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

Implementation and validation of a UHPLC-MS/MS method for quantification of the endocannabinoids AEA and 2-AG in cerebral interstitial fluid and plasma

Endogenous endocannabinoids such as N-arachidonoylethanolamine (AEA) and 2-arachidonoylglycerol (2-AG) are involved in the patho-biochemistry of several neurological diseases and have been associated with mood-enhancing phenomena. Although they have been intensively studied in recent years, accurate and reliable quantification of these analytes in cerebral interstitial fluid (cISF) to elucidate their neuro-modulatory role is still challenging. Moreover, there is a need for an analytical method that can analyze plasma in addition to cISF and is thus able to address research questions in both preclinical and clinical studies. Aim was to implement a method for simultaneous quantification of AEA and 2-AG in cISF and plasma, to validate it by taking the requirements of the U.S. Food and Drug Administration into account, and to test its usability in three different case studies. A UHPLC-MS/MS method with preceding liquid-liquid extraction to determine AEA and 2-AG in cISF and plasma was successfully implemented, and the parameters selectivity, specificity, linearity, accuracy, precision, sensitivity, carry-over and stability met the validation criteria. The usability of the analytical method was demonstrated in an in vitro study with cerebral open flow microperfusion (cOFM), an in vivo cOFM study in rats, and a clinical study in human plasma. The developed method allowed quantification of AEA and 2-AG in the biologically relevant concentration ranges in cISF and plasma. The availability of a reliable, complementary, time-resolved dataset of endocannabinoid concentrations in both matrices can be of considerable future importance for the evaluation of drug efficacy.

Atraumatic Access to Human Glioblastoma in a Xenograft Animal Model by Cerebral Open Flow Microperfusion

BACKGROUND

Orthotopic xenograft studies promote the development of targeted/personalized therapies to improve the still poor life expectancy of glioblastoma patients.

NEW METHOD

We implemented an atraumatic access to glioblastoma with cerebral Open Flow Microperfusion (cOFM) by implantation of xenograft cells in rat brain with intact blood brain barrier (BBB) and subsequent development of a xenograft glioblastoma at the interface between the cOFM probe and surrounding brain tissue. Human glioma U87MG cells were implanted at a well-defined position into immunodeficient Rowett nude rat´s brain via cOFM (cOFM group) and syringe (control group). Characteristics of the mature tumors from both groups were assessed.

RESULTS

For the first time xenograft cells were successfully introduced into rat brain with intact BBB using cOFM, and the tumor tissue developing around the cOFM probe was unaffected by the presence of the probe. Thereby an atraumatic access to the tumor was created. The success rate of glioblastoma development in the cOFM group was high (>70%). The mature cOFM-induced tumors (20-23 days after cell-implantation) resembled the syringe-induced ones and showed typical features of human glioblastoma.

COMPARISON WITH EXISTING METHOD

Examining xenograft tumor microenvironment with currently available methods inevitably causes trauma that could affect the reliability of obtained data.

CONCLUSION

This novel atraumatic access to human glioblastoma in rat brain provides the possibility to collect interstitial fluid from functional tumor tissue in vivo without trauma generation. Thereby, reliable data can be generated promoting drug research, biomarker identification, and enabling investigation of the BBB of an intact tumor.

DATA AVAILABILITY STATEMENT

Original data are available upon request from the corresponding author.

A new electromembrane extraction probe for on-line connection with capillary electrophoresis for determination of substances in biological matrices

A miniature probe for electromembrane extraction is developed and constructed. The tubular probe with an internal volume of 1.1 μL is made of polypropylene hollow fiber with a supported liquid membrane of 85% nitrophenyloctyl ether (NPOE) with 15% bis(2-ethylhexyl)phosphonic acid (DEHP). The probe is connected on-line to the electrophoresis with short separation capillary via an air assisted flow gating interface cast from poly (dimethylsiloxane). The compact instrument is computer controlled via LabView. The probe parameters are tested for extraction of creatinine and basic amino acids from artificial solution and human urine. The sensitivity of the electrophoretic determination after 300 s extraction at 150 V compared to the sensitivity without extraction is 4.9-fold and 2.6-fold higher for creatinine and arginine, respectively. The RSDs for peak area measured from 5 repeated extractions of 50 μM solutions are 7.5%, 7.2%, 8.6% and 9.2% for Crea, Lys, Arg and His, respectively. The probe can be used for all-day measurements. The preparation of the probe is simple and requires no special tool.

