Penetration of piperacillin and tazobactam into pneumonic human lung tissue measured by in vivo microdialysis

Antimicrobial Drug Penetration Is Enhanced by Lung Tissue Inflammation and Injury

Rationale: Pneumonia is a frequent and feared complication in intubated critically ill patients. Tissue concentrations of antimicrobial drugs need to be sufficiently high to treat the infection and also prevent development of bacterial resistance. It is uncertain whether pulmonary inflammation and injury affect antimicrobial drug penetration into lung tissue.Objectives: To determine and compare tissue and BAL fluid concentrations of ceftaroline fosamil and linezolid in a model of unilateral acute lung injury in pigs and to evaluate whether dose adjustment is necessary to reach sufficient antimicrobial concentrations in injured lung tissue.Methods: After induction of unilateral acute lung injury, ceftaroline fosamil and linezolid were administered intravenously. Drug concentrations were measured in lung tissue through microdialysis and in blood and BAL fluid samples during the following 8 hours. The primary endpoint was the tissue concentration area under the concentration curve in the first 8 hours (AUC0-8 h) of the two antimicrobial drugs.Measurements and Main Results: In 10 pigs, antimicrobial drug concentrations were higher in inflamed and injured lung tissue compared with those in uninflamed and uninjured lung tissue (median ceftaroline fosamil AUC0-8 h [and interquartile range] = 26.7 mg ⋅ h ⋅ L-1 [19.7-39.0] vs. 16.0 mg ⋅ h ⋅ L-1 [13.6-19.9], P = 0.02; median linezolid AUC0-8 h 76.0 mg ⋅ h ⋅ L-1 [68.1-96.0] vs. 54.6 mg ⋅ h ⋅ L-1 [42.7-60.9], P = 0.01), resulting in a longer time above the minimal inhibitory concentration and in higher peak concentrations and dialysate/plasma ratios. Penetration into BAL fluid was excellent for both antimicrobials, but without left-to-right differences (ceftaroline fosamil, P = 0.78; linezolid, P = 1.00).Conclusions: Tissue penetration of two commonly used antimicrobial drugs for pneumonia is enhanced by early lung tissue inflammation and injury, resulting in longer times above the minimal inhibitory concentration. Thus, lung tissue inflammation ameliorates antimicrobial drug penetration during the acute phase.

Lung microdialysis and in vivo PK/PD integration of cefquinome against Actinobacillus pleuropneumoniae in a porcine experimental lung infection model

This study aim to explore the application of microdialysis in pharmacokinetic (PK) and pharmacodynamic (PD) integration of cefquinome against Actinobacillus pleuropneumoniae in a porcine experimental lung infection model. The model was established via intratracheal inoculation where average bacterial counts (CFU) in the lungs of infected pigs reached 6.57 log10 CFU/g after 3 h. The PK profiles of unbound cefquinome in lung dialysates were determined following intramuscular injection of single doses of 0.125, 0.25, 0.5, 1, 2, and 4 mg/kg. Lung dialysate samples were collected using microdialysis at a flow rate of 1.5 μL/min until 24 h. The PD studies were conducted over 24 h based on 10 intermittent dosing regimens and total daily doses ranged from 0.25 to 4 mg/kg and dosage intervals included 12 and 24 h. The lung tissue was collected after 24 h of treatment and homogenized for bacterial counts. The relationships between PK/PD parameters derived from lung dialysates and drug efficacy were analyzed using an inhibitory sigmoid Emax model. The percentage of time the free drug concentration exceeded the minimum inhibitory concentration (%fT > MIC) was the PK/PD index best describing the antimicrobial activity (R2 = 0.96) in the porcine experimental infection model. The %fT > MIC values required to achieve net bacterial stasis, 1, 2 and 3 log10 CFU/g reductions in the lung were 22.45, 28.86, 37.62, and 56.46%, respectively. Cefquinome exhibited time-dependent characteristics against A. pleuropneumoniae in vivo. These results provide valuable insights into the application of microdialysis in PK/PD integration model studies and optima regimen of cefquinome for the treatment of porcine respiratory diseases caused by A. pleuropneumoniae.

Understanding the nebulisation of antibiotics: the key role of lung microdialysis studies

BACKGROUND

Nebulisation of antibiotics is a promising treatment for ventilator-associated pneumonia (VAP) caused by multidrug-resistant organisms. Ensuring effective antibiotic concentrations at the site of infection in the interstitial space fluid is crucial for clinical outcomes. Current assessment methods, such as epithelial lining fluid and tissue homogenates, have limitations in providing longitudinal pharmacokinetic data.

MAIN BODY

Lung microdialysis, an invasive research technique predominantly used in animals, involves inserting probes into lung parenchyma to measure antibiotic concentrations in interstitial space fluid. Lung microdialysis offers unique advantages, such as continuous sampling, regional assessment of antibiotic lung concentrations and avoidance of bronchial contamination. However, it also has inherent limitations including the cost of probes and assay development, the need for probe calibration and limited applicability to certain antibiotics. As a research tool in VAP, lung microdialysis necessitates specialist techniques and resource-intensive experimental designs involving large animals undergoing prolonged mechanical ventilation. However, its potential impact on advancing our understanding of nebulised antibiotics for VAP is substantial. The technique may enable the investigation of various factors influencing antibiotic lung pharmacokinetics, including drug types, delivery devices, ventilator settings, interfaces and disease conditions. Combining in vivo pharmacokinetics with in vitro pharmacodynamic simulations can become feasible, providing insights to inform nebulised antibiotic dose optimisation regimens. Specifically, it may aid in understanding and optimising the nebulisation of polymyxins, effective against multidrug-resistant Gram-negative bacteria. Furthermore, lung microdialysis holds promise in exploring novel nebulisation therapies, including repurposed antibiotic formulations, bacteriophages and immunomodulators. The technique's potential to monitor dynamic biochemical changes in pneumonia, such as cytokines, metabolites and inflammation/infection markers, opens avenues for developing theranostic tools tailored to critically ill patients with VAP.

