PET as a potential tool for imaging molecular mechanisms of oncology in man

Assessment of Brain Functional Activity Using a Miniaturized Head-Mounted Scanning Photoacoustic Imaging System in Awake and Freely Moving Rats

Understanding the relationship between brain function and natural behavior remains a significant challenge in neuroscience because there are very few convincing imaging/recording tools available for the evaluation of awake and freely moving animals. Here, we employed a miniaturized head-mounted scanning photoacoustic imaging (hmPAI) system to image real-time cortical dynamics. A compact photoacoustic (PA) probe based on four in-house optical fiber pads and a single custom-made 48-MHz focused ultrasound transducer was designed to enable focused dark-field PA imaging, and miniature linear motors were included to enable two-dimensional (2D) scanning. The total dimensions and weight of the proposed hmPAI system are only approximately 50 × 64 × 48 mm and 58.7 g (excluding cables). Our ex vivo phantom experimental tests revealed that a spatial resolution of approximately 0.225 mm could be achieved at a depth of 9 mm. Our in vivo results further revealed that the diameters of cortical vessels draining into the superior sagittal sinus (SSS) could be clearly imaged and continuously observed in both anesthetized rats and awake, freely moving rats. Statistical analysis showed that the full width at half maximum (FWHM) of the PA A-line signals (relative to the blood vessel diameter) was significantly increased in the selected SSS-drained cortical vessels of awake rats (0.58 ± 0.17 mm) compared with those of anesthetized rats (0.31 ± 0.09 mm) (p < 0.01, paired t-test). In addition, the number of pixels in PA B-scan images (relative to the cerebral blood volume (CBV)) was also significantly increased in the selected SSS-drained blood vessels of awake rats (107.66 ± 23.02 pixels) compared with those of anesthetized rats (81.99 ± 21.52 pixels) (p < 0.01, paired t-test). This outcome may result from a more active brain in awake rats than in anesthetized rats, which caused cerebral blood vessels to transport more blood to meet the increased nutrient demand of the tissue, resulting in an obvious increase in blood vessel volume. This hmPAI system was further validated for utility in the brains of awake and freely moving rats, showing that their natural behavior was unimpaired during vascular imaging, thereby providing novel opportunities for studies of behavior, cognition, and preclinical models of brain diseases.

Elucidating the complementarity of resting-state networks derived from dynamic [18F]FDG and hemodynamic fluctuations using simultaneous small-animal PET/MRI

Functional connectivity (FC) and resting-state network (RSN) analyses using functional magnetic resonance imaging (fMRI) have evolved into a growing field of research and have provided useful biomarkers for the assessment of brain function in neurological disorders. However, the underlying mechanisms of the blood oxygen level-dependant (BOLD) signal are not fully resolved due to its inherent complexity. In contrast, [¹⁸F]fluorodeoxyglucose positron emission tomography ([¹⁸F]FDG-PET) has been shown to provide a more direct measure of local synaptic activity and may have additional value for the readout and interpretation of brain connectivity. We performed an RSN analysis from simultaneously acquired PET/fMRI data on a single-subject level to directly compare fMRI and [¹⁸F]FDG-PET-derived networks during the resting state. Simultaneous [¹⁸F]FDG-PET/fMRI scans were performed in 30 rats. Pairwise correlation analysis, as well as independent component analysis (ICA), were used to compare the readouts of both methods. We identified three RSNs with a high degree of similarity between PET and fMRI-derived readouts: the default-mode-like network (DMN), the basal ganglia network and the cerebellar-midbrain network. Overall, [¹⁸F]FDG connectivity indicated increased integration between different, often distant, brain areas compared to the results indicated by the more segregated fMRI-derived FC. Additionally, several networks exclusive to either modality were observed using ICA. These networks included mainly bilateral cortical networks of a limited spatial extent for fMRI and more spatially widespread networks for [¹⁸F]FDG-PET, often involving several subcortical areas.

