In vivo imaging of tumors with protease-activated near-infrared fluorescent probes

Feasibility of in vivo multichannel optical imaging of gene expression: experimental study in mice

PURPOSE

To develop and test a multichannel reflectance imaging system for small animals on the basis of a previously developed single-channel setup.

MATERIALS AND METHODS

The imaging system was composed of modular parts, including a light source, excitation filters, emission filters, and a charged-coupled device for recording images. On the basis of generated excitation and absorption spectra of green fluorescent protein (GFP), tricarbocyanine 5.5 (Cy5.5), and indocyanine green (ICG), filters were selected to allow spectral separation and optimize resultant recorded signal. The system was tested by using a combination of the fluorochromes to confirm spectral separation. In vivo tests were performed in nude mice with tumors that expressed cathepsin B, which could be evaluated by using a Cy5.5-based activatable probe and GFP. For each in vivo tumor type and channel, statistical analysis was performed on the basis of signal intensity in the region of interest.

RESULTS

The different fluorochromes were readily distinguished with the system; characteristics such as power were determined for all wavelengths. The system demonstrated a linear response for GFP, a monotonic response for Cy5.5 over a range of more than three orders of magnitude of concentration, and a more complex response for ICG. In vivo analysis demonstrated the ability to image GFP expression and cathepsin B expression separately in tumors: As expected, marked differences were observed in GFP-expression imaging between tumor types (1,363 arbitrary units [AU] +/- 236 [SD] vs 110 AU +/- 11 for GFP-positive and GFP-negative tumors, respectively; P <.001), whereas similar cathepsin B expression (1,070 AU +/- 285 vs 1,168 AU +/- 367; P >.5) was observed. Histologic analysis confirmed in vivo findings.

CONCLUSION

Imaging multiple gene expressions simultaneously in vivo by using optical imaging is feasible.

Three-dimensional, Bayesian image reconstruction from sparse and noisy data sets: near-infrared fluorescence tomography

A method for inverting measurements made on the surfaces of tissues for recovery of interior optical property maps is demonstrated for sparse near-infrared (NIR) fluorescence measurement sets on large tissue-simulating volumes with highly variable signal-to-noise ratio. A Bayesian minimum-variance reconstruction algorithm compensates for the spatial variability in signal-to-noise ratio that must be expected to occur in actual NIR contrast-enhanced diagnostic medical imaging. Image reconstruction is demonstrated by using frequency-domain photon migration measurements on 256-cm(3) tissue-mimicking phantoms containing none, one, or two 1-cm(3) heterogeneities with 50- to 100-fold greater concentration of Indocyanine Green dye over background levels. The spatial parameter estimate of absorption owing to the dye was reconstructed from only 160 to 296 surface measurements of emission light at 830 nm in response to incident 785-nm excitation light modulated at 100 MHz. Measurement error of acquired fluence at fluorescent emission wavelengths is shown to be highly variable. Convergence and quality of image reconstructions are improved by Bayesian conditioning incorporating (i) experimentally determined measurement error variance, (ii) recursively updated estimates of parameter uncertainty, and (iii) dynamic zonation. The results demonstrate that, to employ NIR fluorescence-enhanced optical imaging for large volumes, reconstruction approaches must account for the large range of signal-to-noise ratio associated with the measurements.

Establishment of fluorescent lung carcinoma metastasis model and its real-time microscopic detection in SCID mice

Lung cancer is the most prevalent malignant tumor in the world. Metastasis of the disease causes death in lung cancer patients. Recent study has shown that multiple cascades of gene defects occur in lung cancer. In this report, we established a novel H1299/EGFP tumor model to determine whether H1299 transfected with the enhanced green fluorescent protein (EGFP) gene in vitro and xenotransplanted into SCID mouse lung would permit the detection of lung cancer micrometastasis in vivo. We demonstrated that EGFP-transduced H1299 cells maintained stable high-level EGFP expressions during their growth in vivo. EGFP fluorescence clearly demarcated the primary seeding place and readily allowed for the visualization of distant micrometastasis and local invasion at the single-cell level. Small metastatic and locally invasive foci, including those immediately adjacent to the tumor's leading invasive edge, were almost undetectable by routine hematoxylin and eosin staining and immunohistochemistry. The GFP tagged lung cancer model is superior for the detection and study of physiologically relevant patterns of lung cancer invasion and metastasis in vivo.

Detection limit enhancement of fluorescent heterogeneities in turbid media by dual-interfering excitation

We report on a quantitative comparison between the single-source and the dual-interfering-source configurations for the detection of fluorescent heterogeneities embedded in a piecewise highly scattering homogeneous fluorescent background. The study is based on simulations with analytical solutions of the frequency-domain fluorescent diffuse photon density waves and practical signal-to-noise ratio considerations. Results show that dual-interfering sources outperform single-source techniques for the detection of heterogeneities in terms of fluorophore concentration and lifetime contrast. To detect the same inhomogeneity, less concentration and lifetime contrast is required with dual-interfering sources.

