacokinetics (PK), pharmacokinetics/pharmacodynamics (PK/PD), safety and tolerance of ceftobiprole in Chinese participants, to evaluate this dosage regimen for the treatment of community-acquired pneumonia (CAP) and hospital-acquired pneumonia (HAP) in China.

METHODS

The use of ceftobiprole was investigated in a single-center, open-label, single- and multiple-dose study using 12 healthy Chinese participants (6 males and 6 females). Ceftobiprole plasma and urine concentrations were analyzed using a validated liquid chromatography-tandem mass spectrometry assay. The PK/PD characteristics of 500 mg ceftobiprole every 8 h at 1.5-, 2-, 3-, or 4-h infusion time were analyzed by Monte Carlo simulations (MCS).

RESULTS

The maximum plasma concentration of ceftobiprole was observed 2 h after dosage; its terminal half-life was about 3 h. Ceftobiprole was predominantly eliminated in urine, and the cumulative excretion in 24 h was >90%. There was no accumulation after multiple dosing. Both single and multiple doses were well tolerated, with no severe or serious adverse events (AEs). PK/PD analysis indicated that Staphylococcus pneumoniae (S. pneumoniae) and Staphylococcus aureus (S. aureus) were sensitive to ceftobiprole. About half of extended-spectrum β-lactamase (ESBL) non-producing Enterobacteriaceae are sensitive to ceftobiprole, according to PK/PD results of ceftobiprole. For Pseudomonas aeruginosa (P. aeruginosa), no regimen was found to be effective against strains.

CONCLUSIONS

The PK/PD results indicated that 500 mg ceftobiprole every 8 h at 2-h infusion time is expected to achieve good microbiological efficacy in the treatment of CAP and HAP in China.

Comparing probability of target attainment against Staphylococcus aureus for ceftaroline fosamil, vancomycin, daptomycin, linezolid, and ceftriaxone in complicated skin and soft tissue infection using pharmacokinetic/pharmacodynamic models

For recently licensed antibiotics, such as the cephalosporin ceftaroline fosamil, probability of target attainment (PTA) curves, showing the percentage of patients reaching a predefined pharmacokinetic (PK)/pharmacodynamic (PD) target at different bacterial minimum inhibitory concentrations (MICs), have been used to support and justify dose recommendations across patient populations. However, information on PTA for older antibiotics is limited. A retrospective analysis was conducted to construct PTA curves for 4 antibiotics against Staphylococcus aureus in patients with complicated skin and soft tissue infections (cSSTIs). PK models for vancomycin, linezolid, daptomycin, and ceftriaxone were selected from the literature based on large numbers of subjects with covariates representative of patients in Europe and/or the United States. An existing model was available for ceftaroline fosamil. Standard and high-dosage regimens were used to compare the PTA of each antibiotic at MIC values 0.03 to 64 mg/L for a simulated set of patients with cSSTI caused by S. aureus. These were compared to proportions of S. aureus isolates at each MIC from global surveillance data. Ceftaroline achieved PTAs >99.9% for bacteriostatic and bactericidal targets at the MIC90 (1 mg/L), whereas the comparators failed to achieve PTAs >90%, at bacteriostatic or bactericidal targets, even when clinical doses were increased beyond those recommended. PTA analysis can be used to compare different drugs with the same simulated patient dataset, subject to availability of an appropriate PK model and robust exposure targets. This analysis shows that some antibiotics commonly used to treat cSSTIs may fail to reach high PTAs relative to contemporary MIC90 estimates.

Pharmacokinetics and pharmacodynamics of the novel monobactam LYS228 in a neutropenic murine thigh model of infection

Objectives

The neutropenic murine thigh infection model and a dose-fractionation approach were used to determine the pharmacokinetic/pharmacodynamic (PK/PD) relationship of LYS228, a novel monobactam antibiotic with activity against Enterobacteriaceae including carbapenem-resistant strains.

Methods

Mice (n = 4 per group) were inoculated with Enterobacteriaceae strains via intramuscular injection. Two hours post-bacterial inoculation, treatment with LYS228 was initiated. Animals were euthanized with CO2 24 h after the start of therapy and bacterial counts (log10 cfu) per thigh were determined. PK parameters were calculated using free (f) plasma drug levels.

Results

Following a dose-fractionation study, non-linear regression analysis determined that the predominant PK/PD parameter associated with antibacterial efficacy of LYS228 was the percentage of the dosing interval that free drug concentrations remained above the MIC (%fT>MIC). In a dose-dependent manner, LYS228 reduced the thigh bacterial burden in models established with Enterobacteriaceae producing β-lactamase enzymes of all classes (e.g. ESBLs, NDM-1, KPC, CMY-2 and OXA-48). The range of the calculated static dose was 86-649 mg/kg/day for the isolates tested, and the magnitude of the driver of efficacy was 37-83 %fT>MIC. %fT>MIC was confirmed as the parameter predominantly driving efficacy as evidenced by a strong coefficient of determination (r2 = 0.68). Neutrophils had minimal impact on the effect of LYS228 in the murine thigh infection model.

Conclusions

LYS228 is efficacious in murine thigh infection models using β-lactamase-producing strains of Enterobacteriaceae, including those expressing metallo-β-lactamases, ESBLs and serine carbapenemases, with the PK/PD driver of efficacy identified as %T>MIC.

Ceftobiprole: drug evaluation and place in therapy

Introduction: Ceftobiprole is a fifth-generation cephalosporin with a broad spectrum of antimicrobial activity, including also methicillin-resistant Staphylococcus aureus (MRSA). Ceftobiprole is approved for the treatment of community-acquired pneumonia (CAP) and hospital-acquired pneumonia (HAP), excluding ventilator-associated pneumonia, in several European and non-European countries. Areas covered: In this narrative review, we discuss the current place in therapy of ceftobiprole, both within and outside approved indications. An inductive MEDLINE/PubMed search of the available literature was conducted. Expert opinion: There are three main reasons which render ceftobiprole an attractive option for the empirical and targeted treatment of CAP and HAP: (i) its broad spectrum of activity; (ii) its activity against MRSA; (iii) its good safety profile. For these indications, ceftobiprole should be employed thoughtfully, in those scenarios in which its intrinsic advantages could be maximized. The use of ceftobiprole outside approved indications could be justified in specific scenarios, such as when other approved alternatives are ineffective, when the risk of toxicity due to other agents is unacceptable, and for salvage therapy. In the near future, ongoing phase 3 studies and further observational experiences could both enlarge the current panel of approved indications and enrich our knowledge on the use of ceftobiprole for off-label indications.

