A quantitative study of blood-brain barrier permeability ultrastructure in a new rat glioma model

Volumetric Printing Across Melt Electrowritten Scaffolds Fabricates Multi-Material Living Constructs with Tunable Architecture and Mechanics

Major challenges in biofabrication revolve around capturing the complex, hierarchical composition of native tissues. However, individual 3D printing techniques have limited capacity to produce composite biomaterials with multi-scale resolution. Volumetric bioprinting recently emerged as a paradigm-shift in biofabrication. This ultrafast, light-based technique sculpts cell-laden hydrogel bioresins into 3D structures in a layerless fashion, providing enhanced design freedom over conventional bioprinting. However, it yields prints with low mechanical stability, since soft, cell-friendly hydrogels are used. Herein, the possibility to converge volumetric bioprinting with melt electrowriting, which excels at patterning microfibers, is shown for the fabrication of tubular hydrogel-based composites with enhanced mechanical behavior. Despite including non-transparent melt electrowritten scaffolds in the volumetric printing process, high-resolution bioprinted structures are successfully achieved. Tensile, burst, and bending mechanical properties of printed tubes are tuned altering the electrowritten mesh design, resulting in complex, multi-material tubular constructs with customizable, anisotropic geometries that better mimic intricate biological tubular structures. As a proof-of-concept, engineered tubular structures are obtained by building trilayered cell-laden vessels, and features (valves, branches, fenestrations) that can be rapidly printed using this hybrid approach. This multi-technology convergence offers a new toolbox for manufacturing hierarchical and mechanically tunable multi-material living structures.

Vascular permeability in the RG2 glioma model can be mediated by macropinocytosis and be independent of the opening of the tight junction

This study evaluates the extravasation pathways of circulating macromolecules in a rat glioma model (RG2) which was observed by both magnetic resonance imaging using ultrasmall superparamagnetic iron oxide and electron microscopy. Although magnetic resonance imaging signal enhancement was observed as soon as 10 min after injection (9.4% 2 h after injection), electron microscopy showed that endothelial cells were still tightly sealed. However, circulating immunoglobulin G and ultrasmall superparamagnetic iron oxide were found in large membrane compartments of endothelial cells, in the basal lamina (7.4 ± 1.2 gold particles/µm² in the tumor versus 0.38 ± 0.17 in healthy tissue, p = 1.4.10^-5) and between tumoral cells. Altogether, this strongly suggests an active transport mediated by macropinocytosis. To challenge this transport mechanism, additional rats were treated with amiloride, an inhibitor of macropinocytosis, leading to a reduction of membrane protrusions (66%) and of macropinosomes. Amiloride however also opened tumoral tight junctions allowing a larger extravasation of ultrasmall superparamagnetic iron oxide (magnetic resonance imaging signal enhancement of 35.7% 2 h after injection). Altogether, these results suggest that ultrasmall superparamagnetic iron oxide and immunoglobulin G in the RG2 glioma model follow an active extravasation pathway mediated by a macropinocytosis process. Amiloride also appears as a potential strategy to facilitate the extravasation of chemotherapeutic drugs in glioma.

Potential role of the Slit/Robo signal pathway in angiogenesis

Intensive investigations on angiogenesis and vasculogenesis have increased our understanding of molecular mechanisms of blood vessel formation during pathologic and developmental conditions. However, endothelial cells (ECs), the main component of vasculature, are heterogeneous, as revealed by our phenotypic and molecular biological studies in the laboratory, and it is still hard to adequately understand the molecular mechanisms of angiogenesis and vasculogenesis. Indeed, there are several major ligand/receptor signal pathways: VEGF/VEGFR, Jagged-1/Notch, Wnt ligand/frizzled receptor, and ephrin/Eph; each of which having distinct and independent roles during vascular formation. In this review, we focus on the angiogenic effect of the Slit and Robo signal pathway that was formally known as neuronal axon guidance. Among the existing vascular signals, this pathway is the most recently found ligand/receptor vascular signal, and may play important physiological roles as other major receptor/ligand signals do. Here, we briefly address: (1) the background of Slit and Robo families; (2) expression patterns of Slit and Robo; (3) functional roles of the Slit/Robo pathway in vascular formation; and (4) confronting tasks of this novel vascular pathway in the near future. Together, a summary of these data suggest the essential role of the Slit/Robo pathway in angiogenesis, and may explain why multiple vascular signals exist in heterogenic endothelial cells.

Clinical applications of iron oxide nanoparticles for magnetic resonance imaging of brain tumors

Current neuroimaging provides detailed anatomic and functional evaluation of brain tumors, allowing for improved diagnostic and prognostic capabilities. Some challenges persist even with today's advanced imaging techniques, including accurate delineation of tumor margins and distinguishing treatment effects from residual or recurrent tumor. Ultrasmall superparamagnetic iron oxide nanoparticles are an emerging tool that can add clinically useful information due to their distinct physiochemical features and biodistribution, while having a good safety profile. Nanoparticles can be used as a platform for theranostic drugs, which have shown great promise for the treatment of CNS malignancies. This review will provide an overview of clinical ultrasmall superparamagnetic iron oxides and how they can be applied to the diagnostic and therapeutic neuro-oncologic setting.

Tumor-associated edema in brain cancer patients: pathogenesis and management

The long-term treatment of peritumoral edema remains a major challenge in clinical neuro-oncology. Steroids have been and will remain the backbone of any anti-edematous therapy because of their striking activity, convenient oral administration and also because of their cost-effectiveness. Their side effects, however, can compromise quality of life, particularly upon continuous administration. Therapeutic alternatives which may replace or - at least - help to reduce the steroid dose are limited. However, with the development of new agents such as corticorelin acetate, there is a hope that steroid-induced side effects can be delayed and reduced. The administration of anti-angiogenic agents with steroid-sparing effects, for example, bevacizumab, is limited due to their costs. Increased knowledge on boswellic acids and cyclooxygenase-2 inhibitors which are available for clinical application may help to exploit their anti-edema activity more efficiently in the future.

Effect of Naringenin on metabolic markers, lipid profile and expression of GFAP in C6 glioma cells implanted rat's brain

BACKGROUND

Naringenin, a flavanone, has been reported to exhibit a wide range of pharmacological properties including antitumor activity.

PURPOSE

We wanted to test the efficacy of Naringenin on C6 glioma cells-implanted into rats was investigated.

METHODS

Biochemical and immunohistochemical methods were used for analyzing various markers.

RESULTS

Injection of C6 glioma cells into rat brain resulted in increased metabolic markers {Lactatate Dehydrogenase (LDH), 5' Nucleotidase 5'ND), creatine kinase (CK), Hexokinase (HK) and Glucose 6-phosphate dehydrogenase (G6PD)} and lipid profile (triglycerides, free fatty acids, phos-pholipids, total cholesterol and free cholesterol). Oral administration of Naringenin (50 mg /kg of BW for 30 days) significantly altered this biochemical profile. Further, the immuno fluorescence expression of Glial fibrilary acidic protein (GFAP) was also studied.

CONCLUSION

In C6 glioma cells-implanted rats, increased expression of GFAP was noted on treatment with Naringenin. These observations suggest that Naringenin may participate by inhibiting glial cell tumorigenesis.

