Relationship between serum and free interstitial concentrations of cefodizime and cefpirome in muscle and subcutaneous adipose tissue of healthy volunteers measured by microdialysis

Tissue Drug Concentration

Blood flow enables the delivery of oxygen and nutrients to the different tissues of the human body. Drugs follow the same route as oxygen and nutrients; thus, drug concentrations in tissues are highly dependent on the blood flow fraction delivered to each of these tissues. Although the free drug concentration in blood is considered to correlate with pharmacodynamics, the pharmacodynamics of a drug is actually primarily commanded by the concentrations of drug in the aqueous spaces of bodily tissues. However, the concentrations of drug are not homogeneous throughout the tissues, and they rarely reflect the free drug concentration in the blood. This heterogeneity is due to differences in the blood flow fraction delivered to the tissues and also due to membrane transporters, efflux pumps, and metabolic enzymes. The rate of drug elimination from the body (systemic elimination) depends more on the driving force of drug elimination than on the free concentration of drug at the site from which the drug is being eliminated. In fact, the actual free drug concentration in the tissues results from the balance between the input and output rates. In the present paper, we develop a theoretical concept regarding solute partition between intravascular and extravascular spaces; discuss experimental research on aqueous/non-aqueous solute partitioning and clinical research on microdialysis; and present hypotheses to predict in-vivo elimination using parameters of in-vitro metabolism.

Microdialysis techniques and microdialysis-based patient-near diagnostics

This article will debate the usefulness of POCT measurements and the contribution microdialysis can make to generating valuable information. A particular theme will be the rarely considered difference between ex vivo sampling, which typically generates only a static measure of concentration, and in vivo measurements that are subject to dynamic changes due to mass transfer. Those dynamic changes provide information about the patients' physiological state.

Heterogeneous antiretroviral drug distribution and HIV/SHIV detection in the gut of three species

HIV replication within tissues may increase in response to a reduced exposure to antiretroviral drugs. Traditional approaches to measuring drug concentrations in tissues are unable to characterize a heterogeneous drug distribution. Here, we used mass spectrometry imaging (MSI) to visualize the distribution of six HIV antiretroviral drugs in gut tissue sections from three species (two strains of humanized mice, macaques, and humans). We measured drug concentrations in proximity to CD3+ T cells that are targeted by HIV, as well as expression of HIV or SHIV RNA and expression of the MDR1 drug efflux transporter in gut tissue from HIV-infected humanized mice, SHIV-infected macaques, and HIV-infected humans treated with combination antiretroviral drug therapy. Serial 10-μm sections of snap-frozen ileal and rectal tissue were analyzed by MSI for CD3+ T cells and MDR1 efflux transporter expression by immunofluorescence and immunohistochemistry, respectively. The tissue slices were analyzed for HIV/SHIV RNA expression by in situ hybridization and for antiretroviral drug concentrations by liquid chromatography-mass spectrometry. The gastrointestinal tissue distribution of the six drugs was heterogeneous. Fifty percent to 60% of CD3+ T cells did not colocalize with detectable drug concentrations in the gut tissue. In all three species, up to 90% of HIV/SHIV RNA was found to be expressed in gut tissue with no exposure to drug. These data suggest that there may be gut regions with little to no exposure to antiretroviral drugs, which may result in low-level HIV replication contributing to HIV persistence.

Lack of Pharmacokinetic Basis of Weight-Based Dosing and Intra-Operative Re-Dosing with Cefazolin Surgical Prophylaxis in Obese Patients: Implications for Antibiotic Stewardship

