Antipyrine as a dialyzable reference to correct differences in efficiency among and within sampling devices during in vivo microdialysis

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.

A Review on Microdialysis Calibration Methods: the Theory and Current Related Efforts

Microdialysis is a sampling technique first introduced in the late 1950s. Although this technique was originally designed to study endogenous compounds in animal brain, it is later modified to be used in other organs. Additionally, microdialysis is not only able to collect unbound concentration of compounds from tissue sites; this technique can also be used to deliver exogenous compounds to a designated area. Due to its versatility, microdialysis technique is widely employed in a number of areas, including biomedical research. However, for most in vivo studies, the concentration of substance obtained directly from the microdialysis technique does not accurately describe the concentration of the substance on-site. In order to relate the results collected from microdialysis to the actual in vivo condition, a calibration method is required. To date, various microdialysis calibration methods have been reported, with each method being capable to provide valuable insights of the technique itself and its applications. This paper aims to provide a critical review on various calibration methods used in microdialysis applications, inclusive of a detailed description of the microdialysis technique itself to start with. It is expected that this article shall review in detail, the various calibration methods employed, present examples of work related to each calibration method including clinical efforts, plus the advantages and disadvantages of each of the methods.

Microdialysis based device for continuous extravascular monitoring of blood glucose

Glycemic control of intensive care patients can be beneficial for this patient group but the continuous determination of their glucose concentration is challenging. Current continuous glucose monitoring systems based on the measurement of interstitial fluid glucose concentration struggle with sensitivity losses, resulting from biofouling or inflammation reactions. Their use as decision support systems for the therapeutic treatment is moreover hampered by physiological time delays as well as gradients in glucose concentration between plasma and interstitial fluid. To overcome these drawbacks, we developed and clinically evaluated a system based on microdialysis of whole blood. Venous blood is heparinised at the tip of a double lumen catheter and pumped through a membrane based micro-fluidic device where protein-free microdialysate samples are extracted. Glucose recovery as an indicator of long term stability was studied in vitro with heparinised bovine blood and remained highly stable for 72 h. Clinical performance was tested in a clinical trial in eight healthy volunteers undergoing an oral glucose tolerance test. Glucose concentrations of the new system and the reference method correlated at a level of 0.96 and their mean relative difference was 1.9 +/- 11.2%. Clinical evaluation using Clark's Error Grid analysis revealed that the obtained glucose concentrations were accurate and clinically acceptable in 99.6% of all cases. In conclusion, results of the technical and clinical evaluation suggest that the presented device delivers microdialysate samples suitable for accurate and long term stable continuous glucose monitoring in blood.

Investigation of microdialysis sampling calibration approaches for lipophilic analytes: doxorubicin

Microdialysis is an important sampling technique in many pharmacokinetics and pharmacological studies. Applying microdialysis to lipophilic analytes can be difficult as low extraction efficiencies are generally obtained with these types of analytes. In this investigation, the effects of applying microdialysis to the lipophilic compound, doxorubicin are discussed. Using varying concentrations of doxorubicin (DOX) from 1 to 20 microM, in vitro probe calibrations were performed by delivery, recovery and no-net flux. Any changes in the extraction efficiencies calculated were monitored through the addition of an internal standard, antipyrine. DOX was chosen as a representative lipophilic analyte because its red color could be visibly monitored on the probe. At higher concentrations the probe membrane became redder. For delivery experiments, the inlet of the probe was more highly colored than the outlet. The opposite was true for recovery experiments, in which the outlet was more highly colored than the inlet. It was observed that while antipyrine was well-behaved in these experiments, for DOX the extraction efficiency determined by recovery was not the same as the extraction efficiency determined by delivery (p<0.005, 0.05). It was further observed that for DOX the extraction efficiency determined by a no-net flux experiment was in good agreement with the value determined by delivery and not that determined by recovery. However, the only point in which no DOX was present in the perfusate was not on the no-net flux line. In addition, the transport of DOX across the microdialysis membrane was considerably slower than the transport of antipyrine.

