Microdialysis in mice for drug delivery research

Impact of Neurodegenerative Diseases on Drug Binding to Brain Tissues: From Animal Models to Human Samples

Drug efficacy in the central nervous system (CNS) requires an additional step after crossing the blood-brain barrier. Therapeutic agents must reach their targets in the brain to modulate them; thus, the free drug concentration hypothesis is a key parameter for in vivo pharmacology. Here, we report the impact of neurodegeneration (Alzheimer's disease (AD) and Parkinson's disease (PD) compared with healthy controls) on the binding of 10 known drugs to postmortem brain tissues from animal models and humans. Unbound drug fractions, for some drugs, are significantly different between healthy and injured brain tissues (AD or PD). In addition, drugs binding to brain tissues from AD and PD animal models do not always recapitulate their binding to the corresponding human injured brain tissues. These results reveal potentially relevant implications for CNS drug discovery.

A high-throughput cell-based method to predict the unbound drug fraction in the brain

Optimization of drug efficacy in the brain requires understanding of the local exposure to unbound drug at the site of action. This relies on measurements of the unbound drug fraction (f_u,brain), which currently requires access to brain tissue. Here, we present a novel methodology using homogenates of cultured cells for rapid estimation of f_u,brain. In our setup, drug binding to human embryonic kidney cell (HEK293) homogenate was measured in a small-scale dialysis apparatus. To increase throughput, we combined drugs into cassettes for simultaneous measurement of multiple compounds. Our method estimated f_u,brain with an average error of 1.9-fold. We propose that our simple method can be used as an inexpensive, easily available and high-throughput alternative to brain tissues excised from laboratory animals. Thereby, estimates of unbound drug exposure can now be implemented at a much earlier stage of the drug discovery process, when molecular property changes are easier to make.

Contributions of microdialysis to new alternative therapeutics for hepatic encephalopathy

Hepatic encephalopathy (HE) is a common complication of cirrhosis, of largely reversible impairment of brain function occurring in patients with acute or chronic liver failure or when the liver is bypassed by portosystemic shunts. The mechanisms causing this brain dysfunction are still largely unclear. The need to avoid complications caused by late diagnosis has attracted interest to understand the mechanisms underlying neuronal damage in order to find markers that will allow timely diagnosis and to propose new therapeutic alternatives to improve the care of patients. One of the experimental approaches to study HE is microdialysis; this technique allows evaluation of different chemical substances in several organs through the recollection of samples in specific places by semi-permeable membranes. In this review we will discuss the contributions of microdialysis in the understanding of the physiological alterations in human hepatic encephalopathy and experimental models and the studies to find novel alternative therapies for this disease.

Clinical microdialysis in neuro-oncology: principles and applications

Clinical microdialysis allows a discrete volume of the brain to be sampled for neurochemical analysis of neurotransmitters, metabolites, biomarkers, and drugs. The technique can be safely used in humans intraoperatively, in the intensive care unit, and in ambulatory settings. Microdialysis probes, micropumps, and analytical equipment are commercially available and have been used extensively for neurochemical monitoring in traumatic brain injury, stroke, and subarachnoid hemorrhage. There has been very limited use of microdialysis in neuro-oncology, but this technique has great promise in the study of the basic neurochemistry of brain tumors, alterations in neurochemistry in response to therapy, and the pharmacokinetics of chemotherapeutic agents. Microdialysis probes may also be used to deliver drugs while simultaneously permitting monitoring of neurochemical changes induced by this therapy.

Interstitial Fluid and Lymph Formation and Transport: Physiological Regulation and Roles in Inflammation and Cancer

The interstitium describes the fluid, proteins, solutes, and the extracellular matrix (ECM) that comprise the cellular microenvironment in tissues. Its alterations are fundamental to changes in cell function in inflammation, pathogenesis, and cancer. Interstitial fluid (IF) is created by transcapillary filtration and cleared by lymphatic vessels. Herein we discuss the biophysical, biomechanical, and functional implications of IF in normal and pathological tissue states from both fluid balance and cell function perspectives. We also discuss analysis methods to access IF, which enables quantification of the cellular microenvironment; such methods have demonstrated, for example, that there can be dramatic gradients from tissue to plasma during inflammation and that tumor IF is hypoxic and acidic compared with subcutaneous IF and plasma. Accumulated recent data show that IF and its convection through the interstitium and delivery to the lymph nodes have many and diverse biological effects, including in ECM reorganization, cell migration, and capillary morphogenesis as well as in immunity and peripheral tolerance. This review integrates the biophysical, biomechanical, and biological aspects of interstitial and lymph fluid and its transport in tissue physiology, pathophysiology, and immune regulation.