CONCLUSION

In summary, lung microdialysis can be a potential transformative tool, offering real-time insights into nebulised antibiotic pharmacokinetics. Its potential to inform optimal dosing regimen development based on precise target site concentrations and contribute to development of theranostic tools positions it as key player in advancing treatment strategies for VAP caused by multidrug-resistant organisms. The establishment of international research networks, exemplified by LUMINA (lung microdialysis applied to nebulised antibiotics), signifies a proactive step towards addressing complexities and promoting multicentre experimental studies in the future.

Microdialysis as a safe and feasible method to study target-site piperacillin-tazobactam disposition in septic piglets and children

OBJECTIVES

Knowledge on the tissue penetration of piperacillin-tazobactam in children with sepsis is lacking. In this study, the feasibility and performance of microdialysis experiments were explored in septic piglets and children as part of a translational research project.

METHODS

Multiple-day microdialysis investigations were performed in muscle tissue of 22 piglets (of which 11 were septic) and 6 children with sepsis. An in vitro experiment preceded the (pre)clinical trials to derive optimal experimental settings and calibration technique. Linear mixed-effects models quantified the impact of sepsis on relative recovery (RR) and intercatheter, interindividual, interoccasion, and residual variability.

RESULTS

In vivo microdialysis was well tolerated in piglets and children, with no significant adverse events reported. Using identical experimental settings, lower RR values were recorded in healthy and septic piglets (range: piperacillin, 17.2-29.1% and tazobactam, 23.5-29.1%) compared with the in vitro experiment (piperacillin, 43.3% and tazobactam, 55.3%), and there were unacceptably low values in children with sepsis (<10%). As a result, methodological changes were made in the pediatric trial. Realistic tissue concentration-time curves were derived in piglets and children. In piglets, sepsis reduced the RR. The greatest contributors to RR variability were residual (>40%) and interoccasion (>30%) variability. The internal standard method was the preferred calibration technique in both piglets and children.

CONCLUSIONS

Microdialysis is a safe and applicable method for the measurement of tissue drug concentrations in piglets and children. This study demonstrated the impact of experimental settings, sepsis, and target population on individual RR.

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.

β-Lactam pharmacodynamics in Gram-negative bloodstream infections in the critically ill

OBJECTIVES

To determine the β-lactam exposure associated with positive clinical outcomes for Gram-negative blood stream infection (BSI) in critically ill patients.

PATIENTS AND METHODS

Pooled data of critically ill patients with mono-microbial Gram-negative BSI treated with β-lactams were collected from two databases. Free minimum concentrations (fCmin) of aztreonam, cefepime, ceftazidime, ceftriaxone, piperacillin (co-administered with tazobactam) and meropenem were interpreted in relation to the measured MIC for targeted bacteria (fCmin/MIC). A positive clinical outcome was defined as completion of the treatment course or de-escalation, without other change of antibiotic therapy, and with no additional antibiotics commenced within 48 h of cessation. Drug exposure breakpoints associated with positive clinical outcome were determined by classification and regression tree (CART) analysis.

RESULTS

Data from 98 patients were included. Meropenem (46.9%) and piperacillin/tazobactam (36.7%) were the most commonly prescribed antibiotics. The most common pathogens were Escherichia coli (28.6%), Pseudomonas aeruginosa (19.4%) and Klebsiella pneumoniae (13.3%). In all patients, 87.8% and 71.4% achieved fCmin/MIC ≥1 and fCmin/MIC >5, respectively. Seventy-eight patients (79.6%) achieved positive clinical outcome. Two drug exposure breakpoints were identified: fCmin/MIC >1.3 for all β-lactams (predicted difference in positive outcome 84.5% versus 15.5%, P < 0.05) and fCmin/MIC >4.95 for meropenem, aztreonam or ceftriaxone (predicted difference in positive outcome 97.7% versus 2.3%, P < 0.05).

CONCLUSIONS

A β-lactam fCmin/MIC >1.3 was a significant predictor of a positive clinical outcome in critically ill patients with Gram-negative BSI and could be considered an antibiotic dosing target.

