A review of microdialysis sampling for pharmacokinetic applications

Evaluation of D-amino acid oxidase activity in rat kidney using a D-kynurenine derivative, 6-methylthio-D-kynurenine: An in vivo microdialysis study

D-Amino acid oxidase (DAO), a D-amino acid metabolizing enzyme, is reportedly associated with the psychiatric disease schizophrenia, suggesting a role for DAO inhibitors in its treatment. We have previously reported that DAO catalyzes the conversion of nonfluorescent 6-methylthio-D-kynurenine (MeS-D-KYN) to fluorescent 5-methylthiokynurenic acid (MeS-KYNA) in vitro. The present study aimed to determine the potential of MeS-D-KYN in evaluating DAO activity in vivo using renal microdialysis technique in rats. Male Sprague-Dawley rats were subjected to linear microdialysis probe implantation in the left kidney. Continuous perfusion of MeS-D-KYN was maintained, and DAO activity in the kidney cortex was evaluated by measuring the MeS-KYNA content in the microdialysate. The microdialysate was collected every 30 min and analyzed by high-performance liquid chromatography with fluorescence detection, monitored at 450 nm with an excitation wavelength of 364 nm. A significant production of MeS-KYNA was observed during, but not before, infusion of MeS-D-KYN, indicating that this compound is not endogenous. MeS-KYNA production was suppressed by the co-infusion of DAO inhibitor, 5-chlorobenzo[d]isoxazol-3-ol (CBIO), suggesting that MeS-D-KYN was converted to MeS-KYNA by renal DAO. Moreover, oral administration of CBIO effectively suppressed DAO activity in a dose-dependent manner. DAO converted MeS-D-KYN to MeS-KYNA in vivo, suggesting the potential of this compound in evaluating DAO activity. The use of the renal microdialysis technique developed in this study facilitates the monitoring of DAO activity in live experimental animals.

Visualizing and quantifying antimicrobial drug distribution in tissue

The biodistribution and pharmacokinetics of drugs are vital to the mechanistic understanding of their efficacy. Measuring antimicrobial drug efficacy has been challenging as plasma drug concentration is used as a surrogate for tissue drug concentration, yet typically does not reflect that at the intended site(s) of action. Utilizing an image-guided approach, it is feasible to accurately quantify the biodistribution and pharmacokinetics within the desired site(s) of action. We outline imaging modalities used in visualizing drug distribution with examples ranging from in vitro cellular drug uptake to clinical treatment of microbial infections. The imaging modalities of interest are: radio-labeling, magnetic resonance, mass spectrometry imaging, computed tomography, fluorescence, and Raman spectroscopy. We outline the progress, limitations, and future outlook for each methodology. Further advances in these optical approaches would benefit patients and researchers alike, as non-invasive imaging could yield more profound insights with a lower clinical burden than invasive measurement approaches used today.

Xylem sap phosphorus sampling using microdialysis-a non-destructive high sampling frequency method tested under laboratory and field conditions

For a better understanding of plant nutrition processes, it is important to study the flux of nutrients within plants. However, existing xylem sap sampling methods are typically destructive and do not allow for repeated, highly frequent measurements of nutrient concentration. In this paper, we present a novel use of microdialysis (MD) for characterizing xylem sap phosphate (PO43-) concentration as a possible alternative to destructive sampling. First, MD probes were tested under laboratory conditions in vitro, in a stirred solution test, and in vivo, using beech tree stem segments. Exponential decline in the relative recovery (RR) with an increasing MD pumping rate allows for determining an optimal sampling interval (i.e., the maximum amount of sample volume with the minimum required concentration). The RR changed only minimally, with a change in the simulated sap flow velocity during the in vivo stem segment test. This suggests that MD can be applied over a range of naturally occurring sap flow velocities. Differences in the ionic strength between the xylem sap and the perfusate pumped through the MD did not influence the RR. Then, MD was successfully applied in a 24 h field campaign in two beech trees of different ages and allowed for in situ assessments of the diurnal variation of PO43- concentration and (together with xylem flow measurements) flux variability in living trees. Both beech trees exhibited the same diurnal pattern in PO43- concentrations with higher concentrations in the younger tree. The xylem PO43- concentration measured with MD was in the same order of magnitude as that received through destructive sampling in the younger tree. The MD probes did not show a decline in RR after the field application. We showed that MD can be applied to capture the PO43- concentration dynamics in the xylem sap with bihourly resolution under field conditions.

Green Approaches to Sample Preparation Based on Extraction Techniques

Preparing a sample for analysis is a crucial step of many analytical procedures. The goal of sample preparation is to provide a representative, homogenous sample that is free of interferences and compatible with the intended analytical method. Green approaches to sample preparation require that the consumption of hazardous organic solvents and energy be minimized or even eliminated in the analytical process. While no sample preparation is clearly the most environmentally friendly approach, complete elimination of this step is not always practical. In such cases, the extraction techniques which use low amounts of solvents or no solvents are considered ideal alternatives. This paper presents an overview of green extraction procedures and sample preparation methodologies, briefly introduces their theoretical principles, and describes the recent developments in food, pharmaceutical, environmental and bioanalytical chemistry applications.

Liquid chromatography method to assay tretinoin in skin layers: validation and application in skin penetration/retention studies

A liquid chromatography (LC) method for the quantification of tretinoin (TTN) in different matrices (adhesive tape, cotton and porcine skin layers, stratum corneum, viable epidermis, and dermis) was validated and applied in in vitro porcine skin penetration/retention studies. This study proposes, for the first time, a method for assaying TTN in separated porcine skin layers (stratum corneum, viable epidermis, and dermis). The skin studies were carried out using tape stripping and cutaneous retention techniques. The procedures for the extraction of TTN from dermatological formulations (creams and gels) and biological and non-biological matrices used with the tape stripping and retention techniques were also evaluated. The LC method consisted of a mobile phase composed of a mixture of methanol, water, and glacial acetic acid (85:15:1, v/v); a C18 column used as the stationary phase; a flow rate of 1.0 mL min⁻¹; an injection volume of 100 μL; and TTN detection at 342 nm. The method was linear in the range of 0.05–15.00 μg mL⁻¹ (r = 0.9999), and it was precise and accurate. The limit of detection (LOD) and limit of quantification (LOQ) were 0.0165 μg mL⁻¹ and 0.0495 μg mL⁻¹, respectively. TTN was extracted from different matrices, showing good precision [relative standard deviation (RSD) of <5%] and accuracy (89.4–113.9%). This method was successfully applied in the evaluation of TTN skin retention/permeation from dermatological formulations (cream and gel). A higher penetration of TTN through the skin was achieved with the gel rather than the cream, showing the influence of the dosage form. Therefore, the developed method can easily be applied in porcine skin penetration/retention studies of dermatological formulations containing TTN, and it is able to discriminate the behaviours of the different formulations.

