Imaging and analytical methods as applied to the evaluation of vasculature and hypoxia in human brain tumors

Wnt Signaling in Brain Tumors: A Challenging Therapeutic Target

The involvement of Wnt signaling in normal tissue homeostasis and disease has been widely demonstrated over the last 20 years. In particular, dysregulation of Wnt pathway components has been suggested as a relevant hallmark of several neoplastic malignancies, playing a role in cancer onset, progression, and response to treatments. In this review, we summarize the current knowledge on the instructions provided by Wnt signaling during organogenesis and, particularly, brain development. Moreover, we recapitulate the most relevant mechanisms through which aberrant Wnt pathway activation may impact on brain tumorigenesis and brain tumor aggressiveness, with a particular focus on the mutual interdependency existing between Wnt signaling components and the brain tumor microenvironment. Finally, the latest anti-cancer therapeutic approaches employing the specific targeting of Wnt signaling are extensively reviewed and discussed. In conclusion, here we provide evidence that Wnt signaling, due to its pleiotropic involvement in several brain tumor features, may represent a relevant target in this context, although additional efforts will be needed to: (i) demonstrate the real clinical impact of Wnt inhibition in these tumors; (ii) overcome some still unsolved concerns about the potential systemic effects of such approaches; (iii) achieve efficient brain penetration.

Tumor Microenvironment in Glioma Invasion

A major malignant trait of gliomas is their remarkable infiltration capacity. When glioma develops, the tumor cells have already reached the distant part. Therefore, complete removal of the glioma is impossible. Recently, research on the involvement of the tumor microenvironment in glioma invasion has advanced. Local hypoxia triggers cell migration as an environmental factor. The transcription factor hypoxia-inducible factor (HIF) -1α, produced in tumor cells under hypoxia, promotes the transcription of various invasion related molecules. The extracellular matrix surrounding tumors is degraded by proteases secreted by tumor cells and simultaneously replaced by an extracellular matrix that promotes infiltration. Astrocytes and microglia become tumor-associated astrocytes and glioma-associated macrophages/microglia, respectively, in relation to tumor cells. These cells also promote glioma invasion. Interactions between glioma cells actively promote infiltration of each other. Surgery, chemotherapy, and radiation therapy transform the microenvironment, allowing glioma cells to invade. These findings indicate that the tumor microenvironment may be a target for glioma invasion. On the other hand, because the living body actively promotes tumor infiltration in response to the tumor, it is necessary to reconsider whether the invasion itself is friend or foe to the brain.

Adapt to Persist: Glioblastoma Microenvironment and Epigenetic Regulation on Cell Plasticity

Glioblastoma (GBM) is the most frequent and aggressive brain tumor, characterized by great resistance to treatments, as well as inter- and intra-tumoral heterogeneity. GBM exhibits infiltration, vascularization and hypoxia-associated necrosis, characteristics that shape a unique microenvironment in which diverse cell types are integrated. A subpopulation of cells denominated GBM stem-like cells (GSCs) exhibits multipotency and self-renewal capacity. GSCs are considered the conductors of tumor progression due to their high tumorigenic capacity, enhanced proliferation, invasion and therapeutic resistance compared to non-GSCs cells. GSCs have been classified into two molecular subtypes: proneural and mesenchymal, the latter showing a more aggressive phenotype. Tumor microenvironment and therapy can induce a proneural-to-mesenchymal transition, as a mechanism of adaptation and resistance to treatments. In addition, GSCs can transition between quiescent and proliferative substates, allowing them to persist in different niches and adapt to different stages of tumor progression. Three niches have been described for GSCs: hypoxic/necrotic, invasive and perivascular, enhancing metabolic changes and cellular interactions shaping GSCs phenotype through metabolic changes and cellular interactions that favor their stemness. The phenotypic flexibility of GSCs to adapt to each niche is modulated by dynamic epigenetic modifications. Methylases, demethylases and histone deacetylase are deregulated in GSCs, allowing them to unlock transcriptional programs that are necessary for cell survival and plasticity. In this review, we described the effects of GSCs plasticity on GBM progression, discussing the role of GSCs niches on modulating their phenotype. Finally, we described epigenetic alterations in GSCs that are important for stemness, cell fate and therapeutic resistance.

Local delivery to malignant brain tumors: potential biomaterial-based therapeutic/adjuvant strategies

Glioblastoma (GBM) is the most aggressive malignant brain tumor and is associated with a very poor prognosis. The standard treatment for newly diagnosed patients involves total tumor surgical resection (if possible), plus irradiation and adjuvant chemotherapy. Despite treatment, the prognosis is still poor, and the tumor often recurs within two centimeters of the original tumor. A promising approach to improving the efficacy of GBM therapeutics is to utilize biomaterials to deliver them locally at the tumor site. Local delivery to GBM offers several advantages over systemic administration, such as bypassing the blood-brain barrier and increasing the bioavailability of the therapeutic at the tumor site without causing systemic toxicity. Local delivery may also combat tumor recurrence by maintaining sufficient drug concentrations at and surrounding the original tumor area. Herein, we critically appraised the literature on local delivery systems based within the following categories: polymer-based implantable devices, polymeric injectable systems, and hydrogel drug delivery systems. We also discussed the negative effect of hypoxia on treatment strategies and how one might utilize local implantation of oxygen-generating biomaterials as an adjuvant to enhance current therapeutic strategies.

