Syntheses and discovery of a novel class of cinnamic hydroxamates as histone deacetylase inhibitors by multimodality molecular imaging in living subjects

Histone deacetylases (HDAC) that regulate gene expression are being explored as cancer therapeutic targets. In this study, we focused on HDAC6 based on its ability to inhibit cancerous Hsp90 chaperone activities by disrupting Hsp90/p23 interactions. To identify novel HDAC6 inhibitors, we used a dual-luciferase reporter system in cell culture and living mice by bioluminescence imaging (BLI). On the basis of existing knowledge, a library of hydrazone compounds was generated for screening by coupling cinnamic hydroxamates with aldehydes and ketones. Potency and selectivity were determined by in vitro HDAC profiling assays, with further evaluation to inhibit Hsp90(α/β)/p23 interactions by BLI. In this manner, we identified compound 1A12 as a dose-dependent inhibitor of Hsp90(α/β)/p23 interactions, UKE-1 myeloid cell proliferation, p21(waf1) upregulation, and acetylated histone H3 levels. 1A12 was efficacious in tumor xenografts expressing Hsp90(α)/p23 reporters relative to carrier control-treated mice as determined by BLI. Small animal (18)F-FDG PET/CT imaging on the same cohort showed that 1A12 also inhibited glucose metabolism relative to control subjects. Ex vivo analyses of tumor lysates showed that 1A12 administration upregulated acetylated-H3 by approximately 3.5-fold. Taken together, our results describe the discovery and initial preclinical validation of a novel selective HDAC inhibitor.

Light in and sound out: emerging translational strategies for photoacoustic imaging

Photoacoustic imaging (PAI) has the potential for real-time molecular imaging at high resolution and deep inside the tissue, using nonionizing radiation and not necessarily depending on exogenous imaging agents, making this technique very promising for a range of clinical applications. The fact that PAI systems can be made portable and compatible with existing imaging technologies favors clinical translation even more. The breadth of clinical applications in which photoacoustics could play a valuable role include: noninvasive imaging of the breast, sentinel lymph nodes, skin, thyroid, eye, prostate (transrectal), and ovaries (transvaginal); minimally invasive endoscopic imaging of gastrointestinal tract, bladder, and circulating tumor cells (in vivo flow cytometry); and intraoperative imaging for assessment of tumor margins and (lymph node) metastases. In this review, we describe the basics of PAI and its recent advances in biomedical research, followed by a discussion of strategies for clinical translation of the technique.

MicroRNA-regulated non-viral vectors with improved tumor specificity in an orthotopic rat model of hepatocellular carcinoma

In hepatocellular carcinoma (HCC), tumor specificity of gene therapy is of utmost importance to preserve liver function. MicroRNAs (miRNAs) are powerful negative regulators of gene expression and many are downregulated in human HCC. We identified seven miRNAs that are also downregulated in tumors in a rat hepatoma model (P<0.05) and attempted to improve tumor specificity by constructing a panel of luciferase-expressing vectors containing binding sites for these miRNAs. Attenuation of luciferase expression by the corresponding miRNAs was confirmed across various cell lines and in mouse liver. We then tested our vectors in tumor-bearing rats and identified two miRNAs, miR-26a and miR-122, that significantly decreased expression in liver compared with the control vector (6.40 and 0.26%, respectively; P<0.05). In tumor, miR-122 had a nonsignificant trend towards decreased (∼50%) expression, whereas miR-26 had no significant effect on tumor expression. To our knowledge, this is the first work using differentially expressed miRNAs to de-target transgene expression in an orthotopic hepatoma model and to identify miR-26a, in addition to miR-122, for de-targeting liver. Considering the heterogeneity of miRNA expression in human HCC, this information will be important in guiding development of more personalized vectors for the treatment of this devastating disease.

Gold nanoparticles: a revival in precious metal administration to patients

Gold has been used as a therapeutic agent to treat a wide variety of rheumatic diseases including psoriatic arthritis, juvenile arthritis, and discoid lupus erythematosus. Although the use of gold has been largely superseded by newer drugs, gold nanoparticles are being used effectively in laboratory based clinical diagnostic methods while concurrently showing great promise in vivo either as a diagnostic imaging agent or a therapeutic agent. For these reasons, gold nanoparticles are therefore well placed to enter mainstream clinical practice in the near future. Hence, the present review summarizes the chemistry, pharmacokinetics, biodistribution, metabolism, and toxicity of bulk gold in humans based on decades of clinical observation and experiments in which gold was used to treat patients with rheumatoid arthritis. The beneficial attributes of gold nanoparticles, such as their ease of synthesis, functionalization, and shape control are also highlighted demonstrating why gold nanoparticles are an attractive target for further development and optimization. The importance of controlling the size and shape of gold nanoparticles to minimize any potential toxic side effects is also discussed.

Molecular imaging: current status and emerging strategies

In vivo molecular imaging has a great potential to impact medicine by detecting diseases in early stages (screening), identifying extent of disease, selecting disease- and patient-specific treatment (personalized medicine), applying a directed or targeted therapy, and measuring molecular-specific effects of treatment. Current clinical molecular imaging approaches primarily use positron-emission tomography (PET) or single photon-emission computed tomography (SPECT)-based techniques. In ongoing preclinical research, novel molecular targets of different diseases are identified and, sophisticated and multifunctional contrast agents for imaging these molecular targets are developed along with new technologies and instrumentation for multi-modality molecular imaging. Contrast-enhanced molecular ultrasound (US) with molecularly-targeted contrast microbubbles is explored as a clinically translatable molecular imaging strategy for screening, diagnosing, and monitoring diseases at the molecular level. Optical imaging with fluorescent molecular probes and US imaging with molecularly-targeted microbubbles are attractive strategies as they provide real-time imaging, are relatively inexpensive, produce images with high spatial resolution, and do not involve exposure to ionizing irradiation. Raman spectroscopy/microscopy has emerged as a molecular optical imaging strategy for ultrasensitive detection of multiple biomolecules/biochemicals with both in vivo and ex vivo versatility. Photoacoustic imaging is a hybrid of optical and US techniques involving optically-excitable molecularly-targeted contrast agents and quantitative detection of resulting oscillatory contrast agent movement with US. Current preclinical findings and advances in instrumentation, such as endoscopes and microcatheters, suggest that these molecular imaging methods have numerous potential clinical applications and will be translated into clinical use in the near future.

