Probe calibration in transient microdialysis in vivo

On-Demand Nanoliter Sampling Probe for the Collection of Brain Fluid

Continuous fluidic sampling systems allow collection of brain biomarkers in vivo. Here, we propose a new sequential and intermittent sampling paradigm using droplets, called Droplet on Demand (DoD). It is implemented in a microfabricated neural probe and alternates phases of analyte removal from the tissue and phases of equilibration of the concentration in the tissue. It allows sampling droplets loaded with molecules from the brain extracellular fluid punctually, without the long transient equilibration periods typical of continuous methods. It uses an accurately defined fluidic sequence with controlled timings, volumes, and flow rates, and correct operation is verified by the embedded electrodes and a flow sensor. As a proof of concept, we demonstrated the application of this novel approach in vitro and in vivo, to collect glucose in the brain of mice, with a temporal resolution of 1-2 min and without transient regime. Absolute quantification of the glucose level in the samples was performed by direct infusion nanoelectrospray ionization Fourier transform mass spectrometry (nanoESI-FTMS). By adjusting the diffusion time and the perfusion volume of DoD, the fraction of molecules recovered in the samples can be tuned to mirror the tissue concentration at accurate points in time. Moreover, this makes quantification of biomarkers in the brain possible within acute experiments of only 20-120 min. DoD provides a complementary tool to continuous microdialysis and push-pull sampling probes. Thus, the advances allowed by DoD will benefit quantitative molecular studies in the brain, i.e., for molecules involved in volume transmission or for protein aggregates that form in neurodegenerative diseases over long periods.

A rotating operant chamber for use with microdialysis

BACKGROUND

Recently, the time resolution of microdialysis followed by a chemical separation for quantitative analysis has improved. The advent of faster microdialysis measurements promises to aid in behavioral research on awake animals. However, microdialysis with awake animals generally employs a fluidic commutator (swivel). The swivel's volume is inimical to the time resolution of the measurements.

NEW METHOD

Animals can be housed in rotating cages so that the swivel is not required, but rotating operant chambers are not available. Here we describe the design and construction of a rotating operant chamber with microdialysis capability. We modified a rotating cage by adding operant behavior testing components to the side of the bowl-shaped cage. A modular on-board controller facilitates operant component/computer communication. A battery provides power to the controller and the operant components. The battery and controller rotate with the cage, and the controller communicates with the computer wirelessly.

RESULTS

The rotating operant chamber can be used to train a rat to retrieve a sucrose pellet following a cue. Microdialysis and online liquid chromatography can be used to measure dopamine at one minute intervals while the rat moves freely and interacts with operant behavior testing components.

COMPARISON WITH EXISTING METHOD(S)

We are not aware of one-minute dopamine measurements in awake animals in an operant chamber.

CONCLUSIONS

Rotating cage modifications are straightforward. One-minute observations of striatal dopamine can be accomplished while an animal is awake, moving, and interacting with its surroundings.

Determination of acyclovir in renal microdialysis fluid and confirmation of renal function index

Acyclovir (ACV) is a nucleoside antivirus-free agent that was developed and marketed by Burroughs Well-come of the United States. Renal damage from ACV has been a major factor limiting its clinical application. Thus, the renal toxicity mechanism of ACV requires systematic study. In our previous study, we speculated that the nephrotoxicity of ACV may be associated with oxidative stress. In addition to the study of ACV's toxic effect in vivo, it is also necessary to explore the absorption and distribution of ACV in the body to further investigate the changes to ACV in the body. In this study, the toxicokinetics ACV in the kidney of the rat were explored using microdialysis, and the renal function of rats was measured. The results showed that high-dose ACV is associated with renal toxicity after a single intravenous injection or successive administration.

Sex- and SERT-mediated differences in stimulated serotonin revealed by fast microdialysis

In vivo microdialysis is widely used to investigate how neurotransmitter levels in the brain respond to biologically relevant challenges. Here, we combined recent improvements in the temporal resolution of online sampling and analysis for serotonin with a brief high-K(+) stimulus paradigm to study the dynamics of evoked release. We observed stimulated serotonin overflow with high-K(+) pulses as short as 1 min when determined with 2-min dialysate sampling in ventral striatum. Stimulated serotonin levels in female mice during the high estrogen period of the estrous cycle were similar to serotonin levels in male mice. By contrast, stimulated serotonin overflow during the low estrogen period in female mice was increased to levels similar to those in male mice with local serotonin transporter (SERT) inhibition. Stimulated serotonin levels in mice with constitutive loss of SERT were considerably higher yet, pointing to neuroadaptive potentiation of serotonin release. When combined with brief K(+) stimulation, fast microdialysis reveals dynamic changes in extracellular serotonin levels associated with normal hormonal cycles and pharmacologic vs genetic loss of SERT function.

