Interstitial Space Fluid Research Articles

Microdialysis of Voriconazole and its N-Oxide Metabolite: Amalgamating Knowledge of Distribution and Metabolism Processes in Humans

PurposeVoriconazole is an essential antifungal drug whose complex pharmacokinetics with high interindividual variability impedes effective and safe therapy. By application of the minimally-invasive sampling technique microdialysis, interstitial space fluid (ISF) concentrations of VRC and its potentially toxic N-oxide metabolite (NO) were assessed to evaluate target-site exposure for further elucidating VRC pharmacokinetics.MethodsPlasma and ISF samples of a clinical trial with an approved VRC dosing regimen were analyzed for VRC and NO concentrations. Concentration-time profiles, exposure assessed as area-under-the-curve (AUC) and metabolic ratios of four healthy adults in plasma and ISF were evaluated regarding the impact of multiple dosing and CYP2C19 genotype.ResultsVRC and NO revealed distribution into ISF with AUC values being ≤2.82- and 17.7-fold lower compared to plasma, respectively. Intraindividual variability of metabolic ratios was largest after the first VRC dose administration while interindividual variability increased with multiple dosing. The CYP2C19 genotype influenced interindividual differences with a maximum 6- and 24-fold larger AUCNO/AUCVRC ratio between the intermediate and rapid metabolizer in plasma and ISF, respectively. VRC metabolism was saturated/auto-inhibited indicated by substantially decreasing metabolic concentration ratios with increasing VRC concentrations and after multiple dosing.ConclusionThe feasibility of the simultaneous microdialysis of VRC and NO in vivo was demonstrated and provided new quantitative insights by leveraging distribution and metabolism processes of VRC in humans. The exploratory analysis suggested substantial dissimilarities of VRC and NO pharmacokinetics in plasma and ISF. Ultimately, a thorough understanding of target-site pharmacokinetics might contribute to the optimization of personalized VRC dosing regimens.

Microdialysis sampling to monitor target-site vancomycin concentrations in septic infants: a feasible way to close the knowledge gap

This work is dedicated to the memory of Hartmut Derendorf (1953–2020), a pioneer of modern pharmacokinetics and valued mentor of this project.Objectives: Septic infants/neonates need effective antibiotic exposure, but dosing recommendations are challenging as the pharmacokinetics in this age are highly variable. For vancomycin, which is used as a standard treatment, comprehensive pharmacokinetic knowledge especially at the infection site is lacking. Hence, an exploratory clinical study was conducted to assess the feasibility and safety of microdialysis sampling for vancomycin monitoring at the target site.Methods: Nine infants/neonates with therapeutic indications for vancomycin treatment were administered 15 mg/kg as 1-hour infusions every 8–24 hours. Microdialysis catheters were implanted in the subcutaneous interstitial space fluid of the lateral thigh. Samples were collected every 30 minutes over 24 hours, followed by retrodialysis for catheter calibration. Prior in vitro investigations have evaluated impact factors on relative recovery and retrodialysis.Results: In vitro investigations showed the applicability of microdialysis for vancomycin monitoring. Microdialysis sampling was well tolerated in all infants/neonates (23–255 days) without major bleeding or other adverse events. Pharmacokinetic profiles were obtained and showed plausible vancomycin concentration-time courses.Conclusions: Microdialysis as a minimally invasive technique for continuous longer-term sampling is feasible and safe in infants/neonates. Interstitial space fluid profiles were plausible and showed substantial interpatient variation. Hence, a larger microdialysis trial is warranted to further characterise the pharmacokinetics and variability of vancomycin at the target site and ultimately improve vancomycin dosing in these vulnerable patients.

