Non-invasive gamma camera imaging of gene transfer using an adenoviral vector encoding an epitope-tagged receptor as a reporter

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?

Multimodality reporter gene imaging: Construction strategies and application

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

Adenoviral-mediated imaging of gene transfer using a somatostatin receptor-cytosine deaminase fusion protein

Suicide gene therapy is a process by which cells are administered a gene that encodes a protein capable of converting a nontoxic prodrug into an active toxin. Cytosine deaminase (CD) has been widely investigated as a means of suicide gene therapy due to the enzyme’s ability to convert the prodrug 5-fluorocytosine (5-FC) into the toxic compound 5-fluorouracil (5-FU). However, the extent of gene transfer is a limiting factor in predicting therapeutic outcome. The ability to monitor gene transfer, non-invasively, would strengthen the efficiency of therapy. In this regard, we have constructed and evaluated a replication-deficient adenovirus (Ad) containing the human somatostatin receptor subtype 2 (SSTR2) fused with a C-terminal yeast CD gene for the non-invasive monitoring of gene transfer and therapy. The resulting Ad (AdSSTR2-yCD) was evaluated in vitro in breast cancer cells to determine the function of the fusion protein. These studies demonstrated that the both the SSTR2 and yCD were functional in binding assays, conversion assays, and cytotoxicity assays. In vivo studies similarly demonstrated the functionality using conversion assays, biodistribution studies, and small animal positron-emission tomography (PET) imaging studies. In conclusion, the fusion protein has been validated as useful for the non-invasive imaging of yCD expression and will be evaluated in the future for monitoring yCD-based therapy.

The somatostatin analog 188Re-P2045 inhibits the growth of AR42J pancreatic tumor xenografts

P2045 is a peptide analog of somatostatin with picomolar affinity for the somatostatin receptor subtype 2 (SSTR2) upregulated in some pancreatic tumors. Studies were conducted in rat AR42J pancreatic tumor xenograft mice to determine whether (188)Re-P2045 could inhibit the growth of pancreatic cancer in an animal model.

METHODS

(188)Re-P2045 was intravenously administered every 3 d for 16 d to nude mice with AR42J tumor xenografts that were approximately 20 mm(3) at study initiation. Tumor volumes were recorded throughout the dosing period. At necropsy, all tissues were assessed for levels of radioactivity and evaluated for histologic abnormalities. Clinical chemistry and hematology parameters were determined from terminal blood samples. The affinity of nonradioactive (185/187)Re-P2045 for somatostatin receptors was compared in human NCI-H69 and rat AR42J tumor cell membranes expressing predominantly SSTR2.

RESULTS

In the 1.85- and 5.55-MBq groups, tumor growth was inhibited in a dose-dependent fashion. In the 11.1-MBq group, tumor growth was completely inhibited throughout the dosing period and for 12 d after the last administered dose. The radioactivity level in tumors 4 h after injection was 10 percentage injected dose per gram, which was 2-fold higher than in the kidneys. (188)Re-P2045 was well tolerated in all dose groups, with no adverse clinical, histologic, or hematologic findings. The nonradioactive (185/187)Re-P2045 bound more avidly (0.2 nM) to SSTR2 in human than rat tumor membranes, suggesting that these studies are relevant to human studies.

CONCLUSION

(188)Re-P2045 is a promising therapeutic candidate for patients with somatostatin receptor-positive cancer.

Monitoring of biodistribution and persistence of conditionally replicative adenovirus in a murine model of ovarian cancer using capsid-incorporated mCherry and expression of human somatostatin receptor subtype 2 gene

