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.     

Human microdialysis

Microdialysis has been used extensively in animal studies for decades and in human pharmacokinetic studies for about 10 years. Microdialysis is based on the passive diffusion of a compound along its concentration gradient from the tissue through the membrane into the dialysate. Microdialysis samples from the interstitial space which is a defined, anatomical compartment; there is no net loss of body fluid; the sample is "purified" and no enzymatic degradation takes place because proteins do not pass through the probe membrane into the dialysate; microdialysis data relate to the intact molecule; time resolution is high compared to biopsy and skin blister techniques; radioabelling or induction of a magnetic response is not needed; microdialysis is also an alternative method to determine protein binding of a compound in vivo; microdialysis can readily be set up in clinical research units without expensive infrastructure. Microdialysis has been used to measure tissue concentrations of endogenous compounds and to investigate the tissue penetration of drugs in a variety of tissues in humans in vivo in both healthy volunteers and patients. Microdialysis data have also been used in PK-PD modelling and to obtain concentration-response relationships locally in tissues in vivo. There are also studies combining microdialysis with imaging techniques, e.g. PET. Microdialysis data may be used in early studies to select the appropriate compound, to optimise dosing regimens and to investigate the kinetic and dynamic consequences in the tissues of drug-drug and drug-disease interactions. Microdialysis can also be used in late phase studies to provide tissue concentration data in support of therapeutic efficacy trials or to create a niche for an already marketed drug. FDA and CPMP documents emphasise the value and importance of human tissue drug concentration data and support the use of microdialysis in humans to obtain such information. Microdialysis can satisfy regulatory requirements by providing data on drug concentrations in a well-defined anatomical tissue compartment at or close to the effect target site. Microdialysis is a versatile technique because of its multifaceted utility, low cost, ease of use, adaptability to different types of compounds and its feasibility for a number of organs and tissues. Equipment and probes for use in various organs have been commercially available for years.     

微透析技术用于化疗药物——吉西他滨体内药动学的初步研究

目的：建立以微透析法（microdialysis,MD）为采样技术的肿瘤化疗药物在体检测方法,并进行药动学研究。方法：以吉西他滨（GEM）为研究对象,大鼠为实验动物,采用尾静脉注射为给药方式,通过微透析技术进行血管内取样,对血药浓度进行在线、实时、连续监测,求算相关药动学参数。结果：GEM在大鼠体内血液中的探针回收率为（11.9±2.0）%,经大鼠尾静脉给药后GEM体内过程为二室模型,其消除和分布为一级动力学过程。实验过程中大鼠未见明显副作用。结论：微透析技术可用于活体动物体内GEM浓度的连续监测,提示微透析技术可用于抗肿瘤药物的局部药动学研究。     

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.     

微透析技术在抗菌药物药动学和药效学中的应用

微透析技术是一项新兴的体内药物分析技术。通过微透析技术与药动学和药效学模型结合,实现对组织或细胞外游离态药物浓度及其相应药理效应的同时研究,不仅有利于进一步明确药物的剂量-效应关系,制定临床最佳给药方案,而且也为个体化给药提供了科学依据。本文综合近年文献,对微透析技术的基本原理以及在抗菌药物药动学和药效学研究中的应用作一综述。     

提高微透析探针回收率方法的研究进展

微透析是将灌流取样和透析技术结合起来的研究物质动态变化的一种新型采样技术,适合于深部组织和重要器官的活体研究。微透析探针的回收率是影响微透析结果准确性的重要因素之一。微透析探针对物质的回收率通常较低,需结合灵敏的检测方法才能实现物质的定量分析,因此限制了微透析技术在科研中的应用及发展。如何提高微透析探针的回收率是亟待解决的问题,本文综述了国内外研究中提高微透析探针对物质回收率的方法,为探针回收率的提高提供一些参考。     

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.     

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.     

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.     

Human subcutaneous tissue distribution of fluconazole: comparison of microdialysis and suction blister techniques