Microdialysis Sampling of Quorum Sensing Homoserine Lactones during Biofilm Formation

Bacteria communicate chemically through a system called quorum sensing. In this work, microdialysis sampling procedures were optimized to collect quorum sensing molecules produced during in situ biofilm formation directly on the polymeric semipermeable membrane of the microdialysis probe. V. harveyi, a Gram-negative bacterium, was used as the model organism and releases variable chain length acylhomoserine lactones (AHLs) and acyl-oxohomoserine lactones (AOHLs) as signaling molecules during quorum sensing. Eliciting biofilm formation required coating fetal bovine serum onto the poly(ether sulfone) microdialysis membrane. Dialysates were collected in different experiments either during or after biofilm formation directly on a microdialysis probe. Continuous sampling of C4-AHL, C6-AHL, C8-AHL, C6-OXO-AHL, and C12-OXO-AHL was achieved over a period of up to 4 days. The AHLs and AOHLs in dialysates were concentrated with solid-phase extraction and quantified using LC-MS. Dialysate concentrations obtained for the AOHLs and AHLs ranged between 1 and 100 ppb (ng/mL) and varied between sampling days. This work demonstrates the initial use of microdialysis sampling to collect quorum sensing signaling chemicals during biofilm formation by a Gram-negative bacterial species.

Metal-organic frameworks as affinity agents to enhance the microdialysis sampling efficiency of fatty acids

Microdialysis (MD) has been extensively used for in vivo sampling of hydrophilic analytes such as neurotransmitters and drug metabolites. In contrast, there have been few reports on sampling of lipophilic analytes by MD. Lipophilic analytes are easily adsorbed on the surfaces of the dialysis membrane and the inner wall of tubing, which leads to a very low relative recovery (RR). In this work, a strategy to develop an enhanced MD sampling of fatty acids (FAs) by using metal-organic frameworks (MOFs) as affinity agents in the perfusion fluid was investigated. Two MOFs, MIL-101 and ZIF-8, were synthesized and tested for the first time. A 2 times higher RR, about 70% RR, was obtained. The FT-IR experiment showed that the unsaturated metal sites in MOFs could coordinate with FAs, therefore the FAs were encapsulated into MOFs, avoiding FAs to be absorbed on the surfaces of the dialysis membrane and the inner wall of tubing. Moreover, incorporation of FAs into MOFs led to a decrease of free concentration of FAs inside the MD membrane and an increase of concentration gradient, allowing more FAs to diffuse across the membrane. Consequentially, an enhanced RR was obtained. The approach was successfully used to monitor the time profile of targeted FAs in cell culture media after lipopolysaccharide (LPS)-induced inflammation.

Microscale Mass Spectrometry Analysis of Extracellular Metabolites in Live Multicellular Tumor Spheroids

Extracellular compounds in tumors play critical roles in intercellular communication, tumor proliferation, and cancer cell metastasis. However, the lack of appropriate techniques leads to limited studies of extracellular metabolite. Here, we introduced a microscale collection device, the Micro-funnel, fabricated from biocompatible fused silica capillary. With a small probe size (∼25 μm), the Micro-funnel can be implanted into live multicellular tumor spheroids to accumulate the extracellular metabolites produced by cancer cells. Metabolites collected in the Micro-funnel device were then extracted by a microscale sampling and ionization device, the Single-probe, for real-time mass spectrometry (MS) analysis. We successfully detected the abundance change of anticancer drug irinotecan and its metabolites inside spheroids treated under a series of conditions. Moreover, we found that irinotecan treatment dramatically altered the composition of extracellular compounds. Specifically, we observed the increased abundances of a large number of lipids, which are potentially related to the drug resistance of cancer cells. This study provides a novel way to detect the extracellular compounds inside live spheroids, and the successful development of our technique can benefit the research in multiple areas, including the microenvironment inside live tissues, cell-cell communication, biomarker discovery, and drug development.

Monitoring Dopamine Responses to Potassium Ion and Nomifensine by in Vivo Microdialysis with Online Liquid Chromatography at One-Minute Resolution

Recently, our laboratory has demonstrated the technical feasibility of monitoring dopamine at 1 min temporal resolution with microdialysis and online liquid chromatography. Here, we monitor dopamine in the rat striatum during local delivery of high potassium/low sodium or nomifensine in awake-behaving rats. Microdialysis probes were implanted and perfused continuously with or without dexamethasone in the perfusion fluid for 4 days. Dexamethasone is an anti-inflammatory agent that exhibits several positive effects on the apparent health of the brain tissue surrounding microdialysis probes. Dopamine was monitored 1 or 4 days after implantation under basal conditions, during 10 min applications of 60 mM or 100 mM K⁺, and during 15 min applications of 10 μM nomifensine. High K⁺ and nomifensine were delivered locally by adding them to the microdialysis perfusion fluid using a computer-controlled, low-dead-volume six-port valve. Each day/K⁺/dexamethasone combination elicited specific dopamine responses. Dexamethasone treatment increased dopamine levels in basal dialysates (i.e., in the absence of K⁺ or nomifensine). Applications of 60 mM K⁺ evoked distinct responses on days one and four after probe implantation, depending upon the presence or absence of dexamethasone, consistent with dexamethasone's ability to mitigate the traumatic effect of probe implantation. Applications of 100 mM K⁺ evoked dramatic oscillations in dopamine levels that correlated with changes in the field potential at a metal electrode implanted adjacent to the microdialysis probe. This combination of results indicates the role of spreading depolarization in response to 100 mM K⁺. With 1 min temporal resolution, we find that it is possible to characterize the pharmacokinetics of the response to the local delivery of nomifensine. Overall, the findings reported here confirm the benefits arising from the ability to monitor dopamine via microdialysis at high sensitivity and at high temporal resolution.