Quantitative monitoring and modelling of retrodialysis drug delivery in a brain phantom

A vast number of drug molecules are unable to cross the blood-brain barrier, which results in a loss of therapeutic opportunities when these molecules are administered by intravenous infusion. To circumvent the blood-brain barrier, local drug delivery devices have been developed over the past few decades such as reverse microdialysis. Reverse microdialysis (or retrodialysis) offers many advantages, such as a lack of net volume influx to the intracranial cavity and the ability to sample the tumour's micro-environment. However, the translation of this technique to efficient drug delivery has not been systematically studied. In this work, we present an experimental platform to evaluate the performance of microdialysis devices in reverse mode in a brain tissue phantom. The mass of model drug delivered is measured by computing absorbance fields from optical images. Concentration maps are reconstructed using a modern and open-source implementation of the inverse Abel transform. To illustrate our method, we assess the capability of a commercial probe in delivering methylene blue to a gel phantom. We find that the delivery rate can be described by classical microdialysis theory, except at low dialysate flow rates where it is impacted by gravity, and high flow rates where significant convection to the gel occurs. We also show that the flow rate has an important impact not only on the overall size of the drug plume, but also on its shape. The numerical tools developed for this study have been made freely available to ensure that the method presented can be used to rapidly and inexpensively optimise probe design and protocol parameters before proceeding to more in-depth studies.

Neuropharmacological Study of Posaconazole for Glioblastoma: A Phase 0 Clinical Trial Protocol

BACKGROUND

Glioblastoma (GBM) is the most common malignant primary brain tumor with a universally poor prognosis. GBMs express elevated levels of hexokinase 2 (HK2), catalyzing the critical step in glycolysis and influencing several oncogenic pathways. Previous preclinical work has suggested a role for repurposed posaconazole (PCZ) in downregulating HK2 activity, reducing lactate and pyruvate production, interfering with tumor cell metabolism, and increasing mouse survival.

OBJECTIVE

To establish brain tumor penetrance, neuropharmacokinetic profile, and mechanistic effect on tumor cell metabolism of PCZ in adults with GBM.

METHODS

This is an open label, nonrandomized, parallel arm trial involving patients with GBM. Cohorts will receive PCZ (intervention, n = 5) or will not receive PCZ (control, n = 5), followed by tumor resection and microdialysis catheter placement. Dialysate, plasma, and tumor samples will be analyzed for lactate and pyruvate concentrations. Tumor samples will also be assessed for PCZ concentration, HK2 expression, angiogenesis, and apoptosis. PCZ's neuropharmacokinetics will be determined based on the concentration vs time profile and area under the curve 0 to 24 hours of PCZ concentration in the brain interstitium.

EXPECTED OUTCOMES

(1) Increased PCZ concentration in contrast-enhancing brain regions compared with nonenhancing regions; (2) inverse correlation between lactate/pyruvate and PCZ concentrations in dialysate samples from treated patients, over time; and (3) decreased HK2 activity in PCZ-treated tumor samples.

DISCUSSION

A successful trial will support the decision to proceed to advanced phase trials. Any tumor penetration by PCZ, with concomitant effect on glycolysis, warrants further in-depth analysis, as therapeutic options for these deadly tumors are currently limited.

High-grade glioma therapy: adding flexibility in trial design to improve patient outcomes

INTRODUCTION

Outcomes for patients with high grade gliomas have changed little over the past thirty years. This realization prompted renewed efforts to increase flexibility in the design and conduct of clinical brain tumor trials.

AREAS COVERED

This manuscript reviews the development of clinical trial methods, challenges and considerations of flexible clinical trial designs, approaches to improve identification and testing of active agents for high grade gliomas, and evaluation of their delivery to the central nervous system.

EXPERT OPINION

Flexibility can be introduced in clinical trials in several ways. Flexible designs tout smaller sample sizes, adaptive modifications, fewer control arms, and inclusion of multiple arms in one study. Unfortunately, modifications in study designs cannot address two challenges that are largely responsible for the lack of progress in treating high grade gliomas: 1) the identification of active pharmaceutical agents and 2) the delivery of these agents to brain tumor tissue in therapeutic concentrations. To improve the outcomes of patients with high grade gliomas efforts must be focused on the pre-clinical screening of drugs for activity, the ability of these agents to achieve therapeutic concentrations in non-enhancing tumors, and a willingness to introduce novel compounds in minimally pre-treated patient populations.

Perioperative clinical trials for glioma: Raising the bar

Gliomas are a heterogeneous group of primary brain cancers with poor survival despite multimodality therapy that includes surgery, radiation and chemotherapy. Numerous clinical trials have investigated systemic therapies in glioma, but have largely been negative. Multiple factors have contributed to the lack of progress including tumour heterogeneity, the tumour micro-environment and presence of the blood-brain barrier, as well as extrinsic factors relating to trial design, such as the lack of a contemporaneous biopsy at the time of treatment. A number of strategies have been proposed to progress new agents into the clinic. Here, we review the progress of perioperative, including phase 0 and 'window of opportunity', studies and provide recommendations for trial design in the development of new agents for glioma. The incorporation of pre- and post-treatment biopsies in glioma early phase trials will provide valuable pharmacokinetic and pharmacodynamic data and also determine the target or biomarker effect, which will guide further development of new agents. Perioperative 'window of opportunity' studies must use drugs with a recommended-phase-2-dose, known safety profile and adequate blood-brain barrier penetration. Drugs shown to have on-target effects in perioperative trials can then be evaluated further in a larger cohort of patients in an adaptive trial to increase the efficiency of drug development.

