The therapy was tested on 1,296 colon cancer patients in a study directed by Charles G. Moertel of the Mayo Clinic. Over three years, the treatment reduced recurrence of a form of colon cancer known as Dukes' C by 41 percent and the death rate by one-third.
The study builds upon an earlier one of 408 patients that was announced in October by the National Cancer Institute, the University of Pennsylvania Cancer Center and other medical institutions. At that time, researchers said the treatment could cut the death rate of Dukes' C by 10 percent to 15 percent.
Cancer of the colon - which is a part of the large intestine - is the nation's second largest cause of cancer deaths; it is expected to kill 53,300 people this year.
With the latest findings, cancer researchers have significantly increased their estimate of the number of lives that could be saved if the new treatment were followed.
"These findings overwhelmingly confirm that earlier study and show that the new drugs are even more effective than was believed last fall," Macdonald said. "This is a major step forward and certainly one of the major advances in cancer treatment of the past decade."
The two drugs involved in the treatment are levamisole, a veterinary drug used to kill worms in the intestines of farm animals, and 5-fluorouracil, or 5-FU, an anti-cancer drug that has been widely used for more than 30 years.
The National Cancer Institute has begun providing levamisole, which is not yet commercially sold in the United States, to all physicians who wish to use it in on their patients. The drug 5-FU is already widely available.
Daniel C. Haller, an associate professor at Penn's Cancer Center and another author of today's study, cautioned that the new drugs have been found only to be effective for patients who have undergone surgery for colon cancer and whose cancer has spread to adjacent lymph nodes.
An estimated 22,000 of the 110,000 patients expected to be diagnosed with colon cancer this year will have the Dukes' C type.
Both Haller and Macdonald said they thought it is reasonable to believe that further studies might show that the new chemotherapy is also effective against other forms of gastrointestinal cancer.
"We have every reason to believe it may also be beneficial to people with rectal cancer, which affects 45,000 Americans each year," said Haller, who participated in the new study along with Penn researcher, John H. Glick.
The study followed 1,296 patients, including 929 with Dukes' C, for between 2 and 5 1/2 years after they underwent surgery for colon cancer. The study, sponsored by the National Cancer Institute, included men and women of all ages who lived in all parts of the United States.
The researchers said it is too early to assess the effectiveness of the two drugs on patients with another form of colon cancer, Dukes' B, in which the cancer has spread to the colon wall but not yet to the lymph nodes.
The data suggest that the recurrence of cancer has declined in Dukes' B patients who have taken the two drugs but the data have not yet shown that there are fewer cancer deaths.
Moertal said that it is important that the new therapy be started in patients within three to five weeks after surgery for Dukes' C.
"Only about one-third of all Dukes' C patients are receiving the new treatment," said Moertal. "We are hoping that publicizing this new treatment option will prompt the other 14,500 eligible patients to enter a clinical trial or take advantage of this therapy."
Haller said the two drugs can cause a variety of side effects, including nausea, vomiting, diarrhea, hair loss, cold sores and infections.
Last week, the Oncologic Drugs Advisory Committee of the U.S. Food and Drug Administration unanimously recommended that Janssen Pharmaceutica Inc. be given permission to sell the medicine for routine use. Eva Kemper, an FDA spokeswoman, yesterday told the Associated Press that final approval should come within a month or so.
Macdonald said scientists do not yet understand why the combination of the two drugs are effective against colon cancer. Studies have not yet shown any significant benefit of using either levamisole or 5-FU alone.
"We don't yet know what is happening on the cellular level, although researchers at the University of Arizona and other institutions are now studying this," he said.
Moertal said that some researchers believe that the two drugs might somehow fight colon cancer by stimulating the body's immune system.