This is the first study using simultaneous PET/fMRI to report RSNs subject-wise from dynamic [¹⁸F]FDG tracer delivery and BOLD fluctuations with both independent component analysis (ICA) and pairwise correlation analysis in small animals. Our findings support previous studies, which show a close link between local synaptic glucose consumption and BOLD-fMRI-derived FC. However, several brain regions were exclusively attributed to either [¹⁸F]FDG or BOLD-derived networks underlining the complementarity of this hybrid imaging approach, which may contribute to the understanding of brain functional organization and could be of interest for future clinical applications.

Probing carbohydrate metabolism using hyperpolarized 13 C-labeled molecules

Glycolysis is a fundamental metabolic process in all organisms. Anomalies in glucose metabolism are linked to various pathological conditions. In particular, elevated aerobic glycolysis is a characteristic feature of rapidly growing cells. Glycolysis and the closely related pentose phosphate pathway can be monitored in real time by hyperpolarized 13 C-labeled metabolic substrates such as 13 C-enriched, deuterated D-glucose derivatives, [2-13 C]-D-fructose, [2-13 C] dihydroxyacetone, [1-13 C]-D-glycerate, [1-13 C]-D-glucono-δ-lactone and [1-13 C] pyruvate in healthy and diseased tissues. Elevated glycolysis in tumors (the Warburg effect) was also successfully imaged using hyperpolarized [U-13 C6 , U-2 H7 ]-D-glucose, while the size of the preexisting lactate pool can be measured by 13 C MRS and/or MRI with hyperpolarized [1-13 C]pyruvate. This review summarizes the application of various hyperpolarized 13 C-labeled metabolites to the real-time monitoring of glycolysis and related metabolic processes in normal and diseased tissues.

Multimodality reporter gene imaging: Construction strategies and application

Molecular imaging has played an important role in the noninvasive exploration of multiple biological processes. Reporter gene imaging is a key part of molecular imaging. By combining with a reporter probe, a reporter protein can induce the accumulation of specific signals that are detectable by an imaging device to provide indirect information of reporter gene expression in living subjects. There are many types of reporter genes and each corresponding imaging technique has its own advantages and drawbacks. Fused reporter genes or single reporter genes with products detectable by multiple imaging modalities can compensate for the disadvantages and potentiate the advantages of each modality. Reporter gene multimodality imaging could be applied to trace implanted cells, monitor gene therapy, assess endogenous molecular events, screen drugs, etc. Although several types of multimodality imaging apparatus and multimodality reporter genes are available, more sophisticated detectors and multimodality reporter gene systems are needed.

Applications of positron emission tomography in neuro-oncology: a clinical approach

The field of neuro-oncology is concerned with some of the most challenging and difficult to treat conditions in medicine. Despite modern therapies patients diagnosed with primary brain tumours often have a poor prognosis. Imaging can play an important role in evaluating the disease status of such patients. In addition to the structural information derived from MRI and CT scans, positron emission tomography (PET) provides important quantitative metabolic assessment of brain tumours. This review describes the use of PET with radiolabelled glucose and amino acid analogues to aid in the diagnosis of tumours, differentiate between recurrent tumour and radiation necrosis and guide biopsy or treatment. [(18)F]Fluorodeoxyglucose (FDG) is the tracer that has been used most widely because it has a 2 h half life and can be transported to imaging centres remote from the cyclotron and radiochemistry facilities which synthesise the tracers. The high uptake of FDG in normal grey matter however limits its use in some low grade tumours which may not be visualised. [(11)C] methionine (MET) is an amino acid tracer with low accumulation in normal brain which can detect low grade gliomas, but its short 20 min half life has limited its use to imaging sites with their own cyclotron. The emergence of new fluorinated amino acid tracers like [(18)F]Fluoroethyl-l-tyrosine (FET) will likely increase the availability and utility of PET for patients with primary brain tumours. PET can, further, characterise brain tumours by investigating other metabolic processes such as DNA synthesis or thymidine kinase activity, phospholipid membrane biosynthesis, hypoxia, receptor binding and oxygen metabolism and blood flow, which will be important in the future assessment of targeted therapy.