Indirect imaging of cardiac-specific transgene expression using a bidirectional two-step transcriptional amplification strategy

Transcriptional targeting for cardiac gene therapy is limited by the relatively weak activity of most cardiac-specific promoters. We have developed a bidirectional plasmid vector, which uses a two-step transcriptional amplification (TSTA) strategy to enhance the expression of two optical reporter genes, firefly luciferase (fluc) and Renilla luciferase (hrluc), driven by the cardiac troponin T (cTnT) promoter. The vector was characterized in vitro and in living mice using luminometry and bioluminescence imaging to assess its ability to mediate strong, correlated reporter gene expression in a cardiac cell line and the myocardium, while minimizing expression in non-cardiac cell lines and the liver. In vitro, the TSTA system significantly enhanced cTnT-mediated reporter gene expression with moderate preservation of cardiac specificity. After intramyocardial and hydrodynamic tail vein delivery of an hrluc-enhanced variant of the vector, long-term fluc expression was observed in the heart, but not in the liver. In both the cardiac cell line and the myocardium, fluc expression correlated well with hrluc expression. These results show the vector's ability to effectively amplify and couple transgene expression in a cardiac-specific manner. Further replacement of either reporter gene with a therapeutic gene should allow non-invasive imaging of targeted gene therapy in living subjects.

Development of a breast cancer brain metastases model to study 131I radioablative therapy

Abstract #2011

Background: An increasing number of women develop brain metastases (BM) after breast cancer (BC) treatment. A large proportion of these are estrogen/progesterone receptor-negative (ER-/PR-) and/or Her-2/neu overexpressing tumors. 131I radioablative therapy may provide a therapeutic alternative to treat metastases at this anatomic sanctuary since over 70% of invasive breast cancers, including a majority of ER- tumors and some brain metastases (unpublished data) express the sodium-iodide symporter (NIS). This approach relies on the success of radioiodide as a targeted treatment for thyroid cancers. To test this concept, we developed a BC BM model using tumor cells engineered to express NIS.&#x2028; Methods: MDAMB231 and SKBr3 cell lines were transduced with a lentiviral vector carrying a bicistronic cassette with NIS and the firefly luciferase (Fluc) genes separated by an internal ribosomal entry site. Single cell clones were selected and characterized for iodide uptake and bioluminescence. NIS-Fluc-MDAMB231 or NIS-Fluc SKBr3 cells (2.5 x 106 cells) were implanted subcutaneously (sc) in the mammary fat pad (mfp) of nude mice (NCr nude; 5-6 weeks old; n=5). NIS-Fluc mfp tumor xenografts were then explanted, 1x1 mm pieces excised and inserted stereotactically into the basal ganglia of the animal. All tumor development was monitored by serial in vivo bioluminescent imaging. Once established, brain tumors were excised, dissociated, established in tissue culture and re-implanted sc in the mfp of a new set of mice. Successive passages in the mfp then in the brain were performed in an attempt to increase tumor take. A second strategy tested with MDAMB231 cells consisted of direct implantation of cells into the basal ganglia. NIS expression was evaluated on tissue sections with a polyclonal antibody raised against the C-terminus of the human NIS.&#x2028; Results: All mice survived and were healthy in appearance. Intracranial implantation of mfp xenografts was highly successful with 66% take in both MDAMB231 (after two passages) and SKBr3 (after first passage). Bioluminescent imaging revealed sustained growth of tumors for more than 4 weeks. Microscopically, the explanted brain tumors had a cellular appearance without stromal cell or lymphocytic infiltration and were congruent with the histology of mfp xenografts. However, the tumor cell population was heterogeneous as NIS expression was present with plasma membrane staining in about 50% of SKBr3 and 15% of MDAMB231 cells. Direct cell implantations failed as no discernible bioluminescence was noted over a period of 3 weeks and no visible tumor at necropsy.&#x2028; Conclusions: A BCBM model has been developed by implanting intracranially mfp xenografts obtained with ER-/PR- +/- Her-2/neu overexpressing cells. Using this model, it will be possible to evaluate the effects of 131I on NIS-expressing BCBM.

Citation Information: Cancer Res 2009;69(2 Suppl):Abstract nr 2011.

Human adipose tissue-derived mesenchymal stromal cells as vehicles for tumor bystander effect: a model based on bioluminescence imaging

Human adipose tissue mesenchymal stromal cells (AMSCs) share common traits, including similar differentiation potential and cell surface markers, with their bone marrow counterparts. Owing to their general availability, higher abundance and ease of isolation AMSCs may be convenient autologous delivery vehicles for localized tumor therapy. We demonstrate a model for tumor therapy development based on the use of AMSCs expressing renilla luciferase and thymidine kinase, as cellular vehicles for ganciclovir-mediated bystander killing of firefly luciferase expressing tumors, and noninvasive bioluminescence imaging to continuously monitor both, tumor cells and AMSCs. We show that the therapy delivering AMSCs survive long time within tumors, optimize the ratio of AMSCs to tumor cells for therapy, and asses the therapeutic effect in real time. Treatment of mice bearing prostate tumors plus therapeutic AMSCs with the prodrug ganciclovir induced bystander killing effect, reducing the number of tumor cells to 1.5 % that of control tumors. Thus, AMSCs could be useful vehicles to deliver localized therapy, with potential for clinical application in inoperable tumors and surgical borders after tumor resection. This approach, useful to evaluate efficiency of therapeutic models, should facilitate the selection of cell types, dosages, therapeutic agents and treatment protocols for cell-based therapies of specific tumors.

Molecular imaging of phosphorylation events for drug development

PURPOSE

Protein phosphorylation mediated by protein kinases controls numerous cellular processes. A genetically encoded, generalizable split firefly luciferase (FL)-assisted complementation system was developed for noninvasive monitoring phosphorylation events and efficacies of kinase inhibitors in cell culture and in small living subjects by optical bioluminescence imaging.

PROCEDURES

An Akt sensor (AST) was constructed to monitor Akt phosphorylation and the effect of different PI-3K and Akt inhibitors. Specificity of AST was determined using a non-phosphorylable mutant sensor containing an alanine substitution (ASA).

RESULTS

The PI-3K inhibitor LY294002 and Akt kinase inhibitor perifosine led to temporal- and dose-dependent increases in complemented FL activities in 293T human kidney cancer cells stably expressing AST (293T/AST) but not in 293T/ASA cells. Inhibition of endogenous Akt phosphorylation and kinase activities by perifosine also correlated with increase in complemented FL activities in 293T/AST cells but not in 293T/ASA cells. Treatment of nude mice bearing 293T/AST xenografts with perifosine led to a 2-fold increase in complemented FL activities compared to that of 293T/ASA xenografts. Our system was used to screen a small chemical library for novel modulators of Akt kinase activity.

CONCLUSION

This generalizable approach for noninvasive monitoring of phosphorylation events will accelerate the discovery and validation of novel kinase inhibitors and modulators of phosphorylation events.

Noninvasive Raman spectroscopy in living mice for evaluation of tumor targeting with carbon nanotubes

An optimized noninvasive Raman microscope was used to evaluate tumor targeting and localization of single walled carbon nanotubes (SWNTs) in mice. Raman images were acquired in two groups of tumor-bearing mice. The control group received plain-SWNTs, whereas the experimental group received tumor targeting RGD-SWNTs intravenously. Raman imaging commenced over the next 72 h and revealed increased accumulation of RGD-SWNTs in tumor ( p < 0.05) as opposed to plain-SWNTs. These results support the development of a new preclinical Raman imager.

