Imaging transcriptional regulation of p53-dependent genes with positron emission tomography in vivo

Positron emission tomography molecular imaging-based cancer phenotyping

During the past several decades, numerous studies have provided insights into biological characteristics of cancer cells and identified various hallmarks of cancer acquired in the tumorigenic processes. However, it is still challenging to image these distinctive traits of cancer to facilitate the management of patients in clinical settings. The rapidly evolving field of positron emission tomography (PET) imaging has provided opportunities to investigate cancer's biological characteristics in vivo. This article reviews the current status of PET imaging on characterizing hallmarks of cancer and discusses the future directions of PET imaging strategies facilitating in vivo cancer phenotyping.

Various direct and indirect imaging strategies have been developed in positron emission tomography. Positron emission tomography has shown great potential in characterizing cancer hallmarks in vivo.

Engineered circular ADAR-recruiting RNAs increase the efficiency and fidelity of RNA editing in vitro and in vivo

Current methods for programmed RNA editing using endogenous ADAR enzymes and engineered ADAR-recruiting RNAs (arRNAs) suffer from low efficiency and bystander off-target editing. Here, we describe LEAPER 2.0, an updated version of LEAPER that uses covalently closed circular arRNAs, termed circ-arRNAs. We demonstrate on average ~3.1-fold higher editing efficiency than their linear counterparts when expressed in cells or delivered as in vitro-transcribed circular RNA oligonucleotides. To lower off-target editing we deleted pairings of uridines with off-target adenosines, which almost completely eliminated bystander off-target adenosine editing. Engineered circ-arRNAs enhanced the efficiency and fidelity of editing endogenous CTNNB1 and mutant TP53 transcripts in cell culture. Delivery of circ-arRNAs using adeno-associated virus in a mouse model of Hurler syndrome corrected the pathogenic point mutation and restored α-L-iduronidase catalytic activity, lowering glycosaminoglycan accumulation in the liver. LEAPER 2.0 provides a new design of arRNA that enables more precise, efficient RNA editing with broad applicability for therapy and basic research.

Molecular Imaging with Reporter Genes: Has Its Promise Been Delivered?

The first reporter systems were developed in the early 1980s and were based on measuring the activity of an enzyme-as a surrogate measure of promoter-driven transcriptional activity-which is now known as a reporter gene system. The initial objective and application of reporter techniques was to analyze the activity of a specific promoter (namely, the expression of a gene that is under the regulation of the specific promoter that is linked to the reporter gene). This system allows visualization of specific promoter activity with great sensitivity. In general, there are 2 classes of reporter systems: constitutively expressed (always-on) reporter constructs used for cell tracking, and inducible reporter systems sensitive to endogenous signaling molecules and transcription factors that characterize specific tissues, tumors, or signaling pathways.This review traces the development of different reporter systems, using fluorescent and bioluminescent proteins as well as radionuclide-based reporter systems. The development and application of radionuclide-based reporter systems is the focus of this review. The question at the end of the review is whether the "promise" of reporter gene imaging has been realized. What is required for moving forward with radionuclide-based reporter systems, and what is required for successful translation to clinical applications?

Small Animal PET Imaging of hTERT RNA-Targeted HSV1-tk Gene Expression with Trans-Splicing Ribozyme

Background: Trans-splicing ribozymes (TSR) are useful anticancer agents targeting cancer-specific transcripts and replacing the RNA to induce anticancer gene expression specifically and selectively in cancer cells. Similar to other gene therapy methods, it is also important to evaluate the transgene expression for target specificity and ribozyme activity. Materials and Methods: In this study, the authors performed in vivo small animal positron emission tomography (PET) imaging and biodistribution assay to evaluate human telomerase reverse transcriptase (hTERT) RNA-targeting-specific TSR, which directs the expression of herpes simplex virus type 1 thymidine kinase (HSV1-tk) gene selectively in hTERT-positive tumors through targeted RNA replacement of the hTERT transcript. Results: The hTERT RNA-targeted HSV1-tk expression with TSR was monitored by PET imaging with ¹²⁴I labeled 2'-fluoro-2'-deoxy-1-β-D-arabinofuranosyl-5-iodouracil, which is one of the thymidine derivatives acting as substrates for HSV1-tk, in hTERT-positive tumor-bearing mice. Conclusions: Imaging of hTERT RNA-targeted HSV1-tk expression by TSR could be used in the development of advanced gene therapy using tumor-specific TSR.

Programmable RNA editing by recruiting endogenous ADAR using engineered RNAs

Current tools for targeted RNA editing rely on the delivery of exogenous proteins or chemically modified guide RNAs, which may lead to aberrant effector activity, delivery barrier or immunogenicity. Here, we present an approach, called leveraging endogenous ADAR for programmable editing of RNA (LEAPER), that employs short engineered ADAR-recruiting RNAs (arRNAs) to recruit native ADAR1 or ADAR2 enzymes to change a specific adenosine to inosine. We show that arRNA, delivered by a plasmid or viral vector or as a synthetic oligonucleotide, achieves editing efficiencies of up to 80%. LEAPER is highly specific, with rare global off-targets and limited editing of non-target adenosines in the target region. It is active in a broad spectrum of cell types, including multiple human primary cell types, and can restore α-L-iduronidase catalytic activity in Hurler syndrome patient-derived primary fibroblasts without evoking innate immune responses. As a single-molecule system, LEAPER enables precise, efficient RNA editing with broad applicability for therapy and basic research.

PARP-1-Targeted Radiotherapy in Mouse Models of Glioblastoma

The DNA repair enzyme poly(ADP-ribose) polymerase 1 (PARP-1) is overexpressed in glioblastoma, with overall low expression in healthy brain tissue. Paired with the availability of specific small molecule inhibitors, PARP-1 is a near-ideal target to develop novel radiotherapeutics to induce DNA damage and apoptosis in cancer cells, while sparing healthy brain tissue. Methods: We synthesized an ¹³¹I-labeled PARP-1 therapeutic and investigated its pharmacology in vitro and in vivo. A subcutaneous tumor model was used to quantify retention times and therapeutic efficacy. A potential clinical scenario, intratumoral convection-enhanced delivery, was mimicked using an orthotopic glioblastoma model combined with an implanted osmotic pump system to study local administration of ¹³¹I-PARPi (PARPi is PARP inhibitor). Results:¹³¹I-PARPi is a 1(2H)-phthalazinone, similar in structure to the Food and Drug Administration-approved PARP inhibitor AZD-2281. In vitro studies have shown that ¹³¹I-PARPi and AZD-2281 share similar pharmacologic profiles. ¹³¹I-PARPi delivered 134.1 cGy/MBq intratumoral injected activity. Doses to nontarget tissues, including liver and kidney, were significantly lower. Radiation damage and cell death in treated tumors were shown by p53 activation in U87-MG cells transfected with a p53-bioluminescent reporter. Treated mice showed significantly longer survival than mice receiving vehicle (29 vs. 22 d, P < 0.005) in a subcutaneous model. Convection-enhanced delivery demonstrated efficient retention of ¹³¹I-PARPi in orthotopic brain tumors, while quickly clearing from healthy brain tissue. Conclusion: Our results demonstrate ¹³¹I-PARPi's high potential as a therapeutic and highlight PARP's relevance as a target for radionuclide therapy. Radiation plays an integral role in brain tumor therapy, and radiolabeled PARP therapeutics could ultimately lead to improvements in the standard of care.