Fluorescence molecular tomography resolves protease activity in vivo

Systematic efforts are under way to develop novel technologies that would allow molecular sensing in intact organisms in vivo. Using near-infrared fluorescent molecular beacons and inversion techniques that take into account the diffuse nature of photon propagation in tissue, we were able to obtain three-dimensional in vivo images of a protease in orthopic gliomas. We demonstrate that enzyme-activatable fluorochromes can be detected with high positional accuracy in deep tissues, that molecular specificities of different beacons towards enzymes can be resolved and that tomography of beacon activation is linearly related to enzyme concentration. The tomographic imaging method offers a range of new capabilities for studying biological function; for example, identifying molecular-expression patterns by multispectral imaging or continuously monitoring the efficacy of therapeutic drugs.

In vivo imaging of proteolytic activity in atherosclerosis

BACKGROUND

Atherosclerotic plaque rupture, the most important cause of acute cardiovascular incidents, has been strongly associated with vascular inflammation. On the basis of the hypothesis that the inflammatory response and proteolysis lead to plaque rupture, we have examined the role of cathepsin B as a model proteolytic enzyme.

METHODS AND RESULTS

Using western-type diet-fed apoE and apoE/endothelial NO synthase double knockout mice as models of atherosclerosis, we show (1) that cathepsin B is upregulated in atherosclerotic lesions characterized by high degrees of inflammation compared with normal aorta or silent lesions, (2) that intravenously injectable novel cathepsin B imaging beacons are highly activated within active atherosclerotic lesions and colocalize with cathepsin B immunoreactivity, and (3) that cathepsin B activity in atherosclerotic lesions can be imaged in whole animals by using a novel near-infrared tomographic imaging system.

CONCLUSIONS

These studies indicate that cathepsin B, and potentially other proteases, may serve as a biomarker for vulnerable plaques when probed with beacons. The tomographic in vivo imaging method as well as catheter-based optical sensing methods could be readily adapted to screening and potentially to the molecular profiling of a number of proteases in vulnerable plaque in vivo.

Tricarbocyanine cholesteryl laurates labeled LDL: new near infrared fluorescent probes (NIRFs) for monitoring tumors and gene therapy of familial hypercholesterolemia

For monitoring low-density lipoprotein receptors (LDLr) in tumors and in livers of patients with familial hypercholesterolemia (FH) treated with gene therapy, a series of tricarbocyanine cholesteryl laurates were synthesized with the cholesteryl laurate moiety serving as the lipid-chelating anchor for low-density lipoprotein (LDL). One of these conjugates, TCL17, was successfully used to label LDL to give a new NIRF, TCL17-LDL. Ex vivo biological studies on an LDLr overexpressing tumor model, human hepatoblastoma G(2) (HepG(2)), confirmed that this NIRF were internalized selectively by the tumor and detected with high sensitivity by a low-temperature 3-D redox scanner.

Molecular and functional imaging technology for the development of efficient treatment strategies for gliomas

Gliomas are the most common types of brain tumors, which invariably lead to death over months or years. Before new and potentially more effective treatment strategies, such as gene therapy, can be effectively introduced into clinical application the following goals must be reached: (1) the determination of localization, extent and metabolic activity of the glioma; (2) the assessment of functional changes within the surrounding brain tissue; (3) the identification of genetic changes on the molecular level leading to disease; and in addition (4) a detailed non-invasive analysis of both endogenous and exogenous gene expression in animal models and in the clinical setting. Non-invasive imaging of endogenous gene expression by means of positron emission tomography (PET) may reveal insight into the molecular basis of pathogenesis and metabolic activity of the glioma and the extent of treatment response. When exogenous genes are introduced to serve for a therapeutic function, PET imaging techniques may reveal the assessment of the location, magnitude and duration of therapeutic gene expression and its relation to the therapeutic effect. Here, we review the main principles of PET imaging and its key roles in neurooncology research.