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.

Monitoring neurochemical release from astrocytes using in vitro microdialysis coupled with high-speed capillary electrophoresis

We have developed a novel in vitro approach for monitoring fast neurochemical dynamics in model cell systems using microdialysis sampling coupled with high-speed capillary electrophoresis (CE). Cells from an immortalized astrocyte line (C8-D1A) were cultured in direct contact with the porous membrane of a microdialysis probe. Confocal microscopy was used to confirm cell viability and confluency over the microdialysis sampling region. Small molecules released from the astrocytes were efficiently sampled by the probe due to the direct contact with the membrane. Microdialysis sampling was coupled with online, high-speed CE allowing changes in the dialysate concentration of small molecule amine neurochemicals to be monitored with 20 s temporal resolution. Basal release of a number of important analytes was detected including glycine, taurine, D-serine, and glutamate. The ability of the in vitro microdialysis-CE instrument to monitor dynamic changes in analyte concentration was assessed by transferring a probe cultured with astrocytes from a solution containing artificial cerebrospinal fluid (aCSF) to a high K(+) solution (100 mM K(+)-aCSF). Upon stimulation, the observed concentration of a number of key neurochemicals increased dramatically including glycine (700%), taurine (185%), and serine (215%). Amino acids such as phenylalanine and valine, which are not known to respond to cellular swelling mechanisms, were unaffected by the K(+) stimulation.

Dynamic detection of non-protein-bound strychnine and brucine in rabbit muscle and synovial fluid after topical application of total Strychnos alkaloid patches

Semen Strychni, a known toxic drug in Chinese pharmacopoeia, is notable for its therapeutic effects on local muscle and joint pain. However, oral administration can be risky. Topically administered drugs accumulate in the topical muscles and knee joints without any major increase in plasma levels; only non-protein-bound drugs in the biological fluids of target tissues are effective for therapeutic effects. A sensitive and rapid ultra performance liquid chromatography - mass spectrometry (UPLC-MS) method coupled with a microdialysis technique was developed to determine the non-protein-bound strychnine (Str) and brucine (Bru) in rabbit muscle and synovial fluid microdialysate. The UPLC separation was carried out using a 1.7μm BEH C18 column (50 mm × 2.1 mm) with a mobile phase consisting of methanol: water (29.5:70.5, v/v) with 0.1% formic acid and 20 mM ammonium acetate in water. The method was validated at concentrations ranging from 0.58 ng/ml to 467.20 ng/ml for Str and from 0.42 ng/ml to 422.40 ng/ml for Bru. Intra-day and inter-day accuracy ranged from 99.1% to 103.2% for Str and from 95.8% to 108.8% for Bru with intra-day and inter-day precision within 9.7%. The proposed method was successfully applied to determine non-protein-bound Str and Bru, and the analysates concentration remained stable in rabbit muscle and synovial fluid after topical application of total Strychnos alkaloid patches, which indicated that total Strychnos alkaloid patches could substitute for the traditional oral administration of Semen Strychni.

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

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

Assessment of topical bioequivalence using dermal microdialysis and tape stripping methods

PURPOSE

To assess the bioequivalence of two commercial topical formulations of oxytetracycline HCl by tape stripping and microdialysis in healthy volunteers.

METHODS

Tape stripping study was conducted on 12 healthy volunteers. After a 30-minute application of the formulations, adhesive tapes were used to sample stratum corneum at 0.25, 0.5, 1, 1.5, 2, 3, 4 hr. Ten of these volunteers were included in the microdialysis study with a period of 4 weeks between the experiments. Microdialysis probes were inserted into the dermis of the forearm. Following the application of the test and reference simultaneously, dialysates were collected in 30-minute sampling intervals up to 4 hr.

RESULTS

Pharmacokinetic evaluation by microdialysis yielded that the test could not be said to be bioequivalent to the reference at 90% CI. The intersubject variability of oxytetracycline content in stratum corneum was moderate when it was compared to the dermal levels. The test was found to be bioequivalent to reference according to the dermatopharmacokinetic evaluation by tape stripping.

CONCLUSIONS

No significant correlations were found between microdialysis and tape stripping methods as regarding the topical bioequivalence of oxytetracycline HCl formulations.

Microdialysis combined blood sampling technique for the determination of rosiglitazone and glucose in brain and blood of gerbils subjected to cerebral ischemia

Rosiglitazone is a potent synthetic peroxisome proliferator-activated receptor-gamma (PPAR-γ) agonist which improves glucose control in the plasma and reduces ischemic brain injury. However, the pharmacokinetics of rosiglitazone in the brain is still unclear. In this study, a method using liquid chromatography-mass spectrometry coupled with microdialysis and an auto-blood sampling system was developed to determine rosiglitazone and glucose concentration in the brain and blood of gerbils subjected to treatment with rosiglitazone (3.0 mg kg(-1), i.p.). The results showed the limit of detection was 0.04 μg L(-1) and the correlation coefficient was 0.9997 for the determination of rosiglitazone in the brain. The mean parameters, maximum drug concentration (C(max)) and the area under the concentration-time curve from time zero to time infinity (AUC(inf)), following rosiglitazone administration were 1.06±0.28 μg L(-1) and 296.82±44.67 μg min L(-1), respectively. The time to peak concentration (C(max) or T(max)) of rosiglitazone occurred at 105±17.10 min, and the mean elimination half-life (t(1/2)) from brain was 190.81±85.18 min after administration of rosiglitazone. The brain glucose levels decreased to 71% of the basal levels in the rosiglitazone-treated group when compared with those in the control (p<0.01). Treatment with rosiglitazone decreased blood glucose levels to 80% at 1h after pretreatment of rosiglitazone (p<0.05). In addition, pretreatment with rosiglitazone significantly reduced the cerebral infarct volume compared with that of the control group. These findings suggest that this method may be useful for simultaneous and continuous determination of rosiglitazone and glucose concentrations in brain and plasma. Rosiglitazone was effective at penetrating the blood-brain barrier as evidenced by the rapid appearance of rosiglitazone in the brain, and rosiglitazone may contribute to a reduction in the extent of injuries related to cerebral ischemic stroke via its hypoglycemic effect.