Which Analysis Approach Is Adequate to Leverage Clinical Microdialysis Data? A Quantitative Comparison to Investigate Exposure and Response Exemplified by Levofloxacin

PurposeSystematic comparison of analysis methods of clinical microdialysis data for impact on target-site drug exposure and response.Methods39 individuals received a 500 mg levofloxacin short-term infusion followed by 24-h dense sampling in plasma and microdialysate collection in interstitial space fluid (ISF). ISF concentrations were leveraged using non-compartmental (NCA) and compartmental analysis (CA) via (ii) relative recovery correction at midpoint of the collection interval (midpoint-NCA, midpoint-CA) and (ii) dialysate-based integrals of time (integral-CA). Exposure and adequacy of community-acquired pneumonia (CAP) therapy via pharmacokinetic/pharmacodynamic target-attainment (PTA) analysis were compared between approaches.ResultsIndividual AUCISF estimates strongly varied for midpoint-NCA and midpoint-CA (≥52.3%CV) versus integral-CA (≤32.9%CV) owing to separation of variability in PK parameters (midpoint-CA = 46.5%–143%CVPK, integral-CA = 26.4%–72.6%CVPK) from recovery-related variability only in integral-CA (41.0%–50.3%CVrecovery). This also led to increased variability of AUCplasma for midpoint-CA (56.0%CV) versus midpoint-NCA and integral-CA (≤33.0%CV), and inaccuracy of predictive model performance of midpoint-CA in plasma (visual predictive check). PTA analysis translated into 33% of evaluated patient cases being at risk of incorrectly rejecting recommended dosing regimens at CAP-related epidemiological cut-off values.ConclusionsIntegral-CA proved most appropriate to characterise clinical pharmacokinetics- and microdialysis-related variability. Employing this knowledge will improve the understanding of drug target-site PK for therapeutic decision-making.

Location matters: highly divergent protein levels in samples from different CNS compartments in a clinical trial of rituximab for progressive MS

BackgroundThe relationship between proteins in different CNS extracellular compartments is unknown. In this study the levels of selected proteins in three compartments in people with progressive multiple sclerosis (PMS) were compared.MethodsDuring an open label, phase 1b study on intraventricular administration of rituximab for PMS, samples were collected from the interstitial space (ISS) of the brain through microdialysis. Samples were also obtained from ventricular and lumbar cerebrospinal fluid (CSF). These samples were analyzed with a multiplexed proximity extension assay, measuring the levels of 180 proteins split equally between two panels, detecting proteins associated with immunology and neurology, respectively.ResultsConsiderable differences in concentrations were observed between the three analyzed compartments. Compared to ventricular CSF, ISS fluid contained statistically significant higher levels of 25 proteins (84% immunology panel and 16% neurology panel). Ventricular CSF contained significantly higher levels of 54 proteins (31% immunology panel and 69% neurology panel) compared to ISS fluid, and 17 proteins (76% immunology panel and 24% neurology panel) compared to lumbar CSF. Lumbar CSF showed significantly higher levels of 115 proteins (32% immunology panel and 68% neurology panel) compared to ventricular CSF. The three compartments displayed poor correlation with a median Spearman’s rho of -0.1 (IQR 0.4) between ISS and ventricular CSF and 0.3 (IQR 0.4) between ventricular and lumbar CSF.ConclusionA substantial heterogeneity in the protein levels of samples obtained from different CNS compartments was seen. Therefore, data obtained from analysis of lumbar CSF should be interpreted with caution when making conclusions about pathophysiological processes in brain tissue.

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

ObjectivesThe 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. MethodsIn 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. ResultsIn 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. DiscussionThis 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.