Development of an Azoreductase-based Reporter System with Synthetic Fluorogenic Substrates

Enzyme/substrate pairs, such as β-galactosidase with chromogenic x-gal substrate, are widely used as reporters to monitor biological events, but there is still a requirement for new reporter systems, which may be orthogonal to existing systems. Here, we focused on azoreductase (AzoR). We designed and synthesized a library of azo-rhodamine derivatives as candidate fluorogenic substrates. These derivatives were nonfluorescent, probably due to ultrafast conformational change around the N═N bond after photoexcitation. We found that AzoR-mediated reduction of the azo bond of derivatives bearing an electron-donating group on the azobenzene moiety was followed by nonenzymatic cleavage to afford highly fluorescent 2-methyl-rhodamine green (2-Me RG), which was well retained in cells. We show that the AzoR/compound 9 reporter system can detect azoreductase-expressing live cells at the single cell level.

Radiochemical synthesis of 2'-[18F]-labelled and 3'-[18F]-labelled nucleosides for positron emission tomography imaging

This review article considers 2'-labelled and 3'-labelled nucleosides, which are of great importance as positron emission tomography (PET) probes in clinical diagnostics and PET research. Although the radiochemical preparation of several [(18)F]-labelled nucleosides such as [(18)F]fluorothymidine or [(18)F](fluoroarabinofuranosyl)cytosine has been accomplished within the last two decades, a number of potentially interesting nucleoside-based biomarkers are not yet available for automated good manufacturing practice production due to the lack of fast and efficient synthetic methods for late-stage [(18)F]-introduction. In order to meet recent demands for new PET-based biomarkers in various clinical applications, appropriate precursors that can easily be fluorinated and deprotected need to be developed.

In vivo dynamics of estrogen receptor activity: the ERE-Luc model

In recent years several studies demonstrated the presence of estrogen receptors in mammalian tissues and significantly improved our understanding of their ability to control biological processes in reproductive as well as non-reproductive organs. Considering the manifold mechanisms and organs that are involved in estrogen action and the implication of estrogens in human female physiology, innovative approaches are required to shed light on the widespread activities of estrogen receptors in woman physiology. This is particularly relevant for the definition of novel, more efficacious hormonal replacement therapies or for the evaluation of the risk associated with the exposure to endocrine disruptors. The introduction of genetic engineering and the development and application of in vivo imaging techniques offer new tools for pre-clinical studies. The generation of the ERE-Luc mouse, a reporter animal developed for in vivo studies of the estrogen receptor activity, allows assessing the activity state of the ER signaling pathway in all target tissues and organs at once, under physiological stimuli or as a result of a pharmacological treatment. This review summarizes the main steps in the generation and appraisal of the estrogen receptor reporter mouse ERE-Luc, designed for in vivo molecular imaging studies, and describes examples demonstrating the suitability of the ERE-Luc model for drug development and for the investigation of the effects of endogenous, environmental, and dietary estrogens in vivo. This article is part of a Special Issue entitled 'Phytoestrogens'.

An improved strategy for the synthesis of [¹⁸F]-labeled arabinofuranosyl nucleosides

The expression of the herpes simplex virus type-1 thymidine kinase (HSV1-tk) gene can be imaged efficaciously using a variety of 2'-[(18)F]fluoro-2'-deoxy-1-b-D-arabinofuranosyl-uracil derivatives [[(18)F]-FXAU, X=I(iodo), E(ethyl), and M(methyl)]. However, the application of these derivatives in clinical and translational studies has been impeded by their complicated and long syntheses (3-5h). To remedy these issues, in the study at hand we have investigated whether microwave or combined catalysts could facilitate the coupling reaction between sugar and nucleobase and, further, have probed the feasibility of establishing a novel approach for [(18)F]-FXAU synthesis. We have demonstrated that the rate of the trimethylsilyl trifluoromethanesulfonate (TMSOTf)-catalyzed coupling reaction between the 2-deoxy-sugar and uracil derivatives at 90 °C can be significantly accelerated by microwave-driven heating or by the addition of Lewis acid catalyst (SnCl(4)). Further, we have observed that the stability of the α- and β-anomers of [(18)F]-FXAU derivatives differs during the hydrolysis step. Using the microwave-driven heating approach, overall decay-corrected radiochemical yields of 19%-27% were achieved for [(18)F]-FXAU in 120min at a specific activity of >22MBq/nmol (595Ci/mmol). Ultimately, we believe that these high yielding syntheses of [(18)F]-FIAU, [(18)F]-FMAU and [(18)F]-FEAU will facilitate routine production for clinical applications.

Specific biomarkers of receptors, pathways of inhibition and targeted therapies: pre-clinical developments

A deeper understanding of the role of specific genes, proteins, pathways and networks in health and disease, coupled with the development of technologies to assay these molecules and pathways in patients, promises to revolutionise the practice of clinical medicine. Especially the discovery and development of novel drugs targeted to disease-specific alterations could benefit significantly from non-invasive imaging techniques assessing the dynamics of specific disease-related parameters. Here we review the application of imaging biomarkers in the management of patients with brain tumours, especially malignant glioma. In our other review we focused on imaging biomarkers of general biochemical and physiological processes related with tumour growth such as energy, protein, DNA and membrane metabolism, vascular function, hypoxia and cell death. In this part of the review, we will discuss the use of imaging biomarkers of specific disease-related molecular genetic alterations such as apoptosis, angiogenesis, cell membrane receptors and signalling pathways and their application in targeted therapies.