Long-term calibration considerations during subcutaneous microdialysis sampling in mobile rats

The level at which implanted sensors and sampling devices maintain their calibration is an important research area. In this work, microdialysis probes with identical geometry and different membranes, polycarbonate/polyether (PC) or polyethersulfone (PES), were used with internal standards (Vitamin B(12) (MW 1355), antipyrine (MW 188) and 2-deoxyglucose (2-DG, MW 164)) and endogenous glucose to investigate changes in their long-term calibration after implantation into the subcutaneous space of Sprague-Dawley rats. Histological analysis confirmed an inflammatory response to the microdialysis probes and the presence of a collagen capsule. The membrane extraction efficiency (percentage delivered to the tissue space) for antipyrine and 2-DG was not altered throughout the implant lifetime for either PC- or PES membranes. Yet, Vitamin B(12) extraction efficiency and collected glucose concentrations decreased during the implant lifetime. Antipyrine was administered i.v. and its concentrations obtained in both PC- and PES-membrane probes were significantly reduced between the implant day and seven (PC) or 10 (PES) days post-implantation suggesting that solute supply is critical for in vivo extraction efficiency. For the low molecular weight solutes such as antipyrine and glucose, localized delivery is not affected by the foreign body reaction, but recovery is significantly reduced. For Vitamin B(12), a larger solute, the fibrotic capsule formed around the probe significantly restricts diffusion from the implanted microdialysis probes.

Quantitative microdialysis using modified ultraslow microdialysis: direct rapid and reliable determination of free brain concentrations with the MetaQuant technique

The only method to quantify free extracellular levels of drugs in the brain of living animals is microdialysis. However, quantitative microdialysis has been hampered by methodological issues for decades. The problems arise from the need to establish the in vivo recovery for appropriate quantitation. In dealing with these issues the "dynamic no-net-flux" (DNNF) method seemed to be the experimental method of choice. Major disadvantages were, however, the need for a very high degree of bioanalytical precision and accuracy and the need for a large number of animals. Moreover, today we know that the experimental data are not always straightforward. To improve robustness and practicality of quantitative microdialysis sampling we modified the ultraslow microdialysis approach. Ultraslow microdialysis uses very low microdialysis flow rates (<200 nl/min) which increase recovery (both in vivo and in vitro) to over 90%. However, new practical issues arise when attempting to work with these flow rates. The resulting very low volumes and long lag times make this method very impractical for general application. In the modified version, addition of a carrier flow after the dialysis process has been completed, which negates the problems of long lag times and low volumes. The resulting dilution of the dialysis sample concentration can simply be mathematically corrected. In the current study we measured the free brain levels of two CNS compounds using the classic DNNF and the new modified ultraslow dialysis method. Modified ultraslow microdialysis was shown to generate robust data with the use of only small numbers of rats. The method is a promising tool for common straightforward screening of blood-brain barrier penetration of compounds into the brain.

The use of a deuterated calibrator for in vivo recovery estimations in microdialysis studies

One of the crucial issues in quantitative microdialysis is the reliability of recovery estimates to correctly estimate unbound drug tissue concentrations. If a deuterated calibrator is used for retrodialysis, the calibrator has the same properties as the study drug. However, recovery of the calibrator may be affected by the presence of the drug in the tissues. The aim of this study was to investigate the recovery of deuterated morphine with time in the absence and presence of morphine in rat tissues. Microdialysis probes were placed in the brain and blood of eight rats. Ringer's solution containing D3-morphine was perfused throughout the study and recovery was estimated. After a stabilization period of 3 h, an exponential infusion of morphine was administered over 4 h. The presence of morphine did not affect the recovery of D3-morphine from brain or blood. The average recovery values (SD) were 0.145 (0.039) and 0.131 (0.048) during the stabilization and infusion periods, respectively, for the brain probe and 0.792 (0.055) and 0.790 (0.084), respectively, for the blood probe. The recovery of deuterated morphine was stable over time in the brain and in blood, and was not affected by the presence of pharmacologically concentrations of morphine.

Development of tissue-targeted metabonomics. Part 1. Analytical considerations

Tissue-targeted metabonomics provides tissue specific metabolic information while still retaining the profiling approach of traditional metabonomics. Microdialysis sampling is used to generate site-specific samples of endogenous metabolites. The dialysate samples are subjected to proton NMR analysis with data analysis by principal components analysis and partial least squares regression. In this study, sample and data pretreatment methods were examined for their impact on the quality of the data analysis. Specifically, the effects of speed vacuuming, sample solubility, sample pH stability, and sample storage stability were examined. Data pretreatment methods examined included the effects of standardization and normalization to internal standards. In addition, the ability of tissue-targeted metabonomics to generate time trend data was explored and more fully characterized using principal components analysis and partial least squares regression.