Magnetic resonance imaging-guided microdialysis cannula implantation in a spontaneous high-grade glioma murine model

Cerebral microdialysis is used to study anticancer drug penetration in the central nervous system (CNS) and brain tumors in animal models. Genetically engineered murine models (GEMMs) have been recently used to study many aspects of CNS tumors since they represent a more relevant model than orthotopic brain tumor xenograft models. However, it is challenging to implant microdialysis cannula in these animals because T2-weighted magnetic resonance imaging (MRI) does not show the reference point (bregma) traditionally used to obtain stereotactic coordinates. Thus, an alternative reference point that can be visualized on MRI images is needed. In this study, a novel reference point, identified as the intersection between the olfactory bulb/frontal lobe border and the midline between cerebral hemispheres on T2-weighted MRI images, was used to calculate anterior-posterior and medial-lateral coordinates of brain tumors in a GEMM. This point overlies a visible crossover between the rostral rhinal vein and the midline suture on the mouse skull, allowing for the conversion of the MRI coordinates into surgical stereotactic coordinates. Postmortem MRI and histological examination confirmed accurate probe placement. This procedure will facilitate the accurate and precise implantation of microdialysis probes for the study of anticancer drug penetration in brain tumors of GEMMs.

Microdialysis as a method for biochemical and physiological studies of the porcine and human disc

At present, no minimally invasive technique exists for the continuous evaluation of the biochemistry of animal and human intervertebral discs, but for research purposes microdialysis may be such a new technique. Thus, the aims of this study were to (1) evaluate if microdialysis can be used successfully in lumbar porcine disc and (2) develop a suitable procedure for the application of microdialysis in the human disc. Conventional specific pathogen-free pigs were used to evaluate cannulation (n = 2) and then insertion of a 10 mm microdialysis membrane, including recovery of a dialysate from the nucleus pulposus (n = 6). The procedure was performed immediately after euthanasia and aided by fluoroscopy to guide the needle and confirm catheter placement. Access to lumbar porcine disc was obtained with an 18 G 2 in. needle applied at a 35-45 degrees angle from the sagittal plane, and took less than 8 min to perform. At a 0.5 microL/min flush rate, dialysates could be recovered and analysable amounts of glucose, lactate and pyruvate were obtained. In one human cadaver, the L4-L5 disc was accessed by a 19 G 3 in. needle inserted at a 35-40 degrees angle. It was possible to apply 10 mm as well as 30 mm microdialysis membranes in the nucleus. In both species the position of the membranes was verified by direct fluoroscopy and with contrast fluid. The results obtained from porcine and human cadavers are promising, and encourage further in vivo studies using microdialysis technique on intervertebral discs.

Interstitial fluid: the overlooked component of the tumor microenvironment?

Background

The interstitium, situated between the blood and lymph vessels and the cells, consists of a solid or matrix phase and a fluid phase, together constituting the tissue microenvironment. Here we focus on the interstitial fluid phase of tumors, i.e., the fluid bathing the tumor and stromal cells. Novel knowledge on this compartment may provide important insight into how tumors develop and how they respond to therapy.

Results

We discuss available techniques for interstitial fluid isolation and implications of recent findings with respect to transcapillary fluid balance and uptake of macromolecular therapeutic agents. By the development of new methods it is emerging that local gradients exist in signaling substances from neoplastic tissue to plasma. Such gradients may provide new insight into the biology of tumors and mechanistic aspects linked to therapy. The emergence of sensitive proteomic technologies has made the interstitial fluid compartment in general and that of tumors in particular a highly valuable source for tissue-specific proteins that may serve as biomarker candidates. Potential biomarkers will appear locally at high concentrations in the tissue of interest and will eventually appear in the plasma, where they are diluted.

Conclusions

Access to fluid that reliably reflects the local microenvironment enables us to identify substances that can be used in early detection and monitoring of disease.