Comparison of cell-labeling methods with ¹²⁴I-FIAU and ⁶⁴Cu-PTSM for cell tracking using chronic myelogenous leukemia cells expressing HSV1-tk and firefly luciferase

Cell-tracking methods with molecular-imaging modality can monitor the biodistribution of cells. In this study, the direct-labeling method with ⁶⁴Cu-pyruvaldehyde-bis(N4-methylthiosemicarbazone) (⁶⁴Cu-PTSM), indirect cell-labeling methods with herpes simplex virus type 1-thymidine kinase (HSV1-tk)-mediated ¹²⁴I-2'-fluoro-2'-deoxy-1-β-D-arabinofuranosyl-5-iodouracil (¹²⁴I-FIAU) were comparatively investigated in vitro and in vivo for tracking of human chronic myelogenous leukemia cells. K562-TL was established by retroviral transduction of the HSV1-tk and firefly luciferase gene in the K562 cell. K562-TL cells were labeled with ⁶⁴Cu-PTSM or ¹²⁴I-FIAU. Cell labeling efficiency, viability, and radiolabels retention were compared in vitro. The biodistribution of radiolabeled K562-TL cells with each radiolabel and small-animal positron emission tomography imaging were performed. Additionally, in vivo and ex vivo bioluminescence imaging (BLI) and tissue reverse transcriptase-polymerase chain reaction (RT-PCR) analysis were used for confirming those results. K562-TL cells were efficiently labeled with both radiolabels. The radiolabel retention (%) of ¹²⁴I-FIAU (95.2%±1.1%) was fourfold higher than ⁶⁴Cu-PTSM (23.6%±0.7%) at 24 hours postlabeling. Viability of radiolabeled cells was statistically nonsignificant between ¹²⁴I-FIAU and ⁶⁴Cu-PTSM. The radioactivity of each radiolabeled cells was predominantly accumulated in the lungs and liver at 2 hours. Both the radioactivity of ⁶⁴Cu-PTSM- and ¹²⁴I-FIAU-labeled cells was highly accumulated in the liver at 24 hours. However, the radioactivity of ¹²⁴I-FIAU-labeled cells was markedly decreased from the body at 24 hours. The K562-TL cells were dominantly localized in the lungs and liver, which also verified by BLI and RT-PCR analysis at 2 and 24 hours postinjection. The ⁶⁴Cu-PTSM-labeled cell-tracking method is more efficient than ¹²⁴I-FIAU-labeled cell tracking, because of markedly decrease of radioactivity and fast efflux of ¹²⁴I-FIAU in vivo. In spite of a high labeling yield and radiolabel retention of ¹²⁴I-FIAU in vitro, the in vivo cell-tracking method using ⁶⁴Cu-PTSM could be a useful method to evaluate the distribution and targeting of various cell types, especially, stem cells and immune cells.

Gamma camera and optical imaging with a fusion reporter gene using human sodium/iodide symporter and monomeric red fluorescent protein in mouse model

PURPOSE

Multimodality imaging contributes to the activation of translational research by compensating for its weak points. Herein, we developed a noninvasive dual-reporter gene system for nuclear and optical imaging.

MATERIALS AND METHODS

We constructed a fusion reporter vector concurrently expressing the human sodium/iodide symporter (hNIS) and monomeric red fluorescent protein (mCherry), and evaluated the function of this fusion reporter system under in vitro and in vivo conditions.

RESULTS

The expression of hNIS/mCherry fusion gene was confirmed in transfected cells using reverse transcription polymerase chain reaction (RT-PCR) and Western blotting. As the numbers of cells increased, the fluorescence and 125I uptake increased in the hNIS/mCherry-transfected cells, and a high correlation between fluorescence intensity and radioactivity was noted. The fluorescence intensities and radioactivity signals were also well-correlated in HT-29-hNIS/mCherry tumors (R2=0.9304) in in vivo fluorescence and gamma camera imaging.

CONCLUSIONS

The dual-reporter imaging method using hNIS and mCherry genes reflected tumor extent as well as viable cell numbers, and correlated well with one another. This suggests that the hNIS/mCherry dual-reporter system can be a useful tool for multi-modal imaging.

Potential targets for molecular imaging of apoptosis resistance in hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is one of the most common cancers, which is mainly a concern in Southeast Asia. Apoptosis resistance in HCC is one of the significant factors for hepatocarcinogenesis and tumour progression. Recent advances of apoptosis resistance mechanisms in HCC could serve as potential targets for molecular imaging, which would be of considerable value to explore the molecular processes involved in HCC progression and to evaluate responses of certain anti-HCC therapies. Disruptions in the balance of anti-apoptotic and pro-apoptotic processes have been found to be involved in apoptosis resistance in HCC. Loss of response to death receptors, transformation of growth factor-β induced apoptosis, upregulation of anti-apoptotic Bcl-2 subgroup, as well as downregulation of pro-apoptotic Bax subgroup and BH3-only subgroup, are associated with apoptosis resistance in HCC. Mutation of p53 gene, dysregulation of NF-κB and survivin are also of interest because of their contribution to HCC development. In this review, the aim is to identify potential targets for molecular imaging of apoptosis resistance in HCC.

A new pyrimidine-specific reporter gene: a mutated human deoxycytidine kinase suitable for PET during treatment with acycloguanosine-based cytotoxic drugs

In this article, we describe a series of new human-derived reporter genes based on human deoxycytidine kinase (dCK) suitable for clinical PET.

METHODS

Native dCK and its mutant reporter genes were tested in vitro and in vivo for their phosphorylation of pyrimidine- and acycloguanosine-based radiotracers including 2'-deoxy-2'-fluoroarabinofuranosylcytosine, 2'-fluoro-2'-deoxyarabinofuranosyl-5-ethyluracil (FEAU), penciclovir, and 9-[4-fluoro-3-(hydroxymethyl)butyl]guanine (FHBG) and clinically applied antiviral and anticancer drugs.

RESULTS

Cells transduced with dCK mutant reporter genes showed high in vitro and in vivo uptake of pyrimidine-based radiopharmaceuticals ((18)F-FEAU) comparable to that of herpes simplex virus type-1 thymidine kinase (HSV1-tk)-transduced cells. These mutants did not phosphorylate acycloguanosine-based radiotracers ((18)F-FHBG) or antiviral drugs (ganciclovir). Furthermore, the mutants displayed suicidal activation of clinically used pyrimidine-based prodrugs (cytarabine, gemcitabine).