Improved synthesis of 17β-hydroxy-16α-iodo-wortmannin, 17β-hydroxy-16α-iodoPX866, and the [(131)I] analogue as useful PET tracers for PI3-kinase

An improved method for the synthesis of 17β-hydroxy-16α-iodo-wortmannin along with the first synthesis of 17β-hydroxy-16α-iodoPX866 and [(131)I] radiolabeled 17β-hydroxy-16α-[(131)I]iodo-wortmannin, as potential PET tracers for PI3K was also described. The differences between wortmannin and its iodo analogue were compared by covalently docking each structure to L833 in PI3K.

Kinetic quantification of protein polymer nanoparticles using non-invasive imaging

Protein polymers are repetitive amino acid sequences that can assemble monodisperse nanoparticles with potential applications as cancer nanomedicines. Of the currently available molecular imaging methods, positron emission tomography (PET) is the most sensitive and quantitative; therefore, this work explores microPET imaging to track protein polymer nanoparticles over several days. To achieve reliable imaging, the polypeptides were modified by site-specific conjugation using a heterobifunctional sarcophagine chelator, AmBaSar, which was subsequently complexed with (64)Cu. AmBaSar/(64)Cu was selected because it can label particles in vivo over periods of days, which is consistent with the timescales required to follow long-circulating nanotherapeutics. Using an orthotopic model of breast cancer, we observed four elastin-like polypeptides (ELPs)-based protein polymers of varying molecular weight, amino acid sequence, and nanostructure. To analyze this data, we developed a six-compartment image-driven pharmacokinetic model capable of describing their distribution within individual subjects. Surprisingly, the assembly of an ELP block copolymer (78 kD) into nanoparticles (R(h) = 37.5 nm) minimally influences pharmacokinetics or tumor accumulation compared to a free ELP of similar length (74 kD). Instead, ELP molecular weight is the most important factor controlling the fate of these polymers, whereby long ELPs (74 kD) have a heart activity half-life of 8.7 hours and short ELPs (37 kD) have a half-life of 2.1 hours. These results suggest that ELP-based protein polymers may be a viable platform for the development of multifunctional therapeutic nanoparticles that can be imaged using clinical PET scanners.

Engineered nanoparticles for biomolecular imaging

In recent years, the production of nanoparticles (NPs) and exploration of their unusual properties have attracted the attention of physicists, chemists, biologists and engineers. Interest in NPs arises from the fact that the mechanical, chemical, electrical, optical, magnetic, electro-optical and magneto-optical properties of these particles are different from their bulk properties and depend on the particle size. There are numerous areas where nanoparticulate systems are of scientific and technological interest, particularly in biomedicine where the emergence of NPs with specific properties (e.g. magnetic and fluorescence) for contrast agents can lead to advancing the understanding of biological processes at the biomolecular level. This review will cover a full description of the physics of various imaging methods, including MRI, optical techniques, X-rays and CT. In addition, the effect of NPs on the improvement of the mentioned non-invasive imaging methods will be discussed together with their advantages and disadvantages. A detailed discussion will also be provided on the recent advances in imaging agents, such as fluorescent dye-doped silica NPs, quantum dots, gold- and engineered polymeric-NPs, superparamagnetic iron oxide NPs (SPIONs), and multimodal NPs (i.e. nanomaterials that are active in both MRI and optical methods), which are employed to overcome many of the limitations of conventional contrast agents (e.g. gadolinium).

Novel membrane-permeable contrast agent for brain tumor detection by MRI

One of the key challenges hindering the clinical intervention against brain cancer is defined by the inability to detect brain tumors at an early enough stage to permit effective therapy. Furthermore, the rapid growth and severe lethality of this form of cancer predicate the vital importance of monitoring the development of the pathology and its outcome after therapeutic intervention. With this in mind, we designed a novel membrane-permeant contrast agent, MN-MPAP-Cy5.5, which consists of a superparamagnetic iron oxide core, for MRI conjugated to myristoylated polyarginine peptides, as a membrane translocation module and labeled with the near-infrared dye Cy5.5 for correlative microscopy. This probe showed a remarkable uptake by U-87 human glioma cells in vitro and localized and delineated stereotactically injected tumor in vivo by MRI. Our findings suggest that the agent mediates its effects by translocation of the magnetic nanoparticles label across the leaky tumor vasculature, followed by enhanced accumulation in tumor cells. The noninvasive detection of brain tumors when they are still small represents a formidable challenge from an imaging standpoint. Our study describes an improved strategy to detect brain lesions by utilizing a contrast agent with membrane translocation properties.