A significant limiting factor to the human clinical application of conditionally replicative adenovirus (CRAd)-based virotherapy is the inability to noninvasively monitor these agents and their potential persistence. To address this issue, we proposed a novel imaging approach that combines transient expression of the human somatostatin receptor (SSTR) subtype 2 reporter gene with genetic labeling of the viral capsid with mCherry fluorescent protein. To test this dual modality system, we constructed the Ad5/3Δ24pIXcherry/SSTR CRAd and validated its capacity to generate fluorescent and nuclear signals in vitro and following intratumoral injection. Analysis of 64Cu-CB-TE2A-Y3-TATE biodistribution in mice revealed reduced uptake in tumors injected with the imaging CRAd relative to the replication-incompetent, Ad-expressing SSTR2 but significantly greater uptake compared to the negative CRAd control. Optical imaging demonstrated relative correlation of fluorescent signal with virus replication as determined by viral genome quantification in tumors. Positron emission tomography/computed tomography studies demonstrated that we can visualize radioactive uptake in tumors injected with imaging CRAd and the trend for greater uptake by standardized uptake value analysis compared to control CRAd. In the aggregate, the plasticity of our dual imaging approach should provide the technical basis for monitoring CRAd biodistribution and persistence in preclinical studies while offering potential utility for a range of clinical applications.

An animal model allowing controlled receptor expression for molecular ultrasound imaging

Reported in this study is an animal model system for evaluating targeted ultrasound (US) contrast agents binding using adenoviral (Ad) vectors to modulate cellular receptor expression. An Ad vector encoding an extracellular hemagglutinin (HA) epitope tag and a green fluorescent protein (GFP) reporter was used to regulate receptor expression. A low and high receptor density (in breast cancer tumor bearing mice) was achieved by varying the Ad dose with a low plaque forming unit (PFU) on day 1 and high PFU on day 2 of experimentation. Targeted US contrast agents, or microbubbles (MB), were created by conjugating either biotinylated anti-HA or IgG isotype control antibodies to the MB surface with biotin-streptavidin linkage. Targeted and control MBs were administered on both days of experimentation and contrast-enhanced US (CEUS) was performed on each mouse using MB flash destruction technique. Signal intensities from MBs retained within tumor vasculature were analyzed through a custom Matlab program. Results showed intratumoral enhancement attributable to targeted MB accumulation was significantly increased from the low Ad vector dosing and the high Ad vector dosing (p = 0.001). Control MBs showed no significant differences between day 1 and day 2 imaging (p = 0.96). Additionally, targeted MBs showed a 10.5-fold increase in intratumoral image intensity on day 1 and an 18.8-fold increase in image intensity on day 2 compared with their control MB counterparts.

Identification of critical residues involved in ligand binding and G protein signaling in human somatostatin receptor subtype 2

G protein signaling through human somatostatin receptor subtype 2 (SSTR2) is well known, but the amino acids involved in stimulation of intracellular responses upon ligand binding have not been characterized. We constructed a series of point mutants in SSTR2 at amino acid positions 89, 139, and 140 in attempts to disrupt G protein signaling upon ligand binding. The aspartic acid changes at position 89 to either Ala, Leu, or Arg generated mutant receptors with varying expression profiles and a complete inability to bind somatostatin-14 (SST). Mutations to Asp 139 and Arg 140 also led to varying expression profiles with some mutants maintaining their affinity for SST. Mutation of Arg 140 to Ala resulted in a mutated receptor that had a B(max) and dissociation constant (K(d)) similar to wild-type receptor but was still coupled to the G protein as determined in both a cAMP assay and a calcium-release assay. In contrast, mutation of Asp 139 to Asn resulted in a mutated receptor with B(max) and K(d) values that were similar to wild type but was uncoupled from G protein-mediated cAMP signaling, but not calcium release. Thus, we identified mutations in SSTR2 that result in either receptor expression levels that are similar to wild type but is completely ablated for ligand binding or a receptor that maintains affinity for SST and is uncoupled from G protein-mediated cAMP signaling.