Brain Exposure to Piperacillin in Acute Hemorrhagic Stroke Patients Assessed by Cerebral Microdialysis and Population Pharmacokinetics

BackgroundThe broad antibacterial spectrum of piperacillin/tazobactam makes the combination suitable for the treatment of nosocomial bacterial central nervous system (CNS) infections. As limited data are available regarding piperacillin CNS exposure in patients without or with low-grade inflammation, a clinical study was conducted (1) to quantify CNS exposure of piperacillin by cerebral microdialysis and (2) to evaluate different dosing regimens in order to improve probability of target attainment (PTA) in brain.MethodsTen acute hemorrhagic stroke patients (subarachnoid hemorrhage, n = 6; intracerebral hemorrhage, n = 4) undergoing multimodality neuromonitoring received 4 g piperacillin/0.5 g tazobactam every 8 h by 30-min infusions for the management of healthcare-associated pneumonia. Cerebral microdialysis was performed as part of the clinical neuromonitoring routine, and brain interstitial fluid samples were retrospectively analyzed for piperacillin concentrations after the first and after multiple doses for at least 5 days and quantified by high-performance liquid chromatography. Population pharmacokinetic modeling and Monte Carlo simulations with various doses and types of infusions were performed to predict exposure. A T>MIC of 50% was selected as pharmacokinetic/pharmacodynamic target parameter.ResultsMedian peak concentrations of unbound piperacillin in brain interstitial space fluid were 1.16 (range 0.08–3.59) and 2.78 (range 0.47–7.53) mg/L after the first dose and multiple doses, respectively. A one-compartment model with a transit compartment and a lag time (for the first dose) between systemic and brain exposure was appropriate to describe the brain concentrations. Bootstrap median estimates of the parameters were: transfer rate from plasma to brain (0.32 h−1), transfer rate from brain to plasma (7.31 h−1), and lag time [2.70 h (coefficient of variation 19.7%)]. The simulations suggested that PTA would exceed 90% for minimum inhibitory concentrations (MICs) up to 0.5 mg/L and 1 mg/L at a dose of 12–16 and 24 g/day, respectively, regardless of type of infusion. For higher MICs, PTA dropped significantly.ConclusionLimited CNS exposure of piperacillin might be an obstacle in treating patients without general meningeal inflammation except for infections with highly susceptible pathogens. Brain exposure of piperacillin did not improve significantly with a prolongation of infusions.

TREM2: Modulator of Lipid Metabolism in Microglia

Lipid-processing mechanisms during demyelination are poorly understood. In this issue of Neuron,Nugent etal. (2020) show by cell-specific lipidomics that Trem2 deficiency leads to cholesterol ester (CE) overload in microglia. This is mediated by misregulation of lipid metabolism genes and is rescued by modulating CE synthesis or efflux.

This Month in Anesthesiology.

This Month in Anesthesiology.

Lung Pharmacokinetics of Tobramycin by Intravenous and Nebulized Dosing in a Mechanically Ventilated Healthy Ovine Model.

Nebulized antibiotics may be used to treat ventilator-associated pneumonia. In previous pharmacokinetic studies, lung interstitial space fluid concentrations have never been reported. The aim of the study was to compare intravenous and nebulized tobramycin concentrations in the lung interstitial space fluid, epithelial lining fluid, and plasma in mechanically ventilated sheep with healthy lungs. Ten anesthetized and mechanically ventilated healthy ewes underwent surgical insertion of microdialysis catheters in upper and lower lobes of both lungs and the jugular vein. Five ewes were given intravenous tobramycin 400 mg, and five were given nebulized tobramycin 400 mg. Microdialysis samples were collected every 20 min for 8 h. Bronchoalveolar lavage was performed at 1 and 6 h. The peak lung interstitial space fluid concentrations were lower with intravenous tobramycin 20.2 mg/l (interquartile range, 12 mg/l, 26.2 mg/l) versus the nebulized route 48.3 mg/l (interquartile range, 8.7 mg/l, 513 mg/l), P = 0.002. For nebulized tobramycin, the median epithelial lining fluid concentrations were higher than the interstitial space fluid concentrations at 1 h (1,637; interquartile range, 650, 1,781, vs. 16 mg/l, interquartile range, 7, 86, P < 0.001) and 6 h (48, interquartile range, 17, 93, vs. 4 mg/l, interquartile range, 2, 9, P < 0.001). For intravenous tobramycin, the median epithelial lining fluid concentrations were lower than the interstitial space fluid concentrations at 1 h (0.19, interquartile range, 0.11, 0.31, vs. 18.5 mg/l, interquartile range, 9.8, 23.4, P < 0.001) and 6 h (0.34, interquartile range, 0.2, 0.48, vs. 3.2 mg/l, interquartile range, 0.9, 4.4, P < 0.001). Compared with intravenous tobramycin, nebulized tobramycin achieved higher lung interstitial fluid and epithelial lining fluid concentrations without increasing systemic concentrations.