Lung Collapse during Mini-Thoracotomy Reduces Penetration of Cefuroxime to the Tissue: Interstitial Microdialysis Study in Animal Models

Background: Single-lung ventilation facilitates surgical exposure during minimally invasive cardiac surgery. However, a deeper knowledge of antibiotic distribution within a collapsed lung is necessary for effective antibiotic prophylaxis of pneumonia. Patients and Methods: The pharmacokinetics/pharmacodynamics (PK/PD) of cefuroxime were compared between the plasma and interstitial fluid (ISF) of collapsed and ventilated lungs in 10 anesthetized pigs, which were ventilated through a double-lumen endotracheal cannula. Cefuroxime (20 mg/kg) was administered in single 30-minute intravenous infusion. Samples of blood and lung microdialysate were collected until six hours post-dose. Ultrafiltration, in vivo retrodialysis, and high-performance liquid chromatography-tandem mass spectrometry were used to determine plasma and ISF concentrations of free drug. The concentrations were examined with non-compartmental analysis and compartmental modeling. Results: The concentration of free cefuroxime in ISF was lower in the non-ventilated lung than the ventilated one, evidenced by a lung penetration factor of 47% versus 63% (p < 0.05), the ratio between maximum concentrations (65%, p < 0.05), and the ratio between the areas under the concentration-time curve (78%, p = 0.12). The time needed to reach a minimum inhibitory concentration (MIC) was 30%-40% longer for a collapsed lung than for a ventilated one. In addition, a delay of 10-40 minutes was observed for lung ISF compared with plasma. The mean residence time values (ISF collapsed lung > ISF ventilated lung > plasma) could explain the absence of practically important differences in the time interval with the concentration of cefuroxime exceeding the MICs of sensitive strains (≤4 mg/L). Conclusion: The concentration of cefuroxime in the ISF of a collapsed porcine lung is lower than in a ventilated one; furthermore, its equilibration with plasma is delayed. Administration of the first cefuroxime dose earlier or at a higher rate may be warranted, as well as dose intensification of the perioperative prophylaxis of pneumonia caused by pathogens with higher MICs.

Use of microdialysis for the assessment of fluoroquinolone pharmacokinetics in the clinical practice

Antibacterial drugs, including fluoroquinolones, can exert their therapeutic action only with adequate penetration at the infection site. Multiple factors, such as rate of protein binding, drug liposolubility and organ blood-flow all influence ability of antibiotics to penetrate target tissues. Microdialysis is an in vivo sampling technique that has been successfully applied to measure the distribution of fluoroquinolones in the interstitial fluid of different tissues both in animal studies and clinical setting. Tissue concentrations need to be interpreted within the context of the pathogenesis and causative agents implicated in infections. Integration of microdialysis -derived tissue pharmacokinetics with pharmacodynamic data offers crucial information for correlating exposure with antibacterial effect. This review explores these concepts and provides an overview of tissue concentrations of fluoroquinolones derived from microdialysis studies and explores the therapeutic implications of fluoroquinolone distribution at various target tissues.

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.

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.

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.

Penetration of antimicrobials to pulmonary epithelial lining fluid and muscle and impact of drug physicochemical properties determined by microdialysis

INTRODUCTION

The objectives of this study were to characterize antimicrobial drug penetration into the pulmonary epithelial lining fluid (PELF) and extracellular fluid (ECF) of muscle in relation to physicochemical properties of the drugs (molecular mass, Log D, polar surface area and charge), using intrabronchial microdialysis. The series of drugs tested include gentamicin, sulfadiazine, cefquinome, minocycline and colistin.

METHODS

Drug concentrations were measured during 2h of steady state plasma drug concentrations at therapeutic levels in anesthetized pigs. Microdialysis probes were positioned 2 to 4cm distal to the tracheal bifurcature and in M. gluteobiceps and were calibrated by retrodialysis by drug.

RESULTS

Mean AUCPELF/PLASMA(fu) and mean AUCMUSCLE/PLASMA(fu) ratios were respectively for gentamicin (0.8, 0.7), sulfadiazine (1.1, 0.7), cefquinome (1.3, 1.5) minocycline (1.6, 0.7) and colistin (0.26, 0.12). The penetration of drugs into PELF (r(2)=0.55-0.77, p=0.0004-0.0089) and ECF of muscle (r(2)=0.39-0.53, p=0.0108-0.0397) was positively correlated to Log D, whereas molecular mass, polar surface area and charge were negatively correlated to drug penetration. Sulfadiazine, gentamicin, cefquinome and colistin had similar penetration ratios into PELF and ECF of muscle, ranging from 0.12 to 1.50.

DISCUSSION

In conclusion, drug penetration into PELF and ECF of muscle is correlated to mass, lipophilicity, polarity and charge of the drugs. Drug partition into ECF of muscle and PELF are similar for the passively transported drugs gentamicin, sulfadiazine, cefquinome and colistin, whereas minocycline appears to be actively transported into PELF.

Antibiotic Tissue Penetration in Diabetic Foot Infections A Review of the Microdialysis Literature and Needs for Future Research

Although many antimicrobial agents display good in vitro activity against the pathogens frequently implicated in diabetic foot infections, effective treatment can be complicated by reduced tissue penetration in this population secondary to peripheral arterial disease and emerging antimicrobial resistance, which can result in clinical failure. Improved characterization of antibiotic tissue pharmacokinetics and penetration ratios in diabetic foot infections is needed. Microdialysis offers advantages over the skin blister and tissue homogenate studies historically used to define antibiotic penetration in skin and soft-tissue infections by defining antibiotic penetration into the interstitial fluid over the entire concentration versus time profile. However, only a select number of agents currently recommended for treating diabetic foot infections have been evaluated using these methods, which are described herein. Better characterization of the tissue penetration of antibiotic agents is needed for the development of methods for maximizing the pharmacodynamic profile of these agents to ultimately improve treatment outcomes for patients with diabetic foot infections.

