updated 4/9/2008 12:21:16 PM ET 2008-04-09T16:21:16

An increasing number of trials testing cancer drugs are being stopped early, potentially putting patients at risk, according to research published Wednesday.

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European researchers looked at all cancer trials published in three major medical journals between 1997 and 2007. They found 25 that were stopped early, because the drugs were working so well that experts wanted patients to get the medicines as soon as possible. The study was published online in the Annals of Oncology.

In the last three years, nearly 80 percent of cancer trials for experimental drugs stopped prematurely were linked to an application for a marketing license in Europe or the United States.

"That suggests that companies have a commercial interest in stopping these trials, which may not be good for patients," said Dr. Giovanni Apolone, of the Mario Negri Institute for Pharmacological Research in Milan, one of the study's authors.

"Without enough information, we don't know if patients are really benefiting from these new drugs," Apolone said. He and his colleagues were not paid for the study. Apolone's institute receives grants from GlaxoSmithKline PLC and Novartis AG.

But Apolone and colleagues had no proof that financial considerations alone drove the trials to be stopped early.

Decisions to halt drug trials are usually made by independent data monitoring committees, which review the trial information and regularly assess progress. They make recommendations to the pharmaceutical companies, which often hire outside academics to test their drugs.

If the drugs appear to be working, trials are usually stopped early to give those who are receiving fake pills the chance to benefit. Trials are also stopped if drugs seem to be harming patients.

In the European study, 19 out of the 25 trials halted early had independent data monitoring committees.

Drugs are also continually monitored for dangerous side effects, even after they are on the market.

The European Medicines Agency requires that pharmaceutical companies track reports of side effects from their drugs and submit occasional safety reports.

"At the time when marketing authorizations are granted, we are aware that we don't know everything about the medicine," said Monika Bernstetter a spokeswoman for the European Medicines Agency. "In the end, this is an ongoing process and it is an issue of risk management."

Bernstetter said the agency had not received any unusual reports of side effects from cancer drugs in the last few years.

A study published in the Journal of the American Medical Association in 2005 looked at drug trials in cardiology, cancer, and AIDS. Researchers also found that trials were often stopped early because of potential benefits, and warned that the results could have been skewed.

"Just having preliminary promising results is not good enough," Apolone said. It can take years to discover the long-term benefits or side effects of new drugs, yet most studies only lasted about two years. When tests are interrupted at the halfway point, the smaller numbers may exaggerate a drug's benefits.

Many cancer drugs are very toxic and come with unpleasant side effects like hair loss, vomiting, and anemia.

Some experts said that the competition to provide better cancer treatments was behind the accelerated process.

"There's great pressure on the cancer community to get new drugs out as soon as possible," said Dr. David Kerr, editor in chief of Annals of Oncology and a cancer expert at Oxford University.

Kerr said that the process for evaluating new drugs had been greatly strengthened in the last two decades, but that the trend of stopping trials early was worrying.

"Everyone wants new and powerful therapies available quickly," he said. "But we need to be sure we understand the evidence first."

Copyright 2008 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.

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