Extracellular fluid concentrations of cisplatin, carboplatin, and oxaliplatin in brain, muscle, and blood measured using microdialysis in nonhuman primates

PURPOSE

Cisplatin, carboplatin, and oxaliplatin are chemically reactive anticancer drugs with modest activity in brain tumors. Previously, we have demonstrated that drug exposure in cerebrospinal fluid (CSF) for these platinum analogs is <5% of the plasma ultrafiltrate (UF) drug exposure in nonhuman primates. Microdialysis is a minimally invasive in vivo method for sampling small molecules in the blood and tissue extracellular fluid (ECF). The purpose of this study was to estimate the penetration of platinum analogs into the brain ECF.

METHODS

We measured free concentrations of cisplatin, carboplatin, and oxaliplatin in ECF of brain, muscle, and blood of nonhuman primates using microdialysis and compared ECF platinum concentrations in blood and brain to plasma UF and CSF concentrations obtained using conventional sampling methods.

RESULTS

For all three platinum analogs, AUC(0-4h) for microdialysis sampling from the vein was similar to standard plasma UF sampling. The median AUC(0-4h) ratio for vein to plasma UF was 1.1 (range, 0.9-1.4). The platinum analogs had limited distribution (<5%) to the CSF and brain ECF. CSF penetration predicts for the limited penetration of the platinum analogs into brain ECF, but concordance between CSF and brain ECF measurements was poor. CSF oxaliplatin concentrations (AUC(0-4h), 0.4-0.9 microM h) were substantially lower than brain ECF concentrations (AUC(0-4h), 2.0-8.6 microM h).

CONCLUSIONS

The penetration of platinum analogs into CSF and brain is limited. The differences in the CNS penetrations among the three platinum analogs are not clinically significant. For cisplatin and carboplatin, CSF penetration appears to be a surrogate for brain extracellular free drug exposure.

The use of a deuterated calibrator for in vivo recovery estimations in microdialysis studies

One of the crucial issues in quantitative microdialysis is the reliability of recovery estimates to correctly estimate unbound drug tissue concentrations. If a deuterated calibrator is used for retrodialysis, the calibrator has the same properties as the study drug. However, recovery of the calibrator may be affected by the presence of the drug in the tissues. The aim of this study was to investigate the recovery of deuterated morphine with time in the absence and presence of morphine in rat tissues. Microdialysis probes were placed in the brain and blood of eight rats. Ringer's solution containing D3-morphine was perfused throughout the study and recovery was estimated. After a stabilization period of 3 h, an exponential infusion of morphine was administered over 4 h. The presence of morphine did not affect the recovery of D3-morphine from brain or blood. The average recovery values (SD) were 0.145 (0.039) and 0.131 (0.048) during the stabilization and infusion periods, respectively, for the brain probe and 0.792 (0.055) and 0.790 (0.084), respectively, for the blood probe. The recovery of deuterated morphine was stable over time in the brain and in blood, and was not affected by the presence of pharmacologically concentrations of morphine.

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.

AAPS-FDA workshop white paper: microdialysis principles, application and regulatory perspectives

Many decisions in drug development and medical practice are based on measuring blood concentrations of endogenous and exogenous molecules. Yet most biochemical and pharmacological events take place in the tissues. Also, most drugs with few notable exceptions exert their effects not within the bloodstream, but in defined target tissues into which drugs have to distribute from the central compartment. Assessing tissue drug chemistry has, thus, for long been viewed as a more rational way to provide clinically meaningful data rather than gaining information from blood samples. More specifically, it is often the extracellular (interstitial) tissue space that is most closely related to the site of action (biophase) of the drug. Currently microdialysis (microD) is the only tool available that explicitly provides data on the extracellular space. Although microD as a preclinical and clinical tool has been available for two decades, there is still uncertainty about the use of microD in drug research and development, both from a methodological and a regulatory point of view. In an attempt to reduce this uncertainty and to provide an overview of the principles and applications of microD in preclinical and clinical settings, an AAPS-FDA workshop took place in November 2005 in Nashville, TN, USA. Stakeholders from academia, industry and regulatory agencies presented their views on microD as a tool in drug research and development.