Cellular activation of the self-quenched fluorescent reporter probe in tumor microenvironment

The effect of intralysosomal proteolysis of near-infrared fluorescent (NIRF) self-quenched macromolecular probe (PGC-Cy5.5) has been previously reported and used for tumor imaging. Here we demonstrate that proteolysis can be detected noninvasively in vivo at the cellular level. A codetection of GFP fluorescence (using two-photon excitation) and NIRF was performed in tumor-bearing animals injected with PGC-Cy5.5. In vivo microscopy of tumor cells in subdermal tissue layers (up to 160 microm) showed a strong Cy5.5 dequenching effect in GFP-negative cells. This observation was corroborated by flow cytometry, sorting, and reverse transcription polymerase chain reaction analysis of tumor-isolated cells. Both GFP-positive (81% total) and GFP-negative (19% total) populations contained Cy5.5-positive cells. The GFP-negative cells were confirmed to be host mouse cells by the absence of rat cathepsin mRNA signal. The subfraction of GFP-negative cells (2.5-3.0%) had seven times higher NIRF intensity than the majority of GFP-positive or GFP-negative cells (372 and 55 AU, respectively). Highly NIRF-positive, FP-negative cells were CD45- and MAC3-positive. Our results indicate that: 1) intracellular proteolysis can be imaged in vivo at the cellular level using cathepsin-sensitive probes; 2) tumor-recruited cells of hematopoetic origin participate most actively in uptake and degradation of long-circulating macromolecular probes.

Charge-coupled-device based scanner for tomography of fluorescent near-infrared probes in turbid media

We present a novel tomographer for three-dimensional reconstructions of fluorochromes in diffuse media. Photon detection is based on charge-coupled device technology that allows the implementation of a large parallel array of detection channels with high sensitivity. Using this instrument we studied the response and detection limits of near-infrared fluorochromes in diffuse media as a function of light intensity and for a wide range of biologically relevant concentrations. We further examined the resolution of the scanner and the reconstruction linearity achieved. We demonstrate that the instrument attains better than 3 mm resolution, is linear within more than two orders of magnitude of fluorochrome concentration, and can detect fluorescent objects at femto-mole quantities in small animal-like geometries. These measurements delineate detection and reconstruction characteristics associated with imaging of novel classes of fluorescent probes developed for in vivo molecular and functional probing of tissues.

A complete substitutional analysis of VIP for better tumor imaging properties

Since numerous tumor cells overexpress the vasoactive intestinal peptide (VIP) receptor subtype 1 (VPAC(1)), VIP-dye conjugates would be useful as contrast agents for in vivo imaging. However, proteolytic degradation of VIP in vivo limits their diagnostic use and highlights the need for structurally optimized VIP derivatives with improved pharmacokinetics. Here, we applied parallel nano-synthesis of cleavable peptides on cellulose membranes to perform a complete VIP substitutional analysis. The resulting 504 different VIP-dye analogs were tested for cell binding by flow cytometry. They provided a detailed analysis of amino acid positions essential for binding to VPAC(1) overexpressing cells. A generalized VIP-dye binding motif derived from the substitutional analysis results served as a reference point for further optimization. An [Arg8]-VIP-dye analog showed increased stability towards proteolytic degradation, good tumor-to-tissue contrast in mice and a longer half-life in vivo.

Three-dimensional fluorescence enhanced optical tomography using referenced frequency-domain photon migration measurements at emission and excitation wavelengths

The ultimate success of near-infrared optical tomography rests on the precise measurement of light propagation within tissues or random media, the accurate prediction of these measurements from a light propagation model, and an efficient three-dimensional solution of the inverse imaging problem. To date, optical tomography algorithms have focused on frequency-domain photon migration (FDPM) measurements of phase-delay and amplitude attenuation, which are reported relative to the incident light, even though phase-delay and amplitude of incident light are nearly impossible to measure directly. In this contribution, we examine referenced, fluorescence-enhanced frequency-domain photon migration measured at excitation and/or emission wavelengths and report on a measurement strategy to minimize measurement and calibration error for efficient coupling of data to a distorted Born iterative imaging algorithm. We examine three referencing approaches and develop associated inversion algorithms for (1) normalizing detected emission FDPM data to the predicted emission wave arising from a homogeneous medium, (2) referencing detected emission FDPM data to that detected at a reference point, and (3) referencing detected emission FDPM data to detected excitation FDPM data detected at a reference point. Our results show the latter approach to be practical while reducing the nonlinearity of the inverse problem. Finally, in light of our results, we demonstrate the method for eliminating the influence of source strength and instrument functions for effective fluorescence-enhanced optical tomography using FDPM.

In Vivo Tomographic Imaging of Near-Infrared Fluorescent Probes

Fluorescence imaging is increasingly used to probe protein function and gene expression in live animals. This technology could enhance the study of pathogenesis, drug development, and therapeutic intervention. In this article, we focus on three-dimensional fluorescence observations using fluorescence-mediated molecular tomography (FMT), a novel imaging technique that can resolve molecular function in deep tissues by reconstructing fluorescent probe distributions in vivo. We have compared FMT findings with conventional fluorescence reflectance imaging (FRI) to study protease function in nude mice with subsurface implanted tumors. This validation of FMT with FRI demonstrated the spatial congruence of fluorochrome activation as determined by the two techniques.