Antimicrobial chemotherapy and lung microdialysis: a review

Pneumonia is a form of lung infection that may be caused by various micro-organisms. The predominant site of infection in pneumonia is debatable. Advances in the fields of diagnostic and therapeutic medicine have had a less than optimal effect on the outcome of pneumonia and one of the many causes is likely to be inadequate antimicrobial concentrations at the site of infection in lung tissue. Traditional antimicrobial therapy guidelines are based on indirect modelling from blood antimicrobial levels. However, studies both in humans and animals have shown the fallacy of this concept in various tissues. Many different methods have been employed to study lung tissue antimicrobial levels with limited success, and each has limitations that diminish their utility. An emerging technique being used to study the pharmacokinetics of antimicrobial agents in lung tissue is microdialysis. Development of microdialysis catheters, along with improvement in analytical techniques, has improved the accuracy of the data. Unfortunately, very few studies have reported the use of microdialysis in lung tissue, and even fewer antimicrobial classes have been studied. These studies generally suggest that this technique is a safe and effective way of assessing the pharmacokinetics of antimicrobial agents in lung tissue. Further descriptive studies need to be conducted to study the pharmacokinetics and pharmacodynamics of different antimicrobial classes in lung tissue. Data emanating from these studies could inform decisions for appropriate dosing schedules of antimicrobial agents in pneumonia.

Central nervous system penetration for small molecule therapeutic agents does not increase in multiple sclerosis- and Alzheimer's disease-related animal models despite reported blood-brain barrier disruption

Therapy for central nervous system (CNS) diseases requires drugs that can cross the blood-brain barrier (BBB). BBB disruption has been reported in patients with multiple sclerosis (MS) and Alzheimer's disease (AD) and the related animal models as evidenced by increased infiltration of inflammatory cells or increased staining of Igs in the central nervous system. Although CNS penetration of therapeutic agents under pathological conditions has rarely been investigated, it is commonly assumed that BBB disruption may lead to enhanced CNS penetration and also provide a "window of opportunity" through which drugs that do not normally cross BBB are able to do so. In this article, we have compared brain penetration of eight small molecules in naive animals and experimental autoimmune encephalomyelitis (EAE) mice, streptozotocin-induced mice, and TASTPM transgenic mice. The tool compounds are lipophilic transcellular drugs [GlaxoSmithKline (GSK)-A, GSK-B, GSK-C, and naproxen], lipophilic P-glycoprotein (P-gp) substrates (amprenavir and loperamide), and hydrophilic paracellular compounds (sodium fluorescein and atenolol). Our data showed that rate and extent of CNS penetration for lipophilic transcellular drugs and P-gp substrates are similar in naive and all tested animal models. The brain penetration for paracellular drugs in EAE mice is transiently increased but similar to that in naive mice at steady state. Our data suggest that, despite reported BBB disruption, CNS penetration for small molecule therapeutic agents does not increase in MS- and AD-related animal models.

Current separation and detection methods in microdialysis the drive towards sensitivity and speed

This review outlines some of the analytical challenges associated with the analysis of microdialysis (MD) samples, in particular, the minute complex sample volumes that are often encountered. In MD sampling many different low-molecular-weight molecules can be collected, but the research findings are often limited by the sensitivity, specificity, and reliability of the analytical technique that is coupled to the dialysis probe. Therefore it is critical that a lot of consideration is given in selecting the most suitable analytical method including the most appropriate detector. This review aims to highlight the strengths and weaknesses of a range of commonly used analytical methods employed in MD. In Section 1, a brief overview of the MD technique is described, followed by a discussion on some of the advantages and drawbacks of this sampling technique. Sections 2 and 3 examine analytical and other technical considerations regarding analysis, with special emphasis on the factors that specifically influence analytical detection. Section 4 outlines the most commonly employed analytical techniques used in MD, including HPLC coupled with various detectors. Detail is given regarding the LOD and LOQ for many applications using each detector. As MS is of such high importance in MD, a special sub-section has been devoted to it. The importance of CE is also highlighted, with specific applications described. In addition, analytical techniques that do not appear to have found routine use in MD are discussed. Section 5 is concerned with recent innovations in chemical separation techniques, in particular MCE and ultra-performance liquid chromatography. Specific applications of the coupling of these techniques with MD are highlighted, along with technical challenges associated with miniaturization. In the Section 6, the future outlook of MD is discussed. Techniques other than electrophoretic- and chromatographic based separation methods are outside the scope of this review.

Transcutaneous sampling of ciprofloxacin and 8-methoxypsoralen by electroporation (ETS technique)

The novel technique of transcutaneous sampling of drugs by electroporation was developed to study the dermatokinetics of ciprofloxacin and 8-methoxypsoralen. The selected drugs differ in their aqueous solubility and also with respect to the extent of protein binding. Ciprofloxacin (15mg/kg) was administered i.v. through tail vein, whereas 8-methoxypsoralen (5mg/kg) was given by oral administration, in hairless rats and the time course of drug concentration in the plasma was determined. Drug concentration in the dermal extracellular fluid (ECF) was determined by ETS and microdialysis sampling techniques. The extent of penetration into dermal ECF for ciprofloxacin and 8-methoxypsoralen was found to be approximately 19-32% and approximately 13-23%, respectively. The drug concentration in the dermal ECF determined by ETS and microdialysis did not differ significantly from each other and so as were the pharmacokinetic parameters. The results show that ETS can be utilized as a potential technique for sampling of drugs from the dermal ECF.

