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.     

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.     

A review of membrane sampling from biological tissues with applications in pharmacokinetics, metabolism and pharmacodynamics.

This review provides an overview of membrane sampling techniques, microdialysis and ultrafiltration, and cites illustrations of their applications in pharmacokinetics, metabolism and/or pharmacodynamics. The review organizes applications by target tissue and general type of information gleaned. It focuses on recently published microdialysis studies (1999 to this writing) and offers the first review of ultrafiltration sampling studies. The advantages and limitations of using microdialysis and ultrafiltration sampling as tools for obtaining pharmacokinetic and metabolism data are discussed. Numerous examples are described including studies in which several types of data are collected simultaneously. Reports that study local metabolism of drug delivered through the probe are also presented.     

Microdialysis in drug discovery

Microdialysis has been developed during the last 25 years by several authors primarily to study brain function and changes in levels of endogenous compounds such as neurotransmitters or metabolites. The development of microdialysis for the purpose of measuring drugs was initiated during the late eighties. This technique provides a means of continuous plasma sampling without repeated blood sampling and the applicability to the study of drug metabolism and pharmacokinetics in experimental animals and human. Also, the microdialysis technique allows the study of plasma protein binding and the saturation of protein binding. The implantation of the microdialysis probe in other tissues and organs, like central nervous system, adipose tissue and heart, allows the study of drug distribution. On the other hand, the measurement of endogenous substances using the microdialysis technique permits the study of the effect of drugs on neurotransmission and metabolism. Moreover, as this technique allows the simultaneous determination of different physiological parameters such as blood pressure, locomotor and convulsive activity, it is a suitable tool for pharmacokinetic-pharmacodynamic studies of drugs and pharmacokinetic-pharmacodynamic (PK-PD) modeling. Lastly, the reverse microdialysis is a powerful technique for the study of local actions of drugs in different tissues such as specific brain nuclei, myocardium, liver or skeletal muscle. So, this article reviewed the vast applications of the microdialysis technique for the study of pharmacokinetic and pharmacodynamic properties of drugs.     

Applicability of reverse microdialysis in pharmacological and toxicological studies

A recent application of microdialysis is the introduction of a substance into the extracellular space via the microdialysis probe. The inclusion of a higher amount of a drug in the perfusate allows the drug to diffuse through the microdialysis membrane to the tissue. This technique, actually called as reverse microdialysis, not only allows the local administration of a substance but also permits the simultaneous sampling of the extracellular levels of endogenous compounds. Local effects of exogenous compounds have been studied in the central nervous system, hepatic tissue, dermis, heart and corpora luteae of experimental animals by means of reverse microdialysis. In central nervous studies, reverse microdialysis has been extensively used for the study of the effects on neurotransmission at different central nuclei of diverse pharmacological and toxicological agents, such as antidepressants, antipsychotics, antiparkinsonians, hallucinogens, drugs of abuse and experimental drugs. In the clinical setting, reverse microdialysis has been used for the study of local effects of drugs in the adipose tissue, skeletal muscle and dermis. The aim of this review is to describe the principles of the reverse microdialysis, to compare the technique with other available methods and finally to describe the applicability of reverse microdialysis in the study of drugs properties both in basic and clinical research.     

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

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

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.     

Microdialysis in mice for drug delivery research

Intracerebral microdialysis was first performed in the mouse at the end of the 1980s. Most microdialysis studies on mice were confined to neuropharmacology and changes in neurotransmitter concentrations up to 1995, although pharmacological studies were done on other tissues like the skin, kidney and implanted tumors. The use of microdialysis in mice for pharmacokinetic and drug delivery studies owes much to the recent availability of genetically engineered mice, such as mice in which the genes encoding multiple drug resistance have been knocked out. The quantitative microdialysis of blood and various tissue fluids of the mouse is now feasible and the recent development of specific microdialysis devices for use in mice should facilitate its use in these small animals. This review covers the technical aspects of microdialysis in the mouse and includes references to many of the published studies on pharmacokinetics and drug delivery.     