Serine-dependent redox homeostasis regulates glioblastoma cell survival

Background

The amino acid serine is an important substrate for biosynthesis and redox homeostasis. We investigated whether glioblastoma (GBM) cells are dependent on serine for survival under conditions of the tumour microenvironment.

Methods

Serine availability in GBM cells was modulated pharmacologically, genetically and by adjusting serine and glycine concentrations in the culture medium. Cells were investigated for regulation of serine metabolism, proliferation, sensitivity to hypoxia-induced cell death and redox homeostasis.

Results

Hypoxia-induced expression of phosphoglycerate dehydrogenase (PHGDH) and the mitochondrial serine hydroxymethyltransferase (SHMT2) was observed in three of five tested glioma cell lines. Nuclear factor erythroid 2-related factor (Nrf) 2 activation also induced PHGDH and SHMT2 expression in GBM cells. Low levels of endogenous PHGDH as well as PHGDH gene suppression resulted in serine dependency for cell growth. Pharmacological inhibition of PHGDH with CBR-5884 reduced proliferation and sensitised cells profoundly to hypoxia-induced cell death. This effect was accompanied by an increase in reactive oxygen species and a decrease in the NADPH/NADP⁺ ratio. Similarly, hypoxia-induced cell death was enhanced by PHGDH gene suppression and reduced by PHGDH overexpression.

Conclusions

Serine facilitates adaptation of GBM cells to conditions of the tumour microenvironment and its metabolism could be a plausible therapeutic target.

Arterial Spin Labeling for Glioma Grade Discrimination: Correlations with IDH1 Genotype and 1p/19q Status

Since accurate grading of gliomas has important clinical value, the aim of this study is to evaluate the diagnostic efficacy of perfusion values derived from arterial spin labeling (ASL) to grade gliomas. In addition, the correlation between perfusion and isocitrate dehydrogenase 1 (IDH1) genotypes and chromosome arms 1p and 19q (1p/19q) status of gliomas was assessed. A total of 52 cases of supratentorial gliomas in adults who received ASL imaging were enrolled in this retrospective study. The cerebral blood flow (CBF) images derived from ASL and anatomical maps were normalized to the Montreal Neurological Institute coordinate system and matched. The mean CBF (meanCBF), the maximum CBF (maxCBF), and their relative values (rmeanCBF and rmaxCBF, respectively) were assessed in each case. The tumor grades, IDH1 genotypes, and 1p/19q status were diagnosed according to the 2016 WHO criteria. Receiver operating characteristic curves were performed to assess the efficacy of perfusion parameters for grading. Qualitatively, all gliomas were divided into high- and low-perfusion groups. The crosstabs chi-square test of independence was performed to calculate contingency coefficient (C) and Cramer V coefficient to assess the correlation between perfusion and IDH1 genotypes and 1p/19q status of gliomas. The rmaxCBF showed the best diagnostic efficacy; meanwhile, rmeanCBF had the best specificity for grade discrimination. In astrocytoma, there was a mild correlation between IDH1 genotypes and tumor perfusion with the Cramer's V coefficient of 0.378. There was no significant association between 1p/19q codeletion and perfusion in grade II and III gliomas.

A Two-Component Assay for Hypoxia Incorporating Long-Term Nitroreduction and Short-Term DNA-Damage Allows Differentiation of the Three Hypoxia Sub-types

Hypoxia in tumors has many well-characterized effects that are known to prevent optimal cancer treatment. Despite the existence of a large number of assays that have supported hypoxia as an important diagnostic, there is no routine clinical assay in use, and anti-hypoxia therapies have often not included parallel hypoxia measurements. Even with a functioning hypoxia assay, it is difficult to match the oxygen dependence of treatment resistance to that of the assay, and this mismatch can vary substantially from assay to assay and even from tumor to tumor [e.g., caused by endogenous variations in non-protein sulfhydryls (NPSH)]. An underlying concern is the current inability to measure the three types of hypoxia; in particular, cycling hypoxia can affect all aspects of detection and treatment strategy. Here we present data that help validate a new two-component hypoxia assay recently suggested by our laboratory. This assay incorporates the long-term bioreduction of the 2-nitroimidazole, EF5, and the short-term production of γ-H2AX (e.g., time of ionizing radiation exposure). The former can be calibrated to provide the average tissue pO₂ over the EF5 exposure time while the latter provides the combined sum of microenvironmental radiation response modifiers (e.g., oxygen and NPSH) at the time of irradiation. Importantly, formation of γ-H2AX is not dependent on blood flow, while EF5 binding is only minimally so, due to the rapid and extensive diffusion characteristics of lipophilic compounds. While both individual assays have their limitations, which are addressed in this article, their combination can dissect the type of hypoxia present. In particular, a mismatch between the two assays can directly detect cycling hypoxia in a therapeutically relevant manner. Preliminary use of this two-component assay in small PC3 tumors showed essentially no binding of EF5. Similarly, there were no tumor regions (for uniform irradiation with 12 Gy) with the low levels of γ-H2AX expected for a condition of cycling hypoxia. Thus, both assays were consistent with an essentially aerobic, radiation-responsive tumor. In a larger PC3 tumor, all regions of high EF5 binding had low levels of γ-H2AX.