Pharmacokinetic comparisons of tail-bleeding with cannula- or retro-orbital bleeding techniques in rats using six marketed drugs

INTRODUCTION

The evaluation of drug disposition properties of chemical entities in drug discovery research typically involves the conduct of pharmacokinetic studies in rodents that requires blood sampling over several time points, preferably without disrupting the physiological status of the animals. Several blood withdrawal methods have been employed throughout the industry, yet these methods have not been comprehensively evaluated with regard to their effects on pharmacokinetic profiles of the drug investigated to recommend best practices.

METHODS

In this paper, the pharmacokinetics of six marketed drugs from four distinct therapeutic classes were compared using tail-vein, femoral-artery cannula-, and retro-orbital sinus bleeding techniques. The marketed drugs used in these studies were pentoxifylline, gemfibrozil, glipizide, methotrexate, clonidine, and fluoxetine.

RESULTS

Following oral administration, peak plasma concentration (C(max)), and area under the curve (AUC(0-24)) values for all compounds were not significantly different with the tail-vein method when compared to cannula- or retro-orbital sinus bleeding, except for fluoxetine and gemfibrozil for which minor, but statistically significant differences were observed. The effect of arterial versus venous tail-bleeding on the pharmacokinetics of pentoxifylline indicated no statistical differences in either C(max) or AUC(0-24) values. However, for fluoxetine, higher exposures were observed with tail arterial than venous sampling (2-fold with respect to C(max) and 1.7-fold with respect to AUC(0-24), p<0.05).

DISCUSSION

The observed differences with fluoxetine may be due to its pharmacological effects on thermoregulatory responses that influence tail blood flow, a hypothesis that remains to be tested. Based on these observations, we recommend the tail-bleeding technique for pharmacology or toxicology exposure and F% studies, particularly in early discovery work. Retro-orbital bleeding is controversial and is no longer considered a humane method. Cannula-bleeding, especially coupled with automated blood-collection techniques, has become the most efficient way for pharmaceutical industry to perform rat bioavailability studies.

An investigation into the pharmacokinetics of 3-mercaptopropionic acid and development of a steady-state chemical seizure model using in vivo microdialysis and electrophysiological monitoring

OBJECTIVES

The goal of the present study was to develop a chemical seizure model using the convulsant, 3-mercaptopropionic acid (3-MPA). A pharmacodynamics approach was taken, combining in vivo microdialysis sampling with electrophysiological methods to simultaneously monitor, in real-time, the 3-MPA concentration in the brain and the corresponding electrocorticographic (ECoG) activity.

METHODS

The 3-MPA was administered in two doses (50 and 100 mg/kg) in order to study its pharmacokinetics. Microdialysis samples were collected from the striatum, hippocampus, and jugular vein every 5 min. The microdialysates were analyzed using high-performance liquid chromatography with electrochemical detection (HPLC-EC). The ECoG activity was monitored via screws placed onto the cortex. Noncompartmental pharmacokinetics analysis was performed to obtain the elimination constants (K(e)), the maximum concentration (C(max)), the time to achieve maximum concentration (T(max)), and the area under the concentration-time curves (AUC(inf)).

RESULTS

The average brain K(e) for the 50 and the 100mg/kg doses were 0.060 and 0.018 min(-1), respectively. The brain AUC(inf) for the 50 and 100mg/kg doses were 353 and 2168 mg min(-1)mL(-1), respectively. This led to a 67-fold increase in the observed number of seizures in the higher dose with the average seizure intensity double that of the smaller dose. These data led to the dosing scheme for the chemical seizure model of administering a 3-MPA loading dose of 60 mg/kg followed by a constant infusion of 50 mg/(kg min(-1)).

CONCLUSIONS

This study describes, to our knowledge, the first successful attempt to combine in vivo microdialysis with electrophysiology to monitor in real-time, the concentration and effects of 3-MPA in the brain. This led to the development of a steady-state chemical seizure model.

Application of microdialysis technique in the traditional chinese medicine

The concentration of extracellular neurotransmitters can be dynamically measured by in vivo microdialysis. This technique can apply to quantitatively evaluating the beneficial effects of Traditional Chinese Medicine (TCM). In the present study, the protective effects of Puerarin (Pur) on cerebral injuries evoked by ischemia/reperfusion was supported by a decrease of extracellular amino acids, especially glutamate (Glu), which were monitored by microdialysis combined with HPLC throughout the experiments. It suggested that, in addition to being a versatile and practical method in neuroscience, the microdialysis technique would play an important role in assessing and screening the effects of TCM.

A microchip electrophoresis device with on-line microdialysis sampling and on-chip sample derivatization by naphthalene 2,3-dicarboxaldehyde/2-mercaptoethanol for amino acid and peptide analysis

The integration of rapid on-chip sample derivatization employing naphthalene 2,3-dicarboxaldehyde and 2-mercaptoethanol (NDA/2ME) with an easily assembled microdialysis/microchip electrophoresis device was carried out. The microchip device consisted of a glass layer with etched microfluidic channels that was sealed with a layer of poly(dimethylsiloxane) (PDMS) via plasma oxidation. This simple sealing procedure alleviated the need for glass thermal bonding and allowed the device to be re-sealed in the event of blockages within the channels. The device was used for analysis of a mixture of amino acids and peptides derivatized on-chip with NDA/2ME for laser-induced fluorescence (LIF) detection. A 0.6 mM NDA/1.2 mM 2ME mixture was simply added into the buffer reservoir for dynamic on-column derivatization of sample mixtures introduced at a flow rate of 1.0 microl/min. Using this scheme, sample injection plugs were derivatized and separated simultaneously. Injections of ca. 12 fmol of 5 mM amino acid and peptide samples were conducted using the system. Finally, a three-component mixture of Arg, Gly-Pro, and Asp was sampled from a vial using microdialysis, derivatized, separated and detected with the system. The ultimate goal of this effort is the creation of a micro-total analysis system for high-temporal resolution monitoring of primary amines in biological systems.

