Cytoplasmically retargeted HSV1-tk/GFP reporter gene mutants for optimization of noninvasive molecular-genetic imaging

Next Generation Imaging Techniques to Define Immune Topographies in Solid Tumors

In recent years, cancer immunotherapy experienced remarkable developments and it is nowadays considered a promising therapeutic frontier against many types of cancer, especially hematological malignancies. However, in most types of solid tumors, immunotherapy efficacy is modest, partly because of the limited accessibility of lymphocytes to the tumor core. This immune exclusion is mediated by a variety of physical, functional and dynamic barriers, which play a role in shaping the immune infiltrate in the tumor microenvironment. At present there is no unified and integrated understanding about the role played by different postulated models of immune exclusion in human solid tumors. Systematically mapping immune landscapes or "topographies" in cancers of different histology is of pivotal importance to characterize spatial and temporal distribution of lymphocytes in the tumor microenvironment, providing insights into mechanisms of immune exclusion. Spatially mapping immune cells also provides quantitative information, which could be informative in clinical settings, for example for the discovery of new biomarkers that could guide the design of patient-specific immunotherapies. In this review, we aim to summarize current standard and next generation approaches to define Cancer Immune Topographies based on published studies and propose future perspectives.

ERCC2 Helicase Domain Mutations Confer Nucleotide Excision Repair Deficiency and Drive Cisplatin Sensitivity in Muscle-Invasive Bladder Cancer

PURPOSE

DNA-damaging agents comprise the backbone of systemic treatment for many tumor types; however, few reliable predictive biomarkers are available to guide use of these agents. In muscle-invasive bladder cancer (MIBC), cisplatin-based chemotherapy improves survival, yet response varies widely among patients. Here, we sought to define the role of the nucleotide excision repair (NER) gene ERCC2 as a biomarker predictive of response to cisplatin in MIBC.

EXPERIMENTAL DESIGN

Somatic missense mutations in ERCC2 are associated with improved response to cisplatin-based chemotherapy; however, clinically identified ERCC2 mutations are distributed throughout the gene, and the impact of individual ERCC2 variants on NER capacity and cisplatin sensitivity is unknown. We developed a microscopy-based NER assay to profile ERCC2 mutations observed retrospectively in prior studies and prospectively within the context of an institution-wide tumor profiling initiative. In addition, we created the first ERCC2-deficient bladder cancer preclinical model for studying the impact of ERCC2 loss of function.

RESULTS

We used our functional assay to test the NER capacity of clinically observed ERCC2 mutations and found that most ERCC2 helicase domain mutations cannot support NER. Furthermore, we show that introducing an ERCC2 mutation into a bladder cancer cell line abrogates NER activity and is sufficient to drive cisplatin sensitivity in an orthotopic xenograft model.

CONCLUSIONS

Our data support a direct role for ERCC2 mutations in driving cisplatin response, define the functional landscape of ERCC2 mutations in bladder cancer, and provide an opportunity to apply combined genomic and functional approaches to prospectively guide therapy decisions in bladder cancer.See related commentary by Grivas, p. 907.

Imaging of Sleeping Beauty-Modified CD19-Specific T Cells Expressing HSV1-Thymidine Kinase by Positron Emission Tomography

PURPOSE

We have incorporated a positron emission tomography (PET) functionality in T cells expressing a CD19-specific chimeric antigen receptor (CAR) to non-invasively monitor the adoptively transferred cells.

PROCEDURES

We engineered T cells to express CD19-specific CAR, firefly luciferase (ffLuc), and herpes simplex virus type-1 thymidine kinase (TK) using the non-viral-based Sleeping Beauty (SB) transposon/transposase system adapted for human application. Electroporated primary T cells were propagated on CD19⁺ artificial antigen-presenting cells.

RESULTS

After 4 weeks, 90 % of cultured cells exhibited specific killing of CD19⁺ targets in vitro, could be ablated by ganciclovir, and were detected in vivo by bioluminescent imaging and PET following injection of 2'-deoxy-2'-[¹⁸F]fluoro-5-ethyl-1-β-D-arabinofuranosyl-uracil ([¹⁸F]FEAU).

CONCLUSION

This is the first report demonstrating the use of SB transposition to generate T cells which may be detected using PET laying the foundation for imaging the distribution and trafficking of T cells in patients treated for B cell malignancies.

Enhancement of PSMA-Directed CAR Adoptive Immunotherapy by PD-1/PD-L1 Blockade

Chimeric antigen receptor (CAR) T cell therapy in hematologic malignancies has shown remarkable responses, but the same level of success has not been observed in solid tumors. A new prostate cancer model (Myc-CaP:PSMA(+)) and a second-generation anti-hPSMA human CAR T cells expressing a Click Beetle Red luciferase reporter) were used to study hPSMA targeting and assess CAR T cell trafficking and persistence by bioluminescence imaging (BLI). We investigated the antitumor efficacy of human CAR T cells targeting human prostate-specific membrane antigen (hPSMA), in the presence and absence of the target antigen; first alone and then combined with a monoclonal antibody targeting the human programmed death receptor 1 (anti-hPD1 mAb). PDL-1 expression was detected in Myc-CaP murine prostate tumors growing in immune competent FVB/N and immune-deficient SCID mice. Endogenous CD3⁺ T cells were restricted from the centers of Myc-CaP tumor nodules growing in FVB/N mice. Following anti-programmed cell death protein 1 (PD-1) treatment, the restriction of CD3⁺ T cells was reversed, and a tumor-treatment response was observed. Adoptive hPSMA-CAR T cell immunotherapy was enhanced when combined with PD-1 blockade, but the treatment response was of comparatively short duration, suggesting other immune modulation mechanisms exist and restrict CAR T cell targeting, function, and persistence in hPSMA expressing Myc-CaP tumors. Interestingly, an “inverse pattern” of CAR T cell BLI intensity was observed in control and test tumors, which suggests CAR T cells undergo changes leading to a loss of signal and/or number following hPSMA-specific activation. The lower BLI signal intensity in the hPSMA test tumors (compared with controls) is due in part to a decrease in T cell mitochondrial function following T cell activation, which may limit the intensity of the ATP-dependent Luciferin-luciferase bioluminescence signal.