Perfluorocarbon emulsions radiosensitise brain tumors in carbogen breathing mice with orthotopic GL261 gliomas

Background

Tumour hypoxia limits the effectiveness of radiation therapy. Delivering normobaric or hyperbaric oxygen therapy elevates pO₂ in both tumour and normal brain tissue. However, pO2 levels return to baseline within 15 minutes of stopping therapy.

Aim

To investigate the effect of perfluorocarbon (PFC) emulsions on hypoxia in subcutaneous and intracranial mouse gliomas and their radiosensitising effect in orthotopic gliomas in mice breathing carbogen (95%O₂ and 5%CO₂).

Results

PFC emulsions completely abrogated hypoxia in both subcutaneous and intracranial GL261 models and conferred a significant survival advantage orthotopically (Mantel Cox: p = 0.048) in carbogen breathing mice injected intravenously (IV) with PFC emulsions before radiation versus mice receiving radiation alone. Carbogen alone decreased hypoxia levels substantially and conferred a smaller but not statistically significant survival advantage over and above radiation alone.

Conclusion

IV injections of PFC emulsions followed by 1h carbogen breathing, radiosensitises GL261 intracranial tumors.

Energy metabolism in glioblastoma stem cells: PPARα a metabolic adaptor to intratumoral microenvironment

Glioblastoma (GB), the most-common cancer in the adult brain, despite surgery and radio/ chemotherapy, is to date almost incurable. Many hypoxic tumors, including GB, show metabolic reprogramming to sustain uncontrolled proliferation, hypoxic conditions and angiogenesis. Peroxisome Proliferator-activated Receptors (PPAR), particularly the α isotype, have been involved in the control of energetic metabolism. Herein, we characterized patient-derived GB neurospheres focusing on their energetic metabolism and PPARα expression. Moreover, we used a specific PPARα antagonist and studied its effects on the energetic metabolism and cell proliferation/survival of GB stem cells. The results obtained demonstrate that tumor neurospheres are metabolically reprogrammed up-regulating glucose transporter, glucose uptake and glycogen and lipid storage, mainly under hypoxic culture conditions. Treatment with the PPARα antagonist GW6471 resulted in decreased cell proliferation and neurospheres formation. Therefore, PPARα antagonism arises as a potent new strategy as adjuvant to gold standard therapies for GB for counteracting recurrences and opening the way for pre-clinical trials for this class of compounds. When tumor neurospheres were grown in hypoxic conditions in the presence of different glucose concentrations, the most diluted one (0.25g/L) mimicking the real concentration present in the neurosphere core, PPARα increase/PPARγ decrease, increased proliferation and cholesterol content, decreased glycogen particles and LDs were observed. All these responses were reverted by the 72 h treatment with the PPARα antagonist.

Multidrug resistance in glioblastoma stem-like cells: Role of the hypoxic microenvironment and adenosine signaling

Glioblastoma multiforme (GBM) is considered the most common and aggressive tumour of the central nervous system and is characterized for being highly chemoresistant. This property is mainly due to the activation of Multiple Drug Resistance (MDR) mechanisms that protect cancer cells from structurally and morphologically different drugs. Overexpression and increased ABC transporters activity is one of the most important MDR mechanisms at the clinical level, and both its expression and activity are elevated in GBM cells. Within the tumour, there is a subpopulation called glioblastoma stem-like cells (GSCs), which due to its high tumourigenic capacity and chemoresistance, have been postulated as the main responsible for tumour recurrence. The GSCs inhabit hypoxic tumour zones, niches that apart from maintaining and promoting stem phenotype have also been correlated with high chemoresistance. Of the signalling pathways activated during hypoxia, purinergic signalling has been highly associated to the induction of MDR mechanisms. Through its receptors, the nucleoside adenosine has been shown to promotes the chemoresistance mediated by ABC transporters. Therefore, targeting its components is a promising alternative for GBM treatment. In this review, we will discuss chemoresistance in GSCs and the effect of the hypoxic microenvironment and adenosine on MDR mechanisms.

Stress Response Leading to Resistance in Glioblastoma-The Need for Innovative Radiotherapy (iRT) Concepts

Glioblastoma (GBM) is the most common and most aggressive malignant primary brain tumor in adults. In spite of multimodal therapy concepts, consisting of surgery, radiotherapy and chemotherapy, the median survival, merely 15–18 months, is still poor. Mechanisms for resistance of GBM to radio(chemo)therapy are not fully understood yet and due to the genetic heterogeneity within the tumor including radiation-resistant tumor stem cells, there are several factors leading to therapy failure. Recent research revealed that, hypoxia during radiation and miRNAs may adversely affect the therapeutic response to radiotherapy. Further molecular alterations and prognostic markers like the DNA-repair protein O6-methylguanine-DNA methyltransferase (MGMT), anti-apoptotic molecular chaperones, and/or the activity of aldehyde dehydrogenase 1 (ALDH1) have also been identified to play a role in the sensitivity to cytostatic agents. Latest approaches in the field of radiotherapy to use particle irradiation or dose escalation strategies including modern molecular imaging, however, need further evaluation with regard to long-term outcome. In this review we focus on current information about the mechanisms and markers that mediate resistance to radio(chemo)therapy, and discuss the opportunities of Innovative Radiotherapy (iRT) concepts to improve treatment options for GBM patients.