Molecular imaging (PET) of brain tumors

Despite the recognized limitations of (18)Fluorodeoxyglucose positron emission tomography (FDG-PET) in brain tumor imaging due to the high background of normal gray matter, this imaging modality provides critical information for the management of patients with cerebral neoplasms with regard to the following aspects: (1) providing a global picture of the tumor and thus guiding the appropriate site for stereotactic biopsy, and thereby enhancing its accuracy and reducing the number of biopsy samples; and (2) prediction of biologic behavior and aggressiveness of the tumor, thereby aiding in prognosis. Another area, which has been investigated extensively, includes differentiating recurrent tumor from treatment-related changes (eg, radiation necrosis and postsurgical changes). Furthermore, FDG-PET has demonstrated its usefulness in differentiating lymphoma from toxoplasmosis in patients with acquired immune deficiency syndrome with great accuracy, and is used as the investigation of choice in this setting. Image coregistration with magnetic resonance imaging and delayed FDG-PET imaging are 2 maneuvers that substantially improve the accuracy of interpretation, and hence should be routinely employed in clinical settings. In recent years an increasing number of brain tumor PET studies has used other tracers (like labeled methionine, tyrosine, thymidine, choline, fluoromisonidazole, EF5, and so forth), of which positron-labeled amino acid analogues, nucleotide analogues, and the hypoxia imaging tracers are of special interest. The major advantage of these radiotracers over FDG is the markedly lower background activity in normal brain tissue, which allows detection of small lesions and low-grade tumors. The promise of the amino acid PET tracers has been emphasized due to their higher sensitivity in imaging recurrent tumors (particularly the low-grade ones) and better accuracy for differentiating between recurrent tumors and treatment-related changes compared with FDG. The newer PET tracers have also shown great potential to image important aspects of tumor biology and thereby demonstrate ability to forecast prognosis. The value of hypoxia imaging tracers (such as fluoromisonidazole or more recently EF5) is substantial in radiotherapy planning and predicting treatment response. In addition, they may play an important role in the future in directing and monitoring targeted hypoxic therapy for tumors with hypoxia. Development of optimal image segmentation strategy with novel PET tracers and multimodality imaging is an approach that deserves mention in the era of intensity modulated radiotherapy, and which is likely to have important clinical and research applications in radiotherapy planning in patients with brain tumor.

Evaluating the potential of 188Re-SOCTA-trastuzumab as a new radioimmunoagent for breast cancer treatment

INTRODUCTION

Radioimmunotherapy, which utilizes monoclonal antibodies and therapeutic radioisotopes against antigen-expressing tumor tissues, is an attractive therapeutic approach for cancer therapy. Trastuzumab (Herceptin) is a humanized anti-HER-2/neu monoclonal antibody for breast cancer treatment. In this paper, we introduce a new radioimmunoagent, (188)Re-trastuzumab, via a bifunctional ligand, succinimidyl 3,6-diaza-5-oxo-3-[2-((triphenylmethyl)thio)ethyl]-8-[(triphenylmethyl)thio]octanoate (SOCTA), and evaluate its potential to be a therapeutic radiopharmaceutical for breast cancer treatment.

METHODS

Equimolar amounts of SOCTA and trastuzumab were selected to react, and the conjugation ratio of SOCTA-trastuzumab was evaluated by the MALDI-TOF method. The immunoreactivity of SOCTA-trastuzumab was compared with nonconjugated trastuzumab in HER-2/neu overexpressing human breast cancer cell BT-474. Biodistribution experiment and microSPECT/CT images of (188)Re-SOCTA-trastuzumab being administered intravenously to SCID mice bearing xenografted BT-474 breast cancer were investigated to evaluate the tumor-targeting capability.