Assessment of cutaneous drug delivery using microdialysis

During the last decade microdialysis has been successfully applied to assess cutaneous drug delivery of numerous substances, indicating the large potential for bioequivalence/bioavailability evaluation of topical formulations. The technique has been shown to be minimally invasive and supply pharmacokinetic information directly in the target organ for cutaneous drug delivery with high temporal resolution without further intervention with the tissue after implantation. However, there are a few challenges that need to be addressed before microdialysis can be regarded as a generally applicable routine technique for cutaneous drug delivery assessments. Firstly, the technique is currently not suitable for sampling of highly lipophilic compounds and, secondly, more studies are desirable for elucidation of the variables associated with the technique to increase reproducibility. The present literature indicates that the condition of the skin at the individual assessment sites is the main variable, but also variables associated with relative recovery, differentiation between the pharmacokinetic parameters (i.e., lag time, distribution, absorption and elimination rate) can influences the reproducibility of the technique. Furthermore, it has been indicated that cutaneous microdialysis in rats may be useful for prediction of dermal pharmacokinetic properties of novel drugs/topical formulations in man.     

Microdialysis. A novel tool for clinical studies of anti-infective agents

In vivo microdialysis (MD) is an innovative clinical technique that has been employed in preclinical research and metabolic studies in patients for more than a decade. Recently, MD has been adopted for human drug studies and has opened up the opportunity to quantify tissue drug distribution in vivo. The particular advantage of MD for the anti-infective field relates to the fact that MD allows for online measurement of the unbound, pharmacologically active drug fraction in the interstitial space fluid (ISF), the anatomically defined target site for most bacterial infections. The aim of this review is to provide an overview of the current literature about MD in anti-microbial drug studies. It will be shown that MD has become feasible in most human tissues including brain and lung. So far, several MD studies have demonstrated that anti-microbial concentrations at the effect site may be subinhibitory, although effective concentrations are attained in serum, a finding that has significant impact on clinical decision making. In addition to its property as a pharmacokinetic sampling technique, MD offers unique opportunities in pharmacokinetic-pharmacodynamic (PK-PD) research and has the potential to streamline the decision process on proper drug dosing in drug development.     

In vivo microdialysis technique on rat myocardium]

The microdialysis procedure is a technique that has been established for some years. The heart, however, has several difficulties in the application of this technique. The heart is the beating in vivo, in contrast to other organs. I developed the flexibly mounted microdialysis technique, which involves the synchronized movement of the tip of the probe with the beating heart to reduce tissue injury. With this technique, it is feasible to make stable and long-term measurements of interstitial biological substances. By employing the flexibly mounted microdialysis technique, I will demonstrate that the monitoring of free radical generation and the level of adenosine measured during AMP perfusion that gives an index of the activity of ecto-5'-nucleotidase in the tissue. In the future, this technique will have versatile applications for useful studies to elucidate the actual mechanism in the pathogenesis of heart disorders.     

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.     

Assessment of in vitro and in vivo recovery of gallamine using microdialysis

The application of microdialysis technique for the investigation of pharmacokinetics and pharmacodynamics of drugs requires careful assessment of probe performance to ensure validity of the data obtained using this technique. The aim of this study was to establish and validate the microdialysis technique for investigation of the pharmacokinetics and pharmacodynamics of the neuromuscular blocker, gallamine. In vitro recovery of gallamine from the microdialysis probe when different perfusion flow rates were employed was evaluated leading to selection of a flow rate of 2 microl/min with 15-min sampling intervals for the subsequent studies. In vitro recovery of gallamine from the microdialysis probe was independent of concentration, stable over an 8-h period and reproducible. Comparable in vitro recoveries were obtained by different established approaches including recovery estimation by gain, loss and the zero-net flux (ZNF) method. Recovery by loss was used to study the in vivo recovery of gallamine from rat muscle tissue. The in vivo recovery was stable over a 5.5-h sampling period. In vitro performance of the probe subsequent to the in vivo study remained stable supporting reusage of the probe. These data highlight the importance of a systematic examination of microdialysis probe validation.     

