|Metabolism of Four Drugs (in hours)|
Animals will also produce unreliable results if they process a drug in a completely different way, not just at a different speed. One species might metabolise the drug into toxic fragments whilst another produces harmless by-products. A comparative study of 23 chemicals shows that in only four cases did rats and people metabolise the substance in a similar way.11 ’ Despite this, rats are the main species used to study how drugs and other products are metabolised, distributed and excreted by the body.1 As Edward Calabrese, Professor of Toxicology at the University of Massachusetts’ School of Public Health points out, ‘It seems almost incredible that the rat is the model so heavily relied upon when predicting human responses to toxic/carcinogenic agents.’ Calabrese refers to ‘profound differences’ between rats and people.12
Differences in body chemistry between species can be so great that, according to one researcher, ‘... it is often a matter of pure luck that animal experiments lead to clinically useful drugs.’13 In fact, reliance on animal experiments can be dangerously misleading, however ‘rigorous’ the tests may have been. In June 1993, the US National Institutes of Health halted clinical trials of the hepatitis drug failuridine, following deaths and serious complications amongst participants. Patients unexpectedly suffered liver toxicity yet the drug had seemed both safe and effective in laboratory animals, having been tested for toxicity in mice, rats and rhesus monkeys.14 However, the metabolism of antiviral drugs of this type is thought to be very different in animals and people.15
Failuridine is not alone and there are many examples where animals have failed to identify serious hazards of drug therapy.
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