A comprehensive review on the pharmacokinetics of antibiotics in interstitial fluid spaces in humans: implications on dosing and clinical pharmacokinetic monitoring

The objective of the current review was to provide an updated and comprehensive summary on pharmacokinetic data describing the distribution of antimicrobials into interstitial fluid (ISF) by comparing drug concentration versus time profiles between ISF and blood/plasma in healthy individuals and/or diseased populations. An extensive literature search identified 55 studies detailing 87 individual comparisons. For each antibiotic (antibacterial) (or antibiotic class), we comment on dosing implications based on tissue ISF distribution characteristics and determine the suitability of conducting clinical pharmacokinetic monitoring (CPM) using a previously published scoring algorithm. Using piperacillin as an example, there is evidence supporting different degrees of drug penetration into the ISF of different tissues. A higher dose of piperacillin may be required to achieve an adequate ISF concentration in soft tissue infections. To achieve these higher doses, alternative administration regimens such as intravenous infusions may be utilized. Data also suggest that piperacillin can be categorized as a 'likely suitable' agent for CPM in ISF. Regression analyses of data from the published studies, including protein binding, molecular weight, and predicted partition coefficient (using XlogP3) as dependent variables, indicated that protein binding was the only significant predictor for the extent of drug distribution as determined by ratios of the area under the concentration-time curve between muscle ISF/total plasma (R (2) = 0.65, p < 0.001) and adipose ISF/total plasma (R (2) = 0.48, p < 0.004). Although recurrent limitations (i.e., small sample size, lack of statistical comparisons, lack of steady-state conditions, high individual variability) were identified in many studies, these data are still valuable and allowed us to generate general dosing guidelines and assess the suitability of using ISF for CPM.

Interpretation of Epithelial Lining Fluid Concentrations of Antibiotics against Methicillin Resistant Staphylococcus aureus

Although antibiotics whose epithelial lining fluid (ELF) concentrations are reported high tend to be preferred in treatment of pneumonia, measurement of ELF concentrations of antibiotics could be misled by contamination from lysis of ELF cells and technical errors of bronchoalveolar lavage (BAL). In this review, ELF concentrations of anti-methicillin resistant Staphylococcus aureus (MRSA) antibiotics were interpreted considering above confounding factors. An equation used to explain antibiotic diffusion into CSF (cerebrospinal fluid) was adopted: ELF/free serum concentration ratio = 0.96 + 0.091 × ln (partition coefficient / molecular weight(1/2)). Seven anti-MRSA antibiotics with reported ELF concentrations were fitted to this equation to see if their ELF concentrations were explainable by the penetration capacity only. Then, outliers were modeled under the assumption of varying contamination from lysed ELF cells (test range 0-10% of ELF volume). ELF concentrations of oritavancin, telavancin, tigecycline, and vancomycin were well described by the diffusion equation, with or without additional impact from cell lysis. For modestly high ELF/free serum concentration ratio of linezolid, technical errors of BAL should be excluded. Although teicoplanin and iclaprim showed high ELF/free serum ratios also, their protein binding levels need to be cleared for proper interpretation. At the moment, it appears very premature to use ELF concentrations of anti-MRSA antibiotics as a relevant guide for treatment of lung infections by MRSA.

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.

Extended-Infusion versus standard-infusion piperacillin-tazobactam for sepsis syndromes at a tertiary medical center

Piperacillin-tazobactam (PTZ) is frequently used as empirical and targeted therapy for Gram-negative sepsis. Time-dependent killing properties of PTZ support the use of extended-infusion (EI) dosing; however, studies have shown inconsistent benefits of EI PTZ treatment on clinical outcomes. We performed a retrospective cohort study of adult patients who received EI PTZ treatment and historical controls who received standard-infusion (SI) PTZ treatment for presumed sepsis syndromes. Data on mortality rates, clinical outcomes, length of stay (LOS), and disease severity were obtained. A total of 843 patients (662 with EI treatment and 181 with SI treatment) were available for analysis. Baseline characteristics of the two groups were similar, except for fewer female patients receiving EI treatment. No significant differences between the EI and SI groups in inpatient mortality rates (10.9% versus 13.8%; P = 0.282), overall LOS (10 versus 12 days; P = 0.171), intensive care unit (ICU) LOS (7 versus 6 days; P = 0.061), or clinical failure rates (18.4% versus 19.9%; P = 0.756) were observed. However, the duration of PTZ therapy was shorter in the EI group (5 versus 6 days; P < 0.001). Among ICU patients, no significant differences in outcomes between the EI and SI groups were observed. Patients with urinary or intra-abdominal infections had lower mortality and clinical failure rates when receiving EI PTZ treatment. We did not observe significant differences in inpatient mortality rates, overall LOS, ICU LOS, or clinical failure rates between patients receiving EI PTZ treatment and patients receiving SI PTZ treatment. Patients receiving EI PTZ treatment had a shorter duration of PTZ therapy than did patients receiving SI treatment, and EI dosing may provide cost savings to hospitals.