Configurations of a two-tiered amplified gene expression system in adenoviral vectors designed to improve the specificity of in vivo prostate cancer imaging

Effective treatment for recurrent, disseminated prostate cancer is notably limited. We have developed adenoviral vectors with a prostate-specific two-step transcriptional amplification (TSTA) system that would express therapeutic genes at a robust level to target metastatic disease. The TSTA system employs the prostate-specific antigen (PSA) promoter/enhancer to drive a potent synthetic activator, which in turn activates the expression of the therapeutic gene. In this study, we explored different configurations of this bipartite system and discovered that physical separation of the two TSTA components into E1 and E3 regions of adenovirus was able to enhance androgen regulation and cell-discriminatory expression. The TSTA vectors that express imaging reporter genes were assessed by noninvasive imaging technologies in animal models. The improved selectivity of the E1E3 configured vector was reflected in silenced ectopic expression in the lung. Significantly, the enhanced specificity of the E1E3 vector enabled the detection of lung metastasis of prostate cancer. An E1E3 TSTA vector that expresses the herpes simplex virus thymidine kinase gene can effectively direct positron emission tomography (PET) imaging of the tumor. The prostate-targeted gene delivery vectors with robust and cell-specific expression capability will advance the development of safe and effective imaging guided therapy for recurrent metastatic stages of prostate cancer.

Noninvasive molecular neuroimaging using reporter genes: part II, experimental, current, and future applications

SUMMARY

In this second article, we review the various strategies and applications that make use of reporter genes for molecular imaging of the brain in living subjects. These approaches are emerging as valuable tools for monitoring gene expression in diverse applications in laboratory animals, including the study of gene-targeted and trafficking cells, gene therapies, transgenic animals, and more complex molecular interactions within the central nervous system. Further development of more sensitive and selective reporters, combined with improvements in detection technology, will consolidate the position of in vivo reporter gene imaging as a versatile technique for greater understanding of intracellular biologic processes and underlying molecular neuropathology and will potentially establish a future role in the clinical management of patients with neurologic diseases.

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

SUMMARY

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

Initial evaluation of 18F-fluorothymidine (FLT) PET/CT scanning for primary pancreatic cancer

PURPOSE

The aim of this study was to evaluate the potential of (18)F-fluorothymidine (FLT) PET/CT for imaging pancreatic adenocarcinoma.

METHODS

This was a pilot study of five patients (four males, one female) with newly diagnosed and previously untreated pancreatic adenocarcinoma. Patients underwent FLT PET/CT, (18)F-fluorodeoxyglucose (FDG) PET/CT, and contrast-enhanced CT scanning before treatment. The presence of cancer was confirmed by histopathological analysis at the time of scanning in all five patients. The degree of FLT and FDG uptake at the primary tumor site was assessed using visual interpretation and semi-quantitative SUV analyses.

RESULTS

The primary tumor size ranged from 2.5 x 2.8 cm to 3.5 x 7.0 cm. The SUV of FLT uptake within the primary tumor ranged from 2.1 to 3.1. Using visual interpretation, the primary cancer could be detected from background activity in two of five patients (40%) on FLT PET/CT. By comparison, FDG uptake was higher in each patient with a SUV range of 3.4 to 10.8, and the primary cancer could be detected from background in all five patients (100%).

CONCLUSIONS

In this pilot study of five patients with primary pancreatic adenocarcinoma, FLT PET/CT scanning showed poor lesion detectability and relatively low levels of radiotracer uptake in the primary tumor.

2-deoxy-2-[F-18]fluoro-D-glucose accumulation in ovarian carcinoma cell lines

PURPOSE

To evaluate 2-deoxy-2-[F-18]fluoro-D-glucose (FDG) accumulation in human ovarian carcinoma cell lines compared with control tumor cell lines known to accumulate FDG.

PROCEDURES

FDG accumulation assays were performed in 15 different ovarian carcinoma cell lines at 1, 2, and 3 hours after incubation with 1 microCi of FDG. Results were compared with FDG accumulation in six different control tumor cell lines. 2-deoxy-2-[F-18]fluoro-D-glucose accumulation was expressed as counts per minute (cpm) in cells and normalized to initial cpm in medium and total protein content of cell lysates.

RESULTS

FDG accumulation in all 15 ovarian carcinoma cell lines was equal to or higher than 0.0005 +/- 8.6 10(-5) cpm in cells/cpm in medium/mug protein at all three different time points. In two ovarian carcinoma cell lines (ES-2, poorly differentiated clear cell carcinoma, and OVCAR-3, poorly differentiated papillary adenocarcinoma), FDG accumulation was not statistically, significantly different compared to the control cell line with the highest FDG accumulation (LS 174T human colorectal adenocarcinoma) at two or more time points (P > or = 0.07). In 2 of 15 (13%) ovarian carcinoma cell lines (OVCAR5 epithelial carcinoma and SKOV3 clear cell carcinoma), FDG accumulation was lower than that in the control cell line with the lowest FDG accumulation (HT-29 human colorectal adenocarcinoma) at one or more time points (P < 0.05).

CONCLUSIONS

Most human ovarian carcinoma cell lines showed comparable FDG accumulations with control cell lines known to accumulate FDG. This study lays the foundations for further comparisons with other ovarian cancer cell lines and for other positron emission tomography tracers.

2-Deoxy-2-[F-18]fluoro-d-glucose Positron Emission Tomography/Computed Tomography in the Management of Melanoma

OBJECTIVES

2-Deoxy-2-[F-18]fluoro-D-glucose (FDG)-positron emission tomography (PET)/computed tomography (CT) is widely available as a powerful imaging modality, combining the ability to detect active metabolic processes and their morphologic features in a single exam. The role of FDG-PET is proven in a variety of cancers, including melanoma, but the estimates of sensitivity and specificity are based in the majority of the published studies on dedicated PET, not PET/CT. Therefore, we were prompted to review our experience with FDG-PET/CT in the management of melanoma.

METHODS

This is a retrospective study on 106 patients with melanoma (20-87 years old; average: 56.8 +/- 15.9), who had whole-body FDG-PET/CT at our institution from January 2003 to June 2005. Thirty-eight patients (35.9%) were women and 68 patients (64.1%) were men. Reinterpretation of the imaging studies for accuracy and data analysis from medical records were performed.

RESULTS

All patients had the study for disease restaging. The primary tumor depth (Breslow's thickness) at initial diagnosis was available for 76 patients (71.7%) and ranged from 0.4 to 25 mm (average: 3.56 mm). The anatomic level of invasion in the skin (Clark's level) was determined for 70 patients (66%): 3, level II; 13, level III; 43, level IV; 11, level V. The administered dose of (18)F FDG ranged from 9.8 to 21.6 mCi (average: 15.4 +/- 1.8 mCi). FDG-PET/CT had a sensitivity of 89.3% [95% confidence interval (CI): 78.5-95] and a specificity of 88% (95% CI: 76.2-94.4) for melanoma detection.

