NEWS

A One-Two Punch: Light-Based Therapy Finds Tumors, Delivers Treatment

Rabiya S. Tuma

Imagine the power of visualizing all of the metastases in a cancer patient and simultaneously targeting each of these lesions with a therapeutic compound. That ideal may not be achievable in the clinic right now, but it is not an outlandish fairy tale either, say experts in the field of molecular imaging. In fact, by combining transcriptional targeting and in vivo imaging, one researcher demonstrated earlier this month that it is already possible in mouse models.



View larger version (93K):
[in this window]
[in a new window]
 
Mice with human prostate cancer tumors were injected with a viral vector that contained a tissue-specific promoter and luciferase. After 3 weeks, images of the mice revealed that luciferase was expressed in the tumor and in the metastases. (Source: Adams et al., Nat Med 2002 Aug;8(8):891-7.) © 2002 Nature Publishing Group. Reprinted with permission.

 
Lily Wu, M.D., Ph.D., an associate professor of urology at the David Geffen School of Medicine at the University of California at Los Angeles, reasoned that if she could design a viral vector with a luciferase gene under the control of a prostate-specific promoter, then she would be able to turn on expression of the transgene only in cancer cells in mouse models of human prostate cancer. And because the transgenic protein is luminescent, then she would be able to light up the tumor cells. Sure enough, when she used a camera to visualize luciferase in animals that had been injected with such an adenovirus vector, she found the luminescence was concentrated in the tumor. What was less expected, Wu said, was that she also detected smaller metastases in the mice.



View larger version (148K):
[in this window]
[in a new window]
 
Dr. Lily Wu

 
"It was quite surprising that we were actually getting to the metastases, because we did not even know that there were metastases in this model," said Wu. "People had documented it, but we just didn’t know that we could see it. That was a tremendous, unexpected, and exciting finding." Wu and her colleagues published their results in the August issue of Nature Medicine.

This type of in vivo monitoring of gene therapy is important, especially when inducible genes are involved, said Ronald Blasberg, M.D., a leader in molecular imaging at Memorial Sloan-Kettering Cancer Center in New York. Therefore, he believes that Wu’s work is an important proof of principle. "If you can turn a reporter gene [like luciferase] on and off—as Lily [Wu] did—you can turn on a therapeutic gene in patients. It would be nice in patients to see that, to see where the gene is turned on and off, and whether you were really achieving the level of expression" necessary for effective therapy, he said.

Working in that direction, Wu and her collaborators at UCLA’s imaging center and Blasberg’s team are each trying to develop vectors that allow researchers to simultaneously see cancer cells and destroy them. Numerous schemes are being tested. For example, Blasberg’s group, including Juri Gelovani-Tjuvajev, M.D., Ph.D., an associate laboratory member at Sloan-Kettering, has built a vector with a tissue-specific promoter upstream of a green fluorescent protein (GFP) sequence fused to that of wild-type herpes simplex virus thymidine kinase (TK), which makes cells vulnerable to the antibiotic ganciclovir. When the vector is transferred to an animal, the researchers use the fluorescence of GFP to track transgene expression and the TK part of the protein to induce cell death upon ganciclovir treatment.

Although light-based systems like the luminescence Wu used and the GFP Blasberg is working with are adequate for small animal models, the light transmission is not powerful enough to penetrate the depth of human tissues. For that reason, many researchers in the field, including both Blasberg’s team and Wu and her colleagues, are developing radiolabeled probes that can be detected by positron emission tomography (PET), an imaging technique already widely used in clinical practice.

In one vector scheme, Wu plans to use TK for both imaging and treatment. With TK under the control of a tissue-specific promoter like the one used in the mouse model of human prostate cancer, the researchers inject the virus into the animals and then treat them with a small, sub-therapeutic amount of labeled ganciclovir. With PET imaging of the whole animal, the researchers can confirm where the transgene is expressed by tracking the concentration of the labeled probe.

"We light it up first," said Wu, referring to the PET images, "... and you see that it gets to the right place and then you add in the pharmacological doses of ganciclovir and you kill the cells."

To create an even more flexible system that can use a variety of imaging tracers and therapeutic genes, Wu and others have developed a two-step transcriptional amplification system. In such a system, one promoter serves as the major control switch, providing tissue specificity, similar to the prostate-specific promoter used in the luciferase experiments described above. The transcriptional product of that promoter, which is expressed only in the target cells, then turns on other promoters in the vector, thereby restricting their expression to the targeted tissue.

This allows scientists to incorporate multiple genes into an imaging and treatment vector, but they only have to worry about controlling one major promoter switch. Preliminary experiments with such vectors show that the relative rate of transcription of the reporter and therapeutic gene is constant, implying that that the reporter gene is an accurate measure of the therapeutic gene’s activity.

So, how far is all of this imaging and gene therapy technology from the clinic? Wu predicts that vectors that combine transcriptional targeting and imaging are probably 3 to 5 years from the clinic. But Blasberg’s group is already enrolling patients in a trial that will piggyback imaging onto viral TK therapy.

In this upcoming phase II trial, which will be run in collaboration with Savio Woo, Ph.D., director of the Mount Sinai Institute for Gene Therapy and Molecular Medicine in New York, the researchers will test the combined power of gene therapy and molecular imaging in patients with either localized prostate cancer or colorectal cancer metastases in the liver. The researchers will treat the patients with viral TK after surgical resection of their tumors. In this case, the viral TK gene won’t be under the control of a tissue specific promoter, so the researchers will deliver the gene to the residual tumor bed by liposome transfection.

The researchers will administer a small, subtherapeutic dose of radiolabeled ganciclovir, image the region with PET, and then administer larger therapeutic doses of ganciclovir to kill the cancer cells. Although the trial is not testing transcriptional targeting—those vectors are not yet approved for use in humans—the clinicians will have the benefit of seeing where TK is active and monitoring its activity over time.

"The field is moving so rapidly that what you see is targeted therapies in animals, but they have not been translated into patients," said Blasberg. "It is a wide-open field, but for clinical applications to be able to monitor the therapeutic response for gene therapy" would be tremendously powerful.



             
Copyright © 2002 Oxford University Press (unless otherwise stated)
Oxford University Press Privacy Policy and Legal Statement