In Vitro and In Vivo Characterization of a Dual-Function Green Fluorescent Protein–HSV1-Thymidine Kinase Reporter Gene Driven by the Human Elongation Factor 1α Promoter

Toward the goal of monitoring activity of native mammalian promoters with molecular imaging techniques, we stably transfected DU145 prostate carcinoma cells with a fusion construct of enhanced green fluorescent protein (EGFP) and wild-type herpes simplex virus-1 thymidine kinase (HSV1-TK) as a reporter gene driven by the promoter for human elongation factor 1α (EF-1α-EGFP-TK). Using this model system, expression of EGFP was quantified by flow cytometry and fluorescence microscopy, while the HSV1-TK component of the reporter was quantified with 8-[3H]ganciclovir (8-[3H]GCV). As analyzed by flow cytometry, passage of EGFP-TK-DU145 transfected cells (ETK) in vitro resulted in populations of cells with high and low expression of EGFP over time. High and low ETK cells retained 23-fold and 5-fold more GCV, respectively, than control. While differences in uptake and retention of GCV corresponded to relative expression of the reporter gene in each subpopulation of cells as determined by both flow cytometry (EGFP) and quantitative RT-PCR, the correlation was not linear. Furthermore, in high ETK cells, net retention of various radiolabeled nucleoside analogues varied; the rank order was 8-[3H]GCV < 9-(4-fluoro-3-hydroxymethylbutyl)guanine ([18F]FHBG) ≈ 8-[3H]penciclovir (8-[3H]PCV) < 2′-fluoro-2′-deoxy-5-iodouracil-beta-d-arabinofuranoside (2-[14C]FIAU). Xenograft tumors of ETK cells in vivo accumulated 2.5-fold more 8-[3H]GCV per gram of tissue and showed greater fluorescence from EGFP than control DU145 cells, demonstrating that the reporter gene functioned in vivo. These data extend previous reports by showing that a human promoter can be detected in vitro and in vivo with a dual-function reporter exploiting optical and radiotracer techniques.

Imaging of differential protease expression in breast cancers for detection of aggressive tumor phenotypes

PURPOSE

To determine if different expression levels of tumor cathepsin-B activity in well differentiated and undifferentiated breast cancers could be revealed in vivo with optical imaging.

MATERIALS AND METHODS

A well differentiated human breast cancer (BT20, n = 8) and a highly invasive metastatic human breast cancer (DU4475, n = 8) were implanted orthotopically in athymic nude mice. Tumor-bearing animals were examined in vivo with near-infrared fluorescence (NIRF) imaging 24 hours after intravenous injection of an enzyme-sensing imaging probe. Immunohistochemistry, Western blotting (on cells and whole tumor samples), and correlative fluorescence microscopy were performed.

RESULTS

Both types of breast cancers activated the NIRF probe so that tumors became readily detectable. However, in tumors of equal size, there was a 1.5-fold higher fluorescence signal in the highly invasive breast cancer (861 arbitrary units +/- 88) compared with the well differentiated lesion (566 arbitrary units +/- 36, P <.01). Western blotting confirmed a higher cathepsin-B protein content in the highly invasive breast cancer (DU4475) of about 1.4-fold (whole tumor samples) to 1.7-fold (cells). Immunohistochemistry and fluorescence microscopy findings confirmed the imaging findings.

CONCLUSION

Cathepsin-B enzyme activity can be determined in vivo with NIRF optical imaging, while differences in tumoral expression may correlate with tumor aggressiveness.

In vivo imaging of NF-kappa B activity

A wide range of human disorders involves inappropriate regulation of NF-kappaB, including cancers and numerous inflammatory conditions. Toward our goal to define mechanisms through which NF-kappaB leads to the development of disease, we have developed transgenic mice that express luciferase under the control of NF-kappaB, enabling real-time in vivo imaging of NF-kappaB activity in intact animals. We show that in the absence of extrinsic stimulation, strong luminescence is evident in lymph nodes in the neck region, thymus, and Peyer's patches. Treating mice with TNF-alpha, IL-1alpha, or LPS increased the luminescence in a tissue-specific manner, with the strongest activity observed in skin, lungs, spleen, Peyer's patches, and the wall of the small intestine. Liver, kidney, heart, muscle, and adipose tissue displayed less intense activities. Also, exposure of skin to a low dose of UV radiation increased luminescence in the exposed areas. Furthermore, induction of chronic inflammation resembling rheumatoid arthritis produced strong NF-kappaB activity in the affected joints, as revealed by in vivo imaging. Thus, we have developed a versatile model for monitoring NF-kappaB activation in vivo.