Effects of Pulse Interval and Dosing Flux on Cells Varying the Relative Velocity of Micro Droplets and Culture Solution

Microdroplet dosing to cell on a chip could meet the demand of narrow diffusion distance, controllable pulse dosing and less impact to cells. In this work, we studied the diffusion process of microdroplet cell pulse dosing in the three-layer sandwich structure of PDMS (polydimethylsiloxane)/PCTE (polycarbonate) microporous membrane/PDMS chip. The mathematical model is established to solve the diffusion process and the process of rhodamine transfer to micro-traps is simulated. The rhodamine mass fraction distribution, pressure field and velocity field around the microdroplet and cell surfaces are analyzed for further study of interdiffusion and convective diffusion effect. The cell pulse dosing time and drug delivery efficiency could be controlled by adjusting microdroplet and culture solution velocity without impairing cells at micro-traps. Furthermore, the accuracy and controllability of the cell dosing pulse time and maximum drug mass fraction on cell surfaces are achieved and the drug effect on cells could be analyzed more precisely especially for neuron cell dosing.

Foe or friend? Janus-faces of the neurovascular unit in the formation of brain metastases

Despite the potential obstacle represented by the blood-brain barrier for extravasating malignant cells, metastases are more frequent than primary tumors in the central nervous system. Not only tightly interconnected endothelial cells can hinder metastasis formation, other cells of the brain microenvironment (like astrocytes and microglia) can also be very hostile, destroying the large majority of metastatic cells. However, malignant cells that are able to overcome these harmful mechanisms may benefit from the shielding and even support provided by cerebral endothelial cells, astrocytes and microglia, rendering the brain a sanctuary site against anti-tumor strategies. Thus, cells of the neurovascular unit have a Janus-faced attitude towards brain metastatic cells, being both destructive and protective. In this review, we present the main mechanisms of brain metastasis formation, including those involved in extravasation through the brain vasculature and survival in the cerebral environment.

In vivo neurochemical measurements in cerebral tissues using a droplet-based monitoring system

Direct collection of extracellular fluid (ECF) plays a central role in the monitoring of neurological disorders. Current approaches using microdialysis catheters are however drastically limited in term of temporal resolution. Here we show a functional in vivo validation of a droplet collection system included at the tip of a neural probe. The system comprises an advanced droplet formation mechanism which enables the collection of neurochemicals present in the brain ECF at high-temporal resolution. The probe was implanted in a rat brain and could successfully collect fluid samples organized in a train of droplets. A microfabricated target plate compatible with most of the surface-based detection methods was specifically developed for sample analysis. The time-resolved brain-fluid samples are analyzed using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). The results provide a time evolution picture of the cerebral tissues neurochemical composition for selected elements known for their involvement in neurodegenerative diseases.

Ganciclovir concentrations in the cerebral extracellular space after valganciclovir treatment; a case study

Nearly all glioblastomas (GBMs), brain tumours with very poor prognosis, are infected with human cytomegalovirus (CMV). The anti-CMV drug valganciclovir (VGCV) has shown promise as a treatment option for patients with GBM, but its penetration into the central nervous system (CNS) is unknown. Here we describe a patient with GMB receiving VGCV in whom an intracerebral microdialysis catheter was implanted and ganciclovir (GCV) concentrations in brain extracellular fluid (BECF) and serum were monitored. GCV was rapidly absorbed. Cmax values (at 3 h) in serum and BECF were 19.6 and 10.2 µmol/L, T½ values were 3.2 and 4.5 h, and plasma and BECF AUC0-∞ values were 90.7 and 75.9 µmol h/L, respectively. Thus, VGCV treatment results in significant intracerebral levels of GCV that may be sufficient for therapeutic effects. Further studies of this drug in patients with GBM are warranted.

Tumor Interstitial Fluid Formation, Characterization, and Clinical Implications

The interstitium, situated between the blood and lymph vessels and the cells, consists of a solid or matrix phase and a fluid phase representing the tissue microenvironment. In the present review, we focus on the interstitial fluid phase of solid tumors, the tumor interstitial fluid (TIF), i.e., the fluid bathing the tumor and stroma cells, also including immune cells. This is a component of the internal milieu of a solid tumor that has attracted regained attention. Access to this space may provide important insight into tumor development and therapy response. TIF is formed by transcapillary filtration, and since this fluid is not readily available we discuss available techniques for TIF isolation, results from subsequent characterization and implications of recent findings with respect to fluid filtration and uptake of macromolecular therapeutic agents. There appear to be local gradients in signaling substances from neoplastic tissue to plasma that may provide new understanding of tumor biology. The development of sensitive proteomic technologies has made TIF a valuable source for tumor specific proteins and biomarker candidates. Potential biomarkers will appear locally in high concentrations in tumors and may eventually be found diluted in the plasma. Access to TIF that reliably reflects the local tumor microenvironment enables identification of substances that can be used in early detection and monitoring of disease.