Method for improved accuracy in endogenous urea recovery marker calibrations for microdialysis in tumors

INTRODUCTION

Urea has been proposed as an endogenous recovery marker for microdialysis for absolute concentration calculations of analytes in microdialysis samples. Previously we demonstrated a linear relationship between urea concentrations in a rat mammary carcinoma and that in plasma, validating its use as a recovery marker for that particular tumor. In this paper, we have extended the validation to two other tumor lines, thereby providing confidence that the calibration is constant across tumor types. To improve the accuracy in the determination of the plasma/tumor urea relationship from no net flux calibrations, we extended the range of the calibration by adding exogenous urea to tumor bearing animals. This method enabled more accurate calculations of absolute recovery from plasma and dialysate urea concentrations. We confirm that by using this method the calibration is valid across three different tumor lines. The existence of a common calibration between tumors provides rationale for using plasma urea as a recovery marker for clinical trials. The existence of a common calibration between tumor types bypasses the need to perform time consuming calibrations for each patient. This makes the procedure much more practical for clinical studies.

METHODS

The no net flux technique was used to determine the plasma vs. tumor urea relationship for the R3230Ac mammary carcinoma, 9 L glioma, and a fibrosarcoma (FSa), grown in Fischer 344 rats. Plasma urea was stably increased beyond the normally occurring concentration for some of the data points by subcutaneous bolus administration to extend the range of data for the no net flux calibration.

RESULTS

Urea recovery was unaffected by plasma urea concentration and was consistent with other reported values. The relationship between plasma and tumor urea was fit by a line, and linear regressions of the data with the extended plasma urea range had better R2 values than we reported previously. Statistical comparison of the regressions suggests that within reasonable uncertainty limits, they are the same for the different tumor types.

DISCUSSION

Increasing the plasma urea concentration range for no net flux calibrations of urea as an endogenous recovery marker in tumors resulted in more accurate determination of the plasma/tumor urea relationship. A single linear regression may describe the relationship between plasma and tumor urea concentration across tumor lines for a given set of microdialysis parameters.

In vivo microdialysis for PK and PD studies of anticancer drugs

In vivo microdialysis technique has become one of the major tools to sample endogenous and exogenous substances in extracellular spaces. As a well-validated sampling technique, microdialysis has been frequently employed for quantifying drug disposition at the desired target in both preclinical and clinical settings. This review addresses general methodological considerations critical to performing microdialysis in tumors, highlights selected preclinical and clinical studies that characterized drug disposition in tumors by the use of microdialysis, and illustrates the potential application of microdialysis in the assessment of tumor response to cancer treatment.

Assaying protein unbound drugs using microdialysis techniques

Compared with traditional sampling methods, microdialysis is a technique for protein unbound drug sampling without withdrawal of biological fluids and involving minimal disturbance of physiological function. Conventional total drug sample consists of unbound drugs and protein bound drugs, which are loosely bound to plasma proteins such as albumin and alpha-1 acid glycoprotein, forming an equilibrium ratio between bound and unbound drugs. However, only the unbound fraction of drug is available for absorption, distribution, metabolism and elimination, and delivery to the target sites for pharmacodynamic actions. Although several techniques have been used to determine protein unbound drugs from biological fluids, including ultrafiltration, equilibrium dialysis and microdialysis, only microdialysis allows simultaneous sampling of protein unbound chemicals from plasma, tissues and body fluids such as the bile juice and cerebral spinal fluid for pharmacokinetic and pharmacodynamic studies. This review article describes the technique of microdialysis and its application in pharmacokinetic studies. Furthermore, the advantages and limitations of microdialysis are discussed, including the detailed surgical techniques in animal experiments from rat blood, brain, liver, bile duct and in vitro cell culture for unbound drug analysis.

Validation of a new intraarterial microdialysis shunt probe for the estimation of pharmacokinetic parameters

The aim of our study was to compare pharmacokinetic parameters of a highly bound protein drug, irbesartan, obtained from microdialysis data (MD) of arterial blood and conventional blood samples (BS). A new vascular shunt microdialysis probe was inserted into the carotid artery and one femoral vein was cannulated for i.v. administration of irbesartan. Microdialysis samples were collected every 15 min. Blood samples were taken every 15 min. Levels of drug were measured by HPLC. Pharmacokinetic parameters were estimated using TOPFIT program. Corrected MD were compared with BS taken at same time to determine protein binding. The irbesartan protein binding did not change during the experiment. The estimated Ke from MD and BS were similar (MD: 1.8+/-0.3 h(-1), n=5; BS: 1.7+/-0.2 h(-1), n=5). After protein binding correction for the MD, the estimated values of volume of distribution (Vd) (MD: 1.2+/-0.4 l, n=5; BS: 1.1+/-0.4 l, n=5), clearance (Cl) (MD: 32.3+/-7.3 ml min(-1), n=5; BS: 30.7+/-8.2 ml min(-1), n=5) and AUC (MD: 7.7+/-3.2 microg x ml(-1) h, n=5; BS: 8.8+/-3.4 microg x ml(-1) h, n=5) were similar between MD and BS. In conclusion, these results show that our new probe inserted in the carotid artery provides accurate MD to estimate pharmacokinetic parameters of a highly bound protein drug like irbesartan. On the other hand, MD were also useful to the in vivo study of drug protein binding and saturation in protein binding.