RESULTS

The covalent attachment of SOCTA to trastuzumab (at 1:1 molar ratio) resulted in the averaged conjugation ratio of 0.27+/-0.06 (n=3). The complex could easily be labeled with (188)Re and achieve 95% radiochemical purity (RCP) after 1 h of reaction at room temperature. The in vitro stability study also revealed that the RCP of (188)Re-SOCTA-trastuzumab was at a value of more than 85% after 48 h of incubation with human serum. The immunoreactivity evaluation showed that SOCTA-trastuzumab and nonconjugated trastuzumab had similar binding capacity (B(max)) to HER-2/neu receptor in BT-474 cells. The animal experiments showed that (188)Re-SOCTA-trastuzumab accumulated more intensively in the tumor site as compared to normal tissue.

CONCLUSION

We suggest that (188)Re-SOCTA-trastuzumab could be a potential candidate for radioimmunotherapy.

The status of gene therapy for brain tumors

The advent of gene therapy in the early 1990's raised expectations for brain tumor therapies; however, whereas clinical trials in patients with malignant gliomas provided evidence of safety, therapeutic benefit was not convincing. These early forays resembled the historical introductions of other therapies that seemed promising, only to fail in human trials. Nevertheless, re-study in the laboratory and retesting in iterative laboratory-clinic processes enabled therapies with strong biological rationales to ultimately show evidence of success in humans and become accepted. Examples, such as organ transplantation, monoclonal antibody therapy and antiangiogenic therapy, provide solace that a strategy's initial lack of success in humans provides an opportunity for its further refinement in the laboratory and development of solutions that will translate into patient success stories. The authors herein summarize results from clinical trials of gene therapy for malignant gliomas, and discuss the influence of these results on present thought in preclinical research.

The role of PET scanning in determining pharmacoselective doses in oncology drug development

Molecular imaging is the most sensitive and specific method for measuring in vivo molecular pathways in man. Its use in oncology has developed significantly over the last 5-10 years. Molecules can be labelled with positron emitting isotopes and the emitted radiation is detected using sensitive positron emission tomography (PET) cameras. It is now possible to measure in vivo and normal tissue pharmacokinetics of anti-cancer drugs and investigate their mechanism of action. Radiolabelling of tracers can be used to measure specific pharmacodynamic endpoints and target identification. Increasing evidence shows how these technologies, when added to early drug development, can rapidly reduce the time for entry into man and early identification of mechanisms of action. With the move towards more segmented markets and identification of specific subgroups, PET's use for noninvasive biomarkers will become in- creasingly important. However, much international effort between academia and industry is required with prioritisation of development of this technology.

Chemical mapping of tumor progression by FT-IR imaging: towards molecular histopathology

Fourier-transform infrared (FT-IR) spectro-imaging enables global analysis of samples, with resolution close to the cellular level. Recent studies have shown that FT-IR imaging enables determination of the biodistribution of several molecules of interest (carbohydrates, lipids, proteins) for tissue analysis without pre-analytical modification of the sample such as staining. Molecular structure information is also available from the same analysis, notably for protein secondary structure and fatty acyl chain peroxidation level. Thus, several cancer markers can be identified from FT-IR tissue images, enabling accurate discrimination between healthy and tumor areas. FT-IR imaging applications are now able to provide unique chemical and morphological information about tissue status. With the fast image acquisition provided by modern mid-infrared imaging systems, it is now envisaged to analyze cerebral tumor exereses in delays compatible with neurosurgery. Accordingly, we propose to take FT-IR imaging into consideration for the development of new molecular histopathology tools.

Imaging in neurooncology

Imaging in patients with brain tumors aims toward the determination of the localization, extend, type, and malignancy of the tumor. Imaging is being used for primary diagnosis, planning of treatment including placement of stereotaxic biopsy, resection, radiation, guided application of experimental therapeutics, and delineation of tumor from functionally important neuronal tissue. After treatment, imaging is being used to quantify the treatment response and the extent of residual tumor. At follow-up, imaging helps to determine tumor progression and to differentiate recurrent tumor growth from treatment-induced tissue changes, such as radiation necrosis. A variety of complementary imaging methods are currently being used to obtain all the information necessary to achieve the above mentioned goals. Computed tomography and magnetic resonance imaging (MRI) reveal mostly anatomical information on the tumor, whereas magnetic resonance spectroscopy and positron emission tomography (PET) give important information on the metabolic state and molecular events within the tumor. Functional MRI and functional PET, in combination with electrophysiological methods like transcranial magnetic stimulation, are being used to delineate functionally important neuronal tissue, which has to be preserved from treatment-induced damage, as well as to gather information on tumor-induced brain plasticity. In addition, optical imaging devices have been implemented in the past few years for the development of new therapeutics, especially in experimental glioma models. In summary, imaging in patients with brain tumors plays a central role in the management of the disease and in the development of improved imaging-guided therapies.