In Vivo Pharmacodynamic Study of Cefiderocol, a Novel Parenteral Siderophore Cephalosporin, in Murine Thigh and Lung Infection Models

The pharmacokinetic (PK) and pharmacodynamic (PD) parameters which correlated with the in vivo efficacy of cefiderocol were evaluated using neutropenic murine thigh and lung infection models in which the infections were caused by a variety of Gram-negative bacilli. The dose fractionation study using the thigh infection model in which the infection was caused by Pseudomonas aeruginosa showed that the cumulative percentage of a 24-h period that the free drug concentration in plasma exceeds the MIC (%fT>MIC) rather than the free peak level divided by the MIC (fCmax/MIC) and the area under the free concentration-time curve over 24 h divided by the MIC (fAUC/MIC) was the PK/PD parameter that best correlated with efficacy. The study with multiple carbapenem-resistant strains revealed that the %fT>MIC determined in iron-depleted cation-adjusted Mueller-Hinton broth (ID-CAMHB) better reflected the in vivo efficacy of cefiderocol than the %fT>MIC determined in cation-adjusted Mueller-Hinton broth (CAMHB). The mean %fT>MIC of cefiderocol required for a 1-log10 reduction against 10 strains of Enterobacteriaceae and 3 strains of Pseudomonas aeruginosa in the thigh infection models were 73.3% and 77.2%, respectively. The mean %fT>MIC for Enterobacteriaceae, P. aeruginosa, Acinetobacter baumannii, and Stenotrophomonas maltophilia in the lung infection model were 64.4%, 70.3%, 88.1%, and 53.9%, respectively. These results indicate that cefiderocol has potent efficacy against Gram-negative bacilli, including carbapenem-resistant strains, irrespective of the bacterial species, in neutropenic thigh and lung infection models and that the in vivo efficacy correlated with the in vitro MIC under iron-deficient conditions.

The Quinazolinone Allosteric Inhibitor of PBP 2a Synergizes with Piperacillin and Tazobactam against Methicillin-Resistant Staphylococcus aureus

The quinazolinones are a new class of antibacterials with in vivo efficacy against methicillin-resistant Staphylococcus aureus (MRSA). The quinazolinones target cell wall biosynthesis and have a unique mechanism of action by binding to the allosteric site of penicillin-binding protein 2a (PBP 2a). We investigated the potential for synergism of a lead quinazolinone with several antibiotics of different classes using checkerboard and time-kill assays. The quinazolinone synergized with β-lactam antibiotics. The combination of the quinazolinone with commercial piperacillin-tazobactam showed bactericidal synergy at sub-MICs of all three drugs. We demonstrated the efficacy of the triple-drug combination in a mouse MRSA neutropenic thigh infection model. The proposed mechanism for the synergistic activity in MRSA involves inhibition of the β-lactamase by tazobactam, which protects piperacillin from hydrolysis, which can then inhibit its target, PBP 2. Furthermore, the quinazolinone binds to the allosteric site of PBP 2a, triggering the allosteric response. This leads to the opening of the active site, which, in turn, binds another molecule of piperacillin. In other words, PBP 2a, which is not normally inhibited by piperacillin, becomes vulnerable to inhibition in the presence of the quinazolinone. The collective effect is the impairment of cell wall biosynthesis, with bactericidal consequence. Two crystal structures for complexes of the antibiotics with PBP 2a provide support for the proposed mechanism of action.

Breakpoint determination when multiple organisms are tested for effect targets

Determination of the susceptibility breakpoint for antibiotics is important, as it guides the use of agents in the clinical setting. Currently, breakpoints are often evaluated using a Probability of Target Attainment Analysis in which the targets are set through pre-clinical experiments, often by examining a strain of a target pathogen in a murine model such as a neutropenic thigh infection model. However, regulatory authorities are often rightly concerned about the setting of breakpoints when a number of isolates of target pathogens are evaluated and there is a sizeable spread of the drug exposures necessary to achieve the target with a sufficiently high (usually 90%) probability. Here, we propose a method for supporting a breakpoint determination for this circumstance. We examined 8 isolates of resistant Enterobacteriaceae in a neutropenic murine thigh infection model. The stasis exposure was determined and ranged from 5.70 to 43.5 AUC/MIC Ratio. The mean ± standard deviation was 20.05 ± 13.05. A 5000-iterate Monte Carlo simulation was performed to generate a range of stasis targets and Probability of Target Attainment Analyses were calculated at the 1st, 5th, 10th, 25th, 50th, 75th, 90th, 95th, and 99th percentiles of the distribution. Breakpoints were determined at each percentile. Breakpoints ranged from 2 mg/L to 32 mg/L. A weighted (by the percentages of the distribution) breakpoint was calculated and determined to be 4 mg/L. This method is a rational approach to identifying breakpoints when there is substantial between-isolate variability in exposure targets.

Ceftobiprole: pharmacokinetics and PK/PD profile

Ceftobiprole shows many similar pharmacokinetic properties to other cephalosporins, except for not being orally bioactive, and that it is administered by IV infusion as the prodrug ceftobiprole medocaril, which is subsequently hydrolyzed in the blood into the active molecule. Distribution focus in extracellular fluid and active antibiotic concentration has been proven in different corporal tissues using dosing regimen of 500 mg intravenous infusion over 2 h every 8 h. Ceftobiprole is eliminated exclusively into the urine, thus the reason why dose adjustment is required for patients with moderate or severe renal impairment, or increased creatinine clearance. However, there is no need for dose adjustments related with other comorbidities and patients' conditions such as age, body weight. Although considering distribution features, molecular weight and dose fraction, increase dosing regimen might be necessary in patients using renal replacement therapy. The half-life of ceftobiprole is more than 3 h, allowing to easily reach optimal PK/PD parameters with the infusion time of 2 h, using the usual dosing regimen.

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.

Antibacterial drug development program successes and failures: a pharmacometric explanation

My thesis is a simple one. We have not been doing a good enough job selecting dose regimens for serious infections during the drug development process. If we are to do a better job in the future, we need to revisit some uncomfortable places. That is, some notable program failures. To be clear, we are not revisiting program failures to make anyone uncomfortable or cast aspersions - but rather so that we sow the seeds for a better future. To that end, we will examine program failures and successes through a pharmacometric lens. Through this powerful lens, we will come to understand that many of our failures were not only predictable, but perhaps expected and entirely avoidable. The goal of this communication is to set forth the type of thinking and data that is necessary for rational dose selection.