Delivery of 125I-cobrotoxin after intranasal administration to the brain: a microdialysis study in freely moving rats

In order to determine the contribution of intranasal (i.n.) administration to the uptake of large molecular weight (MW) substances into central nervous system (CNS), concentration in brain of the centrally acting polypeptide cobrotoxin (NT-I) versus time profiles were studied using dual-probe microdialysis in awake free-moving rats. NT-I, radiolabeled with sodium (125)I-Iodide ((125)I-NT-I), was administered at the dose of 105 microg/kg intravenously and intranasally in the same set of rat (n=15). The (125)I-NT-Inasal preparations were formulated with borneol/menthol eutectic mixture (+BMEM) as an absorption enhancer and without (-BMEM). After application, the dialysates sampled simultaneously from olfactory bulb and cerebellar nuclei were measured in a gamma-counter for radioactivity. The real concentrations of NT-I were recalculated by in vivo recoveries of microdialysis probes. The results showed that the area under the curve (AUC) value in cerebellar nuclei (2283.51+/-34.54 min ng/ml) following i.n. administration (+BMEM) was significantly larger than those (AUC(olfactory)=1141.92+/-26.42 min ng/ml; AUC(cerebellar)=1364.62+/-19.35 min ng/ml) after intravenous (i.v.) bolus, respectively. A prolonged time values to peak concentrations after i.n. application (+BMEM) were observed compared with those following i.v. administration. Also, following i.n. application (+BMEM) the measured time value to peak concentration in cerebellar nuclei (85 min) was statistically longer than that in olfactory bulb (75 min), which could be plausibly an indication for NT-I delivery into brain via nose-brain pathway in the presence of absorption enhancer. i.n. administration (-BMEM) had little or no ability of NT-I delivering into brain. In conclusion, i.n. administration (+BMEM) significantly enhanced brain transport of NT-I with uneven distribution in discrete regions of brain compared with i.v. administration. Additionally, multi-probe microdialysis technique should be considerably valuable in brain delivery studies.

Continuous multi-element (Cu, Mn, Ni, Se) monitoring in saline and cell suspension using on-line microdialysis coupled with simultaneous electrothermal atomic absorption spectrometry

We have developed a microdialysis sampling technique coupled on-line with simultaneous electrothermal atomic absorption spectrometry (SIMAAS) for the continuous monitoring of copper (Cu), manganese (Mn), nickel (Ni), and selenium (Se) in saline solutions and in cell suspensions. These trace elements are considered to be those associated most significantly with oxidative stress in biological systems. We employed ultrapure saline (0.9% NaCl) as the perfusate and, thus, the dialysate samples contained a high concentration of salt in the matrix. The use of modifiers [Pd coupled with Mg(NO3)2] prevented the target elements from undergoing evaporation at a pyrolysis temperature of 1200 degrees C, a process that effectively eliminated interference from NaCl. The excellent linearity, detection limits, and precision of the SIMAAS technique allowed the Cu, Mn, Ni, and Se concentrations to be determined in saline. For the on-line microdialysis-SIMAAS system, the ultrapure saline was perfused at a flow rate of 1 microL/min. The probe recoveries of Cu, Mn, Ni, and Se in saline were 57.9, 65.0, 65.5, and 67.9%, respectively. A standard saline solution was measured continuously by the on-line system to ensure long-term stability; each measurement fell within a range of two standard deviations. We determined the on-line spiked recoveries of Cu, Mn, Ni, and Se (101.3, 88.8, 91.3, and 98.5%, respectively) by adding a spiking standard into the stirred saline. The spiked recoveries (Cu, 37.5%; Mn, 3.8%; Ni, 71.1%; Se, 33.8%) were also determined through on-line spiking of a standard into the stirred cell suspension; these values demonstrate that Cu, Mn, and Se were depleted in the cell suspension, but Ni was not. The use of this on-line microdialysis-SIMAAS system permitted the in situ, dynamic, and continuous monitoring of Cu, Mn, Ni, and Se in cell suspensions at a temporal resolution of 20 min.

Characterisation of two novel cyclodextrinases using on-line microdialysis sampling with high-performance anion exchange chromatography

In this work, a real-time sampling/analytical method for on-line measurements of two newly discovered cyclomaltodextrinases (CDases) has been developed and evaluated. This novel methodology not only allows the final products to be investigated, but it also reveals enzyme-specific differences in the degradation pathways during the hydrolysis of different substrates, which is a great advantage in the important tasks of investigating the mechanisms of and classifying new hydrolases, and is an advantage that conventional techniques cannot offer. Two different enzymes, one CDase from Laceyella sacchari (LsCda13) and one from Anoxybacillus flavithermus (AfCda13), were investigated during the hydrolysis of alpha-, beta- and gamma-cyclodextrin, and the hydrolysis products were sampled via a microdialysis probe and injected on-line every 30 min into a high-performance anion exchange chromatography system equipped with a pulsed amperometric detector (HPAEC-PAD), where they were identified. The enzymes yielded the same end-products, maltose and glucose, in an approximate molar ratio of 2:1, but they exhibited distinctly different patterns of intermediate product formation before reaching the end-point. LsCda13 had a more random distribution of the intermediate products, whereas AfCda13 showed the distinct intermediate production of maltotriose, which in some cases accumulated.

Coupling microdialysis with flow-injection chemiluminescence detection for a protein–drug interaction study

The interaction of metronidazole (MTZ) and human serum albumin (HSA) was studied using the coupling system of on-line microdialysis sampling with flow-injection chemiluminescence detection (FI-MD-CL). The interested drug and HSA were mixed in different molar ratios in 0.067 mol L(-1) phosphate buffer (pH 7.4) and incubated at 37 degrees C in a water-bath. Then the microdialysis probe was put into the MTZ-HSA mixed solution and sampled at a perfusion rate of 5 microL min(-1). The microdialysates was determined using flow-injection chemiluminescence. In vitro recovery (R) of MTZ under experimental conditions was approximately 25.2% with a R.S.D. of about 3.2%. The values estimated for the binding constant (K) and the number of the binding sites (n) were found to be 1.50 x 10(3)L mol(-1) and 1.89, respectively. The values of nK obtained using Scatchard analysis and Klotz plot were found to be quite similar. The method provided a reliable and simple technique for the study of drug-protein interaction.