Comparative Analysis of T Cell Imaging with Human Nuclear Reporter Genes

Monitoring genetically altered T cells is an important component of adoptive T cell therapy in patients, and the ability to visualize their trafficking/targeting, proliferation/expansion, and retention/death using highly sensitive reporter systems that do not induce an immunologic response would provide useful information. Therefore, we focused on human reporter gene systems that have the potential for translation to clinical studies. The objective of the in vivo imaging studies was to determine the minimum number of T cells that could be visualized with the different nuclear reporter systems. We determined the imaging sensitivity (lower limit of T cell detection) of each reporter using appropriate radiolabeled probes for PET or SPECT imaging.

METHODS

Human T cells were transduced with retroviral vectors encoding for the human norepinephrine transporter (hNET), human sodium-iodide symporter (hNIS), a human deoxycytidine kinase double mutant (hdCKDM), and herpes simplex virus type 1 thymidine kinase (hsvTK) reporter genes. After viability and growth were assessed, 10(5) to 3 × 10(6) reporter T cells were injected subcutaneously on the shoulder area. The corresponding radiolabeled probe was injected intravenously 30 min later, followed by sequential PET or SPECT imaging. Radioactivity at the T cell injection sites and in the thigh (background) was measured.

RESULTS

The viability and growth of experimental cells were unaffected by transduction. The hNET/meta-(18)F-fluorobenzylguanidine ((18)F-MFBG) reporter system could detect less than 1 × 10(5) T cells because of its high uptake in the transduced T cells and low background activity. The hNIS/(124)I-iodide reporter system could detect approximately 1 × 10(6) T cells; (124)I-iodide uptake at the T cell injection site was time-dependent and associated with high background. The hdCKDM/2'-(18)F-fluoro-5-ethyl-1-β-d-arabinofuranosyluracil ((18)F-FEAU) and hsvTK/(18)F-FEAU reporter systems detected approximately 3 × 10(5) T cells, respectively. (18)F-FEAU was a more efficient probe (higher uptake, lower background) than (124)I-1-(2-deoxy-2-fluoro-1-d-arabinofuranosyl)-5-iodouracil for both hdCKDM and hsvTK.

CONCLUSION

A comparison of different reporter gene-reporter probe systems for imaging of T cell number was performed, and the hNET/(18)F-MFBG PET reporter system was found to be the most sensitive and capable of detecting approximately 35-40 × 10(3) T cells at the site of T cell injection in the animal model.

Rational design of a triple reporter gene for multimodality molecular imaging

Multimodality imaging using noncytotoxic triple fusion (TF) reporter genes is an important application for cell-based tracking, drug screening, and therapy. The firefly luciferase (fl), monomeric red fluorescence protein (mrfp), and truncated herpes simplex virus type 1 thymidine kinase SR39 mutant (ttksr39) were fused together to create TF reporter gene constructs with different order. The enzymatic activities of TF protein in vitro and in vivo were determined by luciferase reporter assay, H-FEAU cellular uptake experiment, bioluminescence imaging, and micropositron emission tomography (microPET). The TF construct expressed in H1299 cells possesses luciferase activity and red fluorescence. The tTKSR39 activity is preserved in TF protein and mediates high levels of H-FEAU accumulation and significant cell death from ganciclovir (GCV) prodrug activation. In living animals, the luciferase and tTKSR39 activities of TF protein have also been successfully validated by multimodality imaging systems. The red fluorescence signal is relatively weak for in vivo imaging but may expedite FACS-based selection of TF reporter expressing cells. We have developed an optimized triple fusion reporter construct DsRedm-fl-ttksr39 for more effective and sensitive in vivo animal imaging using fluorescence, bioluminescence, and PET imaging modalities, which may facilitate different fields of biomedical research and applications.

A multimodality reporter gene for monitoring transplanted stem cells

INTRODUCTION

The aim of this study is to explore the feasibility of a triple-fused reporter gene, termed TGF [herpes simplex virus type 1 thymidine kinase (HSV1-tk), enhanced green fluorescent protein (eGFP) and firefly luciferase (Fluc)], to monitor stem cells using multimodality molecular imaging.

METHODS

A recombinant adenovirus vector carrying the triple-fused reporter gene (Ad5-TGF) was constructed. Bone marrow mesenchymal stem cells (BMSCs) were transfected with different virus titers of Ad5-TGF [multiplicities of infection (MOIs) were 0, 50, 100, 150, 200 and 250]. The mRNA and protein expressions of HSV1-tk, eGFP and Fluc in the transfected BMSCs were evaluated using polymerase chain reaction and Western blot. After the transfection of the BMSCs with different virus titers of Ad5-TGF (MOIs were 25, 50, 75, 100 and 125), their uptake rates of (131)I-FIAU were measured. Whole-body fluorescence, bioluminescence and micro-positron emission tomography (PET) images were acquired 1 day after the transfected BMSCs were injected into the left forelimb of rats.