Pharmacokinetic drug evaluation of ceftobiprole for the treatment of MRSA

INTRODUCTION

Methicillin-resistant Staphylococcus aureus (MRSA), while decreasing in overall incidence, is still a prominent concern world-wide. New agents coming to market in the last 10 years allow practitioners to optimize treatment for MRSA infections. Ceftobiprole is a cephalosporin agent with MRSA activity, currently approved in selected countries for the treatment of community-acquired pneumonia and hospital-acquired pneumonia. Areas covered: Relevant literature regarding spectrum of activity, pharmacokinetics, pharmacodynamics, and clinical trials will be discussed. Expert opinion: Ceftobiprole is an addition to a growing number of antimicrobials with activity against MRSA. Concern for appropriate dosing in critically ill patients remains due to its ineffectiveness for the treatment of ventilator-associated pneumonia (VAP). While ceftobiprole has activity against gram-negative organisms, the allowance for use of an additional agent for gram-negative infections in clinical trials limits recommendations for monotherapy for empirical treatment of HAP. Ceftobiprole's place in therapy will lie in its activity against gram positive organisms, such as Streptococcus spp. and Staphylococcus spp.

Comparative In Vivo Efficacies of Tedizolid in Neutropenic versus Immunocompetent Murine Streptococcus pneumoniae Lung Infection Models

Given that tedizolid exhibits substantial lung penetration, we hypothesize that it could achieve good efficacy against Streptococcus pneumoniae lung infections. We evaluated the pharmacodynamics of tedizolid for treatment of S. pneumoniae lung infections and compared the efficacies of tedizolid human-simulated epithelial lining fluid (ELF) exposures in immunocompetent and neutropenic murine lung infection models. ICR mice were rendered neutropenic via intraperitoneal cyclophosphamide injections and then inoculated intranasally with S. pneumoniae suspensions. Immunocompetent CBA/J mice were inoculated similarly. Single daily tedizolid doses were administered 4 h postinoculation (termed 0 h). Changes in log₁₀ CFU at 24 h compared with 0-h controls were estimated. Ratios of area under the free-drug concentration-time curve to MIC (fAUC_0-24/MIC) required to achieve various efficacy endpoints against each isolate were estimated using the Hill equation. Tedizolid doses in neutropenic and immunocompetent mice that mimic the human-simulated ELF exposure were examined. Stasis, 1-log reduction, and 2-log reduction were achieved at fAUC_0-24/MIC of 8.96, 24.62, and 48.34, respectively, in immunocompetent mice and 19.21, 48.29, and 103.95, respectively, in neutropenic mice. Tedizolid at 40 mg/kg of body weight/day and 55 mg/kg/day in immunocompetent and neutropenic mice, respectively, resulted in ELF AUC_0-24 comparable to that achieved in humans following a 200-mg once-daily clinical dose. These human-simulated ELF exposures were adequate to attain >2-log reduction in bacterial burden at 24 h in 3 out of 4 isolates in both models and 1.58- and 0.74-log reductions with the fourth isolate in immunocompetent and neutropenic mice, respectively. Tedizolid showed potent in vivo efficacy against S. pneumoniae in both immunocompetent and neutropenic lung infection models, which support its consideration for S. pneumoniae lung infections.

Activity of cefquinome against extended-spectrum β-lactamase-producing Klebsiella pneumoniae in neutropenic mouse thigh model

Increasing prevalence of extended-spectrum β-lactamase (ESBL)-producing Klebsiella pneumoniae (K. pneumoniae) is of clinical concern. The objective of our study was to examine the in vivo activity of cefquinome against ESBL-producing K. pneumoniae strain using a neutropenic mouse thigh infection model. Cefquinome kinetics and protein binding in infected neutropenic mice were measured by liquid chromatography-tandem mass spectrometry (LC-MS/MS). Dose-fractionation studies over a 24-h dose range of 2.5-320 mg/kg were administered every 3, 6, 12, or 24 h. The percentage of the dosing interval that the free-drug serum levels exceed the MIC (%fT > MIC) was the PK-PD index that best correlated with cefquinome efficacy (R²  = 86%). Using a sigmoid E_max model, the magnitudes of %fT > MIC producing net bacterial stasis, a 1-log₁₀ kill and a 2-log₁₀ kill over 24 h, were estimated to be 20.07%, 29.57%, and 55.12%, respectively. These studies suggest that optimal cefquinome PK/PD targets are not achieved in pigs, sheep, and cattle at current recommended doses (1˜2 mg/kg). Further studies with higher doses in the target species are needed to ensure therapeutic concentration, if cefquinome is used for treatment of K. pneumoniae infection.

Pharmacokinetic/Pharmacodynamic Profiles of Tiamulin in an Experimental Intratracheal Infection Model of Mycoplasma gallisepticum

Mycoplasma gallisepticum is the most important pathogen in poultry among four pathogenic Mycoplasma species. Tiamulin is a pleuromutilin antibiotic that shows a great activity against M. gallisepticum and has been approved for use in veterinary medicine particularly for poultry. However, the pharmacokinetic/pharmacodynamics (PK/PD) profiles of tiamulin against M. gallisepticum are not well understood. Therefore, in the current studies, we investigated the in vivo PK/PD profiles of tiamulin using a well-established experimental intratracheal infection model of M. gallisepticum. The efficacy of tiamulin against M. gallisepticum was studied in 8-day-old chickens after intramuscular (i.m.) administration at 10 doses between 0–80 mg/kg. Liquid chromatography-tandem mass spectrometry (LC-MS/MS) was used to evaluate the PK parameters of tiamulin following i.m. administration at doses of 5, 40, and 80 mg/kg in Mycoplasma gallisepticum-infected neutropenic chickens. Real-time PCR (RT-PCR) was used for quantitative detection of M. gallisepticum. The MIC of tiamulin against M. gallisepticum strain S6 was 0.03 μg/mL. The PK/PD index, AUC_24h/MIC, correlated well with the in vivo antibacterial efficacy. The in vivo data suggest that animal dosage regimens should supply AUC_24h/MIC of tiamulin of 382.68 h for 2 log₁₀ ccu equivalents M. gallisepticum reduction. To attain that goal, the administered dose is expected to be 45 mg/kg b.w. for treatment of M. gallisepticum infection with an MIC₉₀ of 0.03 μg/mL.

