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Bmj Usa

Which drugs cause cancer?For and against: Cancer bioassays

BMJ 2005; 331 doi: (Published 13 October 2005) Cite this as: BMJ 2005;331:E389

Which drugs cause cancer?

  1. Andrew Knight, research scientist (,
  2. Jarrod Bailey, medical scientist2,
  3. Jonathan Balcombe, research scientist3
  1. 1 Animal Consultants International 91 Vanbrugh Court Wincott St London SE11 4NR, UK
  2. 2 School of Population and Health Sciences University of Newcastle upon Tyne, UK
  3. 3 Physicians Committee for Responsible Medicine Washington, DC
  4. US Environmental Protection Agency Washington, DC
  1. Correspondence to: A Knight

    Animal tests yield misleading results

    FOR Despite President Nixon's War on Cancer, launched in 1971, and billions of dollars spent since then, cancer remains the second-leading killer of Americans. Around 40% of us will get cancer, and half of us will die from it.1 This cease-less tide of human suffering starkly questions the effectiveness of our strategies, including the accuracy of our methods for identifying human carcinogens.

    Millions of laboratory animals have been sacrificed for this purpose. Thousands of chemicals, including ever-increasing numbers of therapeutic drugs, are consequently described as potentially carcinogenic. Yet, are animal experiments really predictive of human carcinogenicity?

    The agency most responsible for protecting Americans from environmental contaminants is the Environmental Protection Agency (EPA), and the chemicals of greatest public health concern are described within its Integrated Risk Information System (IRIS) toxic chemicals database. We recently surveyed this database to assess the human utility of animal carcinogenicity data. Most chemicals lack human exposure data and possess only animal carcinogenicity data. In the majority of cases, however—58.1% (93/160)—we found that the EPA considered the animal data inadequate to support the useful human carcinogenicity classifications of probable carcinogen or non-carcinogen.2

    But at least the animal data were predictive for 42% of chemicals. Or were they? A comparison of EPA carcinogenicity classifications with those assigned by the World Health Organization's International Agency for Research on Cancer (IARC) yielded disturbing results. For the 128 chemicals with human or animal data assessed by both agencies, human carcinogenicity classifications were similar only for those 17 possessing significant human data. For the 111 primarily reliant on animal data, the EPA was far likelier than the IARC to assign carcinogenicity classifications indicative of greater human risk.2

    The IARC is widely recognized as the world's leading authority on carcinogenicity assessments. Such profound differences …

    Correspondence to:

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