CONCLUSION

The mutants of human dCK can be used as pyrimidine-specific PET reporter genes for imaging with (18)F-FEAU during treatment with acycloguanosine-based antiviral drugs. Additionally, the prosuicidal activity of these reporters with pyrimidine-based analogs will allow for the safe elimination of transduced cells.

Multimodality imaging of gene transfer with a receptor-based reporter gene

Gene therapy trials have traditionally used tumor and tissue biopsies for assessing the efficacy of gene transfer. Noninvasive imaging techniques offer a distinct advantage over tissue biopsies in that the magnitude and duration of gene transfer can be monitored repeatedly. Human somatostatin receptor subtype 2 (SSTR2) has been used for the nuclear imaging of gene transfer. To extend this concept, we have developed a somatostatin receptor-enhanced green fluorescent protein fusion construct (SSTR2-EGFP) for nuclear and fluorescent multimodality imaging.

METHODS

An adenovirus containing SSTR2-EGFP (AdSSTR2-EGFP) was constructed and evaluated in vitro and in vivo. SCC-9 human squamous cell carcinoma cells were infected with AdEGFP, AdSSTR2, or AdSSTR2-EGFP for in vitro evaluation by saturation binding, internalization, and fluorescence spectroscopy assays. In vivo biodistribution and nano-SPECT imaging studies were conducted with mice bearing SCC-9 tumor xenografts directly injected with AdSSTR2-EGFP or AdSSTR2 to determine the tumor localization of (111)In-diethylenetriaminepentaacetic acid (DTPA)-Tyr3-octreotate. Fluorescence imaging was conducted in vivo with mice receiving intratumoral injections of AdSSTR2, AdSSTR2-EGFP, or AdEGFP as well as ex vivo with tissues extracted from mice.

RESULTS

The similarity between AdSSTR2-EGFP and wild-type AdSSTR2 was demonstrated in vitro by the saturation binding and internalization assays, and the fluorescence emission spectra of cells infected with AdSSTR2-EGFP was almost identical to the spectra of cells infected with wild-type AdEGFP. Biodistribution studies demonstrated that the tumor uptake of (111)In-DTPA-Tyr3-octreotate was not significantly different (P > 0.05) when tumors (n = 5) were injected with AdSSTR2 or AdSSTR2-EGFP but was significantly greater than the uptake in control tumors. Fluorescence was observed in tumors injected with AdSSTR2-EGFP and AdEGFP in vivo and ex vivo but not in tumors injected with AdSSTR2. Although fluorescence was observed, there were discrepancies between in vivo imaging and ex vivo imaging as well as between nuclear imaging and fluorescent imaging.

CONCLUSION

These studies showed that the SSTR2-EGFP fusion construct can be used for in vivo nuclear and optical imaging of gene transfer.

Novel molecular imaging platform for monitoring oncological kinases

Recent advances in oncology have lead to identification of a plethora of alterations in signaling pathways that are critical to oncogenesis and propagation of malignancy. Among the biomarkers identified, dysregulated kinases and associated changes in signaling cascade received the lion's share of scientific attention and have been under extensive investigations with goal of targeting them for anti-cancer therapy. Discovery of new drugs is immensely facilitated by molecular imaging technology which enables non-invasive, real time, dynamic imaging and quantification of kinase activity. Here, we review recent development of novel kinase reporters based on conformation dependent complementation of firefly luciferase to monitor kinase activity. Such reporter system provides unique insights into the pharmacokinetics and pharmacodynamics of drugs that modulate kinase signaling and have a huge potential in drug discovery, validation, and drug-target interactions.

Iodine-124: a promising positron emitter for organic PET chemistry

The use of radiopharmaceuticals for molecular imaging of biochemical and physiological processes in vivo has evolved into an important diagnostic tool in modern nuclear medicine and medical research. Positron emission tomography (PET) is currently the most sophisticated molecular imaging methodology, mainly due to the unrivalled high sensitivity which allows for the studying of biochemistry in vivo on the molecular level. The most frequently used radionuclides for PET have relatively short half-lives (e.g. ¹¹C: 20.4 min; ¹⁸F: 109.8 min) which may limit both the synthesis procedures and the time frame of PET studies. Iodine-124 (¹²⁴I, t_1/2 = 4.2 d) is an alternative long-lived PET radionuclide attracting increasing interest for long term clinical and small animal PET studies. The present review gives a survey on the use of ¹²⁴I as promising PET radionuclide for molecular imaging. The first part describes the production of ¹²⁴I. The second part covers basic radiochemistry with ¹²⁴I focused on the synthesis of ¹²⁴I-labeled compounds for molecular imaging purposes. The review concludes with a summary and an outlook on the future prospective of using the long-lived positron emitter ¹²⁴I in the field of organic PET chemistry and molecular imaging.

Improved synthesis of 2'-deoxy-2'-[18F]-fluoro-1-beta-D-arabinofuranosyl-5-iodouracil ([18F]-FIAU)

An improved synthesis of 2'-[(18)F]-fluoro-2'-deoxy-1-beta-D-arabinofuranosyl-5-iodouracil ([(18)F]-FIAU) has been developed. The method utilizes trimethylsilyl trifluoromethanesulfonate (TMSOTf) catalyzed coupling of 2-deoxy-2-[(18)F]-fluoro-1,3,5-tri-O-benzoyl-d-arabinofuranose with 2,4-bis(trimethylsilyloxy)-5-iodouracil to yield the protected dibenzoyl-[(18)F]-FIAU. Dibenzoyl-[(18)F]-FIAU was deprotected with sodium methoxide to yield a mixture of alpha- and beta-anomers in a ratio of 1:1, which were purified by HPLC. The procedure described in this article eliminates the need for HBr activation of the sugar prior to coupling with silylated iodouracil and is suitable for automation. The total reaction time was about 110 min, starting from [(18)F]-fluoride. The average isolated yield of the required beta-anomer was 10+/-6% (decay corrected) with average specific activity of 125 mCi/micromol.