CONCLUSION

This study confirms the good results of FDG-PET/CT for residual/recurrent melanoma detection, as well as for distant metastases localization. PET/CT should be an integral part in evaluation of patients with high-risk melanoma, prior to selection of the most appropriate therapy.

Merkel Cell Carcinoma: Is there a Role for 2-Deoxy-2-[F-18]fluoro-d-glucose-Positron Emission Tomography/Computed Tomography?

PURPOSE

2-Deoxy-2-[F-18]fluoro-D-glucose (FDG)-positron emission tomography (PET)/computed tomography (CT) is becoming widely available as a powerful imaging modality, combining the ability to detect active metabolic processes and their morphologic features in a single study. The role of FDG-PET/CT is proven in lymphoma, melanoma, colorectal carcinoma, and other cancers. However, there are rare malignancies such as Merkel cell carcinoma that can potentially be evaluated with PET/CT. We were therefore prompted to review our experience with FDG-PET/CT in the management of patients with Merkel cell carcinoma.

PROCEDURES

This is a retrospective case series of six patients with Merkel cell carcinoma, 58-81 years old (average 69 +/- 8.3), who had whole-body PET/CT at our institution from January 1st, 2003 to August 31st, 2005. Two patients were women and four were men. Reinterpretation of the imaging studies for accuracy and data analysis from medical records were performed.

RESULTS

Twelve examinations were acquired for the six patients (one patient had six PET/CT, one patient had two PET/CT, and four patients had one PET/CT). The injected FDG doses ranged 381.1-669.7 MBq (average 573.5 +/- 70.3). Four patients had the PET/CT as part of initial staging, and two patients had the exam for restaging (after surgery and XRT). A total of six Merkel lesions (pancreas, adrenal, lip, submandibular lymph nodes, cervical lymph nodes, and parapharyngeal soft tissue) were identified in three patients and confirmed on histopathological examination. The FDG uptake in these areas was intense, with maximum standardized uptake value (SUVmax) values of 5-14 (average 10.4 +/- 3.8). In one patient, the PET/CT scan identified abnormal focal distal sigmoid uptake that was biopsied and diagnosed as adenocarcinoma. Two patients had negative scans and had no clinical evidence of disease on follow-up office visits (up to one year after PET/CT).

CONCLUSIONS

This case series suggests that FDG-PET/CT may have a promising role in the management of patients with Merkel cell carcinoma.

Diagnosis of aseptic deep venous thrombosis of the upper extremity in a cancer patient using fluorine-18 fluorodeoxyglucose positron emission tomography/ computerized tomography (FDG PET/CT)

We describe a patient with a history of recurrent squamous cell carcinoma of the tongue and abnormal FDG uptake in the left arm during a re-staging FDG PET/CT. After revision of the patient's clinical history, tests and physical exam, the abnormal FDG uptake was found to correspond to an extensive aseptic deep venous thrombosis of the upper extremity.

Non-invasive imaging of a transgenic mouse model using a prostate-specific two-step transcriptional amplification strategy

Non-invasive assessment of transgenic animals using bioluminescence imaging offers a rapid means of evaluating disease progression in animal models of disease. One of the challenges in the field is to develop models with robust expression to image repetitively live intact animals through solid tissues. The prostate-specific antigen (PSA) promoter is an attractive model for studying gene regulation due to its hormonal response and tissue-specificity permitting us to measure signaling events that occur within the native tissues. The use of the GAL4-VP16 activator offers a powerful means to augment gene expression levels driven by a weak promoter. We have used a two-step transcriptional amplification (TSTA) system to develop a transgenic mouse model to investigate the tissue-specificity and developmental regulation of firefly luciferase (fl) gene expression in living mice using bioluminescence imaging. We employed an enhanced prostate-specific promoter to drive the yeast transcriptional activator, GAL4-VP16 (effector). The reporter construct carries five Gal4 binding sites upstream of the fl gene. We generated a transgenic mouse model using a single vector carrying the effector and reporter constructs. The transgenic mice show prostate-specific expression as early as three weeks of age. The bioluminescence signal in the prostate is significantly higher than in other organs. We also demonstrate that blocking androgen availability can downregulate the fl expression in the prostate. The transgenic mice display normal physical characteristics and developmental behavior, indicating that the high level of GAL4 driven expression is well tolerated. These findings suggest that the GAL4-VP16 transactivator can be used to amplify reporter gene expression from a relatively weak promoter in a transgenic mouse model. The transgenic TSTA model in conjunction with other transgenic cancer models should also help to detect and track malignancies. The strategies developed will be useful for transgenic research in general by allowing for amplified tissue specific gene expression.

Quantum dots for molecular imaging and cancer medicine

Extract: The past few decades have witnessed technical advances that have introduced cell biologists and physicians to a new, dynamic, subcellular world where genes and gene products can be visualized to interact in space and time and in health and disease. The accelerating field of molecular imaging has been critically dependent on indicator probes which show when and where genetically or biochemically defined molecules, signals or processes appear, interact and disappear, with high spatial and temporal resolution in living cells and whole organisms. For example, the use of radionuclide tracers combined with 3-dimensional (3-D) imaging systems such as Positron Emission Tomography (PET) and Single Photon Emission Computed Tomography (SPECT) are now helping clinicians to characterize the molecular status of tumors deep within patients. Other types of imaging probes rely on the bioluminescence and fluorescence of genetically encoded proteins (originally found in fireflies and jellyfish, respectively) or entirely synthetic fluorochromes, or a combination of both. New powerful biological fluorescence microscopes provide the ability to study single molecules within single cells. Multiphoton confocal microscopy has been developed to allow for the capturing of high-resolution, 3-D images of living tissues that have been tagged with highly specific fluorophores.

Quantum dots for live cells, in vivo imaging, and diagnostics

Research on fluorescent semiconductor nanocrystals (also known as quantum dots or qdots) has evolved over the past two decades from electronic materials science to biological applications. We review current approaches to the synthesis, solubilization, and functionalization of qdots and their applications to cell and animal biology. Recent examples of their experimental use include the observation of diffusion of individual glycine receptors in living neurons and the identification of lymph nodes in live animals by near-infrared emission during surgery. The new generations of qdots have far-reaching potential for the study of intracellular processes at the single-molecule level, high-resolution cellular imaging, long-term in vivo observation of cell trafficking, tumor targeting, and diagnostics.