Pharmacokinetics of piperacillin and tazobactam in plasma and subcutaneous interstitial fluid in critically ill patients receiving continuous venovenous haemodiafiltration

This prospective pharmacokinetic study aimed to describe plasma and interstitial fluid (ISF) pharmacokinetics of piperacillin and tazobactam in critically ill patients on continuous venovenous haemodiafiltration (CVVHDF). Piperacillin/tazobactam (4g/0.5g) was administered every 8h and CVVHDF was performed as a 3-3.5L/h exchange using a polyacrylonitrile filter with a surface area of 1.05m(2). Serial blood (pre- and post-filter), filtrate/dialysate, urine and ISF concentrations were measured. Subcutaneous tissue ISF concentrations were determined using microdialysis. A total of 407 samples were collected. Median peak plasma concentrations were 210.5 (interquartile range=161.5-229.0) and 29.4 (27.9-32.0) mg/L and median trough plasma concentrations were 64.3 (49.0-68.9) and 12.3 (7.7-13.7) mg/L for piperacillin and tazobactam, respectively. The plasma elimination half-life was 6.4 (4.6-8.7) and 7.3 (4.6-11.8) h, volume of distribution 0.42 (0.29-0.49) and 0.32 (0.24-0.36) L/kg, total clearance 5.1 (4.2-6.2) and 3.8 (3.3-4.2) L/h and CVVHDF clearance 2.5 (2.3-3.1) and 2.5 (2.3-3.2) L/h for piperacillin and tazobactam, respectively. The tissue penetration ratio or ratio of area under the concentration-time curve of the unbound drug in ISF to plasma (unbound AUCISF/AUCplasma) was ca. 1 for both piperacillin and tazobactam. This is the first report of concurrent plasma and ISF concentrations of piperacillin and tazobactam during CVVHDF. For the CVVHDF settings used in this study, a dose of 4.5g piperacillin/tazobactam administered evry 8h resulted in piperacillin concentrations in plasma and ISF >32mg/L throughout most of the dosing interval.

Pharmacokinetics of ganciclovir during continuous venovenous hemodiafiltration in critically ill patients

Ganciclovir is an antiviral agent that is frequently used in critically ill patients with cytomegalovirus (CMV) infections. Continuous venovenous hemodiafiltration (CVVHDF) is a common extracorporeal renal replacement therapy in intensive care unit patients. The aim of this study was to investigate the pharmacokinetics of ganciclovir in anuric patients undergoing CVVHDF. Population pharmacokinetic analysis was performed for nine critically ill patients with proven or suspected CMV infection who were undergoing CVVHDF. All patients received a single dose of ganciclovir at 5 mg/kg of body weight intravenously. Serum and ultradiafiltrate concentrations were assessed by high-performance liquid chromatography, and these data were used for pharmacokinetic analysis. Mean peak and trough prefilter ganciclovir concentrations were 11.8 ± 3.5 mg/liter and 2.4 ± 0.7 mg/liter, respectively. The pharmacokinetic parameters elimination half-life (24.2 ± 7.6 h), volume of distribution (81.2 ± 38.3 liters), sieving coefficient (0.76 ± 0.1), total clearance (2.7 ± 1.2 liters/h), and clearance of CVVHDF (1.5 ± 0.2 liters/h) were determined. Based on population pharmacokinetic simulations with respect to a target area under the curve (AUC) of 50 mg · h/liter and a trough level of 2 mg/liter, a ganciclovir dose of 2.5 mg/kg once daily seems to be adequate for anuric critically ill patients during CVVHDF.

Importance of relating efficacy measures to unbound drug concentrations for anti-infective agents

For the optimization of dosing regimens of anti-infective agents, it is imperative to have a good understanding of pharmacokinetics (PK) and pharmacodynamics (PD). Whenever possible, drug efficacy needs to be related to unbound concentrations at the site of action. For anti-infective drugs, the infection site is typically located outside plasma, and a drug must diffuse through capillary membranes to reach its target. Disease- and drug-related factors can contribute to differential tissue distribution. As a result, the assumption that the plasma concentration of drugs represents a suitable surrogate of tissue concentrations may lead to erroneous conclusions. Quantifying drug exposure in tissues represents an opportunity to relate the pharmacologically active concentrations to an observed pharmacodynamic parameter, such as the MIC. Selection of an appropriate specimen to sample and the advantages and limitations of the available sampling techniques require careful consideration. Ultimately, the goal will be to assess the appropriateness of a drug and dosing regimen for a specific pathogen and infection.

In vivo microdialysis in pharmacological studies of antibacterial agents in the brain

Cerebral microdialysis (MD) has proven to be a valuable clinical and research tool in neuroscience. It allows sampling of endogenous and exogenous molecules of interest from the extracellular fluid (ECF) of the brain. MD has also been successfully used to assess drug delivery to the target tissues in pharmacokinetic (PK) studies. There is a concern that due to the blood-brain barrier (BBB), current regimens of commonly used antibiotics might be inadequate. Although PK/pharmacodynamic (PK/PD) studies play an important role in drug evaluation, PK MD studies of antibacterial agents in cerebral tissue are few in number. These studies demonstrate a significant variation in drug penetration in the presence of intracranial pathology. Antibacterial agents from the same chemical group have significantly different PK profiles due to different affinity to the transport proteins of the BBB. Some studies suggest that commonly used antibiotics do not reach a therapeutic concentration range in brain ECF. Studies reviewed in this article are small and performed in different patient populations (brain tumour, head injury, epilepsy) using different methodological approaches to the drug recovery estimation. Nevertheless, they provide interesting and important data on the variability of antibiotic penetration that could be utilized for PK/PD studies and which may have clinical relevance.