In Vitro and In Vivo Synergy of the Oxadiazole Class of Antibacterials with β-Lactams

The oxadiazole antibacterials target the bacterial cell wall and are bactericidal. We investigated the synergism of ND-421 with the commonly used β-lactams and non-β-lactam antibiotics by the checkerboard method and by time-kill assays. ND-421 synergizes well with β-lactam antibiotics, and it also exhibits a long postantibiotic effect (4.7 h). We also evaluated the in vivo efficacy of ND-421 in a murine neutropenic thigh infection model alone and in combination with oxacillin. ND-421 has in vivo efficacy by itself in a clinically relevant infection model (1.49 log10 bacterial reduction for ND-321 versus 0.36 log10 for linezolid with NRS119) and acts synergistically with β-lactam antibiotics in vitro and in vivo, and the combination of ND-421 with oxacillin is efficacious in a mouse neutropenic thigh methicillin-resistant Staphylococcus aureus (MRSA) infection model (1.60 log10 bacterial reduction). The activity of oxacillin was potentiated in the presence of ND-421, as the strain would have been resistant to oxacillin otherwise.

Pharmacokinetics of Intraperitoneal Cefalothin and Cefazolin in Patients Being Treated for Peritoneal Dialysis-Associated Peritonitis

♦

BACKGROUND

The standard treatment of peritoneal dialysis (PD)-associated peritonitis (PD-peritonitis) is intraperitoneal (IP) administration of antibiotics. Only limited data on the pharmacokinetics and appropriateness of contemporary dose recommendations of IP cefalothin and cefazolin exist. The aim of this study was to describe the pharmacokinetics of IP cefalothin and cefazolin in patients treated for PD-peritonitis. ♦

METHODS

As per international guidelines, IP cefalothin or cefazolin 15 mg/kg once daily was dosed with gentamicin in a 6-hour dwell to patients with PD-peritonitis during routine care. Serial plasma and PD effluent samples were collected over the first 24 hours of therapy. Antibiotic concentrations were quantified using a validated chromatographic method with pharmacokinetic analysis performed using a non-compartmental approach. ♦

RESULTS

Nineteen patients were included (cefalothin n = 8, cefazolin n = 11). The median bioavailability for both antibiotics exceeded 92%, but other pharmacokinetic parameters varied markedly between antibiotics. Both antibiotics achieved high PD effluent concentrations throughout the antibiotic dwell. Cefazolin had a smaller volume of distribution compared with cefalothin (14 vs 40 L, p = 0.003). The median trough total plasma antibiotic concentration for cefazolin and cefalothin during the dwell differed (plasma 56 vs 13 mg/L, p < 0.0001) despite a similar concentration in PD effluent (37 vs 38 mg/L, p = 0.58). Lower antibiotic concentrations were noted during PD dwells not containing antibiotic, particularly cefalothin, which was frequently undetectable in plasma and PD effluent. The median duration that the unbound antibiotic concentration was above the minimum inhibitory concentration (MIC) was approximately 13% (plasma) and 25% (IP) for cefalothin, and 100% (plasma and IP) for cefazolin, of the dosing interval. ♦

CONCLUSIONS

When IP cefalothin or cefazolin is allowed to dwell for 6 hours, sufficient PD effluent concentrations are present for common pathogens during this time. However, with once-daily IP dosing, in contrast to cefazolin, there is a risk of subtherapeutic plasma and PD effluent cefalothin concentrations, so more frequent dosing may be required.

Defining Clinical Exposures of Cefepime for Gram-Negative Bloodstream Infections That Are Associated with Improved Survival

The percentage of time that free drug concentrations remain above the MIC (fT>MIC) that is necessary to prevent mortality among cefepime-treated patients with Gram-negative bloodstream infections (GNBSI) is poorly defined. We conducted a retrospective study of adult patients with GNBSI. Eligible cases were frequency matched to ensure categorical representation from all MICs. Organism, MIC, infection source, gender, age, serum creatinine, weight, antibiotic history, and modified APACHE II score were collected from hospital records. Two population pharmacokinetic models (models 1 and 2) were used to impute exposures over the first 24 h in each patient from mean model parameters, covariates, and dosing history. From the imputed exposures, survival thresholds for fT>MIC were identified using classification and regression tree (CART) analysis and analyzed as nominal variables for univariate and multivariate regressions. A total of 180 patients were included in the analysis, of whom 13.9% died and 86.1% survived. Many patients (46.7% [n = 84/180]) received combination therapy with cefepime. Survivors had higher mean (standard deviation [SD]) fT>MIC than those who died (model 1, 74.2% [29.6%] versus 52.1% [33.8%], P < 0.001; model 2, 85.9% [24.0%] versus 64.4% [31.4%], P < 0.001). CART identified fT>MIC threshold values for greater survival according to models 1 and 2 at >68% and >74%, respectively. Survival was improved for those with fT>MIC of >68% (model 1 adjusted odds ratio [aOR], 7.12; 95% confidence interval [CI], 1.90 to 26.7; P = 0.004) and >74% (model 2 aOR, 6.48; 95% CI, 1.90 to 22.1) after controlling for clinical covariates. Similarly, each 1% increase in cefepime fT>MIC resulted in a 2% improvement in multivariate survival probability (P = 0.015). Achieving a cefepime fT>MIC of 68 to 74% was associated with a higher odds of survival for patients with GNBSI. Regimens targeting this exposure should be aggressively pursued.

In Vivo Pharmacodynamics of Cefquinome in a Neutropenic Mouse Thigh Model of Streptococcus suis Serotype 2 at Varied Initial Inoculum Sizes

Streptococcus suis serotype 2 is an emerging zoonotic pathogen and causes severe disease in both pigs and human beings. Cefquinome (CEQ), a fourth-generation cephalosporin, exhibits broad-spectrum activity against Gram-positive bacteria such as S. suis. This study evaluated the in vitro and in vivo antimicrobial activities of CEQ against four strains of S. suis serotype 2 in a murine neutropenic thigh infection model. We investigated the effect of varied inoculum sizes (10(6) to 10(8) CFU/thigh) on the pharmacokinetic (PK)/pharmacodynamic (PD) indices and magnitudes of a particular PK/PD index or dose required for efficacy. Dose fractionation studies included total CEQ doses ranging from 0.625 to 640 mg/kg/24 h. Data were analyzed via a maximum effect (Emax) model using nonlinear regression. The PK/PD studies demonstrated that the percentage of time that serum drug levels were above the MIC of free drug (%ƒT>MIC) in a 24-h dosing interval was the primary index driving the efficacy of both inoculum sizes (R(2) = 91% and R(2) = 63%). CEQ doses of 2.5 and 40 mg/kg body weight produced prolonged postantibiotic effects (PAEs) of 2.45 to 8.55 h. Inoculum sizes had a significant influence on CEQ efficacy. Compared to the CEQ exposure and dosages in tests using standard inocula, a 4-fold dose (P = 0.006) and a 2-fold exposure time (P = 0.01) were required for a 1-log kill using large inocula of 10(8) CFU/thigh.