Traditionally, there have been uniform antibiotic dosing guidelines for prophylaxis for clean-clean-contaminated surgery in both non-obese and obese adults. All other factors predisposing to surgical site infections (SSIs) being equal, over time, the preferred drug is cefazolin. The usual dose, given immediately pre-procedure, has been 1 g intravenously (IV) in non-penicillin-allergic patients, which has been highly effective, Recently, it has become common practice to use high-dose cefazolin; i.e., 3 g IV, in obese patients. This article reviews the literature on high-dose cefazolin prophylactic regimens in the obese from a pharmacokinetic (PK) point of view. There are no comparative studies to support this approach, which is based largely on the theory "more must be better." Weight-based dosing of cefazolin in the obese is flawed, because it does not take into account PK factors, which are critical in the obese. Cefazolin is a water-soluble (hydrophilic) antibiotic that does not penetrate adipose tissue regardless of IV dose. Importantly, adipose tissue is not a valid target tissue in clean-clean-contaminated SSI prophylaxis, as it does not become infected. Higher doses result in proportionately higher serum/non-adipose tissue concentrations, but adipose tissue concentrations are unaffected. Cefazolin displays time-dependent killing kinetics so that as long as serum/tissue concentrations are above the minimum inhibitory concentration (MIC) of SSI pathogens, there is no enhanced killing with higher concentrations relative to concentration-dependent antibiotics. Taking into account PK principles, a cefazolin 1 g IV bolus results in peak serum concentrations of ∼185 mcg/mL, provides at least six hours of intra-operative protection, aside from any post-antibiotic effects, and eliminates any rationale for intra-operative re-dosing for procedures lasting six hours or less. Some have argued that a cefazolin 3 g IV dose in the obese does not matter, as more must necessarily be better. However, from an antibiotic stewardship program (ASP) perspective, unneeded antibiotics are unnecessary. Moreover, the costs of cefazolin 1 g (IV push) at $0.75 versus 2 g (IV piggyback) at $ 6.83 can be significant in large centers using cefazolin prophylaxis for cardiothoracic, orthopedic, obstetric/gynecology, and bariatric surgery. Excessive antibiotics also expose the patient to potential adverse effects; i.e., Clostridium difficile. There is no dose-dependent or duration of exposure effect on resistance with one or two pre-operative or intra-operative doses. Well-done PK-based studies in obese patients clearly demonstrate the lack of benefit of using a 3-g dose or intra-operative re-dosing and show no incremental increase in adipose tissue concentrations with high doses. From an ASP point of view, antibiotic dosing recommendations should be reviewed and revised on the basis of PK principles that indicate that weight-based dosing has no basis for pre-operative prophylaxis in obese patients.

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.

Antimicrobial considerations in the perioperative patient

Surgical site infections are among the complications that can be reduced with the timely implementation of appropriate antimicrobial therapy. A 3-D approach to judicious antimicrobial use focuses on the de-escalation of systemic antimicrobial therapy, design of dosing regimens, and decontamination of the surgeon, patient, and environment. De-escalation can be accomplished in part through proper antimicrobial prophylaxis. Dosing regimens should be designed to maximize efficacy and minimize resistance. Decontamination includes disinfection of inanimate surfaces and timely application of appropriate antiseptics at concentrations that maximize efficacy.

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.

Effect of Simulated Microgravity on the Disposition and Tissue Penetration of Ciprofloxacin in Healthy Volunteers

This study evaluated the effects of simulated microgravity (smuG) on the pharmacokinetics of ciprofloxacin. Six healthy volunteers participated in a crossover study to compare the pharmacokinetics of ciprofloxacin after a single 250-mg oral dose in normal gravity (1G) and smuG. Plasma and urine samples were collected, and in vivo microdialysis was employed to obtain the free interstitial concentrations in the thigh muscle. Tissue penetration (f) was determined as the ratio of the free tissue area under the concentration versus time curve (AUC(tiss,free))/AUC(plasma,free). Plasma and free interstitial ciprofloxacin concentrations were simultaneously fit to a 1-compartment body model after correction for protein binding and tissue penetration. Total and free plasma concentrations were very similar in smuG and 1G. Tissue penetration in smuG (f =0.61 +/- 0.36) was slightly lower than in 1G (f =0.92 +/- 0.63); however, the difference was not significant. The authors conclude that the disposition of ciprofloxacin was not affected by simulated microgravity.

