Pharmacometric approaches to assess antibiotic dosing in special patient populations: Towards therapeutic decision support

Almost one century has passed since the discovery of the first antibiotic drug, yet bacterial infections remain a major threat to public health. Two alarming trends have been observed in the last decades: While no truly novel antibiotic drugs were developed, the emergence and spread of antimicrobial resistance dramatically increased. Therefore, a rational use of the existing antibiotic drugs is crucial. One key pillar of rational antibiotic treatment is the choice of an appropriate dosing regimen resulting in adequate antibiotic exposure at the site of infection. In special patient populations, such as critically ill patients or morbidly obese patients, appropriate dosing is particularly challenging since these patients commonly show certain patient-specific characteristics altering antibiotic exposure. The objective of the present thesis was to leverage pharmacometric modelling and simulation approaches in order to (i) enhance the understanding of the pharmacokinetics of antibiotic drugs in special patient populations, but also of the variability in the microdialysis technique – as the method of choice to determine drug exposure at target site, (ii) to evaluate and optimise antibiotic dosing regimens via adequate antibiotic exposure, and (iii) to translate the research results into the clinics supporting future therapeutic decisions. The thesis focused on the two antibiotic drugs ‘linezolid’ (Project I, II) and ‘meropenem’ (Project III, IV) in the selected special populations of ‘obese surgical patients’ and ‘critically ill patients’, respectively. Project I characterised the pharmacokinetics (PK) of linezolid in plasma as well as at the target site (interstitial space fluid of s.c. adipose tissue, representing a common location of infections) in obese compared to nonobese surgical patients: The distribution of linezolid to the target site was delayed and exposure was reduced compared to plasma. The body size descriptor ‘lean body weight’ together with the obesity status of the patient were identified as factors which had an impact on linezolid PK. Both factors led to lower exposure in obese patients compared to nonobese patients, with a particularly pronounced difference at the target site. Interestingly, also anaesthesia and the related haemodynamic changes were found to impact linezolid PK, which resulted in reduced linezolid tissue fluid distribution and excretion. In addition to the PK-related findings, Project I characterised the variability in the microdialysis technique by integrating all available microdialysis data into the pharmacometric model and by dissecting and quantifying various levels of variability (interpatient, intercatheter, intracatheter). While the interpatient variability was almost fully explained by the obesity status of the patient, the quantified inter- and intracatheter variability highlighted the importance of special care in the performance of microdialysis (calibration of catheter, placement of catheter etc.). Project II, a simulation analysis, applied the developed pharmacometric model of linezolid to assess standard linezolid dosing regarding the attainment of effective linezolid exposure (i.e. attainment of a predefined PK/Pharmacodynamic target). In the setting of perioperative infection prophylaxis, single standard linezolid dosing only resulted in effective target site exposure for susceptible pathogens and/or for surgical procedures of short durations. Overall, an increase in the risk of ineffective exposure was observed with increasing body size. In the setting of acute therapy, standard linezolid dosing was related to high risk of ineffective linezolid exposure at the target site, partly even for susceptible pathogens and/or in plasma. Increasing the daily dose (from 1200 mg to 2400 mg) clearly reduced the risk of ineffective exposure. In general, also prolongation of the infusion duration (from 30 min to 4 h) or shortening of the dosing interval (from 12 h to 8 h) reduced the risk of ineffective exposure, yet, less pronounced than the intensification of the daily dose. For resistant pathogens, none of the investigated dosing alterations resulted in effective linezolid exposure, neither in obese nor in nonobese patients. Project III and IV characterised the PK of meropenem in a heterogenous critically ill patient population with severe infections. A large PK variability was observed between patients, which was to a large extent explained by the wide disparity in the patient characteristics: creatinine clearance (according to Cockcroft and Gault, CLCRCG), body weight and serum albumin concentration. Of these three characteristics, CLCRCG showed by far the strongest impact on the (non)-attainment of