Ceftobiprole medocaril in the treatment of hospital-acquired pneumonia

Ceftobiprole medocaril is a fifth-generation cephalosporin approved in Europe as single-agent therapy for hospital-acquired pneumonia (HAP), excluding ventilator-associated pneumonia (VAP). It is rapidly converted to the active metabolite ceftobiprole following intravenous administration. Ceftobiprole has a broad spectrum of activity, notably against methicillin-resistant Staphylococcus aureus, ampicillin-susceptible enterococci, penicillin-resistant pneumococci and Enterobacteriaceae not producing extended-spectrum β-lactamase. Ceftobiprole is primarily excreted renally by glomerular filtration, with minimal propensity for interaction with co-administered drugs. Normal dose is ceftobiprole 500 mg, administered by 2-h intravenous infusion every 8 h, with dose adjustment according to renal function. In a pivotal Phase III trial in patients with HAP, ceftobiprole monotherapy was as efficacious as ceftazidime/linezolid for clinical and microbiological cure and was noninferior to ceftazidime/linezolid in the subgroup of patients with HAP excluding VAP. Ceftobiprole and ceftazidime/linezolid were similarly well tolerated. Ceftobiprole is an efficacious and well-tolerated option for empirical treatment of patients with HAP (excluding VAP).

Ceftobiprole medocaril: a review of its use in patients with hospital- or community-acquired pneumonia

Ceftobiprole, the active metabolite of the prodrug ceftobiprole medocaril (Zevtera(®)), is a new generation broad-spectrum intravenous cephalosporin with activity against methicillin-resistant Staphylococcus aureus. Ceftobiprole exhibits potent in vitro activity against a number of Gram-positive and Gram-negative pathogens associated with hospital-acquired pneumonia (HAP) and community-acquired pneumonia (CAP). It is the first cephalosporin monotherapy approved in the EU for the treatment of both HAP (excluding ventilator associated-pneumonia [VAP]) and CAP. In phase III trials, ceftobiprole medocaril was noninferior, in terms of clinical cure rates at the test-of-cure visit, to ceftazidime plus linezolid in patients with HAP and to ceftriaxone ± linezolid in patients with CAP severe enough to require hospitalization. In patients with HAP, noninferiority of ceftobiprole medocaril to ceftazidime plus linezolid was not demonstrated in a subset of patients with VAP. In patients with CAP, ceftobiprole medocaril was effective in those at risk for poor outcomes (pneumonia severity index ≥91, Pneumonia Patient Outcomes Research Team score IV-V or bacteraemic pneumonia). In the phase III trials, ceftobiprole medocaril was generally well tolerated, with ≈10 % of patients discontinuing the treatment because of adverse events. The most common treatment-related adverse events occurring in ceftobiprole recipients in the trials in patients with HAP or CAP included nausea, diarrhoea, infusion site reactions, vomiting, hepatic enzyme elevations and hyponatraemia. Therefore, ceftobiprole medocaril monotherapy offers a simplified option for the initial empirical treatment of patients with HAP (excluding VAP) and in those with CAP requiring hospitalization.

In vivo pharmacokinetic/pharmacodynamic profiles of valnemulin in an experimental intratracheal Mycoplasma gallisepticum infection model

Valnemulin, a semisynthetic pleuromutilin antibiotic derivative, is greatly active against Mycoplasma. The objective of our study was to evaluate the effectiveness of valnemulin against Mycoplasma gallisepticum in a neutropenic intratracheal model in chickens using a pharmacokinetic/pharmacodynamic (PK-PD) method. The PK of valnemulin after intramuscular (i.m.) administration at doses of 1, 10, and 20 mg/kg of body weight in M. gallisepticum-infected neutropenic chickens was evaluated by liquid chromatography-tandem mass spectrometry (LC-MS/MS). Real-time PCR (RT-PCR) was used for quantitative detection of M. gallisepticum. The ratio of the 24-h area under the concentration-time curve divided by the MIC (AUC24/MIC) correlated well with the in vivo antibacterial effectiveness of valnemulin (R(2) = 0.9669). The AUC24/MIC ratios for mycoplasmastasis (a reduction of 0 log10 color-changing unit [CCU] equivalents/ml), a reduction of 1 log10 CCU equivalents/ml, and a reduction of 2.5 log10 CCU equivalents/ml are 28,820, 38,030, and 56,256, respectively. In addition, we demonstrated that valnemulin at a dose of 6.5 mg/kg resulted in a reduction of 2.5 log10 CCU equivalents/ml. These investigations provide a solid foundation for the usage of valnemulin in poultry with M. gallisepticum infections.

Population Pharmacokinetics of Cefazolin in Serum and Tissue for Patients with Complicated Skin and Soft Tissue Infections (cSSTI)

Introduction

Cefazolin is commonly used to treat complicated skin and soft tissue infections (cSSTI) caused by methicillin-susceptible Staphylococcus aureus (MSSA) and Enterobacteriaceae. We aimed to determine the variability of cefazolin exposure in interstitial fluid (ISF) of tissue and evaluate its dosing recommendations.

Methods

Population pharmacokinetics were performed to co-model serum and ISF concentration data from six patients enrolled in a previous in vivo microdialysis study. A 5,000 patient Monte Carlo simulation was then conducted for 1 and 2 g every 8 h (q8h) regimens to calculate the penetration ratio and probability of target attainment (PTA) at 30% and 50% of the dosing interval that free drug concentrations remain above the minimum inhibitory concentration (fT > MIC) in ISF of tissue.

Results

A three-compartment model, with one of the compartments representing ISF concentrations, fits the data best. The final model resulted in the mean ± SD parameter values: Clearance = 3.8 ± 2.1 L/h, volume of distribution in central compartment = 8.6 ± 6.4 L and volume of distribution in ISF = 36.6 ± 17.9 L. The mean ± SD and median penetration ratios were 1.36 ± 4.57 and 0.80, respectively. At the MIC₉₀ for MSSA of 1 mg/L, PTAs for the 1 g q8h dose in ISF were 96% and 91% for 30% and 50% fT > MIC targets, respectively, which decreased to 87% and 71% at 2 mg/L. For the same respective targets, a 2 g q8h dosing regimen increased PTA to 96% and 91% at 2 mg/L.