RESULTS

After the transfection with different titers of Ad5-TGF, the positive transfection rate reached a peak (70%) when the MOI was 100. HSV1-tk, eGFP and Fluc mRNA and protein were detected in the Ad5-TGF-transfected BMSCs, which implies their successful transfection and expression. The BMSCs uptake of (131)I-FIAU increased with the adenovirus titer and incubation time and reached a plateau (approximately 5.3%) after 3 h. Strong signals were observed in the injected left forearms in the fluorescence, bioluminescence and micro-PET images.

CONCLUSIONS

A triple-fused reporter gene, TGF, can be used as a multifunctional molecular probe for multimodality imaging.

Noninvasive and real-time monitoring of the therapeutic response of tumors in vivo with an optimized hTERT promoter

BACKGROUND

Telomerase is commonly recognized as an effective anticancer target. The human telomerase reverse transcriptase (hTERT), the rate-limiting component of telomerase, is expressed in most malignant tumors, but it is not found in most normal somatic cells. Here, we report a real-time and noninvasive method to monitor tumor response to a lentivirus-based hTERT-conditional suicidal gene therapy.

METHODS

In this study, we constructed a lentivirus system in which an optimized hTERT promoter was used to drive the expression of the cytosine deaminase (CD) gene, one of the suicide genes, and a green fluorescent protein (GFP) reporter gene (pLenti-CD/GFP). The lentivirus was used to infect telomerase-positive or telomerase-negative cell lines. In vitro and in vivo experiments were conducted to analyze the dynamic processes of exogenous gene expression noninvasively in cell culture and living animals in real time via optical imaging.

RESULTS

The lentivirus was able to express the CD gene and GFP in telomerase-positive tumor cells and significantly decrease cell proliferation after the use of prodrug 5-flucytosine. However, it could not express GFP and CD in telomerase-negative cell lines, nor could it induce any suicidal effect in those cells. The in vivo study showed that telomerase-positive tumors can be visualized after intratumor injection of the lentivirus in tumor-bearing nude mice via an optical imaging system. Significant tumor growth suppression was observed in telomerase-positive tumors.

CONCLUSIONS

Collectively, this technology provides a valuable, noninvasive method to evaluate the real-time therapeutic response of tumors in vivo.

Optical probes and the applications in multimodality imaging

Optical imaging essentially refers to in vivo fluorescence imaging and bioluminescence imaging. These types of imaging are widely used visualization methods in biomedical research and are important in molecular imaging. A new generation of imaging agents called multimodal probes have emerged in the past few years. These probes can be detected by two or more imaging modalities, which harnesses the strengths of the different modalities and enables researchers to obtain more information than can be achieved using only one modality. Owing to its low cost and the large number of probes available, the optical method plays an important role in multimodality imaging. In this mini-review, we describe the available multimodal imaging probes for in vivo imaging that combine optical imaging with other modalities.

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.

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.

Combined radionuclide-chemotherapy and in vivo imaging of hepatocellular carcinoma cells after transfection of a triple-gene construct, NIS, HSV1-sr39tk, and EGFP

The sodium iodine symporter (NIS) or mutant Herpes-simplex virus type1 sr39 thymidine kinase (HSV1-sr39tk) gene is used for in vivo imaging and cancer therapy. Transfection of both NIS and HSV1-sr39tk genes to hepatocellular carcinoma cells (Huh-7/NTG) could enhance intracellular accumulation of therapeutic radionuclides and guanosine nucleoside analogue prodrugs to produce better outcomes than single gene therapy. Non-invasive imaging with I-124, F-18 FHBG and combination therapy with I-131 and GCV were performed in hepatocellular carcinoma cells transfected with NIS, HSV1-sr39tk and GFP. Our results show that: (1) all three genes are stably expressed in Huh-7/NTG cells, (2) I-125 and H3-PCV uptake were markedly increased in the Huh-7/NTG cells in vitro, (3) cellular survival and tumor growth of Huh-7/NTG was inhibited by I-131 or GCV both in vitro and in vivo, and was much prominent with combination therapy, (4) in vivo imaging with I-124 and F-18 FHBG revealed increased uptake in the Huh-7/NTG tumor. Our results demonstrated the potential of combination gene therapy using NIS and HSV1-sr39tk followed by radioiodine treatment and chemotherapy in human hepatocellular carcinoma cells.

Real-time imaging of HIF-1alpha stabilization and degradation

HIF-1α is overexpressed in many human cancers compared to normal tissues due to the interaction of a multiplicity of factors and pathways that reflect specific genetic alterations and extracellular stimuli. We developed two HIF-1α chimeric reporter systems, HIF-1α/FLuc and HIF-1α(ΔODDD)/FLuc, to investigate the tightly controlled level of HIF-1α protein in normal (NIH3T3 and HEK293) and glioma (U87) cells. These reporter systems provided an opportunity to investigate the degradation of HIF-1α in different cell lines, both in culture and in xenografts. Using immunofluorescence microscopy, we observed different patterns of subcellular localization of HIF-1α/FLuc fusion protein between normal cells and cancer cells; similar differences were observed for HIF-1α in non-transduced, wild-type cells. A dynamic cytoplasmic-nuclear exchange of the fusion protein and HIF-1α was observed in NIH3T3 and HEK293 cells under different conditions (normoxia, CoCl₂ treatment and hypoxia). In contrast, U87 cells showed a more persistent nuclear localization pattern that was less affected by different growing conditions. Employing a kinetic model for protein degradation, we were able to distinguish two components of HIF-1α/FLuc protein degradation and quantify the half-life of HIF-1α fusion proteins. The rapid clearance component (t_1/2 ∼4–6 min) was abolished by the hypoxia-mimetic CoCl_2, MG132 treatment and deletion of ODD domain, and reflects the oxygen/VHL-dependent degradation pathway. The slow clearance component (t_1/2 ∼200 min) is consistent with other unidentified non-oxygen/VHL-dependent degradation pathways. Overall, the continuous bioluminescence readout of HIF-1α/FLuc stabilization in vitro and in vivo will facilitate the development and validation of therapeutics that affect the stability and accumulation of HIF-1α.

