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.     

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.     

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.     

微透析技术在靶组织局部药物浓度检测中的应用

微透析技术是一项越来越被广泛使用的在体研究技术,其遵循透析原理,以探针为基础取样,可连续检测局部组织细胞外液的药物浓度,能满足常规的药代动力学/药效学(PK-PD)研究。本文就其在靶组织局部药物浓度检测中的应用作一简要介绍。     

Intracerebral microdialysis technique and its application on brain pharmacokinetic-pharmacodynamic study

Intracerebral microdialysis (IC-MD) has been developed as a well-validated and powerful technique for decades. As a practical sampling tool, it can gain the continuous dialysates of endogenous and exogenous substances in extracellular fluid (ECF) of awake freely moving animals. Also, variform IC-MD probes (IC-MDPs) have grown more exquisite. The implantation of the IC-MDP in certain tissue of brain allows monitor drug distribution and measure drug and corresponding neurotransmitters levels in brain ECF after administration for brain pharmacokinetic-pharmacodynamic (B-PK-PD) study. So it is suitable for IC-MD to B-PK-PD study (IC-MD/B-PK-PD). The performance of IC-MD/B-PK-PD can not only elevate the degree of precision and accuracy of experimental data, minimize the individual difference by reduced number of animals, but also give important information for the prediction and optimization of drug effective dose in preclinical study. In this review, we have discussed various IC-MD/B-PK-PD studies of analgesic, antiepileptic and antidepressant drug. The role of IC-MD/B-PK-PD in confirming and assessing the drug effect before clinic trials is highlighted.     

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.     

The use of microdialysis for the determination of plasma protein binding of drugs.

Microdialysis sampling was used for the determination of the protein binding and the free therapeutic drug concentration of drugs in plasma in vitro. Several drugs with varying extent of protein binding and for which the plasma monitoring is important were studied. To mimic the in vivo situation, an artificial blood vessel was constructed and filled with spiked plasma circulating at the flow rate of human blood at 37 degrees C. The microdialysis probe (16 mm membrane length, 20000 MW cut off) was placed in the vessel and perfused with 0.9% NaCl at 5 microliters min-1. Dialysates were collected every 10 min and were analysed by reversed-phase LC with UV detection. The free concentration of the drug was calculated by correcting the concentration in the dialysate for the recovery of the probe, which was also determined in the artificial blood vessel after the experiment. The data confirm that microdialysis is a valid alternative technique for the determination of protein binding or free therapeutic plasma concentration of drugs on a comparative basis. Reference to literature values indicates that the results of the proposed method correspond reasonably well with accepted values.     

Application of an in vivo brain microdialysis technique to studies of drug transport across the blood-brain barrier

There is a wide range of methods available for studying the transport of drugs across the blood-brain barrier (BBB) which is equipped with several systems to transport drugs as well as endogenous nutrients and waste products. The in vivo brain microdialysis technique, which allows direct sampling of the brain interstitial fluid (ISF), is a powerful means of characterizing influx and efflux transport across the BBB. In this paper, we review our results from the successful application of this technique to BBB drug transport studies. The drugs investigated include novel and CNS-active peptides, some agents that are actively removed from the brain ISF across the BBB, and a brain-directed prodrug.     

Application of microdialysis in clinical pharmacology

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 in different laboratory animals. However, in the last ten years microdialysis sampling has been introduced as a versatile technique in the clinical setting. Although, microdialysis sampling has been extensively used for metabolic monitoring in patients, it was also employed for the study of distribution of different therapeutic agents especially anti-infective and antineoplasic drugs. In addition, clinical effect of drugs in patients could be also determined by means of microdialysis. So, this article reviewed the vast applications of the microdialysis technique for the study of pharmacokinetic and pharmacodynamic properties of drugs in the clinical setting.     

Microdialysis: current applications in clinical pharmacokinetic studies and its potential role in the future

Microdialysis is a probe-based sampling method, which, if linked to analytical devices, allows for the measurement of drug concentration profiles in selected tissues. During the last two decades, microdialysis has become increasingly popular for preclinical and clinical pharmacokinetic studies. The advantage of in vivo microdialysis over traditional methods relates to its ability to continuously sample the unbound drug fraction in the interstitial space fluid (ISF). This is of particular importance because the ISF may be regarded as the actual target compartment for many drugs, e.g. antimicrobial agents or other drugs mediating their action through surface receptors. In contrast, plasma concentrations are increasingly recognised as inadequately predicting tissue drug concentrations and therapeutic success in many patient populations. Thus, the minimally invasive microdialysis technique has evolved into an important tool for the direct assessment of drug concentrations at the site of drug delivery in virtually all tissues. In particular, concentrations of transdermally applied drugs, neurotransmitters, antibacterials, cytotoxic agents, hormones, large molecules such as cytokines and proteins, and many other compounds were described by means of microdialysis. The combined use of microdialysis with non-invasive imaging methods such as positron emission tomography and single photon emission tomography opened the window to exactly explore and describe the fate and pharmacokinetics of a drug in the body. Linking pharmacokinetic data from the ISF to pharmacodynamic information appears to be a straightforward approach to predicting drug action and therapeutic success, and may be used for decision making for adequate drug administration and dosing regimens. Hence, microdialysis is nowadays used in clinical studies to test new drug candidates that are in the pharmaceutical industry drug development pipeline.     