Glioblastoma Stem Cells Microenvironment: The Paracrine Roles of the Niche in Drug and Radioresistance

Among all solid tumors, the high-grade glioma appears to be the most vascularized one. In fact, "microvascular hyperplasia" is a hallmark of GBM. An altered vascular network determines irregular blood flow, so that tumor cells spread rapidly beyond the diffusion distance of oxygen in the tissue, with the consequent formation of hypoxic or anoxic areas, where the bulk of glioblastoma stem cells (GSCs) reside. The response to this event is the induction of angiogenesis, a process mediated by hypoxia inducible factors. However, this new capillary network is not efficient in maintaining a proper oxygen supply to the tumor mass, thereby causing an oxygen gradient within the neoplastic zone. This microenvironment helps GSCs to remain in a "quiescent" state preserving their potential to proliferate and differentiate, thus protecting them by the effects of chemo- and radiotherapy. Recent evidences suggest that responses of glioblastoma to standard therapies are determined by the microenvironment of the niche, where the GSCs reside, allowing a variety of mechanisms that contribute to the chemo- and radioresistance, by preserving GSCs. It is, therefore, crucial to investigate the components/factors of the niche in order to formulate new adjuvant therapies rendering more efficiently the gold standard therapies for this neoplasm.

High-uptake areas on positron emission tomography with the hypoxic radiotracer (18)F-FRP170 in glioblastomas include regions retaining proliferative activity under hypoxia

Objective

The aim was to evaluate the proliferative activity of high-uptake areas on positron emission tomography (PET) with the hypoxic cell radiotracer, 1-(2-[¹⁸F]fluoro-1-[hydroxymethyl]ethoxy)methyl-2-nitroimidazole (FRP170).

Methods

Thirteen patients with glioblastoma underwent FRP170 PET before tumor resection. During surgery, tumor specimens were stereotaxically obtained from regions corresponding to high (high-uptake areas, HUAs) and relatively low (low-uptake areas, LUAs) accumulation of FRP170. We compared immunohistochemical staining for Ki-67 and hypoxia-inducible factor (HIF)-1α between HUA and LUA.

Results

HIF-1α index was significantly higher in HUAs than in LUAs. In contrast, mean Ki-67 indices did not differ significantly between HUAs and LUAs.

Conclusions

Findings for HIF-1α index clearly indicated that HUAs on FRP170 PET represented hypoxic regions in glioblastoma. However, findings of Ki-67 index suggest that HUAs on FRP170 PET include regions retaining proliferative activity regardless of tissue hypoxia.

Radiation therapy for glioma stem cells

Radiation therapy is the most effective adjuvant treatment modality for virtually all patients with high-grade glioma. Its ability to improve patient survival has been recognized for decades. Cancer stem cells provide new insights into how tumor biology is affected by radiation and the role that this cell population can play in disease recurrence. Glioma stem cells possess a variety of intracellular mechanisms to resist and even flourish in spite of radiation, and their proliferation and maintenance appear tied to supportive stimuli from the tumor microenvironment. This chapter reviews the basis for our current use of radiation to treat high-grade gliomas, and addresses this model in the context of therapeutically resistant stem cells. We discuss the available evidence highlighting current clinical efforts to improve radiosensitivity, and newer targets worthy of further development.

Automated measurement of microcirculatory blood flow velocity in pulmonary metastases of rats

Because the lung is a major target organ of metastatic disease, animal models to study the physiology of pulmonary metastases are of great importance. However, very few methods exist to date to investigate lung metastases in a dynamic fashion at the microcirculatory level, due to the difficulty to access the lung with a microscope. Here, an intravital microscopy method is presented to functionally image and quantify the microcirculation of superficial pulmonary metastases in rats, using a closed-chest pulmonary window and automated analysis of blood flow velocity and direction. The utility of this method is demonstrated to measure increases in blood flow velocity in response to pharmacological intervention, and to image the well-known tortuous vasculature of solid tumors. This is the first demonstration of intravital microscopy on pulmonary metastases in a closed-chest model. Because of its minimized invasiveness, as well as due to its relative ease and practicality, this technology has the potential to experience widespread use in laboratories that specialize on pulmonary tumor research.

Imaging tumor hypoxia to advance radiation oncology

SIGNIFICANCE

Most solid tumors contain regions of low oxygenation or hypoxia. Tumor hypoxia has been associated with a poor clinical outcome and plays a critical role in tumor radioresistance.

RECENT ADVANCES

Two main types of hypoxia exist in the tumor microenvironment: chronic and cycling hypoxia. Chronic hypoxia results from the limited diffusion distance of oxygen, and cycling hypoxia primarily results from the variation in microvessel red blood cell flux and temporary disturbances in perfusion. Chronic hypoxia may cause either tumor progression or regressive effects depending on the tumor model. However, there is a general trend toward the development of a more aggressive phenotype after cycling hypoxia. With advanced hypoxia imaging techniques, spatiotemporal characteristics of tumor hypoxia and the changes to the tumor microenvironment can be analyzed.