MicroPET imaging of Cre-loxP-mediated conditional activation of a herpes simplex virus type 1 thymidine kinase reporter gene

Site-specific recombination tools such as the Cre-loxP system are used to create animal models where conditional gene deletion/activation studies are required. In the current proof of principle study, we have demonstrated that a PET reporter gene (PRG), the herpes simplex virus type 1 thymidine kinase (HSV1-tk), can be made to remain silent and can be activated by Cre-loxP-mediated recombination in cell culture and in living mice. An adenovirus carrying a silent HSV1-tk was tail-vein injected (1 x 10(9) PFU) in six transgenic mice that express Cre recombinase in their liver (Cre+) and in four control mice (Cre-). The liver-specific expression of the PRG in Cre+ mice was detected in the microPET following injection of the reporter probe, 9-[4-fluoro-3-(hydroxymethyl)butyl]guanine ([(18)F]-FHBG). The [(18)F]-FHBG accumulation in the liver in terms of percent-injected dose per gram of tissue was 7.72+/-1.13 for the Cre+ mice and 0.10+/-0.02 for the Cre- mice (P<0.05) 48 h after adenoviral injection. These results were further validated by quantitative RT-PCR, western blotting and by in vitro assays for herpes simplex virus type 1 thymidine kinase enzyme activity. Thus by using the Cre-loxP system it is possible to modulate a PRG and noninvasively monitor the extent of Cre-loxP-mediated gene activation by imaging in a microPET scanner.

Molecular imaging of gene expression in living subjects by spliceosome-mediated RNA trans-splicing

Spliceosome-mediated RNA trans-splicing (SMaRT) provides an effective means to reprogram mRNAs and the proteins they encode. SMaRT technology has a broad range of applications, including RNA repair and molecular imaging, each governed by the nature of the sequences delivered by the pre-trans-splicing molecule. Here, we show the ability of SMaRT to optically image the expression of an exogenous gene at the level of pre-mRNA splicing in cells and living animals. Because of the modular design of pre-trans-splicing molecules, there is great potential to employ SMaRT to image the expression of any arbitrary gene of interest in living subjects. In this report, we describe a model system that demonstrates the feasibility of imaging gene expression by transsplicing in small animals. This represents a previously undescribed approach to molecular imaging of mRNA levels in living subjects.

Optical imaging of Renilla luciferase, synthetic Renilla luciferase, and firefly luciferase reporter gene expression in living mice

We have recently demonstrated that Renilla luciferase (Rluc) is a promising bioluminescence reporter gene that can be used for noninvasive optical imaging of reporter gene expression in living mice, with the aid of a cooled charged couple device (CCD) camera. In the current study, we explore the expression of a novel synthetic Renilla luciferase reporter gene (hRluc) in living mice, which has previously been reported to be a more sensitive reporter than native Rluc in mammalian cells. We explore the strategies of simultaneous imaging of both Renilla luciferase enzyme (RL) and synthetic Renilla luciferase enzyme (hRL):coelenterazine (substrate for RL/hRL) in the same living mouse. We also demonstrate that hRL:coelenterazine can yield a higher signal when compared to Firefly luciferase enzyme (FL): D-Luciferin, both in cell culture studies and when imaged from cells at the surface and from lungs of living mice. These studies demonstrate that hRluc should be a useful primary reporter gene with high sensitivity when used alone or in conjunction with other bioluminescence reporter genes for imaging in living rodents.

Gene Therapy Progress and Prospects: Noninvasive imaging of gene therapy in living subjects

Recent progress in the development of noninvasive imaging technologies should allow molecular imaging to play a major role in the field of gene therapy. These tools have recently been validated in gene therapy models for continuous quantitative monitoring of the location(s), magnitude, and time variation of gene delivery and/or expression. This article reviews the use of radionuclide, magnetic resonance, and optical imaging technologies, as they have been used in imaging gene delivery and gene expression for gene therapy applications. The studies published to date lend support that noninvasive imaging tools will help to accelerate preclinical model validation, as well as allow for clinical monitoring of human gene therapy.

Optical imaging of transferrin targeted PEI/DNA complexes in living subjects

Noninvasive optical bioluminescence imaging systems are important tools for evaluating gene expression in vivo for study of individual and temporal variation in a living animal. In this report, we demonstrate that expression of the firefly luciferase reporter gene (fl) delivered by transferrin (Tf) targeted polyethylenimine (PEI) complexes with, or without, poly(ethylene glycol) (PEG) modifications can be imaged in living A/J mice bearing N2A tumors using a cooled charged coupled device (CCD) camera. Tf-PEI-PEG, Tf-PEI, and PEI (positive control) complexes were tail-vein injected and mice were imaged at 5, 24, 48, and 72 h after complex injection. After imaging, the organs were analyzed ex vivo for firefly luciferase protein (FL) activity. The Tf and PEG modified formulations show significantly (P<0.05) higher FL activity in vivo and ex vivo at the tumor as compared to other organs, including the lungs (a site of high expression with PEI, the positive control). Furthermore, the in vivo bioluminescent signal correlated well (R(2)=0.83) with ex vivo FL activity. These data support that noninvasive imaging of fl reporter expression can be used to monitor the specificity of Tf-PEI and Tf-PEI-PEG polyplex targeting of N2A tumors in A/J mice.

Monitoring protein-protein interactions using split synthetic renilla luciferase protein-fragment-assisted complementation

In this study we developed an inducible synthetic renilla luciferase protein-fragment-assisted complementation-based bioluminescence assay to quantitatively measure real time protein-protein interactions in mammalian cells. We identified suitable sites to generate fragments of N and C portions of the protein that yield significant recovered activity through complementation. We validate complementation-based activation of split synthetic renilla luciferase protein driven by the interaction of two strongly interacting proteins, MyoD and Id, in five different cell lines utilizing transient transfection studies. The expression level of the system was also modulated by tumor necrosis factor alpha through NFkappaB-promoter/enhancer elements used to drive expression of the N portion of synthetic renilla luciferase reporter gene. This new system should help in studying protein-protein interactions and when used with other split reporters (e.g., split firefly luciferase) should help to monitor different components of an intracellular network.

Monitoring adenoviral DNA delivery, using a mutant herpes simplex virus type 1 thymidine kinase gene as a PET reporter gene

Current gene therapy protocols often suffer from an inability to monitor the site, level and persistence of gene expression following somatic DNA delivery. Herpes simplex virus 1 thymidine kinase (HSV1-tk) is currently under intensive investigation as a reporter gene for in vivo imaging of reporter gene expression. The presence of the HSV1-tk reporter gene is repetitively and non-invasively monitored by systemic injection of positron-emitting, radionuclide-labeled thymidine analogues or acycloguanosine HSV1-TK substrates and subsequent detection, by positron emission tomography, of trapped, phosphorylated product. To improve the efficacy of the HSV1-tk PET reporter gene system, both alternative substrates and mutations in the HSV1-tk gene have been described. We used a replication defective adenovirus to deliver the HSV1-sr39tk mutant enzyme and the wild-type HSV1-tk enzyme to mice. HSV1-sr39TK demonstrates greater sensitivity than wild-type HSV1-TK enzyme in vivo, using 9-[(4-[(18)F]fluoro-3-hydroxymethylbutyl)guanine as probe, following adenovirus-mediated hepatic expression in mice. Using this adenoviral delivery system, the location, magnitude and duration of HSV1-sr39tk PET reporter gene expression could be non-invasively, quantitatively and repetitively monitored for over 3 months by microPET.