The technique of intracerebral microdialysis

Intracerebral microdialysis is an increasingly popular experimental technique. A brief description of the principles of microdialysis is presented and the terms relevant to the procedure are defined. The methodology involved in conducting intracerebral microdialysis is described in detail. Factors influencing the outcome of analysis such as external stimuli, perfusion fluid, perfusion rate, temperature, probe placement, membrane characteristics, and timing of sample collection are discussed. The importance of maintaining the uniformity of the above-mentioned factors is stressed.

Application of microdialysis to characterize drug disposition in tumors

Microdialysis is an in vivo sampling technique that was initially developed to measure endogenous substances in the field of neurotransmitter research. In the past decade, microdialysis has been increasingly applied to study the pharmacokinetics and drug metabolism in the blood and various tissues of both animals and humans. This paper describes the general aspects of this in vivo sampling technique followed by the survey of the recent papers regarding the application of microdialysis to characterize anticancer drug disposition in solid tumors. It can be concluded that microdialysis is a very suitable method to obtain drug concentration-time profiles in the interstitial fluid of solid tumors as well as of other variety of tissues.

Methodological issues in microdialysis sampling for pharmacokinetic studies

Microdialysis is an in vivo technique that permits monitoring of local concentrations of drugs and metabolites at specific sites in the body. Microdialysis has several characteristics, which makes it an attractive tool for pharmacokinetic research. About a decade ago the microdialysis technique entered the field of pharmacokinetic research, in the brain, and later also in peripheral tissues and blood. Within this period much has been learned on the proper use of this technique. Today, it has outgrown its child diseases and its potentials and limitations have become more or less well defined. As microdialysis is a delicate technique for which experimental factors appear to be critical with respect to the validity of the experimental outcomes, several factors should be considered. These include the probe; the perfusion solution; post-surgery interval in relation to surgical trauma, tissue integrity and repeated experiments; the analysis of microdialysate samples; and the quantification of microdialysate data. Provided that experimental conditions are optimized to give valid and quantitative results, microdialysis can provide numerous data points from a relatively small number of individual animals to determine detailed pharmacokinetic information. An example of one of the added values of this technique compared with other in vivo pharmacokinetic techniques, is that microdialysis reflects free concentrations in tissues and plasma. This gives the opportunity to assess information on drug transport equilibration across membranes such as the blood-brain barrier, which already has provided new insights. With the progress of analytical methodology, especially with respect to low volume/low concentration measurements and simultaneous measurement of multiple compounds, the applications and importance of the microdialysis technique in pharmacokinetic research will continue to increase.

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.

In vivo microdialysis sampling: theory and applications

During the last two decades, a number of methods have been developed for in vivo collection, separation and characterization of biological samples and analytes. The capability and reliability of the microdialysis technique for measuring endogenous substances (such as neurotransmitters and their metabolites) as well as exogenous therapeutic agents in various tissue systems have brought it to the forefront of the in vivo tissue sampling methods. The usability of this technique is demonstrated by its application as reported in almost 3600 scientific papers (as of January 1998). This paper describes the general aspects and various applications of this fast growing technique. Emphasis has been given to analytical considerations with regards to microdialysis probe recovery and newer HPLC techniques.