New imaging approaches to phaeochromocytomas and paragangliomas

Formerly used concepts for phaeochromocytomas and paragangliomas have been challenged by recent discoveries that at least 24% of tumours are familial and thereby often multiple in various locations throughout the body. Furthermore, tumours are often malignant and perhaps more aggressive if associated with SDHB gene mutations. Some paragangliomas are clinically silent and may present only with dopamine hypersecretion. In the current era where CT and MRI are more commonly used, tumours are more often found as incidentalomas and MRI may be less specific for phaeochromocytoma and paraganglioma than previously thought. Because of unique tumour characteristics (e.g. the presence of cell membrane and intracellular vesicular norepinephrine transporters) these tumours were 'born' to be imaged by means of specific functional imaging approaches. Moreover, additional recent discoveries related to apoptosis, hypoxia, acidosis, anaerobic glycolysis and angiogenesis, often disturbed in tumour cells, open new options and challenges to specifically image phaeochromocytomas and paragangliomas and possibly link those results to their pathophysiology, genotypic alterations and metastatic potential. Functional imaging, especially represented by positron emission tomography (PET), offers an excellent approach by which tumour-specific processes can be detected, evaluated and seen in the context of tumour-specific behaviour and its genetic signature. In this review, we address the recent developments in new functional imaging modalities for phaeochromocytoma and paraganglioma and provide the reader with suggested imaging approaches in various phaeochromocytomas and paragangliomas of sympathetic origin. Current imaging algorithms of head and neck parasympathetic paragangliomas are not discussed. Finally, this review outlines some future perspectives of functional imaging of these tumours.

Construction of mutant TKGFP for real-time imaging of temporal dynamics of HIF-1 signal transduction activity mediated by hypoxia and reoxygenation in tumors in living mice

The herpes simplex virus type 1 thymidine kinase (HSV1-tk)/green fluorescent protein (TKGFP) dual-reporter gene and a multimodality imaging approach play a critical role in monitoring therapeutic gene expression, immune cell trafficking, and protein-protein interactions in translational molecular-genetic imaging. However, the cytotoxicity and low temporal resolution of TKGFP limits its application in studies that require a rapid turnover of the reporter. The purpose of this study was to construct a novel mutant TKGFP fusion reporter gene with low cytotoxicity and high temporal resolution for use in the real-time monitoring of temporal dynamics and spatial heterogeneity of hypoxia-inducible factor 1 (HIF-1) signal transduction activity mediated by hypoxia and reoxygenation in vitro and in vivo.

METHODS

Destabilized TKGFP was produced by inserting the nuclear export signal (NES) sequence at the N terminus and fusing the degradation domain of mouse ornithine decarboxylase (dMODC) at the C terminus. The stability of TKGFP in living NG4TL4 cells was determined by Western blot analysis, HSV1-tk enzyme activity assay, and flow cytometric analysis. The suitability of NESTKGFP:dMODC as a transcription reporter was investigated by linking it to a promoter consisting of 8 copies of hypoxia-responsive elements, whose activities depend on HIF-1. The dynamic transcriptional events mediated by hypoxia and reoxygenation were monitored by NESTKGFP:dMODC or TKGFP and determined by optical imaging and PET.

RESULTS

Unlike TKGFP, NESTKGFP:dMODC was unstable in the presence of cycloheximide and showed a short half-life of protein and enzyme activity. Rapid turnover of NESTKGFP:dMODC occurred in a 26S proteasome-dependent manner. Furthermore, NESTKGFP:dMODC showed an upregulated expression and low cytotoxicity in living cells. Studies of hypoxia-responsive TKGFP and NESTKGFP:dMODC expression showed that NESTKGFP:dMODC as a reporter gene had better temporal resolution than did TKGFP for monitoring the dynamic transcriptional events mediated by hypoxia and reoxygenation; the TKGFP expression level was not optimal for the purpose of monitoring.

CONCLUSION

In translational molecular-genetic imaging, NESTKGFP:dMODC as a reporter gene, together with optical imaging and PET, allows the direct monitoring of transcription induction and easy determination of its association with other biochemical changes.

Visualization of gene expression in the live subject using the Na/I symporter as a reporter gene: applications in biotherapy

Biotherapies involve the utilization of antibodies, genetically modified viruses, bacteria or cells for therapeutic purposes. Molecular imaging has the potential to provide unique information that will guarantee their biosafety in humans and provide a rationale for the future development of new generations of reagents. In this context, non-invasive imaging of gene expression is an attractive prospect, allowing precise, spacio-temporal measurements of gene expression in longitudinal studies involving gene transfer vectors. With the emergence of cell therapies in regenerative medicine, it is also possible to track cells injected into subjects. In this context, the Na/I symporter (NIS) has been used in preclinical studies. Associated with a relevant radiotracer ((123)I(-), (124)I(-), (99m)TcO4(-)), NIS can be used to monitor gene transfer and the spread of selectively replicative viruses in tumours as well as in cells with a therapeutic potential. In addition to its imaging potential, NIS can be used as a therapeutic transgene through its ability to concentrate therapeutic doses of radionuclides in target cells. This dual property has applications in cancer treatment and could also be used to eradicate cells with therapeutic potential in the case of adverse events. Through experience acquired in preclinical studies, we can expect that non-invasive molecular imaging using NIS as a transgene will be pivotal for monitoring in vivo the exact distribution and pharmacodynamics of gene expression in a precise and quantitative way. This review highlights the applications of NIS in biotherapy, with a particular emphasis on image-guided radiotherapy, monitoring of gene and vector biodistribution and trafficking of stem cells.

Molecular imaging: a primer for interventionalists and imagers

The characterization of human diseases by their underlying molecular and genomic aberrations has been the hallmark of molecular medicine. From this, molecular imaging has emerged as a potentially revolutionary discipline that aims to visually characterize normal and pathologic processes at the cellular and molecular levels within the milieu of living organisms. Molecular imaging holds promise to provide earlier and more precise disease diagnosis, improved disease characterization, and timely assessment of therapeutic response. This primer is intended to provide a broad overview of molecular imaging with specific focus on future clinical applications relevant to interventional radiology.

Use of radionuclides in cancer research and treatment

Cancer occurs as a result of misregulation of cell growth, which appears to be a consequence of alteration in the function of oncogenes and tumour suppressor genes. Ionising radiation has been used, since the discovery of X-rays in 1896 by Roentgen, both in cancer research and treatment of the disease. The main purpose of cancer research is to understand the molecular alterations involved in the development and progression of the disease in order to improve diagnosis and develop personalised therapies, by focusing on the features of the tumoral cell and the biological events associated to carcinogenesis. Radioisotopic techniques have been used routinely for in vitro research in the molecular and cellular biology of cancer for more than 20 years and are in the process of being substituted by alternative non-radioactive techniques. However in vivo techniques such as irradiation of cells in culture and/or experimental animal models and radioactive labelling are in development, due in part to advances in molecular imaging technologies. The objective of this review is to analyse in an integrative way the applications of ionising radiation in cancer research and therapy. It had been divided into two parts. The first one will approach the techniques applied to cancer research and the second will summarise how ionising radiation is applied to the treatment of neoplastic disease.

