Five researchers are among those leading efforts to bring advances in molecular imaging into broad clinical application. Although the five investigators are concentrating on different phases of research—from pursuing the initial eureka moments through conducting early testing to developing human diagnostic agents—they share a commitment to unlocking the power of the smallest tools to detect disease.

As the imaging community moves continually toward the day when looking for changes in anatomy is naturally accompanied by the evaluation of changes in physiology, these five projects are likely to serve as important stepping stones. All five projects are being funded through the National Institute of Biomedical Imaging and Bioengineering of the US National Institutes of Health.

In the long run, it is the ability to erase the line dividing structure from function that could become the legacy of these and other pioneering efforts in molecular imaging. It might one day be as routine to view the activity of a tissue or organ as it now is to look at its shape. The reason for that change is as important as its consequences: In medicine, the early detection of a problem often means that its solution will be more effective, less costly, and easier for the patient to undergo.

Contrast patterns labeled with two different contrast agents. In vivo MRI of macrophage accumulation on POD 6 of an allograft heart with MPIO-particle labeling (D) an isograft heart with MPIO-particle labeling (E), and an allograft heart with USPIO labeling (C). Click on image for larger view.

Optical Molecular Imaging

The methods and agents involved in tissue-specific optical imaging are being investigated by Samuel Achilefu, PhD, professor of radiology at Washington University School of Medicine, St Louis.1 He is working toward earlier, more accurate diagnosis of cancer through molecular imaging of the genes of proteins found within tumors, which diverge from similar genes found in normal tissues. In turn, of course, earlier and better cancer diagnosis can improve patient outcomes.

Tumors and similar pathologies have conventionally been subjected to nuclear methods of molecular imaging. Achilefu, however, is attempting to complement these established types of imaging by applying the newer diagnostic abilities found in optical imaging. This is still an emerging field; it uses light from the near-infrared sector of the spectrum to assess tissues at a depth of up to several centimeters. In addition to superficial lesions and tissues exposed through surgery, the areas evaluated can be deeper organs reached using endoscopes.

Achilefu is developing new agents for visualizing tumors in living tissues. These novel methods of optical imaging rely on small peptides that are specific to certain receptors. These can be made to accumulate, quickly and selectively, in the cell membranes of tumors having the target receptor proteins. As a result, the intensity of fluorescence will vary according to the expression of the receptor in the tumor, allowing it to be visualized against the background of normal tissues.

In vivo MRI of a rat allograft heart over time. MPIO particles are administered once on POD 3.5, and the same animal is imaged on POD 3.5 (A), 4.5 (B), and 5.5 (C). Click on image for larger view.

Nuclear methods of molecular imaging already have indicated that somatostatin receptor subtype 2 (STR2) is clinically useful. In order to decrease the time needed to bring research on optical imaging agents into the clinical realm, Achilefu is beginning with STR2 as his basis for the design and synthesis of new optical molecular probes. These agents will be tumor specific, but also will be applicable to the detection of multiple tumor types and other forms of disease.

To obtain the best possible selectivity and binding affinity for STR2, molecular modeling and 2D nuclear magnetic resonance (NMR) will be used. Then, the candidates that appear most useful will be assessed for suitability before proceeding to in vivo studies; their cytotoxicity, receptor-binding affinity, receptor-mediated internalization, and subcellular distribution will be evaluated.

Tumor xenografts will be used to test the sensitivity and specificity of the new STR2 agents through both biodistribution and planar optical imaging. Then, histological analysis will be used to validate the optical images. Using these methods, Achilefu expects to identify at least one optical molecular probe that can proceed to translational research.

Magnetomotive Probes

Stephen A. Boppart, MD, PhD, associate professor in the department of electrical and computer engineering at the Beckman Institute for Advanced Science and Technology at the University of Illinois, Urbana?Champaign, is conducting research on magnetomotive optical probes for molecular imaging.2 He is developing a new class of probes that respond to the presence of a controllable external magnetic field in such a way that they change visible optical contrast.

Optical coherence tomography (OCT) is being used investigationally to perform in vivo molecular imaging with the magnetomotive probes. Several applications of this work are possible because the resolution scale of imaging using this method is unique, with a high detection sensitivity and dynamic range. The intended uses of magnetomotive OCT include basic biomedical research into the pharmacokinetic and other properties of the probes themselves, as well as disease characterization in living tissue and evaluation of drug efficacy.

