Tumor Painting

A Toxin From Scorpion Venom Combined With A Fluorescent Beacon Specifically Lights Up A Variety Of Cancers

By Chris Hubert

You finally have a clear view of the patient's brain and begin carefully inspecting the tissue. The MRI scans show most of the tumor in the left rear quadrant of the brain. The patterned folds present elsewhere in the brain are disrupted here, and the tissue is irregular, slightly less firm. You recognize this as not normal brain tissue, but the surface of tumor you must remove. It's important to remove the entire tumor; otherwise the cancer can return and may become even harder to treat. But if you remove too much, the damages can be severe and permanent. If only it didn't all look about the same. You hold a silent breath and start to cut.

"You guys need to come up with some way to make these cancer cells light up so that we can see them,” said Richard Ellenbogen, the chief of neurosurgery at Children's Hospital and Regional Medical Center in Seattle. It was time for "tumor board,” a conference where surgeons, oncologists, radiologists, nurses, and other medical staff discuss any complicated cancer cases in the hospital. Topics could include anything from assessing scans for potential relapse to planning upcoming difficult treatments.

That day, Ellenbogen was discussing a particularly troublesome case in which surgeons left behind part of a young girl's brain tumor because it looked just like normal brain. For the last 20 years, Ellenbogen had thought about how to light up brain cancer cells. He turned to Jim Olson and said, "I believe your lab can come up with the answer.”

Jim Olson runs a brain cancer research lab at the Fred Hutchinson Cancer Research Center. Last year, his lab released a paper describing the development of "Tumor Paint,” a novel molecule to make cancer cells glow, and his lab is currently testing its use in a variety of mouse cancer models.

Tumor Paint has given Olson a great feeling of satisfaction. Like Ellenbogen, he too had been dreaming of identifying cancer cells for close to 20 years. In 1989, Olson remembers, he was defending his thesis project describing a technique to make tumors detectable by hospitals' scanners. When asked what was next for his project, he replied that he wanted to make the cells actually change color. In 1989, technology hadn't evolved to a point where this feat was possible. Now, after being challenged in the tumor board meeting, Olson started to think there might finally be a way.

Olson soon brought Patrik Gabikian, a neurosurgery resident, into the lab to start looking for a way to specifically mark tumor cells. Gabikian started sifting through the computer databases. Cancer reports. Neuroscience journals. National Library of Medicine listings. Text books. He hated it. "Every day Patrik would ask me for reagents [to start his experiments]” Olson recalls. "And I'd tell him no.” There was considerable amount of research already published and Olson thought there must be something buried in the literature that they could use.

Several weeks wore by before Gabikian found a few reports using a small protein called chlorotoxin, found in the venom of the Israeli desert scorpion. Researchers were studying how chlorotoxin inhibits the flow of salts through channels in the membrane of glioma (a type of brain cancer) cells. Most exciting was a report that proved radioactive chlorotoxin could bind to gliomas in the brain while bypassing most normal tissues.

Chlorotoxin had all the signs of a superb targeting protein. The scorpions had already spent millions of years evolving a protein that could cross from the blood into the brain– a major hurdle for human drug developers. Researchers now believe that it binds to a cell surface protein called MMP2 which acts like a molecular brush cutter clearing space around the cell. Since cancer cells are constantly growing, they need proteins like MMP2 to make space for themselves in the body. Chlorotoxin could therefore stick to many different kinds of cancer. Maybe.

With a targeting molecule in hand, the next decision was how to light it up. Purple, blue, green, and yellow light do not pass through tissue well, so a tumor emitting these colors would be missed if brain tissue got in the way. Red light shines through tissue better but is harder to see and is obscured by blood. "Dyes at longer wavelengths can penetrate through the tissue better and have lower background,” explains Mark Stroud, a senior scientist in the Olson lab. The solution was to go beyond red and use near-infrared light. Although it cannot be seen with the naked eye, near-infrared shines through tissue better than any color in the rainbow and is easily seen by special cameras.

For light, Olson decided to utilize a fluorescent dye called Cy5.5 to emit the near-infrared light. Since his lab did not have the chemistry experience to join the two molecules, Olson partnered with Miqin Zhang at the University of Washington. Zhang returned the linked molecules now named Chlorotoxin:Cy5.5.

After about 6 months of testing and evaluations in the lab, Gabikian injected about 1/300th of an ounce of Chlorotoxin:Cy5.5 into the bloodstream of a mouse with a glioma. "As soon as we saw the first image, that was the moment I knew it would work,” Olson recalls.

To test Chlorotoxin:Cy5.5 in mouse models of cancer, postdoctoral researchers Mandana Vieseh and Barham Bahrami took on the job of developing operating procedures, dye synthesis and purification schemes, and imaging methods. Throughout their testing, cancers kept lighting up one after the other. Gliomas, other brain cancers, prostate cancer, muscle cancer, colon cancer, skin cancer. Chlorotoxin:Cy5.5 quickly earned the name "tumor paint.”

Olson's team was imaging a tumor in a lymph node, when they noticed what appeared to be a section of fat highlighted by the tumor paint. This was important since up until now fat had never lit up. They put the tissue section under a microscope and focused in. It was indeed fat tissue, but running through the fat was a tiny lymph channel connecting the cancerous lymph node to other healthy ones. Glowing in that channel was a cluster of about 200 cancer cells that had broken away from the main tumor and had made it halfway to the healthy lymph nodes. Current MRI scanners can only detect cancers when more than 1 million cells are together.

The future seems bright for tumor paint, and Olson hopes to begin human trials within a few years. Stroud is now designing methods to synthesize tumor paint in sufficient size and purity for FDA approval. "Ultimately, we want it in a sterile bottle where you can stick in a needle and inject it into your patient [before surgery].”

Stroud is currently testing other near-infrared light molecules that may work better and be easier to use in patients than Cy5.5. Another set of projects is investigating if Chlorotoxin can target drugs, nanoparticles, or other therapies to the cancer cells while bypassing the rest of the body.

Chris Hubert is a graduate student in Molecular and Cellular Biology at the University of Washington. He is currently a member of the Olson lab but is conducting unrelated research and has not contributed to any work described in this article.

Image:

Four mouse brains: The two mouse brains on the left are normal and have no tumors. One of the mice was treated with Tumor Paint, but the Tumor Paint gives little color to this normal brain. The two brains on the right are from mice with tumors towards the rear (bottom of this image) of the brain and both have been treated with Tumor Paint. Photo: Stacey Hansen