5-[18F]Fluoroalkyl pyrimidine nucleosides: probes for positron emission tomography imaging of herpes simplex virus type 1 thymidine kinase gene expression

INTRODUCTION

The preliminary in vivo evaluation of novel 5-[(18)F]fluoroalkyl-2'-deoxyuridines ([(18)F]FPrDU, [(18)F]FBuDU, [(18)F]FPeDU; [(18)F]1a-c, respectively) and 2'-fluoro-2'-deoxy-5-[(18)F]fluoroalkyl-1-beta-d-arabinofuranosyl uracils ([(18)F]FFPrAU, [(18)F]FFBuAU, [(18)F]FFPeAU; [(18)F]1d-f, respectively) as probes for imaging herpes simplex virus type 1 thymidine kinase (HSV1-tk) gene expression is described.

METHODS

[(18)F]1a-f were successfully synthesized by a rapid and efficient two-step one-pot nucleophilic fluorination reaction using 5-O-mesylate precursors and [(18)F]F(-). For in vivo studies, tumor xenografts were grown in nude mice by implanting RG2 cells stably expressing HSV1-tk (RG2TK+) and wild-type cells (RG2).

RESULTS

Biodistribution studies at 2 h pi revealed that the uptake of [(18)F]1a-b and [(18)F]1d-e in RG2TK+ tumors was not significantly different from control tumors. However, [(18)F]1c and [(18)F]1f had an average 1.6- and 1.7-fold higher uptake in RG2TK+ tumors than control RG2 tumors. Blood activity curves for [(18)F]1c and [(18)F]1f highlight rapid clearance of radioactivity in the blood. Dynamic small animal PET (A-PET) imaging studies of tumor-bearing mice with [(18)F]1c and [(18)F]1f showed higher initial uptake (3.5- and 1.4-fold, respectively) in RG2TK+ tumors than in control tumors, with continued washout of activity from both tumors over time.

CONCLUSIONS

Biological evaluations suggest that [(18)F]1c and [(18)F]1f may have limited potential for imaging HSV1-tk gene expression due to fast washout of activity from the blood, thus significantly decreasing sensitivity and specificity of tracer accumulation in HSV1-tk-expressing tumors.

Multimodality imaging of TGFbeta signaling in breast cancer metastases

The skeleton is a preferred site for breast cancer metastasis. We have developed a multimodality imaging approach to monitor the transforming growth factor beta (TGFbeta) signaling pathway in bone metastases, sequentially over time in the same animal. As model systems, two MDA-MB-231 breast cancer cells lines with different metastatic tropisms, SCP2 and SCP3, were transduced with constitutive and TGFbeta-inducible reporter genes and were tested in vitro and in living animals. The sites and expansion of metastases were visualized by bioluminescence imaging using a constitutive firefly luciferase reporter, while TGFbeta signaling in metastases was monitored by microPET imaging of HSV1-TK/GFP expression with [(18)F]FEAU and by a more sensitive and cost-effective bioluminescence reporter, based on nonsecreted Gaussia luciferase. Concurrent and sequential imaging of metastases in the same animals provided insight into the location and progression of metastases, and the timing and course of TGFbeta signaling. The anticipated and newly observed differences in the imaging of tumors from two related cell lines have demonstrated that TGFbeta signal transduction pathway activity can be noninvasively imaged with high sensitivity and reproducibility, thereby providing the opportunity for an assessment of novel treatments that target TGFbeta signaling.

PET imaging of HSV1-tk mutants with acquired specificity toward pyrimidine- and acycloguanosine-based radiotracers

PURPOSE

The aim of this study was to create an alternative mutant of the herpes simplex virus type 1 thymidine kinase (HSV1-tk) reporter gene with reduced phosphorylation capacity for acycloguanosine derivatives, but not pyrimidine-based compounds that will allow for successful PET imaging.

METHODS

A new mutant of HSV1-tk reporter gene, suitable for PET imaging using pyrimidine-based radiotracers, was developed. The HSV1-tk mutant contains an arginine-to-glutamine substitution at position 176 (HSV1-R176Qtk) of the nucleoside binding region of the enzyme.

RESULTS

The mutant-gene product showed favorable enzymatic characteristics toward pyrimidine-based nucleosides, while exhibiting reduced activity with acycloguanosine derivatives. In order to enhance HSV1-R176Qtk reporter activity with pyrimidine-based radiotracers, we introduced the R176Q substitution into the more active HSV1-sr39tk mutant. U87 human glioma cells transduced with the HSV1-R176Qsr39tk double mutant reporter gene showed high (3)H-FEAU pyrimidine nucleoside and low (3)H-penciclovir acycloguanosine analog uptake in vitro. PET imaging also demonstrated high (18)F-FEAU and low (18)F-FHBG accumulation in HSV1-R176Qsr39tk+ xenografts. The feasibility of imaging two independent nucleoside-specific HSV1-tk mutants in the same animal with PET was demonstrated. Two opposite xenografts expressing the HSV1-R176Qsr39tk reporter gene and the previously described acycloguanosine-specific mutant of HSV1-tk, HSV1-A167Ysr39tk reporter gene, were imaged using a short-lived pyrimidine-based (18)F-FEAU and an acycloguanosine-based (18)F-FHBG radiotracer, respectively, administered on 2 consecutive days.

CONCLUSION

We conclude that in combination with acycloguanosine-specific HSV1-A167Ysr39tk reporter gene, a HSV1-tk mutant containing the R176Q substitution could be used for PET imaging of two different cell populations or concurrent molecular biological processes in the same living subject.

Comparative positron-emission tomography (PET) imaging and phototherapeutic potential of 124I- labeled methyl- 3-(1'-iodobenzyloxyethyl)pyropheophorbide-a vs the corresponding glucose and galactose conjugates

In our present study, 3-(1(')-m-iodobenzyloxyethyl)pyropheophorbide-a methyl ester 1, 3-(1'-m-iodobenzyloxyethyl)-17(2)-{(2-deoxy)glucose}pyropheophorbide-a 2, and 3-(1'-m-iodobenzyloxyethyl)-17(2)-{(1-deoxy)galactose}pyropheophorbide-a 3 were synthesized and converted into the corresponding (124)I-labeled analogues by reacting the intermediate trimethyltin analogues with Na(124)I. Photosensitizers 1-3 were evaluated for the PDT efficacy in C3H mice bearing RIF tumors at variable doses and showed a significant long-term tumor cure. Among the compounds investigated, the non-carbohydrate analogue 1 was most effective. These results were in contrast to the in vitro data, where compared to the parent analogue the corresponding galactose and glucose derivatives showed enhanced cell kill. Among the corresponding (124)I-labeled analogues, excellent tumor images were obtained from compound 1 in both tumor models (RIF and Colon-26) and the best tumor contrast was observed at 72 h after injection. Conjugating a glucose moiety to photosensitizer 1 initially diminished its tumor uptake, whereas with time the corresponding galactose analogue showed improved tumor contrast.