A modified surgical procedure for microdialysis probe implantation in the lateral ventricle of a FVB mouse

A modified surgical procedure is described to implant a microdialysis probe to sample ventricular cerebrospinal fluid (vCSF) in FVB mice. Microdialysis sampling of drugs in vCSF provides insight into drug penetration into the brain across the blood brain barrier (BBB) and the blood CSF barrier (BCB); however, this method has been reported primarily in larger animal species. Implanting a microdialysis probe in the lateral ventricle of a mouse is technically very challenging. The modification consisted of changes in the stereotaxic coordinates and insertion of the cannula and ultimately the probe at a 20 degrees angle. Exact placement of the probe was confirmed using ultrasound (US), micro-computed tomography (CT), and histologic review of serial paraffin sections. Additionally, studies of topotecan CSF penetration in the FVB mouse were conducted. With this modified procedure, the ventricular CSF to plasma AUC ratio of unbound topotecan lactone was greater than that previously reported using conventional methods. We speculate this is due to changes incorporated by the modified procedure that places the probe directly into the lateral ventricle allowing sampling of that discrete compartment. Thus, we propose that this modified procedure for placement of the microdialysis probe is superior to the conventional perpendicular method previously reported.

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

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

Local controlled drug delivery to the brain: mathematical modeling of the underlying mass transport mechanisms

The mass transport mechanisms involved in the controlled delivery of drugs to living brain tissue are complex and yet not fully understood. Often the drug is embedded within a polymeric or lipidic matrix, which is directly administered into the brain tissue, that is, intracranially. Different types of systems, including microparticles and disc- or rod-shaped implants are used to control the release rate and, thus, to optimize the drug concentrations at the site of action in the brain over prolonged periods of time. Most of these dosage forms are biodegradable to avoid the need for the removal of empty remnants after drug exhaustion. Various physical and chemical processes are involved in the control of drug release from these systems, including water penetration, drug dissolution, degradation of the matrix and drug diffusion. Once the drug has been released from the delivery system, it has to be transported through the living brain tissue to the target site(s). Again, a variety of phenomena, including diffusion, drug metabolism and degradation, passive or active uptake into CNS tissue and convection can be of importance for the fate of the drug. An overview is given of the current knowledge of the nature of barriers to free access of drug to tumour sites within the brain and the state of the art of: (i) mathematical modeling approaches describing the physical transport processes and chemical reactions which can occur in different types of intracranially administered drug delivery systems, and of (ii) theories quantifying the mass transport phenomena occurring after drug release in the living tissue. Both, simplified as well as complex mathematical models are presented and their major advantages and shortcomings discussed. Interestingly, there is a significant lack of mechanistically realistic, comprehensive theories describing both parts in detail, namely, drug transport in the dosage form and in the living brain tissue. High quality experimental data on drug concentrations in the brain tissue are difficult to obtain, hence this is itself an issue in testing mathematical approaches. As a future perspective, the potential benefits and limitations of these mathematical theories aiming to facilitate the design of advanced intracranial drug delivery systems and to improve the efficiency of the respective pharmacotherapies are discussed.

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

INTRODUCTION

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

METHODS

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

RESULTS

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

DISCUSSION

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

In vivo microdialysis sampling of cytokines produced in mice given bacterial lipopolysaccharide

Cytokines are proteins that mediate communication between cells of the immune system as well as certain other non-immune host cells. These proteins are produced by many cell types and they mediate immune and inflammatory responses. However, the direct site analysis of these critical proteins is hampered by the lack of site-specific tools available for such direct measurements. In this study, both in vitro and in vivo microdialysis sampling of different cytokines (tumor necrosis factor-alpha [TNF-alpha], interferon-gamma [IFN-gamma], interleukin-6 [IL-6], IL-12p70, and macrophage chemoattractant protein-1 [MCP-1]) was performed. A mouse model of bacterial lipopolysaccharide (LPS) administration and response pattern was used for in vivo studies. Three cytokines, TNF-alpha, IL-6, and MCP-1 were quantified in the serum from mice given LPS. In vivo studies demonstrated the ability to monitor increasing levels of these cytokines (TNF-alpha, IL-6, and MCP-1) via microdialysis probes placed in the peritoneal cavity of mice given LPS. All three cytokines were quantified simultaneously in 15 muL of dialysate using a multiplexed bead-based immunoassay for flow cytometry. The detected dialysate cytokine concentrations varied between 200 pg/mL and 1500 pg/mL for TNF-alpha, between 600 pg/mL and 3000 pg/mL for MCP-1, and between 2700 pg/mL and more than 5000 pg/mL for IL-6. The detected serum cytokine concentrations ranged from 5700 pg/mL to 35,000 pg/mL for TNF-alpha, from 40,000 pg/mL to 65,000 pg/mL for MCP-1, and greater than than 100,000 pg/mL for IL-6. This work demonstrates that microdialysis sampling can be used in vivo to collect temporal profiles of cytokine production.

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.