Noninvasive imaging of protein-protein interactions in living subjects by using reporter protein complementation and reconstitution strategies

In this study we have developed bioluminescence-imaging strategies to noninvasively and quantitatively image protein-protein interactions in living mice by using a cooled charge-coupled device camera and split reporter technology. We validate both complementation and intein-mediated reconstitution of split firefly luciferase proteins driven by the interaction of two strongly interacting proteins, MyoD and Id. We use transient transfection of cells and image MyoD-Id interaction after induction of gene expression in cell culture and in cells implanted into living mice. Techniques to study protein-protein interactions in living subjects will allow the study of cellular networks, including signal transduction pathways, as well as development and optimization of pharmaceuticals for modulating protein-protein interactions.

Noninvasive quantitative imaging of protein-protein interactions in living subjects

We are developing methods to image molecular and cellular events in living subjects. In this study, we validate imaging of protein-protein interactions in living mice by using bioluminescent optical imaging. We use the well studied yeast two-hybrid system adapted for mammalian cells and modify it to be inducible. We employ the NF-kappaB promoter to drive expression of two fusion proteins (VP16-MyoD and GAL4-ID). We modulate the NF-kappaB promoter through tumor necrosis factor alpha. Firefly luciferase reporter gene expression is driven by the interaction of MyoD and ID through a transcriptional activation strategy. We demonstrate the ability to detect this induced protein-protein interaction in cell culture and image this induced interaction in living mice by using transiently transfected cells. The current approach will be a valuable and potentially generalizable tool to noninvasively and quantitatively image protein-protein interactions in living subjects. The approaches validated should have important implications for the study of protein-protein interactions in cells maintained in their natural in vivo environment as well as for the in vivo evaluation of new pharmaceuticals targeted to modulate protein-protein interactions.

Optical imaging of Renilla luciferase reporter gene expression in living mice

Imaging reporter gene expression in living subjects is a rapidly evolving area of molecular imaging research. Studies have validated the use of reporter genes with positron emission tomography (PET), single photon emission computed tomography (SPECT), MRI, fluorescence with wild-type and mutants of green fluorescent protein, as well as bioluminescence using Firefly luciferase enzyme/protein (FL). In the current study, we validate for the first time the ability to image bioluminescence from Renilla luciferase enzyme/protein (RL) by injecting the substrate coelenterazine in living mice. A highly sensitive cooled charge-coupled device camera provides images within a few minutes of photon counting. Cells, transiently expressing the Rluc were imaged while located in the peritoneum, s.c. layer, as well as in the liver and lungs of living mice tail-vein injected with coelenterazine. Furthermore, d-luciferin (a substrate for FL) does not serve as a substrate for RL, and coelenterazine does not serve as a substrate for FL either in cell culture or in living mice. We also show that both Rluc and Fluc expression can be imaged in the same living mouse and that the kinetics of light production are distinct. The approaches validated will have direct applications to various studies where two molecular events need to be tracked, including cell trafficking of two cell populations, two gene therapy vectors, and indirect monitoring of two endogenous genes through the use of two reporter genes.

Two-step transcriptional amplification as a method for imaging reporter gene expression using weak promoters

We are developing assays to image tissue-specific reporter gene expression in living mice by using optical methods and positron emission tomography. Approaches for imaging reporter gene expression depend on robust levels of mRNA and reporter protein. Attempts to image reporter gene expression driven by weak promoters are often hampered by the poor transcriptional activity of such promoters. Most tissue-specific promoters are weak relative to stronger but constitutively expressing viral promoters. In this study, we have validated methods to enhance the transcriptional activity of the prostate-specific antigen promoter for imaging by using a two-step transcriptional amplification (TSTA) system. We used the TSTA system to amplify expression of firefly luciferase (fl) and mutant herpes simplex virus type 1 thymidine kinase (HSV1-sr39tk) in a prostate cancer cell line (LNCaP). We demonstrate approximately 50-fold (fl) and approximately 12-fold (HSV1-sr39tk) enhancement by using the two-step approach. The TSTA system is observed to retain tissue selectivity. A cooled charge-coupled device optical imaging system was used to visualize the amplified fl expression in living mice implanted with LNCaP cells transfected ex vivo. These imaging experiments reveal a approximately 5-fold gain in imaging signal by using the TSTA system over the one-step system. The TSTA approach will be a valuable and generalizable tool to amplify and noninvasively image reporter gene expression in living animals by using tissue-specific promoters. The approaches validated should have important implications for study of gene therapy vectors, cell trafficking, transgenic models, as well as studying development of eukaryotic organisms.

Positron emission tomography in evaluation of dementia: Regional brain metabolism and long-term outcome

CONTEXT

Deficits in cerebral glucose utilization have been identified in patients with cognitive dysfunction attributed to various disease processes, but their prognostic and diagnostic value remains to be defined.

OBJECTIVE

To assess the sensitivity and specificity with which cerebral metabolic patterns at a single point in time forecast subsequent documentation of progressive dementia.

DESIGN, SETTING, AND PATIENTS

Positron emission tomography (PET) studies of [(18)F]fluorodeoxyglucose in 146 patients undergoing evaluation for dementia with at least 2 years' follow-up for disease progression at the University of California, Los Angeles, from 1991 to 2000, and PET studies in 138 patients undergoing evaluation for dementia at an international consortium of facilities, with histopathological diagnoses an average of 2.9 years later, conducted from 1984 to 2000.

MAIN OUTCOME MEASURES

Regional distribution of [(18)F]fluorodeoxyglucose in each patient, classified by criteria established a priori as positive or negative for presence of a progressive neurodegenerative disease in general and of Alzheimer disease (AD) specifically, compared with results of longitudinal or neuropathologic analyses.

RESULTS

Progressive dementia was detected by PET with a sensitivity of 93% (191/206) and a specificity of 76% (59/78). Among patients with neuropathologically based diagnoses, PET identified patients with AD and patients with any neurodegenerative disease with a sensitivity of 94% and specificities of 73% and 78%, respectively. The negative likelihood ratio of experiencing a progressive vs nonprogressive course over the several years following a single negative brain PET scan was 0.10 (95% confidence interval, 0.06-0.16), and the initial pattern of cerebral metabolism was significantly associated with the subsequent course of progression overall (P<.001).

CONCLUSION

In patients presenting with cognitive symptoms of dementia, regional brain metabolism was a sensitive indicator of AD and of neurodegenerative disease in general. A negative PET scan indicated that pathologic progression of cognitive impairment during the mean 3-year follow-up was unlikely to occur.

