PET for diagnosis and therapy of brain tumors

Main contribution of PET in the management of brain tumors is at the therapeutic level. Specific reasons explain this role of molecular imaging in the therapeutic management of brain tumors, especially gliomas. Gliomas are by nature infiltrating neoplasms and the interface between tumor and normal brain tissue may not be accurately defined on CT and MRI. Also, gliomas are often histologically heterogeneous with anaplastic areas evolving within a low-grade tumor, and the contrast-enhancement on CT or MRI does not represent a good marker for anaplastic tissue detection. Finally, assessment of tumor residue, recurrence or progression may be altered by different signals related to inflammation or adjuvant therapies, even on contrast-enhanced CT and MRI. These limitations of the conventional neuroimaging in delineating tumor and detecting anaplastic tissue lead to potential inaccuracy in lesion targeting at different steps of the management (diagnostic, surgical, and post-therapeutic stages). Molecular information provided by PET has proved helpful to supplement morphological imaging data in this context. ¹⁸F-FDG (FDG) and amino-acid tracers such as ¹¹C-methionine (MET), provides complementary metabolic data that are independent from the anatomical MR information. These tracers help in the definition of glioma extension, in the detection of anaplastic areas and in the postoperative follow-up. Additionally, PET data have an independent prognostic value. To take advantage of PET data in glioma treatment, PET might be integrated in the planning of image-guided biopsies, radiosurgery and resection.     

Molecular-genetic imaging: a nuclear medicine-based perspective

Molecular imaging is a relatively new discipline, which developed over the past decade, initially driven by in situ reporter imaging technology. Noninvasive in vivo molecular-genetic imaging developed more recently and is based on nuclear (positron emission tomography [PET], gamma camera, autoradiography) imaging as well as magnetic resonance (MR) and in vivo optical imaging. Molecular-genetic imaging has its roots in both molecular biology and cell biology, as well as in new imaging technologies. The focus of this presentation will be nuclear-based molecular-genetic imaging, but it will comment on the value and utility of combining different imaging modalities. Nuclear-based molecular imaging can be viewed in terms of three different imaging strategies: (1) "indirect" reporter gene imaging; (2) "direct" imaging of endogenous molecules; or (3) "surrogate" or "bio-marker" imaging. Examples of each imaging strategy will be presented and discussed. The rapid growth of in vivo molecular imaging is due to the established base of in vivo imaging technologies, the established programs in molecular and cell biology, and the convergence of these disciplines. The development of versatile and sensitive assays that do not require tissue samples will be of considerable value for monitoring molecular-genetic and cellular processes in animal models of human disease, as well as for studies in human subjects in the future. Noninvasive imaging of molecular-genetic and cellular processes will complement established ex vivo molecular-biological assays that require tissue sampling, and will provide a spatial as well as a temporal dimension to our understanding of various diseases and disease processes.     

Preclinical TSPO Ligand PET to Visualize Human Glioma Xenotransplants: A Preliminary Study

Current positron emission tomography (PET) imaging biomarkers for detection of infiltrating gliomas are limited. Translocator protein (TSPO) is a novel and promising biomarker for glioma PET imaging. To validate TSPO as a potential target for molecular imaging of glioma, TSPO expression was assayed in a tumor microarray containing 37 high-grade (III, IV) gliomas. TSPO staining was detected in all tumor specimens. Subsequently, PET imaging was performed with an aryloxyanilide-based TSPO ligand, [¹⁸F]PBR06, in primary orthotopic xenograft models of WHO grade III and IV gliomas. Selective uptake of [¹⁸F]PBR06 in engrafted tumor was measured. Furthermore, PET imaging with [¹⁸F]PBR06 demonstrated infiltrative glioma growth that was undetectable by traditional magnetic resonance imaging (MRI). Preliminary PET with [¹⁸F]PBR06 demonstrated a preferential tumor-to-normal background ratio in comparison to 2-deoxy-2-[¹⁸F]fluoro-D-glucose ([¹⁸F]FDG). These results suggest that TSPO PET imaging with such high-affinity radiotracers may represent a novel strategy to characterize distinct molecular features of glioma growth, as well as better define the extent of glioma infiltration for therapeutic purposes.     