effective meropenem exposure. Patients with normal or augmented renal function were at highest risk of ineffective exposure. Increasing the daily dose of meropenem, but particularly increasing the infusion duration (from 30-min to 3-h prolonged and/or continuous infusion regimens) reduced the risk of ineffective exposure. In order to translate the findings into the clinics, two easy-to-use tools – the ‘MeroRisk Calculator’ and the ‘3-level dosing algorithm’ – were developed. By providing a simple and intuitive interface, both tools enable the application of the pharmacometric modelling and simulation results by health care professionals. The MeroRisk calculator is an Excel® tool, which allows assessing the risk of ineffective exposure when administering standard meropenem dosing, by considering a patient’s CLCRCG and the susceptibility of the identified/suspected pathogen. The 3-level dosing algorithm provides an intuitive dosing overview, which recommends dosing regimens likely to result in effective exposure. The algorithm is based on a patient’s CLCRCG, considers four different levels of knowledge about the infecting pathogen and for the first time the uncertainty in the underlying pharmacometric model for selection of meropenem dosing regimens. To conclude, the present thesis contributed to a better understanding of the PK of clinically relevant antibiotic drugs in special patient populations and identified patient- and surgery-specific influencing factors altering antibiotic exposure in plasma and at the target site. By assessing the adequacy of standard and alternative antibiotic dosing regimens and translating the results into easy-to-use tools for clinical application, the present thesis has taken substantial steps towards therapeutic decision support to combat bacterial infections in the context of model-informed precision dosing. Future clinical studies are required to evaluate the tools with respect to clinical efficacy and safety before widespread application of the tools in clinical practice.Seit der Entdeckung des ersten Antibiotikums ist fast ein Jahrhundert vergangen, dennoch stellen bakterielle Infektionen weiterhin eine ernsthafte Bedrohung für die öffentliche Gesundheit dar. In den letzten Jahrzehnten wurden zwei alarmierende Trends beobachtet: Während keine neuartigen Antibiotika entwickelt wurden, nahm das Auftreten und die Verbreitung von Antibiotikaresistenzen dramatisch zu. Daher ist eine rationale Anwendung der aktuell vorhandenen Antibiotika von entscheidender Bedeutung. Eine wichtige Säule in der rationale Antibiotikatherapie ist die Wahl eines geeigneten Dosierungsschematas, welches in einer adäquaten Antibiotikaexposition am Infektionsort resultiert. In speziellen Patientenpopulationen wie Intensiv- oder krankhaft adipösen Patienten ist eine geeignete Dosierung besonders herausfordernd, da diese Patienten häufig bestimmte patientenspezifische Merkmale aufweisen, welche die Antibiotikaexposition verändern. Ziel der vorliegenden Arbeit war es, pharmakometrische Modellierungs- und Simulationsansätze zu nutzen, um (i) das Verständnis der Pharmakokinetik von Antibiotika in speziellen Patientenpopulationen, aber auch der Variabilität in der Mikrodialysetechnik - als Methode der Wahl zur Bestimmung der Arzneistoffexposition am Wirkort – zu verbessern, (ii) die Antibiotikadosierung im Hinblick auf eine adäquate Antibiotikaexposition zu evaluieren und zu optimieren und (iii) die Forschungsergebnisse in die Klinik zu übertragen, um zukünftige klinische Dosierungsentscheidungen zu unterstützen. Die Arbeit konzentrierte sich auf die zwei Antibiotika ‚Linezolid‘ (Projekt I, II) und ‚Meropenem‘ (Projekt III, IV) in den speziellen Populationen der ‚adipösen chirurgischen Patienten‘ bzw. der ‚Intensivpatienten‘. In Projekt I wurde die Pharmakokinetik (PK) von Linezolid im Plasma sowie am Wirkort (Interstitialflüssigkeit des s.c. Fettgewebes, welche einen häufigen Infektionsort darstellt) in adipösen im Vergleich zu nicht-adipösen chirurgischen Patienten charakterisiert: Die Verteilung von Linezolid zum Wirkort war verzögert und die Exposition im Vergleich zum Plasma verringert. Die Körpermassenkennzahl ‚Lean Body Weight‘ wurde zusammen mit dem Adipositasstatus des Patienten als Einflussfaktoren für die PK von Linezolid identifiziert. Beide Faktoren führten in adipösen Patienten zu einer geringeren Linezolidexposition verglichen mit nicht-adipösen Patienten, wobei der Unterschied am Wirkort besonders ausgeprägt war. Interessanterweise zeigten auch die Anästhesie und die damit verbundenen hämodynamischen Veränderungen einen Einfluss auf die PK von Linezolid, was zu einer verminderten Gewebsverteilung und Ausscheidung von Linezolid führte. Neben den Ergebnissen zur Pharmakokinetik charakterisierte Projekt I zusätzlich die Variabilität in der Mikrodialysetechnik, indem