Noninvasive optical imaging of firefly luciferase reporter gene expression in skeletal muscles of living mice

The ability to monitor reporter gene expression noninvasively offers significant advantages over current techniques such as postmortem tissue staining or enzyme activity assays. Here we demonstrate a novel method of repetitively tracking in vivo gene expression of firefly luciferase (FL) in skeletal muscles of mice using a cooled charged coupled device (CCD) camera. We first show that the cooled CCD camera provides consistent and reproducible results within +/-8% standard deviation from mean values, and a detection sensitivity (range tested: 1 x 10(4) - 1 x 10(9) plaque form-ing units (pfu)) of 1 x 10(6) pfu of E1-deleted adenovirus expressing FL driven by a cytomegalovirus promoter (Ad-CMV-FL). The duration and magnitude of adenoviral mediated (1 x 10(9) pfu) FL gene expression were then followed over time. FL gene expression in immunocompetent Swiss Webster mice peaks within the first 48 hours, falls by 98% after 20 days, and persists for >150 days. In contrast, FL activity in nude mice remains elevated for >110 days. Finally, transduced Swiss Webster and nude mice were sacrificed to show that the in vivo CCD signals correlate well with in vitro luciferase enzyme assays (r(2)=0.91 and 0.96, respectively). Our findings demonstrate the ability of the cooled CCD camera to sensitively and noninvasively track the location, magnitude, and persistence of FL gene expression. Monitoring of gene therapy studies in small animals may be aided considerably with further extensions of this technique.

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.

Quantitative imaging of gene induction in living animals

Methods to repeatedly, non-invasively, and quantitatively image gene expression in living animals are rapidly emerging and should fundamentally change studies of gene expression in vivo. We previously developed assays utilizing positron emission tomography (PET) to image reporter gene expression. In this paper we: (1) describe a new bi-directional, tetracycline-inducible system that can be used to pharmacologically induce target gene expression and to quantitatively image induced expression by using a PET reporter gene; (2) demonstrate the potential of this system in transient and stable cell transfection assays; and (3) demonstrate the ability to repetitively and quantitatively image tetracycline and tetracycline analog induction of gene expression in living animals. We utilize the dopamine type-2 receptor (D(2)R) and the mutant herpes-simplex virus type 1 thymidine kinase (HSV1-sr39tk) reporter genes to validate this system. We utilize microPET technology to show that quantitative tomographic imaging of gene induction is possible. We find a high correlation (r(2) = 0.98) between 'target' and reporter gene expression. This work establishes a new technique for imaging time-dependent variation of gene expression both from vectors with inducible promoters and in transgenic animals in which pharmacologic induction of gene expression must be monitored. These techniques may be applied both in gene therapy and for the study of gene expression in transgenic animals.

Noninvasive, quantitative imaging in living animals of a mutant dopamine D2 receptor reporter gene in which ligand binding is uncoupled from signal transduction

The dopamine D2 receptor (D2R) has been used in adenoviral delivery systems and in tumor cell xenografts as an in vivo reporter gene. D2R reporter gene expression has been non-invasively, repetitively and quantitatively imaged by positron emission tomography (PET), following systemic injection of a positron-labeled ligand (3-(2'-[18F]-fluoroethyl)-spiperone; FESP) and subsequent D2R-dependent sequestration. However, dopamine binding to the D2R can modulate cyclic AMP levels. For optimal utilization of D2R as a reporter gene, it is important to uncouple ligand-binding from Gi-protein-mediated inhibition of cAMP production. Mutation of Asp80 or Ser194 produces D2Rs that still bind [3H]spiperone in transfected cells. The D2R80A mutation completely eliminates the ability of the D2R to suppress forskolin-stimulated cAMP accumulation in response to dopamine, in cells transfected with a D2R80A expression plasmid and in cells infected with replication-defective adenovirus expressing D2R80A. The D2R194A mutation substantially reduces, but does not completely eliminate, dopamine modulation of cAMP levels. Cultured cells infected with adenoviruses expressing D2R and D2R80A demonstrated equivalent [3H]spiperone binding activity. Moreover, hepatic FESP sequestration is equivalent, following intravenous injection of adenoviruses expressing D2R and D2R80A. The D2R80A mutant, which can no longer modulate cAMP levels following ligand binding, has full capability as a PET reporter gene.

Impact of whole-body 18F-FDG PET on staging and managing patients with breast cancer: the referring physician's perspective

FDG PET has emerged as an important clinical imaging modality for diagnosing and staging cancer. However, the impact of FDG PET on staging and managing patients with breast cancer from the referring physician's point of view is unknown.

METHODS

The referring physicians of 160 breast cancer patients received standardized questionnaires inquiring if and how PET findings altered their patient's stage and their clinical management decisions. Management changes were classified as intermodality if the change was from one modality to another (e.g., medical to surgical, surgical to radiation, medical to no treatment, and vice versa) or as intramodality if the change was within the same modality (e.g., altered medical or radiotherapy approach).

RESULTS

Fifty of the 160 surveys were completed (31% response rate). PET changed the clinical stage in 36% of patients (28% upstaged, 8% downstaged) and resulted in intermodality changes in 28% of patients and intramodality changes in 30% of patients.

CONCLUSION

The results of this prospective survey show that FDG PET has a major impact on the management of breast cancer patients, influencing both clinical stage and management in more than 30% of patients.

Human pharmacokinetic and dosimetry studies of [(18)F]FHBG: a reporter probe for imaging herpes simplex virus type-1 thymidine kinase reporter gene expression

9-[4-[(18)F]fluoro-3-(hydroxymethyl)butyl]guanine ([(18)F]FHBG) has been used as a reporter probe to image expression of herpes simplex virus type-1 thymidine kinase (HSV1-tk) reporter gene in living animals. Our aim was to study the kinetics, biodistribution, stability, dosimetry, and safety of [(18)F]FHBG in healthy human volunteers, preparatory to imaging patients undergoing HSV1-tk gene therapy.

METHODS

[(18)F]FHBG was synthesized with a specific activity of 37,000--444,000 GBq/mmol and a radiochemical purity > 99%. Ten healthy volunteers consented to participate in the study. A transmission scan was obtained before bolus injection of 70.3--229.4 MBq [(18)F]FHBG into a hand vein, followed by dynamic PET imaging with 4 consecutive emission scans. Warmed hand-vein blood was withdrawn at various times after injection for blood time--activity measurements. Electrocardiography, blood pressure, and blood and urine pharmacologic parameters were measured before and after injection of the [(18)F]FHBG tracer (n = 5). The stability of [(18)F]FHBG in the urine was analyzed. Attenuation-corrected images were reconstructed using the ordered-subsets expectation maximization algorithm. Image region-of-interest time-activity data were used with the MIRD program to estimate absorbed radiation dosages.

RESULTS

[(18)F]FHBG had rapid blood clearance; only 8.42% +/- 4.76% (mean +/- SD) of the peak blood activity remained at approximately 30 min. The average ratio of plasma activity to whole-blood activity during the study was 0.91 +/- 0.04. Penetration of [(18)F]FHBG across the blood-brain barrier was not observed. The primary routes of clearance were renal and hepatobiliary. High activities were observed in the bladder, gut, liver, and kidneys, but <0.0002% of the injected dose per gram was observed in other tissues. In the urine, 83% of activity 180 min after injection was stable [(18)F]FHBG. Blood and urine pharmacologic parameters did not change significantly after injection of the [(18)F]FHBG tracer. The bladder absorbed the highest radiation dose.