Overton's rule helps to estimate the penetration of anti-infectives into patients' cerebrospinal fluid

In 1900, Ernst Overton found that the entry of anilin dyes through the cell membranes of living cells depended on the lipophilicity of the dyes. The brain is surrounded by barriers consisting of lipid layers that possess several inward and outward active transport systems. In the absence of meningeal inflammation, the cerebrospinal fluid (CSF) penetration of anti-infectives in humans estimated by the ratio of the area under the concentration-time curve (AUC) in CSF (AUC(CSF)) to that in serum (AUC(CSF)/AUC(S)) correlated positively with the lipid-water partition coefficient at pH 7.0 (log D) (Spearman's rank correlation coefficient r(S) = 0.40; P = 0.01) and negatively with the molecular mass (MM) (r(S) = -0.33; P = 0.04). The ratio of AUC(CSF) to the AUC of the fraction in serum that was not bound (AUC(CSF)/AUC(S,free)) strongly correlated with log D (r(S) = 0.67; P < 0.0001). In the presence of meningeal inflammation, AUC(CSF)/AUC(S) also correlated positively with log D (r(S) = 0.46; P = 0.002) and negatively with the MM (r(S) = -0.37; P = 0.01). The correlation of AUC(CSF)/AUC(S,free) with log D (r(S) = 0.66; P < 0.0001) was as strong as in the absence of meningeal inflammation. Despite these clear correlations, Overton's rule was able to explain only part of the differences in CSF penetration of the individual compounds. The site of CSF withdrawal (lumbar versus ventricular CSF), age of the patients, underlying diseases, active transport, and alterations in the pharmacokinetics by comedications also appeared to strongly influence the CSF penetration of the drugs studied.

Comparable population pharmacokinetics and pharmacodynamic breakpoints of cefpirome in cystic fibrosis patients and healthy volunteers

Cystic fibrosis (CF) patients are often reported to have higher clearances and larger volumes of distribution per kilogram of total body weight (WT) for beta-lactams than healthy volunteers. As pharmacokinetic (PK) data on cefpirome from studies of CF patients are lacking, we systematically compared its population PK and pharmacodynamic breakpoints for CF patients and healthy volunteers of similar body size. Twelve adult CF patients (median lean body mass [LBM] = 45.7 kg) and 12 healthy volunteers (LBM = 50.0 kg) received a single 10-min intravenous infusion of 2 g cefpirome. Plasma and urine concentrations were determined by high-performance liquid chromatography (HPLC). Population PK and Monte Carlo simulations were performed using NONMEM and S-ADAPT and a duration of an unbound plasma concentration above the MIC ≥ 65% of the dosing interval as a pharmacodynamic target. Unscaled clearances for CF patients were similar to those seen with healthy volunteers, and the volume of distribution was 6% lower for CF patients. Linear scaling of total clearance by WT resulted in clearance that was 20% higher (P ≤ 0.001 [nonparametric bootstrap]) in CF patients. Allometric scaling by LBM explained the differences between the two subject groups with respect to average clearance and volume of distribution and reduced the unexplained between-subject variability of renal and nonrenal clearance by 10 to 14%. For the CF patients, robust (>90%) probabilities of target attainment (PTA) were achieved by the administration of a standard dose of 2 g/70 kg WT every 12 h (Q12h) given as 30-min infusions for MICs ≤ 1.5 mg/liter. As alternative dosage regimens, a 5-h infusion of 1.33 g/70 kg WT Q8h achieved robust PTAs for MICs ≤ 8 to 12 mg/liter and a continuous infusion of 4 g/day for MICs ≤ 12 mg/liter. Prolonged infusion of cefpirome is expected to be superior to short-term infusions for MICs between 2 and 12 mg/liter.

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.

In vivo time-course changes in ethanol levels sampled with subcutaneous microdialysis

BACKGROUND

The objective of this study was to determine time-course changes in in vivo ethanol (EtOH) concentrations using a novel subcutaneous (s.c.) microdialysis sampling technique. The hypothesis to be tested was that EtOH concentrations in the s.c. fluid would reflect blood EtOH concentrations. If this is the case, then s.c. microdialysis could allow a more detailed analysis of changes in in vivo levels of EtOH under different drinking paradigms.