Molecular imaging with copper-64

Molecular imaging is expected to change the face of drug discovery and development. The ability to link imaging to biology for guiding therapy should improve the rate at which novel imaging technologies, probes, contrast agents, drugs and drug delivery systems can be transferred into clinical practice. Nuclear medicine imaging, in particular, positron emission tomography (PET) allows the detection and monitoring of a variety of biological and pathophysiological processes, at tracer quantities of the radiolabelled target agents, and at doses free from pharmacological effects. In the field of drug discovery and development, the use of radiotracers for radiolabelling target agents has now become one of the essential tools in identifying, screening and development of new target agents. In this regard, (64)Cu (t(1/2)=12.7 h) has been identified as an emerging PET isotope. Its half-life is sufficiently long for radiolabelling a range of target agents and its ease of production and adaptable chemistry make it an excellent radioisotope for use in molecular imaging. This review describes recent advances, in the routes of (64)Cu production, design and application of bi-functional ligands for use in radiolabelling with (64/67)Cu(2+), and their significance and anticipated impact on the field of molecular imaging and drug development.

Molecular imaging in oncology

Cancer is a genetic disease that manifests in loss of normal cellular homeostatic mechanisms. The biology and therapeutic modulation of neoplasia occurs at the molecular level. An understanding of these molecular processes is therefore required to develop novel prognostic and early biomarkers of response. In addition to clinical applications, increased impetus for the development of such technologies has been catalysed by pharmaceutical companies investing in the development of molecular therapies. The discipline of molecular imaging therefore aims to image these important molecular processes in vivo. Molecular processes, however, operate at short length scales and concentrations typically beyond the resolution of clinical imaging. Solving these issues will be a challenge to imaging research. The successful implementations of molecular imaging in man will only be realised by the close co-operation amongst molecular biologists, chemists and the imaging scientists.

Molecular imaging of gene therapy for cancer

Gene therapy of cancer has been one of the most exciting and elusive areas of scientific and clinical research in the past decade. One of the most critical issues for ensuring success of this therapy is the development of technology for noninvasive monitoring of the location, magnitude and duration of vector-mediated gene expression, as well as the distribution and targeting of vector particles in vivo. In recent years many advances have been made in high-resolution, in vivo imaging methods, including: radionuclide imaging, such as positron emission tomography (PET) and single photon emission tomography (SPECT), magnetic resonance (MR) imaging and spectroscopy, bioluminescence imaging and various fluorescence imaging techniques, including fluorescence-mediated tomography (FMT) and near-infrared fluorescence (NIRF) reflectance imaging. A variety of factors determine the choice of specific imaging system, some of them are the imaging requirements (single or repeated), intended use (animal or human) and spatial requirements (organs versus cellular resolution and depth). This review provides descriptions of modalities applicable to imaging different parameters of vector-mediated gene expression in tumors and stem cell tracking in vivo.

Imaging in gene therapy of patients with glioma

Over 10 years ago, the first successful gene therapy paradigms for experimental brain tumors models have been conducted, and they were thought to revolutionize the treatment of patients with gliomas. Application of gene therapy has been quickly forced into clinical trials, the first patients being enrolled in 1994, with overall results being disappointing. However, single patients seemed to benefit from gene therapy showing long-term treatment response, and most of these patients bearing small glioblastomas. Whereas the gene therapy itself has been performed with high sophistication, limited attention has been paid on technologies, which (i) allow an identification of viable target tissue in heterogenous glioma tissue and which (ii) enable an assessment of successful vector administration and vector-mediated gene expression in vivo. However, these measures are a prerequisite for the development of successful gene therapy in the clinical application. As biological treatment strategies such as gene and cell-based therapies hold promise to selectively correct disease pathogenesis, successful clinical implementation of these treatment strategies rely on the establishment of molecular imaging technology allowing the non-invasive assessment of endogenous and exogenous gene expression in vivo. Imaging endogenous gene expression will allow the characterization and identification of target tissue for gene therapy. Imaging exogenously introduced cells and genes will allow the determination of the 'tissue dose' of transduced cell function and vector-mediated gene expression, which in turn can be correlated to the induced therapeutic effect. Only these combined strategies of non-invasive imaging of gene expression in vivo will enable the establishment of safe and efficient vector administration and gene therapy protocols for clinical application. Here, we review some aspects of imaging in gene therapy trials for glioblastoma, and we present a 'proof-of-principle' 2nd-generation gene therapy protocol integrating molecular imaging technology for the establishment of efficient gene therapy in clinical application.