CONCLUSION

[(18)F]FHBG has the desirable in vivo characteristics of stability, rapid blood clearance, low background signal, biosafety, and acceptable radiation dosimetry in humans. This study forms the foundation for using [(18)F]FHBG in applications to monitor HSV1-tk reporter gene expression.

Use of positron emission tomography in animal research

Among the several imaging technologies applied to in vivo studies of research animals, positron emission tomography (PET) is a nuclear imaging technique that permits the spatial and temporal distribution of compounds labeled with a positron-emitting radionuclide to be determined noninvasively. It can be viewed as an in vivo analog of classic autoradiographic methods. Many different positron-labeled compounds have been synthesized as tracers that target a range of specific markers or pathways. These tracers permit the measurement of quantities of biological interest ranging from glucose metabolism to gene expression. PET has been extensively used in imaging studies of larger research animals such as dogs and nonhuman primates. Now, using newly developed high-resolution dedicated animal PET scanners, these types of studies can be performed in small laboratory animals such as mice and rats. The entire whole-body biodistribution kinetics can be determined in a single imaging study in a single animal. This technique should enable statistically significant biodistribution data to be obtained from a handful of animals, compared with the tens or hundreds of animals that might be required for a similar study by autoradiography. PET also enables repeat studies in a single subject, facilitating longitudinal study designs and permitting each animal to serve as its own control in experiments designed to evaluate the effects of a particular interventional strategy. This paper provides a basic overview of the methodology of PET imaging, a discussion of the advantages and drawbacks of PET as a tool in animal research, a description of the latest generation of dedicated animal PET scanners, and a review of a few of the many applications of PET in animal research to date.

Direct correlation between positron emission tomographic images of two reporter genes delivered by two distinct adenoviral vectors

Biodistribution, magnitude and duration of a therapeutic transgene's expression may be assessed by linking it to the expression of a positron emission tomography (PET) reporter gene (PRG) and then imaging the PRG's expression by a PET reporter probe (PRP) in living animals. We validate the simple approach of co-administering two distinct but otherwise identical adenoviruses, one expressing a therapeutic transgene and the other expressing the PRG, to track the therapeutic gene's expression. Two PET reporter genes, a mutant herpes simplex virus type 1 thymidine kinase (HSV1-sr39tk) and dopamine-2 receptor (D(2)R), each regulated by the same cytomegalovirus (CMV) promoter, have been inserted into separate adenoviral vectors (Ad). We demonstrate that cells co-infected with equivalent titers of Ad-CMV-HSV1-sr39tk and Ad-CMV-D(2)R express both reporter genes with good correlation (r(2) = 0.93). Similarly, a high correlation (r(2) = 0.97) was observed between the expression of both PRGs in the livers of mice co-infected via tail-vein injection with equivalent titers of these two adenoviruses. Finally, microPET imaging of HSV1-sr39tk and D(2)R expression with 9-(4-[(18)F]fluoro-3-hydroxymethylbutyl) guanine ([(18)F]FHBG) and 3-(2-[(18)F]fluoroethyl)spiperone ([(18)F]FESP), utilizing several adenovirus-mediated delivery routes, illustrates the feasibility of evaluating relative levels of transgene expression in living animals, using this approach.

A fast nonlinear method for parametric imaging of myocardial perfusion by dynamic (13)N-ammonia PET

A parametric image of myocardial perfusion (mL/min/g) is a quantitative image generated by fitting a tracer kinetic model to dynamic (13)N-ammonia PET data on a pixel-by-pixel basis. There are several methods for such parameter estimation problems, including weighted nonlinear regression (WNLR) and a fast linearizing method known as Patlak analysis. Previous work showed that sigmoidal networks can be used for parameter estimation of mono- and biexponential models. The method used in this study is a hybrid of WNLR and sigmoidal networks called nonlinear regression estimation (NRE). The purpose of the study is to compare NRE with WNLR and Patlak analysis for parametric imaging of perfusion in the canine heart by (13)N-ammonia PET.

METHODS

A simulation study measured the statistical performance of NRE, WNLR, and Patlak analysis for a probabilistic model of time-activity curves. Four canine subjects were injected with 740 MBq (13)N-ammonia and scanned dynamically. Images were reconstructed with filtered backprojection and resliced into short-axis cuts. Parametric images of a single midventricular plane per subject were generated by NRE, WNLR, and Patlak analysis. Small regions of interest (ROIs) were drawn on each parametric image (8 ROIs per subject for a total of 32).

RESULTS

For the simulation study, the median absolute value of the relative error for a perfusion value of 1.0 mL/min/g was 16.6% for NRE, 17.9% for WNLR, 19.5% for Patlak analysis, and 14.5% for an optimal WNLR method (computable by simulation only). All methods are unbiased conditioned on a wide range of perfusion values. For the canine studies, the least squares line fits comparing NRE (y) and Patlak analysis (z) with WNLR (x) for all 32 ROIs were y = 1.02x - 0.028 and z = 0.90x + 0.019, respectively. Both NRE and Patlak analysis generate 128 x 128 parametric images in seconds.

CONCLUSION

The statistical performance of NRE is competitive with WNLR and superior to Patlak analysis for parametric imaging of myocardial perfusion. NRE is a fast nonlinear alternative to Patlak analysis and other fast linearizing methods for parametric imaging. NRE should be applicable to many other tracers and tracer kinetic models.

Impact of 18F-FDG PET on managing patients with colorectal cancer: the referring physician's perspective

Whole-body PET imaging with 18F-FDG has been used successfully to stage colorectal cancer. However, the impact of FDG PET on patient management from the referring physician's point of view has not been determined.

METHODS

A questionnaire was sent to referring physicians to determine whether and how PET altered the management of colorectal cancer patients. Management changes, when present, were classified as intermodality (e.g., medical to surgical, surgical to radiation, medical to no treatment) or intramodality (e.g., altered medical, surgical, or radiotherapy approach).

RESULTS

Of 60 responses from referring physicians, changes in clinical stage were reported for 25 patients (42%). Among these, the disease was upstaged in 20 patients (80%) and downstaged in 5 patients (20%). The PET findings contributed to intermodality management changes in 22 of the 60 patients (37%), intramodality changes in 11 patients (18%), a combination of management changes in 4 patients (7%), and no change in 19 patients (32%). Two of the 60 patients (3%) had other changes, and no response to this question was received for the remaining 2 patients (3%). As a result of PET findings, physicians avoided major surgery in 41% of patients for whom surgery was the intended treatment.

CONCLUSION

This survey-based study of referring physicians shows that FDG PET had a major impact on the management of colorectal cancer patients and contributed to changes in clinical stage and major management decisions in >40% of patients.