Induction of the fibrinolytic system by cartilage extract mediates its antiangiogenic effect in mouse glioma

Both the antiangiogenic and antitumoral activity of shark cartilage extracts (SCE) have been demonstrated in animal models and clinical trials. Studies reported that SCE induces the expression of tissue plasminogen activator gene (PLAT) in endothelial cells and increases the activity of the protein (t-PA) in vitro. The aim of this study was to demonstrate the crucial role of t-PA induction in the antiangiogenic and antitumor activity of SCE in experimental glioma. This study showed antiangiogenic and antitumoral effects of SCE in three mice glioma models (C6, HGD and GL26). Histological examination suggested perivascular proteolysis and edema as well as important intratumoral necrosis, which artefactually increased the tumor volume at high doses. Thus, the antiangiogenic effect of SCE correlated with the presence of t-PA and angiostatin in degenerating vessels. Functional in vivo experiments were conducted to modulate the plasminogen pathway. No antiangiogenic effect was observed on tumors overexpressing the plasminogen activator inhibitor-1 (PAI-1). Moreover, therapeutical effects were neutralized in mice that were cotreated with ε-aminocaproic acid (EACA, 120 mg/kg p.o.), an inhibitor that blocks the high-affinity lysine binding sites of both plasminogen and plasmin. In contrast, cotreatment with N-acetylcysteine (NAC, 7,5mg/kg i.p.), a sulfhydril donor that reduces plasmin into angiostatin or other antiangiogenic fragments, increased the benefit of SCE on mice survival. In subcutaneous models, NAC prevented the increase in tumor volume caused by high doses of cartilage extract. In conclusion, this study indicates that induction of t-PA by shark cartilage extract plays an essential role in its antiangiogenic activity, but that control of excessive proteolysis by a plasmin reductor could prevent edema and uncover the full benefit of shark cartilage extract in the treatment of intracranial tumors.

Measurements of diagnostic examination performance and correlation analysis using microvascular leakage, cerebral blood volume, and blood flow derived from 3T dynamic susceptibility-weighted contrast-enhanced perfusion MR imaging in glial tumor grading

INTRODUCTION

To assess the diagnostic accuracy of microvascular leakage (MVL), cerebral blood volume (CBV) and blood flow (CBF) values derived from dynamic susceptibility-weighted contrast-enhanced perfusion MR imaging (DSC-MR imaging) for grading of cerebral glial tumors, and to estimate the correlation between vascular permeability/perfusion parameters and tumor grades.

METHODS

A prospective study of 79 patients with cerebral glial tumors underwent DSC-MR imaging. Normalized relative CBV (rCBV) and relative CBF (rCBF) from tumoral (rCBVt and rCBFt), peri-enhancing region (rCBVe and rCBFe), and the value in the tumor divided by the value in the peri-enhancing region (rCBVt/e and rCBFt/e), as well as MVL, expressed as the leakage coefficient K(2) were calculated. Hemodynamic variables and tumor grades were analyzed statistically and with Pearson correlations. Receiver operating characteristic (ROC) curve analyses were also performed for each of the variables.

RESULTS

The differences in rCBVt and the maximum MVL (MVL(max)) values were statistically significant among all tumor grades. Correlation analysis using Pearson was as follows: rCBVt and tumor grade, r = 0.774; rCBFt and tumor grade, r = 0.417; MVL(max) and tumor grade, r = 0.559; MVL(max) and rCBVt, r = 0.440; MVL(max) and rCBFt, r = 0.192; and rCBVt and rCBFt, r = 0.605. According to ROC analyses for distinguishing tumor grade, rCBVt showed the largest areas under ROC curve (AUC), except for grade III from IV.

CONCLUSION

Both rCBVt and MVL(max) showed good discriminative power in distinguishing all tumor grades. rCBVt correlated strongly with tumor grade; the correlation between MVL(max) and tumor grade was moderate.

Diagnostic examination performance by using microvascular leakage, cerebral blood volume, and blood flow derived from 3-T dynamic susceptibility-weighted contrast-enhanced perfusion MR imaging in the differentiation of glioblastoma multiforme and brain metastasis

INTRODUCTION

Conventional magnetic resonance (MR) imaging has limited capacity to differentiate between glioblastoma multiforme (GBM) and metastasis. The purposes of this study were: (1) to compare microvascular leakage (MVL), cerebral blood volume (CBV), and blood flow (CBF) in the distinction of metastasis from GBM using dynamic susceptibility-weighted contrast-enhanced perfusion MR imaging (DSC-MRI), and (2) to estimate the diagnostic accuracy of perfusion and permeability MR imaging.

METHODS

A prospective study of 61 patients (40 GBMs and 21 metastases) was performed at 3 T using DSC-MRI. Normalized rCBV and rCBF from tumoral (rCBVt, rCBFt), peri-enhancing region (rCBVe, rCBFe), and by dividing the value in the tumor by the value in the peri-enhancing region (rCBVt/e, rCBFt/e), as well as MVL were calculated. Hemodynamic and histopathologic variables were analyzed statistically and Spearman/Pearson correlations. Receiver operating characteristic curve analysis was performed for each of the variables.

RESULTS

The rCBVe, rCBFe, and MVL were significantly greater in GBMs compared with those of metastases. The optimal cutoff value for differentiating GBM from metastasis was 0.80 which implies a sensitivity of 95%, a specificity of 92%, a positive predictive value of 86%, and a negative predictive value of 97% for rCBVe ratio. We found a modest correlation between rCBVt and rCBFt ratios.

CONCLUSION

MVL measurements in GBMs are significantly higher than those in metastases. Statistically, both rCBVe, rCBVt/e and rCBFe, rCBFt/e were useful in differentiating between GBMs and metastases, supporting the hypothesis that perfusion MR imaging can detect infiltration of tumor cells in the peri-enhancing region.

AQP-4 in peritumoral edematous tissue is correlated with the degree of glioma and with expression of VEGF and HIF-alpha

It is recognized that expression of AQP4 protein is much greater in gliomas than in normal tissue. The relationship between AQP4 and glioma-associated brain edema is affected by osmotic pressure and hypoxia. In this study, we detected changes of AQP4 expression in tumor and peritumoral edematous tissues to analyze the relationship between AQP4 protein and the edema index (EI). We also detected expression of vascular endothelial growth factor (VEGF) and hypoxia-inducible factor-1α (HIF-1α) to investigate their relationship with AQP4 protein, and thus to uncover the molecular biological mechanisms of AQP4 expression in glioma-associated brain edema. Sixty-five patients with brain glioma were divided into tumor and peritumor groups. Fresh tumor specimens, including six cases of grade I glioma, 18 of grade II, 11 of grade III and 30 of grade IV, and peritumoral edematous tissue specimens (1 cm distant from the tumor) were resected from these patients, and AQP4 protein expression levels were detected by western blot. Different AQP4 expression in the tumor and peritumor groups were compared. The relationship between AQP4 expression levels and the degree of peritumoral edema, and expression differences in different grades, were analyzed. Immunofluorescence cytochemistry was used to detect positive expression of AQP4 protein, VEGF protein, and HIF-1α protein in tumor tissue, and differences between expression were analyzed. Western blot showed that AQP4 expression in the peritumor (0.7697 ± 0.0941) and tumor (0.6934 ± 0.0625) groups was higher than in the control group (0.6215 ± 0.0884), and was highest in the peritumor group (both P < 0.01). AQP4 expression level in the peritumor group was positively correlated with EI (r = 0.677, P < 0.001) whereas AQP4 expression level in the tumor group was not correlated with EI (r = 0.096, P > 0.05). AQP4 expression increased with higher tumor grades in the peritumor group, but differences were not significant in the tumor group. Immunofluorescence cytochemical staining revealed that AQP4 protein in normal brain tissue was mainly expressed in the cell membrane surface, and that cytoplasm and nuclear staining was shallow. In glioma cells, AQP4 was widely distributed in the cytoplasm, particularly in the edematous area around the tumor. AQP4 protein expression in the tumor was significantly positively correlated with both VEGF protein (r = 0.877, P < 0.001) and HIF-1α protein (r = 0.876, P < 0.001). AQP4 expression was higher in brain tumor, especially peritumor. The degree of peritumoral edema correlates with AQP4 protein expression only in peritumor, whereas AQP4 expression is in accordance with expression of VEGF and HIF-1α. In glioma-associated brain edema, AQP4 is coregulated by osmotic pressure and hypoxia, with predominance of osmotic regulation, and is redistributed in glioma cells, mainly in the cytoplasm, and its expression level increased with higher glioma grades.