alle verfügbaren Mikrodialysedaten in das pharmakometrische Modell integriert und verschiedene Variabilitätsniveaus (Inter-Patienten, Inter-Katheter, Intra-Katheter) separiert und quantifiziert wurden. Während die Inter-Patienten Variabilität fast vollständig furch den Adipositasstatus des Patienten erklärt wurde, betonte die quantifizierte Inter- und Intra-Katheter Variabilität die Wichtigkeit einer besonderen Sorgfalt bei der Durchführung der Mikrodialyse (Kalibrierung des Katheters, Platzierung des Katheters usw.). Projekt II, eine Simulationsstudie, nutzte das entwickelte pharmakometrische Linezolidmodell um die Standarddosierung von Linezolid hinsichtlich des Erreichens einer effektiver Linezolid Exposition (d.h., Erreichen eines vordefinierten PK/Pharmakodynamischen Zielwertes) zu evaluieren. In der perioperativen Infektionsprophylaxe führte die einmalige Standarddosierung von Linezolid nur im Falle von empfindlichen Krankheitserregern und/oder chirurgischen Eingriffen von kurzer Dauer mit hoher Wahrscheinlichkeit zu einer effektiven Exposition am Wirkort. Insgesamt wurde ein höheres Risiko für ineffektive Exposition mit zunehmender Körpermasse beobachtet. In der Akuttherapie war die Standarddosierung von Linezolid mit einem hohen Risiko für ineffektiven Linezolidexposition am Wirkort verbunden, teilweise sogar für empfindliche Krankheitserreger und/oder im Plasma. Die Erhöhung der Tagesdosis von Linezolid (von 1200 mg auf 2400 mg), verringerte das Risiko einer ineffektiven Exposition deutlich. Im Allgemeinen verringerten auch eine Verlängerung der Infusionsdauer (von 30 min auf 4 h) oder eine Verkürzung des Dosierungsintervalls (von 12 h auf 8 h) das Risiko einer ineffektiven Exposition, jedoch weniger ausgeprägt als die Intensivierung der Tagesdosis. Bei resistenten Erregern führte keine der untersuchten Dosierungsänderungen zu effektiver Linezolid Exposition, weder bei adipösen noch bei nicht-adipösen Patienten. Projekt III und IV charakterisierten die PK von Meropenem in einer heterogenen Population von Intensivpatienten mit schweren Infektionen. Es wurde eine hohe PK-Variabilität zwischen den Patienten beobachtet, die sich zu einem Großteil durch starke Unterschiede in Patienteneigenschaften erklären ließ: Kreatinin-Clearance (gemäß Cockcroft und Gault, CLCRCG), Körpergewicht und Serumalbumin-Konzentration. Von diesen drei Charakteristika zeigte die CLCRCG bei weitem den stärksten Einfluss auf das (Nicht-)Erreichen einer effektiven Meropenemexposition. Patienten mit normaler oder erhöhter Nierenfunktion zeigten das höchste Risiko einer ineffektiven Exposition. Die Erhöhung der Tagesdosis von Meropenem, insbesondere aber die Verlängerung der Infusionsdauer (von 30-min auf 3-h prolongierte und/oder kontinuierliche Infusionsschemata) verringerten das Risiko einer ineffektiven Exposition. Um die Ergebnisse in die Klinik zu übertragen, wurden zwei einfach zu bedienende Tools entwickelt - der ‚MeroRisk Calculator‘ und der ‚3-stufige Dosierungsalgorithmus‘. Durch eine simple und intuitive Benutzeroberfläche ermöglichen beide Tools die Anwendung der pharmakometrischen Modellierungs- und Simulationsergebnisse durch Fachpersonal im Gesundheitswesen. Der MeroRisk Calculator ist ein Excel®-Tool, mit welchem das Risiko einer ineffektiven Exposition bei Verabreichung einer Standarddosierung von Meropenem beurteilt werden kann, indem die CLCRCG des Patienten und die Empfindlichkeit des identifizierten/vermuteten Erregers berücksichtigt werden. Der 3-stufige Dosierungsalgorithmus bietet eine intuitive Dosierungsübersicht, welche Dosierungsschemata empfiehlt, die mit hoher Wahrscheinlichkeit zu einer effektiven Exposition führen. Der Algorithmus basiert auf der CLCRCG des Patienten, berücksichtigt vier verschiedene Wissensniveaus über den infektiösen Erreger, sowie erstmals die Unsicherheit in dem zugrunde liegenden pharmakometrischen Modell für die Auswahl von Dosierungsschemata. Zusammenfassend hat die vorliegende Arbeit zu einem besseren Verständnis der PK von zwei klinisch relevanten Antibiotika in speziellen Patientenpopulationen beigetragen und patienten- und operationsspezifische Einflussfaktoren identifiziert, welche die Antibiotikaexposition im Plasma und am Wirkort verändern. Durch die Beurteilung der Angemessenheit von Standard- und alternativen Antibiotika-Dosierungsschemata und der Translation der Ergebnisse in einfach zu nutzende Tools für die klinische Anwendung hat die vorliegende Arbeit wesentliche Schritte in Richtung einer therapeutischen Entscheidungshilfe zur Bekämpfung bakterieller Infektionen, im Kontext von modellgestützte Präzisionsdosierung, unternommen. Zukünftige klinische Studien sind erforderlich, um die Tools im Hinblick auf die klinische Wirksamkeit und Sicherheit zu bewerten, bevor sie in der klinischen Praxis eingesetzt werden