Assaying protein unbound drugs using microdialysis techniques

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

Demonstration of low flow push-pull perfusion

Methods to follow in vivo chemical composition provide information regarding the processes of intercellular communication. There is a need for methods that provide chemical information from small volumes of the central nervous system (CNS) without sacrificing neurochemical recovery. One method that offers potential for providing such information is push-pull perfusion. In this study a low flow push-pull perfusion system is introduced that provides high (70-80%) in vitro recoveries. A concentric probe design is used with a 27-gauge stainless steel outer cannula for saline infusion and an inner fused silica capillary for fluid withdrawal. Flow rates of 10-50 nl/min were reliably generated and were well matched in vitro. Sampling was performed in the striatum of an anesthetized rat generating a 0.5 microl sample every 12 min. Capillary electrophoresis was used to determine glutamate levels in each sample; the basal level was found to be 1.97+/-0.70 microM. The method described was also demonstrated to deliver L-trans-pyrrolidine-2,4-dicarboxylic acid through the perfusion solution while sampling. Post-sampling histological analysis demonstrates little tissue disturbance to the sampled region. These data provide evidence that low flow push-pull method is a viable alternative for studying neurochemical signaling in the CNS.

Determination of local tissue concentrations of bupivacaine released from biodegradable microspheres and the effect of vasoactive compounds on bupivacaine tissue clearance studied by microdialysis sampling

PURPOSE

Incorporation of bupivacaine, a short acting local analgesic, into injectable microspheres provides a long acting formulation. Co-incorporation of dexamethasone into the microspheres results in extended activity. The purpose of this study is to compare tissue concentrations of bupivacaine resulting from the two types of microspheres and to determine if the observed sustained tissue concentration of bupivacaine is due to changes in its tissue clearance.

METHODS

Microdialysis probes were implanted into rat muscle. Following microsphere injection, bupivacaine tissue concentration was monitored (HPLC-UV), and the tissues histologically examined. The effect of vasoactive compounds on the tissue concentration of bupivacaine, not formulated in microspheres, was monitored.

RESULT

Hind muscle histology showed significant granulomatous reactions around the probe and both types of microspheres. A higher, prolonged bupivacaine concentration was observed from microspheres. A higher, prolonged bupivacaine concentration was observed from microspheres with co-incorporated dexamethasone relative to those without dexamethasone. Addition of vasoconstrictors to the perfusate containing bupivacaine resulted in decreased bupivacaine delivery compared to bupivacaine alone, whereas the addition of a vasodilator increased bupivacaine delivery.

CONCLUSIONS

The longer lasting effect of microspheres with co-incorporated dexamethasone results from higher, prolonged tissue concentrations of bupivacaine. Dexamethasone, a vasoconstrictor, decreases the clearance rate of bupivacaine from the tissue resulting in a higher, prolonged tissue concentration of bupivacaine.

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

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

Microdialysis in freely moving mice: determination of acetylcholine, serotonin and noradrenaline release in galanin transgenic mice

In the present study, we describe micro-surgical methods for simultaneous implantation of a microdialysis probe and an intraventricular injection cannula via their respective guide cannulas into the mouse brain. Basal and stimulated release of acetylcholine (ACh), serotonin (5-HT) and noradrenaline (NA) was determined in the ventral hippocampus of freely moving mice. NA and 5-HT were determined in one run by a newly developed HPLC method based on precolumn derivatization with benzylamine and fluorescence detection. The mice with a loss-of-function mutation of the galanin gene (KO) and the mice that over-expressed galanin (OE) were studied. No significant differences in basal, potassium-stimulated or scopolamine-induced extracellular ACh levels were observed in 4-month-old wild-type (WT) and KO mice. In the aged, 10-month-old animals, the basal extracellular ACh levels were significantly reduced in both WT and KO groups. Galanin (1 nmol i.c.v.) caused a significant reduction of basal extracellular NA by about 40% in both WT and galanin OE mice, however, in the latter group the effect was delayed by almost 2 h. A 10-min forced swimming stress caused a higher increase in release of NA and 5-HT in the OE group than in the corresponding WT mice. Finally, venlafaxin (10 mg/kg i.p.) increased extracellular NA to 400% of the control values in the CBA mice, but only to 250% in the C57BL mice. It is concluded that galanin may play an important role in the cholinergic mechanisms underlying cognitive disorders. Furthermore, modulation by galanin and by behavioral activation, of NA and 5-HT neurotransmission in galanin over-expressing mice indicates its possible role in the aetiology of mood disorders.