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.

Molecular-genetic PET imaging using an HSV1-tk mutant reporter gene with enhanced specificity to acycloguanosine nucleoside analogs

Imaging 2 different molecular-genetic events in a single subject by PET is essential in a variety of in vivo applications. Using herpes simplex virus-1 thymidine kinase (HSV1-tk) mutants with narrower substrate specificities in combination with wild-type HSV1-tk (wtHSV1-tk) would enable differential imaging with corresponding radiotracers, namely 2'-deoxy-2'-(18)F-fluoro-5-ethyl-1-beta-d-arabinofuranosyl-uracil ((18)F-FEAU) and the acycloguanosine derivative 9-(4-(18)F-fluoro-3-[hydroxymethyl]butyl)guanine ((18)F-FHBG). In this study, we evaluated wtHSV1-tk and the A168H mutant, which has been reported to exhibit enhanced acycloguanosine substrate catalytic activity and diminished pyrimidine phosphorylating activity, as PET reporter genes.

METHODS

Computational analysis was performed to assess the binding mode of FHBG and FEAU to wtHSV1-tk and the A168H variant. U87 cells were stably transduced with wtHSV1-tk or HSV1-tk(A168H) fused with green fluorescent protein and sorted to obtain equivalent transgene expression. In vitro uptake studies were performed to determine rates of substrate accumulation and retention. Nude mice bearing tumors expressing HSV1-tk variants were subsequently imaged using (18)F-FHBG and (18)F-FEAU.

RESULTS

Docking results indicate that binding of FHBG to the A168H variant is unaffected whereas the binding of FEAU is hindered because of a steric clash with the bulkier mutant residues. U87 cells expressing HSV1-tk(A168H) accumulated (18)F-FHBG in in vitro uptake studies at a 3-fold higher rate than did cells expressing wtHSV1-tk without any detectable accumulation of (3)H-FEAU. Furthermore, HSV1-tk(A168H) demonstrated no thymidine phosphorylation activity. In contrast, U87 cells expressing wtHSV1-tk preferentially accumulated (3)H-FEAU at an 18-fold higher rate than they did (18)F-FHBG. Tumors expressing wtHSV1-tk or HSV1-tk(A168H) were distinctly imaged with (18)F-FEAU or (18)F-FHBG, respectively. Hence, tumors expressing HSV1-tk(A168H) accumulated 8.4-fold more (18)F-FHBG than did tumors expressing wtHSV1-tk. In addition, wtHSV1-tk tumors, compared with HSV1-tk(A168H)-expressing tumors (which retained baseline levels of the radiotracer), preferentially accumulated (18)F-FEAU.

CONCLUSION

The FEAU and FHBG substrate discrimination capacity of the wtHSV1-tk and HSV1-tk(A168H) reporter enzymes was validated in vivo by PET of mice with tumor xenografts established from U87 cells expressing these different reporters. Thus, HSV1-tk(A168H) may potentially be used as a second reporter gene in combination with wtHSV1-tk to achieve differential PET.

Imaging mesenchymal stem cell migration and the implications for stem cell-based cancer therapies

Mesenchymal stem cells are promising cellular vehicles for the delivery of therapeutic proteins to sites of cancer growth upon transplantation. To better understand the physiology and biology of the transplanted stem cells, it is necessary and desirable to track the fate of stem cells noninvasively and longitudinally. Reporter gene imaging is a powerful tool to monitor live stem cells in vivo. In this special report, we review currently investigated reporter genes used for tracking stem cells in vivo by optical, radionuclide, magnetic resonance and multimodality imaging techniques. We also discuss the possibility and feasibility of applying reporter gene imaging to monitor stem-cell-based therapeutic gene delivery efficiency and treatment efficacy.

Human breast tumor cells express multimodal imaging reporter genes

OBJECTIVE

Human ZR75-1 cells were among the first few characterized estrogen-dependent mammary gland carcinoma cell lines and had been utilized in various studies for the pro- or antitumor effect of xenoestrogens and antiestrogens. The objective of this study was to establish a breast tumor model in ZR75-1 cells bearing multimodal reporter genes to allow noninvasive imaging of tumor growth using fluorescence and nuclear imaging platforms.