Antimicrobial chemotherapy and lung microdialysis: a review

Pneumonia is a form of lung infection that may be caused by various micro-organisms. The predominant site of infection in pneumonia is debatable. Advances in the fields of diagnostic and therapeutic medicine have had a less than optimal effect on the outcome of pneumonia and one of the many causes is likely to be inadequate antimicrobial concentrations at the site of infection in lung tissue. Traditional antimicrobial therapy guidelines are based on indirect modelling from blood antimicrobial levels. However, studies both in humans and animals have shown the fallacy of this concept in various tissues. Many different methods have been employed to study lung tissue antimicrobial levels with limited success, and each has limitations that diminish their utility. An emerging technique being used to study the pharmacokinetics of antimicrobial agents in lung tissue is microdialysis. Development of microdialysis catheters, along with improvement in analytical techniques, has improved the accuracy of the data. Unfortunately, very few studies have reported the use of microdialysis in lung tissue, and even fewer antimicrobial classes have been studied. These studies generally suggest that this technique is a safe and effective way of assessing the pharmacokinetics of antimicrobial agents in lung tissue. Further descriptive studies need to be conducted to study the pharmacokinetics and pharmacodynamics of different antimicrobial classes in lung tissue. Data emanating from these studies could inform decisions for appropriate dosing schedules of antimicrobial agents in pneumonia.

Piperacillin penetration into tissue of critically ill patients with sepsis--bolus versus continuous administration?

OBJECTIVE

To describe a pharmacokinetic model of piperacillin concentrations in plasma and subcutaneous tissue when administered by bolus dosing and continuous infusion in critically ill patients with sepsis on days 1 and 2 of antibiotic therapy and to compare results against previous results for piperacillin from a cohort of patients with septic shock.

DESIGN

Prospective randomized controlled trial.

SETTING

Eighteen-bed intensive care unit at 918-bed tertiary referral hospital.

PATIENTS

Thirteen critically ill adult patients with known or suspected sepsis in whom the treating physician deemed piperacillin-tazobactam appropriate therapy were conveniently sampled.

INTERVENTIONS

Patients were randomized to receive different daily doses of piperacillin-tazobactam by bolus dosing or continuous infusion (continuous infusion--six patients; bolus dosing--seven patients). Serial plasma and tissue concentrations were determined on days 1 and 2 of treatment. Tissue concentrations of piperacillin were determined using a subcutaneously inserted microdialysis catheter. Separate pharmacokinetic models were developed for both bolus and continuous dosing.

MEASUREMENTS AND MAIN RESULTS

This is the first known article to report concurrent plasma and subcutaneous tissue concentrations of a beta-lactam antibiotic administered by bolus and continuous dosing in critically ill patients with sepsis. With a 25% lower piperacillin dose administered to the continuous infusion group, the infusion group had statistically significantly higher median plasma concentrations than the bolus group on day 2 (16.6 vs. 4.9 mg/L; p = 0.007). There was a trend to higher median plasma concentrations on day 1 in the bolus dosing group (8.9 vs. 4.9 mg/L; p = 0.078). Median tissue concentrations were not statistically different on day 1 (infusion group 2.4 mg/L vs. bolus group 2.2 mg/L; p = 0.48) and day 2 (infusion group 5.2 mg/L vs. bolus group 0.8 mg/L; p = 0.45). A two-compartment pharmacokinetic model was found to describe the data best. Tissue pharmacodynamic targets were achieved more successfully with infusion dosing.

CONCLUSIONS

Patients with sepsis do not seem to have the same level of impairment of tissue distribution as described for patients with septic shock. A 25% lower dose of piperacillin administered by continuous infusion seems to maintain higher trough concentrations compared with standard bolus dosing. It is likely that the clinical advantages of continuous infusion are most likely to be evident when treating pathogens with high minimum inhibitory concentration, although without therapeutic drug monitoring and subsequent dose adjustment, infusions may never achieve target concentrations of organisms with very high minimum inhibitory concentrations in a small number of patients.

Development of amoxicillin enzyme-linked immunosorbent assay and measurements of tissue amoxicillin concentrations in a pigeon microdialysis model

A sensitive ELISA was developed for the detection of amoxicillin (AMX) in serum, urine, and milk. The ELISA used an indirect competitive method produced by coating the plate with ovalbumin conjugated with AMX hapten. Antibodies against AMX-BSA were detected by a goat-antirabbit antibody conjugated with peroxidase. Calibration standard curves ranged from 1.28 ng/mL to 20 microg/mL [IC(50) (inhibition concentration 50%) = 100 ng/mL], and the limits of detection were 1.3, 2.7, and 4.8 ng/mL for urine, milk, and serum, respectively. The intra- and interassay variations were less than 4 and 9.6%. The antibody produced against AMX cross-reacted highly with penicillin G (77%); cross-reacted moderately with ampicillin, oxacillin, and cloxacillin (56.9, 51.4, and 48.8%, respectively); but was considered non-cross-reactive with dicloxacillin (7.4%), cefadroxil (<1%), and cefazolin (<1%). Concentrations of AMX were measured simultaneously in venous blood and muscles by using the developed AMX ELISA in an in vivo microdialysis model designed for pigeons. Following i.m. injection (25 mg/kg), AMX attained a peak blood level of 4.74 +/-0.30 mu g/mL and decreased with a half-life of 2.38 +/-0.16 h. In contrast, measurements in pectoral and femoral muscles exhibited delayed appearances, reduced peak concentrations, and prolonged half-lives of 4.07 +/-0.48 (pectoral) and 3.01 +/-0.26 (femoral) that were significantly different from each other and those in the blood (P < 0.05). Blood protein binding was calculated to be 27.9 +/-5.7%. This study demonstrated the semiquantitative application of a selective AMX ELISA in the first microdialysis procedure for continuous monitoring of drug levels in specific tissues of pigeons and maybe useful for related studies in other poultry species.