Gene therapy in cancer

Gene therapy, recently frequently investigated, is an alternative treatment method that introduces therapeutic genes into a cancer cell or tissue to cause cell death or slow down the growth of the cancer. This treatment has various strategies such as therapeutic gene activation or silencing of unwanted or defective genes; therefore a wide variety of genes and viral or nonviral vectors are being used in studies. Gene therapy strategies in cancer can be classified as inhibition of oncogene activation, activation of tumor suppressor gene, immunotherapy, suicide gene therapy and antiangiogenic gene therapy. In this review, we explain gene therapy, gene therapy strategies in cancer, approved gene medicines for cancer treatment and future of gene therapy in cancer. Today gene therapy has not yet reached the level of replacing conventional therapies. However, with a better understanding of the mechanism of cancer to determine the right treatment and target, in the future gene therapy, used as monotherapy or in combination with another existing treatment options, is likely to be used as a new medical procedure that will make cancer a controllable disease.     

Real time non-invasive imaging of receptor-ligand interactions in vivo

Non-invasive longitudinal detection and evaluation of gene expression in living animals can provide investigators with an understanding of the ontogeny of a gene's biological function(s). Currently, mouse model systems are used to optimize magnetic resonance imaging (MRI), positron emission tomography (PET), single-photon emission computed tomography (SPECT), and optical imaging modalities to detect gene expression and protein function. These molecular imaging strategies are being developed to assess tumor growth and the tumor microenvironment. In addition, pre-labeling of progenitor cells can provide invaluable information about the developmental lineage of stem cells both in organogenesis and tumorigenesis. The feasibility of this approach has been extensively tested by targeting of endogenous tumor cell receptors with labeled ligand (or ligand analog) reporters and targeting enzymes with labeled substrate (or substrate analog). We will primarily discuss MRI, PET, and SPECT imaging of cell surface receptors and the feasibility of non-invasive imaging of gene expression using the tumor microenvironment (e.g., hypoxia) as a conditional regulator of gene expression.     

Switching on the lights for gene therapy

Strategies for non-invasive and quantitative imaging of gene expression in vivo have been developed over the past decade. Non-invasive assessment of the dynamics of gene regulation is of interest for the detection of endogenous disease-specific biological alterations (e.g., signal transduction) and for monitoring the induction and regulation of therapeutic genes (e.g., gene therapy). To demonstrate that non-invasive imaging of regulated expression of any type of gene after in vivo transduction by versatile vectors is feasible, we generated regulatable herpes simplex virus type 1 (HSV-1) amplicon vectors carrying hormone (mifepristone) or antibiotic (tetracycline) regulated promoters driving the proportional co-expression of two marker genes. Regulated gene expression was monitored by fluorescence microscopy in culture and by positron emission tomography (PET) or bioluminescence (BLI) in vivo. The induction levels evaluated in glioma models varied depending on the dose of inductor. With fluorescence microscopy and BLI being the tools for assessing gene expression in culture and animal models, and with PET being the technology for possible application in humans, the generated vectors may serve to non-invasively monitor the dynamics of any gene of interest which is proportionally co-expressed with the respective imaging marker gene in research applications aiming towards translation into clinical application.     

Structure-guided engineering of human thymidine kinase 2 as a positron emission tomography reporter gene for enhanced phosphorylation of non-natural thymidine analog reporter probe

Positron emission tomography (PET) reporter gene imaging can be used to non-invasively monitor cell-based therapies. Therapeutic cells engineered to express a PET reporter gene (PRG) specifically accumulate a PET reporter probe (PRP) and can be detected by PET imaging. Expanding the utility of this technology requires the development of new non-immunogenic PRGs. Here we describe a new PRG-PRP system that employs, as the PRG, a mutated form of human thymidine kinase 2 (TK2) and 2'-deoxy-2'-18F-5-methyl-1-β-L-arabinofuranosyluracil (L-18F-FMAU) as the PRP. We identified L-18F-FMAU as a candidate PRP and determined its biodistribution in mice and humans. Using structure-guided enzyme engineering, we generated a TK2 double mutant (TK2-N93D/L109F) that efficiently phosphorylates L-18F-FMAU. The N93D/L109F TK2 mutant has lower activity for the endogenous nucleosides thymidine and deoxycytidine than wild type TK2, and its ectopic expression in therapeutic cells is not expected to alter nucleotide metabolism. Imaging studies in mice indicate that the sensitivity of the new human TK2-N93D/L109F PRG is comparable with that of a widely used PRG based on the herpes simplex virus 1 thymidine kinase. These findings suggest that the TK2-N93D/L109F/L-18F-FMAU PRG-PRP system warrants further evaluation in preclinical and clinical applications of cell-based therapies.     