Observational Evaluations of Mice during Cerebral Microdialysis for Pediatric Brain Tumor Research

In vivo animal experiments are critical in the process of finding and developing new treatments for children with CNS tumors. Cerebral microdialysis, which enables researchers to measure drug concentrations in the brain or tumor tissue of unanesthetized mice, is a highly specialized procedure that provides valuable information that cannot be gained by using an in vitro system. When designing any in vivo animal study, 3 Rs principles (replacement, reduction, and refinement) must be considered to ensure that the highest standards of care are followed. As part of the refinement process, the objectives of this study were to collect behavioral monitoring data from mice undergoing cerebral microdialysis, to identify any behaviors predictive of significant pain or distress that could affect the animal's welfare, and to use these data to refine the existing monitoring checklist and schedule for its use by others performing this procedure. We developed a monitoring checklist for assessing wellbeing and distress of mice during cerebral microdialysis experiments. Comparison of 79 mice that underwent cerebral microdialysis experiments with a control group of 20 mice revealed that cerebral microdialysis and tethering of mice are well tolerated for as long as 24 h with only minor evidence of stress.

Microdialysis Monitoring of CSF Parameters in Severe Traumatic Brain Injury Patients: A Novel Approach

Background: Neuro-intensive care following traumatic brain injury (TBI) is focused on preventing secondary insults that may lead to irreversible brain damage. Microdialysis (MD) is used to detect deranged cerebral metabolism. The clinical usefulness of the MD is dependent on the regional localization of the MD catheter. The aim of this study was to analyze a new method of continuous cerebrospinal fluid (CSF) monitoring using the MD technique. The method was validated using conventional laboratory analysis of CSF samples. MD-CSF and regional MD-Brain samples were correlated to patient outcome.

Materials and Methods: A total of 14 patients suffering from severe TBI were analyzed. They were monitored using (1) a MD catheter (CMA64-iView, n = 7448 MD samples) located in a CSF-pump connected to the ventricular drain and (2) an intraparenchymal MD catheter (CMA70, n = 8358 MD samples). CSF-lactate and CSF-glucose levels were monitored and were compared to MD-CSF samples. MD-CSF and MD-Brain parameters were correlated to favorable (Glasgow Outcome Score extended, GOSe 6–8) and unfavorable (GOSe 1–5) outcome.

Results: Levels of glucose and lactate acquired with the CSF-MD technique could be correlated to conventional levels. The median MD recovery using the CMA64 catheter in CSF was 0.98 and 0.97 for glucose and lactate, respectively. Median MD-CSF (CMA 64) lactate (p = 0.0057) and pyruvate (p = 0.0011) levels were significantly lower in the favorable outcome group compared to the unfavorable group. No significant difference in outcome was found using the lactate:pyruvate ratio (LPR), or any of the regional MD-Brain monitoring in our analyzed cohort.

Conclusion: This new technique of global MD-CSF monitoring correlates with conventional CSF levels of glucose and lactate, and the MD recovery is higher than previously described. Increase in lactate and pyruvate, without any effect on the LPR, correlates to unfavorable outcome, perhaps related to the presence of erythrocytes in the CSF.

Molecularly targeted therapies for malignant gliomas: advances and challenges

The identification of molecular markers associated with tumor but not with normal tissue has allowed the development of highly specific, targeted therapies for the treatment of cancer. Over the last several years, tremendous advances in our understanding of the genetic and molecular changes involved in the progression of malignant gliomas have triggered a large effort in the development of targeted therapies to treat these tumors. However, to date only a modest clinical benefit, limited to subsets of patients, has been demonstrated. Furthermore, despite a high degree of target selectivity, the use of targeted therapies often has systemic toxicity. The reasons behind this limited clinical success are complex and include the intricacy of the signaling pathways in gliomas and the heterogeneity of the disease process, compounded by existing limitations in assessing the efficacy of these novel agents when conventional end points and clinical trial designs are utilized. However, despite these difficulties targeted therapies remain a very attractive avenue of treatment for malignant gliomas. Three basic approaches are needed to overcome the hurdles associated with targeted therapies: first, further development of genetic profiling techniques will help to better determine the genetic changes and molecular pathways involved in gliomas and will potentially allow the design of individualized therapies based on the genetic and molecular signature of each tumor. Second, there is a need for the development of better combination strategies (complementary targeted agents or targeted agents with chemotherapy drugs) directed towards disease heterogeneity. Third, we need to optimize the design of preclinical and clinical trials to obtain the maximum amount of information in the shortest period of time.

Large-volume sample stacking for in vivo monitoring of trace levels of γ-aminobutyric acid, glycine and glutamate in microdialysates of periaqueductal gray matter by capillary electrophoresis with contactless conductivity detection