Microdialysis for in vivo pharmacokinetic/pharmacodynamic characterization of anti-infective drugs

Inadequate tissue penetration of antibiotics can lead to therapeutic failure and bacterial resistance. Pharmacokinetic evaluation of antibiotics should therefore be based on tissue rather than serum concentrations. Over several years, tissue concentration data obtained by methods such as tissue biopsies have flawed the correct interpretation of antibiotic tissue distribution. Microdialysis--a semi-invasive catheter-based sampling technique--has been employed for the in vivo measurement of antibiotic tissue pharmacokinetics. Owing to selective access to the target site for most anti-infective drugs, microdialysis satisfies regulatory requirements for pharmacokinetic distribution studies and might become a reference technique for tissue distribution studies in the near future. Furthermore, microdialysis might contribute to the definition of meaningful surrogate markers for antibiotic efficiency during drug development.     

How Minimally Invasive is Microdialysis Sampling? A Cautionary Note for Cytokine Collection in Human Skin and other Clinical Studies

It is common to refer to microdialysis as a minimally invasive procedure, likening it to insertion of an artificial capillary. While a comparison of this type allows the process to be easily visualized by those outside the field, it tends to provide a false impression of the localized perturbation of the tissue space that is caused by catheter insertion. With the increased acceptance of microdialysis sampling as a viable in vivo sampling method, many researchers have begun to use the technique to explore inflammatory and immune-mediated diseases in the skin and other organs. Unfortunately, many of the molecules of interest, particularly chemokines and cytokines, are known to be generated during the inflammatory response to wounding and the subsequent cellular events leading to wound repair. With more than 11,000 reports citing the use of microdialysis sampling, only a few researchers have sought to assess the tissue damage that is incurred by probe insertion. For this reason, caution is warranted when collecting these molecules and inferring a role for them in clinical disease states. This commentary seeks to remind the research community of the confounding effects that signaling molecules related to the wounding response will have on clinical studies. Proper controls must be incorporated into all studies in order to assess whether or not particular molecules are truly related to the disease state under investigation or have been generated as part of the tissue response to the wound incurred by microdialysis catheter implantation.     

Peritoneal microdialysis in freely moving rodents: an alternative to blood sampling for pharmacokinetic studies in the rat and the mouse.

By performing microdialysis in the peritoneal cavity, we studied the pharmacokinetics of Tramadol in awake, freely moving small laboratory animals. The systemic exposure to Tramadol was determined using both microdialysis sampling and collection of whole blood following a single intravenous injection (10 mg/kg) or a single oral dose (100 mg/kg) of Tramadol HCl. The sampling frequency of the dialysate was 10 min (mouse study) or 20 min (rat study). In rats and in mice, intraperitoneal microdialysis sampling gets reliable pharmacokinetic results without taking blood. The concentration-time curves obtained from peritoneal microdialysis were parallel to the concentration-time curves obtained from classical blood sampling. Accordingly, dose independent pharmacokinetic parameters were similar. A scaling factor, however, has to be introduced (e.g. peritoneal versus plasma AUC ratio) in order to obtain comparable pharmacokinetic results also with dose-dependent parameters. As there was no blood loss during the experiment, peritoneal microdialysis allowed the investigation of complete concentration-time curve profiles. Thus, the number of animals could be kept to a minimum. In conclusion, in vivo peritoneal microdialysis is a unique tool to obtain a complete set of free drug concentrations to determine reliable pharmacokinetic parameters from awake, freely moving rodents. Therefore, we suppose that the technique will have relevance for pharmacokinetic studies in future.     