METHODS

Adult male and female Wistar rats and male alcohol-preferring (P) rats were used in this study. A loop-style microdialysis probe was designed for s.c. applications. After initial in vitro characterization, probes were implanted under the skin between the shoulder blades. Animals were allowed to recover 4 to 24 hours prior to microdialysis collection (2.0 microl/min flow rate with isotonic saline). In vivo microdialysis experiments were then conducted to determine (i) the extraction fraction (or clearance) using EtOH no-net-flux (NNF) coupled with the alcohol clamp method, (ii) the dose-response and time-course effects after systemic EtOH administration and to compare with blood EtOH levels, and (iii) the time-course changes in EtOH levels during and after an EtOH drinking episode.

RESULTS

In vivo probe recovery (extraction fraction) obtained using the alcohol clamp method was 69 +/- 3%, and was comparable to the in vitro recovery of 73 +/- 2%. For the EtOH dose-response experiment, rats injected i.p. with 0.5, 1.0, or 2.0 g/kg EtOH showed a clear dose-response effect in the s.c. dialysate samples. Peak concentrations (70, 123, and 203 mg%, respectively) were reached by 15 minutes after injection. In an experiment comparing levels of EtOH in s.c. dialysis and arterial blood samples in rats administered 1.0 g/kg EtOH, similar time-course changes in in vivo EtOH concentrations were observed with both i.g. and i.p. EtOH administration. In P rats drinking 15% EtOH during a 1-hour scheduled access period, EtOH levels in s.c. microdialysates rose rapidly over the session and peaked at approximately 50 mg% at 60 to 80 minutes.

CONCLUSIONS

Overall, these experiments indicate that s.c. EtOH and blood EtOH concentrations follow a similar time course. Moreover, s.c. microdialysis can be useful as an experimental approach for determining detailed time-course changes in in vivo EtOH concentrations associated with alcohol drinking episodes.

Disposition kinetics and urinary excretion of cefpirome after intravenous injection in buffalo calves

We investigated the disposition kinetics and urinary excretion of cefpirome in buffalo calves after a single intravenous administration of 10 mg/kg. Also, an appropriate dosage regimen was calculated. At 1 min after injection, the concentration of cefpirome in the plasma was 57.4 +/- 0.72 microg/ml, which declined to 0.22 +/- 0.01 microg/ml at 24 h. The cefpirome was rapidly distributed from the blood to the tissue compartment as shown by the high distribution coefficient values (8.67 +/- 0.46/h), and by the drug's rate of transfer constant from the central to the peripheral compartment, K(12) (4.94 +/- 0.31/h). The elimination halflife and the volume of distribution were 2.14 +/- 0.02 h and 0.42 +/- 0.005 l/kg, respectively. Once the distribution equilibrium was reached between the tissues and plasma, the total body clearance (Cl(B)) and the ratio of the drug present in the peripheral to the central compartment (T/P ratio) were 0.14 +/- 0.002 l/kg/h and 1.73 +/- 0.06, respectively. Based on the pharmacokinetic parameters we obtained, an appropriate intravenous cefpirome dosage regimen for treating cefpiromesensitive bacteria in buffalo calves would be 8.0 mg/kg repeated at 12 h intervals for 5 days, or until persistence of the bacterial infection occurred.

Electroporation and transcutaneous extraction (ETE) for pharmacokinetic studies of drugs