CRITICAL ISSUES

In this review, we focus on the biological and clinical consequences of chronic and cycling hypoxia on radiation treatment. We also discuss the advanced non-invasive imaging techniques that have been developed to detect and monitor tumor hypoxia in preclinical and clinical studies.

FUTURE DIRECTIONS

A better understanding of the mechanisms of tumor hypoxia with non-invasive imaging will provide a basis for improved radiation therapeutic practices.

Bringing the heavy: carbon ion therapy in the radiobiological and clinical context

Radiotherapy for the treatment of cancer is undergoing an evolution, shifting to the use of heavier ion species. For a plethora of malignancies, current radiotherapy using photons or protons yields marginal benefits in local control and survival. One hypothesis is that these malignancies have acquired, or are inherently radioresistant to low LET radiation. In the last decade, carbon ion radiotherapy facilities have slowly been constructed in Europe and Asia, demonstrating favorable results for many of the malignancies that do poorly with conventional radiotherapy. However, from a radiobiological perspective, much of how this modality works in overcoming radioresistance, and extending local control and survival are not yet fully understood. In this review, we will explain from a radiobiological perspective how carbon ion radiotherapy can overcome the classical and recently postulated contributors of radioresistance (α/β ratio, hypoxia, cell proliferation, the tumor microenvironment and metabolism, and cancer stem cells). Furthermore, we will make recommendations on the important factors to consider, such as anatomical location, in the future design and implementation of clinical trials. With the existing data available we believe that the expansion of carbon ion facilities into the United States is warranted.

The role of factor inhibiting HIF (FIH-1) in inhibiting HIF-1 transcriptional activity in glioblastoma multiforme

Glioblastoma multiforme (GBM) accounts for about 38% of primary brain tumors in the United States. GBM is characterized by extensive angiogenesis induced by vascular growth factors and cytokines. The transcription of these growth factors and cytokines is regulated by the Hypoxia-Inducible-Factor-1(HIF-1), which is a key regulator mediating the cellular response to hypoxia. It is known that Factor Inhibiting HIF-1, or FIH-1, is also involved in the cellular response to hypoxia and has the capability to physically interact with HIF-1 and block its transcriptional activity under normoxic conditions. Delineation of the regulatory role of FIH-1 will help us to better understand the molecular mechanism responsible for tumor growth and progression and may lead to the design of new therapies targeting cellular pathways in response to hypoxia. Previous studies have shown that the chromosomal region of 10q24 containing the FIH-1 gene is often deleted in GBM, suggesting a role for the FIH-1 in GBM tumorigenesis and progression. In the current study, we found that FIH-1 is able to inhibit HIF-mediated transcription of GLUT1 and VEGF-A, even under hypoxic conditions in human glioblastoma cells. FIH-1 has been found to be more potent in inhibiting HIF function than PTEN. This observation points to the possibility that deletion of 10q23-24 and loss or decreased expression of FIH-1 gene may lead to a constitutive activation of HIF-1 activity, an alteration of HIF-1 targets such as GLUT-1 and VEGF-A, and may contribute to the survival of cancer cells in hypoxia and the development of hypervascularization observed in GBM. Therefore FIH-1 can be potential therapeutic target for the treatment of GBM patients with poor prognosis.

Usefulness of FMISO-PET for glioma analysis

Glioma is one of the most common brain tumors in adults. Its diagnosis and management have been determined by histological classifications. It is difficult to establish new paradigms because the pathology has matured and a great deal of knowledge has accumulated. On the other hand, we understand that there are limitations to this gold-standard because of the heterogeneity of glioma. Thus, it is necessary to find new criteria independent of conventional morphological diagnosis. Molecular imaging such as positron emission tomography (PET) is one of the most promising approaches to this challenge. PET provides live information of metabolism through the behavior of single molecules. The advantage of PET is that its noninvasive analysis does not require tissue sample, therefore examination can be performed repeatedly. This is very useful for capturing changes in the biological nature of tumor without biopsy. In the present clinical practice for glioma, ¹⁸F-fluorodeoxyglucose (FDG) PET is the most common tracer for predicting prognosis and differentiating other malignant brain tumors. Amino acid tracers such as ¹¹C-methionine (MET) are the most useful for detecting distribution of glioma, including low-grade. Tracers to image hypoxia are under investigation for potential clinical use, and recently, ¹⁸F-fluoromisonidazole (FMISO) has been suggested as an effective tracer to distinguish glioblastoma multiforme from others.

