In vivo localization of diglycylcysteine-bearing synthetic peptides by nuclear imaging of oxotechnetate transchelation

PET and SPECT

Assessment of gene function following the completion of human genome sequencing may be done using radionuclide imaging procedures. These procedures are needed for the evaluation of genetically manipulated animals or newly designed biomolecules which require a thorough understanding of physiology, biochemistry and pharmacology. The experimental approaches will involve many new technologies, including in-vivo imaging with SPECT and PET. Nuclear medicine procedures may be applied for the determination of gene function and regulation using established and new tracers or using in-vivo reporter genes, such as genes encoding enzymes, receptors, antigens or transporters. Visualization of in-vivo reporter gene expression can be done using radiolabeled substrates, antibodies or ligands. Combinations of specific promoters and in-vivo reporter genes may deliver information about the regulation of the corresponding genes. Furthermore, protein-protein interactions and the activation of signal transduction pathways may be visualized noninvasively. The role of radiolabeled antisense molecules for the analysis of mRNA content has to be investigated. However, possible applications are therapeutic interventions using triplex oligonucleotides with therapeutic isotopes, which can be brought near to specific DNA sequences to induce DNA strand breaks at selected loci. After the identification of new genes, functional information is required to investigate the role of these genes in living organisms. This can be done by analysis of gene expression, protein-protein interaction or the biodistribution of new molecules and may result in new diagnostic and therapeutic procedures, which include visualization of and interference with gene transcription, and the development of new biomolecules to be used for diagnosis and treatment. Furthermore, the characterization of tumor cell-specific properties allows the design of new treatment modalities, such as gene therapy, which circumvent resistance mechanisms towards conventional chemotherapeutic drugs.

Breast Cancer Treatment in the Era of Molecular Imaging

Molecular imaging employs molecularly targeted probes to visualize and often quantify distinct disease-specific markers and pathways. Modalities like intravital confocal or multiphoton microscopy, near-infrared fluorescence combined with endoscopy, surface reflectance imaging, or fluorescence-mediated tomography, and radionuclide imaging with positron emission tomography (PET) and single-photon emission computed tomography (SPECT) are increasingly used for small animal high-throughput screening, drug development and testing, and monitoring gene therapy experiments. In the clinical treatment of breast cancer, PET and SPECT as well as magnetic resonance-based molecular imaging are already established for the staging of distant disease and intrathoracic nodal status, for patient selection regarding receptor-directed treatments, and to gain early information about treatment efficacy. In the near future, reporter gene imaging during gene therapy and further spatial and qualitative characterization of the disease can become clinically possible with radionuclide and optical methods. Ultimately, it may be expected that every level of breast cancer treatment will be affected by molecular imaging, including screening.

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

A model epitope-tagged receptor was constructed by fusing the hemagglutinin (HA) sequence on the extracellular N-terminus of the human somatostatin receptor subtype 2 (hSSTr2) gene. This construct was placed in an adenoviral (Ad-HAhSSTr2) vector. This study evaluated Ad-HAhSSTr2 in vitro and in vivo using FACS, fluorescent microscopy, radioactive binding assays, and gamma camera imaging techniques. Infection of A-427 non-small cell lung cancer cells with Ad-HAhSSTr2 or Ad-hSSTr2 resulted in similar expression of hSSTr2 by FACS analysis and binding assays using a (99m)Tc-labeled somatostatin analogue ((99m)Tc-P2045). HAhSSTr2 expression in A-427 cells was specific for infection with Ad-HAhSSTr2. FITC-labeled anti-HA antibody (FITC-HA) confirmed surface expression in live A-427 cells and the absence of internalization. Gamma camera imaging and gamma counter analysis of normal mice showed significantly greater (P<0.05) liver uptake of (99m)Tc-labeled anti-HA antibody ((99m)Tc-anti-HA) in mice injected i.v. 48 h earlier with Ad-HAhSSTr2 (53.6+/-6.9% ID/g) as compared to mice similarly injected with Ad-hSSTr2 (9.0+/-1.3% ID/g). In a mouse tumor model, imaging detected increased tumor localization of (99m)Tc-anti-HA due to direct intratumor injection Ad-HAhSSTr2. Gamma counter analysis confirmed significantly greater (P<0.05) uptake of (99m)Tc-anti-HA in tumors injected with Ad-HAhSSTr2 (12.5+/-4.1% ID/g) as compared to Ad-hSSTr2-infected tumors (5.1+/-1.5% ID/g). These studies demonstrate the feasibility of using an epitope-tagged reporter receptor for non-invasively imaging gene transfer.

Viral imaging in gene therapy noninvasive demonstration of gene delivery and expression

Gene therapy is a rapidly developing modality of treatment, with applications in acquired and inherited disorders. Gene delivery vehicles ("vectors") are the main impediment in the evolution of gene therapy into a clinically acceptable mainstream therapy. Vectors based on viral particles are the most commonly used vehicles to carry genes to the organs and tissues of interest. Despite initial promise and substantial progress in the development of experimental gene therapy protocols, human gene therapy still is based on technologies that so far do not allow for routine clinical use. Recent progress in viral vector production and better understanding of molecular aspects of vector delivery and targeting issues has created the need for imaging techniques that would be useful in addressing the problems and opportunities inherent in viral gene therapy development. Two integral components of gene therapy monitoring, the imaging of gene delivery and the imaging of resultant exogenous gene expression, are recognized. These molecular imaging components provide a realistic means for assessment of safety and efficacy of preclinical and clinical development of gene therapy.

