February 9, 2009
Molecular Imaging — A New View on Immunotherapy
By Dan Harvey
Vol. 10 No. 3 P. 18
Adoptive cellular gene immunotherapy (ACGT) seeks to harness a patient’s own white blood cells to hunt down and destroy disease-compromised cells. However, its clinical development has been impeded by shortcomings, including inadequate ways to monitor cells. Existing monitoring methods don’t offer users enough information about the in vivo status of genetically modified white blood cells over time.
To surmount these hurdles, collaborating researchers from Stanford University School of Medicine’s Molecular Imaging Program, the City of Hope Cancer Center, and UCLA devised a technique that includes an innovative PET application and the creation of new imaging agents to obtain sequential images that actively reveal critical information about the location, effectiveness, and survival of these disease-destroying cells.
They initially tested this new technique in a small-scale study involving a brain cancer patient. Their results point toward the possibility of more effective cell-based therapies for a range of diseases.
For decades, scientists and clinicians have researched and clinically deployed complex strategies that utilize the human body’s own immune cells (or white blood cells) to function as disease eradicators. So far, they’ve experienced considerable success with creating microscopic stalkers that efficiently track, attack, and kill disease-compromised cells. Scientists are rehabilitating a malign impulse into benign purpose through ACGT.
ACGT involves blood extraction followed by isolation and the removal of the immune cells from the serum. Typically, the removed cells include the T cells that belong to a white blood cell group called lymphocytes, which protect the body from infection.
When applying adoptive immunotherapy to cancer treatment, scientists combine a patient’s white blood cells with a naturally generated growth factor that enhances the cells’ cancer-combating capability. “The overall idea is that, instead of employing chemotherapy or radiation therapy, which are ‘one time deals,’ clinicians reengineer the body’s immune system, training it to track and kill cancer cells but enabling the immunotherapy to function for months and even years, which would wipe out any small cancer cells remaining in the body,” explains Sanjiv S. Gambhir, MD, PhD, director of Stanford’s Molecular Imaging Program.
However, a lack of effective imaging and tracking methodology poses a significant hurdle. Better tools in both areas could reveal exactly what happens to these reengineered cells once they’ve been reinjected into the body. Without a comprehensive picture, researchers essentially are flying blind, says Gambhir, who is also a professor of radiology and a member of Stanford’s Cancer Center. “Until now, for almost any therapy involving genes and cells, they didn’t know what exactly happened to these cells.”
In cancer treatment, researchers have been able to assess downstream effects, such as tumor shrinkage, but questions remained regarding the killing cells’ status: Where exactly did they go once they were reinjected? Maybe they reach the tumor and destroy it. Then again, maybe they don’t. But then do they go into unwanted areas? And if therapy was unsuccessful, was it because the cells fell short of their intended destination, or did the cells just die?
Such questions can be answered by molecular imaging, Gambhir says. “Like any form of therapy, immunotherapy is optimized by imaging,” he says. “In this case, imaging T cells in action adds a lot of power toward successful therapy, as it can provide a direct way to appropriately customize or individualize the therapy.”
Gambhir and colleagues came up with a solution that effectively addresses the aforementioned issues. Their method provides the repeated “snapshots” that reveal cell location and survival status over the course of months and even years using PET scans, a new imaging agent, and cell modification.
The first step in their process involves the initial modification of therapeutic cells to express a unique reporter gene not present in other body cells. The reporter gene is expressed throughout the life of a cell, unlike external radioactive tags that decay within a short period and also can’t provide information about whether a cell lives or dies.
“One of the advantages this technique has over previous tracking methods involves the reporter gene, which is expressed through a cell’s lifetime but not beyond,” says Gambhir, the study’s lead researcher.
Also, the reporter gene, unlike an external radioactive tag, can be duplicated and passed along during subsequent cellular divisions. Further, reporter genes provide researchers and clinicians with exactly what their name implies: They report information about cell location and, like an embedded news reporter in a war zone, send back dispatches about what’s being accomplished at the front.
For the second step, the collaborating researchers developed an imaging agent called 18F-FHBG that will only become entrapped in cells expressing the reporter gene. When researchers inject this unbound imaging agent into the patient, they get an up-to-date map that reveals the cells’ location and viability related to the mission at hand.