Mechanisms of blood flow and hypoxia production in rat 9L-epigastric tumors

Classical descriptions of tumor physiology suggest two origins for tumor hypoxia; steady-state (diffusion-limited) hypoxia and cycling (perfusion-modulated) hypoxia. Both origins, primarily studied and characterized in murine models, predict relatively small, isolated foci or thin shells of hypoxic tissue interspersed with contrasting oxic tissue. These foci or shells would not be expected to scale with overall tumor size since the oxygen diffusion distance (determined by oxygen permeability and tissue oxygen consumption rate) is not known to vary dramatically from tumor to tumor. We have identified much larger (macroscopic) regions of hypoxia in rat gliosarcoma tumors and in larger human tumors (notably sarcomas and high-grade glial tumors), as indicated by biochemical binding of the hypoxia marker, EF5. Thus, we considered an alternative cause of tumor hypoxia related to a phenomenon first observed in window-chamber tumor models: namely longitudinal arteriole gradients. Although longitudinal arteriole gradients, as originally described, are also microscopic in nature, it is possible for them to scale with tumor size if tumor blood flow is organized in an appropriate manner. In this organization, inflowing blood would arise from relatively well-oxygenated sources and would branch and then coalesce to poorly-oxygenated outflowing blood over distances much larger than the length of conventional arterioles (multi-millimeter scale). This novel concept differs from the common characterization of tumor blood flow as disorganized and/or chaotic. The organization of blood flow to produce extended longitudinal gradients and macroscopic regional hypoxia has many important implications for the imaging, therapy and biological properties of tumors. Herein, we report the first experimental evidence for such blood flow, using rat 9L gliosarcoma tumors grown on the epigastric artery/vein pair.

Differential expression of HIF-1 in glioblastoma multiforme and anaplastic astrocytoma

Hypoxia is an important factor mediating tumor progression and therapeutic resistance, in part through proteome changes mediated by the transcription factor hypoxia-inducible factor (HIF)-1. Since glioblastoma multiforme is the epitome of a highly aggressive tumor entity, while lower-grade astrocytomas often show a prolonged clinical course, a profound difference in the extent of hypoxic tissue areas and corresponding magnitude of HIF-1 activity may exist between these entities. In this study, to address this question, serial sections of 11 glioblastomas and 10 anaplastic astrocytomas were immunostained for HIF-1α, glucose transporter (GLUT)-1, carbonic anhydrase (CA) IX (i.e., hypoxia-related markers), Ki67 (proliferation), phosphorylated ribosomal protein S6 [p-rpS6; mammalian target of rapamycin (mTOR) activity] and CD34 (microvascular endothelium). Digital scans of whole tumor sections were registered to achieve geometric correspondence for subsequent morphometric operations. HIF-1α-, GLUT-1- and CA IX-positive staining was found in all 11 glioblastomas, showing a preferential expression in tissue areas adjacent to necroses. A considerable spatial overlap between GLUT-1 and CA IX, and a colocalization of these proteins with areas of enlarged mean diffusion distances were observed. Conversely, 8 of the 10 anaplastic astrocytomas were completely negative for hypoxia-related markers. The glioblastomas also showed significantly greater heterogeneity of intercapillary distances, larger diffusion-limited tissue fractions, significantly higher mTOR activity and a trend for higher proliferation rates. Microregionally, mTOR and proliferation showed a significant spatial overlap with areas of shorter mean diffusion distances. In conclusion, diffusion-limited hypoxia, leading to the expression of hypoxia-related markers is a pivotal element of the glioblastoma phenotype and may be driven by dysregulated growth and proliferation in normoxic subregions.

Electron paramagnetic resonance imaging of tumor pO₂

Electron paramagnetic resonance imaging (EPRI) can be used to noninvasively and quantitatively obtain three-dimensional maps of tumor pO₂. The paramagnetic tracer triarylmethyl (TAM), a substituted trityl radical moiety, is not toxic to animals and provides narrow isotropic spectra, which is ideal for in vivo EPR imaging experiments. From the oxygen-induced spectral broadening of TAM, pO₂ maps can be derived using EPRI. The instrumentation consists of an EPRI spectrometer and 7T magnetic resonance imaging (MRI) system both operating at a common radiofrequency of 300 MHz. Anatomic images obtained by MRI can be overlaid with pO₂ maps obtained from EPRI. With imaging times of less than 3 min, it was possible to monitor the dynamics of oxygen changes in tumor and distinguish chronically hypoxic regions from acutely hypoxic regions. In this article, the principles of pO₂ imaging with EPR and some relevant examples of tumor imaging are reviewed.

Review of positron emission tomography tracers for imaging of tumor hypoxia

Hypoxia plays a critical role in tumor development and aggressiveness and is an important prognostic factor for resistance to antineoplastic treatments; therefore, it is required to measure the hypoxic level of tumor for a favorable outcome. The pretherapy information on the oxygenation status of a tumor microenvironment should also have implications for treatment selection. A diffuse distribution of hypoxia in a tumor might suggest a benefit from a systemic approach, such as a hypoxic cell cytotoxin, tirapazamine, or antigrowth factor drugs to combat the limitations of hypoxia. Alternatively, a more focal hypoxia might benefit from a local/regional approach, such as intensity-modulated radiation therapy-based radiation dose escalation to the hypoxic subvolume. This review anticipates that (18)F-FMISO ((18)F-fluoromisonodazole) and (64)Cu-ATSM-positron emission tomography (PET) will prove useful for selecting individual patients for the most appropriate treatment. The advent of new radiotracers has allowed noninvasive assessment of hypoxia, with the most extensively investigated and validated PET radiotracer for hypoxia to date being (18)F-FMISO. This article discusses the relevance and biology of hypoxia in cells and organ systems and reviews the laboratory and clinical applications of (18)F-FMISO and other agents in oncology.

