November 2005

JHU ICMIC spurs advanced research

Fusion mapping strategies prove key for study of tumor response

By: Don Rauf

Discoveries relating to the imaging of hypoxia, angiogenesis, and ligand receptors demonstrate the scientific prowess of the In vivo Cellular Molecular Imaging Center at Johns Hopkins University and justify its reputation as one of the top molecular imaging laboratories in the world.

"The vision of our JHU ICMIC is to combine state-of-the-art imaging capabilities with powerful molecular biology techniques to define strategies with 'intent to cure,'" said Dr. Zaver Bhujwalla, director of the ICMIC and a professor of radiology and oncology at JHU.

Bhujwalla's lab has recently been analyzing hypoxia and hypoxia inducible factor, or HIF-1 (a protein that regulates how cancer cells respond in a hypoxic environment). The researchers are studying how to exploit the hypoxia response element to target cancer cells through choline kinase inhibition.

Choline kinase is an enzyme that is overexpressed in human breast, lung, colorectal, and prostate tumors, and Bhujwalla has been pursuing the cause of this altered choline metabolism in cancer. Using MRI, MR spectroscopy, and optical imaging, she found that hypoxia plays a role in creating more total choline. (Hypoxia often occurs within tumors because tumor vasculature is highly chaotic compared with normal tissue.)

Using fusion mapping strategies and green fluorescence protein (GFP) expression under control of a hypoxia response element, JHU scientists have measured vascular volume, permeability, choline distribution, and hypoxia distribution.

"Because we could create a cell line that fluoresces under hypoxia, we were able to see that areas of hypoxia showed elevated total choline," Bhujwalla said.

Dr. Gregg Semenza is investigating the role of HIF-1 in breast cancer invasion and metastasis; Dr. Dmitri Artemov is leading studies on imaging HER-2/neu receptor expression with MRI and determining the role of hypoxia and pH in receptor expression; and Dr. Venu Raman is exploring the use of receptor ligand-based imaging systems to target hypoxia in solid tumors.

Raman's work reflects the novel imaging approaches under development at JHU. His team has devised a nonmammalian receptor ligand system (ER-1mem-ER-1) to be used in optical, MR, and PET imaging in animal models.

Jeff J.W. Bulte, Ph.D. an associate professor of radiology at JHU, has pioneered the field of stem-cell trafficking to aid in the treatment of multiple sclerosis and other neurodegenerative diseases. By developing iron oxide contrast agents that tag cells, Bulte has tracked stem cells implanted in a living animal, using MRI viewed with 3D imaging software. In collaboration with the University of Nijmegen in the Netherlands, he is conducting the first clinical trial of magnetically labeled immune therapeutic cells for treating melanoma patients.

In molecular imaging applications for cardiovascular medicine, Dr. David Bluemke, clinical director of MRI at JHU, has led many clinical trials pertaining to myocardial function. He notes that cardiac MR imaging is becoming central to the management of patients with cardiovascular disease.

"We've especially had potential for great advances with new cardiovascular agents," he said. "Contrast agents that stay in the blood stream and circulate for 10 to 15 minutes allow us to study arterial anatomy with greater resolution."

Ongoing novel investigations include his work with MR assessment of unstable plaque in arteries. Bluemke has been conducting clinical trials of a marker developed by Epix Pharmaceuticals that would tag the fibrin associated with blood clot and potentially identify unstable plaque.

Clinical trials have lagged behind developments in basic research labs because of their complexity and increasing regulation by the FDA. It takes 10 to 15 years from the development of a new agent in the laboratory to its approval for human use, Bluemke said.

The JHU ICMIC also interacts closely with JHU's small animal imaging resource program (SAIRP). Led by Dr. Martin Pomper, the SAIRP works hand in hand with the ICMIC and focuses much of its work on radiopharmaceutical development for PET scanning and SPECT. Pomper's latest research focuses on synthesizing new small-molecule PET and SPECT tracers based on the prostate-specific membrane antigen used for prostate cancer. In addition, his team has produced new ligands for inflammation and infection imaging, and they are investigating a molecular genetic imaging approach for studying the specific "hedgehog signaling cascade" that is active in various cancers.

In technological developments, the SAIRP has contributed to the creation of new scanning equipment. The researchers have been working with Jefferson Lab in Newport News, VA, to invent a SPECT scanner that will be able to image animals while they are awake, and with the University of Virginia to develop a trimodality device with optical, gamma, and x-ray capabilities. The active participation of Dr. Richard Wahl, director of nuclear medicine, facilitates close integration of the JHU ICMIC with that department.

Discoveries have arisen from the creative interactions of ICMIC's interdisciplinary staff. The center has assembled a highly trained and collaborative group of chemists, who make the probes, molecular biologists, who help create molecular genetic reporters, and technologists, who help develop new imaging equipment.

Bhujwalla has brought people from different disciplines together, said assistant professor Kristine Glunde, Ph.D. Researchers who formerly worked in six different departments at JHU were recruited to perform molecular imaging research.

Glunde is creating optical probes incorporated into lysosomes for breast cancer analysis. When she needed to synthesize a contrast agent, she readily turned to one of the lab's synthetic chemists for assistance.

"It's an open-door mentality here, and people are very willing to collaborate," Glunde said.

The lab's success also stems from collaborations with numerous departments at JHU and the longstanding support of its radiology departments, according to Bhujwalla. Dr. Elias Zerhouni was an early proponent of the ICMIC before he left JHU in 2000 to become director of the National Institutes of Health. The current chairperson, Dr. Jonathan S. Lewin, is another strong supporter.

"Dr. Chi Dang, the vice dean for research, has been a major proponent of molecular imaging; we are also fortunate to have a high level of interaction with several members of our cancer center," Bhujwalla said.

The center's collaborative efforts extend beyond JHU's campus in Baltimore. Bhujwalla has worked closely with Robert Gillies, a professor of biochemistry and molecular physics and radiology at the University of Arizona, for example. Gillies' research on tumor pH has been essential for understanding the tumor microenvironment.

Novel pilot projects and career development awards have helped the JHU ICMIC attract research talent. The lab is geared toward cultivating the work of promising researchers, according to Bhujwalla.

"This is our investment in the future," she said.

In terms of current investment, Bhujwalla is an adept fund-raiser. Through her leadership, the lab was selected by the National Cancer Institute in 2000 as one of six designated ICMICs. That designation has thus far funneled at least $8 million in NCI grants into her program.

"Dr. Bhujwalla has really taken the lead in terms of setting up objectives, priorities, and novel development for these laboratory applications," Bluemke said. "She has brought so many people together in a large enterprise, and she is keeping us moving forward."