When the U.S. Food and Drug Administration approved implanting microchips in humans, the manufacturer said it would save lives, letting doctors scan the tiny transponders to access patients' medical records almost instantly. The FDA found "reasonable assurance" the device was safe, and a sub-agency even called it one of 2005's top "innovative technologies."

But neither the company nor the regulators publicly mentioned this: A series of veterinary and toxicology studies, dating to the mid-1990s, stated that chip implants had "induced" malignant tumors in some lab mice and rats.

"The transponders were the cause of the tumors," said Keith Johnson, a retired toxicologic pathologist, explaining in a phone interview the findings of a 1996 study he led at the Dow Chemical Co. in Midland, Mich.

Leading cancer specialists reviewed the research for The Associated Press and, while cautioning that animal test results do not necessarily apply to humans, said the findings troubled them. Some said they would not allow family members to receive implants, and all urged further research before the glass-encased transponders are widely implanted in people.

To date, about 2,000 of the so-called radio frequency identification, or RFID, devices have been implanted in humans worldwide, according to VeriChip Corp. The company, which sees a target market of 45 million Americans for its medical monitoring chips, insists the devices are safe, as does its parent company, Applied Digital Solutions, of Delray Beach, Fla.

"We stand by our implantable products which have been approved by the FDA and/or other U.S. regulatory authorities," Scott Silverman, VeriChip Corp. chairman and chief executive officer, said in a written response to AP questions.

The company was "not aware of any studies that have resulted in malignant tumors in laboratory rats, mice and certainly not dogs or cats," but he added that millions of domestic pets have been implanted with microchips, without reports of significant problems.

"In fact, for more than 15 years we have used our encapsulated glass transponders with FDA approved anti-migration caps and received no complaints regarding malignant tumors caused by our product."

The FDA also stands by its approval of the technology.

Did the agency know of the tumor findings before approving the chip implants? The FDA declined repeated AP requests to specify what studies it reviewed.

The FDA is overseen by the Department of Health and Human Services, which, at the time of VeriChip's approval, was headed by Tommy Thompson. Two weeks after the device's approval took effect on Jan. 10, 2005, Thompson left his Cabinet post, and within five months was a board member of VeriChip Corp. and Applied Digital Solutions. He was compensated in cash and stock options.

Thompson, until recently a candidate for the 2008 Republican presidential nomination, says he had no personal relationship with the company as the VeriChip was being evaluated, nor did he play any role in FDA's approval process of the RFID tag.

"I didn't even know VeriChip before I stepped down from the Department of Health and Human Services," he said in a telephone interview.

Also making no mention of the findings on animal tumors was a June report by the ethics committee of the American Medical Association, which touted the benefits of implantable RFID devices.

Had committee members reviewed the literature on cancer in chipped animals?

No, said Dr. Steven Stack, an AMA board member with knowledge of the committee's review.

Was the AMA aware of the studies?

No, he said.

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Published in veterinary and toxicology journals between 1996 and 2006, the studies found that lab mice and rats injected with microchips sometimes developed subcutaneous "sarcomas" — malignant tumors, most of them encasing the implants.

_ A 1998 study in Ridgefield, Conn., of 177 mice reported cancer incidence to be slightly higher than 10 percent — a result the researchers described as "surprising."

_ A 2006 study in France detected tumors in 4.1 percent of 1,260 microchipped mice. This was one of six studies in which the scientists did not set out to find microchip-induced cancer but noticed the growths incidentally. They were testing compounds on behalf of chemical and pharmaceutical companies; but they ruled out the compounds as the tumors' cause. Because researchers only noted the most obvious tumors, the French study said, "These incidences may therefore slightly underestimate the true occurrence" of cancer.

_ In 1997, a study in Germany found cancers in 1 percent of 4,279 chipped mice. The tumors "are clearly due to the implanted microchips," the authors wrote.

Caveats accompanied the findings. "Blind leaps from the detection of tumors to the prediction of human health risk should be avoided," one study cautioned. Also, because none of the studies had a control group of animals that did not get chips, the normal rate of tumors cannot be determined and compared to the rate with chips implanted.

Still, after reviewing the research, specialists at some pre-eminent cancer institutions said the findings raised red flags.

"There's no way in the world, having read this information, that I would have one of those chips implanted in my skin, or in one of my family members," said Dr. Robert Benezra, head of the Cancer Biology Genetics Program at the Memorial Sloan-Kettering Cancer Center in New York.

Before microchips are implanted on a large scale in humans, he said, testing should be done on larger animals, such as dogs or monkeys. "I mean, these are bad diseases. They are life-threatening. And given the preliminary animal data, it looks to me that there's definitely cause for concern."

Dr. George Demetri, director of the Center for Sarcoma and Bone Oncology at the Dana-Farber Cancer Institute in Boston, agreed. Even though the tumor incidences were "reasonably small," in his view, the research underscored "certainly real risks" in RFID implants.

In humans, sarcomas, which strike connective tissues, can range from the highly curable to "tumors that are incredibly aggressive and can kill people in three to six months," he said.

At the Jackson Laboratory in Maine, a leader in mouse genetics research and the initiation of cancer, Dr. Oded Foreman, a forensic pathologist, also reviewed the studies at the AP's request.

At first he was skeptical, suggesting that chemicals administered in some of the studies could have caused the cancers and skewed the results. But he took a different view after seeing that control mice, which received no chemicals, also developed the cancers. "That might be a little hint that something real is happening here," he said. He, too, recommended further study, using mice, dogs or non-human primates.

Dr. Cheryl London, a veterinarian oncologist at Ohio State University, noted: "It's much easier to cause cancer in mice than it is in people. So it may be that what you're seeing in mice represents an exaggerated phenomenon of what may occur in people."

Tens of thousands of dogs have been chipped, she said, and veterinary pathologists haven't reported outbreaks of related sarcomas in the area of the neck, where canine implants are often done. (Published reports detailing malignant tumors in two chipped dogs turned up in AP's four-month examination of research on chips and health. In one dog, the researchers said cancer appeared linked to the presence of the embedded chip; in the other, the cancer's cause was uncertain.)

Nonetheless, London saw a need for a 20-year study of chipped canines "to see if you have a biological effect." Dr. Chand Khanna, a veterinary oncologist at the National Cancer Institute, also backed such a study, saying current evidence "does suggest some reason to be concerned about tumor formations."

Meanwhile, the animal study findings should be disclosed to anyone considering a chip implant, the cancer specialists agreed.

To date, however, that hasn't happened.

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