Factors influencing caspofungin plasma concentrations in patients of a surgical intensive care unit

BACKGROUND

Co-morbidity, medical and surgical interventions often cause alterations to drug plasma concentrations and pharmacokinetic parameters in critically ill patients. In the present study, we investigated parameters influencing plasma caspofungin concentrations in patients of a surgical intensive care unit (SICU).

METHODS

In a monocentre open study, caspofungin trough concentrations (C(24)) were determined for a group of SICU patients. A linear-mixed model was then used to assess factors influencing caspofungin plasma concentrations.

RESULTS

A total of 40 SICU patients were enrolled. Age and body weight ranged from 22 to 76 years and 47 to 108 kg, respectively. All participants received a caspofungin loading dose of 70 mg and a maintenance dose of 50 mg/day. The median duration of therapy was 10 days. Caspofungin C(24) in SICU patients varied more than those determined for healthy subjects reported in previous studies (0.52-4.08 microg/mL versus 1.12-1.78 microg/mL). According to our model, caspofungin C(24) were predicted to be significantly higher in patients with body weight <75 kg (P=0.019) and patients with albumin concentration >23.6 g/L (P=0.030).

CONCLUSIONS

Our results show that body weight and albumin concentration influence caspofungin C(24) in SICU patients and should therefore be considered prognostic factors.

A pilot study testing whether concentrations of levofloxacin in interstitial space fluid of soft tissues may serve as a surrogate for predicting its pharmacokinetics in lung

Recent observations indicate that pharmacokinetics of beta-lactam antibiotics in the lung can be predicted by the use of concentration versus time profiles in peripheral soft tissues. If this observation is transferred to other classes of antimicrobials, measurement of antimicrobial concentrations in peripheral tissues would enable prediction of the pharmacokinetics of antimicrobials at the site of the respiratory tract infection. We set out to test the hypothesis that concentrations of the fluoroquinolone levofloxacin in the respiratory tract can be predicted on the basis of knowledge of its pharmacokinetics in peripheral soft tissues. After administration of a single intravenous dose of 500mg of levofloxacin, microdialysis was used to describe the concentration versus time profiles of levofloxacin in the interstitial space fluid of lung tissue of patients (n=5) undergoing elective lung surgery. These data were compared with the concentration versus time courses in the interstitial space fluid of skeletal muscle and subcutaneous adipose tissue of healthy volunteers (n=7). The median AUC(0-infinity) of free levofloxacin in lung (2267mg x min/L, 1980-2355) was about 2-fold and 1.5-fold lower compared with skeletal muscle (4381mg x min/L, range 1720-8195) and adipose tissue (3492mg x min/L, range 1323-6420) of healthy controls, respectively. Concentrations in the interstitial space fluid of the lung were descriptively lower compared with corresponding concentrations in peripheral soft tissues. This is in contrast to previous observations made for the class of beta-lactam antibiotics, and indicates that pharmacokinetics of levofloxacin derived from soft tissues may not be used uncritically for prediction of levofloxacin concentrations in the interstitium of the lung.

Use of Pharmacodynamic Principles to Optimise Dosage Regimens for Antibacterial Agents in the Elderly

Throughout most of the world we are witnessing an ever increasing number of aged people as a percentage of the general population. In the coming years, the unique spectrum of infections presented by an elderly population, particularly those in long-term care facilities, will challenge our ability to maintain an effective battery of antibacterials. The pharmacokinetic parameters of most antibacterial agents are altered when assessed in the elderly due in part to non-pathological physiological changes. The inability to clear a drug from the body due to declining lung, kidney/bladder, gastrointestinal and circulatory efficiency can cause accumulation in the body of drugs given in standard dosages. While this may have the potential benefit of achieving therapeutic concentrations at a lower dose, there is also a heightened risk of attaining toxic drug concentrations and an increased chance of unfavourable interactions with other medications. Pharmacodynamic issues in the elderly are related to problems that arise from treating elderly patients who may have a history of previous antibacterial treatment and exposure to resistant organisms from multiple hospitalisations. Furthermore, the elderly often acquire infections in tandem with other common disease states such as diabetes mellitus and heart disease. Thus, it is essential that optimised dosage strategies be designed specifically for this population using pharmacodynamic principles that take into account the unique circumstances of the elderly. Rational and effective dosage and administration strategies based on pharmacodynamic breakpoints and detailed understanding of the pharmacokinetics of antibacterials in the elderly increase the chances of achieving complete eradication of an infection in a timely manner. In addition, this strategy helps prevent selection of drug-resistant bacteria and minimises the toxic effects of antibacterial therapy in the elderly patient.

Optimizing antibiotic treatment for ventilator-associated pneumonia

Ventilator-associated pneumonia (VAP) is the most common infectious complication in patients receiving mechanical ventilation and accounts for exorbitant use of resources in the intensive care unit. Antimicrobial management of VAP incorporates an initial broad-spectrum, empiric regimen to ensure appropriate coverage with deescalation of therapy after 48-72 hours based on culture results and sensitivities. When VAP clinically responds to treatment, antimicrobials should be discontinued after 7-8 days to reduce overall antibiotic consumption and the selection pressure on flora observed in the intensive care unit and thus minimize the development and spread of antimicrobial resistance.

Penetration of piperacillin and tazobactam into inflamed soft tissue of patients with diabetic foot infection

We investigated the pharmacokinetics of piperacillin and tazobactam in the extracellular space fluid of inflamed soft tissues of six patients with diabetic foot infection using in vivo microdialysis and found similar penetration for piperacillin but not for tazobactam into inflamed and noninflamed soft tissue.