Monitoring gene therapy with reporter gene imaging

Rapid advances in imaging technologies and gene transfer strategies offer a great opportunity to optimize clinical trials of human gene therapy. Reporter genes are emerging as very powerful tools to monitor the delivery, magnitude, and time variation of therapeutic gene transfer in vivo. Several reporter genes, such as the herpes simplex virus type 1 thymidine kinase, the dopamine type 2 receptor, and the somatostatin receptor type 2, are currently being successfully used with gamma camera, single photon emission computed tomography, and positron emission tomography imaging. These reporter genes can be coupled with a therapeutic gene of interest to indirectly monitor the expression of the therapeutic gene. Finally, applications of the reporter gene technology to other areas, such as cell trafficking studies and transgenic animal models, are now possible.

Imaging Techniques for Gene Therapy: SPECT, PET, and MRI

Gene therapy by the transfer and expression of suicide genes is performed using genes coding for nonmammalian enzymes that transform nontoxic prodrugs into toxic metabolites. Employing radiolabeled specific substrates and scintigraphic procedures to determine the functional activity of the recombinant enzyme in vivo, a therapeutic window of maximal gene expression and consecutive drug administration may be defined. If the gene therapy approach is based on the transduction of receptor genes, the recombinant gene expression in tumor cells can be monitored with radiolabeled ligands. Transfer of transporter genes as the sodium iodide transporter may also lead to the visualization of transduction via accumulation of iodide or pertechnetate. Furthermore, imaging based on transchelation of oxotechnetate to a polypeptide motif from a biocompatible complex with a higher dissociation constant than that of a diglycilcysteine complex or tyrosinase gene transfer for metal ion scavenging have been described. In addition, therapy effects may be assessed by the evaluation of the morphological changes of the tumor using magnetic resonance imaging or, more effectively, by the measurement of changes in metabolism with positron emission tomography employing tracers of tumor metabolism and proliferation. Finally, enzyme or receptor genes may serve as noninvasive reporter genes, if applied in the context of bicistronic vectors leading to coexpression of the therapeutic gene and the noninvasive reporter gene.

Molecular imaging

The term molecular imaging can be broadly defined as the in vivo characterization and measurement of biologic processes at the cellular and molecular level. In contradistinction to "classical" diagnostic imaging, it sets forth to probe the molecular abnormalities that are the basis of disease rather than to image the end effects of these molecular alterations. While the underlying biology represents a new arena for many radiologists, concomitant efforts such as development of novel agents, signal amplification strategies, and imaging technologies clearly dovetail with prior research efforts of our specialty. Radiologists will play a leading role in directing developments of this embryonic but burgeoning field. This article presents some recent developments in molecular sciences and medicine and shows how imaging can be used, at least experimentally, to assess specific molecular targets. In the future, specific imaging of such targets will allow earlier detection and characterization of disease, earlier and direct molecular assessment of treatment effects, and a more fundamental understanding of the disease process.

Design of metal-binding green fluorescent protein variants

Diglycylcysteine motifs bind reduced oxo-compounds of technetium-99m, an important isotope in nuclear imaging. We suggested a system for detecting gene expression employing the effect of oxo[99mTc]technetate (Tc(V)O3+) transchelation and coordination with redox amino acid motifs. DNA fragments encoding diglycylcysteine (GGC) binding motifs were prepared by PCR and positioned downstream from the green fluorescent protein (GFP) cDNA insert. Using a Bluescript (+) vector with the fusion protein positioned under the control of a lac promoter, we obtained several E. coli clones expressing the following GFP fusion peptides: (1) GFP-P1 bearing a 'hydrophilic' C-terminal peptide (LEGGGCEGGC) containing two residues of glutamic acid and C-terminal cysteine (2) GFP-P2 carrying a 'hydrophobic' (LGGGGCGGGCGI) peptide (3) a control GFP fusion peptide with deleted C-terminal portion. Bacterial lysates obtained from the corresponding clones were tested for oxo[99mTc] technetate transchelation from a glucoheptonate complex. We found, using a solid phase assay, that radioactivity associated with protein lysates obtained from clones expressing GFP-P2 fusions were 3-4 fold higher than lysates prepared from a clone expressing a truncated GFP fusion protein lacking the C-terminal GGC motifs. High expression of GFP fusions (5-21% of total protein) was demonstrated by electrophoresis and verified by immunoblotting. Specific association of the isotope with GFP-P2 fusion proteins was detected upon incubation of gels in the presence of [99mTc]glucoheptonate, while no binding of oxo[99mTc]technetate to GFP-P1 was revealed. We demonstrated, by using semi-quantitative autoradiography, that there is a 10-fold higher binding of oxotechnetate to GFP-P2 than to a control GFP fusion protein. The implications of the study for in vivo gene expression imaging are discussed.