Microdialysis for pharmacokinetic analysis of drug transport to the brain

The intracerebral microdialysis technique represents an important tool for monitoring free drug concentrations in brain extracellular fluid (brain(EcF)) as a function of time. With knowledge of associated free plasma concentrations, it provides information on blood-brain barrier (BBB) drug transport. However, as the implantation of the microdialysis probe evokes tissue reactions, it should be established if the BBB characteristics are maintained under particular microdialysis experimental conditions. Several studies have been performed to evaluate the use of intracerebral microdialysis as a technique to measure drug transport across the BBB and to measure regional pharmacokinetics of drugs in the brain. Under carefully controlled conditions, the intracerebral microdialysis data did reflect passive BBB transport under normal conditions, as well as changes induced by hyperosmolar opening or by the presence of a tumor in the brain. Studies on active BBB transport by the mdr1a-encoded P-glycoprotein (Pgp) were performed, comparing mdr1a(-/-) with wild-type mice. Microdialysis surgery and experimental procedures did not affect Pgp functionality, but the latter did influence in vivo concentration recovery, which was in line with theoretical predictions. It is concluded that intracerebral microdialysis provides meaningful data on drug transport to the brain, only if appropriate methods are applied to determine in vivo concentration recovery.

The design and validation of a novel intravenous microdialysis probe: application to fluconazole pharmacokinetics in the freely-moving rat model

PURPOSE

The purpose of this study was to design and validate a concentric, flexible intravenous microdialysis probe to determine drug concentrations in blood from the inferior vena cava of a freely-moving animal model.

METHODS

An intravenous microdialysis probe was constructed using fused-silica tubing and an acrylonitrile/sodium methallyl sulfonate copolymer hollow fiber. The probe was tested in vitro for the recovery of fluconazole and UK-54,373, a fluconazole analog used for probe calibration by retrodialysis. Subsequent in vivo validation was done in rats (n = 7) that had a microdialysis probe inserted into the inferior vena cava via the femoral vein, and the femoral artery was cannulated for simultaneous blood sampling. Comparisons of fluconazole pharmacokinetic parameters resulting from the two sampling methods were performed at 2 and 10 days after probe implantation.

RESULTS

There were no statistical differences between the microdialysis sampling and conventional blood sampling methods for the T1/2, Cl, Vdss, and dose-normalized AUC by paired t-test (p > 0.05) for repeated dosing at day 2 and day 10 after probe placement. The probe recovery, as determined by retrodialysis, significantly decreased over the ten day period. This finding indicates the necessity for frequent recovery determinations during a long-term blood microdialysis experiment.

CONCLUSIONS

These results show that microdialysis sampling in the inferior vena cava using this unique and robust probe design provides an accurate method of determining blood pharmacokinetics in the freely-moving rat for extended experimental periods. The probe design allows for a simple surgical placement into the inferior vena cava which results in a more stable animal preparation for long-term sampling and repeated-measures experimental designs.

Aluminum citrate is transported from brain into blood via the monocarboxylic acid transporter located at the blood-brain barrier

Aluminum citrate transport across the blood-brain barrier was assessed in rats by in vivo microdialysis. Microdialysis probes were implanted in the jugular vein as well as the left and right frontal cortex. It was demonstrated previously (Allen et al., 1995), in this study, that the steady-state aluminum citrate brain-to-blood-ratio (BBr) is less than 1, suggesting the presence of a process other than diffusion. The addition of 2,4-dinitrophenol (10 microM) to the dialysate perfusing a microdialysis probe in the brain increased the steady-state aluminum citrate brain-to-blood-ratio to a value (1.14) not significantly different from 1, suggesting the presence of an active transporter that is blocked by the metabolic inhibitor. The addition of valproic and pyruvic acid, as putative and known substrates for the monocarboxylic acid transporter, respectively, to brain dialysate (10 and 100 mM) had different outcomes. Valproic acid was ineffective at either concentration, whereas pyruvic acid (100 mM) significantly increased the aluminum citrate brain-to-blood-ratio from 0.19 to 0.31. Pyruvic acid (1 M in the dialysate) increased the aluminum citrate brain-to-blood-ratio to a value not different from unity, suggesting competition between aluminum citrate and pyruvic acid for transport. The only energy-dependent, pyruvic acid-inhibitable transporter is the monocarboxylic acid transporter. Theoretical, pharmacokinetic modeling suggests that the transporter producing an aluminum citrate brain-to-blood-ratio less than 1 is predominantly located at the blood-brain barrier, rather than at neuronal or glial cell membranes. We propose that the monocarboxylic acid transporter at the blood-brain barrier maintains a steady-state aluminum citrate brain-to-blood-ratio much less than 1.