Monitoring neurotransmitter release from isolated retinas using online microdialysis-capillary electrophoresis

Release of neurotransmitters and other primary amine-containing analytes from intact, isolated larval salamander (Ambystoma tigrinum) retinas maintained in a 6.5-microL perfusion chamber was monitored using online microdialysis-capillary electrophoresis with laser-induced fluorescence detection (CE-LIF). Primary amines were derivatized online with o-phthaldialdehyde (OPA) and beta-mercaptoethanol. With the use of overlapping injections, the perfusate was sampled every approximately 10 s. Although separation conditions were optimized using 20 mM hydroxypropyl-gamma-cyclodextrin (HP-gamma-CD) for a number of important neuromessengers including D- and L-serine, D- and L-asparate, glutamate, GABA, serotonin, dopamine, norepinephrine, and taurine, only glutamate (0.48 +/- 0.27 microM), GABA (0.25 +/- 0.12 microM), taurine (5.5 +/- 2.1 microM), and l-serine (2.8 +/- 1.0 microM) were identified in the perfusate. Elevated levels of glutamate, GABA, and taurine were detected during stimulation with 60 mM K+. This method is the first to directly sample multiple neurotransmitters from perfused, isolated retinas and to observe changes in efflux of these neurotransmitters as a result of pharmacological stimulation.

Microfluidic Electrophoresis Chip Coupled to Microdialysis for in Vivo Monitoring of Amino Acid Neurotransmitters

Microfluidic electrophoresis devices were coupled on-line to microdialysis for in vivo monitoring of primary amine neurotransmitters in rat brain. The devices contained a sample introduction channel for dialysate, a precolumn reactor for derivatization with o-phthaldialdehyde, a flow-gated injector, and a separation channel. Detection was performed using confocal laser-induced fluorescence. In vitro testing revealed that the initial device design had detection limits for amino acids of approximately 200 nM, relative standard deviation of peak heights of 2%, and separations within 95 s with up to 30,200 theoretical plates when applying an electric field of 370 V/cm. A second device design that allowed electric fields of 1320 V/cm to be applied while preserving the reaction time allowed separations within 20 s with up to 156,000 theoretical plates. Flow splitting into the electrokinetic network from hydrodynamic flow in the sample introduction channel was made negligible for sampling flow rates from 0.3 to 1.2 microL/min by placing a 360-microm-diameter fluidic access hole that had flow resistance (0.15-7.2) x 10(8)-fold lower than that of the electrokinetic network at the junction of the sample introduction channel and the electrokinetic network. Using serial injections, the device allowed the dialysate stream to be analyzed at 130-s intervals. In vivo monitoring was demonstrated by using the microdialysis/microfluidic device to record glutamate concentrations in the striatum of an anesthetized rat during infusion of the glutamate uptake inhibitor l-trans-pyrrolidine-2,4-dicarboxylic acid. These results prove the feasibility of using a microfabricated fluidic system coupled to sampling probes for chemical monitoring of complex media such as mammalian brain.

No delayed temporal response to sample concentration changes during enhanced microdialysis sampling using cyclodextrins and antibody-immobilized microspheres

The temporal response to concentration changes external to a microdialysis probe containing trapping agents in the perfusion fluid was studied. Native beta-cyclodextrin and a water-soluble beta-cyclodextrin polymer were used as trapping agents in the microdialysis perfusion fluid to study the temporal concentration response to carbamazepine, a hydrophobic analyte. The temporal response of microdialysis probes containing antibody-immobilized microspheres against five different cytokines (tumor necrosis factor-alpha (TNF-alpha), interferon-gamma (IFN-gamma), interleukin-2 (IL-2), IL-4, and IL-5) to concentration changes outside of the probe was also determined. In both cases, no delayed temporal response of enhanced microdialysis was observed for either carbamazepine or the cytokines as compared to standard microdialysis sampling procedures.

Determination and pharmacokinetics of ergometrine maleate in rabbit blood with on line microdialysis sampling and fluorescence detection

The study describes a flow injection on-line microdialysis system for in vivo monitoring of ergometrine maleate in rabbit blood with fluorescence detection. A flow-through microdialysis probe was used for intravenous sampling by pumping of the blood from the tested rabbit through the flow-through microdialysis probe located outside the living system at a flow rate of 15 microl min-1. The perfusion rate is 5 microl min-1. The ergometrine maleate in the dialysate was detected on-line with a flow injection fluorescence system after the ergometrine maleate administration (0.2 mg kg-1, i.v.). The dialysate sample volume was about 15 microl. The system was linearly related to the concentration of ergometrine maleate in the range 1-140 ng ml-1 (r=0.9989) with a detection limit 0.3 ng ml-1 (3sigma). The pharmacokinetic parameters of ergometrine maleate were calculated utilizing the pharmacokinetic software 'NDST-21' by a one-compartmental open model.

Microdialysis sampling and high-performance liquid chromatography with chemiluminescence detection for in-vivo on-line determination and study of the pharmacokinetics of levodopa in blood

A simple, reliable, and reproducible method for in-vivo on-line separation and determination of levodopa has been based on microdialysis then high-performance liquid chromatography with chemiluminescence detection. The perfusate is perfused at a flow rate of 5 microL min(-1). The concentration of levodopa in the dialysate is determined on line with a chemiluminescence system. The dialysate sample volume is approximately 20 microL. The response of the system is linearly related to the concentration of levodopa in the range 1 x 10(-8) to 1 x 10(-6) g mL(-1) (r2 = 0.9995) with a detection limit (3sigma) of 3 x 10(-9) g mL(-1) and sample throughput of 12 h(-1); RSD is 2.8% (n = 11). The method has been successfully used to study the pharmacokinetics of levodopa in vivo; the values of the pharmacokinetics parameters Cmax, AUC(0-t) and Tmax were 16.60, 20.92 ng mL(-1), and 90 min, respectively.