Conclusion

Cefazolin penetration into the ISF of a lower limb infection varied across this simulated patient population. Based on these data, a 1 g q8h regimen should be sufficient to obtain 30% fT > MIC exposure against most MSSA causing cSSTI. However, a 2 g q8h dose is required to obtain 50% fT > MIC pharmacodynamic targets at the current breakpoint for Enterobacteriaceae (2 mg/L).

Electronic supplementary material

The online version of this article (doi:10.1007/s40121-014-0049-3) contains supplementary material, which is available to authorized users.

Vaccine protection of leukopenic mice against Staphylococcus aureus bloodstream infection

The risk for Staphylococcus aureus bloodstream infection (BSI) is increased in immunocompromised individuals, including patients with hematologic malignancy and/or chemotherapy. Due to the emergence of antibiotic-resistant strains, designated methicillin-resistant S. aureus (MRSA), staphylococcal BSI in cancer patients is associated with high mortality; however, neither a protective vaccine nor pathogen-specific immunotherapy is currently available. Here, we modeled staphylococcal BSI in leukopenic CD-1 mice that had been treated with cyclophosphamide, a drug for leukemia and lymphoma patients. Cyclophosphamide-treated mice were highly sensitive to S. aureus BSI and developed infectious lesions lacking immune cell infiltrates. Virulence factors of S. aureus that are key for disease establishment in immunocompetent hosts-α-hemolysin (Hla), iron-regulated surface determinants (IsdA and IsdB), coagulase (Coa), and von Willebrand factor binding protein (vWbp)-are dispensable for the pathogenesis of BSI in leukopenic mice. In contrast, sortase A mutants, which cannot assemble surface proteins, display delayed time to death and increased survival in this model. A vaccine with four surface antigens (ClfA, FnBPB, SdrD, and SpAKKAA), which was identified by genetic vaccinology using sortase A mutants, raised antigen-specific immune responses that protected leukopenic mice against staphylococcal BSI.

Impact of MIC range for Pseudomonas aeruginosa and Streptococcus pneumoniae on the ceftolozane in vivo pharmacokinetic/pharmacodynamic target

Ceftolozane is a novel cephalosporin with activity against drug-resistant pathogens, including Pseudomonas aeruginosa and Streptococcus pneumoniae. The in vivo investigation reported here tested the limits of this drug against 20 P. aeruginosa and S. pneumoniae isolates across a wide MIC range and defined resistance mechanisms. The times above the MIC (T>MIC) targets for stasis and 1- and 2-log reductions were 31%, 39%, and 42% for P. aeruginosa and 18%, 24%, and 27% for S. pneumoniae, respectively. The 1-log endpoint was achieved for strains with MICs as high as 16 μg/ml.

In vivo activity of cefquinome against Escherichia coli in the thighs of neutropenic mice

Cefquinome is a cephalosporin with broad-spectrum antibacterial activity, including activity against enteric Gram-negative bacilli such as Escherichia coli. We utilized a neutropenic mouse model of colibacillosis to examine the pharmacodynamic (PD) characteristics of cefquinome, as measured by organism number in homogenized thigh cultures after 24 h of therapy. Serum drug levels following 4-fold-escalating single doses of cefquinome were measured by liquid chromatography-tandem mass spectrometry (LC-MS/MS). The pharmacokinetic (PK) properties of cefquinome were linear over a dose range of 10 to 640 mg/kg of body weight. Serum half-lives ranged from 0.29 to 0.32 h. Dose fractionation studies over a 24-h dose range of 2.5 to 320 mg/kg were conducted every 3, 6, 12, or 24 h. Nonlinear regression analysis was used to determine which pharmacodynamic parameter best correlated with efficacy. The free percentage of the dosing interval that the serum levels exceed the MIC (fT>MIC) was the PK-PD index that best correlated with efficacy (R(2) = 73% for E. coli, compared with 13% for the maximum concentration of the free drug in serum [fCmax]/MIC and 45% for the free-drug area under the concentration-time curve from 0 to 24 h [fAUC0-24]/MIC). Subsequently, we employed a similar dosing strategy by using 4-fold-increasing total cefquinome doses administered every 4 h to treat animals infected with four additional E. coli isolates. A sigmoid maximum-effect (Emax) model was used to estimate the magnitudes of the %fT>MIC associated with net bacterial stasis, a 1-log10 CFU reduction from baseline, and a 2-log10 CFU reduction from baseline; the corresponding values were 28.01% ± 2.27%, 37.23% ± 4.05%, and 51.69% ± 9.72%. The potent bactericidal activity makes cefquinome an attractive option for the treatment of infections caused by E. coli.

Pharmacodynamics of cefquinome in a neutropenic mouse thigh model of Staphylococcus aureus infection

Cefquinome is a cephalosporin with broad-spectrum antibacterial activity, including activity against Staphylococcus aureus. The objective of our study was to examine the in vivo activity of cefquinome against S. aureus strains by using a neutropenic mouse thigh infection model. Cefquinome kinetics and protein binding in infected neutropenic mice were measured by liquid chromatography-tandem mass spectrometry (LC-MS/MS). In vivo postantibiotic effects (PAEs) were determined after a dose of 100 mg/kg of body weight in mice infected with S. aureus strain ATCC 29213. The animals were treated by subcutaneous injection of cefquinome at doses of 2.5 to 320 mg/kg of body weight per day divided into 1, 2, 3, 6, or 12 doses over 24 h. Cefquinome exhibited time-dependent killing and produced in vivo PAEs at 2.9 h. The percentage of time that serum concentrations were above the MIC (%T>MIC) was the pharmacokinetic-pharmacodynamic (PK-PD) index that best described the efficacy of cefquinome. Subsequently, we employed a similar dosing strategy by using increasing total cefquinome doses that increased 4-fold and were administered every 4 h to treat animals infected with six additional S. aureus isolates. A sigmoid maximum effect (Emax) model was used to estimate the magnitudes of the ratios of the %T that the free-drug serum concentration exceeded the MIC (%T>fMIC) associated with net bacterial stasis, a 0.5-log10 CFU reduction from baseline, and a 1-log10 CFU reduction from baseline; the respective values were 30.28 to 36.84%, 34.38 to 46.70%, and 43.50 to 54.01%. The clear PAEs and potent bactericidal activity make cefquinome an attractive option for the treatment of infections caused by S. aureus.