Detection of dysplastic intestinal adenomas using enzyme-sensing molecular beacons in mice

BACKGROUND & AIMS

Proteases play key roles in the pathogenesis of tumor growth and invasion. This study assesses the expression of cathepsin B in dysplastic adenomatous polyps.

METHODS

Aged Apc(Min/+) mice served as an experimental model for familial adenomatous polyposis. The 4 experimental groups consisted of (a) animals injected with a novel activatable, cathepsin B sensing near infrared fluorescence (NIRF) imaging probe; (b) animals injected with a nonspecific NIRF; (c) uninjected control animals; and (d) non-APC(Min/+) mice injected with the cathepsin B probe. Lesions were analyzed by immunohistochemistry, Western blotting, reverse transcription-polymerase chain reaction, and optical imaging.

RESULTS

Cathepsin B was consistently overexpressed in adenomatous polyps. When mice were injected intravenously with the cathepsin reporter probe, intestinal adenomas became highly fluorescent indicative of high cathepsin B enzyme activity. Even microscopic adenomas were readily detectable by fluorescence, but not light, imaging. The smallest lesion detectable measured 50 microm in diameter. Adenomas in the indocyanine green and/or noninjected group were only barely detectable above the background.

CONCLUSIONS

The current experimental study shows that cathepsin B is up-regulated in a mouse model of adenomatous polyposis. Cathepsin B activity can be used as a biomarker to readily identify such lesions, particularly when contrasted against normal adjacent mucosa. This detection technology can be adapted to endoscopy or tomographic optical imaging methods for screening of suspicious lesions and potentially for molecular profiling in vivo.

Arthritis critically dependent on innate immune system players

K/BxN T cell receptor transgenic mice are a model of inflammatory arthritis, similar to rheumatoid arthritis. Disease in these animals is focused specifically on the joints but stems from autoreactivity to a ubiquitously expressed antigen, glucose-6-phosphate isomerase (GPI). T and B cells are both required for disease initiation, but anti-GPI immunoglobulins (Igs), alone, can induce arthritis in lymphocyte-deficient recipients. Here, we show that the arthritogenic Igs act through both Fc receptors (in particular, FcgammaRIII) and the complement network (C5a). Surprisingly, the alternative pathway of complement activation is critical, while classical pathway components are entirely dispensable. We suggest that autoimmune disease, even one that is organ specific, can occur when mobilization of an adaptive immune response results in runaway activation of the innate response.

New optical technologies for earlier endoscopic diagnosis of premalignant gastrointestinal lesions

Gastrointestinal malignancies continue to be the second leading cause of cancer-related deaths in the developed world. The early detection and treatment of gastrointestinal preneoplasms has been demonstrated to significantly improve patient survival. Conventional screening tools include standard white light endoscopy (WLE) and frequent surveillance with biopsy. Well-defined endoscopic surveillance biopsy protocols aimed at early detection of dysplasia and malignancy have been undertaken for groups at high risk. Unfortunately, the poor sensitivity associated with WLE is a significant limitation. In this regard, major efforts continue in the development and evaluation of alternative diagnostic techniques. This review will focus on notable developments made at the forefront of research in modern gastrointestinal endoscopy based on novel optical endoscopic modalities, which rely on the interactions of light with tissues. Here we present the 'state-of-the-art' in fluorescence endoscopic imaging and spectroscopy, Raman spectroscopy, optical coherence tomography, light scattering spectroscopy, chromoendoscopy, confocal fluorescence endoscopy, and immunofluorescence endoscopy. These new developments may offer significant improvements in the diagnosis of early lesions by allowing for targeted mucosal excisional biopsies, and perhaps may even provide 'optical biopsies' of equivalent histological accuracy. This enhancement of the endoscopist's ability to detect subtle preneoplastic changes in the gastrointestional mucosa in real time and improved staging of lesions could lead to curative endoscopic ablation of these lesions and, in the long term, improve patient survival and quality of life.

Luminescent quantum dots for multiplexed biological detection and imaging

Recent advances in nanomaterials have produced a new class of fluorescent labels by conjugating semiconductor quantum dots with biorecognition molecules. These nanometer-sized conjugates are water-soluble and biocompatible, and provide important advantages over organic dyes and lanthanide probes. In particular, the emission wavelength of quantum-dot nanocrystals can be continuously tuned by changing the particle size, and a single light source can be used for simultaneous excitation of all different-sized dots. High-quality dots are also highly stable against photobleaching and have narrow, symmetric emission spectra. These novel optical properties render quantum dots ideal fluorophores for ultrasensitive, multicolor, and multiplexing applications in molecular biotechnology and bioengineering.