The therapeutic activity and toxicity of drugs often depends on the accumulation of drugs in the peripheral anatomical compartment rather than the central compartment. In the routine practice of therapeutic drug monitoring (TDM) and pharmacokinetic studies, drug concentration determined by intermittent blood sampling is used as a surrogate for calculating the drug concentration in the peripheral compartment tissues. Microdialysis, a relatively less invasive procedure, has been used for estimation of free drug levels in dermal, subcutaneous and muscle tissues. Transcutaneous extraction of drugs from the dermal tissue is a good noninvasive alternative to phlebotomy and microdialysis. This requires a technique, which can facilitate the extraction of significant and reproducible amounts of drugs from the dermal extracellular fluid (ECF) within a short sampling duration. In the present work, we assessed the feasibility of electroporation and transcutaneous extraction (ETE) method for determining the time course of drugs in dermal ECF, using salicylic acid (SA) as a test drug. Electroporation protocol was optimized based on the in vitro diffusion studies of salicylic acid across rat skin. The concentration-time profile of total SA was determined in rats after a single i.v. bolus administration. The in vivo permeability coefficient (P(in vivo)) of rat skin was determined under steady state plasma concentration of drug created by i.v. bolus followed by constant rate infusion of SA. The pharmacokinetic parameters of the drug were determined using a two-compartment pharmacokinetic model. The theoretical predicted time course of free SA in the dermal ECF after a single i.v. bolus administration was calculated using standard formulae. The concentration of free SA determined by ETE is in good agreement with that calculated using two-compartment pharmacokinetic model. This study thus provides a credible evidence for the validity of ETE technique for determining the concentration of SA in the dermal ECF.

Antimicrobial tissue concentrations

The clinical outcome of anti-infective treatment is determined by both PK and PD properties of the antibiotic. Only the free tissue concentrations of antibiotics at the target site, which are usually lower than the total plasma concentrations, are responsible for therapeutic effect. The free antibiotic concentrations at the site of action are a more appropriate PK input value for PK-PD analysis. The unbound tissue concentrations can be measured directly by microdialysis. Using plasma concentrations overestimates the target site concentrations and its clinical efficacy. The optimal dosing regimens of antibiotics have an impact on patients' outcome and cost of therapy, and reduce the emergence of resistance.

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

OBJECTIVES

The pharmacokinetic profile of antibiotics at the site of anti-infective action is one of the most important determinants of drug response, since it correlates with antimicrobial effect. Up to now, only limited information on the lung tissue pharmacokinetics of antibiotic agents has been available. The aim of this study was to measure, using a new microdialysis-based approach, antibiotic penetration into the extracellular space fluid of pneumonic human lung parenchyma.

PATIENTS AND METHODS

The lung penetration of a combination of piperacillin and tazobactam, substances with low protein binding, was determined in five patients suffering from pneumonia and metapneumonic pleural empyema. The condition was treated by decortication after lateral thoracotomy. Intra-, or post-operatively, respectively, two microdialysis probes were inserted into pneumonic lung tissue, and into healthy skeletal muscle to obtain reference values. Serum and microdialysis samples were collected at 20-min intervals for at last 8 h following i.v. administration of a single dose of 4 g piperacillin and 500 mg tazobactam.

RESULTS

The mean free interstitial concentration profiles of piperacillin in infected lung tissue and serum showed a maximal tissue concentration (Cmax) of 176.0 +/- 105.0 mg l-1 and 326.0 +/- 60.6 mg l-1, respectively. The mean AUC (area under the curve) for infected lung tissue was 288.0 +/- 167.0 mg.h l-1 and for serum 470.0 +/- 142.0 mg.h l-1. There was a statistically significant difference between AUC (lung) and AUC (serum) (P = 0.018) as well as between AUC (lung) and AUC (muscle) (P = 0.043). The intrapulmonary concentrations of piperacillin and tazobactam exceeded the minimum inhibitory concentrations (MIC) for most relevant bacteria for 4-6 h. The procedure was well tolerated by all patients and no adverse events or microdialysis-associated side-effects were observed.

CONCLUSION

This microdialysis technique enabled continuous tissue pharmacokinetic measurement of free, unbound anti-infective agents in the lung tissue of patients with pneumonia. The present data corroborate the use of piperacillin and tazobactam in the treatment of lung infections caused by extracellular bacteria and demonstrate the distribution of piperacillin and tazobactam in the interstitial space of pneumonic lung tissue.