Adenovirus-mediated gene expression imaging to directly detect sentinel lymph node metastasis of prostate cancer

The accurate assessment of nodal involvement in prostate cancer is crucial to planning treatment, yet there is a shortage of noninvasive imaging techniques capable of visualizing nodal lesions directly. This study demonstrates the feasibility of using recombinant human adenoviral vectors to detect nodal metastases in a human prostate cancer model. This was achieved by the prostate-restricted expression of optical and positron emission tomography (PET) imaging reporter genes by the viral vector coupled with the innate lymphotropic properties of adenovirus. We show that peritumoral administration of these vectors results in the direct detection of reporter gene expression in metastatic lesions within sentinel lymph nodes. Notably, this approach parallels the current lymphoscintigraphy method but enables the direct PET visualization of sentinel lymph node metastases, eliminating the need for invasive lymphadenectomy. These findings may lead to more effective diagnostic and therapeutic strategies for individuals with advanced-stage prostate cancer.     

Advancing molecular therapies through in vivo bioluminescent imaging

Effective development of therapeutics that target the molecular basis of disease is dependent on testing new therapeutic moieties and delivery strategies in animal models of human disease. Accelerating the analyses of these models and improving their predictive value through whole animal imaging methods, which provide data in real time and are sensitive to the subtle changes, are crucial for rapid advancement of these approaches. Modalities based on optics are rapid, sensitive, and accessible methods for in vivo analyses with relatively low instrumentation costs. In vivo bioluminescent imaging (BLI) is one of these optically based imaging methods that enable rapid in vivo analyses of a variety of cellular and molecular events with extreme sensitivity. BLi is based on the use of light-emitting enzymes as internal biological light sources that can be detected externally as biological indicators. BLI has been used to test spatio-temporal expression patterns of both target and therapeutic genes in living laboratory animals where the contextual influences of whole biological systems are preserved. BLI has also been used to analyze gene delivery, immune cell therapies, and the in vivo efficacy of inhibitory RNAs. New tools for BLI are being developed that will offer greater flexibility in detection and analyses. BLI can be used to accelerate the evaluation of experimental therapeutic strategies and whole body imaging offers the opportunity of revealing the effects of novel approaches on key steps in disease processes.     

Transcriptionally targeted gene therapy to detect and treat cancer

The greatest challenge in cancer treatment is to achieve the highest levels of specificity and efficacy. Cancer gene therapy could be designed specifically to express therapeutic genes to induce cancer cell destruction. Cancer-specific promoters are useful tools to accomplish targeted expression; however, high levels of gene expression are needed to achieve therapeutic efficacy. Incorporating an imaging reporter gene in tandem with the therapeutic gene will allow tangible proof of principle that gene expression occurs at the correct location and at a sufficient level. Gene-based imaging can advance cancer detection and diagnosis. By combining the cancer-targeted imaging and therapeutic strategies, the exciting prospect of a 'one-two punch' to find hidden, disseminated cancer cells and destroy them simultaneously can potentially be realized.     

Emerging mammalian gene switches for controlling implantable cell therapies

Engineered cell-based therapies have emerged as a new paradigm in modern medicine, with several engineered T cell therapies currently approved to treat blood cancers and many more in clinical development. Tremendous progress in synthetic biology over the past two decades has allowed us to program cells with sophisticated sense-and-response modules that can effectively control therapeutic functions. In this review, we highlight recent advances in mammalian synthetic gene switches, focusing on devices designed for therapeutic applications. Although many gene switches responding to endogenous or exogenous molecular signals have been developed, the focus is shifting towards achieving remote-controlled production of therapeutic effectors by stimulating implanted engineered cells with traceless physical signals, such as light, electrical signals, magnetic fields, heat or ultrasound.     