Moving smaller in drug discovery and delivery

Advances in new micro- and nanotechnologies are accelerating the identification and evaluation of drug candidates, and the development of new delivery technologies that are required to transform biological potential into medical reality. This article will highlight the emerging micro- and nanotechnology tools, techniques and devices that are being applied to advance the fields of drug discovery and drug delivery. Many of the promising applications of micro- and nanotechnology are likely to occur at the interfaces between microtechnology, nanotechnology and biochemistry.

Scaling down imaging: molecular mapping of cancer in mice

The development of miniaturized imaging equipment and reporter probes has improved our ability to study animal models of disease, such as transgenic and knockout mice. These technologies can now be used to continuously monitor in vivo tumour development, the effects of therapeutics on individual populations of cells, or even specific molecules. If these techniques prove effective in mice, they might be translated into the clinic in the future, where they could be used to non-invasively detect and monitor treatment of human cancers.

Magnetic labeling of activated microglia in experimental gliomas

Microglia, as intrinsic immunoeffector cells of the central nervous system (CNS), play a very sensitive, crucial role in the response to almost any brain pathology where they are activated to a phagocytic state. Based on the characteristic features of activated microglia, we investigated whether these cells can be visualized with magnetic resonance imaging (MRI) using ultrasmall superparamagnetic iron oxides (USPIOs). The hypothesis of this study was that MR microglia visualization could not only reveal the extent of the tumor, but also allow for assessing the status of immunologic defense. Using USPIOs in cell culture experiments and in a rat glioma model, we showed that microglia can be labeled magnetically. Labeled microglia are detected by confocal microscopy within and around tumors in a typical border-like pattern. Quantitative in vitro studies revealed that microglia internalize amounts of USPIOs that are significantly higher than those incorporated by tumor cells and astrocytes. Labeled microglia can be detected and quantified with MRI in cell phantoms, and the extent of the tumor can be seen in glioma-bearing rats in vivo. We conclude that magnetic labeling of microglia provides a potential tool for MRI of gliomas, which reflects tumor morphology precisely. Furthermore, the results suggest that MRI may yield functional data on the immunologic reaction of the CNS.

Optical imaging of matrix metalloproteinase-2 activity in tumors: feasibility study in a mouse model

PURPOSE

To develop an optical imaging method to determine the expression level of tumoral matrix metalloproteinase-2 (MMP-2) in vivo.

MATERIALS AND METHODS

An optical contrast agent was developed that was highly activatable by means of MMP-2-induced conversion. Signal characteristics of the probe were quantified ex vivo with a recombinant enzyme. Animal tumor models were established with MMP-2-positive (human fibrosarcoma cell line, n = 4) and MMP-2-negative (well-differentiated mammary adenocarcinoma, n = 4) tumor cell lines. Both tumors were implanted into nude mice and were optically imaged after intravenous administration of the MMP-2-sensitive probe.

RESULTS

The MMP-2-sensitive probe was activated by MMP-2 in vitro, producing up to an 850% increase in near-infrared fluorescent signal intensity. This activation could be blocked by MMP-2 inhibitors. MMP-2-positive tumors were easily identified as high-signal-intensity regions as early as 1 hour after intravenous injection of the MMP-2 probe, while contralateral MMP-2-negative tumors showed little to no signal intensity. A nonspecific control probe showed little to no activation in MMP-2-positive tumors.

CONCLUSION

It is feasible to image MMP-2 enzyme activity in vivo by using near-infrared optical imaging technology and "smart" matrix metalloproteinase-sensitive probes.

In vivo imaging of gene and cell therapies

Molecular imaging can be broadly defined as the in vivo characterization and measurement of biological processes at the cellular and molecular level. In contrast to commonly used clinical imaging, it sets forth to probe the molecular abnormalities that are the basis of disease, rather than imaging the end effects of these molecular alterations. Development of new imaging technologies requires a multidisciplinary collaboration between biologists, chemists, physicists, and imaging scientists to create novel agents, signal amplification strategies, and imaging techniques that successfully address these questions. In this article we attempt to present some of the recent developments and show how molecular imaging can be used, at least experimentally, to assess specific molecular targets for gene- and cell-based therapies. In particular, we place emphasis on the development and use of experimental small-animal models, which are particularly inclined toward this approach, primarily in combination with magnetic resonance (MR), radionuclide, and optical imaging. In the future, specific imaging of disease targets will allow earlier detection and characterization of disease, as well as earlier and direct molecular assessment of treatment efficacy.

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.

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.

