STANFORD, Calif. — Coaxing a patient's own cells to hunt down and tackleinfected or diseased cells is a promising therapeutic approach for manydisorders. But until now, efforts to follow these specially modifiedcells after their reintroduction to the body have relied on short-termmonitoring techniques that don't give a complete picture of the cells'status.
Now, for the first time, researchers at the Stanford University Schoolof Medicine have devised a way to obtain repeated "snapshots" of thelocation and survival of such cells in a living human patient months andpossibly years later. This is good news for individual patients andclinicians who may want to assess the cells' disease-fightingperformance over time, as well as for researchers trying to design moreeffective cell-based therapies.
"This has never before been done in a human," said the senior author ofthe research, Sanjiv Gambhir, MD, PhD, director of Stanford's MolecularImaging Program. "Until now, we've been shooting blind—never knowing whyfailed therapies didn't work. Did the cells die? Did they not get wherewe wanted them to go? Now we can repeatedly monitor them throughouttheir lifetime." Gambhir is a professor of radiology and a member ofStanford's Cancer Center. He collaborated with researchers at City ofHope in Los Angeles and at UCLA to conduct the research.
Gambhir and his colleagues tested the technique in a middle-aged manwith an aggressive brain tumor (called a glioblastoma) who was enrolledin a clinical trial of cell-based therapy at City of Hope. However, theybelieve similar strategies will work to monitor cell-based therapies formany disorders. The results of the case study will be published onlineNov. 18 in Nature Clinical Practice Oncology.
The new approach relies on a two-step process: first, the therapeuticcells are modified to express a unique reporter gene shared by no othercells in the body. Second, an imaging agent that is trapped only incells expressing the reporter gene is injected into the patient. Theunbound imaging agent is otherwise quickly cleared from the body. Eachtime the imaging agent is used, the researchers get a new, up-to-datemap showing the cells' locations.
The technique has several advantages over previous tracking methods.Unlike an external radioactive tag, which decays over a short time anddoes not indicate whether a cell is living or dead, the reporter gene isexpressed throughout a cell's lifetime, but not beyond. Furthermore,unlike an external tag, the reporter gene is duplicated and passed alongif the original cell divides. Finally, different reporter genes can beused that could indicate not only the location of the cells, but alsowhat they're up to.
"In this patient, the reporter gene was always on," said Gambhir. "Butthe beauty of this approach is that we could make it so the reportergene is expressed only if the cell differentiates, or finds a certaintarget. Has the T cell found a tumor? Has it activated its cell-killingmachinery?"
In the current study, Gambhir collaborated with Michael Jensen, MD,associate chair of the cancer immunotherapeutics & tumor immunologyprogram at City of Hope, and others to remove cytotoxic, or "killer," Tcells from the man with glioblastoma. These cells naturally seek out anddestroy infected or dysfunctional cells in the body. The researchersthen inserted a circle of DNA encoding two key genes into these T cells.One endowed the cells with the ability to home in on the cancer cells.The other encoded a gene from a herpes simplex virus called thymidekinase, or HSV1-tk. The product of the HSV1-tk gene traps aradioactively labeled imaging molecule that can be visualized on a PETscan. Any imaging molecule that is not trapped in the modified T cellsis eliminated from the body. A clinical PET-CT scanner tracks thelocations of the imaging molecule and therefore the modified T cells.
The researchers then returned the modified T cells to the site of thepatient's brain tumor over a period of five weeks. The patient receivedthe imaging agent three days after the last infusion of cells. As theresearchers had hoped, the subsequent PET-CT scan showed that the Tcells had homed in on the tumor. However, they also migrated through thepatient's brain to highlight a second, previously unsuspected tumorsite. Although this study did not assess the ability of the T cells tokill the tumor cells, the imaging results suggested they at least got totheir targets.
"The cells were actually good at finding the tumor," said Gambhir, whopointed out that the same technique could be used to follow other immunecells or eventually stem cells throughout the body. He plans tocollaborate with other researchers at Stanford and elsewhere to not onlycontinue his study with T cells and other tumor types, but also toinvestigate the movement of therapeutic cells in patients with arthritisand diabetes.
The study could not have been done without the concerted efforts ofresearchers at Stanford, UCLA, City of Hope and the Food and DrugAdministration, Gambhir emphasized. Genetically modifying cells forre-infusion into a human patient requires rigorous quality-controlmeasures and extensive ethical review. The researchers selected theglioblastoma patient for their first attempt because this cell-basedtherapy trial was already approved by the FDA. Gambhir also had FDAapproval on the PET imaging agent.
"It took all of the institutions to come together to make such a complextrial work," said Gambhir, "but, because we're not limited to just onecell population, the results are tremendously exciting."