Application of microdialysis in pharmacokinetic studies

The objective of this review is to survey the recent literature regarding the various applications of microdialysis in pharmacokinetics. Microdialysis is a relatively new technique for sampling tissue extracellular fluid that is gaining popularity in pharmacokinetic and pharmacodynamic studies, both in experimental animals and humans. The first part of this review discusses various aspects of the technique with regard to its use in pharmacokinetic studies, such as: quantitation of the microdialysis probe relative recovery, interfacing the sampling technique with analytical instrumentation, and consideration of repeated procedures using the microdialysis probe. The remainder of the review is devoted to a survey of the recent literature concerning pharmacokinetic studies that apply the microdialysis sampling technique. While the majority of the pharmacokinetic studies that have utilized microdialysis have been done in the central nervous system, a growing number of applications are being found in a variety of peripheral tissue types, e.g. skin, muscle, adipose, eye, lung, liver, and blood, and these are considered as well. Given the rising interest in this technique, and the ongoing attempts to adapt it to pharmacokinetic studies, it is clear that microdialysis sampling will have an important place in studying drug disposition and metabolism.

Hippocampal acetylcholine increases during eyeblink conditioning in the rabbit

The classically conditioned rabbit nictitating membrane reflex (NMR) is modulated by the septohippocampal cholinergic system. Disruption of this system retards NMR acquisition. Aluminium (Al) is a neurotoxin that interferes with hippocampal acetylcholine (ACh) synthesis and release. Using microdialysis, this study tested the hypothesis that NMR acquisition in the rabbit is associated with hippocampal ACh release. This was conducted by measuring ACh release in control and A1-intoxicated rabbits during NMR training. NMR training consisted of four sessions of 100 conditioning trials/session in a delay paradigm. The percentage of conditioned responses (CRs) increased with each conditioning session for both groups, although percent CRs was significantly greater in the control group. Acetylcholine release in the ventral hippocampus increased significantly over baseline in the control group during the second and third conditioning sessions. In the Al-intoxicated group, ACh release did not increase significantly during any conditioning session. A separate group of rabbits was pseudoconditioned, receiving the same conditioning stimuli, although explicitly unpaired. This group did not acquire the CR. Acetylcholine release did not significantly increase during any conditioning session, suggesting that the increase in ACh release observed in the control group was not merely a product of conditioning stimuli presentation. The lack of increased ACh release in the Al-intoxicated rabbits was associated with a CR acquisition deficit. The results of this study are consistent with a role of hippocampal cholinergic function in NMR acquisition in the rabbit.

Fluconazole distribution to the brain: a crossover study in freely-moving rats using in vivo microdialysis

PURPOSE

The purpose of this study was to determine if the microdialysis sampling technique is feasible to study the central nervous system distributional kinetics of a novel triazole antifungal agent, fluconazole, in an awake, freely-moving rat model, and to determine fluconazole distribution to the extracellular fluid (ECF) of the brain.

METHODS

The relative recovery of the microdialysis probes (CMA-12) was determined in vitro and in vivo by retrodialysis using UK-54,373, a fluorinated analog of fluconazole. Sprague-Dawley rats received 10 mg/kg and 20 mg/kg fluconazole IV bolus doses in a crossover design, and brain extracellular fluid fluconazole concentrations were monitored using microdialysis and on-line HPLC analysis. The plasma fluconazole concentration vs. time data were determined using sequential blood sampling and HPLC analysis.

RESULTS

There was no statistical difference between relative probe recoveries for both fluconazole and UK-54,373, either in vitro or in vivo, and probe recoveries did not change during the course of the in vivo crossover experiment. Fluconazole rapidly distributes into in the brain ECF and the average brain distribution coefficient (brain/plasma AUC ratio) was 0.60 +/- 0.18 and was independent of dose. Plasma pharmacokinetic parameters were linear in the dose range studied.

CONCLUSIONS

Fluconazole rapidly reaches a distributional equilibrium between brain extracellular fluid and plasma, and the distribution to the brain is substantial and not dependent on dose over a two-fold range. Furthermore, the results indicate that microdialysis utilizing UK-54,373 as the in vivo retrodialysis probe calibrator is a feasible method to study the transport of fluconazole into the central nervous system.