Molecular Imaging of Gliomas

Gliomas are the most common types of brain tumors. Although sophisticated regimens of conventional therapies are being carried out to treat patients with gliomas, the disease invariably leads to death over months or years. Before new and potentially more effective treatment strategies, such as gene- and cell-based therapies, can be effectively implemented in the clinical application, certain prerequisites have to be established. First of all, the exact localization, extent, and metabolic activity of the glioma must be determined to identify the biologically active target tissue for a biological treatment regimen; this is usually performed by imaging the expression of up-regulated endogenous genes coding for glucose or amino acid transporters and cellular hexokinase and thymidine kinase genes, respectively. Second, neuronal function and functional changes within the surrounding brain tissue have to be assessed in order to save this tissue from therapy-induced damage. Third, pathognomonic genetic changes leading to disease have to be explored on the molecular level to serve as specific targets for patient-tailored therapies. Last, a concerted noninvasive analysis of both endogenous and exogenous gene expression in animal models as well as the clinical setting is desirable to effectively translate new treatment strategies from experimental into clinical application. All of these issues can be addressed by multimodal radionuclide and magnetic resonance imaging techniques and fall into the exciting and fast growing field of molecular and functional imaging. Noninvasive imaging of endogenous gene expression by means of positron emission tomography (PET) may reveal insight into the molecular basis of pathogenesis and metabolic activity of the glioma and the extent of treatment response. When exogenous genes are introduced to serve for a therapeutic function, PET imaging may reveal the assessment of the “location,” “magnitude,” and “duration” of therapeutic gene expression and its relation to the therapeutic effect. Detailed reviews on molecular imaging have been published from the perspective of radionuclide imaging (Gambhir et al., 2000; Blasberg and Tjuvajev, 2002) as well as magnetic resonance and optical imaging (Weissleder, 2002). The present review focuses on molecular imaging of gliomas with special reference on the status and perspectives of imaging of endogenous and exogenously introduced gene expression in order to develop improved diagnostics and more effective treatment strategies of gliomas and, in that, to eventually improve the grim prognosis of this devastating disease.

Molecular and functional imaging technology for the development of efficient treatment strategies for gliomas

Gliomas are the most common types of brain tumors, which invariably lead to death over months or years. Before new and potentially more effective treatment strategies, such as gene therapy, can be effectively introduced into clinical application the following goals must be reached: (1) the determination of localization, extent and metabolic activity of the glioma; (2) the assessment of functional changes within the surrounding brain tissue; (3) the identification of genetic changes on the molecular level leading to disease; and in addition (4) a detailed non-invasive analysis of both endogenous and exogenous gene expression in animal models and in the clinical setting. Non-invasive imaging of endogenous gene expression by means of positron emission tomography (PET) may reveal insight into the molecular basis of pathogenesis and metabolic activity of the glioma and the extent of treatment response. When exogenous genes are introduced to serve for a therapeutic function, PET imaging techniques may reveal the assessment of the location, magnitude and duration of therapeutic gene expression and its relation to the therapeutic effect. Here, we review the main principles of PET imaging and its key roles in neurooncology research.

Special techniques for imaging blood flow to tumors

In this review, the current imaging methods of assessing tumor perfusion are discussed and compared. Because most of the techniques depend on the administration of contrast agents, we first consider some general principles underlying the fate of contrast agents and their behavior inside the unique vascular environment of tumors.