APRIL 2003

Research moves from bench to bedside

Work revolves around new imaging techniques, probes, and gene delivery systems

By: Karen Sandrick

Clinicians and researchers have long recognized the need for noninvasive in vivo imaging alternatives to serial biopsies and pathology slides. Molecularly focused therapy is the newest way to assess whether genes or drugs are finding their intended targets and eliciting the expected response.

For the last half-dozen years, research centers in the U.S. and Europe have adapted existing imaging modalities and radiopharmaceuticals to look not just at structural anatomy but at intracellular molecular processes, organelles, and cell surface receptors. On an even more fundamental level, these centers are developing new imaging techniques and agents to delve into the biological processes underlying disease. Research explores how cells live or die, how cellular signals spur or blunt tissue differentiation, and how growth factors and other mechanisms provoke tumor expansion, inflammation, and autoimmune reactions.

While molecular imaging laboratories in Europe are usually privately funded, their counterparts in the U.S. enjoy strong support from the Biological Imaging Program (BIP) of the National Cancer Institute. The NCI's budget for sites that are creating small-animal imaging methods, new radiopharmaceutical tracers, and gene delivery tracking strategies has grown from about $20 million to more than $65 million over the last five years, according to Dr. John M. Hoffman, chief of the Molecular Imaging Branch of the BIP.

The NCI's investment is beginning to pay off in innovations relevant to imaging practices. Optical imaging, a research technology that is barely five years old, has improved its ability to probe beneath surface tissues well enough to be studied at the University of Texas M.D. Anderson Cancer Center as a potential means of identifying potentially malignant areas in women, Hoffman said.

At Washington University in St. Louis, technetium-99m sestamibi, a well-accepted radiopharmaceutical agent for cardiac perfusion studies, is being exploited to analyze the functional expression of the multiple drug resistance gene and its gene product P-glycoprotein in a prospective clinical trial of women with locally advanced breast cancer.

Dr. Ralph Weissleder, director of the Center for Molecular Imaging Research (CMIR) at Massachusetts General Hospital, has spearheaded the development of monocrystalline iron oxide nanocompounds as one of many promising agents used with MR for molecular imaging. Weissleder and colleagues are conducting phase III clinical trials.

Nuclear medicine programs at several U.S. institutions are investigating how FDG-PET can be used to predict cancer case outcome and assess the effectiveness of therapy. These include Duke University, Memorial Sloan-Kettering Cancer Center, the University of Michigan, New York Weill Medical College of Cornell University, the University of California, Los Angeles, and Washington University.

Meanwhile, researchers in Switzerland are experimenting with fusion PET/CT. When combined with CT, PET can evaluate the utility of cellular markers such as choline in prostate cancer, said Dr. Gustav von Schulthess, director of nuclear medicine at the University of Zurich.

Much of the work in these laboratories involves fashioning basic imaging tools, such as technologies for observing disease models in small animals. University research centers in Ferrara and Pisa, Italy, have built a hybrid PET-SPECT tomograph that has four detecting heads, each with a scintillator matrix of 20 x 20-cm matchlike crystals. It provides a spatial resolution of 1.6 mm for PET and 3.5 mm for SPECT and a 4-cm field-of-view. The Center for In Vivo Imaging in Cancer Biology at UCLA, under the direction of Dr. Harvey Herschman, has pioneered the development of microPET. With a resolution 10-fold greater than a standard scanner, microPET can visualize structures 2 mm in diameter. CMIR is using a dedicated 4.7T mouse MRI, a near-infrared tomographic system that has a laser-diode source and CCD detector, a microCT, and a whole-body bioluminescence unit.

RADIOPHARMACEUTICAL PROBES

Molecular imaging researchers are also tinkering with existing radiopharmaceutical probes and creating new ones. The In Vivo Cellular and Molecular Imaging Center at Washington University has changed the formulation of the single-photon agent Tc-99m sestamibi to produce the Tc-94 isotope, which emits positrons that can be quantified by PET to identify malignancies in deep regions of the body such as the lung, said Dr. David Piwnica-Worms, the center's director. The center is also synthesizing gallium complexes that provide a scaffolding for recognizing P-glycoprotein with PET or SPECT.

Researchers are using a multifaceted imaging approach to analyze reporter gene systems that monitor endogenous genes and their signal transduction pathways from in vitro fluorescence assays in cell culture all the way through imaging in humans, said Dr. Ronald Blasberg, director of the Center for Multidisciplinary In Vivo Molecular Imaging in Cancer at Sloan-Kettering. Blasberg is monitoring the downstream expression of the p53 gene signaling pathway, which can lead to apoptosis, to test the effect of chemotherapy involving alkylating agents such as tyrosine kinase inhibitors. Blasberg is also studying hypoxia inducible factor, which modulates gene expression in solid tumors.

Many molecular imaging research projects over the last five years have been proof-of-principle experiments to show that experimental techniques can measure gene transfer to tumors or the expression and repetitive monitoring of genes in transgenic mice. Herschman points out that now these techniques are being applied to answer pivotal questions in biology and therapeutics: how vascular endothelial growth factor directs the development of angiogenesis and how adenoviruses that carry gene treatments can be directed away from the liver to avoid toxicity.

"There are many tools in cell biology to study how cells work under a microscope," Piwnica-Worms said. "We are taking those tools and developing new ones to study cellular interactions noninvasively in higher orders of whole system regulation in the context of intact animals."