Hypoxia and fetal heart development

Fetal hearts show a remarkable ability to develop under hypoxic conditions. The metabolic flexibility of fetal hearts allows sustained development under low oxygen conditions. In fact, hypoxia is critical for proper myocardial formation. Particularly, hypoxia inducible factor 1 (HIF-1) and vascular endothelial growth factor play central roles in hypoxia-dependent signaling in fetal heart formation, impacting embryonic outflow track remodeling and coronary vessel growth. Although HIF is not the only gene involved in adaptation to hypoxia, its role places it as a central figure in orchestrating events needed for adaptation to hypoxic stress. Although "normal" hypoxia (lower oxygen tension in the fetus as compared with the adult) is essential in heart formation, further abnormal hypoxia in utero adversely affects cardiogenesis. Prenatal hypoxia alters myocardial structure and causes a decline in cardiac performance. Not only are the effects of hypoxia apparent during the perinatal period, but prolonged hypoxia in utero also causes fetal programming of abnormality in the heart's development. The altered expression pattern of cardioprotective genes such as protein kinase c epsilon, heat shock protein 70, and endothelial nitric oxide synthase, likely predispose the developing heart to increased vulnerability to ischemia and reperfusion injury later in life. The events underlying the long-term changes in gene expression are not clear, but likely involve variation in epigenetic regulation.

Glioblastoma, cancer stem cells and hypoxia

Glioblastoma (GBM) prognosis remains dismal, with most patients succumbing to disease within 1 or 2 years of diagnosis. Recent studies have suggested that many solid tumors, including GBM, are maintained by a subset of cells termed cancer stem cells (CSCs). It has been shown that these cells are inherently radio- and chemotherapy resistant, and may be maintained in vivo in a niche characterized by reduced oxygen tension (hypoxia). This review examines the recently described effects of hypoxia on CSC in GBM, and the potential promise in targeting the hypoxic pathway therapeutically.

Hypoxia increases the expression of stem-cell markers and promotes clonogenicity in glioblastoma neurospheres

Hypoxia promotes the expansion of non-neoplastic stem and precursor cell populations in the normal brain, and is common in malignant brain tumors. We examined the effects of hypoxia on stem-like cells in glioblastoma (GBM). When GBM-derived neurosphere cultures are grown in 1% oxygen, hypoxia-inducible factor 1alpha (HIF1alpha) protein levels increase dramatically, and mRNA encoding other hypoxic response genes, such as those encoding hypoxia-inducible gene-2, lysyl oxidase, and vascular endothelial growth factor, are induced over 10-fold. Hypoxia increases the stem-like side population over fivefold, and the percentage of cells expressing CD133 threefold or more. Notch pathway ligands and targets are also induced. The rise in the stem-like fraction in GBM following hypoxia is paralleled by a twofold increase in clonogenicity. We believe HIF1alpha plays a causal role in these changes, as when oxygen-stable HIF1alpha is expressed in normoxic glioma cells CD133 is induced. We used digoxin, which has been shown to lower HIF protein levels in vitro and in vivo, to inhibit the hypoxic response. Digoxin suppressed HIF1alpha protein expression, HIF1alpha downstream targets, and slowed tumor growth in vivo. In addition, pretreatment with digoxin reduced GBM flank xenograft engraftment of hypoxic GBM cells, and daily intraperitoneal injections of digoxin were able to significantly inhibit the growth of established subcutaneous glioblastoma xenografts, and suppressed expression of vascular endothelial growth factor.

Biodistribution and dosimetry of (18)F-EF5 in cancer patients with preliminary comparison of (18)F-EF5 uptake versus EF5 binding in human glioblastoma

PURPOSE

The primary purpose of this study was to assess the biodistribution and radiation dose resulting from administration of (18)F-EF5, a lipophilic 2-nitroimidazole hypoxia marker in ten cancer patients. For three of these patients (with glioblastoma) unlabeled EF5 was additionally administered to allow the comparative assessment of (18)F-EF5 tumor uptake with EF5 binding, the latter measured in tumor biopsies by fluorescent anti-EF5 monoclonal antibodies.

METHODS

(18)F-EF5 was synthesized by electrophilic addition of (18)F(2) gas, made by deuteron bombardment of a neon/fluorine mixture in a high-pressure gas target, to an allyl precursor in trifluoroacetic acid at 0° then purified and administered by intravenous bolus. Three whole-body images were collected for each of ten patients using an Allegro (Philips) scanner. Gamma counts were determined in blood, drawn during each image, and urine, pooled as a single sample. PET images were analyzed to determine radiotracer uptake in several tissues and the resulting radiation dose calculated using OLINDA software and standard phantom. For three patients, 21 mg/kg unlabeled EF5 was administered after the PET scans, and tissue samples obtained the next day at surgery to determine EF5 binding using immunohistochemistry techniques (IHC).

RESULTS

EF5 distributes evenly throughout soft tissue within minutes of injection. Its concentration in blood over the typical time frame of the study (∼3.5 h) was nearly constant, consistent with a previously determined EF5 plasma half-life of ∼13 h. Elimination was primarily via urine and bile. Radiation exposure from labeled EF5 is similar to other (18)F-labeled imaging agents (e.g., FDG and FMISO). In a de novo glioblastoma multiforme patient, focal uptake of (18)F-EF5 was confirmed by IHC.