Evaluation of in vivo and in vitro recovery rate of anatoxin-a through the microdialysis probe

In vivo microdialysis is a versatile sampling technique commonly employed to observe changes in neurotransmitters levels that occur in response to different treatments, being these treatments administered through a microdialysis probe implanted into a specific brain region in living animals. In previous works we have used this technique to study the effects of the drug anatoxin-a, a nicotinic acetylcholine receptor agonist, on dopamine release in striatum. The aim of the present study was to assess the recovery of anatoxin-a through the microdialysis probe. This information allows knowing the exact amount of the drug crossing the microdialysis membrane, acting on extracellular tissue. High Performance Liquid Chromatography (HPLC) with Fluorescence Detection (FLD) has been used for the analysis of anatoxin-a. We observed that the recovery of anatoxin-a was about 0.5%. Under our experimental conditions, the results suggest that anatoxin-a can be used as an important tool in the study of neuronal nicotinic receptors by in vivo microdialysis technique and also show a reliable estimation of the anatoxin-a recovery through the microdialysis probe under both in vivo and in vitro conditions.     

Cerebral microdialysis in clinical studies of drugs: pharmacokinetic applications

The ability to deliver drug molecules effectively across the blood–brain barrier into the brain is important in the development of central nervous system (CNS) therapies. Cerebral microdialysis is the only existing technique for sampling molecules from the brain extracellular fluid (ECF; also termed interstitial fluid), the compartment to which the astrocytes and neurones are directly exposed. Plasma levels of drugs are often poor predictors of CNS activity. While cerebrospinal fluid (CSF) levels of drugs are often used as evidence of delivery of drug to brain, the CSF is a different compartment to the ECF. The continuous nature of microdialysis sampling of the ECF is ideal for pharmacokinetic (PK) studies, and can give valuable PK information of variations with time in drug concentrations of brain ECF versus plasma. The microdialysis technique needs careful calibration for relative recovery (extraction efficiency) of the drug if absolute quantification is required. Besides the drug, other molecules can be analysed in the microdialysates for information on downstream targets and/or energy metabolism in the brain. Cerebral microdialysis is an invasive technique, so is only useable in patients requiring neurocritical care, neurosurgery or brain biopsy. Application of results to wider patient populations, and to those with different pathologies or degrees of pathology, obviously demands caution. Nevertheless, microdialysis data can provide valuable guidelines for designing CNS therapies, and play an important role in small phase II clinical trials. In this review, we focus on the role of cerebral microdialysis in recent clinical studies of antimicrobial agents, drugs for tumour therapy, neuroprotective agents and anticonvulsants.     

Microdialysis Sampling for the Investigation of Dermal Drug Transport

Microdialysis perfusion in vivo has the potential to be a powerful sampling technique in dermal and transdermal drug delivery studies. Characterization of a commercially available microdialysis probe in vitro considering relevant physiological parameters is a vital first step in the evaluation of microdialysis as a dermal sampling technique. In previous microdialysis studies, analyte concentration and neutrality have been implicated in altering microdialysis recovery. The recovery of a model compound 5-fluorouracil (5-FU) was investigated at several pH values and donor concentrations. The relative recovery of 5-FU by the microdialysis probe was affected by pH but not by donor concentration. To confirm further that the changing concentration and pH profile presented by the flux of 5-FU was not significantly altering microdialysis recovery, an experiment comparing direct and microdialysis sampling of a Franz diffusion cell receptor compartment was performed. Although the 5-FU concentration (0-686 ng/ml) and pH (7.40-7.24) changed substantially, the recovery of 5-FU was not adversely affected. To demonstrate the feasibility of dermal microdialysis, the flux of a commercial preparation of 5-fluorouracil was monitored utilizing a microdialysis probe implanted in excised rat skin in vitro. The results from the dermally implanted probe demonstrate the potential of the technique while establishing the limitations of the current microdialysis system.