Positron emission tomography reporter genes and reporter probes: gene and cell therapy applications

Positron emission tomography (PET) imaging reporter genes (IRGs) and PET reporter probes (PRPs) are amongst the most valuable tools for gene and cell therapy. PET IRGs/PRPs can be used to non-invasively monitor all aspects of the kinetics of therapeutic transgenes and cells in all types of living mammals. This technology is generalizable and can allow long-term kinetics monitoring. In gene therapy, PET IRGs/PRPs can be used for whole-body imaging of therapeutic transgene expression, monitoring variations in the magnitude of transgene expression over time. In cell or cellular gene therapy, PET IRGs/PRPs can be used for whole-body monitoring of therapeutic cell locations, quantity at all locations, survival and proliferation over time and also possibly changes in characteristics or function over time. In this review, we have classified PET IRGs/PRPs into two groups based on the source from which they were derived: human or non-human. This classification addresses the important concern of potential immunogenicity in humans, which is important for expansion of PET IRG imaging in clinical trials. We have then discussed the application of this technology in gene/cell therapy and described its use in these fields, including a summary of using PET IRGs/PRPs in gene and cell therapy clinical trials. This review concludes with a discussion of the future direction of PET IRGs/PRPs and recommends cell and gene therapists collaborate with molecular imaging experts early in their investigations to choose a PET IRG/PRP system suitable for progression into clinical trials.

Noninvasive cell-tracking methods

Cell-based therapies, such as adoptive immunotherapy and stem-cell therapy, have received considerable attention as novel therapeutics in oncological research and clinical practice. The development of effective therapeutic strategies using tumor-targeted cells requires the ability to determine in vivo the location, distribution, and long-term viability of the therapeutic cell populations as well as their biological fate with respect to cell activation and differentiation. In conjunction with various noninvasive imaging modalities, cell-labeling methods, such as exogenous labeling or transfection with a reporter gene, allow visualization of labeled cells in vivo in real time, as well as monitoring and quantifying cell accumulation and function. Such cell-tracking methods also have an important role in basic cancer research, where they serve to elucidate novel biological mechanisms. In this Review, we describe the basic principles of cell-tracking methods, explain various approaches to cell tracking, and highlight recent examples for the application of such methods in animals and humans.

Experimental models to study lymphatic and blood vascular metastasis

As a model system for the understanding of human cancer, the mouse has proved immensely valuable. Indeed, studies of mouse models have helped to define the nature of cancer as a genetic disease and demonstrated the causal role of genetic events found in tumors. As an experimental platform, they have provided critical insight into the process of tumor metastasis in the lymphovascular system. Once viewed with skepticism, mouse models are now an integral arm of basic and clinical cancer research. The use of a genetically tractable organism that shares organ systems and an immense degree of genetic similarity to humans provides a means to examine multiple features of human disease. Mouse models enable development and testing of new approaches to disease prevention and treatment, identification of early diagnostic markers and novel therapeutic targets, and an understanding of the in vivo biology and genetics of tumor initiation, promotion, progression, and metastasis. This review summarizes recent mouse models for lymphangiogenesis and the process of lymphovascular metastasis, focusing on the use of the cornea as an experimental platform for lymphangiogenesis in inflammation and immunity, and on the use of molecular and viral vector mediated imaging and to identify and monitor lymph node metastases of prostate cancer.

Noninvasive repetitive imaging of somatostatin receptor 2 gene transfer with positron emission tomography

Noninvasive in vivo imaging of gene expression is desirable to monitor gene transfer in both animal models and humans. Reporter transgenes with low endogenous expression levels are instrumental to this end. The human somatostatin receptor 2 (hSSTR2) has low expression levels in a variety of tissues, including muscle and liver. We tested the possibility of noninvasively and quantitatively monitoring hSSTR2 transgene expression, following adeno-associated viral (AAV) vector-mediated gene delivery to murine muscle and liver by positron emission tomography (PET) using (68)gallium-DOTA-Tyr(3)-Thr(8)-octreotate ((68)Ga-DOTATATE) as a highly specific SSTR2 ligand. Repetitive PET imaging showed hSSTR2 signal up to 6 months, which corresponds to the last time point of the analysis, after gene delivery in both transduced tissues. The levels of tracer accumulation measured in muscle and liver after gene delivery were significantly higher than in control tissues and correlated with the doses of AAV vector administered. As repetitive, quantitative, noninvasive imaging of AAV-mediated SSTR2 gene transfer to muscle and liver is feasible and efficient using PET, we propose this system to monitor the expression of therapeutic genes coexpressed with SSTR2.