Endocytosis at the blood–brain barrier: From basic understanding to drug delivery strategies

The blood-brain barrier (BBB) protects the central nervous system (CNS) from potentially harmful xenobiotics and endogenous molecules. Anatomically, it comprises the brain microvasculature whose functionality is nevertheless influenced by associated astrocyte, pericyte and neuronal cells. The highly restrictive paracellular pathway within brain microvasculature restricts significant CNS penetration to only those drugs whose physicochemical properties afford ready penetration into hydrophobic cell membranes or are capable of exploiting endogenous active transport processes such as solute carriers or endocytosis pathways. Endocytosis at the BBB is an essential pathway by which the brain obtains its nutrients and affords communication with the periphery. The development of strategies to exploit these endocytic pathways for the purposes of drug delivery to the CNS is still an immature field although some impressive results have been documented with the targeting of particular receptors. This current article initially provides an overview of general endocytosis processes and pathways showing evidence of their functional existence within the BBB. Subsequent sections provide, in an entity-specific manner, comprehensive reviews on BBB transport investigations of endocytosis involving: transferrin and the targeting of the transferrin receptor; hormones; cytokines; cell penetrating peptides; microorganisms and toxins, and nanoparticles aimed at more effectively delivering drugs to the CNS.

Primary porcine brain microvascular endothelial cells: biochemical and functional characterisation as a model for drug transport and targeting

The blood-brain barrier (BBB) remains a significant obstacle to the delivery of therapeutic agents into the central nervous system (CNS). Primary cell cultures of brain capillary endothelial cells represent the closest possible phenotype to the in vivo BBB cell providing a convenient model for the study of transport systems and events that mediate solute delivery to the CNS. In this investigation we have characterized an in vitro primary BBB model from porcine brain microvascular endothelial capillary (PBMVEC) cells after recovery from cryopreservation of upto 12 months and studied their modulation by astrocytes. Co-cultures of PBMVECs with astrocytes (C6 astroglioma) resulted in trans-endothelial electrical resistance of up to approximately 900Omega cm2 and marked discrimination between the para- and trans- cellular markers sucrose and propranolol. Micrographs of confluent monolayers of PBMVECs showed the presence of tight junction complexes and vesicles with the morphological characteristics of either caveolae or clathrin coated pits. Extensive RT-PCR evaluation highlighted the expression of tight junction transcripts, ABC transporters, leptin receptor and select nutrient transporters. Functional studies examined the kinetics of transport of glucose, large neutral amino acids and p-glycoprotein (P-gp). Our findings indicate primary PBMVECs retain many barrier characteristics and transport pathways of the in vivo BBB. Further, primary cells can be stored as frozen stocks which can be thawed and cultured without phenotypic drift many months after isolation. Frozen PBMVECs therefore serve as a robust and convenient in vitro cell culture tool for research programs involving CNS drug delivery and targeting and in studies addressing blood-brain barrier transport mechanisms.

Mechanisms of tumor-related brain edema

✓Cerebral edema contributes strongly to symptoms associated with brain tumors. Although the introduction of corticosteroids has greatly simplified treatment of patients with newly diagnosed tumors, these drugs are associated with marked side effects during the long-term treatment that is often necessary in the recurrences. Therefore, a better understanding of mechanisms related to the evolution and clearance of tumor-related edema with the aid of modern imaging and molecular methodology is clearly necessary. Recently, researchers have focused on molecular mechanisms of edema development and have demonstrated alternative routes—such as the inhibition of vascular endothelial growth factor receptor inhibitors—to be explored for treating edema. In this review the author focuses on established and current concepts regarding the pathophysiology of cerebral edema and its treatment.

Blood tumor permeability of experimental brain tumor: an electron microscopic study using lanthanum

In an attempt to assess the permeability of microvessels in the experimental brain tumor model, lanthanum ion (La3+) was used as a low-molecular weight electron microscopic probe. Rat glioma 9 L and adenocarcinoma ACL15 were transplanted to the brain and subflank of rats. The rats were then anesthetized sequentially perfused with saline, saline plus La3+ followed by a fixative in phosphate buffer. The brain and subcutaneous tumors were removed, further fixed, and processed for electron microscopy. La3 did not pass through the tight junctions of the normal cerebral endothelium. Similarly, La3+ did not penetrate the endothelial cell wall of the microvessels in the transplanted brain tumors. In contrast, extravasation of La3+ from the microvessels in the transplanted subcutaneous tumors was observed. The electron microscopy examination results indicate that the vesicular transport was a predominant mechanism in the penetration of La3+ through the endothelial cell wall. Since most chemotherapeutic agents similar as La3+ are of low molecular weight, we can suggest from the results of our present study that the blood tumor permeability of the anti-cancer agents in the rat model of brain glioma transplantation differs from that in the rat model of subcutaneous glioma transportation. In other words, our results indicate that when the subcutaneous glioma transplantation model is used in sensitivity tests of anti-cancer agents, it will possibly be very difficult to predict the anti-neoplastic effect in vivo.

Molecular mechanisms of brain tumor edema

Despite their diverse histological types, most brain tumours cause brain oedema, which is a significant cause of patient morbidity and mortality. Brain tumour oedema occurs when plasma-like fluid enters the brain extracellular space through impaired capillary endothelial tight junctions in tumours. Under-expression of the tight junction proteins occludin, claudin-1 and claudin-5 are key molecular abnormalities responsible for the increased permeability of tumour endothelial tight junctions. Recent evidence suggests that the membrane water channel protein aquaporin-4 (AQP4) also plays a role in brain tumour oedema. AQP4-deficient mice show remarkably altered brain water balance after various insults, including brain tumour implantation. AQP4 expression is strongly upregulated around malignant human brain tumours in association with reduced extracellular volume, which may restrict the flow of extracellular fluid from the tumour bed into the brain parenchyma. Elimination of excess fluid leaking into brain parenchyma requires passage across three AQP4-rich barriers: a) the glia limitans externa, b) the glia limitans interna/ependyma, and c) the blood-brain barrier. Modulation of the expression and/or function of endothelial tight junction proteins and aquaporins may provide novel therapeutic options for reducing brain tumour oedema.

Proton radiation and TNF-alpha/Bax gene therapy for orthotopic C6 brain tumor in Wistar rats

High-grade tumors of the brain remain virtually incurable with current therapeutic regimens, new approaches to augment existing therapies need to be explored. The major goal of this pilot study was to evaluate the feasibility of gene therapy using plasmid DNA encoding tumor necrosis factor-alpha and bax together with proton radiation in an immunocompetent animal model with orthotopic brain tumor. C6 glioma cells were stereotactically implanted into the left hemibrain of Wistar rats (day 0). On day 5, the appropriate groups received intratumoral pGL1-TNF-a and pGL1-Bax (10 microg each), parental plasmid pWS4 (20 microg), or PBS. Hemibrain proton irradiation (10 Gy, 90 MeV, single fraction) was delivered 18-20 hr later. Rats were euthanized when signs of illness appeared. In addition, a subset of animals from each group was euthanized on day 9 for immune and other assays. By day 9, 25%, 20%, and 10% of rats treated with PBS, pWS4, or pGL1-TNF-alpha/pGL1-Bax, respectively, had been euthanized due to weight loss or other signs of illness, whereas all rats treated with pGL1-TNF-alpha/pGL1-Bax + radiation or radiation alone were healthy (P<0.05). At this same time, the pGL1-TNF-alpha/pGL1-Bax + radiation group had significantly elevated lymphocyte percentages (P<0.005 or less) and a relatively high level of lymphocytic infiltrate within tumors. Although the rats treated with pGL1-TNF-alpha/pGL1-Bax had the highest levels of activated T helper (CD4+/CD71+) and T cytotoxic (CD8+/CD71+) cells, the values were not significantly different compared to the pWS4-injected control group. Splenocytes in all tumor cell-injected groups had higher mean values for DNA and protein synthesis compared to the non-tumor cell injected control group, whereas oxygen radical production by phagocytes was consistently higher in groups injected with plasmid or treated with radiation. Body, hemibrain, and spleen masses, white blood cell, red blood cell and platelet counts, hemoglobin, hematocrit, and transforming growth factor-beta1 levels in plasma were similar among groups. The results demonstrate that treatment with pGL1-TNF-alpha/pGL1-Bax combined with proton hemibrain irradiation is safe under the conditions used. Overall, these data support further investigation of this unique combination therapy.

