"We think this is because the immune system is being re-educated," Pardoll said. "If that's the case, will we be able to discontinue the antibody and have the patient's immune system take over and keep the cancer at bay?"
Although the drugs have great promise, the researchers said they also were keeping an eye on the adverse events they can cause, some of which have been very serious.
PD stands for programmed death, and together the two molecules work to switch off the body's immune response. Blocking one or the other keeps the immune system active, which is good for fighting cancer, but there are also early signs that manipulating this response may have a downside.
Some patients had side effects that researchers believe are caused by autoimmunity -- the body mistakenly attacking its own organs and tissues. Those side effects include lung and liver inflammation, rashes and hypoglycemia (low blood sugar), perhaps because of a problem with the thyroid gland.
In a study published in a June 2012 issue of the New England Journal of Medicine, three patients who were taking an anti-PD-1 drug died from pneumonitis, or inflammation of the lungs.
The researchers said they're working to understand why the drugs seem to be particularly toxic to the lungs and to mitigate their adverse effects.
"We need to be cautious about the toxicities," Herbst said. "It's great that we're making progress, and now we need to go to randomized trials."
To find clinical trials that are testing immunotherapy drugs, head to the Cancer Immunotherapy Trials Network.
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