Gambhir and colleagues tested their method in a middle-aged man afflicted with an aggressive brain tumor (as reported in the November 18, 2008, edition of Nature Clinical Practice Oncology). “As far as I know, this was the first demonstration of such a reporter gene/imaging solution conducted in a human subject,” says Gambhir.
Even though researchers have been developing reporter gene technology for more than one decade, this collaborative study and its deployed solution was the first that could answer questions about the activities and fate of the reinjected cells with any degree of satisfaction, adds Gambhir.
In March 2005, the 57-year-old-subject had been diagnosed with grade IV glioblastoma multiforme (GBM), a commonly diagnosed malignant brain tumor in which median patient survival is roughly one to three years following diagnosis. Before agreeing to participate in the Stanford trial, the patient underwent surgical resection and radiotherapy. The patient enrolled himself in an ACGT clinical trial following removal of GBM tumor recurrence.
The Stanford study involved the infusion of ex vivo expanded autologous cytolytic CD8+ T cells that were genetically engineered to express the interleukin 13 zetakine gene, which encodes a receptor protein that targets these T cells toward tumor cells—that is, they zero in on cancer cells, as well as the herpes simplex virus type 1 thymidine kinase (HSV1-tk) suicide gene, and 18F-FHBG.
The HSV1-tk gene generates a product that traps a radioactively labeled imaging molecule that can be visualized on a PET scan. Utilizing a PET/CT scanner, the researchers tracked locations of the imaging molecule and, in turn, the modified T cells.
Modified T cells had been returned to the patient’s brain tumor site over a five-week period. Three days after the final cell infusion, the patient received the imaging agent. A subsequent PET/CT scan revealed that the T cells had homed in on the tumor. Also, the cells migrated through the patient’s brain and revealed a second, unsuspected tumor site. Thus, the study shows T cells’ ability to reach intended targets.
The trial was begun nine months after diagnosis, and the patient survived an additional 14 months. Fourteen weeks after the ACGT treatment began, MRI showed significant regression of the tumor.
“In this study, the genetically reengineered T cells did a good job as far as homing in on cancer cells,” says Gambhir, adding that this technique could be used to track other immune cells and, down the road, stem cells throughout the body.
During the imaging portion of the study, the CT component revealed anatomy, but PET revealed the most crucial information. “The PET component, with the imaging agent that we developed ... that goes into all body cells but is only retained in cells with the reporter gene, reveals the emitted background signal that reflects location of the genetically engineered cells,” Gambhir explains.
He says the FHGB is significantly different than FDG-PET because it selectively goes into only the genetically reengineered cells. “FDG goes into all body cells based on their glucose utilization,” he explains. “But with FDG-PET, we have no way of determining the location of the genetically reengineered cells. What makes FHBG-PET quite different is that the agent becomes entrapped only in reintroduced cells, and that’s what allows us to find them. As such, if we want to image a patient later to determine a cell’s location, we re-inject FHBG and, again, it only homes in on and gets entrapped in cells attached with the reporter gene. So it provides a unique way to find what can be called ‘bar-coded’ cells, where the bar code is a gene placed within a cell that allows PET to track the appropriate cells.”
Gambhir concedes that the study’s complexities were challenging. “As the study involved removal of cells, their genetic reengineering, and their reintroduction into the body to accomplish a tumor-destroying mission, as well as utilizing imaging agents and imaging technology, it was a very difficult study to accomplish,” he says. “It required a great deal of approval, first from the FDA and then the internal review boards of the organizations involved.”
Researchers believe similar strategies will work to monitor cell-based therapies for many disorders. Gambhir plans to collaborate with researchers at Stanford and other facilities in continuing the work on T cells and other tumor types, as well as to investigate the way therapeutic cells move within the bodies of patients with arthritis and diabetes.
“We’d like to work with patients with different kinds of therapies. We’re looking at other cancers and other cells beyond T cells,” he says. “The plan is to ramp up research by involving more patients and to determine how and why these cell-based therapies succeed or fail.”
— Dan Harvey is a freelance writer based in Wilmington, Del., and a frequent contributor to Radiology Today.