METHODS AND RESULTS

Enhanced green fluorescent protein (eGFP) cDNA was fused at the C-terminus with herpes simplex virus type 1 thymidine kinase (HSV1-tk) to form the fusion reporter gene (eGFP-tk). In vitro proliferation, migration, and invasion assays revealed that eGFP-tk-transfected ZR75-1 cells exhibited decreased proliferation rate, migratory activity, and invasion ability compared to the wild-type cells. The functional HSV1-tk enzymatic activity in stably transfected cells were confirmed by in vitro ganciclovir (GCV) sensitivity and [123I]2-fluoro-2-deoxy-1-beta-D-arabinofuranosyl-5-iodouracil (FIAU) accumulation assays. In vivo fluorescence and nuclear imaging were performed on nude mice bearing multiple subcutaneous xenografts established from ZR75-1-eGFP-tk and wild-type cells. Optical imaging was able to detect the green fluorescence of eGFP-tk tumor. The eGFP-tk reporter gene-specific imaging was achieved by single photon emission computed tomography (SPECT) using [123I]FIAU as a radiotracer and demonstrated decreased FIAU uptake in eGFP-tk tumor by GCV treatment. Probably due to a flare reaction after GCV treatment, micro-positron emission tomography (micro-PET) imaging using 2-deoxy-2-[18F]fluoro-D-glucose (FDG) could not demonstrate decreases in FDG uptake. However, in vitro metabolic assay also revealed that eGFP-tk cells transiently increased [3H]-deoxyglucose uptake in response to GCV treatment.

CONCLUSIONS

This study confirmed the usefulness of eGFP-tk in many applications by providing, in vitro and in vivo, the sensitive and reporter gene-specific imaging. ZR75-1-eGFP-tk cells that are ready to incorporate in various imaging platforms constitute a useful model in breast cancer research.

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.

A fusion PET-MRI system with a high-resolution research tomograph-PET and ultra-high field 7.0 T-MRI for the molecular-genetic imaging of the brain

We have developed a positron emission tomography (PET) and magnetic resonance imaging (MRI) fusion system for the molecular-genetic imaging (MGI) of the in vivo human brain using two high-end imaging devices: the HRRT-PET, a high-resolution research tomograph dedicated to brain imaging on the molecular level, and the 7.0 T-MRI, an ultra-high field version used for morphological imaging. HRRT-PET delivers high-resolution molecular imaging with a resolution down to 2.5 mm full width at half maximum (FWHM), which allows us to observe the brain's molecular changes using the specific reporter genes and probes. On the other front, the 7.0 T-MRI, with submillimeter resolution images of the cortical areas down to 250 mum, allows us to visualize the fine details of the brainstem areas as well as the many cortical and subcortical areas. The new PET-MRI fusion imaging system will provide many answers to the questions on neurological diseases as well as cognitive neurosciences. Some examples of the answers are the quantitative visualization of neuronal functions by clear molecular and genetic bases, as well as diagnoses of many neurological diseases such as Parkinson's and Alzheimer's. The salient point of molecular-genetic imaging and diagnosis is the fact that they precede the morphological manifestations, and hence, the early and specific diagnosis of certain diseases, such as cancers.

Monitoring the efficacy of adoptively transferred prostate cancer-targeted human T lymphocytes with PET and bioluminescence imaging

Noninvasive imaging technologies have the potential to enhance the monitoring and improvement of adoptive therapy with tumor-targeted T lymphocytes. We established an imaging methodology for the assessment of spatial and temporal distributions of adoptively transferred genetically modified human T cells in vivo for treatment monitoring and prediction of tumor response in a systemic prostate cancer model.

METHODS

RM1 murine prostate carcinoma tumors transduced with human prostate-specific membrane antigen (hPSMA) and a Renilla luciferase reporter gene were established in SCID/beige mice. Human T lymphocytes were transduced with chimeric antigen receptors (CAR) specific for either hPSMA or human carcinoembryonic antigen (hCEA) and with a fusion reporter gene for herpes simplex virus type 1 thymidine kinase (HSV1tk) and green fluorescent protein, with or without click beetle red luciferase. The localization of adoptively transferred T cells in tumor-bearing mice was monitored with 2'-(18)F-fluoro-2'-deoxy-1-beta-d-arabinofuranosyl-5-ethyluracil ((18)F-FEAU) small-animal PET and bioluminescence imaging (BLI).

RESULTS

Cotransduction of CAR-expressing T cells with the reporter gene did not affect CAR-mediated cytotoxicity. BLI of Renilla and click beetle red luciferase expression enabled concurrent imaging of adoptively transferred T cells and systemic tumors in the same animal. hPSMA-specific T lymphocytes persisted longer than control hCEA-targeted T cells in lung hPSMA-positive tumors, as indicated by both PET and BLI. Precise quantification of T-cell distributions at tumor sites by PET revealed that delayed tumor progression was positively correlated with the levels of (18)F-FEAU accumulation in tumor foci in treated animals.

CONCLUSION

Quantitative noninvasive monitoring of genetically engineered human T lymphocytes by PET provides spatial and temporal information on T-cell trafficking and persistence. PET may be useful for predicting tumor response and for guiding adoptive T-cell therapy.

A new acycloguanosine-specific supermutant of herpes simplex virus type 1 thymidine kinase suitable for PET imaging and suicide gene therapy for potential use in patients treated with pyrimidine-based cytotoxic drugs

The herpes simplex virus type 1 thymidine kinase (HSV1-tk) gene is widely used as a suicide gene in combination with ganciclovir (GCV) and as a nuclear imaging reporter gene with an appropriate reporter probe. Wild-type HSV1-tk recognizes a variety of pyrimidine and acycloguanosine nucleoside analogs, including clinically used antiviral drugs. PET of HSV1-tk reporter gene expression will be compromised in patients receiving nucleoside-based antiviral treatment. With the use of an acycloguanosine-specific mutant of the enzyme, PET of HSV1-tk reporter gene expression can be successfully performed with acycloguanosine-based radiotracers without interference from pyrimidine-based antiviral drugs.