Pharmacokinetic modeling of free amoxicillin concentrations in rat muscle extracellular fluids determined by microdialysis

The aim of the present study was to investigate amoxicillin (AMX) distribution in muscle interstitial fluid by microdialysis in healthy, awake rats. Microdialysis probes were inserted into the jugular vein and hind leg muscle. Probe recoveries in each rat were determined by retrodialysis with cefadroxil. AMX was administered as a bolus dose of 50 mg.kg(-1), and microdialysis samples were collected during 180 min. Concentrations of unbound drug in blood and muscle were analyzed simultaneously by a population approach. Simulations were conducted using a hybrid, physiologically based pharmacokinetic model to investigate the potential impact of tissue blood flow on muscle AMX distribution. A two-compartment pharmacokinetic model described adequately the unbound amoxicillin concentration-time profiles in blood and muscle. Muscle AMX distribution equilibrium was rapidly achieved. Consequently, the best results were obtained by considering concentrations in muscle as part of the central compartment. The ratio of the concentration of unbound drug in muscle to that in blood (Rmodel) was estimated to 0.80 by the model, which is close to the mean value obtained by noncompartmental data analysis (Rarea= 0.86 +/- 0.29). Simulations conducted with a hybrid, physiologically based pharmacokinetic model suggest that a muscle blood flow reduction of 30% to 50%, such as could be encountered in critical care patients, has virtually no effect on muscle AMX concentration profiles. In conclusion, this study has clearly demonstrated that AMX distributes rapidly and extensively within muscle interstitial fluid, consistent with theory, and that altered muscle blood flow seems unlikely to have a major effect on these distribution characteristics.

Lung microdialysis--a powerful tool for the determination of exogenous and endogenous compounds in the lower respiratory tract (mini-review)

In vivo measurement of concentrations of drugs and endogenous substances at the site of action has become a primary focus of research. In this context the minimal invasive microdialysis (MD) technique has been increasingly employed for the determination of pharmacokinetics in lung. Although lung MD is frequently employed to investigate various drugs and endogenous substances, the majority of lung MD studies were performed to determine the pharmacokinetic profile of antimicrobials that can be related to the importance of respiratory tract infections. For the lower respiratory tract various methods, such as surgical collection of whole lung tissue and bonchoalveolar lavage (BAL), are currently available for the determination of pharmacokinetics of antimicrobials. Head-to-head comparison of pharmacokinetics of antibiotics in lung revealed high differences between MD and conventional methods. MD might be regarded as a more advantageous approach because of its higher anatomical resolution and the ability to obtain dynamic time-vs-concentration profiles within one subject. However, due to ethical objections lung MD is limited to animals or patients undergoing elective thoracic surgery. From these studies it was speculated that the concentrations in healthy lung tissue may be predicted reasonably by the measurement of concentrations in skeletal muscle tissue. However, until now this was only demonstrated for beta-lactam antibiotics and needs to be confirmed for other classes of antimicrobials. In conclusion, the present review shows that MD is a promising method for the determination of antimicrobials in the lung, but might also be applicable for measuring a wide range of other drugs and for the investigation of metabolism in the lower respiratory tract.

Microdialysis study of imipenem distribution in skeletal muscle and lung extracellular fluids of noninfected rats

The aim of this study was to investigate the imipenem distribution in muscle and lung interstitial fluids by microdialysis in rats and to compare the free concentrations in tissue with the free concentrations in blood. Microdialysis probes were inserted into the jugular vein, hind leg muscle, and lung. Imipenem recoveries in these three media were determined in each rat by retrodialysis by drug period before drug administration. Imipenem was infused intravenously at a dose of 120 mg . kg-1 over 30 min, and microdialysis samples were collected for 150 min. The whole study was conducted with nonhydrated rats (n=4) and hydrated rats (n=6) while the animals were under isoflurane anesthesia. The decay of free concentrations in blood, muscle, and lung with time were monoexponential; and the concentration profiles in these three media were virtually superimposed in both groups. Accordingly, the ratios of the area under the curve (AUC) for tissue (muscle or lung) to the AUC for blood were always virtually equal to 1. Compared to values previously determined with awake rats, clearance was reduced by 2 and 1.5 in nonhydrated and hydrated rats, respectively, but the volume of distribution was unchanged. By combining microdialysis in blood and tissues, it was possible to demonstrate that free imipenem concentrations were virtually identical in blood, muscle, and lung.

Microdialysis—theoretical background and recent implementation in applied life-sciences

In the past decade microdialysis has become a method of choice in the study of unbound tissue concentrations of both endogenous and exogenous substances. Microdialysis has been shown to offer information about substances directly at the site of action while being well tolerable and safe. The large variety of its field of application has been demonstrated. However, a few challenges have to be met to make this method generally applicable in routine applications. This review will provide an overview over theoretical aspects that have to be considered during the implementation of microdialysis. Moreover, a comparison between microdialysis and other tissue sampling techniques will demonstrate advantages and limitations of the methods mentioned. Subsequently, it will present a critical synopsis of a variety of scientific/biomedical applications of this method with emphasis on the most recent literature, focussing on target tissues while giving examples of substances examined. It is concluded that microdialysis will be of great value in future investigations of pharmacokinetics, pharmacodynamics and in monitoring of disease status and progression.