Vascular microenvironment in gliomas

Structural and functional abnormalities of the vascular microenvironment determine pathophysiological characteristics of gliomas, such as loss of blood-brain barrier function, tumor cell invasiveness, or permselectivity for large molecules. Moreover, the effectiveness of various therapeutic strategies critically depends upon the successful transvascular delivery of molecules. In order to shed more light on the vascular microenvironment in gliomas, a variety of experimental and clinical techniques have been applied to study the glioma microvasculature, including histology, vascular corrosion casts, microangiography, autoradiography, tracer washout techniques, magnetic resonance imaging, as well as intravital fluorescence microscopy. This review summarizes the characteristic features of vascular morphology, angio-architecture, tumor perfusion, microvascular permeability, and microvessel-related immunological competence in gliomas. An improved understanding of the vascular microenvironment in gliomas will help in the future to optimize glioma imaging and to improve delivery of vectors for gene therapy or encapsulated drug carriers for pharmacotherapy in patients.

Combined inhibition of VEGF and PDGF signaling enforces tumor vessel regression by interfering with pericyte-mediated endothelial cell survival mechanisms

Destruction of existing tumor blood vessels may be achieved by targeting vascular endothelial growth factor (VEGF) signaling, which mediates not only endothelial cell proliferation but also endothelial cell survival. In this study, however, intravital microscopy failed to demonstrate that targeting of VEGFR-2 (by the tyrosine kinase inhibitor SU5416) induces significant regression of experimental tumor blood vessels. Immunohistochemistry, electron microscopy, expression analyses, and in situ hybridization provide evidence that this resistance of tumor blood vessels to VEGFR-2 targeting is conferred by pericytes that stabilize blood vessels and provide endothelial cell survival signals via the Ang-1/Tie2 pathway. In contrast, targeting VEGFR-2 plus the platelet-derived growth factor receptor (PDGFR)-beta system (PDGFR-beta) signaling (by SU6668) rapidly forced 40% of tumor blood vessels into regression, rendering these tumors hypoxic as shown by phosphorescence quenching. TUNEL staining, electron microscopy, and apoptosis blocking experiments suggest that VEGFR-2 plus PDGFR-beta targeting enforced tumor blood vessel regression by inducing endothelial cell apoptosis. We further show that this is achieved by an interference with pericyte-endothelial cell interaction. This study provides novel insights into the mechanisms of how 1) pericytes may provide escape strategies to anti-angiogenic therapies and 2) novel concepts that target not only endothelial cells but also pericyte-associated pathways involved in vascular stabilization and maturation exert potent anti-vascular effects.

Evaluation of TNF-alpha/Bax gene therapy and radiation against C6 glioma xenografts

Successful therapy of high-grade tumors of the brain is likely to require a combination of new therapeutic approaches. The major goal of the present study was to construct a plasmid-based bax gene vector (pGL1-Bax) and evaluate its expression in vitro and in vivo using athymic mice with subcutaneously growing C6 glioma. Preliminary experiments of efficacy and safety were also performed using pGL1-Bax alone and in combination with previously constructed pGL1-TNF-alpha, as well as with radiation. pGL1-Bax was expressed by C6 cells and was correlated with apoptosis, indicating that the construct and the bax protein were functional. Although intratumoral injections of pGL1-Bax alone, up to total doses of 450 micro g, did not significantly affect tumor growth, consistently smaller tumors were obtained when pGL1-TNF-alpha plus pGL1-Bax were injected 16-18 hr prior to tumor irradiation. Furthermore, in mice with two tumors, one treated and one untreated, progression of the untreated tumor was delayed in the animals receiving all three modalities. No prohibitive toxicities were noted, based on mouse body weights and in vitro assays of blood and spleen. Significant increases in spleen mass, total leukocyte counts, percentage of granulocytes, spontaneous blastogenesis, and CD71-expressing B cells were primarily associated with tumor presence and not treatment type. Overall, the results are promising and suggest that TNF-alpha/Bax gene therapy may be beneficial against highly malignant tumors of the brain. To our knowledge, this is the first report of bax gene therapy used together with radiation in an in vivo glioma model.

Recognition of ADP-ribosylation factor 4-like by HLA-A2-restricted and tumor-reactive cytotoxic T lymphocytes from patients with brain tumors

Although specific immunotherapy is one candidate treatment of brain tumor, the molecular basis of T-cell-mediated recognition of brain tumors has not yet been elucidated. In this study, we tried to identify brain tumor antigens using HLA-A2-restricted and tumor-reactive cytotoxic T lymphocytes (CTLs). As an HLA-A2-restricted OK-CTL line contained CTLs capable of responding to HLA-A2+ malignant glioma cells, this cell line was used for identification of brain tumor antigens. After screening a cDNA library from brain tumor cells, this CTL line was found to produce interferon (IFN)-gamma when cultured with COS-7 cells, which were cotransfected with both a cDNA clone (clone 1) and HLA-A0207 cDNA. Data base searches indicated that the clone 1 was 98% identical to that of the human ADP-ribosylation factor 4-like (ARF4L). Two peptides, ARF4L 15-24 and ARF4L 69-77, possessed the ability to induce HLA-A2-restricted and tumor-reactive CTLs from peripheral blood mononuclear cells of patients with brain tumors. Although ARF4L seemed to be ubiquitously expressed at the mRNA level, ARF4L-reactive CTLs failed to exhibit cytotoxicity against normal lymphoid blasts. These results indicate that these two ARF4L peptides could be targets for immunotherapy of HLA-A2+ patients with brain tumors.

Increased endothelial vesicular transport correlates with increased blood-tumor barrier permeability induced by bradykinin and leukotriene C4

Bradykinin and leukotriene C4 (LTC4) have been shown to increase molecular transport across the blood-tumor barrier (BTB). The aim of this study was to quantitatively assess whether an increase in vesicular transport or opening of tight junctions was responsible for this increase in permeability. Wistar rats bearing RG2 or C6 gliomas were infused with bradykinin or LTC4 through the right carotid artery for 15 min and then perfused to achieve fixation. Prepared specimens were observed using transmission electron microscopy. Infusion of either bradykinin or LTC4 resulted in significantly increased vesicular density in capillary endothelial cells of the BTB but not in normal brain capillaries. The opening of tight junctions, assessed by determining a cleft index, was found to be greater in tumor capillaries compared to normal controls. However, neither bradykinin nor LTC4 produce variations in the cleft index. A significant accumulation of horseradish peroxidase was seen in the intercellular peri-capillary spaces and in endothelial transport vesicles after infusion of bradykinin, demonstrating that the formation of vesicles was associated with macromolecular transcytosis. These findings suggest that pinocytotic vesicular transport is the primary means by which luminal to abluminal transport occurs in response to vasomodulation with bradykinin or LTC4.

Vascular microenvironment in gliomas

Structural and functional abnormalities of the vascular microenvironment determine pathophysiological characteristics of gliomas, such as loss of blood-brain barrier function, tumor cell invasiveness, or permselectivity for large molecules. Moreover, the effectiveness of various therapeutic strategies critically depends upon the successful transvascular delivery of molecules. In order to shed more light on the vascular microenvironment in gliomas, a variety of experimental and clinical techniques have been applied to study the glioma microvasculature, including histology, vascular corrosion casts, microangiography by injection of dyes, blood flow measurements by autoradiography, tracer washout techniques, magnetic resonance imaging, as well as intravital fluorescence microscopy. This review summarizes the characteristic features of vascular morphology, angio-architecture, tumor perfusion, microvascular permeability, as well as microvessel-related immunological competence in gliomas. An improved understanding of the vascular microenvironment in gliomas will help in the future to optimize glioma imaging and delivery of vectors for gene therapy or encapsulated drug carriers in patients.