METHODS

The levels of expression of wild-type HSV1-tk and HSV1-A167Ytk, HSV1-sr39tk, and HSV1-A167Ysr39tk mutants fused with green fluorescent protein (GFP) and transduced into U87 cells were normalized to the mean fluorescence of GFP measured by fluorescence-activated cell sorting. The levels of enzymatic activities of wild-type HSV1-tk and its mutants were compared by 2-h in vitro radiotracer uptake assays with (3)H-2'-fluoro-2'-deoxy-1-beta-d-arabinofuranosyl-5-ethyluracil ((3)H-FEAU), (3)H-pencyclovir ((3)H-PCV), and (3)H-GCV and by drug sensitivity assays. PET with (18)F-FEAU and (18)F-9-[4-fluoro-3-(hydroxymethyl)butyl]guanine ((18)F-FHBG) was performed in mice with established subcutaneous tumors, expressing wild-type HSV1-tk and its mutants, followed by tissue sampling.

RESULTS

FEAU accumulation was not detected in HSV1-A167Ysr39tk-expressing cells and xenografts. Lack of conversion of pyrimidine derivatives by the HSV1-A167Ysr39tk supermutant was also confirmed by a drug sensitivity assay, in which the 50% inhibitory concentrations for thymine 1-beta-d-arabinofuranoside and bromovinyldeoxyuridine were found to be similar to those in nontransduced cells. In contrast, we found that HSV1-A167Ysr39tk could readily phosphorylate (3)H-GCV at levels similar to those of wild-type HSV1-tk and HSV1-A167Ytk but showed enhanced activity with (3)H-PCV in vitro and with (18)F-FHBG in vivo.

CONCLUSION

We developed a new reporter gene, HSV1-A167Ysr39tk, which exhibits specificity and high phosphorylation activity for acycloguanosine derivatives. The resulting supermutant can be used for PET with (18)F-FHBG and suicidal gene therapy protocols with GCV in patients treated with pyrimidine-based cytotoxic drugs.

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.

Imaging of hypoxia-driven gene expression in an orthotopic liver tumor model

The purpose of this study was to monitor hypoxia in an orthotopic liver tumor model using a hypoxia-sensitive reporter imaging system and to image enhanced gene expression after clamping the hepatic artery. C6 and RH7777 Morris hepatoma cells were transduced with a triple reporter gene (HSV1-tk/green fluorescent protein/firefly luciferase-triple fusion), placed under the control of a HIF-1-inducible hypoxia responsive element (HRE). The cells showed inducible luciferase activity and green fluorescent protein expression in vitro. Isolated reporter-transduced Morris hepatoma cells were used to produce tumors in livers of nude rats, and the effect of hepatic artery clamping was evaluated. Tumor hypoxia was shown by immunofluorescence microscopy with the hypoxia marker EF5 [2-(2-nitro-1H-imidazol-1-yl)-N-(2,2,3,3,3-pentafluoropropyl acetamide)] and the fluorescent perfusion marker Hoechst 33342, and by pO(2) electrode measurements. For tumor hypoxia imaging with the HRE-responsive reporter, both luciferase bioluminescence and [(18)F]2'-fluoro-2'-deoxyarabinofuranosyl-5-ethyluracil positron emission tomography was done, and the presence of hypoxia in Morris hepatoma tumors were successfully imaged by both techniques. Transient clamping of the hepatic artery caused cessation of tumor perfusion and severe hypoxia in liver tumors, but not in adjacent liver tissue. These results show that the orthotopic reporter-transduced RH7777 Morris hepatomas are natively hypoxic and poorly perfused in this animal model, and that the magnitude of hypoxia can be monitored using a HRE-responsive reporter system for both bioluminescence and positron emission tomography imaging. However, the severity of tumor ischemia after permanent ligation of the hepatic artery limits our ability to image severe hypoxia in this animal model.

Construction and validation of improved triple fusion reporter gene vectors for molecular imaging of living subjects

Multimodality imaging using several reporter genes and imaging technologies has become an increasingly important tool in determining the location(s), magnitude, and time variation of reporter gene expression in small animals. We have reported construction and validation of several triple fusion genes composed of a bioluminescent, a fluorescent, and a positron emission tomography (PET) reporter gene in cell culture and in living subjects. However, the bioluminescent and fluorescent components of fusion reporter proteins encoded by these vectors possess lesser activities when compared with the bioluminescent and fluorescent components of the nonfusions. In this study, we first created a mutant (mtfl) of a thermostable firefly luciferase (tfl) bearing the peroxisome localization signal to have greater cytoplasmic localization and improved access for its substrate, d-luciferin. Comparison between the three luciferases [mtfl, tfl, and firefly luciferase (fl)] both in cell culture and in living mice revealed that mtfl possessed 6- to 10-fold (in vitro) and 2-fold (in vivo) higher activity than fl. The improved version of the triple fusion vector carrying mtfl as the bioluminescent reporter component showed significantly (P < 0.05) higher bioluminescence than the previous triple fusion vectors. Of the three different red fluorescent reporter genes (jred, hcred, and mrfp1, isolated from jellyfish chromophore, coral Heteractis crispa, and coral Discosoma, respectively) evaluated, mrfp1 was able to preserve highest expression as a component of the triple fusion reporter gene for in vivo fluorescence imaging. A truncated version of wild-type herpes simplex virus 1 (HSV1) thymidine kinase gene (wttk) retained a higher expression level than the truncated mutant HSV1-sr39 TK (ttk) as the third reporter component of this improved triple fusion vector. Multimodality imaging of tumor-bearing mice using bioluminescence and microPET showed higher luciferase activity [(2.7 +/- 0.1 versus 1.9 +/- 0.1) x (10(6) p/s/cm(2)/sr)] but similar level of fluorine-18-labeled 2'-fluoro-2'-deoxyarabinofuranosyl-5-ethyluracil (18F-FEAU) uptake (1.37 +/- 0.15 versus 1.37 +/- 0.2) percentage injected dose per gram] by mtfl-mrfp1-wttk-expressing tumors compared with the fl-mrfp1-wttk-expressing tumors. Both tumors showed 4- to 5-fold higher accumulation (P < 0.05) of 18F-FEAU than fluorine-18-labeled 9-(4-fluoro-3-hydroxymethylbutyl)guanine. This improved triple fusion reporter vector will enable high sensitivity detection of lower numbers of cells from living animals using the combined bioluminescence, fluorescence, and microPET imaging techniques.