An immunotolerant HER-2/neu transgenic mouse model of metastatic breast cancer

PURPOSE

Animal models of breast cancer metastases that recapitulate the pattern of metastatic progression seen in patients are lacking; metastatic breast cancer models do not currently exist for evaluation of immune-mediated therapies. We have developed and characterized a preclinical model for the evaluation of immune-mediated metastatic breast cancer therapies.

EXPERIMENTAL DESIGN

The NT2.5 mammary tumor cell line was injected into the left cardiac ventricle of immunotolerant transgenic neu-N mice and athymic nu/nu mice. Metastatic progression was monitored by bioluminescent, small-animal magnetic resonance imaging, positron emission tomography, single-photon emission computed tomography/computed tomography imaging, and also by histopathology. Antigen expression in normal organs and tumor metastases was evaluated by Western blot analysis and flow cytometry.

RESULTS

Left cardiac ventricle injection of NT2.5 cells yielded widespread metastases in bones, liver, and spleen. Three to four weeks after injection, mice exhibited hind limb paralysis and occasional abdominal enlargement. Bioluminescence imaging of metastatic progression was successful in nude mice but the bioluminescent cells were rejected in immunocompetent mice. Other imaging modalities allowed successful imaging of nonbioluminescent cells. Small-animal positron emission tomography imaging allowed visualization of disease, in vivo, in the bones and liver. Magnetic resonance imaging revealed initial dissemination of the tumor cells to the bone marrow. Small-animal single-photon emission computed tomography/computed tomography imaging identified metastatic bone lesions targeted by a radiolabeled antibody.

CONCLUSION

The model closely recapitulates the pattern of metastatic spread in breast cancer. This immunotolerant metastatic model is a novel addition to existing breast cancer models and coupling the model with in vivo imaging greatly facilitates the evaluation of targeted immunotherapies of metastasis.

Molecular-genetic imaging based on reporter gene expression

Molecular imaging includes proteomic, metabolic, cellular biologic process, and genetic imaging. In a narrow sense, molecular imaging means genetic imaging and can be called molecular-genetic imaging. Imaging reporter genes play a leading role in molecular-genetic imaging. There are 3 major methods of molecular-genetic imaging, based on optical, MRI, and nuclear medicine modalities. For each of these modalities, various reporter genes and probes have been developed, and these have resulted in successful transitions from bench to bedside applications. Each of these imaging modalities has its unique advantages and disadvantages. Fluorescent and bioluminescent optical imaging modalities are simple, less expensive, more convenient, and more user friendly than other imaging modalities. Another advantage, especially of bioluminescence imaging, is its ability to detect low levels of gene expression. MRI has the advantage of high spatial resolution, whereas nuclear medicine methods are highly sensitive and allow data from small-animal imaging studies to be translated to clinical practice. Moreover, multimodality imaging reporter genes will allow us to choose the imaging technologies that are most appropriate for the biologic problem at hand and facilitate the clinical application of reporter gene technologies. Reporter genes can be used to visualize the levels of expression of particular exogenous and endogenous genes and several intracellular biologic phenomena, including specific signal transduction pathways, nuclear receptor activities, and protein-protein interactions. This technique provides a straightforward means of monitoring tumor mass and can visualize the in vivo distributions of target cells, such as immune cells and stem cells. Molecular imaging has gradually evolved into an important tool for drug discovery and development, and transgenic mice with an imaging reporter gene can be useful during drug and stem cell therapy development. Moreover, instrumentation improvements, the identification of novel targets and genes, and imaging probe developments suggest that molecular-genetic imaging is likely to play an increasingly important role in the diagnosis and therapy of cancer.

Imaging transgene activity in vivo

The successful translation of gene therapy for clinical application will require the assessment of transgene activity as a measure of the biological function of a therapeutic transgene. Although current imaging permits the noninvasive detection of transgene expression, the critical need for quantitative imaging of the action of the expressed transgene has not been met. In vivo magnetic resonance spectroscopic imaging (MRSI) was applied to quantitatively delineate both the concentration and activity of a cytosine deaminase-uracil phosphoribosyltransferase (CD-UPRT) fusion enzyme expressed from a transgene. MRSI enabled the generation of anatomically accurate maps of the intratumoral heterogeneity in fusion enzyme activity. We observed an excellent association between the CD-UPRT concentration and activity and the percentage of CD-UPRT(+) cells. Moreover, the regional levels of UPRT activity, as measured by imaging, correlated well with the biological affect of the enzyme. This study presents a translational imaging paradigm for precise, in vivo measurements of transgene activity with potential applications in both preclinical and clinical settings.

Invincible, but Not Invisible: Imaging Approaches Toward In Vivo Detection of Cancer Stem Cells

With evidence emerging in support of a cancer stem-cell model of carcinogenesis, it is of paramount importance to identify and image these elusive cells in their natural environment. The cancer stem-cell hypothesis has the potential to explain unresolved questions of tumorigenesis, tumor heterogeneity, chemotherapeutic and radiation resistance, and even the metastatic phenotype. Intravital imaging of cancer stem cells could be of great value for determining prognosis, as well as monitoring therapeutic efficacy and influencing therapeutic protocols. Cancer stem cells represent a rare population of cells, as low as 0.1% of cells within a human tumor, and the phenotype of isolated cancer stem cells is easily altered when placed under in vitro conditions. This represents a challenge in studying cancer stem cells without manipulation or extraction from their natural environment. Advanced imaging techniques allow for the in vivo observation of physiological events at cellular resolution. Cancer stem-cell studies must take advantage of such technology to promote a better understanding of the cancer stem-cell model in relation to tumor growth and metastasis, as well as to potentially improve on the principles by which cancers are treated. This review examines the opportunities for in vivo imaging of putative cancer stem cells with regard to currently accepted cancer stem-cell characteristics and advanced imaging technologies.