微透析技术在肿瘤药物研究中的应用进展

微透析技术具有连续动态微创取样优势,能实时在线研究正常生理和肿瘤病理情况下肿瘤药物在体内,尤其是肿瘤组织局部的分布、代谢和消除,利用PK/PD参数模型设计个体化给药方案,预防和减少肿瘤药物的毒性反应。同时,微透析技术还可以应用于肿瘤细胞外间质微环境中生化物质的监测以及肿瘤药物局部给药治疗,是肿瘤药物研究的重要技术手段。本文对近年来微透析在肿瘤药物研究中的应用进展进行检索和归纳,为微透析技术在取样、监测以及治疗领域的进一步研究应用提供参考。     

Microdialysis as a tool in local pharmacodynamics

In many cases the clinical outcome of therapy needs to be determined by the drug concentration in the tissue compartment in which the pharmacological effect occurs rather than in the plasma. Microdialysis is an in vivo technique that allows direct measurement of unbound tissue concentrations and permits monitoring of the biochemical and physiological effects of drugs throughout the body. Microdialysis was first used in pharmacodynamic research to study neurotransmission, and this remains its most common application in the field. In this review, we give an overview of the principles, techniques, and applications of microdialysis in pharmacodynamic studies of local physiological events, including measurement of endogenous substances such as acetylcholine, catecholamines, serotonin, amino acids, peptides, glucose, lactate, glycerol, and hormones. Microdialysis coupled with systemic drug administration also permits the more intensive examination of the pharmacotherapeutic effect of drugs on extracellular levels of endogenous substances in peripheral compartments and blood. Selected examples of the physiological effects and mechanisms of action of drugs are also discussed, as are the advantages and limitations of this method. It is concluded that microdialysis is a reliable technique for the measurement of local events, which makes it an attractive tool for local pharmacodynamic research.     

RIA-linked microdialysis sampling in the awake rat: Application to free-drug pharmacokinetics of hydrocortisone

The purpose of this research was to combine microdialysis sampling techniques with a highly sensitive radioimmunoassay (RIA) to study the in vivo kinetic response of pharmacologically important substances. This technique allowed for a dense sampling regimen from an awake, free-roaming experimental animal with no loss of blood and with rapid analysis of the dialysate. An important methodological criterion for accurate quantitation of a test drug in the extracellular space was knowledge of the relative recovery of the sampling system at the time of experimentation. Accordingly, the factors which influenced the recovery of drug during dense in vivo microdialysis sampling were examined and an analytical technique was developed to measure the instantaneous recovery of drug from the extracellular space. This information was applied to in vivo (iv) sampling experiments on anaesthetized and awake, free-roaming rats following bolus and multiple long-term iv administrations of the highly protein bound steroid (i.e. greater than 90%), hydrocortisone-21-phosphate. These studies indicated that unbound hydrocortisone levels as determined by the RIA-linked microdialysis (RIALM) technique fluctuated rapidly between each 2-min sampling interval, but nevertheless decreased to predose endogenous concentrations in a first-order fashion (t1/2 = 17-29 min). The rapid fluctuations of unbound hydrocortisone may reflect real pharmacokinetic or pharmacodynamic phenomena, attributed, perhaps, to reequilibration of the unbound drug pool with proteins and tissues in the blood.     

Peripheral tissue microdialysis technique in unrestrained, conscious animals

The microdialysis procedure had been developed in the past 2 to 3 decades to determine levels of drug and endogenous compounds in several organs under physiological conditions. Advantages of microdialysis include: minimal stress on the experimental animals; studies may be done in unrestrained, conscious animals, and multiple determination can be made without concern for excess blood loss from small animals; and measurements can be made of drug and/or metabolites at multiple sites in the animal. By employing the microdialysis technique, I developed the method of multiple sampling for a long term period from an animal under the freely moving condition. In addition, I developed a novel microdialysis probe that was applied to several peripheral tissues and/or organs. In this paper, I will describe the fundamental procedure for peripheral tissues and/or organs such as the jugular vein and liver, using subcutaneous and ocular microdialysis sampling in unrestrained, conscious animals.     