Exposure to ceftobiprole is associated with microbiological eradication and clinical cure in patients with nosocomial pneumonia

The percentage of the dosing interval that the non-protein-bound plasma concentration is above the MIC (%fT>MIC) for cephalosporins has been shown to correlate with microbiological outcomes in preclinical studies. However, clinical data are scarce. Using data from a randomized double-blind phase 3 clinical trial, we explored the relationship of ceftobiprole exposure with microbiological and clinical outcomes in patients with nosocomial pneumonia. The individual ceftobiprole exposure was determined for different pharmacokinetic (PK)/pharmacodynamic (PD) indices using individual pharmacokinetic data and a previously published population model. The MICs used in the analysis were the highest MICs for any bacterium cultured at baseline or the end of treatment (EOT). Outcomes were microbiological cure at EOT and clinical cure at test of cure (TOC). Multiple logistic regression (MLR) and classification and regression tree (CART) analyses were applied to determine the relationships among exposure, patient characteristics, and outcomes. MLR indicated that the %fT>MIC of ceftobiprole was the best predictor for both microbiological eradication and clinical cure. CART analysis showed a breakpoint value of 51.1% (n = 159; P = 0.0024) for clinical cure, whereas it was 62.2% (n = 251; P < 0.0001) for microbiological eradication. Other factors also contributed, particularly to clinical outcome. These included the difference between VAP and non-VAP patients, systemic inflammatory response syndrome (SIRS), creatinine clearance, the use of anti-Pseudomonas combination therapy, and Acute Physiology and Chronic Health Evaluation II (APACHE-II) score. There is a strong correlation between microbiological eradication and clinical cure with exposure to ceftobiprole. The %fT>MIC required to result in a favorable clinical outcome is >51% of the dosing interval, which is in line with the values found for microbiological eradication, the comparator ceftazidime, and preclinical models.

Application of pharmacokinetic/pharmacodynamic modelling and simulation for the prediction of target attainment of ceftobiprole against meticillin-resistant Staphylococcus aureus using minimum inhibitory concentration and time-kill curve based approaches

The purpose of this report was to compare two different methods for dose optimisation of antimicrobials. The probability of target attainment (PTA) was calculated using Monte Carlo simulation to predict the PK/PD target of fT>MIC or modelling and simulation of time-kill curve data. Ceftobiprole, the paradigm compound, activity against two MRSA strains was determined, ATCC 33591 (MIC=2mg/L) and a clinical isolate (MIC=1mg/L). A two-subpopulation model accounting for drug degradation during the experiment adequately fit the time-kill curve data (concentration range 0.25-16× MIC). The PTA was calculated for plasma, skeletal muscle and subcutaneous adipose tissue based on data from a microdialysis study in healthy volunteers. A two-compartment model with distribution factors to account for differences between free serum and tissue interstitial space fluid concentration appropriately fit the pharmacokinetic data. Pharmacodynamic endpoints of fT>MIC of 30% or 40% and 1- or 2-log kill were used. The PTA was >90% in all tissues based on the PK/PD endpoint of fT>MIC >40%. The PTAs based on a 1- or 2-log kill from the time-kill experiments were lower than those calculated based on fT>MIC. The PTA of a 1-log kill was >90% for both MRSA isolates for plasma and skeletal muscle but was slightly below 90% for subcutaneous adipose tissue (both isolates ca. 88%). The results support a dosing regimen of 500mg three times daily as a 2-h intravenous infusion. This dose should be confirmed as additional pharmacokinetic data from various patient populations become available.

Pharmacokinetic-pharmacodynamic modeling of antibacterial drugs

Pharmacokinetic-pharmacodynamic (PKPD) modeling and simulation has evolved as an important tool for rational drug development and drug use, where developed models characterize both the typical trends in the data and quantify the variability in relationships between dose, concentration, and desired effects and side effects. In parallel, rapid emergence of antibiotic-resistant bacteria imposes new challenges on modern health care. Models that can characterize bacterial growth, bacterial killing by antibiotics and immune system, and selection of resistance can provide valuable information on the interactions between antibiotics, bacteria, and host. Simulations from developed models allow for outcome predictions of untested scenarios, improved study designs, and optimized dosing regimens. Today, much quantitative information on antibiotic PKPD is thrown away by summarizing data into variables with limited possibilities for extrapolation to different dosing regimens and study populations. In vitro studies allow for flexible study designs and valuable information on time courses of antibiotic drug action. Such experiments have formed the basis for development of a variety of PKPD models that primarily differ in how antibiotic drug exposure induces amplification of resistant bacteria. The models have shown promise for efficacy predictions in patients, but few PKPD models describe time courses of antibiotic drug effects in animals and patients. We promote more extensive use of modeling and simulation to speed up development of new antibiotics and promising antibiotic drug combinations. This review summarizes the value of PKPD modeling and provides an overview of the characteristics of available PKPD models of antibiotics based on in vitro, animal, and patient data.

Pharmacodynamics of ceftaroline against Staphylococcus aureus studied in an in vitro pharmacokinetic model of infection

An in vitro single-compartment dilutional pharmacokinetic model was used to study the pharmacodynamics of ceftaroline against Staphylococcus aureus (both methicillin-susceptible S. aureus [MSSA] and methicillin-resistant S. aureus [MRSA]). Mean serum free concentrations of ceftaroline (the active metabolite of the prodrug ceftaroline fosamil) dosed in humans at 600 mg every 12 h (q12h) were simulated, and activities against 12 S. aureus strains (3 MSSA strains and 9 MRSA strains, 3 of which had a vancomycin-intermediate phenotype) were determined. Ceftaroline produced 2.5- to 4.0-log10-unit reductions in viable counts by 24 h with all strains and a 0.5- to 4.0-log-unit drop in counts at 96 h. The antibacterial effect could not be related to the strain MIC across the ceftaroline MIC range from 0.12 to 2.0 μg/ml. In dose-ranging studies, the cumulative percentage of a 24-h period that the free drug concentration exceeded the MIC under steady-state pharmacokinetic conditions (fT(MIC)) of 24.5% ± 8.9% was associated with a 24-h bacteriostatic effect, one of 27.8% ± 9.5% was associated with a -1-log-unit drop, and one of 32.1% ± 8.1% was associated with a -2-log-unit drop. The MSSA and MRSA strains had similar fT(MIC) values. fT(MIC) values increased with increasing duration of exposure up to 96 h. Changes in ceftaroline population analysis profiles were related to fT(MIC). fT(MIC)s of <50% were associated with growth on 4× MIC recovery plates at 96 h of drug exposure. These data support the use of ceftaroline fosamil at doses of 600 mg q12h to treat S. aureus strains with MICs of ≤ 2 μg/ml. An fT(MIC) of 25 to 30% would make a suitable pharmacodynamic index target, but fTMIC values of ≥ 50% are needed to suppress the emergence of resistance and require clinical evaluation.