Endothelial vesicles in the blood-brain barrier: are they related to permeability?

1. Macromolecules cross capillary walls via large vascular pores that are thought to be formed by plasmalemmal vesicles. Early hypotheses suggested that vesicles transferred plasma constituents across the endothelial wall either by a "shuttle" mechanism or by fusing to form transient patent channels for diffusion. Recent evidence shows that the transcytotic pathway involves both movement of vesicles within the cell and a series of fusions and fissions of the vesicular and cellular membranes. 2. The transfer of macromolecules across the capillary wall is highly specific and is mediated by receptors incorporated into specific membrane domains. Therefore, despite their morphological similarity, endothelial vesicles from heterogeneous populations in which the predominant receptor proteins incorporated in their membranes define the functions of individual vesicles. 3. Blood-brain barrier capillaries have very low permeabilities to most hydrophilic molecules. Their low permeability to macromolecules has been presumed to be due to an inhibition of the transcytotic mechanism, resulting in a low density of endothelial vesicles. 4. A comparison of vesicular densities and protein permeabilities in a number of vascular beds shows only a very weak correlation, therefore vesicle numbers alone cannot be used to predict permeability to macromolecules. 5. Blood-brain barrier capillaries are fully capable of transcytosing specific proteins, for example, insulin and transferrin, although the details are still somewhat controversial. 6. It has recently been shown that the albumin binding protein gp60 (also known as albondin), which facilitates the transcytosis of native albumin in other vascular beds, is virtually absent in brain capillaries. 7. It seems likely that the low blood-brain barrier permeability to macromolecules may be due to a low level of expression of specific receptors, rather than to an inhibition of the transcytosis mechanism.

Determination of intracranial tumor volumes in a rodent brain using magnetic resonance imaging, Evans blue, and histology: a comparative study

The measurement of tumor volumes is a practical and objective method of assessing the efficacy of a therapeutic agent. However, the relative accuracy of different methods of assessing tumor volume has been unclear. Using T1-weighted, gadolinium-enhanced magnetic resonance Imaging (T1-MRI), Evans Blue infusion and histology we measured intracranial tumor volumes in a rodent brain tumor model (RT2) at days 10, 16 and 18 after implantation of cells in the caudate putamen. There is a good correlation between tumor volumes comparing T1-MRI and Evans Blue (r2 = 0.99), T1-MRI and Histology (r2 = 0.98) and histology and Evans Blue (r2 = 0.93). Each of these methods is reliable in estimating tumor volumes in laboratory animals. There was significant uptake of gadolinium and Evans Blue in the tumor suggesting a wide disruption of the blood-brain barrier.

Expression of the tumor-rejection antigen SART1 in brain tumors

We have reported a tumor-rejection antigen, SART1(259), possessing tumor epitopes capable of inducing cytotoxic T lymphocytes (CTLs) in epithelial-cancer patients. This study investigated the expression of SART1(259) antigen in brain tumors, to explore for a potential molecule for use in specific immunotherapy of patients with brain tumors. The SART1(259) antigen was detected in the cytosol fraction of 13 of 18 (72%) glioma cell lines and in 12 of 34 (35%) brain-tumor tissues, with a higher rate of expression among malignant gliomas (5/10, 50%) and schwannomas (3/4). HLA-A24-restricted and SART1-specific CTLs recognized the HLA-A24+ and SART1(259)+ glioma cells, and the levels of recognition correlated both with HLA-A24-antigen expression level and with the concentration of the SART1 peptide antigen. Therefore, the SART1(259) antigen could be a target molecule for specific immunotherapy of patients with brain tumors expressing HLA-class-1 antigens.

Microvessel organization and structure in experimental brain tumors: microvessel populations with distinctive structural and functional properties

We studied microvessel organization in five brain tumor models (ENU, MSV, RG-2, S635cl15, and D-54MG) and normal brain, including microvessel diameter (LMVD), intermicrovessel distance (IMVD), microvessel density (MVD), surface area (S(v)), and orientation. LMVD and IMVD were larger and MVD was lower in tumors than normal brain. S(v) in tumors overlapped normal brain values and orientation was random in both tumors and brain. ENU and RG-2 tumors and brain were studied by electron microscopy. Tumor microvessel wall was thicker than that of brain. ENU and normal brain microvessels were continuous and nonfenestrated. RG-2 microvessels contained fenestrations and endothelial gaps; the latter had a maximum major axis of 3.0 microm. Based on anatomic measurements, the pore area of RG-2 tumors was estimated at 7.4 x 10(-6) cm(2) g(-1) from fenestrations and 3.5 x 10(-5) cm(2) g(-1) from endothelial gaps. Increased permeability of RG-2 microvessels to macromolecules is most likely attributable to endothelial gaps. Three microvessel populations may occur in brain tumors: (1) continuous nonfenestrated, (2) continuous fenestrated, and (3) discontinuous (with or without fenestrations). The first group may be unique to brain tumors; the latter two are similar to microvessels found in systemic tumors. Since structure-function properties of brain tumor microvessels will affect drug delivery, studies of microvessel function should be incorporated into clinical trials of brain tumor therapy, especially those using macromolecules.

Pharmacokinetics of methotrexate in the extracellular fluid of brain C6-glioma after intravenous infusion in rats

PURPOSE

Establishment of the pharmacokinetic profile of methotrexate (MTX) in the extracellular fluid (ECF) of a brain C6-glioma in rats.

METHODS

Serial collection of plasma samples and ECF dialysates after i.v. infusion of MTX (50 or 100 mg/kg) for 4 h. HPLC assay.

RESULTS

Histological studies revealed the presence of inflammation, edema, necrosis, and hemorrhage in most animals. In vivo recovery (reverse dialysis) was 10.8 +/- 5.3%. MTX concentrations in tumor ECF represented about 1-2% of the plasma concentrations. Rapid equilibration between MTX levels in brain tumor ECF and plasma. ECF concentrations almost reached steady-state by the end of the infusion (4 h), then decayed in parallel with those in plasma. Doubling of the dose did not modify MTX pharmacokinetic parameters (t1/2alpha, t1/2beta, MRT, fb, Vd, and CL(T)), except for a 1.7-fold increase of AUC(Plasma) and a 3.8-fold increase in AUC(ECF), which resulted in a 2.3-fold increase in penetration (AUC(ECF)/AUC(Plasma)). In spite of an important interindividual variability, a relationship between MTX concentrations in plasma and tumor ECF could be established from mean pharmacokinetic parameters.

CONCLUSIONS

High plasma concentrations promote the penetration of MTX into brain tissue. However, free MTX concentrations in tumor ECF remain difficult to predict consistently.

Ultrastructural and morphometric investigation of human brain capillaries in normal and peritumoral tissues

Capillaries of peritumoral and normal brain tissues were ultrastructurally and morphometrically investigated to evaluate the changes in peritumoral capillaries connected with the tumor-associated vasogenic edema. The endothelial cells of peritumoral capillaries showed varying thickness, electron-lucent cytoplasm, and structurally normal tight junctions. The basal lamina was thickened, rarefied, and vacuolated. The pericytes were provided with pinocytotic vesicles and phagocytic bodies. The astrocytic glia appeared empty or swollen, with few glycogen granules and a disarranged cytoskeleton; well-preserved glia was occasionally observed. The brain tissue was slightly edematous. No statistically significant differences were observed between normal and peritumoral capillaries as regards diameter, wall thickness, endothelial thickness, and endothelial vesicle density. Instead, the peritumoral capillaries displayed three times as many endothelial surface-connected vesicles, a markedly thicker basal lamina, and significantly reduced extension of pericytic and glial investments. The kind and severity of the vascular modifications, compared with the slight edematous appearance of the nervous tissue, strengthen the hypothesis that peritumoral capillaries could be involved in the edema resolution process.