Linezolid Concentrations in Plasma and Subcutaneous Tissue are Reduced in Obese Patients, Resulting in a Higher Risk of Underdosing in Critically Ill Patients: A Controlled Clinical Pharmacokinetic Study

Background: Linezolid is used for the treatment of soft tissue infections in critically ill patients. However, data for characterizing the pharmacokinetics (PK) and assessing whether effective concentrations are reached at the target site are lacking. We hypothesized that current dosing regimens do not lead to effective concentrations in the plasma and interstitial fluid (ISF) of subcutaneous tissue in obese patients. Methods: As a controlled clinical model, critically ill obese and non-obese patients undergoing intra-abdominal surgery received 600 mg linezolid as a single infusion. Concentrations in the plasma and microdialysate from the ISF of subcutaneous tissue were determined up to 8 h after dosing. Pharmacokinetic analysis was performed by non-compartmental methods. As a therapeutic target, we used fAUC/MIC > 80. Results: Fifteen obese (BMI: 48.7 +/- 11.2 kg/m(2)) and 15 non-obese (23.9 +/- 2.1 kg/m(2)) patients were analyzed. AUC(0-8) in ISF decreased by -1.69 mg*h/L (95% CI: -2.59 to -0.79, p = 1 mg/L in ISF and >= 2 mg/L in plasma. Conclusions: Increasing the weight led to a decrease of linezolid concentrations in the plasma and subcutaneous tissue. The current dosing regimen does not seem to produce sufficient concentrations to kill bacteria with MIC >= 2 mg/L, especially as empirical antimicrobial therapy in critically ill obese patients

