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The findings come from an early stage trial of just 31 patients. But experts were cautiously optimistic about what the study showed: The drug's side effects were manageable, and four patients saw their tumors shrink.
That's a small number, but a trial like this is largely aimed at seeing whether a drug is safe and finding a tolerable dose.
Much more remains to be learned about the drug -- known for now as IMCgp100. "But," Weber said, "it is good enough to show some activity in an early stage trial. It's definitely worth pursuing."
IMCgp100 is just one of several "immunotherapies" under study for advanced melanoma. The general idea behind immunotherapy is to help the body's immune system do a better job of recognizing and killing off cancer cells.
Melanoma is the rarest, but deadliest, form of skin cancer. Caught early, it's curable with surgery. But once it spreads to distant lymph nodes or other organs -- known as stage 4 melanoma -- the disease is very hard to treat.
In just the past few years, though, researchers have made headway. In 2011, the United States and Europe approved an immunotherapy drug called ipilimumab (Yervoy) -- the first treatment shown to prolong the lives of some patients with advanced melanoma.
Still, only a minority of patients respond to the drug, and it can cause severe side effects -- including life-threatening inflammation of the liver or digestive tract.
So there's an acute need for additional therapies, said Dr. Mark Middleton, who was to present the new findings on Tuesday at the annual meeting of the American Association for Cancer Research, in San Diego.
"The reality is, despite exciting advances in treatment, most of the patients we see with advanced melanoma will die of the disease," said Middleton, of Oxford University in England.
IMCgp100 targets melanoma in two ways: It attaches to a specific, tiny protein found on some melanoma cells, and it activates nearby T-cells to attack the tumor.
The catch is that a patient's cancer must be positive for that protein, called HLA A2 -- which is true in about 45 percent of melanomas, Middleton said.
The 31 patients in this study were all HLA A2-positive, and all but one had stage 4 melanoma. They were split into eight groups and given different doses of IMCgp100. Patients who were able to tolerate the first infusion received six more weekly treatments.
In the end, there were four patients who showed a "partial response" to the drug, which meant their tumors shrunk. One patient continued to see a regression with further treatment, and is still stable after more than 10 months, according to Middleton.
The most common side effects were rash, fever and "tumor flare" -- swelling and tenderness at the site of a tumor. Two of four patients who got the highest drug dose did have an immediate drop in blood pressure, so the researchers have since set the maximum dose below that.
Middleton said his team is now studying the drug in a larger group of patients, and trying to find the most effective regimen.
"The other obvious question is, where would this drug fit in?" Middleton said.
Besides Yervoy and other immunotherapies under development, there are also newer "targeted" drugs that directly attack proteins found on some melanomas -- including drugs called BRAF inhibitors.
Weber said researchers will have to figure out whether combinations of different therapies work better than a single one -- and which patients stand to benefit from a particular combination.
Middleton agreed. "We now have a whole range of therapies coming out, which is exciting," he said. But the difficult part, he added, will be understanding how to best use them.
In the United States, about 76,000 people will be diagnosed with melanoma this year, and 9,700 will die of the disease, according to the American Cancer Society. While melanoma is relatively uncommon, its incidence has been rising for the past few decades, the cancer society notes.
Immunocore, the company developing IMCgp100, funded the current study. Middleton reports no financial interests in the work.
The data and conclusions of research presented at medical meetings should be viewed as preliminary until published in a peer-reviewed journal.
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SOURCES: Mark Middleton, M.D., Ph.D., professor, experimental cancer medicine, University of Oxford, U.K.; Jeffrey Weber, M.D., Ph.D., director, Melanoma Research Center of Excellence, Moffitt Cancer Center, Tampa, Fla.; April 8, 2014, presentation, American Association for Cancer Research annual meeting, San Diego