Internalization of microbubbles by tumor cells in vivo and in vitro

Lipid-coated microbubbles (LCM) administered intravenously (i.v.) to rats bearing brain tumor, specifically enhance tumor visualization by ultrasound [1]. In order to understand the basis for this observation, we have examined the interactions of LCM with glioblastoma (C6) and gliosarcoma (9L) tumor cells in vivo and in vitro. LCM and LCM labeled with the fluorescent lipophilic dye 3,3'-dioctadecyloxacarbocyanine perchlorate (diO) were administered to rats bearing brain tumor. LCM and diO-labeled LCM were found principally at the tumor site with no evidence of label in the surrounding normal brain tissue. Analysis of the tumor by confocal laser scanning microscopy revealed that labeled LCM were inside the tumor cells. Similar analysis of LCM interactions with C6 and 9L cells in culture showed that LCM first adsorb at the surface of the cells, and with time became localized inside the cells. Binding and internalization proceeded faster at 37 degrees C than at room temperature (RT). Staining of live cells with N-(3-((2,4-dinitrophenyl)amino)propyl)-N-(3-aminopropyl) methylamine dihydrochloride (DAMP), a dye that recognizes acidic compartments, showed that the majority of internalized LCM was associated with compartments containing DAMP. If the same uptake mechanism were operative in vivo, it would indicate that a portion of LCM bypasses the reticuloendothelial system and become endocytosed directly by tumor cells.

Cell adhesion molecules acting between C6 glioma and endothelial cells

The interactions between tumor cells and endothelium play a key role in the process of tumor growth, local invasion, and distant metastasis. In the present study, we examined the adhesion of C6 glioma cells to bovine endothelial cell (EC) monolayers and defined the cell adhesion molecules acting between these cells. Pretreatment of the EC monolayer with cytokines, tumor necrosis factor (TNF)-alpha, interleukin (IL)-1 beta, and interferon (INF)-gamma, significantly increased the adhesion of C6 glioma cells to the EC monolayer. The effect lasted more than 24 hours and was protein-synthesis dependent. The adhesion of C6 glioma cells to TNF-activated ECs was blocked by the monoclonal antibody to the intercellular adhesion molecule-1 (ICAM-1) or beta 2 integrin, whereas that of melanoma cells was not. These findings provide evidence that ICAM-1 and beta 2 integrin function as inducible cell surface molecules that can support the adhesion of C6 glioma cells to ECs, and may contribute to the characteristic growth of glial tumors in vivo.

Quantitation of blood-brain barrier ultrastructure

This paper describes a quantitative approach to evaluating the ultrastructural features of brain capillaries that relate to the low non-specific permeability of the blood-brain barrier (BBB). Critical features in this approach include examination of large numbers of tissue samples and consistent, objective means of measuring features of interest. Junctional clefts, i.e., continuous channels between tight junctional regions correlate well with the know vascular permeability, being low in normal adult blood-brain barrier, high in fetal brain, and high in tumours, both human and rat. Endothelial vesicles do not always correlate with vascular permeability. They have a low density in normal adult BBB, but are also low in fetal BBB and low in some intracranial tumour vessels. However, they have a high density in muscle capillaries, and others have shown that they increase in BBB vessels damaged by hypertension. Fenestrations are consistently high in leaky vessels, but not all leaky vessels have fenestrations. The density of mitochondria in endothelial cells is high in BBB vessels of some species but not in others. Glut-1, the glucose transporter of the BBB is asymmetrically distributed between the luminal and abluminal membranes of BBB capillaries, being almost four times as numerous on the abluminal face. A large intracellular pool of glucose transporter may provide a means for rapid upregulation of the surface transporters.

Blood-eye barriers in the rat: correlation of ultrastructure with function

The function of different vascular beds in the rat eye and brain was evaluated by measuring the transfer of a vascular tracer, 14C-alpha-amino-isobutyric acid, from blood to tissue. The density of vascular pores was measured in electron micrographs of perfusion-fixed, age-matched tissue to determine whether the differences in tracer transfer were paralleled by differences in ultrastructure. Tracer transfer in retina was approximately four times that in brain of the same animal. The transfer constant was not changed by the inclusion of cold alpha-amino-isobutyric acid, showing that transport across retinal vessels is not saturable, and indicating that, as in brain, transport is due to passive diffusion. Ultrastructurally, retinal vessels have a higher density of interendothelial junctions and of endothelial vesicles, both of which suggest higher vascular permeability. However, pericytes, which contribute to a second line of defence in the blood-brain barrier, are approximately four times as numerous in retina as in brain, and we suggest that in the retina, they act to compensate for a more permeable endothelial barrier. Ciliary body vessels had a high transfer of tracer, probably as a consequence of the fenestrations in their walls. Iridial vessels had a relatively low transfer of tracer, similar to that in retina even though a proportion of the interendothelial junctions in iridial vessels had expanded junctional clefts suggestive of open paracellular channels. However, both iris and ciliary body may lose tracer to the anterior chamber fluid, leading us to underestimate the vascular permeability in these sites.

The relationship of astrocyte-like cells to the vessels that contribute to the blood-ocular barriers

Brain capillaries form a selective interface, the blood-brain barrier (BBB), between the neural parenchyma and the blood. The factors which regulate this interface are poorly understood. Both the iris and retina possess vascular beds that express some BBB characteristics; therefore, they provide attractive models to further our understanding of how blood-tissue interfaces are regulated. We have determined whether three BBB markers: the transferrin receptor, P-glycoprotein, and gamma-glutamyl transpeptidase (gamma-GTP), can be localized in the capillaries of the rat retina and iris. We have also compared, in retina and iris, the relationship which GFAP-positive cells have with the blood vessels to the expression of the three BBB markers by the vessels. Immunocytochemistry revealed that capillaries throughout the retina express P-glycoprotein and the transferrin receptor. Retinal vessels do not show detectable gamma-GTP activity. GFAP-positive cells ensheath capillaries in the nerve fibre layer of the retina. Of the three BBB characteristics we examined, iridial vessels expressed only one of them: P-glycoprotein. In the iris, GFAP-positive cells do not ensheath capillaries. From our results we conclude that all BBB characteristics do not have to be expressed and regulated in capillaries as a unit. Our results, in combination with those of earlier studies, suggest that the expression of some BBB features does not require intimate contact between capillaries and astrocytes or astrocyte-like cells. Barrier maintenance appears to be a complex process which involves the integration of several factors.

Growth of cultured human glioma tumour cells can be regulated with histamine and histamine antagonists

The 50% survival time for low grade astrocytomas is 50 months and for high grade astrocytomas it is 13 months, underlining the need for new therapies. Several reports show that in vivo histamine antagonists cause retardation of tumour growth in some animal models and prolonged survival in cancer patients. Therefore we have tested the growth modulating effects of histamine and histamine antagonists on human glioma cultures. Twelve freshly excised human gliomas were cultured and tested for their in vitro sensitivity to histamine and histamine antagonists. Four continuous glioma cell lines were used to confirm the glioma-specificity of the effects observed in the primary cell lines. In low serum concentration (0 or 1%) the growth of 5/9 primary glioma-derived cultures could be stimulated with 0.2 mM histamine, and in 4/5 cases with 0.2 microM histamine. One mM of the histamine H2-receptor antagonist cimetidine could inhibit the growth of 4/5 primary glioma cultures when tested in 1% human AB serum, and of 6/13 cases when tested in 1% FCS. Lower concentrations (down to 1 microM) were less effective. The histamine H1-receptor antagonist pyrilamine gave variable results. The specificity of the effects is indicated by the absence of a generalised toxic effect, by the observation that the antagonist-induced inhibition could be reversed with histamine, and by the correlation of the obtained cimetidine-induced growth inhibition with the maximal growth rate of the primary cell lines in 10% FCS. The observed cimetidine-induced inhibition of the in vitro proliferation of gliomas suggests that cimetidine is a relevant candidate for the in vivo growth inhibition of these tumours.