SSTR2-based reporters for assessing gene transfer into non-small cell lung cancer: evaluation using an intrathoracic mouse model

The most common cause of cancer-related deaths in North America is lung cancer, 85% of which is non-small cell lung cancer (NSCLC). Gene therapy is a promising approach, but has been hindered by lack of methods for localizing and quantifying gene expression in vivo. Human somatostatin receptor subtype-2 (SSTR2)-based reporters can be used to follow gene expression in vivo using ligands with greater affinity for this subtype. NSCLCs can express SSTR subtypes, which may interfere with SSTR2-based reporters. We assessed whether a SSTR2-based reporter can serve as a reporter of gene transfer into NSCLCs. SSTR subtype expression was assessed in NSCLC cell lines A549, H460, and H1299 using RT-PCR. After infection with an adenovirus containing hemagglutinin-A-tagged-SSTR2 (Ad-HA-SSTR2) or control insert, expression was assessed by immunologic techniques and binding to clinically-approved (111)In-octreotide. In vivo, after magnetic resonance (MR) imaging, intrathoracic H460 tumors were injected with Ad-HA-SSTR2 or control virus (n = 6 mice/group) under ultrasound guidance. Intravenous injection of (111)In-octreotide 2 days later was followed by planar and single-photon emission computed tomography (SPECT) imaging. Biodistribution into tumors was assessed in vivo using anatomic MR and functional gamma-camera images and ex vivo using excised organs/tumors. In human lung tumor samples (n = 70), SSTR2 expression was assessed using immunohistochemistry. All three NSCLC cell lines expressed different SSTR subtypes, but none expressed SSTR2. Upon Ad-HA-SSTR2 infection, HA-SSTR2 expression was seen in all three cell lines using antibodies targeting the HA domain or (111)In-octreotide targeting the receptor domain (p < 0.05). Intrathoracic tumors infected with Ad-HA-SSTR2 were clearly visible by gamma-camera imaging; expression was quantified by both in vivo and ex vivo biodistribution analysis and demonstrated greater uptake in tumors infected with Ad-HA-SSTR2 compared with control virus (p < 0.05). Immunohistochemistry found that 78% of NSCLCs are negative for and 13% have low levels of SSTR2 expression. It is concluded that SSTR2-based reporters can serve as reporters of gene transfer into NSCLCs.

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.

Tumor receptor imaging

Tumor receptors play an important role in carcinogenesis and tumor growth and have been some of the earliest targets for tumor-specific therapy, for example, the estrogen receptor in breast cancer. Knowledge of receptor expression is key for therapy directed at tumor receptors and traditionally has been obtained by assay of biopsy material. Tumor receptor imaging offers complementary information that includes evaluation of the entire tumor burden and characterization of the heterogeneity of tumor receptor expression. The nature of the ligand-receptor interaction poses a challenge for imaging--notably, the requirement for a low molecular concentration of the imaging probe to avoid saturating the receptor and increasing the background because of nonspecific uptake. For this reason, much of the work to date in tumor receptor imaging has been done with radionuclide probes. In this overview of tumor receptor imaging, aspects of receptor biochemistry and biology that underlie tumor receptor imaging are reviewed, with the estrogen-estrogen receptor system in breast cancer as an illustrative example. Examples of progress in radionuclide receptor imaging for 3 receptor systems--steroid receptors, somatostatin receptors, and growth factor receptors-are highlighted, and recent investigations of receptor imaging with other molecular imaging modalities are reviewed.

[Cell tracking. Principles and applications]

Cell based therapies such as stem cell therapies or adoptive immunotherapies are currently being explored as a potential treatment for a variety of diseases such as Parkinson's disease, diabetes or cancer. However, quantitative and qualitative evaluation of adoptively transferred cells is indispensable for monitoring the efficiency of the treatment. Current approaches mostly analyze transferred cells from peripheral blood, which cannot assess whether transferred cells actually home to and stay in the targeted tissue. Using cell-labeling methods such as direct labeling or transfection with a marker gene in conjunction with various imaging modalities (MRI, optical or nuclear imaging), labeled cells can be followed in vivo in real-time, and their accumulation as well as function in vivo can be monitored and quantified accurately. This method is usually referred to as "cell tracking" or "cell trafficking" and is also being applied in basic biological sciences, exemplified in the evaluation of genes contributing to metastasis. This review focuses on principles of this promising methodology and explains various approaches by highlighting recent examples.