Single- and multiple-dose pharmacokinetics of oral creatine

Supplementation with exogenous creatine (Cr) has shown physiological benefits in humans, but little is known about the pharmacokinetics of Cr in humans. Six healthy males completed an open-label study consisting of a full pharmacokinetic analysis following a single oral dose of Cr monohydrate (71 mg kg-1) and at steady-state after 6 days of Cr administration (71 mg kg-1 qid). After the single oral dose, the clearance (CL/F) was 0.20 +/- 0.066 L h-1 kg-1, tmax was 1.9 +/- 0.88 hours, and Cmax = 102.1 +/- 11.2 mg h L-1. At steady-state, CL/F decreased to 0.12 +/- 0.016 L h-1 kg-1, tmax did not change, and Cmax increased to 162.2 +/- 30.0 mg L-1. Penetration (AUCMUSCLE/AUCPLASMA) of Cr into the interstitial muscle space, as determined by microdialysis, was 0.47 +/- 0.09 and 0.37 +/- 0.27 for the single dose and at steady-state, respectively. Plasma and muscle data were simultaneously fitted with a model incorporating a saturable absorption and first-order elimination process. In conclusion, repeated dosing of Cr caused a reduction in clearance that could result from saturation of the skeletal muscle pool of Cr.

Closed-chest microdialysis to measure antibiotic penetration into human lung tissue

The majority of bacterial lung infections are localized to the interstitial space fluid, which is therefore an important target site for antimicrobial chemotherapy. Direct measurement of interstitial concentrations of antimicrobial agents in human lung tissue would allow for a more informed approach to appropriate dosing of antimicrobial agents, but until now this was beyond technical reach. In this exploratory pharmacokinetic study, we measured the time versus concentration profile of cefpirome after a single intravenous dose administration of 2 g in the lung interstitial fluid by flexible microdialysis catheters, which were implanted during lung surgery for pulmonary tumors in five patients. Cefpirome concentrations in lung interstitial fluid were 66% of corresponding plasma values within the first 240 min, and exceeded minimal inhibitory concentrations of most relevant bacteria. The experimental procedure was well tolerated by the patients and no adverse events were observed. The present study provides evidence for the first time that closed chest microdialysis of the human lung is a feasible and safe method to measure lung concentrations in patients in vivo. The present data also corroborate the use of cefpirome as a valuable agent in the treatment of lung infections with most extracellular bacteria.

Validation of subcutaneous microdialysis sampling for pharmacokinetic studies of flurbiprofen in the rat

The objective of this study was to validate subcutaneous (sc) microdialysis sampling to study flurbiprofen pharmacokinetics and plasma protein binding in the awake freely moving rat. A linear microdialysis probe was manufactured using a Hemophane hollow fiber which was tested in vitro and in vivo for the recovery of flurbiprofen and naproxen used as retrodialysis marker. Flurbiprofen was administered intraperitoneally and intravenously at a dose of 20 mg/kg in rats. In both cases, conventional blood sampling and sc microdialysis sampling were simultaneously performed. The microdialysates were analyzed on-line by high-pressure liquid chromatography. Naproxen, which was shown to have a similar in vivo loss by retrodialysis as flurbiprofen (71.5 +/- 0.9% and 71.0 +/- 0.8% respectively, n = 3), was used to continuously monitor probe recovery. Concentration-dependent protein binding of flurbiprofen was demonstrated in vivo based on experiments with a simultaneous sc microdialysis and blood sampling. Values of unbound fraction were similar to those reported previously by intravenous microdialysis sampling, demonstrating that the sc unbound concentrations are very similar to those in the central compartment. There was no significant difference among pharmacokinetic parameters (AUC, CL, t(1/2z), Vd) for total or unbound flurbiprofen determined after intraperitoneal and intravenous administration. Subcutaneous microdialysis is a simple yet powerful tool to study the pharmacokinetics and the in vivo plasma protein binding of flurbiprofen in the awake unrestrained rat.

Comparison of tissue concentrations after intramuscular and topical administration of ketoprofen

PURPOSE

To assess whether topical ketoprofen, which has been reported to provide analgesic effects in clinical studies, reaches predictable tissue concentrations high enough to account for the reported analgesia. Intramuscular ketoprofen was used as positive control.