Magnetomotive OCT is expected by the investigator to meet today’s need for in vivo molecular imaging that is both higher in sensitivity and more specific. It will penetrate tissues on a scale measured in millimeters but provide resolution on a scale measured in microns. The extravasation and diffusion of the probes under various conditions are being researched. Boppart, for example, is studying the effects on probe movement of dimensions and surfactants. Changes that optimize the external magnetomotive forces applied to the probes are being used to determine the strength with which probes bind to their targets and the best probe-to-target ratios for molecular imaging.

Boppart sees magnetomotive probes as likely to become new contrast agents for eventual use in ultrasound and MRI studies. They also show promise for use in hyperthermic therapy, and might be good candidates for ultrasound-mediated drug delivery to target tissues. Both of these applications could add significantly to the power of the current cancer-fighting arsenal. Most likely, probes would be made available in different forms according to whether they were intended for use in the early diagnosis of cancer or in its treatment.

Tumor uptake of magnetomotive probes varies according to the tumor’s stage of growth, the investigator noted,2 so magnetomotive OCT could prove valuable in cancer staging and tumor characterization. In addition to developing probes having high circulation times and low uptake by nontargeted tissues, Boppart is focusing on ensuring that the probes yield more of the desired data by creating various types, each optimized for use in a different type of tissue. Mammary tumors in rats are being used to assess probe delivery, extravasation, and uptake.

In vivo MRI of allograft hearts and lungs, 1 day after IV injection of MPIO particles. Shown with 156- m in-plane resolution at 4.7 Tesla by using a Bruker Biospec AVANCE-DBX MRI instrument. Click on image for larger view.

The Reporter Gene

Noninvasive imaging of cell migration is the focus of work being done by Jeff W.M. Bulte, PhD, professor of radiology at Johns Hopkins University School of Medicine, Baltimore, and director of cellular imaging at the Institute for Cell Engineering.3 This emerging field may be the key to a deeper understanding of the ways in which cells and tissues interact. This knowledge, in turn, could lead to the exploration of new kinds of cell therapies—particularly those involving stem cells and progenitors.

Using a new chemical exchange saturation transfer (CEST) reporter gene will allow the detection of labeled cells using MRI. CEST imaging can detect artificial proteins in the nanomolar range. These amide-enriched proteins, called lysine-rich protein and arginine-rich protein, allow direct visualization of the gene product, without any need for tissue penetration. Cell proliferation does not limit detection sensitivity, and CEST contrast is provided only by living cells.

Histological examination of allograft hearts on POD 6 after in vivo MPIO-particle labeling. (A?C) Optical micrograph (200 magnification) of three neighboring 5- m tissue sections of a POD 6 allograft heart stained with Perl’s Prussian blue for iron (D and E) Optical micrograph (400 magnification) of two neighboring 5- m tissue sections of another POD 6 allograft heart stained with Perl’s Prussian blue for iron (F and G) Partial view of optical micrograph (400 magnification) of two neighboring 5- m tissue sections of a POD 6 allograft heart stained with Perl’s Prussian blue for iron and anti-rat ED1 for macrophage (H) Electron micrograph of an allograft heart harvested on POD 6 after in vivo MPIO-particle labeling.

Also, the utility of CEST imaging might be increased because this type of contrast can be made active and inactive—in a sense, turned on and off—repeatedly. In addition, two or three different labeling strategies can be used at the same time in order to make one imaging session yield more information.

Bulte’s work already has shown that a CEST reporter gene can be cloned successfully, and that it is expressed in transfected cells without altering either the viability of those cells or their proliferation. The reporter gene is detected (using MRI) in phantoms. The investigation is proceeding under the hypothesis that the same kind of detection will be possible in living tissue.

The detection of transfected glioma cells and neural stem cells doubly labeled with lysine-rich protein and arginine-rich protein in animal models will follow the synthesis of new CEST reporter genes that are more efficient than older types.

The long-range goal of Bulte’s work is to overcome the high signal-to-noise ratio of MRI when it is used to detect cells using labels or tracers. This would allow users to take full advantage of MRI’s high spatial resolution and its ability to obtain data concerning both anatomy and function. Intracellular endosomal tagging with superparamagnetic nanoparticles has been of some use in this respect, but this type of contrast can create areas of hypointensity that hide tissue structure. In addition, cell proliferation dilutes the label, making it hard to tell living cells from dead ones. Possible metal toxicity is another of the problems that Bulte is working to solve.