Molecular imaging of novel cell- and viral-based therapies

Drugs, surgery, and radiation are the traditional modalities of therapy in medicine. To these are being added new therapies based on cells and viruses or their derivatives. In these novel therapies, a cell or viral vector acts as a drug in its own right, altering the host or a disease process to bring about healing. Most of these advances originate from the significant recent advances in molecular medicine, but some have been around for some time. Blood transfusions and cowpox vaccinations are part of the history of medicine...but nevertheless are examples of cell- and viral-based therapies. This article focuses on the modern molecular incarnations of these therapies, and specifically on how imaging is used to track and guide these novel agents. We survey the literature dealing with imaging these new cell and viral particle therapies and provide a framework for understanding publications in this area. Leading technology of gene modifications are the fundamental modifications applied to make these new therapies amenable to imaging.

Oncolytic vaccinia virus expressing the human somatostatin receptor SSTR2: molecular imaging after systemic delivery using 111In-pentetreotide

Oncolytic vaccinia viruses (VV) have demonstrated tumor specificity, high levels of transgene expression, and anti-tumor effects. The ability to visualize vector biodistribution noninvasively will be necessary as gene therapy vectors come to clinical trials, and the creation of a VV that can both treat tumors and permit noninvasive imaging after systemic delivery is therefore an exciting concept. To facilitate imaging, a VV expressing the human somatostatin receptor type 2 (SSTR2) was created. Cells infected with the SSTR2-expressing VV or controls were incubated with the somatostatin analog 111In-pentetreotide with or without an excess of nonradiolabeled pentetreotide. The SSTR2-infected cells bound 111In-pentetreotide sixfold more efficiently than control virus-infected cells and this binding was specifically blocked by nonradiolabeled pentetreotide. Nude mice bearing subcutaneous murine colon CA xenografts were injected intraperitoneally with the SSTR2-expressing VV or control VV. After 6 days, mice were injected with 111In-pentetreotide and imaged. Mice were sacrificed and organs collected and counted in a gamma counter. The uptake of radioactivity in tumors and normal tissues (percentage injected dose per gram) and tumor-to-normal tissue ratios were determined. Tumors infected with the SSTR2-expressing VV accumulated significantly higher concentrations of radioactivity compared to tumors in animals receiving the control virus. SSTR2-infected tumors were visible on imaging 6 days after VV injection and could be visualized for up to 3 weeks post-viral injection using repeat injections of 111In-pentetreotide. This reporter gene imaging strategy could be a very effective method to visualize vector distribution, expression, and persistence over time and enhances the potential of VV as a novel anti-cancer therapeutic.

Noninvasive imaging of reporter gene expression in living subjects

The development of noninvasive imaging technologies designed specifically for use with small animals has provided new paradigms for cancer research. Traditional molecular biology techniques are being melded with noninvasive imaging technologies to develop a new research domain, "molecular imaging." One of the most exciting advances in this research area is the adaptation and application of conventional reporter-gene imaging techniques, used extensively by cell and molecular biologists, to living animals. Using these new assays, investigators can image noninvasively, repeatedly, and quantitatively the location, magnitude, and duration of reporter-gene expression in living animals. This review will describe the instrumentation used for noninvasive imaging of reporter genes, the reporter genes developed for noninvasive imaging with radio-nuclide-based assays such as positron emission tomography, and the reporter genes used for optically based noninvasive assays using sensitive charged-coupled device cameras. Applications of noninvasive, whole-animal imaging to gene therapy for cancer, to cell-based therapy for cancer, to lymphocyte activation, to cancer progression and dissemination in engrafted models, to tumor initiation, promotion and metastasis in conditional murine models of cancer induction, and to the noninvasive monitoring of tumor responses to a variety of therapies are described. New developments in multimodality molecular imaging are discussed, and the potential utility of noninvasive reporter gene expression in the diagnosis and management of human cancer is presented.