Targeted molecular imaging in oncology: focus on radiation therapy

Anatomically based technologies (computed tomography scans, magnetic resonance imaging, and so on) are in routine use in radiotherapy for planning and assessment purposes. Even with improvements in imaging, however, radiotherapy is still limited in efficacy and toxicity in certain applications. Further advances may be provided by technologies that image the molecular activities of tumors and normal tissues. Possible uses for molecular imaging include better localization of tumor regions and early assay for the radiation response of tumors and normal tissues. Critical to the success of this approach is the identification and validation of molecular probes that are suitable in the radiotherapy context. Recent developments in molecular-imaging probes and integration of functional imaging with radiotherapy are promising. This review focuses on recent advances in molecular imaging strategies and probes that may aid in improving the efficacy of radiotherapy.

Non-invasive genetic imaging for molecular and cell therapies of cancer

Gene therapy is a very attractive strategy in experimental cancer therapy. Ideally, the approach aims to deliver therapeutic genes selectively to cancer cells. However, progress in the improvement of gene therapy formulations has been hampered by difficulties in measuring transgene delivery and in quantifying transgene expression in vivo. In clinical trials, endpoints rely almost exclusively on the analysis of biopsies, which provide limited information. Non-invasive monitoring of gene delivery and expression is a very attractive approach as it can be repeated over time in the same patient to provide spatiotemporal information on gene expression on a whole body scale. Thus, imaging methods can uniquely provide researchers and clinicians the ability to directly and serially assess morphological, functional and metabolic changes consequent to molecular and cellular based therapies. This review highlights the various methods currently being developed in preclinical models.

Molecular imaging of reporter gene expression in prostate cancer: an overview

Prostate cancer remains an important and growing health problem. Advances in imaging of prostate cancer may help to achieve earlier and more accurate diagnosis and treatment. We review the various strategies using reporter genes for molecular imaging of prostate cancer. These approaches are emerging as valuable tools for monitoring gene expression in laboratory animals and humans. Further development of more sensitive and selective reporters, combined with improvements in detection technology, will consolidate the position of reporter gene imaging as a versatile method for understanding of intracellular biological processes and the underlying molecular basis of prostate cancer, as well as potentially establishing a future role in the clinical management of patients afflicted with this disease.

New approaches for imaging tumour responses to treatment

Tumour responses to treatment are still largely assessed from imaging measurements of reductions in tumour size. However, this can take several weeks to become manifest and in some cases may not occur at all, despite a positive response to treatment. There has been considerable interest, therefore, in non-invasive techniques for imaging tissue function that can give early evidence of response. These can be used in clinical trials of new drugs to give an early indication of drug efficacy, and subsequently in the clinic to select the most effective therapy at an early stage of treatment.

Mammalian cis-reporting plasmid may alter activities due to the derivation of host Escherichia coli strains

Methylation status of CpG dinucleotides in the promoter/regulatory region contributes to regulation of transcriptional activities of downstream genes. Nearly all plasmid vectors used in mammalian cells are generated from transformed Escherichia coli. However, these E. coli hosts may have different DNA methylation activities. For instance E. coli JM109 and DH5alpha contain Dam and Dcm methylases, which are absent in E. coli JM110 and GM2163. It has not been determined whether plasmids propagated from E. coli of different methylation activities result in altered expression in mammalian cells when transient transfection is conducted. In this report, cis-reporting plasmids were tested. When promoter/enhancer of tested plasmids contained several Dam/Dcm sites, the cis-reporting activity was 2 to 3 fold lower for those plasmids isolated from JM109 than from JM110. In contrast, the E. coli-derived methylation had little effect on transcription when the sites of methylation resided in the coding region. These findings suggest that cis-reporting plasmids used in comparative or successive experiments are required to be derived from the E. coli strain of the same methylation status. The plasmid for promoter-transcription factor studies should be Dam/Dcm negative E. coli strain.

Molecular imaging: reporter gene imaging

Non-invasive in-vivo molecular genetic imaging developed over the past decade and predominantly utilises radiotracer (PET, gamma camera, autoradiography), magnetic resonance and optical imaging technology. Molecular genetic imaging has its roots in both molecular biology and cell biology. The convergence of these disciplines and imaging modalities has provided the opportunity to address new research questions, including oncogenesis, tumour maintenance and progression, as well as responses to molecular-targeted therapy. Three different imaging strategies are described: (1) "bio-marker" or "surrogate" imaging; (2) "direct" imaging of specific molecules and pathway activity; (3) "indirect" reporter gene imaging. Examples of each imaging strategy are presented and discussed. Several applications of PET- and optical-based reporter imaging are demonstrated, including signal transduction pathway monitoring, oncogenesis in genetic mouse models, endogenous molecular genetic/biological processes and the response to therapy in animal models of human disease. Molecular imaging studies will compliment established ex-vivo molecular-biological assays that require tissue sampling by providing a spatial and a temporal dimension to our understanding of disease development and progression, as well as response to treatment. Although molecular imaging studies are currently being performed primarily in experimental animals, we optimistically expect they will be translated to human subjects with cancer and other diseases in the near future.

Applications of lentiviral vectors in noninvasive molecular imaging

Noninvasive imaging of molecular-genetic and cellular processes is an effective way to determine the location(s), magnitude, and time variation of action of gene products used for many therapeutic strategies. Lentiviral vectors provide effective means for the delivery, integration, and expression of transgenes in cultured mammalian cells as well as in vivo. Therefore, the combination of lentiviral vector-mediated therapeutic and imaging-targeted reporter gene delivery to various target organs holds promise for the future treatment of diseases. In this chapter, we provide protocols for developing lentiviral vectors that can be utilized for noninvasive monitoring/imaging of reporter gene expression. We have described the procedures to perform cellular assays and animal imaging based on positron emission tomography (PET), optical bioluminescence, and fluorescence reporter genes. The protocols described here are standardized for mouse models, which can also be adapted for other small animal models (e.g., rats).

Noninvasive molecular neuroimaging using reporter genes: part I, principles revisited

SUMMARY

In this first article, we review the basic principles of using reporter genes for molecular imaging of the brain in living subjects. This approach is emerging as a valuable tool for monitoring gene expression in diverse applications in laboratory animals, including the study of gene-targeted and trafficking cells, gene therapies, transgenic animals, and more complex molecular interactions within the central nervous system. Further development of more sensitive and selective reporters, combined with improvements in detection technology, will consolidate the position of in vivo reporter gene imaging as a versatile method for greater understanding of intracellular biologic processes and underlying molecular neuropathology and will potentially establish a future role in the clinical management of patients with neurologic diseases.