Nebulized Antibiotics: Epithelial Lining Fluid Concentrations Overestimate Lung Tissue Concentrations.

Nebulized Antibiotics: Epithelial Lining Fluid Concentrations Overestimate Lung Tissue Concentrations.

Repeated determination of moxifloxacin concentrations in interstitial space fluid of muscle and subcutis in septic patients.

For an effective antimicrobial treatment, it is crucial that antibiotics reach sufficient concentrations in plasma and tissue. Currently no data exist regarding moxifloxacin plasma concentrations and exposure levels in tissue under septic conditions. To determine the pharmacokinetics of moxifloxacin in plasma and interstitial space fluid over a prolonged period. Ten septic patients were treated with 400 mg of moxifloxacin once a day; on days 1, 3 and 5 of treatment plasma sampling and microdialysis in the subcutis and muscle of the upper thigh were performed to determine concentrations of moxifloxacin in different compartments. This trial was registered in the German Clinical Trials Register (DRKS, register number DRKS00012985). Mean unbound fraction of moxifloxacin in plasma was 85.5±3.4%. On day 1, Cmax in subcutis and muscle was 2.8±1.8 and 2.5±1.3 mg/L, respectively, AUC was 24.8±15.1 and 21.3±10.5 mg·h/L, respectively, and fAUC0-24/MIC was 100.9±62.9 and 86.5±38.3 h, respectively. Cmax for unbound moxifloxacin in plasma was 3.5±0.9 mg/L, AUC was 23.5±7.5 mg·h/L and fAUC0-24/MIC was 91.6±24.8 h. Key pharmacokinetic parameters on days 3 and 5 showed no significant differences. Clearance was higher than in healthy adults, but tissue concentrations were comparable, most likely due to a lower protein binding. Surprisingly, the first dose already achieved exposure comparable to steady-state conditions. The approved daily dose of 400 mg was adequate in our patient population. Thus, it seems that in septic patients a loading dose on the first day of treatment with moxifloxacin is not required.

High voriconazole target-site exposure after approved sequence dosing due to nonlinear pharmacokinetics assessed by long-term microdialysis

Voriconazole, a broad-spectrum antifungal drug used to prevent and treat invasive fungal infections, shows complex pharmacokinetics and is primarily metabolised by various CYP enzymes. An adequate unbound antibiotic concentration-time profile at the target-site of an infection is crucial for effective prophylaxis or therapy success. Therefore, the aim was to evaluate the pharmacokinetics of voriconazole after the approved sequence dosing in healthy volunteers in interstitial space fluid, assessed by microdialysis, and in plasma. Moreover, potential pharmacogenetic influences of CYP2C19 polymorphisms on pharmacokinetics were investigated. The prospective, open-labelled, uncontrolled long-term microdialysis study included 9 healthy male individuals receiving the approved sequence dosing regimen for voriconazole. Unbound voriconazole concentrations were sampled over 84 h in interstitial space fluid of subcutaneous adipose tissue and in plasma and subsequently quantified via high-performance liquid chromatography. For pharmacokinetic data analysis, non-compartmental analysis was used. High interindividual variability in voriconazole concentration-time profiles was detected although dosing was adapted to body weight for the first intravenous administrations. Due to nonlinear pharmacokinetics, target-site exposure of voriconazole in healthy volunteers was found to be highly comparable to plasma exposure, particularly after multiple dosing. Regarding the CYP2C19 genotype-predicted phenotype, the individuals revealed a broad spectrum, ranging from poor to rapid metaboliser status. A strong relation between CYP2C19 genotype-predicted phenotype and voriconazole clearance was identified.