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.     

Microdialysis Sampling for Determination of Plasma Protein Binding of Drugs

The use of microdialysis sampling to study the binding of drugs to plasma proteins was evaluated. Microdialysis sampling is accomplished by placing a short length of dialysis fiber in the sample and perfusing the fiber with a vehicle. Small molecules in the sample, such as drugs, diffuse into the fiber and are transported to collection vials for analysis. Larger molecules, such as proteins and protein-bound drugs are excluded by the dialysis membrane. Microdialysis was found to give values for in vitro protein binding in plasma equivalent to those determined by ultrafiltration. Microdialysis offers advantages in terms of maintaining equilibria and experimental versatility. Microdialysis sampling also provides potential use for in vivo determinations of protein binding.     

Compilation of 222 drugs' plasma protein binding data and guidance for study designs

The binding of a drug to plasma protein reduces free drug in the blood circulation that would otherwise be available for penetration into tissues to reach the therapeutic target or the kidney for elimination. Therefore, the binding event affects drug elimination from the body, efficacy, duration of action and toxicity. Co-administration of other drugs, food and pathological conditions of patients can significantly change percentage binding of the drug and result in serious consequences. Here, we present the largest and newest information on plasma protein binding for 222 drugs, of which 50% show 90-100% binding, a range that could be considered as a favorable element for future lead selection. We also provide critical and comprehensive evaluations on the methods and techniques established to determine plasma protein binding, pinpoint advantages and pitfalls of individual approaches, and offer detailed guidance for experimental designs, including ultrafiltration, equilibrium dialysis, ultracentrifugation, charcoal adsorption, high-performance affinity chromatography, high-performance frontal analysis, solid-phase microextraction and in vivo microdialysis.     

Protein binding displacement interactions and their clinical importance

The binding of drugs to proteins is an important pharmacokinetic parameter. Many methods are available for the study of drug protein binding phenomena and there are also many ways to interpret the binding data. Although much emphasis has been placed on the binding of drugs in the plasma, binding also takes place in the tissues. Displacement interactions involving plasma or tissue binding sites have been implicated as the causative mechanisms in many drug interactions. However, the importance of plasma binding displacement as a mechanism of drug interactions. However, the importance of plasma binding displacement as a mechanism of drug interaction has been overestimated and overstated, being based largely on in vitro data. Because displaced drug can normally distribute out of the plasma compartment, increases of free drug concentrations are usually transient and therefore will not give rise to changed pharmacological effects in the patient. Those clinically important drug interactions formerly considered to be caused via displacement from plasma binding sites usually have another interaction mechanism involved; commonly decreased metabolism or renal elimination also takes place. Plasma binding displacement interactions, however, do become important clinically in certain specific situations, namely, when the displacing drug is administered quickly to the patient by the intravenous route, during therapeutic drug monitoring, and in certain drug disposition studies which involve the use of a heparin lock for blood sampling. Tissue binding displacement interactions have a greater potential to cause adverse effects in the patient as in this case drug will be forced from extravascular sites back into the plasma. The resulting increased drug plasma levels will lead to enhanced pharmacological effects and, possibly, frank toxicity. Displacement of drugs from binding sites simultaneously in both the plasma and in the tissues will combine the effects seen after displacement from the separate areas. Due to decreased binding in both areas, the free drug concentration in the plasma will increase leading to overactivity of the displaced drug.     

The relevance of microdialysis for clinical on:cology

Conventional pharmacokinetic trials in oncology measure the total amount of drug and its metabolites in the plasma of patients. These total drug concentrations are then correlated with clinical parameters such as toxicity and/or drug efficacy. However, total drug concentrations in plasma are often not directly related to tumor or tissue concentrations and the kinetics in (tumor) tissue may be very different from the kinetics of the drug in plasma. As only the unbound fraction of drugs can passively diffuse or can be actively transported across the cell membrane of endothelial cells and penetrate the tissues and tumor cells, binding of drugs to plasma components may, in part, be responsible for the variability in clinical outcomes. Also, variation in binding to cellular components may be important. As an overall consequence, in in vivo models high inter-individual variabilities of drug concentrations in the tumor have been observed, which affected drug activity. Many current cancer drug-therapies aim for a clinical benefit at the edge of acceptable toxicity, without paying attention to (variability in) plasma and tumor or tissue pharmacokinetics. This review discusses the potential applicability of microdialysis, a minimally invasive method used to determine the unbound fraction of drugs in blood or in the interstitial fluid of tissues, in clinical oncology. Although faced with some methodological challenges, the technique offers many advantages over others, and enables direct tumor assessment for pharmacokinetics and even pharmacodynamics.