Monitoring of metabolite gradients in tissue-engineered constructs

At present, the assessment of developing tissue-engineered constructs is almost always carried out destructively using biochemical or histological methods to determine cell number, viability and tissue growth throughout the construct. Since many of these experiments are long, taking weeks or even months to complete, simple and readily applicable non-destructive methods of monitoring changes in cell metabolism, viability and tissue deposition within the construct would be invaluable; such methods could point out adverse responses during the early stages of culture. Here, we describe the use of microdialysis for detecting local changes in cellular metabolism within a tissue-engineered construct. Three-dimensional constructs consisting of bovine articular chondrocytes entrapped in an alginate gel were cultured in a bioreactor for two weeks. Glucose and lactate were monitored by microdialysis, as the major nutrient and metabolite, respectively. Concentration gradients within the construct were evident, with the highest lactate concentrations in the construct centre. The local lactate concentration was a measure of cellular metabolic activity, decreasing as cellular activity fell and increasing as cellular activity was stimulated. Nutrient starvation and cell death in the construct centre could be readily detected in constructs deliberately cultured under adverse conditions. The results show that probe measurements can give an early warning of inappropriate local metabolic changes. Such information during the growth of tissue-engineered constructs would allow either corrective action or else an early end to an unsuccessful test.

Microdialysis evaluation of the brain distribution of stavudine following intranasal and intravenous administration to rats

Intranasal (IN) administration as a potential route of enhancing brain delivery of stavudine (d4T) was investigated in rats using microdialysis as a sampling technique. Sprague-Dawley rats were divided into two groups (n = 7 per group). One group of animals received IN administration of 5 mg/kg d4T (50 microL); the other group was dosed intravenously (IV) at the same dose. Following IN administration, d4T was rapidly and completely absorbed into the systemic circulation with a T(max) of 14 min and an IN bioavailability of 105%. The brain/plasma AUC ratios in the lateral ventricle, caudate putamen, and frontal cortex in the anesthetized and nasal surgery-operated rats were 0.36 +/- 0.090, 0.47 +/- 0.089, and 0.41 +/- 0.087, respectively, whereas they were 0.63 +/- 0.077, 0.62 +/- 0.17, 0.60 +/- 0.13, respectively, following IV dosing to sham animals. The half-life of d4T in the various brain regions was significantly longer than that in plasma (p < 0.05). Moreover, the systemic clearance of d4T was significantly reduced in these anesthetized and nasal surgery-operated animals. Further studies of the effect of anesthesia suggest the additive role of anesthesia, possibly in additional to nasal surgery, in decreasing the systemic clearance. The extent of the brain distribution, however, was not significantly affected by anesthesia. Lack of enhancement of the brain delivery of d4T following IN administration over systemic dosing cannot be attributed to its absorption into systemic circulation, since direct nose-brain transport, if fully functional and effective, should be a parallel and competing process with systemic absorption. The current study results along with several physiological considerations raise a question regarding the overall effectiveness of IN administration for direct delivery of small molecules into brain tissues, particularly where passive diffusion predominates.

Measurement of drug release from microcarriers by microdialysis

The purpose of this study was to examine the feasibility of the microdialysis sampling technique as a method to precisely and conveniently measure drug release from microcarrier systems such as liposomes and microspheres. Release of 5-fluorouracil (5-FU) from liposomes and microspheres was evaluated in vitro using microdialysis. Retrodialysis calibration using 5-chlorouracil (5-CU) was performed in conjunction with on-line HPLC analysis. At a microdialysis perfusate flow rate of 0.5 muL/min, concurrent 5-FU gain and 5-CU loss ranged from 72% to 75%, while concurrent 5-FU loss and 5-CU ranged from 69% to 71%. After calibration, simultaneous 5-FU release profiles were obtained by continuous microdialysis and discrete equilibrium dialysis sampling using a side-by-side diffusion apparatus. Release rates were characterized by a first-order release model. The release rate constants for a representative liposomal formulation were 0.30 and 1.85/h by microdialysis in the acceptor and donor compartments, respectively, and 0.39/h by equilibrium dialysis in the acceptor compartment. The calculated release rate constant determined by equilibrium dialysis in the donor compartment (1.98/h) agrees with that determined by microdialysis (1.85/h) when the resistance of the equilibrium dialysis membrane with associated first-order rate constant of transfer of 0.42/h is taken into account. Release profiles of 5-FU from a number of different liposome and microsphere formulations were determined. The results indicate that a convenient and reproducible characterization of drug release from various liposome and microsphere formulations is readily obtainable by microdialysis.