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 μL/min) of an in vivo pump and afterwards switching to the flow rate of 1 and 2 μL/min for 120 min, the time necessary for relative recovery to reach equilibrium was determined.(I) High relative recovery values (flow rate 2 μL/min: ≥84%; flow rate 5 μL/min: ≥65%) were observed for all investigated single drugs. Intra- 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 μL/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.

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

BackgroundNebulised antibiotics are frequently used for the prevention or treatment of ventilator-associated pneumonia. Many factors may influence pulmonary drug concentrations with inaccurate dosing schedules potentially leading to therapeutic failure and/or the emergence of antibiotic resistance. We describe a research pathway for studying the pharmacokinetics of a nebulised antibiotic during mechanical ventilation using in vitro methods and ovine models, using tobramycin as the study antibiotic.MethodsIn vitro studies using a laser diffractometer and a bacterial-viral filter were used to measure the effect of the type and size of tracheal tubes and antibiotic concentration on the particle size distribution of the tobramycin 400 mg (4 ml; 100 mg/ml) and 160 mg (4 ml, 40 mg/ml) aerosol and nebulised mass delivered. To compare the regional drug distribution in the lung of two routes (intravenous and nebulised) of drug administration of tobramycin 400 mg, technetium-99m-labelled tobramycin 400 mg with planar nuclear medicine imaging was used in a mechanically ventilated ovine model. To measure tobramycin concentrations by intravenous and nebulised tobramycin 400 mg (4 ml, 100 mg/ml) administration in the lung interstitial space (ISF) fluid and blood of mechanically ventilated sheep, the microdialysis technique was used over an 8-h duration.ResultsTobramycin 100 mg/ml achieved a higher lung dose (121.3 mg) compared to 40 mg/ml (41.3 mg) solution. The imaging study with labelled tobramycin indicated that nebulised tobramycin distributed more extensively into each lung zone of the mechanically ventilated sheep than intravenous administration. A higher lung ISF peak concentration of tobramycin was observed with nebulised tobramycin (40.8 mg/l) compared to intravenous route (19.0 mg/l).ConclusionsThe research methods appear promising to describe lung pharmacokinetics for formulations intended for nebulisation during mechanical ventilation. These methods need further validation in an experimental pneumonia model to be able to contribute toward optimising dosing regimens to inform clinical trials and/or clinical use.

Characterization of the Ionic Profile of the Extracellular Space of the Injured and Ischemic Brain: A Microdialysis Study.

Traumatic brain injury (TBI) and ischemic stroke cause a variable disruption of ionic homeostasis and massive ionic fluxes with subsequent osmotic water movement across the cells that causes edema, brain swelling, and deformation of the damaged tissue. Although cerebral microdialysis (CMD) has been used to study the brain neurochemistry, the ionic profiles of brain interstitial space fluid have rarely been reported in humans. We studied the ionic profile in injured areas of the brain by using CMD. As a control group, we included seven patients who had undergone surgical treatment of posterior fossa lesions, without abnormalities in the supratentorial compartment. Inductively coupled plasma mass spectrometry (ICP-MS) was used for ion determination. No significant differences were found in the [Na+]o, [K+]o, and [Cl-]o between normal injured brains and controls. The ionic profile of the ischemic core was characterized by very high [K+]o and an increase in [Na+]o, whereas [Cl-]o was linearly related to [Na+]o. In the traumatic core (TC), significantly higher levels of [Na+]o, [Cl-]o, and [K+]o were found. The main finding in the penumbra was a completely normal ionic profile for [Na+]o and [K+]o in 60% of the samples. ICP-MS coupled to ionic assays creates a powerful tool for a better understanding of the complex ionic disturbances that occur after severe TBI and ischemic stroke.