METHODS

Muscle and subcutaneous tissue concentrations were assessed by microdialysis. Plasma and tissue concentrations after intramuscular injection were described using a three-compartment population pharmacokinetic model. The prediction performance of the model was assessed by superimposing tissue concentrations of 12 subjects that did not participate in the present study.

RESULTS

Most dialysate concentrations after topical dosing of ketoprofen (100 mg) were below the quantification limit of 0.47 ng/ml. Plasma concentrations increased slowly and reached an apparent plateau of 7-40 ng/ml at 10-12h. No decline was observed up to 16 h. Tissue concentrations after intramuscular injection (100 mg) were about 10 times higher than those after topical dosing. Tissue concentrations measured in the majority of the 12 subjects that did not participate in the present study were found within the range of two-thirds of the predicted concentrations.

CONCLUSION

Predictable and cyclooxygenase-inhibiting concentrations of ketoprofen were achieved in subcutaneous and muscle tissue after intramuscular but not after topical dosing. Thus, the tissue concentrations of ketoprofen after topical administration can hardly explain the reported clinical efficacy of topical ketoprofen.

Microdialysis in peripheral tissues

The objective of this review is to survey the recent literature regarding the applications of microdialysis in pharmacokinetic studies and facilitating many other studies in peripheral tissues such as muscle, subcutaneous adipose tissue, heart, lung, etc. It has been reported extensively that microdialysis is a useful technique for monitoring free concentrations of compounds in extracellular fluid (ECF), and it is gaining popularity in pharmacokinetic and pharmacodynamic studies, both in experimental animals and humans. The first part of this review discusses the use of microdialysis technique for ECF sampling in peripheral tissues in animal studies. The second part of the review describes the use of microdialysis for ECF sampling in peripheral tissues in human studies. Microdialysis has been applied extensively to measure both endogenous and exogenous compounds in ECF. Of particular benefit is the fact that microdialysis measures the unbound concentrations in the peripheral tissue fluid which have been shown to be responsible for the pharmacological effects. With the increasing number of applications of microdialysis, it is obvious that this method will have an important place in studying drug pharmacokinetics and pharmacodynamics.

Microdialysis in clinical drug delivery studies

The introduction of in vivo microdialysis (MD) to clinical pharmacological studies has opened the opportunity to obtain previously inaccessible information about the drug distribution process to the clinically relevant target site. The aim of this review is to provide a comprehensive overview of the current literature about MD in drug delivery studies from a clinical perspective. In particular the application of MD in clinical--antimicrobial, oncological and transdermal--and neurological research will be described and the scope of MD in pharmacokinetic-pharmacodynamic (PK-PD) studies will be discussed. It is concluded that MD has a great potential for both academic and industrial research, and may become the method of choice for drug distribution studies in humans.

Blood microdialysis in pharmacokinetic and drug metabolism studies

Microdialysis is a sampling technique allowing measurement of endogenous and exogenous substances in the extracellular fluid surrounding the probe. In vivo microdialysis sampling offers several advantages over conventional methods of studying the pharmacokinetics and metabolism of xenobiotics, both in experimental animals and humans. In the first part of this review article various practical aspects related to blood microdialysis will be discussed, such as: probe design, surgical implantation techniques, methods to determine the in vivo relative recovery of the analyte of interest by the probe, special analytical considerations related to small volume microdialysate samples, and pharmacokinetic calculations based on microdialysis data. In the second part of this review a few selected applications of in vivo microdialysis sampling to investigate pharmacokinetic processes are briefly discussed: determination of in vivo plasma protein binding in small laboratory animals, distribution of drugs across the blood-brain barrier, the use of microdialysis sampling to study biliary excretion and enterohepatic cycling, blood microdialysis sampling in man and in the mouse, and in vivo drug metabolism studies.

Surgery and intensive care procedures affect the target site distribution of piperacillin

OBJECTIVE

Therapeutic failure of antibiotic therapy has been ascribed to pharmacokinetic alterations in compromised patient populations. The present study, therefore, aimed at examining the influences of cardiac surgery and intensive care procedures on the postoperative target site distribution of piperacillin. For this purpose, the penetration of piperacillin to the interstitial space fluid, the relevant target site for most bacterial infections, was compared between patients after aortic valve replacement and healthy volunteers.