Analytical solutions for time-resolved fluorescence lifetime imaging in a turbid medium such as tissue

An analytical solution is developed to quantify a site-specific fluorophore lifetime perturbation that occurs, for example, when the local metabolic status is different from that of surrounding tissue. This solution may be used when fluorophores are distributed throughout a highly turbid media and the site of interest is embedded many mean scattering distances from the source and the detector. The perturbation in lifetime is differentiated from photon transit delays by random walk theory. This analytical solution requires a priori knowledge of the tissue-scattering and absorption properties at the excitation and emission wavelengths that may be obtained from concurrent time-resolved reflection measurements. Additionally, the solution has been compared with the exact, numerically solved solution. Thus the presented solution forms the basis for practical lifetime imaging in turbid media such as tissue.

Experimental three-dimensional fluorescence reconstruction of diffuse media by use of a normalized Born approximation

We present a normalized Born expansion that facilitates fluorescence reconstructions in turbid, tissuelike media. The algorithm can be particularly useful for tissue investigations of fluorochrome distributionin vivo, since it does not require absolute photon-field measurements or measurements before contrast-agent administration. This unique advantage can be achieved only in fluorescence mode. We used this algorithm to three-dimensionally image and quantify an indocyanine fluorochrome phantom, using a novel fluorescence tomographic imager developed for animals.

In vivo molecular target assessment of matrix metalloproteinase inhibition

A number of different matrix metalloproteinase (MMP) inhibitors have been developed as cytostatic and anti-angiogenic agents and are currently in clinical testing. One major hurdle in assessing the efficacy of such drugs has been the inability to sense or image anti-proteinase activity directly and non-invasively in vivo. We show here that novel, biocompatible near-infrared fluorogenic MMP substrates can be used as activatable reporter probes to sense MMP activity in intact tumors in nude mice. Moreover, we show for the first time that the effect of MMP inhibition can be directly imaged using this approach within hours after initiation of treatment using the potent MMP inhibitor, prinomastat (AG3340). The developed probes, together with novel near-infrared fluorescence imaging technology will enable the detailed analysis of a number of proteinases critical for advancing the therapeutic use of clinical proteinase inhibitors.

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.

Three-dimensional unconstrained and constrained image-reconstruction techniques applied to fluorescence, frequency-domain photon migration

The development of near-infrared (NIR) optical imaging for biomedical optical imaging is hampered by the computational intensiveness of large-scale three-dimensional (3-D) image reconstruction and the potential lack of endogenous contrast for detection of relevant tissue features. In this contribution the inverse optical imaging problem is formulated in three dimensions in a noncompressive geometry as a simple-bound constrained minimization problem in order to recover the interior fluorescence properties of exogenous contrast agent from frequency-domain photon migration measurements at the boundary. The solution of the forward optical diffusion problem for the frustum shape containing fluorescence inclusions of 10:1 contrast is accomplished by use of the Galerkin finite-element formulation. The inverse approach employs the truncated Newton method with trust region and a modification of automatic reverse differentiation to speed the computation of the optimization problem. The image-reconstruction results confirm that the constrained minimization may offer a more logical approach for the 3-D optical imaging problem than unconstrained optimization.

Novel fluorescent contrast agents for optical imaging of in vivo tumors based on a receptor-targeted dye-peptide conjugate platform

We have designed, synthesized, and evaluated the efficacy of novel dye-peptide conjugates that are receptor specific. Contrary to the traditional approach of conjugating dyes to large proteins and antibodies, we used small peptide-dye conjugates that target over-expressed receptors on tumors. Despite the fact that the peptide and the dye probe have similar molecular mass, our results demonstrate that the affinity of the peptide for its receptor and the dye fluorescence properties are both retained. The use of small peptides has several advantages over large biomolecules, including ease of synthesis of a variety of compounds for potential combinatorial screening of new targets, reproducibility of high purity compounds, diffusiveness to solid tumors, and the ability to incorporate a variety of functional groups that modify the pharmacokinetics of the peptide-dye conjugates. The efficacy of these new fluorescent optical contrast agents was evaluated in vivo in well-characterized rat tumor lines expressing somatostatin (sst(2)) and bombesin receptors. A simple continuous wave optical imaging system was employed. The resulting optical images clearly show that successful specific tumor targeting was achieved. Thus, we have demonstrated that small peptide-dye conjugates are effective as contrast agents for optical imaging of tumors.

Receptor-targeted optical imaging of tumors with near-infrared fluorescent ligands

We report here the in vivo diagnostic use of a peptide-dye conjugate consisting of a cyanine dye and the somatostatin analog octreotate as a contrast agent for optical tumor imaging. When used in whole-body in vivo imaging of mouse xenografts, indotricarbocyanine-octreotate accumulated in tumor tissue. Tumor fluorescence rapidly increased and was more than threefold higher than that of normal tissue from 3 to 24 h after application. The targeting conjugate was also specifically internalized by primary human neuroendocrine tumor cells. This imaging approach, combining the specificity of ligand/receptor interaction with near-infrared fluorescence detection, may be applied in various other fields of cancer diagnosis.