The pharmacokinetics and blood-brain barrier permeation of the chelators 1,2 dimethly-, 1,2 diethyl-, and 1-[ethan-1'ol]-2-methyl-3-hydroxypyridin-4-one in the rat

The 3-hydroxypyridin-4-ones (HPs) are iron and aluminum chelators. Their ability to enter the brain had not previously been directly determined. To determine whether they cross the blood-brain barrier (BBB), three HPs possessing a wide range of lipophilicity were examined: 1-[ethan-1'ol]-2-methyl-HP (CP40), 1,2-dimethyl-HP (CP20, L1, deferiprone), and 1,2-dimethyl-HP (CP94, EL1NEt). Their pharmacokinetics were determined in rats to establish dosing parameters for microdialysis studies of BBB permeation. Studies were then conducted with microdialysis probes in the blood, frontal cortex, and lateral ventricle to determine the rate and extent of HP BBB permeability. All three HPs were detectable in brain dialysate samples collected 0-7 min after HP injection, demonstrating rapid entry into the brain. The extent of unbound distribution (an indicator of the mechanism of BBB permeation) was 0.9 and 1.2 for the frontal cortex and lateral ventricle for CP20, and was 1.1 and 1.6 for CP94, suggesting diffusion across the BBB. The extent of unbound distribution of CP40 was 0.2 for both the frontal cortex and lateral ventricle, suggesting the presence of a transporter moving it out of brain extracellular fluid. Introduction of cyanide into the brain did not affect the brain to blood CP40 ratio, suggesting that the transporter is not energy-dependent. Both CP94 and CP40 caused death due to respiratory failure, whereas CP20 did not. The ability of less toxic bidentate HP chelators, such as CP20, to enter the brain may enable their use in the treatment of metal-induced diseases and iron-facilitated oxidative injury involving the central nervous system.

Delayed elevation of platelet activating factor in ischemic hippocampus

We used in vivo microdialysis to define the chronological relationship between release of thromboxane and platelet activating factor (PAF) into the extracellular space of ischemic hippocampus. The thromboxane level peaked after 20 min of postischemic reperfusion, followed by a delayed PAF response 120 min later. We conclude that cerebral ischemia causes delayed elevation of PAF in the extracellular space, long after the immediate synthesis and release of thromboxane metabolites.

The use of microdialysis for the study of drug kinetics: some methodological considerations illustrated with antipyrine in rat frontal cortex

1. The neuropharmacokinetics of antipyrine, a readily dialysable drug, in rat frontal cortex were studied and the effect of sampling time and contribution of period sampling and dialysate dead volume investigated in relation to tmax, Cmax, AUC and t1/2 values. 2. After i.p. administration, antipyrine (35 mg kg-1, n = 5) concentrations rose rapidly in rat frontal cortex (tmax, 12 min) and then declined exponentially tmax, Cmax, AUC and t1/2 values were determined after 2 min dialysate sampling and compared to values obtained from simulated sampling times of 4, 6, 8, 10 and 20 min. 3. Antipyrine tmax and Cmax values were directly dependent on sampling frequency. Thus, mean 2 min sampling tmax and Cmax values were 63% lower and 27% higher, respectively, compared to 20 min sampling values. AUC and t1/2 values were unaffected. 4. Adjustment for dialysate dead volume (the volume of dialysate within the dialysis probe and sampling tube) reduced tmax values significantly but did not affect the other neuropharmacokinetic parameters. 5. Contribution of period sampling on neuropharmacokinetic parameters were investigated by comparing plots of antipyrine concentration data at midpoint and at endpoint of sampling time interval. Only tmax values were affected with values decreasing with increasing sampling time interval. 6. In conclusion, although microdialysis is a useful method for monitoring events at the extracellular level and for kinetic studies, it is important to understand its inherent characteristics so that data can be interpreted appropriately. Sampling frequency, particularly during monitoring of periods of rapid change, is very important since Cmax and tmax values will be significantly underestimated and overestimated respectively, if sampling time is longer rather than shorter. These considerations are particularly important in relation to microdialysis studies of pharmacokinetic-pharmacodynamic interrelationships and modelling.

Application of microdialysis to the pharmacokinetics of analgesics: problems with reduction of dialysis efficiency in vivo

Microdialysis in freely moving rats coupled to high-performance liquid chromatography (HPLC) was used to measure the free concentration of acetaminophen (APAP) in blood and cerebrospinal fluid (CSF) after an intravenous bolus dose (25 mg/kg). In vitro calibration of two commercially available probe types was performed in 0.9% NaCl solution and blood. The influence of these media on recovery was tested by retrodialysis. This technique was also used for in vivo calibration and to monitor the dynamics of the performance of implanted probes. The results were compared with data obtained from conventional sampling techniques of direct withdrawal of blood and CSF, and also with the results obtained by correcting dialysate concentrations using in vitro recovery values. The data demonstrate that whole blood lowers recovery not only by reducing the free concentration of drug, but also by directly influencing dialysis efficiency (mean reduction of recovery: 50.1%). By contrast, low transport capacities of CSF surrounding the implanted probe lead to suboptimal conditions and, therefore, to a reduction of in vivo recovery (mean reduction of recovery: 65.5%). After correction of recovery values using in vivo retrodialysis prior to dosing the animal, we obtained similar data as compared to conventional sampling techniques. These results demonstrate that microdialysis may provide a minimally invasive method to monitor the free concentrations of drugs, such as acetaminophen, in different compartments, and allow a multitude of pharmacokinetic data to be obtained from freely moving animals.

