LEAD: The AIDS drug AZT has been found to cause cancer in laboratory animals, the drug's manufacturer said today. But doctors emphasized that AIDS patients should continue to take the drug because the risk is still only theoretical.
The AIDS drug AZT has been found to cause cancer in laboratory animals, the drug's manufacturer said today. But doctors emphasized that AIDS patients should continue to take the drug because the risk is still only theoretical.
In a letter sent to thousands of physicians who treat people with AIDS, the Burroughs Wellcome Company, the maker of the drug, reported that studies with mice and rats over the past two years had shown AZT to be a "weak" carcinogen that did not kill any of the test animals.
It is not known whether the drug causes cancer in humans, and doctors treating AIDS patients said the findings would have little effect on most decisions about whether to use the drug.
"It is not surprising that the drug might cause cancer," said Dr. Jerome Groopman an AIDS and cancer specialist at Harvard University and the New England Deaconness Hospital. He said other toxic drugs, even those used in cancer treatment, can cause cancer in a small percentage of patients over a long period of time.
Benefits and Risks
But for most people infected with the human immunodeficiency virus, which causes AIDS, the risk of deteriorating without the drug would far outweigh any theoretical risk raised by the animal experiments, said Dr. Bernard Bihari, executive director of the Community Research Initiative, an advocacy group for patients with AIDS. The group helps carry on private research into AIDS treatments.
"These studies are enough to raise concern, and to require some long-term studies," he said.
AZT, or azidothymidine, is the only drug approved for the treatment of AIDS. Doctors say it does not cure AIDS but can delay the onset of symptoms. It is sold by Burroughs Wellcome under the brand name Zidovudine.
Dr. Mathilde Krim, of the American Foundation for AIDS Research, said the results were generally positive.
"It could have been worse," she said. "What they have observed is a very low carcinogenicity. These tumors all came on the highest dose, much higher than would be given to human beings, and were small and noninvasive, and came at the end of life. This should not change anything in the way AZT is being used."
Doctors who treat many AIDS patients said the findings raised concerns about only one group: pregnant women infected with the AIDS virus.
Studies of Pregnant Women
The letter to the physicians was signed by Dr. David W. Barry, vice president for research at Burroughs Wellcome. In a telephone interview today, he said that the company would assemble a committee of experts to determine whether two studies of AZT in pregnant women should go forward. One study is under way and the other is planned.
In those studies, pregnant women were to receive the drug in the weeks just before delivery, or the infant would get the drug immediately after delivery, in hopes of preventing the virus from infecting the babies.
Only about one-third of infants born of to mothers who are infected with the virus now become infected, and it is impossible to tell which infants will be infected. As a result, giving pregnant women or infants the drug means that two-thirds of the babies would be treated with a toxic drug even though they would not become infected.
The Burroughs Wellcome studies involved 960 mice or rats; 720 of them got the drug, while 240 did not get the drug and were used for comparison.
Among those that got the drug, 10 developed cancers. In a group of this size and over this time, the researchers said, they would not have expected to see any cancer of that type.
None of the animals died of the disease. The tumors were all found on examination after death.
Seven of the cancers were found in mice taking the highest dose of AZT for their entire lives, or 19 months to 22 months. One was found in a mouse taking a medium dosage for a lifetime. Two were found in rats that had received the highest doses.
Dr. James O. Mason, Assistant Secretary for Health, said that while the results of the study appeared to be significant, they did not mean that the drug causes cancer in humans. He said the Public Health Service, which is under his jurisdiction, "still strongly recommends that AIDS patients continue their Zidovudine therapy."
Burroughs Wellcome and the Public Health Service have set up a toll-free telephone line for physicians who have questions about the findings: (800) 443-6763.
Representative Ted Weiss, the Manhattan Democrat who is chairman of the Human Resources subcommittee, said it was important to continue clinical trials with AIDS drugs to determine just what effects they have in the long term.
"Clearly the potential harm to women receiving AZT cannot be taken lightly," he said. "However, until a safer and less toxic treatment is found for AIDS, AZT remains the only useful antiviral."
Peter Staley, a spokesman for the AIDS Coalition to Unleash Power, said: "I am taking AZT and I do find this fairly worrisome, but I am more fearful of HIV than I am of cancer. This shifts the equation of benefit and risk, but not enough to tilt it away from using the drug."