Invasion of experimental rat brain tumor: early morphological changes following microinjection of C6 glioma cells

We present morphological data of the early stage of tumor invasion in the central nervous system. C6 rat glioma cells were injected into the caudate-putamen of rat brain using glass micropipettes to minimize traumatic reactions. Four days after the inoculation, we examined the tumor-brain interface using light and electron microscopy. Ultrastructurally the tumor processes were attached to the perivascular basement membrane instead of the astroglial end-feet. At the tumor periphery, the vessel walls were in contact with both tumor processes and astroglial end-feet. Astrocytes withdrew their processes from the vascular walls and changed into a reactive phenotype, while the neuronal cells remained virtually intact, even when surrounded by tumor cells. Immunohistochemical study using C6 cells labeled with bromodeoxyuridine showed migration of the cells toward the perivascular space that was distant from the site of injection. These observations represent the earliest morphologically detectable changes of the tumor-brain interface, and suggest that the C6 cells possess the characteristics of high affinity to the endothelial basement membrane and invade along the preexisting blood vessels with brain parenchymal infiltration.

Re-evaluating the role of astrocytes in blood-brain barrier induction

Neural tissue induces brain capillary endothelial cells to express a diverse array of characteristics that allow them to regulate the passage of solutes between the blood and the brain; these features are collectively referred to as the blood-brain barrier (BBB). Because astrocytes are intimately associated with brain capillaries, they have been thought to be the cell type responsible for barrier induction. Widely accepted support of this hypothesis has been derived from experiments showing that astrocytes implanted into the anterior chamber of the rat eye, or onto the chorioallantoic membrane of the chicken embryo, remain unstained by circulating Evan's blue, while grafts of fibroblasts in these sites stain intensely. We have found several limitations associated with placing grafts in either site, leading us to believe that previously reported results are inconclusive. Astrocytes implanted into the anterior chamber form grafts that are poorly vascularized, whereas fibroblast grafts are richly vascularized by vessels which are often fenestrated. This likely accounts for apparent differences in vessel permeability reported by others. We have found that iridial vessels associated with astrocyte grafts do not change their ultrastructure to resemble brain capillaries. Grafting of cells to the chorioallantoic membrane elicits an extensive inflammatory response. Inflammation results in poor delivery of tracers to graft vasculature as well as altering vessel permeability. Treatment of hosts with steroidal anti-inflammatory agents in doses compatible with survival of the host does allow improved graft survival. Even after treatment with anti-inflammatory agents, however, astrocyte graft vasculature fails to express high levels of a barrier marker, the GLUT-1 isoform of the glucose transporter. Transplantation of avascular embryonic spinal cord, that induces robust vessel ingrowth and GLUT-1 expression in intra-embryonic vessels, was unable to elicit the ingrowth of more than a few vessels from the chorioallantoic membrane vasculature, and none of these expressed glucose transporter. We conclude that the anterior chamber and chorioallantoic membrane are not suitable sites for studying BBB induction, and that there is, at present, no conclusive evidence that mature astrocytes play a significant role in the initial expression of the BBB.

A transmission and scanning electron microscopic study of tumoral and peritumoral microblood vessels in human gliomas

The tumor microblood vessels (MBVs) of 25 cases of gliomas of varying grades were studied and compared with those in peritumoral region using both transmission and scanning electron microscopy (TEM and SEM). The TEM study revealed numerous villous projections with pinocytotic vesicles (PCVs) and large vacuoles (LVs) concentrated mainly at the luminal aspect in tumor MBVs which increased with increasing severity of edema. The peritumoral MBVs, in addition to showing some increase in villous projections on the luminal surface, also showed increased number of PCVs and LVs concentrated at the abluminal aspect with some of them even communicating with the extravascular space. The SEM study largely corroborated the TEM findings. The sites of formation of PCVs and LVs appeared as small pits or large craters on the luminal surface of the endothelial cells of tumor MBVs. We feel that the morphological evidence of increased permeability in tumor MBVs represents their role in the development of edema and that the occurrence of reverse pinocytosis in peritumoral MBVs is a distinct possibility which may be associated with resorption of edema fluid.

Differences in contrast enhancement between intra- and extra-axial lesions after two hours of iodine injection

In order to try to improve discrimination between the brain lesions that produce damage to the brain-blood barrier and those which are irrigated by extracerebral vessels, the behavior of contrast enhancement up to two hours after iodine injection was studied in two groups of patients. Eight patients had extra-axial lesions (7 meningiomas, one acoustic neuroma) and eleven had intra-axial lesions (6 of astrocytic type, 3 metastases, 2 inflammatory). In the first group, mean values of the lesioned areas in HU decreased progressively with time. However, in many cases in the second group the values at the end of the study were equal to or higher than those observed immediately after contrast medium injection. Highly significant differences were observed between the two groups. The contrast medium in the intra-axial lesions conform to the behavior of all other fluids in a damaged barrier, remaining for a very long time in the interstitial space. In the extra-axial lesions, the contrast moved freely and the return from the interstitial to the intravascular space was also unobstructed, since values decayed immediately after injection. For these reasons, the observation of scans after two hours of contrast injection might be useful for differential diagnosis in those cases where doubt exists.

Radiobiology of radiosurgery: Part II. The rat C6 glioma model

We developed an experimental animal model to evaluate the potential role of stereotactic radiosurgery for glial neoplasms. Rats were randomized to control or treatment groups after implantation of C6 glioma cells into the right frontal region; 14 days later, 19 rats underwent stereotactic radiosurgical treatment of the induced tumor, using the 4-mm collimator of the gamma unit. Both groups were observed for up to 65 days after implantation. Treated animals had a mean survival of 39.2 days; the 22 control animals lived a mean of 29.4 days before death from tumor growth (P = 0.07). Six treated animals (32%), but only one control animal, survived the full observation period (P = 0.07). The mean tumor diameter in the control group was 9.64 mm; in the radiosurgery group, it was 6.47 mm (P = 0.001). Compared with tumors in control animals, treated tumors had a hypocellular appearance (P less than 0.001) and demonstrated cellular edema (P less than 0.005) under light microscopy, indicating a direct cytotoxic response to treatment. No difference was identified in the amount of tumor necrosis, intratumor hemorrhage, or degree of brain invasion between the two groups. Variations in the maximum treatment dose (30, 40, 50, 70, or 100 Gy) did not result in observed differences in tumor response. This in vivo rat malignant glioma model is a valuable tool to evaluate the tumoricidal effects of single-fraction, focused irradiation. Additional studies are warranted to evaluate dose-response relationships, radiation sensitizers, and use of radiosurgery with other adjuvant treatments.

Comparison of the blood-brain barrier and liver penetration of acridine antitumor drugs

The blood-brain barrier penetration of amsacrine and its analogs 9-([2-methoxy-4-[(methylsulfonyl)-amino]phenyl]amino)-,5-dimethyl- 4-acridine carboxamide (CI-921) and M-[2-(dimethylamino)ethyl]-acridine-4-carboxamide (AC) was measured in the barbiturate-anesthetized mouse. After intracarotid administration, AC was almost completely extracted (90%) in a single transit through the brain capillaries, whereas CI-921 (20%) and amsacrine (15%) were moderately extracted. AC is retained in the brain; no loss of AC from the brain was apparent at 1, 2, 4, or 8 min after injection. In contrast, after intraportal administration, 75% of the AC, 94% of the CI-921, and 57% of the amsacrine was extracted in a single transit through the hepatic vasculature. Rather than being retained in the mouse liver, these acridine antitumor agents show time-dependent loss (t1/2 = 10 min for amsacrine and AC, 24 min for CI-921). We conclude that unlike most antitumor agents, these acridine drugs appear to penetrate the blood-brain barrier readily.