Multi-Modality Molecular Imaging of Tumors

Noninvasive in vivo molecular-genetic imaging uses nuclear, magnetic resonance, and optical imaging techniques. Described and discussed are "direct" imaging of specific molecules and pathway activity, "indirect" reporter gene imaging, and "bio-marker" or "surrogate" imaging. Applications of PET- and optical-based reporter imaging are demonstrated, including imaging of oncogenesis in genetic mouse models, endogenous molecular-genetic-biological properties, and response to therapy in animal models of human disease. Molecular imaging studies complement established ex vivo molecular-biological assays that require tissue sampling by providing a spatial as well as temporal dimension to our understanding of oncogenesis, and the progression and treatment of cancer. Molecular imaging studies being performed in experimental animals will be translated to animals in the near future.

Reporter gene imaging: potential impact on therapy

Positron emission tomography (PET)-based molecular-genetic imaging in living organisms has enjoyed exceptional growth over the past 5 years; this is particularly striking since it has been identified as a new discipline only within the past decade. Positron emission tomography is one of three imaging technologies (nuclear, magnetic resonance and optical) that has begun to incorporate methods that are established in molecular and cell biology research. The convergence of these disciplines and the wider application of multi-modality imaging are at the heart of this success story. Most current molecular-genetic imaging strategies are "indirect," coupling a "reporter gene" with a complimentary "reporter probe." Reporter gene constructs can be driven by constitutive promoter elements and used to monitor gene therapy vectors and the efficacy of trans gene targeting and transduction, as well as to monitor adoptive cell-based therapies. Inducible promoters can be used as "sensors" to regulate the magnitude of reporter gene expression and can be used to provide information about endogenous cell processes. Reporter systems can also be constructed to monitor mRNA stabilization and specific protein-protein interactions. Promoters can be cell specific and restrict transgene expression to certain tissue and organs. The translation of reporter gene imaging to specific clinical applications is discussed. Several examples that have potential for patient imaging studies in the near future include monitoring adenoviral-based gene therapy, oncolytic herpes virus therapy, adoptive cell-based therapies and Salmonella-based tumor-targeted cancer therapy and imaging. The primary translational applications of noninvasive in vivo reporter gene imaging are likely to be (a) quantitative monitoring of the gene therapy vector and the efficacy of transduction in clinical protocols, by imaging the location, extent and duration of transgene expression; (b) monitoring cell trafficking, targeting, replication and activation in adoptive therapies, involving ex vivo transduction of harvested immune-competent cells and stem/progenitor cells; (c) assessments of endogenous molecular events using different reporter gene imaging technologies following the development of safe, efficient and target-specific vectors for "diagnostic transductions."

Quality analysis of in vivo near-infrared fluorescence and conventional gamma images acquired using a dual-labeled tumor-targeting probe

The cyclic peptide, cyclopentapeptide cyclo(lys-Arg-Gly-Asp-phe) (c(KRGDf)), which is known to target alpha(v)beta3 integrin, is dual-labeled with a radiotracer, (111)indium, for gamma scintigraphy as well as with a near-infrared dye, IRDye800, for continuous-wave (cw) imaging of alpha(v)beta3 positive human M21 melanoma in xenografts. Twenty-four hours after administration of the dual-labeled peptide at a dose equivalent to 90 microCi of (111)In and 5 nmol of near-infrared (NIR) dye, whole-body gamma scintigraphy and cw imaging was conducted. Image acquisition time was 15 min for the gamma scintigraphy images and 800 ms for the optical images acquired using an NIR sensitive intensified charge-coupled device. The results show that while the target-to-background ratio (TBR) of nuclear and optical imaging were similar for surface regions of interest and consistent with the origin of gamma and NIR radiation from a common targeted peptide, the signal-to-noise ratio (SNR) was significantly higher for optical than nuclear imaging. Furthermore, an analysis of SNR versus contrast showed greater sensitivity of optical over nuclear imaging for the subcutaneous tumor targets. While tomographic reconstructions are necessary to probe TBR, SNR, and contrast for interior tissues, this work demonstrates for the first time the direct comparison of molecular optical and planar nuclear imaging for surface and subsurface cancers.

