Dr. Daniel Haberr of the Massachusetts General Cancer Center and the senior author of one of the two studies, said "This discovery will help us significantly improve the treatment of many lung cancer patients and is also an important next step in the molecular targeting of cancer drugs."
Iressa is one of a new wave of cancer drugs aimed at the molecular-level defects at the root of cancers.
"Get a molecular definition of a tumor and you'll know what it will respond to," Dr. Brian Druker of Oregon Health & Science University, a pioneer in such research, told the journal Science.
The studies are being published in the April 30 issue of Science and in the May 20 issue of the New England Journal of Medicine. They found that a specific mutation in a gene called EGFR was present in almost all patients who responded to AstraZeneca's Iressa.
The drug has been used on patients with non-small cell lung cancer, the form of the disease present in 85 percent of all lung cancer cases. Lung cancer kills some 160,000 people in the United States each year.
Iressa was the first drug to work dramatically well against lung cancer, but it helped only about 10 percent of patients.
In the study published in Science, researchers scanned lung tumors from 58 Japanese and 61 American patients for gene mutations. One American patient and 15 Japanese patients had a mutation in the gene for EGFR.
Iressa had previously been found to be more effective in Japanese patients.
Meanwhile, researchers found that tumor tissue from a woman with cancer that had spread to the lining around her lungs was very responsive to the drug when it was tested in a laboratory dish. When the DNA was analyzed, it was found to have the same EGFR gene mutation that was found in the one American and 15 Japanese patients.
Investigators then analyzed tumor samples from five patients who had been successfully treated with Iressa and four patients whose tumors did not respond. All of the responders were found to have EGFR mutations, and none of the four whose lung cancers did not respond had mutations.
"Our results suggest that screening patients for EGFR mutations can help predict whether they will be responsive to treatment with Iressa," commented Dr. Bruce Johnson, a co-senior author of the study and a researcher at the Dana-Farber Cancer Institute.
In the study to be published in the New England Journal of Medicine, researchers from Massachusetts General Hospital reported that eight of nine patients who responded to Iressa were found to genetic mutations affecting the same area of the EGFR protein.
Researchers are hopeful that these results will help doctors determine which patients could benefit from Iressa, since the drug can cost $2,000 to $3,000 a month.
"People have not understood how Iressa is working or who it should be deployed in. This gives us a major clue," said Dr. William Sellers, a co-senior author of the Science study.
Neither of the studies were funded by Iressa's maker AstraZeneca Plc.