Multidisciplinary NMR

Chien Ho, PhD, is professor and director of the NMR Center in the department of biological sciences at Carnegie Mellon University, Pittsburgh.4 He is involved in at least 17 molecular-imaging projects through the NMR Center, which is a multidisciplinary operation involving medicine, NMR, electrical engineering, and computer engineering. The center’s goal is to use MRI and MR spectroscopy to address major biomedical problems by acquiring accurate data on biochemistry, physiology, and morphology.

Seven of the center’s projects are classified as service work, relying on use of the facilities and knowledge of the center and covering a range of biomedical topics. The center trains graduate and postgraduate students, postdoctoral research associates, and medical fellows in the biomedical applications of NMR.

Five more projects concern technological research and development. They involve high-resolution dynamic imaging, monitoring the function of transplanted organs using MR bioimaging, quantitative perfusion imaging, MR microscopy in living tissue, and the detection of allograft rejection using noninvasive methods.

Another five projects are collaborative research, Ho noted. These investigations address the use of MR microscopy to determine structural alterations in the mouse forebrain that depend on gender and age; the use of MRI for in vivo monitoring of immune-cell migration in rats; the use of MRI to detect tissue rejection in large animals; the noninvasive measurement of antitumor activity and tissue distribution of interleukin-2 activated natural killer cells, transferred adoptively; and the use of MRI in rats to assess traumatic brain injury.

Ultrasonic Contrast

Samuel A. Wickline, MD, professor of internal medicine, biomedical engineering, and physics in the Molecular Cell Biology and Computational Biology programs at Washington University, is researching the development of a tissue-targeted contrast agent for ultrasonography.5

The underlying hypothesis of this work is the premise that specific epitopes (the antigen areas where antibodies become attached) are necessary for tumor growth and implantation. Fibrin, for example, is the provisional matrix of a tumor; transmembrane glycoproteins are present during the early proliferation of new blood vessels that a primary or metastatic tumor needs in order to grow. Other epitopes of interest in this research are also in evidence during the early stages of cancer.

The expression of particular epitopes will be the target of Wickline’s newly developed contrast agent: a liquid-perfluorocarbon nanoparticle. Through the use of monoclonal antibodies or small-molecule peptidomimetics to create targeting ligands, selected molecular epitopes will become detectable. Eventually, this could lead to clinical applications involving several pathologies.

Wickline has chosen to work toward development of this contrast for use with ultrasound because of the modality’s broad availability worldwide. Since ultrasound equipment is generally inexpensive and portable, it is available at most medical facilities. This factor, according to Wickline, will carry clinical molecular imaging into global use, as well as make it useful in the diagnosis of many types of disease.

In defining the role played by targeted nanoparticles in the characterization of tumors, Wickline is working with ultrasound units at both high power levels (used for research) and at the power levels now commonly found in clinical use. As a surrogate endpoint for the study of drug delivery, ultrasound is being used to monitor the local delivery of antiangiogenic agents by drug-carrying nanoparticles.

Noncavitating focused ultrasound—applied using commercially available units—is being used to investigate ways of boosting the therapeutic efficacy and drug deposition of drug-carrying nanoparticles. The same work is expected to help improve the accuracy of diagnosis using targeted nanoparticles.

Another part of this project involves ultrasound scientists from Philips Medical Systems, Andover, Mass, who are working with Wickline to develop and apply new thermal and imaging probes (TIPs). Because they combine methods, TIPs might be superior for visualization of some kinds of nanoparticles using thermal flash imaging. They also could be used for therapies that involve the focused phase conversion of targeted liquid-perfluorocarbon nanoparticles.


Although the early detection of primary and metastatic tumors might be the first thing that comes to mind when molecular imaging is mentioned, it is clear from the work of these five researchers and their counterparts elsewhere that even more is to be expected from this field, both now and in the future. Most likely, cancer diagnosis will remain the driving force behind much activity in imaging on the molecular scale; however, the clinical applications that can be predicted thus far will be both deeper (addressing response to therapy and other oncology questions in addition to initial diagnosis) and broader (dealing with many other disease entities and dysfunctions). Over the long term, molecular imaging may well touch every subspecialty within imaging, whether those divisions are defined by body part or modality.

Kris Kyes is technical editor of  Medical Imaging. For more information, contact .


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