AIMS: To investigate uptake of fluconazole into the interstitial fluid of human subcutaneous tissue using the microdialysis and suction blister techniques. METHODS: A sterile microdialysis probe (CMA/60) was inserted subcutaneously into the upper arm of five healthy volunteers following an overnight fast. Blisters were induced on the lower arm using gentle suction prior to ingestion of a single oral dose of fluconazole (200 mg). Microdialysate, blister fluid and blood were sampled over 8 h. Fluconazole concentrations were determined in each sample using a validated HPLC assay. In vivo recovery of fluconazole from the microdialysis probe was determined in each subject by perfusing the probe with fluconazole solution at the end of the 8 h sampling period. Individual in vivo recovery was used to calculate fluconazole concentrations in subcutaneous interstitial fluid. A physiologically based pharmacokinetic (PBPK) model was used to predict fluconazole concentrations in human subcutaneous interstitial fluid. RESULTS: There was a lag-time (approximately 0.5 h) between detection of fluconazole in microdialysate compared with plasma in each subject. The in vivo recovery of fluconazole from the microdialysis probe ranged from 57.0 to 67.2%. The subcutaneous interstitial fluid concentrations obtained by microdialysis were very similar to the unbound concentrations of fluconazole in plasma with maximum concentration of 4.29 +/- 1.19 microg ml(-1) in subcutaneous interstitial fluid and 3.58 +/- 0.14 microg ml(-1) in plasma. Subcutaneous interstitial fluid-to-plasma partition coefficient (Kp) of fluconazole was 1.16 +/- 0.22 (95% CI 0.96, 1.35). By contrast, fluconazole concentrations in blister fluid were significantly lower (P < 0.05, paired t-test) than unbound plasma concentrations over the first 3 h and maximum concentrations in blister fluid had not been achieved at the end of the sampling period. There was good agreement between fluconazole concentrations derived from microdialysis sampling and those estimated using a blood flow-limited PBPK model. CONCLUSIONS: Microdialysis and suction blister techniques did not yield comparable results. It appears that microdialysis is a more appropriate technique for studying the rate of uptake of fluconazole into subcutaneous tissue. PBPK model simulation suggested that the distribution of fluconazole into subcutaneous interstitial fluid is dependent on tissue blood flow.     

Microdialysis of triamcinolone acetonide in rat muscle

The objective of this study was to compare plasma and muscle concentrations of triamcinolone acetonide (TA) in the rat by microdialysis. Microdialysis experiments were carried out at steady state in rats after an initial I.V. bolus 50 mg/kg of the phosphate ester of TA (TAP) followed by 23 mg/kg/h infusion. In vivo recovery was calculated by retrodialysis. The concentration determined at steady state in microdialysate, corrected for recovery, was 2.73 +/- 0.42 microg/mL compared to 21.9 +/- 2.3 microg/mL in plasma. The pharmacokinetics of TA in plasma was described by an open two-compartment model with a terminal half-life of 2.7 h. The clearance of TA in rats determined by compartmental analysis was 0.94 L/h/kg. The measured microdialysate levels of TA in muscle, corrected for recovery, were comparable to the predicted free drug levels in the peripheral compartment. Protein binding in rat plasma, measured by ultrafiltration, was 90.1%. The microdialysis in vivo recovery in muscle was similar to the in vitro recovery under stirred conditions. The results show the applicability of microdialysis to measure free tissue concentrations of TA in rats.     

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.     

微透析技术探针回收率的影响因素研究进展

微透析技术是近年来发展起来的动态生物取样技术,具有"活体、微创、实时、高效"等特点,其与现代分析技术联用,实现了连续取样和动态测定,可进行微量的定性、定量分析。微透析探针回收率的准确校正,是测定生物体中待测组分确切浓度的关键步骤,可提高大分子、难溶性物质的回收率,使微透析的应用更为广泛。本文对微透析探针回收率的影响因素、校正方法以及近年来提高回收率所取得的进展做一综述。     

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.     

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。     

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。     

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.     

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.     

In Vivo Microdialysis Sampling for Pharmacokinetic Investigations

In vivo microdialysis sampling coupled to liquid chromatography was used to study acetaminophen disposition in anesthetized rats. The pharmacokinetics of acetaminophen and its sulfate and glucuronide metabolites were determined using both microdialysis sampling and collection of whole blood. For microdialysis, samples were continuously collected for over 5 hr without fluid loss using a single experimental animal. Microdialysis sampling directly assesses the free drug concentration in blood. The pharmacokinetic results obtained with microdialysis sampling were the same as those obtained from blood collection. The administration of heparin, necessary when collecting blood samples, was found to double the elimination half-life of acetaminophen. Microdialysis sampling is a powerful tool for pharmacokinetic studies, providing accurate and precise pharmacokinetic data.     

Pitfalls of the application of microdialysis in clinical oncology: controversial findings with docetaxel

Microdialysis is a novel and minimally invasive sampling technique, based on the diffusion of analytes from the interstitial compartment through a semi-permeable membrane, and enables direct assessment of tissue disposition and penetration of drugs. Variable antitumor responses may be associated with differences in tumor vascularity, capillary permeability or tumor interstitial pressure resulting in variable delivery of anticancer agents. In preparation of pharmacokinetic studies, aimed at measuring docetaxel concentrations in healthy and malignant tissues in vivo, in pre-clinical as well as clinical studies, in vitro recovery experiments were performed. In contrast to published data, the recovery experiments suggest that docetaxel has a very low recovery as a result of non-specific binding to currently available microdialysis catheters. Here we discuss our findings with docetaxel in a historical perspective and we report on our experience using polysorbate 80 to eliminate the non-specific binding and its effects on the recovery of docetaxel.