A prospective observational study

Background: Severe bacterial infections remain a major challenge in intensive care units because of their high prevalence and mortality. Adequate antibiotic exposure has been associated with clinical success in critically ill patients. The objective of this study was to investigate the target attainment of standard meropenem dosing in a heterogeneous critically ill population, to quantify the impact of the full renal function spectrum on meropenem exposure and target attainment, and ultimately to translate the findings into a tool for practical application. Methods: A prospective observational single-centre study was performed with critically ill patients with severe infections receiving standard dosing of meropenem. Serial blood samples were drawn over 4 study days to determine meropenem serum concentrations. Renal function was assessed by creatinine clearance according to the Cockcroft and Gault equation (CLCRCG). Variability in meropenem serum concentrations was quantified at the middle and end of each monitored dosing interval. The attainment of two pharmacokinetic/pharmacodynamic targets (100%T>MIC, 50%T>4×MIC) was evaluated for minimum inhibitory concentration (MIC) values of 2 mg/L and 8 mg/L and standard meropenem dosing (1000 mg, 30-minute infusion, every 8 h). Furthermore, we assessed the impact of CLCRCG on meropenem concentrations and target attainment and developed a tool for risk assessment of target non- attainment. Results: Large inter- and intra-patient variability in meropenem concentrations was observed in the critically ill population (n = 48). Attainment of the target 100%T>MIC was merely 48.4% and 20.6%, given MIC values of 2 mg/L and 8 mg/L, respectively, and similar for the target 50%T>4×MIC. A hyperbolic relationship between CLCRCG (25–255 ml/ minute) and meropenem serum concentrations at the end of the dosing interval (C8h) was derived. For infections with pathogens of MIC 2 mg/L, mild renal impairment up to augmented renal function was identified as a risk factor for target non- attainment (for MIC 8 mg/L, additionally, moderate renal impairment). Conclusions: The investigated standard meropenem dosing regimen appeared to result in insufficient meropenem exposure in a considerable fraction of critically ill patients. An easy- and free-to-use tool (the MeroRisk Calculator) for assessing the risk of target non-attainment for a given renal function and MIC value was developed

Role of renal function in risk assessment of target non-attainment after standard dosing of meropenem in critically ill patients: a prospective observational study

Background: Severe bacterial infections remain a major challenge in intensive care units because of their high prevalence and mortality. Adequate antibiotic exposure has been associated with clinical success in critically ill patients. The objective of this study was to investigate the target attainment of standard meropenem dosing in a heterogeneous critically ill population, to quantify the impact of the full renal function spectrum on meropenem exposure and target attainment, and ultimately to translate the findings into a tool for practical application. Methods: A prospective observational single-centre study was performed with critically ill patients with severe infections receiving standard dosing of meropenem. Serial blood samples were drawn over 4 study days to determine meropenem serum concentrations. Renal function was assessed by creatinine clearance according to the Cockcroft and Gault equation (CLCRCG). Variability in meropenem serum concentrations was quantified at the middle and end of each monitored dosing interval. The attainment of two pharmacokinetic/pharmacodynamic targets 100%T >MIC,50%T >4×MIC) was evaluated for minimum inhibitory concentration (MIC) values of 2 mg/L and 8 mg/L and standard meropenem dosing (1000 mg, 30-minute infusion, every 8 h). Furthermore, we assessed the impact of CLCRCG on meropenem concentrations and target attainment and developed a tool for risk assessment of target non-attainment. Results: Large inter- and intra-patient variability in meropenem concentrations was observed in the critically ill population (n = 48). Attainment of the target 100%T >MIC was merely 48.4% and 20.6%, given MIC values of 2 mg/L and 8 mg/L, respectively, and similar for the target 50%T >4×MIC. A hyperbolic relationship between CLCRCG (25–255 ml/minute) and meropenem serum concentrations at the end of the dosing interval (C8h) was derived. For infections with pathogens of MIC 2 mg/L, mild renal impairment up to augmented renal function was identified as a risk factor for target non-attainment (for MIC 8 mg/L, additionally, moderate renal impairment). Conclusions: The investigated standard meropenem dosing regimen appeared to result in insufficient meropenem exposure in a considerable fraction of critically ill patients. An easy- and free-to-use tool (the MeroRisk Calculator) for assessing the risk of target non-attainment for a given renal function and MIC value was developed. Trial registration Clinicaltrials.gov, NCT01793012 . Registered on 24 January 2013

An RNA replicon system to investigate promising inhibitors of feline coronavirus.