Axially symmetric semi-infinite domain models of microdialysis and their application to the determination of nutritive flow in rat muscle

Theoretical models for the description of microdialysis outflow:inflow (O/I) ratio for 3H2O and [14C]ethanol were developed, taking into account the nutritive fraction of total blood flow in muscle. The models yielded an approximately exponential decay expression for the O/I ratio, dependent on the physical dimensions of a linear probe (length and radius), the flow rate through the probe, muscle blood flow (including the nutritive fraction) and the diffusion coefficients for the tracer in the probe and muscle. The models compared favourably with experimental data from the constant-flow perfused rat hindlimb. Estimates of the nutritive fraction of total blood flow from experimental data were determined by minimizing the error between model and experimental data. The nutritive fraction was found to be 0.22 +/- 0.04 under basal perfusion conditions. When 70 nM noradrenaline (norepinephrine) was included in the perfusion medium, the nutritive fraction was 0.91 +/- 0.06 (P < 0.05). The inclusion of 300 nM serotonin, decreased the nutritive fraction to 0.05 +/- 0.01 (P < 0.05). This model can be applied to the determination of nutritive fraction of skeletal muscle blood flow in physiologically relevant microvascular conditions such as during exercise and in disease states.

Review of microdialysis in brain tumors, from concept to application: first annual Carolyn Frye-Halloran symposium

In individuals with brain tumors, pharmacodynamic and pharmacokinetic studies of therapeutic agents have historically used analyses of drug concentrations in serum or cerebrospinal fluid, which unfortunately do not necessarily reflect concentrations within the tumor and adjacent brain. This review article introduces to neurological and medical oncologists, as well as pharmacologists, the application of microdialysis in monitoring drug metabolism and delivery within the fluid of the interstitial space of brain tumor and its surroundings. Microdialysis samples soluble molecules from the extracellular fluid via a semipermeable membrane at the tip of a probe. In the past decade, it has been used predominantly in neurointensive care in the setting of brain trauma, vasospasm, epilepsy,and intracerebral hemorrhage. At the first Carolyn Frye-Halloran Symposium held at Massachusetts General Hospital in March 2002, the concept of microdialysis was extended to specifically address its possible use in treating brain tumor patients. In doing so we provide a rationale for the use of this technology by a National Cancer Institute consortium, New Approaches to Brain Tumor Therapy, to measure levels of drugs in brain tissue as part of phase 1 trials.

Noradrenaline transporter blockers raise extracellular dopamine in medial prefrontal but not parietal and occipital cortex: differences with mianserin and clozapine

This study compared the interaction between noradrenaline (NA) and dopamine (DA) mechanisms in the prefrontal (PFCX) and in the parietal (ParCX) and occipital (OccCX) cortex. The effect of reboxetine and desipramine, two NA transporter blockers, of mianserin, an antagonist of alpha2 and 5-HT2 receptors, and of clozapine, an atypical antipsychotic, on dialysate DA in the medial PFCX, ParCX and OccCX was studied. We also assessed the influence of a prior 6-hydroxydopamine (6-OHDA) lesion of the dorsal noradrenergic bundle (DNAB) on the effect of reboxetine and clozapine on dialysate DA in the PFCX and ParCX. Systemic administration of reboxetine and desipramine dose-dependently increased dialysate DA in the PFCX but not in the ParCX and OccCX. In contrast, mianserin and clozapine raised dialysate DA in the ParCX and OccCX to an even larger extent than in the PFCX. 6-OHDA lesions of DNAB abolished the increase of dialysate DA elicited by reboxetine in the PFCX and by clozapine both in the PFCX and in the ParCX. It is concluded that, although PFCX and ParCX/OccCX share the presence of a strong control of DA transmission by NA through alpha2 receptors, they differ in the extent to which DA is cleared from the extracellular compartment by uptake through the NA transporter. This process, although extensive in the PFCX, appears insignificant in the ParCX and OccCX, probably as a result of the higher ratio of NA to DA resulting in exclusion of DA from NA transporter.

Effects of blood flow on the in vivo recovery of a small diffusible molecule by microdialysis in human skin

The aim of this study was to investigate the impact of changes in local blood flow on the recovery of a small, diffusible molecule (sodium fluorescein) from the extravascular tissue space of the skin, by microdialysis in vivo. Loss and recovery of fluorescein by linear microdialysis probes (5-kDa molecular mass cutoff, 0.2 mm diameter) inserted 1 mm apart in pairs, at three sites in the skin of the volar surface of the forearm of healthy volunteers, was measured under conditions of basal, reduced (noradrenaline, 0.005 mg/ml), and increased (glyceryl trinitrate, patch) blood flow. Whereas loss of tracer from the delivery probe appeared unaffected by changes in local blood flow, retrieval of fluorescein by the second probe was directly related to blood flux, measured using scanning laser Doppler imaging. Steady-state recovery at vasoconstricted sites was 4.0 +/- 0.7 microg. ml(-1) compared with 1.8 +/- 0.7 microg. ml(-1) at control sites (p < 0.001). Local vasodilatation reduced the retrieval of fluorescein by approximately 50% to give a steady-state concentration of fluorescein in the dialysate at 40 to 50 min after the start of perfusion of 0.9 +/- 0.3 microg. ml(-1) (p = 0.05). These studies in the skin are consistent with microdialysis theory. They suggest that clearance of solute by the blood will have a significant impact on microdialysis probe recovery and that, in the skin, the magnitude of this clearance is directly related to blood flow.