A new variant of large-volume sample stacking injection (LVSS) was used in the capillary electrophoresis with capacitively coupled contactless conductivity detection (CE/C(4)D) determination of the neurotransmitters γ-aminobutyric acid (GABA), glycine (Gly) and glutamate (Glu) in microdialysates of periaqueductal gray matter (PAG). The separation capillary was filled to 98% from the injection side with a sample of microdialysate in acetonitrile. Simultaneously with turning on the separation voltage, the sample zone was forced out by the background electrolyte by increasing the pressure in the terminal capillary outlet vessel. As a consequence of the stacking effect, the analyte was concentrated from the large sample volume into a narrow zone at the sample/background electrolyte boundary close to the injection end of the capillary. Under these conditions, LOD values of 9, 10 and 15nM were determined in the model samples for GABA, Gly and Glu, respectively; RSD equalled 0.5% for the migration times and 1.0-1.9% for the peak areas, respectively. In analysis of microdialysates of PAG, LOD values of 29, 29 and 37nM were determined for GABA, Gly and Glu, respectively; RSD equalled 0.5-0.7% for the migration times and 2.6-8.2% for the peak areas, respectively. The determined basal levels of the neurotransmitters in PAG microdialysates are 0.08, 4.7 and 0.8μM for GABA, Gly and Glu, respectively. Carrageenan-induced hyperalgesia increases the Gly and Glu levels and reduces GABA in PAG microdialysate. Peroral administration of paracetamol in hyperalgesia effectively reduces the Gly value and has no effect on Glu and GABA.

Interstitial fluid-a reflection of the tumor cell microenvironment and secretome

The interstitium or interstitial space describes the space outside the blood and lymphatic vessels. It contains two phases; the interstitial fluid (IF) and the extracellular matrix. In this review we focus on the interstitial fluid phase, which is the physical and biochemical microenvironment of the cells, and more specifically that of tumors. IF is created by transcapillary filtration and cleared by lymphatic vessels, and contains substances that are either produced and secreted locally, thus denoted secretome, or brought to the organ by the circulation. The structure of the interstitium is discussed briefly and moreover techniques for IF isolation focusing on those that are relevant for studies of the secretome. Accumulated data show that tumor IF is hypoxic and acidic compared with subcutaneous IF and plasma, and that there are gradients between IF and plasma giving information on where substances are produced and thereby reflecting the local microenvironment. We review recent data on the origin of tissue specific substances, challenges related to isolating a representative secretome and the use of this as a substrate for biomarker identification. Finally we perform a comparative analysis across human tumor types and techniques and show that there is great variation in the results obtained that may at least partially be due to the isolation method used. We conclude that when care is taken in isolation of substrate, analysis of the secretome may give valuable biological insight and result in identification of biomarker candidates. This article is part of a Special Issue entitled: An Updated Secretome.

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.

Microdialysis for assessing intratumoral drug disposition in brain cancers: a tool for rational drug development

IMPORTANCE OF THE FIELD

many promising targeted agents and combination therapies are being investigated for brain cancer. However, the results from recent clinical trials have been disappointing. A better understanding of the disposition of drug in the brain early in drug development would facilitate appropriate channeling of new drugs into brain cancer clinical trials.

AREAS COVERED IN THIS REVIEW

barriers to successful drug activity against brain cancer and issues affecting intratumoral drug concentrations are reviewed. The use of the microdialysis technique for extracellular fluid (ECF) sampling and its application to drug distribution studies in brain are reviewed using published literature from 1995 to the present. The benefits and limitations of microdialysis for performing neuorpharmacokinetic (nPK) and neuropharmacodynamic (nPD) studies are discussed.

WHAT THE READER WILL GAIN

the reader will gain an appreciation of the challenges involved in identifying agents likely to have efficacy in brain cancer, an understanding of the general principles of microdialysis, and the power and limitations of using this technique in early drug development for brain cancer therapies.

TAKE HOME MESSAGE

a major factor preventing efficacy of anti-brain cancer drugs is limited access to tumor. Intracerebral microdialysis allows sampling of drug in the brain ECF. The resulting nPK/nPD data can aid in the rational selection of drugs for investigation in brain tumor clinical trials.

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.

Influence of blood-brain barrier efflux pumps on the distribution of vincristine in brain and brain tumors

Vincristine (VCR) is efficacious in some but not all brain cancers and an established substrate of Pgp and Mrp1. However, the extent to which such transporters affect the VCR penetration through the blood-brain barrier (BBB) is poorly understood. To evaluate the role of Pgp and Mrp1 in VCR CNS distribution, VCR concentrations were analyzed under steady-state conditions in normal brain, brain tumor, and bone marrow in wild-type (WT), Mrp1 ko (mrp1-/-), Pgp ko (mdr1a-/-:mdr1b-/-), and TKO (mdr1a-/-:mdr1b-/-:mrp1-/-) mice. VCR normal brain partition coefficients (i.e. tissue/plasma VCR concentrations) in TKO mice were greater than those in WT mice at both targeted 10 and 50 ng/mL plasma VCR concentrations, and ranged from 1.3- to 3.6-fold. VCR brain tumor partition coefficients in Mrp1 mice were greater than WT mice at both doses, being 1.5- and 2.4-fold higher at low and high doses, respectively. TKO mice also showed elevated VCR brain tumor penetration with a brain tumor partition coefficient of 1.9-fold greater than that in WT mice at the high-dose level. The bone marrow partition coefficient in Mrp1 ko mice was 1.65-fold greater than that in WT mice. Within strain comparisons revealed that VCR brain tumor concentrations were significantly greater than normal brain in all strains, ranging from 9- to 40-fold. These findings indicate that disruption of the BBB caused the largest enhancement in VCR tumor concentrations, yet the absence of Mrp1 on the brain tumor vasculature could enhance the penetration compared with that in normal brain.