Simultaneous Semimechanistic Population Analyses of Levofloxacin in Plasma, Lung, and Prostate To Describe the Influence of Efflux Transporters on Drug Distribution following Intravenous and Intratracheal Administration.

Levofloxacin (LEV) is a broad-spectrum fluoroquinolone used to treat pneumonia, urinary tract infections, chronic bacterial bronchitis, and prostatitis. Efflux transporters, primarily P-glycoprotein (P-gp), are involved in LEV's tissue penetration. In the present work, LEV free lung and prostate interstitial space fluid (ISF) concentrations were evaluated by microdialysis in Wistar rats after intravenous (i.v.) and intratracheal (i.t.) administration (7 mg/kg of body weight) with and without coadministration of the P-gp inhibitor tariquidar (TAR; 15 mg/kg administered i.v.). Plasma and tissue concentration/time profiles were evaluated by noncompartmental analysis (NCA) and population pharmacokinetics (popPK) analysis. The NCA showed significant differences in bioavailability (F) for the control group (0.4) and the TAR group (0.86) after i.t. administration. A four-compartment model simultaneously characterized total plasma and free lung (compartment 2) and prostate (compartment 3) ISF concentrations. Statistically significant differences in lung and prostate average ISF concentrations and levels of kidney active secretion in the TAR group from those measured for the control group (LEV alone) were observed. The estimated population means were as follows: volume of the central compartment (V1), 0.321 liters; total plasma clearance (CL), 0.220 liters/h; TAR plasma clearance (CLTAR), 0.180 liters/h. The intercompartmental distribution rate constants (K values) were as follows: K12, 8.826 h(-1); K21, 7.271 h(-1); K13, 0.047 h(-1); K31, 7.738 h(-1); K14, 0.908 h(-1); K41, 0.409 h(-1); K21 lung TAR (K21LTAR), 8.883 h(-1); K31 prostate TAR (K31PTAR), 4.377 h(-1). The presence of P-gp considerably impacted the active renal secretion of LEV but had only a minor impact on the efflux from the lung following intratracheal dosing. Our results strongly support the idea of a role of efflux transporters other than P-gp contributing to LEV's tissue penetration into the prostrate.

Target site antimicrobial activity of colistin might be misestimated if tested in conventional growth media.

Cation-dependent inhibition of antimicrobial activity was reported for polymyxin antibiotics. Ca(2+) and Mg(2+) concentrations recommended by the Clinical and Laboratory Standards Institute (CLSI) for the supplementation of Müller-Hinton broth (MHB) are markedly lower than interstitial space fluid (ISF) concentrations in vivo. Hence, it was speculated that the antimicrobial activity of colistin might be overestimated if tested using conventional cation-adjusted MHB. The antimicrobial activity of colistin against n = 100 clinical isolates of Pseudomonas aeruginosa, Acinetobacter baumannii, Klebsiella pneumoniae and Escherichia coli (n = 25 each) was evaluated by broth microdilution and, for selected isolates, by time-kill curves, in MHB without cations (MHB(ONLY)), MHB supplemented with 25 mg/L Ca(2+) and 12.5 mg/L Mg(2+) according to CLSI recommendations (MHB(CLSI)), and in MHB adjusted to 50 mg/L Ca(2+) and 20 mg/L Mg(2+) simulating ISF concentrations (MHB(ISF)). The minimum inhibitory concentration (MIC) values of colistin against the vast majority of isolates of both P. aeruginosa and A. baumannii increased significantly with higher cation concentrations. The susceptibility of K. pneumoniae isolates to colistin did not show significant changes between cation-supplemented media, while the MICs of E. coli decreased with ascending cation concentrations. These findings were confirmed in time-kill studies, where colistin killing against P. aeruginosa 1514 and A. baumannii 1485 declined with increasing cation concentrations. Contrarily, the killing of selected concentrations of colistin against K. pneumoniae 15 and E. coli 16 was enhanced in the presence of increasing cation concentrations. The present data suggest that the clinical antimicrobial activity of colistin might be misestimated in vitro if tested in conventional growth media.