Microdialysis calibration using retrodialysis and zero-net flux: application to a study of the distribution of zidovudine to rabbit cerebrospinal fluid and thalamus

A retrodialysis (RD) method for the real-time calibration of on-line microdialysis (MD) procedures was investigated in vitro and in vivo. Calibration by retrodialysis was simultaneously validated through the use of a zero-net flux (ZNF) method, which assumes directional independence of diffusion of the solute. In RD, a calibrator with dialysance (PeA; effective permeability-surface area product) similar to that of the compound of interest is introduced into the perfusate. If the calibrator is suitable, its loss from the perfusate during RD is identical to the recovery of the solute of interest determined simultaneously by normal MD. Two antiviral nucleosides (AZT and AZdU) which differ structurally by only a methylene group were utilized as solute and calibrator, respectively. Both nucleosides exhibited similar recovery and loss at flow rates of 0.5 to 5 microL/min in vitro, indicating a similar PeA product in this flow domain. Furthermore, both compounds showed similar loss into the lateral ventricle or thalamus of rabbits (n = 4) during RD at a flow rate of 1 microL/min for 6 hr. The relative loss decreased rapidly within the first hour, reaching a relatively stable value after 2 hr. The significant reduction in the loss of AZdU and AZT in vivo compared with that in vitro likely results from a lower diffusion coefficient in tissue. The distribution of AZT between plasma and cerebrospinal fluid (CSF) in the ventricle and extracellular fluid (ECF) in thalamus was determined at steady state using calibration by RD and ZNF simultaneously.(ABSTRACT TRUNCATED AT 250 WORDS)

A survey on quantitative microdialysis: theoretical models and practical implications

The existing methods of quantitative microdialysis are reviewed. The methods are divided into 8 groups, depending on the mathematical models and theoretical principles used to describe convective diffusion in the extracellular space of the brain. Special emphasis is made to describe each method from a historical perspective, showing its main contribution to recent knowledge, as well as its limitations and drawbacks. It is concluded that those methods based on explicitly derived equations for in vivo recovery are still too approximative and not suitable for routine application. Therefore, empirical models based on varying perfusion flow rates or concentrations of substances in the perfusion solution, found several practical implications. Methods using a reference substance as a marker of in vivo recovery are also discussed. The paper stresses the increasing importance of methods allowing the quantitative evaluation of microdialysis data whenever measuring neurotransmitter release, drug concentrations or pharmacokinetic variables.

4-Trimethylammonium antipyrine: a quaternary ammonium nonradionuclide marker for blood-brain barrier integrity during in vivo microdialysis

The well-controlled microdialysis (MD) study of substance permeation into brain extracellular fluid (ECF) and cerebrospinal fluid requires consideration of blood-brain barrier (BBB) integrity, which might be compromised by microdialysis probe implantation. Others have assessed BBB integrity with radionuclide markers. A nonradionuclide marker may be desirable in many studies. A charged antipyrine analogue may be useful to determine BBB integrity with concomitant antipyrine characterization of probe efficiency (Yokel et al., 1992, J Pharmacol Toxicol Methods 27:135-142), and may not require another analytical technique. We synthesized, validated, and evaluated 4-trimethylammonium antipyrine (4TMA-AP) as a BBB integrity marker. BBB permeation was determined by calculation of a BBB integrity percentage (Pi) from brain/blood concentrations. The PiS of Evan's blue, which does not permeate the intact BBB, and 4TMA-AP were not significantly different in rats without known BBB disruption, suggesting a lack of 4TMA-AP permeation through the intact BBB. When MD probes were slowly implanted into the frontal cortex, 4TMA-AP PiS were usually zero. Intracarotid oleic acid injection to open the BBB significantly increased 4TMA-AP PiS, suggesting that 4TMA-AP entered brain ECF when the BBB was compromised. Rapid probe implantation produced increased 4TMA-AP PiS, suggesting BBB disruption. The predicted appearance of 4TMA-AP in brain ECF suggests that it is a BBB integrity marker.