In vivo monitoring of the transfer kinetics of trace elements in animal brains with hyphenated inductively coupled plasma mass spectrometry techniques

The roles of metal ions to sustain normal function and to cause dysfunction of neurological systems have been confirmed by various studies. However, because of the lack of adequate analytical method to monitor the transfer kinetics of metal ions in the brain of a living animal, research on the physiopathological roles of metal ions in the CNS remains in its early stages and more analytical efforts are still needed. To explicitly model the possible links between metal ions and physiopathological alterations, it is essential to develop in vivo monitoring techniques that can bridge the gap between metalloneurochemistry and neurophysiopathology. Although inductively coupled plasma mass spectrometry (ICP-MS) is a very powerful technique for multiple trace element analyses, when dealing with chemically complex microdialysis samples, the detection capability is largely limited by instrumental sensitivity, selectivity, and contamination that arise from the experimental procedure. As a result, in recent years several high efficient and clean on-line sample pretreatment systems have been developed and combined with microdialysis and ICP-MS for the continuous and in vivo determination of the concentration-time profiles of metal ions in the extracellular space of rat brain. This article reviews the research relevant to the development of analytical techniques for the in vivo determination of dynamic variation in the concentration levels of metal ions in a living animal.

Capillary and microchip electrophoresis in microdialysis: recent applications

The theme of this review is to highlight the importance of microscale electrophoretic-based separation systems in microdialysis (microD). The ability of CE and MCE to yield very rapid and highly efficient separations using just nanolitre volumes of microdialysate samples will also be discussed. Recent advances in this area will be highlighted, by illustration of some exciting new applications while the need for further innovation will be covered. The first section briefly introduces the concept of microD sampling coupled with electrophoresis-based separation and the inherent advantages of this approach. The following section highlights some specific applications of CE separations in the detection of important biomarkers such as low-molecular-weight neurotransmitters, amino acids, and other molecules that are frequently encountered in microD. Various detection modes in CE are outlined and some of the advantages and drawbacks thereof are discussed. The last section introduces the concepts of micro-total analysis systems and the coupling of MCE and microD. Some of the latest innovations will be illustrated. The concluding section reflects on the future of this important chemical alliance between microD and CE/MCE.

Intra-operative, real-time, three-dimensional ultrasound assisted positioning of catheters in the microdialysis of glial tumours

Microdialysis allows sampling of the extra cellular fluid of normal and pathological tissues. Accurate positioning of catheters in viable, representative tumour tissue is crucial for the accuracy and effectiveness of the technique. We have performed microdialysis with the aid of intra-operative three-dimensional ultrasonography (3D-US) to guide the placement of catheters in seven patients undergoing resection for supratentorial high-grade astrocytoma. The final position of the catheter tip membrane was confirmed by intra-operative ultrasound scanning. The accuracy of the spatial targeting was validated by pathological examination and the quality of the microdialysate was checked with ultra performance liquid chromatography-mass spectrometry. Our results indicate that intra-operative 3D-US can be used to correctly position catheters for microdialysis and allows adjustment to the catheters, when necessary, prior to the dialysis of viable target tumour tissue.

The neuropharmacokinetics of temozolomide in patients with resectable brain tumors: potential implications for the current approach to chemoradiation

PURPOSE

Intracerebral microdialysis (ICMD) is an accepted method for monitoring changes in neurochemistry from acute brain injury. The goal of this pilot study was to determine the feasibility of using ICMD to examine the neuropharmacokinetics of temozolomide in brain interstitium following oral administration.

EXPERIMENTAL DESIGN

Patients with primary or metastatic brain tumors had a microdialysis catheter placed in peritumoral brain tissue at the time of surgical debulking. Computerized tomography scan confirmed the catheter location. Patients received a single oral dose of temozolomide (150 mg/m2) on the first postoperative day, serial plasma and ICMD samples were collected over 24 hours, and temozolomide concentrations were determined by tandem mass spectrometry.

RESULTS

Nine patients were enrolled. Dialysate and plasma samples were successfully collected from seven of the nine patients. The mean temozolomide areas under the concentration-time curve (AUC) in plasma and brain interstitium were 17.1 and 2.7 microg/mL x hour, with an average brain interstitium/plasma AUC ratio of 17.8%. The mean peak temozolomide concentration in the brain was 0.6 +/- 0.3 microg/mL, and the mean time to reach peak level in brain was 2.0 +/- 0.8 hours.

CONCLUSIONS

The use of ICMD to measure the neuropharmacokinetics of systemically administered chemotherapy is safe and feasible. Concentrations of temozolomide in brain interstitium obtained by ICMD are consistent with published data obtained in a preclinical ICMD model, as well as from clinical studies of cerebrospinal fluid. However, the delayed time required to achieve maximum temozolomide concentrations in brain suggests that current chemoradiation regimens may be improved by administering temozolomide 2 to 3 hours before radiation.

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.