Pharmacokinetic and pharmacodynamics evaluation of ceftobiprole medocaril for the treatment of hospital-acquired pneumonia

INTRODUCTION

Ceftobiprole is a cephalosporin with activity against methicillin-resistant Staphylococcus aureus, Enterobacteriaceae, and Pseudomonas aeruginosa with a promising role in the treatment of hospital-acquired pneumonia (HAP). Cure rates, however, with ceftobiprole at the doses studied may be inferior to conventional treatment in the ventilator-acquired subset of HAP.

AREAS COVERED

Literature was sought using PubMed and through abstracts from the Interscience Conference on Antimicrobial Agents and Chemotherapy (2006 - 2012) and the European Congress of Clinical Microbiology and Infectious Diseases (2007 - 2012). The authors used the search terms "ceftobiprole," "BAL9141," "RO63-9141," "BAL5788," and 'RO5788." The article discusses the activity, mechanism of action, pharmacokinetics (PK), pharmacodynamics (PD), and clinical trials of ceftobiprole in HAP. The article also provides discussion of how PK/PD parameters play a role in the outcome of HAP treatment and how dosing in ventilator-associated pneumonia (VAP) should be reconsidered in light of altered PK/PD.

EXPERT OPINION

In patients with normal PK and non-VAP, ceftobiprole is effective for the treatment of HAP in the recommended doses, ceftobiprole is unlikely to achieve the desired PD targets when PK parameters are altered in VAP (e.g., increased volume of distribution and clearance). In these settings, off-label use at higher doses may overcome these limitations; but in the presence of alternative therapies, it cannot be currently recommended.

Inoculum effects of ceftobiprole, daptomycin, linezolid, and vancomycin with Staphylococcus aureus and Streptococcus pneumoniae at inocula of 10(5) and 10(7) CFU injected into opposite thighs of neutropenic mice

Reduced bactericidal efficacy at a high inoculum is known as the inoculum effect (IE). We used neutropenic mice to compare the IEs of ceftobiprole (CFB), daptomycin (DAP), linezolid (LZD), and vancomycin (VAN) against 6 to 9 strains of Staphylococcus aureus and 4 strains of Streptococcus pneumoniae at 2 inocula in opposite thighs of the same mice. Neutropenic mice had 10(4.5) to 10(5.7) CFU/thigh (low inoculum [LI]) in one thigh and 10(6.4) to 10(7.2) CFU/thigh (high inoculum [HI]) in the opposite thigh when treated for 24 h with subcutaneous (s.c.) doses every 12 h of DAP at 0.024 to 100 mg/kg of body weight and LZD at 0.313 to 320 mg/kg and s.c. doses every 6 h of CFB at 0.003 to 160 mg/kg and VAN at 0.049 to 800 mg/kg. Dose-response data were analyzed by a maximum effect (E(max)) model using nonlinear regression. Static doses for each drug and at each inoculum were calculated, and the difference between HI and LI (IE index) gave the magnitude of IE for each drug-organism combination. Mean (range) IE indexes of S. aureus were 2.9 (1.7 to 4.6) for CFB, 4.1 (2.6 to 9.3) for DAP, 4.6 (1.7 to 7.1) for LZD, and 10.1 (6.3 to 20.3) for VAN. In S. pneumoniae, the IE indexes were 2.5 (1.3 to 3.3) for CFB, 2.0 (1.6 to 2.8) for DAP, 1.9 (1.7 to 2.2) for LZD, and 1.5 (0.8 to 3.2) for VAN; these values were similar for all drugs. In S. aureus, the IE was much larger with VAN than with CFB, DAM, and LZD (P < 0.05). An in vivo time course of vancomycin activity showed initiation of killing at 4- to 16-fold-higher doses at HI than at LI despite similar initial growth of controls.

In vivo activities of ceftolozane, a new cephalosporin, with and without tazobactam against Pseudomonas aeruginosa and Enterobacteriaceae, including strains with extended-spectrum β-lactamases, in the thighs of neutropenic mice

Ceftolozane is a new cephalosporin with potent activity against Pseudomonas aeruginosa and Enterobacteriaceae. A neutropenic murine thigh infection model was used to determine which pharmacokinetic/pharmacodynamic index and magnitude drives the efficacy of ceftolozane with Gram-negative bacilli, to compare the rates of in vivo killing of P. aeruginosa by ceftolozane and ceftazidime, and to determine the impact of different ratios of ceftolozane plus tazobactam on Enterobacteriaceae containing extended-spectrum β-lactamases (ESBLs). Neutropenic mice had 10(6.2-7.1) CFU/thigh when treated with ceftolozane for 24 h with (i) various doses (3.12 to 1,600 mg/kg) and dosage intervals (3, 6, 12, and 24 h) against two Enterobacteriaceae strains, (ii) 0.39 to 800 mg/kg every 6 h for four Enterobacteriaceae and four P. aeruginosa strains, and (iii) 400 or 800 mg/kg with 2:1. 4:1, and 8:1 ratios of tazobactam against five Enterobacteriaceae strains with ESBLs. The pharmacokinetics of ceftolozane at 25, 100, and 400 mg/kg were linear with peak/dose values of 1.0 to 1.4 and half-lives of 12 to 14 min. T>MIC was the primary index driving efficacy. For stasis (1 log kill), T>MIC was 26.3% ± 2.1% (31.6% ± 1.6%) for wild-type Enterobacteriaceae, 31.1% ± 4.9% (34.8% ± 4.4%) for Enterobacteriaceae with ESBLs, and 24.0% ± 3.3% (31.5% ± 3.9%) for P. aeruginosa. At 200 mg/kg every 3 h, the rate of in vivo killing of P. aeruginosa was faster with ceftolozane than with ceftazidime (-0.34 to -0.41 log10 CFU/thigh/h versus -0.21 to -0.24 log10 CFU/thigh/h). The 2:1 ratio of ceftolozane with tazobactam was the most potent combination studied. The T>MIC required for ceftolozane is less than with other cephalosporins and may be due to more rapid killing.