FIPV is of great significance in the cat population around the world, causing 0.3%-1.4% of feline deaths in veterinary practices (2). As there are neither effective preventive measures nor approved treatment options available, there is an urgent need to identify antiviral drugs against FIPV. Our FCoV replicon system provides a valuable tool for drug discovery in vitro. Due to the lack of cell culture systems for serotype I FCoVs (the serotype most prevalent in the feline population) (2), a different system is needed to study these viruses. A viral replicon system is a valuable tool for studying FCoVs. Overall, our results demonstrate the utility of the serotype I feline coronavirus replicon system for antiviral screening as well as to study this virus in general. We propose several compounds representing promising candidates for future clinical trials and ultimately with the potential to save cats suffering from FIP

Drug combinations and impact of experimental conditions on relative recovery in in vitro microdialysis investigations

The need for pharmacokinetic knowledge about antibiotics directly at the site of infection, typically the interstitial space fluid (ISF) of tissues, is gaining acceptance for effective and safe treatment. One option to acquire such data is the microdialysis technique employing a catheter with a semipermeable membrane inserted directly in the ISF. A prerequisite is catheter calibration, e.g. via retrodialysis, yielding a conversion factor from measured to true ISF concentrations, termed relative recovery. This value can be influenced by various factors. The present investigation assessed the impact of three of them on relative recovery using seven drugs: (I) drug combinations/order, (II) air in the microdialysis system, (III) flow rate changes inherent when using common in vivo microdialysis pumps. All experiments were performed in a standardised in vitro microdialysis system. (I) Relative recovery of single antibiotics (linezolid, meropenem, cefazolin, metronidazole, tigecycline) was determined in microdialysis and retrodialysis settings and compared with values using either antibiotic or antibiotic + analgesic (acetaminophen and metamizole) combinations or single drugs with reversed microdialysis order. For assessing these factors for lower relative recovery values (as in in vivo), these were mimicked by increasing the flow rate for linezolid. (II) For the impact of air, linezolid relative recovery of freshly carbonated solutions was compared to degassed ones in microdialysis and retrodialysis settings. For each condition in (I) and (II), summary statistics of relative recovery were calculated and for the impact of the factors a linear mixed-effect model developed. (III) From samples taken during an automatic flush sequence (15 mu L/min) of an in vivo pump and afterwards switching to the flow rate of 1 and 2 mu L/min for 120 min, the time necessary for relative recovery to reach equilibrium was determined. (I) High relative recovery values (flow rate 2 mu L/min: >= 84%; flow rate 5 mu L/min: >= 65%) were observed for all investigated single drugs. Infra- and intercatheter variability ranged from 0.3%-11% and 3%-25%, respectively. Based on these values and on the statistical model, the impact of drug combination versus single drug as well as of reversed order was small with changes in relative recovery of smaller equal 9%. (II) Compared to degassed solutions, relative recovery in carbonated solutions was 23% and 19% lower (relative reduction) in the microdialysis and retrodialysis setting, respectively, with increased intercatheter variability (up to 37%). (III) As expected, relative recovery increased after the flush sequence and was constant 10-15 min after the switch to the typical 1 and 2 mu L/min flow rate. Given the intercatheter variability, combinations and the order of drugs showed minor but clinically negligible impact on relative recovery. In contrast, air in the microdialysis catheter/system caused falsely low and inconsistent relative recovery values and must be avoided when performing a trial. Also changes in flow rate at the end of pump flush sequence impacted relative recovery. Hence, a sufficient equilibration time of 10-15 min prior to sampling should be implemented in sampling protocols. In vitro microdialysis presents a highly valuable complementary platform to clinical microdialysis studies impacting the design, sampling schedule and data analysis of such trials to gain knowledge of target-site pharmacokinetics for contributing to better informed decisions in the individual patient/special populations in future

Risk of target non-attainment in obese compared to non-obese patients in calculated linezolid therapy