Unbound drug concentration in brain homogenate and cerebral spinal fluid at steady state as a surrogate for unbound concentration in brain interstitial fluid

The objective of the present study was to examine the accuracy of using unbound brain concentration determined by a brain homogenate method (C(ub)), cerebral spinal fluid concentration (C(CSF)), and unbound plasma concentration (C(up)) as a surrogate for brain interstitial fluid concentration determined by brain microdialysis (C(m)). Nine compounds-carbamazepine, citalopram, ganciclovir, metoclopramide, N-desmethylclozapine, quinidine, risperidone, 9-hydroxyrisperidone, and thiopental-were selected, and each was administered as an intravenous bolus (up to 5 mg/kg) followed by a constant intravenous infusion (1-9 mg/kg/h) for 6 h in rats. For eight of the nine compounds, the C(ub)s were within 3-fold of their C(m); thiopental had a C(m) 4-fold of its C(ub). The C(CSF)s of eight of the nine compounds were within 3-fold of their corresponding C(m); 9-hydroxyrisperidone showed a C(CSF) 5-fold of its C(m). The C(up)s of five of the nine compounds were within 3-fold of their C(m); four compounds (ganciclovir, metoclopramide, quinidine, and 9-hydroxyrisperidone) had C(up)s 6- to 14-fold of their C(m). In conclusion, the C(ub) and C(CSF) were within 3-fold of the C(m) for the majority of the compounds tested. The C(up)s were within 3-fold of C(m) for lipophilic non-P-glycoprotein (-P-gp) substrates and greater than 3-fold of C(m) for hydrophilic or P-gp substrates. The present study indicates that the brain homogenate and cerebral spinal fluid methods may be used as surrogate methods to predict brain interstitial fluid concentrations within 3-fold of error in drug discovery and development settings.

Droplet-based microdialysis-Concept, theory, and design considerations

The capability of continuously sampling the extracellular fluid opens up a wide range of applications of microdialysis in biological, pharmaceutical, and clinical studies. Existing microdialysis, however, faces challenges in sampling analytes with fast clearance and limited diffusivity because sampling resolution is limited by device size. Size reduction in probes and interconnected cannulae is a promising solution to improve temporal and spatial resolution. But the back pressure produced by resistance to laminar flows will be magnified in smaller channels, raising a concern as to whether it is feasible to operate continuous perfusion for miniaturized microdialysis. We demonstrate that a 10-fold smaller channel will exhibit 100-fold larger back pressure in response to the increase in the flow rate to maintain the relative recovery. In order to overcome the foreseen back pressure issue, this paper discusses a new concept using discrete droplets instead of continuous flows to operate dialysis in a miniaturized probe. This conceptual design is referred to as droplet-based digital microdialysis, in which droplets are produced, controlled and advanced within microchannels at a rate that in theory should allow for analytes to equilibrate with the extracellular fluid under no flow conditions. Expecting that a digital droplet design will entirely eliminate back pressure by introducing air between droplets, we numerically compare the equilibration kinematics of droplets to that of continuous flow. Results suggest equilibration of low molecular weight analytes between intermittently stationary droplets and the extracellular fluid in a few seconds. Considerations in design, prototyping, calibration and quantification, and the integration with other devices are suggested.

Effects of tissue trauma on the characteristics of microdialysis zero-net-flux method sampling neurotransmitters

Microdialysis has been used for studying neurochemistry in brain regions that respond to afferent inputs or administered drugs. As the knowledge derived from and concerning microdialysis grows, so do the concerns over its invasiveness and, hence, the credibility of resulting data. Recent experimental and theoretical studies impugned the validity of the microdialysis zero-net-flux (ZNF) method in measuring brain extracellular neurotransmitters, suggesting that the tissue trauma resulting from probe implantation seriously compromises its worth. This paper developed a theoretical model to study the influences of two categories of tissue trauma on microdialysis ZNF operation: (1) morphological alterations in tissue extracellular structure and (2) physiological impairment of neurotransmitter release and uptake processes. Model results show that alterations of tissue extracellular structure negligibly affect the accuracy of the ZNF method in determining the basal level of extracellular neurotransmitter but do affect the fundamental characteristics of microdialysis: the extraction efficiency and relative recovery. An inhibited or damaged neurotransmitter uptake process always decreases the efficiency of microdialysis extraction, but rise of the relative recovery of neurotransmitters with the same uptake inhibition/damage occurs only when there is far more damage to the neurotransmitter release than to the uptake process in the tissue. A criterion for this rising trend of microdialysis relative recovery is discussed in terms of trauma parameters and neurotransmitter uptake inhibition.

Microdialysis—theoretical background and recent implementation in applied life-sciences

In the past decade microdialysis has become a method of choice in the study of unbound tissue concentrations of both endogenous and exogenous substances. Microdialysis has been shown to offer information about substances directly at the site of action while being well tolerable and safe. The large variety of its field of application has been demonstrated. However, a few challenges have to be met to make this method generally applicable in routine applications. This review will provide an overview over theoretical aspects that have to be considered during the implementation of microdialysis. Moreover, a comparison between microdialysis and other tissue sampling techniques will demonstrate advantages and limitations of the methods mentioned. Subsequently, it will present a critical synopsis of a variety of scientific/biomedical applications of this method with emphasis on the most recent literature, focussing on target tissues while giving examples of substances examined. It is concluded that microdialysis will be of great value in future investigations of pharmacokinetics, pharmacodynamics and in monitoring of disease status and progression.