CONCLUSION

These results confirm predictions of biodistribution and safety based on EF5's characteristics (high biological stability, high lipophilicity). EF5 is a novel hypoxia marker with unique pharmacological characteristics allowing both noninvasive and invasive measurements.

The radiation response of cells from 9L gliosarcoma tumours is correlated with [F18]-EF5 uptake

PURPOSE

Tumour hypoxia affects cancer biology and therapy-resistance in both animals and humans. The purpose of this study was to determine whether EF5 ([2-(2-nitro-1-H-imidazol-1-yl)-N-(2,2,3,3,3-pentafluoropropyl)-acetamide]) binding and/or radioactive drug uptake correlated with single-dose radiation response in 9L gliosarcoma tumours.

MATERIALS AND METHODS

Twenty-two 9L tumours were grown in male Fischer rats. Rats were administered low specific activity (18)F-EF5 and their tumours irradiated and assessed for cell survival and hypoxia. Hypoxia assays included EF5 binding measured by antibodies against bound-drug adducts and gamma counts of (18)F-EF5 tumour uptake compared with uptake by normal muscle and blood. These assays were compared with cellular radiation response (in vivo to in vitro assay). In six cases, uptake of tumour versus muscle was also assayed using images from a PET (positron emission tomography) camera (PENN G-PET).

RESULTS

The intertumoural variation in radiation response of 9L tumour-cells was significantly correlated with uptake of (18)F-labelled EF5 (i.e., including both bound and non-bound drug) using either tumour to muscle or tumour to blood gamma count ratios. In the tumours where imaging was performed, there was a significant correlation between the image analysis and gamma count analysis. Intertumoural variation in cellular radiation response of the same 22 tumours was also correlated with mean flow cytometry signal due to EF5 binding.

CONCLUSION

To our knowledge, this is the first animal model/drug combination demonstrating a correlation of radioresponse for tumour-cells from individual tumours with drug metabolism using either immunohistochemical or non-invasive techniques.

Fusion of images in multistep analysis of neovascularisation

PURPOSE

The aim of our study was to develop a method for the fusion of images received after repeated staining of the same sample taking into account spatial differences between the images.

MATERIAL AND METHODS

A method of objective fusion performance was investigated on the images receiving during multistep staining of the xenograft tumour cross-sections.

RESULTS

It was shown that several images receiving from different steps of staining procedures may be successfully fused by fluorescent marking of slide position with Trout red blood cells before analysis.

CONCLUSIONS

Proposed technique provides an accurate rigid fusion of light and fluorescent images receiving during multistep image analysis under microscope and may be applied for study of neovascularisation.

Initial results of hypoxia imaging using 1-alpha-D: -(5-deoxy-5-[18F]-fluoroarabinofuranosyl)-2-nitroimidazole ( 18F-FAZA)

PURPOSE

Tumour hypoxia is thought to play a significant role in the outcome of solid tumour therapy. Positron emission tomography (PET) is the best-validated noninvasive technique able to demonstrate the presence of hypoxia in vivo. The locally developed PET tracer for imaging hypoxia, 1-alpha-D: -(5-deoxy-5-[(18)F]-fluoroarabinofuranosyl)-2-nitroimidazole ((18)F-FAZA), has been shown to accumulate in experimental models of tumour hypoxia and to clear rapidly from the circulation and nonhypoxic tissues. The safety and general biodistribution patterns of this radiopharmaceutical in patients with squamous cell carcinoma of the head and neck (HNSCC), small-cell lung cancer (SCLC) or non-small-cell lung cancer (NSCLC), malignant lymphoma, and high-grade gliomas, were demonstrated in this study.

METHODS

Patients with known primary or suspected metastatic HNSCC, SCLC or NSCLC, malignant lymphoma or high-grade gliomas were dosed with 5.2 MBq/kg of (18)F-FAZA, then scanned 2-3 h after injection using a PET or PET/CT scanner. Images were interpreted by three experienced nuclear medicine physicians. The location and relative uptake scores (graded 0 to 4) of normal and abnormal (18)F-FAZA biodistribution patterns, the calculated tumour-to-background (T/B) ratio, and the maximum standardized uptake value were recorded.

RESULTS

Included in the study were 50 patients (32 men, 18 women). All seven patients with high-grade gliomas showed very high uptake of (18)F-FAZA in the primary tumour. In six out of nine patients with HNSCC, clear uptake of (18)F-FAZA was observed in the primary tumour and/or the lymph nodes in the neck. Of the 21 lymphoma patients (15 with non-Hodgkin's lymphoma and 6 with Hodgkin's disease), 3 demonstrated moderate lymphoma-related uptake. Of the 13 lung cancer patients (12 NSCLC, 1 SCLC), 7 had increased (18)F-FAZA uptake in the primary lung tumour. No side effects of the administration of (18)F-FAZA were observed.

CONCLUSION

This study suggests that (18)F-FAZA may be a very useful radiopharmaceutical to image hypoxia in the tumour types selected. Especially the high uptake by gliomas was encouraging. Given the good imaging properties, including acceptable T/B ratios in the tumour categories studied, (18)F-FAZA could be considered as a very promising agent for assessing the hypoxic fraction of these tumour types.