Objectives: The aim was to characterize linezolid population pharmacokinetics in plasma and interstitial space fluid of subcutaneous adipose tissue (target site) of obese compared with non-obese patients and to determine dosing regimens enabling adequate therapy using Monte Carlo simulations. Methods: In this prospective, parallel group, open-label, controlled, single-centre trial, 30 surgery patients (15 obese, 15 non-obese) received 600 mg of intravenous linezolid. A population pharmacokinetic analysis characterizing plasma and microdialysis-derived target site pharmacokinetics was followed by Monte Carlo simulations using twice/thrice daily 600-1200 mg short-term and extended infusions of linezolid. Adequacy of therapy was assessed by the probability of pharmacokinetic/pharmacodynamic target attainment for time and exposure-related indices. Results: In the model, lean body weight and obesity status largely explained between-patient variability in linezolid PK parameters (12.0-44.9%). Both factors caused lower area under the concentration-time curve in typical obese patients in plasma (-20.4%, 95% CI -22.0% to -15.9%) and at target-site (-37.7%, 95% CI -47.1% to -24.2%) compared with non-obese patients. Probability of target attainment showed improvement with increasing linezolid doses. Depending on lean body weight, adequate therapy was partially attained for 900- and 1200-mg linezolid doses and minimum inhibitory concentrations (MICs) <= 2 mg/L (probability of target attainment 62.5-100%) but could not be reached for MIC = 4 mg/L (probability of target attainment <= 82.3%). Additionally, lower linezolid distribution into the target site in obese patients as described above might compromise the plasma-based probability of target attainment analysis. Discussion: This analysis revealed risks of linezolid underdosing in empirical antibiotic therapy of most resistant bacteria for obese and non-obese patients. Doubling the standard dose is associated with adequate probability of target attainment throughout most body masses for MIC <= 2 mg/L. Further clinical studies with adjusted dosing regimens in for example intensive care patients are needed. L. Ehmann, Clin Microbiol Infect 2020;26:1222 (C) 2020 Published by Elsevier Ltd on behalf of European Society of Clinical Microbiology and Infectious Diseases

Quantification of microdialysis related variability in humans: Clinical trial design recommendations

Objective: Target-site concentrations obtained via the catheter-based minimally invasive microdialysis technique often exhibit high variability. Catheter calibration is commonly performed via retrodialysis, in which a transformation factor, termed relative recovery (RR), is determined. Leveraging RR values from a rich data set of a very large clinical microdialysis study, promised to contribute critical insight into the origin of the reportedly high target-site variability. The present work aimed (i) to quantify and explain variability in RR associated with the patient (including non-obese vs. obese) and the catheter, and (ii) to derive recommendations on the design of future clinical microdialysis studies. Methods: A prospective, age- and sex-matched parallel group, single-centre trial in non-obese and obese patients (BMI=18.7-86.9 kg/m2) was performed. 1-3 RR values were obtained in the interstitial fluid of the subcutaneous fat tissue in one catheter per upper arm of 120 patients via the retrodialysis method (nRR=1008) for a panel of drugs (linezolid, meropenem, tigecycline, cefazolin, fosfomycin, piperacillin and acetaminophen). A linear mixed-effects model was developed to quantify the different types of variability in RR and to explore the association between RR and patient body size descriptors. Results: Estimated RR was highest for acetaminophen (69.7%, 95%CI=65.0% to 74.3%) and lowest for piperacillin (40.4%, 95%CI=34.6% to 46.0%). The linear mixed-effects modelling analysis showed that variability associated with the patient (σ=15.9%) was the largest contributor (46.7%) to overall variability, whereas the contribution of variability linked to the catheter (σ=5.55%) was ~1/6 (16.8%). The relative contribution of residual unexplained variability (σ=12.0%, including intracatheter variability) was ~1/3 (36.4%). The limits of agreement of repeated RR determinations in a single catheter ranged from 0.694-1.64-fold (linezolid) to 0.510-3.02-fold (cefazolin). Calculated fat mass affected RR, explaining the observed lower RR in obese (ΔRRmean= -29.7% relative reduction) versus non-obese patients (p<0.001); yet only 15.8% of interindividual variability was explained by this effect. No difference in RR was found between catheters implanted into the left or right arm (p=0.732). Conclusions: Three recommendations for clinical microdialysis trial design were derived: 1) High interindividual variability underscored the necessity of measuring individual RR per patient. 2) The low relative contribution of intercatheter variability to overall variability indicated that measuring RR with a single catheter per patient is sufficient for reliable catheter calibration. 3) The wide limits of agreement from multiple RR in the same catheter implied an uncertainty of a factor of two in target-site drug concentration estimation necessitating to perform catheter calibration (retrodialysis sampling) multiple times per patient. To allow routine clinical use of microdialysis, research efforts should aim at further understanding and minimising the method-related variability. Optimised study designs in clinical trials will ultimately yield more informative microdialysis data and increase our understanding of this valuable sampling technique to derive target-site drug exposure