Development of a population pharmacokinetic model characterizing the tissue distribution of azithromycin in healthy subjects.

Recent clinical trials indicate that the use of azithromycin is associated with the emergence of macrolide resistance. The objective of our study was to simultaneously characterize free target site concentrations and correlate them with the MIC90s of clinically relevant pathogens. Azithromycin (500 mg once daily [QD]) was administered orally to 6 healthy male volunteers for 3 days. The free concentrations in the interstitial space fluid (ISF) of muscle and subcutaneous fat tissue as well as the total concentrations in plasma and polymorphonuclear leukocytes (PMLs) were determined on days 1, 3, 5, and 10. All concentrations were modeled simultaneously in NONMEM 7.2 using a tissue distribution model that accounts for nonlinear protein binding and ionization state at physiological pH. The model performance and parameter estimates were evaluated via goodness-of-fit plots and nonparametric bootstrap analysis. The model we developed described the concentrations at all sampling sites reasonably well and showed that the overall pharmacokinetics of azithromycin is driven by the release of the drug from acidic cell/tissue compartments. The model-predicted unionized azithromycin (AZM) concentrations in the cytosol of PMLs (6.0 ± 1.2 ng/ml) were comparable to the measured ISF concentrations in the muscle (8.7 ± 2.9 ng/ml) and subcutis (4.1 ± 2.4 ng/ml) on day 10, whereas the total PML concentrations were >1,000-fold higher (14,217 ± 2,810 ng/ml). The total plasma and free ISF concentrations were insufficient to exceed the MIC90s of the skin pathogens at all times. Our results indicate that the slow release of azithromycin from low pH tissue/cell compartments is responsible for the long terminal half-life of the drug and thus the extended period of time during which free concentrations reside at subinhibitory concentrations.

Reduced subcutaneous tissue distribution of cefazolin in morbidly obese versus non-obese patients determined using clinical microdialysis

As morbidly obese patients are prone to surgical site infections, adequate blood and subcutaneous tissue concentrations of prophylactic antibiotic agents during surgery are imperative. In this study we evaluated cefazolin subcutaneous adipose tissue distribution in morbidly obese and non-obese patients, thereby quantifying the influence of morbid obesity on cefazolin pharmacokinetics and enabling Monte Carlo simulations for subsequent dose adjustments. Nine morbidly obese patients [body mass index (BMI) 47 ± 6 kg/m(2)], of whom eight were evaluable, and seven non-obese patients (BMI 28 ± 3 kg/m(2)) received cefazolin 2 g intravenously before surgery (NCT01309152). Using microdialysis, interstitial space fluid (ISF) samples of subcutaneous adipose tissue were collected together with total and unbound plasma cefazolin samples until 240 min after dosing. Using NONMEM, population pharmacokinetic modelling, covariate analysis and Monte Carlo simulations were performed. The unbound (free) cefazolin ISF penetration ratio (fAUC(tissue)/fAUC(plasma)) was 0.70 (range 0.68-0.83) in morbidly obese patients versus 1.02 (range 0.85-1.41) in non-obese patients (P < 0.05). A two-compartment model with saturable protein binding was identified in which the central volume of distribution and cefazolin distribution from the central compartment to the ISF compartment proved dependent on body weight (P < 0.001 and P < 0.01, respectively). Monte Carlo simulations showed reduced probability of target attainment for morbidly obese versus non-obese patients for MIC values of 2 and 4 mg/L. This study shows that cefazolin tissue distribution is lower in morbidly obese patients and reduces with increasing body weight, and that dose adjustments are required in this patient group.

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.