In vivo molecular-genetic imaging: multi-modality nuclear and optical combinations

Multi-modality, noninvasive in vivo imaging is increasingly being used in molecular-genetic studies and will soon become the standard approach for reporter gene imaging studies in small animals. The coupling of nuclear and optical reporter genes, as described here, represents only the beginning of a far wider application of this technology in the future. Optical imaging and optical reporter systems are cost-effective and time-efficient; they require less resources and space than PET or MRI, and are particularly well suited for imaging small animals, such as mice. Optical reporter systems are also very useful for the quantification and selection of transduced cells using FACS, and for performing in vitro assays to validate the function and sensitivity of constitutive and specific-inducible reporter systems. However, optical imaging techniques are limited by depth of light penetration and do not yet provide optimal quantitative or tomographic information. These issues are not limiting for PET- or MRI-based reporter systems, and PET- and MRI-based animal studies are more easily generalized to human applications. Many of the shortcomings of each modality alone can be overcome by the use of dual- or triple-modality reporter constructs that incorporate the opportunity for PET, fluorescence and bioluminescence imaging.

In vivo imaging of MLC2v-luciferase, a cardiac-specific reporter gene expression in mice

RATIONALE AND OBJECTIVES

A reporter or marker gene that is detectable by in vivo imaging permits longitudinal monitoring of certain fundamental biological processes (eg, differentiation) within the context of physiologically authentic environments. Tissue-specific expression of a reporter gene can be achieved when it is under the transcriptional control of a tissue-specific promoter. The objective of this study was to construct a plasmid vector containing firefly luciferase (Fluc) marker gene downstream of the promoter sequence of rat ventricular myosin light chain 2 (MLC2v); to detect the in vivo expression of this cardiac-specific reporter (MLC2v-Fluc) in the mouse heart by bioluminescent imaging; and to correlate the bioluminescent signal with postmortem luminometer assay.

MATERIALS AND METHODS

MLC2v-Fluc plasmid was generated by molecular cloning of 3 kb promoter sequence into a pGL3-Basic vector containing the Fluc reporter. Twenty microg of MLC2v-Fluc plasmid DNA in phosphate-buffered saline was directly injected into mouse myocardium through a midline sternotomy.

RESULTS

At 1 week after injection, MLC2v-Fluc expression was detected by in vivo bioluminescent imaging in 60% of injected animals; the average in vivo signal intensity was (1.5 +/- 0.6) x 10(4) radiance (p/sec/cm2/sr); in vivo signal was well above the detection threshold over 3 weeks after injection. In vivo bioluminescent signal is correlated (r2 = 0.8) with the luminometer assay results from homogenized heart samples.

CONCLUSION

The capability of noninvasive imaging of the MLC2v-Fluc in the heart will encourage applications that aim at monitoring and tracking the marker gene expression over time in cells undergoing cardiac differentiation.

A novel triple-modality reporter gene for whole-body fluorescent, bioluminescent, and nuclear noninvasive imaging

Two genetic reporter systems were developed for multimodality reporter gene imaging of different molecular-genetic processes using fluorescence, bioluminescence (BLI), and nuclear imaging techniques. The eGFP cDNA was fused at the N-terminus with HSV1-tk cDNA bearing a nuclear export signal from MAPKK (NES-HSV1-tk) or with truncation at the N-terminus of the first 45 amino acids (Delta45HSV1-tk) and with firefly luciferase at the C-terminus. A single fusion protein with three functional subunits is formed following transcription and translation from a single open reading frame. The NES-TGL (NES-TGL) or Delta45HSV1-tk/GFP/luciferase (Delta45-TGL) triple-fusion gene cDNAs were cloned into a MoMLV-based retrovirus, which was used for transduction of U87 human glioma cells. The integrity, fluorescence, bioluminescence, and enzymatic activity of the TGL reporter proteins were assessed in vitro. The predicted molecular weight of the fusion proteins (~130 kDa) was confirmed by western blot. The U87-NES-TGL and U87-Delta45-TGL cells had cytoplasmic green fluorescence. The in vitro BLI was 7- and 13-fold higher in U87-NES-TGL and U87-Delta45-TGL cells compared to nontransduced control cells. The Ki of (14)C-FIAU was 0.49+/-0.02, 0.51+/-0.03, and 0.003+/-0.001 ml/min/g in U87-NES-TGL, U87-Delta45-TGL, and wild-type U87 cells, respectively. Multimodality in vivo imaging studies were performed in nu/ nu mice bearing multiple s.c. xenografts established from U87-NES-TGL, U87-Delta45-TGL, and wild-type U87 cells. BLI was performed after administration of d-luciferin (150 mg/kg i.v.). Gamma camera or PET imaging was conducted at 2 h after i.v. administration of [(131)I]FIAU (7.4 MBq/animal) or [(124)I]FIAU (7.4 MBq/animal), respectively. Whole-body fluorescence imaging was performed in parallel with the BLI and radiotracer imaging studies. In vivo BLI and gamma camera imaging showed specific localization of luminescence and radioactivity to the TGL transduced xenografts with background levels of activity in the wild-type xenografts. Tissue sampling yielded values of 0.47%+/-0.08%, 0.86%+/-0.06%, and 0.03%+/-0.01%dose/g [(131)I]FIAU in U87-NES-TGL, U87-Delta45-TGL, and U87 xenografts, respectively. The TGL triple-fusion reporter gene preserves the functional activity of its subunits and is very effective for multimodality imaging. It provides for the seamless transition from fluorescence microscopy and FACS to whole-body bioluminescence imaging, to nuclear (PET, SPET, gamma camera) imaging, and back to in situ fluorescence image analysis.