Development of a Syngeneic Rat Brain Tumor Model Expressing EGFRvIII and Its Use for Molecular Targeting Studies with Monoclonal Antibody L8A4

Purpose: The goals of the present study were 2-fold: (a) to develop and characterize a rat brain tumor model that could be used for studies of molecular targeting of EGFRvIII and (b) to study the tumor localizing properties of radiolabeled monoclonal antibody (mAb) L8A4, specifically directed against EGFRvIII, following systemic, i.t., and convection enhanced delivery to brain tumor–bearing rats.

Experimental Design and Results: F98 wild-type (F98WT) rat glioma cells were transfected with a gene encoding human EGFRvIII, and following selection and cloning, a cell line, designated F98npEGFRvIII, was identified, which expressed a nonconstitutively phosphorylated form of the receptor. As determined by a radioligand binding assay, there were 1.2×105 EGFRvIII sites per cell compared with an undetectable number on F98WT cells. The tumorigenicity of the F98npEGFRvIIIglioma was studied following i.c. implantation of 103, 104, or 105 cells into CD-Fischer rats. Mean survival times were 23, 17, and 13 days, respectively, which were equivalent to those obtained with F98EGFR and F98WT cells. As determined by magnetic resonance imaging, the mean doubling times for the F98WT and F98npEGFRvIII gliomas were similar (59.8 ± 4.8 versus 52 ± 3.3 hours). Following i.v. administration to glioma-bearing rats, mAb L8A4 specifically targeted the F98npEGFRvIII glioma, and at 24 hours, 7.7% of the injected dose per gram (ID/g) localized in the tumor. This increased 5-fold to 39.5% ID/g following i.t. injection and 7-fold to 59.8% ID/g at 24 hours following convection enhanced delivery.

Conclusions: Based on these data, we have concluded that the F98npEGFRvIII glioma should be a valuable animal model for therapy studies focusing on molecular targeting of EGFRvIII by receptor specific mAbs.

Boron-Containing Nucleosides as Potential Delivery Agents for Neutron Capture Therapy of Brain Tumors

The purpose of the present study was to evaluate both in vitro and in vivo a series of boron-containing nucleosides that potentially could be used as delivery agents for neutron capture therapy. The rationale for their synthesis was based on the fact that proliferating neoplastic cells have increased requirements for nucleic acid precursors, and, therefore, they should preferentially localize in the tumor. A series of 3-carboranlyalkyl thymidine analogs has been synthesized and a subset, designated N4, N5, and N7, and the corresponding 3-dihydroxypropyl derivatives, designated N4-2OH, N5-2OH, and N7-2OH, have been selected for evaluation. Using these compounds as substrates for recombinant human thymidine kinase-1 and the mitochondrial isoenzyme thymidine kinase-2, the highest phosphorylation levels relative to thymidine were seen with N5 and the corresponding dihydroxypropyl analog N5-2OH. In contrast, N4, N4-OH, N7, and N7-OH had substantially lower phosphorylation levels. To compare compounds with high and low thymidine kinase-1 substrate activity, N5 and N7 and the corresponding dihydroxypropyl derivatives were selected for evaluation of their cellular toxicity, uptake and retention by the F98 rat glioma, human MRA melanoma, and murine L929 cell lines, all of which are thymidine kinase-1(+), and a mutant L929 cell line that is thymidine kinase-1(-). N5-2OH was the least toxic (IC50, 43-70 microm), and N7 and N7-2OH were the most toxic (IC50, 18-49 microm). The highest boron uptake was seen with N7-2OH by the MRA 27 melanoma and L929 wild-type (wt) cell lines. The highest retention was seen with L929 (wt) cells, and this ranged from 29% for N5-2OH to 46% for N7. Based on the in vitro toxicity and uptake data, N5-2OH was selected for in vivo biodistribution studies either in rats bearing intracerebral implants of the F98 glioma or in mice bearing either s.c. or intracerebral implants of L929 (wt) tumors. At 2.5 hours after convection-enhanced delivery, the boron values for the F98 glioma and normal brain were 16.2 +/- 2.3 and 2.2 microg/g, respectively, and the tumor to brain ratio was 8.5. Boron values at 4 hours after convection-enhanced delivery of N5-2OH to mice bearing intracerebral implants of L929 (wt) or L929 thymidine kinase-1(-) tumors were 39.8 +/- 10.8 and 12.4 +/- 1.6 microg/g, respectively, and the corresponding normal brain values were 4.4 and 1.6 microg/g, thereby indicating that there was selective retention by the thymidine kinase-1(+) tumors. Based on these favorable in vitro and in vivo data, neutron capture therapy studies will be initiated using N5-2OH in combination with two non-cell cycle dependent boron delivery agents, boronophenylalanine and sodium borocaptate.