Molecular imaging of gene expression and efficacy following adenoviral-mediated brain tumor gene therapy

Cancer gene therapy is an active area of research relying upon the transfer and subsequent expression of a therapeutic transgene into tumor cells in order to provide for therapeutic selectivity. Noninvasive assessment of therapeutic response and correlation of the location, magnitude, and duration of transgene expression in vivo would be particularly useful in the development of cancer gene therapy protocols by facilitating optimization of gene transfer protocols, vector development, and prodrug dosing schedules. In this study, we developed an adenoviral vector containing both the therapeutic transgene yeast cytosine deaminase (yCD) along with an optical reporter gene (luciferase). Following intratumoral injection of the vector into orthotopic 9 L gliomas, anatomical and diffusion-weighted MR images were obtained over time in order to provide for quantitative assessment of overall therapeutic efficacy and spatial heterogeneity of cell kill, respectively. In addition, bioluminescence images were acquired to assess the duration and magnitude of gene expression. MR images revealed significant reduction in tumor growth rates associated with yCD/5-fluorocytosine (5FC) gene therapy. Significant increases in mean tumor diffusion values were also observed during treatment with 5FC. Moreover, spatial heterogeneity in tumor diffusion changes were also observed revealing that diffusion magnetic resonance imaging could detect regional therapeutic effects due to the nonuniform delivery and/or expression of the therapeutic yCD transgene within the tumor mass. In addition, in vivo bioluminescence imaging detected luciferase gene expression, which was found to decrease over time during administration of the prodrug providing a noninvasive surrogate marker for monitoring gene expression. These results demonstrate the efficacy of the yCD/5FC strategy for the treatment of brain tumors and reveal the feasibility of using multimodality molecular and functional imaging for assessment of gene expression and therapeutic efficacy.     

Cancer gene suppression strategies: issues and potential

Oncogenes are ideal targets for therapies which down-regulate gene expression. However, effective modalities for altering gene expression in vivo have thus far proven to be elusive. Whilst there has been recent success with small molecule inhibitors of oncoprotein function, evolution of resistance to these agents has been observed in the clinical setting, indicating the need for combinations of therapies for cancer treatment. Strategies for in vivo gene down-regulation still hold promise for the treatment of cancer. The technologies relevant to such therapeutic strategies are discussed in terms of molecular action, delivery and choice of target gene. Consideration is given to the pre-clinical and clinical efficacy these agents have demonstrated to date.     

Theranostics: combining imaging and therapy

Employing theranostic nanoparticles, which combine both therapeutic and diagnostic capabilities in one dose, has promise to propel the biomedical field toward personalized medicine. This review presents an overview of different theranostic strategies developed for the diagnosis and treatment of disease, with an emphasis on cancer. Herein, therapeutic strategies such as nucleic acid delivery, chemotherapy, hyperthermia (photothermal ablation), photodynamic, and radiation therapy are combined with one or more imaging functionalities for both in vitro and in vivo studies. Different imaging probes, such as MRI contrast agents (T(1) and T(2) agents), fluorescent markers (organic dyes and inorganic quantum dots), and nuclear imaging agents (PET/SPECT agents), can be decorated onto therapeutic agents or therapeutic delivery vehicles in order to facilitate their imaging and, in so doing, gain information about the trafficking pathway, kinetics of delivery, and therapeutic efficacy; several such strategies are outlined. The creative approaches being developed for these classes of therapies and imaging modalities are discussed, and the recent developments in this field along with examples of technologies that hold promise for the future of cancer medicine are highlighted.     

Prostate cancer and the genomic revolution: Advances using microarray analyses

The emerging technology of microarray analysis allows the establishment of molecular portraits of prostate cancer and the discovery of novel genes involved in the carcinogenesis process. Many novel genes have already been identified using this technique, and functional analyses of these genes are currently being tested. The combination of microarray analysis with other recently developed high-throughput techniques, such as proteomics, tissue arrays, and gene promoter-methylation, especially using tissue microdissection methods, will provide us with more comprehensive insights into how prostate cancer develops and responds to gene-targeted therapies. Animal models of prostate cancer are being characterized by high throughput techniques to better define the similarities and differences between those models and the human disease, and to determine whether particular models may be useful for specific targeted therapies in pre-clinical studies. Although profiling of mRNA expression provides important information of gene expression, the development of proteomic technologies will allow for an even more precise global insight into cellular signaling and structural alterations during prostate carcinogenesis. Not only will the "omic" revolution change basic science, but it will lead to a new era of molecular medicine.