Zidovudine concentration in brain extracellular fluid measured by microdialysis: steady-state and transient results in rhesus monkey

We measured zidovudine concentrations in blood, muscle, and brain extracellular fluid (ECF) by microdialysis and in serum ultrafiltrate and cerebrospinal fluid (CSF) samples during a continuous intravenous infusion (15 mg/kg/h) and after bolus dosing (50-80 mg/kg over 15 min) in nonhuman primates to determine whether CSF drug penetration is a valid surrogate for blood-brain barrier penetration. Recovery was estimated in vivo by zero net flux for the continuous infusion and retrodialysis for the bolus dosing. In vivo recovery was tissue-dependent and was lower in brain than in blood or muscle. Mean (+/-S.D.) steady-state blood, muscle, and brain zidovudine concentrations by microdialysis were 112 +/- 63.8, 105 +/- 51.1, and 13.8 +/- 10.4 microM, respectively; and steady-state serum ultrafiltrate and CSF concentrations were 81.2 +/- 40.2 and 14.1 +/- 8.0 microM, respectively. Brain ECF penetration (microdialysis brain/blood ratio) and CSF penetration (standard sampling CSF/serum ratio) at steady state were 0.13 +/- 0.06 and 0.17 +/- 0.02, respectively. With bolus dosing the mean (+/-S.D.) zidovudine area under concentration-time curve (AUC) normalized to a dose of 80 mg/kg was 577 +/- 103 microM. h in blood, 528 +/- 202 microM. h in muscle, and 108 +/- 74 microM. h in brain (brain/blood ratio of 0.18 +/- 0.10) by microdialysis. Serum ultrafiltrate AUC was 446 +/- 72 microM. h and the CSF AUC was 123 +/- 4.7 microM. h (CSF/serum ratio of 0.28 +/- 0.06). In conclusion, recovery was tissue-dependent. CSF and brain ECF zidovudine concentrations were comparable at steady state, and the corresponding AUCs were comparable after bolus injection. Thus, zidovudine penetration in brain ECF and CSF in nonhuman primates is limited to a similar extent, presumably by active transport, as in other species.

In vivo monitoring of amino acids by direct sampling of brain extracellular fluid at ultralow flow rates and capillary electrophoresis

Extracellular levels of glutamate (GLU), aspartate (ASP), glycine (GLY), phosphoethanolamine (PEA), and gamma-aminobutyric acid (GABA) were measured in the striatum of anesthetized rats using a novel sampling approach in which extracellular fluid (ECF) was removed at 1-50 nl/min using a fused silica capillary tube with 18-40 microm inner diameter and a outer diameter of 90 microm. The samples of ECF were analyzed by capillary electrophoresis with laser-induced fluorescence detection. Basal levels for GABA, GLY, and GLU measured using direct sampling at 1 nl/min were 270 +/- 40, 4950 +/- 1100, and 1760 +/- 150 nM, respectively in good agreement with the values obtained using microdialysis sampling calibrated by the low-flow rate method. ASP levels were approximately four-fold higher in directly sampled fluid than in dialysate. At higher direct sampling flow rates (10-50 nl/min), detected levels of the amino acids were lower by 70-90% indicating depletion of analyte under these conditions. PEA, an indicator of membrane disruption, was 5.5-fold higher in dialysate than in directly sampled ECF indicating greater tissue damage associated with microdialysis. In addition to the basal measurements, the direct sampling technique was applied to monitoring concentration changes of GLU and ASP in the striatum with better than 90 s temporal resolution after perfusion of either 120 mM K(+) or 400 microM L-trans-pyrrolidine-2,4-dicarboxylic acid (PDC) through a microdialysis probe immediately adjacent to the direct sampling capillary. Levels of GLU and ASP increased 615 +/- 95 and 542 +/- 96%, respectively (n=4) upon addition of 120 mM K(+) to the perfusate and 622 +/- 234 and 672 +/- 218% (n=5) for PDC. It is concluded that direct sampling at low-flow rates allows determination of extracellular levels of the amino acids with spatial resolution that is at least 500-fold better than microdialysis.