DESIGN

Comparative study in two study populations.

SETTING

The intensive care unit and research ward of a university hospital.

PATIENTS

The study population included six otherwise healthy patients scheduled to undergo aortic valve replacement and a control group of six healthy male volunteers.

INTERVENTIONS

After the administration of a single i.v. infusion of 4.0 g piperacillin, free piperacillin concentrations were measured in the interstitium of skeletal muscle and subcutaneous tissue by in vivo microdialysis and in venous serum. Piperacillin concentrations were assayed with reversed phase high-performance liquid chromatography.

MEASUREMENTS AND MAIN RESULTS

Interstitial piperacillin concentrations in muscle and subcutaneous adipose tissue were significantly lower in patients compared with volunteers with the area under the curve for the interstitium/area under the curve for serum concentration ratios ranging from 0.25 to 0.27 and from 0.43 to 1.22 in patients and volunteers, respectively (p < .05 between groups). The terminal elimination half-life was markedly prolonged in patients, leading to a concomitant increase in t > minimal inhibitory concentration (MIC) values, the relevant surrogate for therapeutic success of therapy with beta-lactam antibiotics, for strains with MIC50 <4 microg/mL. For strains with MIC50 >20 microl/mL, however, inadequate target site concentrations were attained in the patient population.

CONCLUSIONS

During the postoperative and intensive care periods, target site concentrations of piperacillin are markedly altered and decreased. This may also be true for other antibiotic agents and may have clinical implications in that current dosing guidelines may result in inadequate target site concentrations for high-MIC strains. Conceivably, this could lead to therapeutic failure in some patients.

Simultaneous blood and brain sampling of cephalexin in the rat by microdialysis and microbore liquid chromatography: application to pharmacokinetics studies

To circumvent the need for laborious sample clean-up and multiple blood sampling, a system was developed consisting of on-line microdialysis coupled to microbore liquid chromatography and ultraviolet detection. The system was designed for the simultaneous and continuous monitoring of unbound blood and brain cephalexin in the rat following single bolus intravenous administrations (10 mg/kg, n = 6). Microdialysis probes were inserted into the jugular vein and brain striatum, respectively, for blood and brain sampling. Chromatographic conditions consisted of a mobile phase of methanol-100 mM monosodium phosphoric acid (20:80, v/v, pH 5.0) pumped through a microbore reversed-phase column at a flow-rate of 0.05 ml/min. Detection wavelength was set at 260 nm. The method was validated for response linearity as well as intra- and inter-day variabilities. Rapid appearance of cephalexin in the striatal dialysate suggested good blood-brain barrier penetration. This study provided pharmacokinetics information for cephalexin as well as demonstrated the applicability of this continuous sampling method for pharmacokinetics studies.

Assessment of pharmacodynamic and pharmacokinetic characteristics of drugs using microdialysis sampling and capillary electrophoresis

Microdialysis sampling combined with capillary electrophoresis is emerging as a new approach in drug studies. It allows the continuous monitoring, in vivo or in vitro, of changes in free endogenous compounds as well as in drug substances, following the administration of pharmacological agents. The low volume requirement of capillary electrophoresis for injection allows the collection of dialysates during short sampling times, leading to a precise temporal description of drug-induced biochemical changes or pharmacokinetics. Various protocols can be used for analyzing endogenous compounds and drug substances in microdialysis samples. Capillary electrophoresis with laser-induced fluorescence detection often affords the high sensitivity level which is needed in most studies. Furthermore, the direct on-line coupling of microdialysis, derivatization of samples, and electrophoretic analysis now brings a separation-based biosensor, allowing a real-time description of chemical events with a high molecular specificity. Microdialysis sampling combined with capillary electrophoresis has recently been used to assess pharmacodynamic and pharmacokinetic characteristics of various drugs in animal studies; it may also represent a new approach in clinical pharmacology in the near future.