Continuous in vivo Monitoring of Blood Diffusible Calcium Using On-line Microdialysis Sampling Coupled with Flame Atomic Absorption Spectrometry

A direct, rapid and continuous in vivo monitoring of diffusible calcium in the blood of living rabbits has been developed using microdialysis sampling coupled on-line with flame atomic absorption spectrometry. Microdialysates perfused through implanted microdialysis probes were collected with a sample loop on an injection valve and directly introduced into the flame atomizer by a carrier solution. An ultrapure saline solution (0.9% NaCI, pH 7.2) was used as the perfusion solution at a flow rate of 20 microI min(-1) via the microdialysis probe. A 0.1% La solution in 0.5% HNO3 solution was employed as the carrier solution at a nebulizer uptake flow rate of 2.5 ml min(-1). The interval for each determination was 2.5 min (2 min of sampling time, 20 s of read time and 10 s of washing time). The performance characteristics of the on-line microdialysis-FAAS system were validated as follows: linearity range, 0 - 100 mg l(-1); detection limit (3a, n = 7), 3.66 mg l(-1); precision (RSD, n = 50), 6.2%. For the evaluation of analytical accuracy, the proposed on-line method was compared with the in vivo no net flux method. The use of an on-line microdialysis-FAAS system permitted the in situ, dynamic and continuous in vivo monitoring of diffusible calcium in the blood of the living rabbits after CaCl2 administration with a temporal resolution of 2.5 min.

Droplet-based Micro-flow Chemiluminescence System for in vivo Glucose Determination by Microdialysis Sampling

A micro-flow chemiluminescence (CL) system in vivo for glucose determination by the on-line microdialysis sampling is described in this paper. The micro-flow CL system uses discrete sample droplets, which formed at the tip of the capillary with the sampling volume of 4.5 microl. The sol-gel method is introduced to co-immobilize horseradish peroxidase (HRP) and glucose oxidase (GOD) on the inside surface of the micro-flow cell which was fabricated in polymethyl methacrylate (PMMA). The CL detection involved enzymatic oxidation of glucose to D-gluconic acid and H2O2, then H2O2 oxidizing luminol to produce CL in presence of HRP. The microdialysis probe was utilized for sampling in the rabbit blood; the sample throughput was 20 h(-1). The glucose level in blood of the rabbit was on-line monitored with good results.

A feasibility study of solid supported enhanced microdialysis

For the first time, a solid supported enhanced microdialysis methodology for analysis of neuropeptides is described. The microdialysis samples were, in this study, subsequently collected in fractions, dissolved from the solid particles, dried, and resolved in a formic acid buffer in order to make them suitable for capillary liquid chromatography-mass spectrometry. Different microdialysis flow profiles were evaluated where air-gapped continuous flow was considered most suitable for the solid supported microdialysis mode. Six endogenous neuropeptides were initially used to investigate the feasibility of this enhanced microdialysis methodology. The improved relative recovery obtained from the solid supported enhanced microdialysis was varying from no effect to 10 times higher as compared to ordinary microdialysis. The most efficient enrichment was obtained for luteinizing hormone releasing hormone, which was the largest but also the most hydrophilic of the peptides. In contrast, no significant difference in recovery was observed for Leu-enkephalin being the smallest and the most hydrophobic peptide tested. These results indicate an increased flux and selective uptake of hydrophilic peptides across the membrane and enrichment on the particles in solid supported microdialysis.

Monitoring d-Serine Dynamics in the Rat Brain Using Online Microdialysis-Capillary Electrophoresis

D-Serine was detected in dialysate collected from the rat striatum using an online microdialysis-CE-LIF instrument. Dialysate can be analyzed every 12.5 s using the online instrument, giving much better temporal resolution than previously possible for D-serine. Basal concentrations of D-serine (8 +/- 2 microM), glutamate (0.8 +/- 0.2 microM), GABA (0.11 +/- 0.04 microM), and L-serine (23 +/- 4 microM) were measured. Increases in the concentrations of these neurochemicals induced by the introduction of high-K+ aCSF were quantitated. Notably, an increase in D-serine concentration in response to high-K+ aCSF was observed for the first time. The identity of the D-serine peak was confirmed unambiguously using D-amino acid oxidase to selectively remove D-serine from a dialysate sample. The microdialysis-CE-LIF instrument was able to monitor this enzymatic reaction as it proceeded over a period of 60 min, demonstrating that online microdialysis-CE-LIF is not only useful in monitoring in vivo dynamics but can also be used to monitor other chemical systems.

Microdialysis combined with liquid chromatography–tandem mass spectrometry for the determination of 6-aminobutylphthalide and its main metabolite in the brains of awake freely-moving rats

6-Aminobutylphthalide (ABP) is a new drug candidate which is currently being developed for the treatment of cerebral ischemia. The pharmacokinetics and metabolism of ABP were studied using in situ microdialysis sampling in the brains of awake freely-moving rats. Two LC-MS/MS methods were used for the quantitative and qualitative analysis of microdialysate. For comparison and confirmation, brain tissue samples were also analyzed by LC-MS/MS and GC/MS. The results described provide more authentic information in pharmacokinetics and metabolism at the site of action by using the coupling of microdialysis to LC-MS/MS technique than the traditional sampling methods.

Miniaturization: an overview of biotechnologies for monitoring the physiology and pathophysiology of rodent animal models

Recent advances in bioengineering technologies have made it possible to collect high-quality reproducible data quantitatively in a wide range of laboratory animal species, including rodents. Several of these technologies are incorporated into a plan called Miniaturization, which aims to design, develop, and maintain rodent animal models to study the pathophysiology and therapy of human diseases. Laser Doppler flowmetry, digital sonomicrometry, bioelectrical impedance, and microdialysis are some of the most widely used methods under the plan because they cause minimal pain and distress, reduce the number of animals used in biomedical research, and allow chronic, nonterminal assessment of physiological parameters in rodents. An overview of each of these technologies and their major applications in rodents used for biomedical research is provided.