Effect of an inhibitor of lipid peroxidation, U78517F, on C6 astrocytoma growth

A potent in vitro inhibitor of lipid peroxidation, U78517F, has been employed to assess its influence on C6 astrocytoma growth in monolayer culture and tumor growth and protein extravasation in the rat C6 astrocytoma spheroid implantation model. Results demonstrate that 1 microM concentration of U78517F inhibits cell division, but is not tumoricidal to C6 astrocytoma cells in monolayer culture. Concentrations of 5 microM and above significantly decreased astrocytoma cell viability. Following spheroid implantation, rats were treated with one of these U78517F regimes: 12 mg/kg/day for 13 days post-implantation, 4 mg/kg/day for 13 days post-implantation or 12 mg/kg/day commencing seven days post-implantation until 13 days post-implantation. Tumor wet and dry weights were lower in all treatment groups, but these decreases were not statistically significant. Protein extravasation as measured by Evans blue extravasation was not significantly reduced by any treatment regime used. It is concluded that U78517F inhibits C6 astrocytoma growth in monolayer culture at and above 5 microM concentrations, but the treatment regimes utilized did not significantly decrease tumor growth or permeability in the C6 astrocytoma spheroid implantation model.

The glucose transporter and blood-brain barrier of human brain tumors

The glucose transporter of the human brain has been localized to endothelial cells expressing the blood-brain barrier, but little is known regarding its mechanism of induction or whether its expression is exclusively linked with restricted vascular permeability. We investigated glucose transporter expression by vessels in human astrocytic tumors and pulmonary metastases to the brain using immunohistochemical techniques. Vessels in 9 of 10 low-grade astrocytomas and 8 of 10 anaplastic astrocytomas were positive for glucose transporter. Glioblastoma vessels were transporter-positive in only 2 of 10 specimens. Vessels in all three metastatic tumors were negative for the glucose transporter. The decrease in transporter expression observed in higher-grade tumors occurred independently of increases in vascular permeability. In low-grade astrocytomas and glioblastomas transporter expression and contrast enhancement were inversely related, but vessels in 6 of 9 anaplastic astrocytomas were transporter-positive despite contrast enhancement. These findings suggest that separate mechanisms induce the glucose transporter and the permeability restrictions of the human blood-brain barrier. They also have potential implications for the therapy and prognosis of astroglial neoplasms.

Cortical microvessels during brain development: a morphometric study in the rat

In order to determine whether any structural changes occur in the blood-brain barrier during development which can be related to changes in barrier function over the same period, morphometric methods were used on pieces of visual cortex taken from rats between 16 days gestation and adult and processed for light and electron microscopy. Capillary volume fraction, numbers per unit area, and surface density increased in two phases, before birth and between 10 and 20 days after birth, with no subsequent change after 20 days. Brain parenchymal mitochondria, assumed to be a measure of brain oxidative activity, did not change before birth but increased gradually and continuously after birth from 1.7% to 5.7% in adults. The capillary endothelial mitochondrial volume fraction as a percentage of the endothelial cell cytoplasm, previously thought to be important for ion transport, did not change with age (mean = 5.4%), although there was evidence that mitochondria either divide or change in shape up to 30 days after birth. The endothelial cell thickness decreased with age from 0.6 to 0.2 microns, probably through cell elongation during vascular growth. By combining the data on endothelial mitochondria and vascularity, it was shown that the total endothelial mitochondrial volume per unit length of capillary decreased with age, whereas per unit volume of tissue it increased.

Mechanisms of tumor-associated edema: a review

An understanding of the mechanisms responsible for tumor-associated edema involves the elucidation of the role played by a number of intra-related processes. These include (i) the permeability of new tumor microvessels that are associated with tumor angiogenesis; (ii) alterations in microvascular permeability due to factors secreted by tumor cells; (iii) immunological mechanisms and (iv) increased microvessel permeability associated with inflammation. The rationale for a role for inflammatory processes in tumor-associated edema has been outlined and the role of non-steroidal anti-inflammatory drugs in modulating experimental and human tumor-associated edema has been explored.

Investigation of an inhibitor of lipid peroxidation U74006F on tumor growth and protein extravasation in the C6 astrocytoma spheroid implantation glioma model

The role played by lipid peroxidation in cerebral tumor growth and protein extravasation has not been defined. A potent in vitro inhibitor of lipid peroxidation (21-[4-(2,6-di-1-pyrrolidinyl-4-pyrimidinyl)-1-piperazinyl]-16a- methylpregna-1,4,9(11)-triene-3,20-dione monomethane sulfonate (U74006F), has been used to assess its influence on both tumor growth and protein extravasation in a rat C6 astrocytoma implantation model. Following spheroid implantation, rats were treated with one of these U74006F regimens: 12 mg/kg/day for 13 days post-implantation, 3.6 mg/kg/day for 13 days post-implantation or 12 mg/kg/day commencing seven days post-implantation until 13 days post-implantation. Tumor wet weights, dry weight and protein flux as measured by Evans blue dye extravasation were assessed. Neither wet weights nor dry weights of any of the three treatment groups was significantly lower when compared to control tumor wet weights and dry weights. Protein extravasation was not significantly reduced by any of the treatments. It is concluded that U74006F did not significantly decrease tumor growth or permeability in the C6 astrocytoma spheroid implantation model.

Immunology of central nervous system tumors

With progress in cellular immunology and the development of hybridoma technology, the idea of manipulating host-tumor immune interactions to improve the prognosis of brain tumors has aroused renewed interest. Although no brain tumor-specific antigens have been found, and in spite of the wide antigenic heterogeneity of brain tumor cells, some monoclonal antibodies possessing restricted specificity have been isolated and their potential clinical applications investigated. One of the most pronounced changes in the cellular immune responses of brain tumor patients is a profound depression of the T4-helper lymphocytes. Clinical and laboratory trials are under way to assess the ability of lymphokines, such as gamma-interferon or interleukin-2, to restore immunologic competence in these patients and potentiate a specific anti-tumor immunologic response. Recent work suggests that the endothelium-astrocyte complex may have a pivotal role in assisting the escape of brain tumors from the host's immunologic responses, since it is responsible for the intracerebral sequestration of antigens and local anti-tumor responses. In this review, the data on the antigenic properties of central nervous system tumors and the host's humoral and cellular immune responses to them are analyzed and potential immunologic therapies are discussed.

Ultrastructural study of micro-blood vessels in human brain tumors and peritumoral tissue

Ultrastructural and tracer studies have demonstrated that vasogenic edema, a serious complication of brain tumor is the result of increased permeability of tumor vessels. However, not much information is available on the alterations in the vessels in the peritumoral areas. Therefore, we studied the ultrastructural changes in the tumor micro-blood vessels (MBVs) in 20 cases of glioma and compared these with the changes in the peritumoral MBVs in 10 of these cases. The tumor MBVs showed remarkable structural changes, viz, increase in pinocytotic vesicles, large vacuoles and microvilli in the endothelial cells, varying degrees of endothelial attenuation and fenestration, an occasional partially or completely opened-up junction and some pale and edematous endothelial cells, which can adequately explain their increased permeability. The peritumoral MBVs also showed evidence of increased permeability in the form of increased pinocytotic vesicles, large vacuoles and microvilli associated with pale and edematous cytoplasm of some endothelial cells. Thickened multilayered basement membrane, absence of ensheathment of capillary basement membrane by astrocytic cell processes and widened perivascular space were observed in both tumoral and peritumoral MBVs. An interesting observation was that in the peritumoral MBVs, the pinocytotic vesicles were most conspicuously seen on the abluminal side of the endothelial cells often fused with the abluminal plasma membrane. Although a static study like this cannot indicate any definite direction of movement of fluid, we feel that the occurrence of reverse pinocytosis is a distinct possibility in the peritumoral MBVs and that it may be an important means of resorption of edema fluid.