Near-infrared fluorescent dyes for enhanced contrast in optical mammography: phantom experiments

Optical mammography with near-infrared (NIR) light using time-domain, frequency-domain, or continuous-wave techniques is a novel imaging modality to locate human breast tumors. By investigating excised specimens of normal and diseased mamma tissue we were able to demonstrate that differences in their scattering properties are a poor predictive parameter for normal and diseased mamma tissue. This paper describes the application of a NIR dye to improve the differentiation between breast tumors and normal tissue in a rat model. The NIR dye furnished a high tumor-to-tissue contrast ratio (6:1) in fluorescence images. Furthermore, this dye was used to develop liquid scattering phantoms with absorbing and fluorescent inhomogeneities. Using frequency-domain and time-domain instrumentation these inhomogeneities were localized at sufficient contrast by their increased absorption and fluorescence. Contrast between inhomogeneities and surrounding medium could be improved by combining fluorescence and transmittance images.

Detection and measurement of in vitro gene transfer by gamma camera imaging

The purpose of this work was to develop a high capacity method to image gene transfer to cancer cells growing as monolayers in cell culture plates. A sensitive and high capacity nuclear-imaging method for detection of gene transfer in vitro will allow rapid validation of vectors in different cell lines under various conditions. Human cancer cell lines (A-427 non-small cell lung, SKOV3.ip1 ovarian, MDA-MB-468 breast, and BxPC-3 pancreatic) were infected with a replication-incompetent adenoviral vector encoding the human type 2 somatostatin receptor (Ad-hSSTr2). Expression of the hSSTr2 reporter protein in cells was detected by imaging an internalized 99mTc-labeled, hSSTr2 binding peptide (P2045, Diatide, Inc.). Imaging provided an accurate measure of internally bound 99mTc as evidenced by equivalence of results for imaging region of interest (ROI) analyses and gamma counter measurements. Internally bound 99mTc-P2045 was linearly correlated (R2 = 0.98) with the percentage of hSSTr2-positive cells following gene transfer. Excess P2045 blocked binding and internalization of the 99mTc-P2045, indicating the specificity of the technique. Up to four 96-well plates could be imaged simultaneously, thereby demonstrating the high capacity of the system. This novel in vitro approach provides a new method to test enhanced gene transfer as new vectors are developed.

Simultaneous evaluation of dual gene transfer to adherent cells by gamma-ray imaging

A gamma camera imaging method was developed to detect dual gene transfer to adherent cells growing as monolayers in cell culture plates. Human cancer cells were infected with replication-incompetent adenoviral vectors encoding the human type 2 somatostatin receptor (Ad-hSSTr2) and/or herpes virus thymidine kinase (Ad-TK). The hSSTr2 and TK reporter proteins were detected by imaging internally bound (99m)Tc-P2045 peptide (Diatide, Inc.) and radioiodinated 2'-deoxy-2'-fluoro-beta-D-arabinofuranosyl-5-iodouracil (FIAU), respectively. Following gene transfer, expression of hSSTr2 and TK were accurately imaged in vitro.

Interfering diffusive photon-density waves with an absorbing-fluorescent inhomogeneity

This work reports an investigation of the fluorescent field re-emitted by an object embedded in a highly scattering media illuminated by two-interfering sources. Simulations in the frequency domain with a finite difference method solving the diffusion equation were performed. The media considered had features typical of a soft-compressed breast. An absorbing-fluorescent inhomogeneity was embedded in the center of the slab. A qualitative study of the re-emitted field was achieved. The re-emitted field was found to possess unique features characteristic of the two-interfering sources excitation, i.e. null intensity when the object was between the two sources and a 180 degrees transition crossing this position. Those features, when performing a scan of the two sources, permitted accurate localization of the inhomogeneity. Moreover, even when the detector was not placed on the mid-plane of the two sources, the re-emitted field still exhibited the interfering characteristic pattern, which was not seen at the excitation wavelength. Thus, for such configurations, the re-emitted field still possessed the specific sensitivity of phased array emission conversely to the excitation wavelength.

In vivo imaging of gene expression:

With the ability to readily engineer genes, create knock-in and knock-out models of human disease, and replace and insert genes in clinical trials of gene therapy, it has become clear that imaging will play a critical role in these fields. Imaging is particularly helpful in recording temporal and spatial resolution of gene expression in vivo, determining vector distribution, and, ultimately, understanding endogenous gene expression during disease development. While endeavors are under way to image targets ranging from DNA to entire phenotypes in vivo, this short review focuses on in vivo imaging of gene expression with magnetic resonance and optical techniques.