Animal ‘models’ are not predictive

Open up a rat, a dog, a pig and a human and you will find much the same terrain, but with many intricate differences. It is precisely these differences which have an impact when it comes to assimilating drugs. For example, rats, the species most commonly used in vivisection (1), have no gall bladder and excrete bile very effectively. “Many drugs are excreted via bile, so this affects the half-life of the drug,” explain Ray and Jean Greek.

“Drugs bind to rat plasma much less efficiently. Rats always breathe through the nose. Because some chemicals are absorbed in the nose, some are filtered. So rats get a different mix of substances entering their systems. Also, they are nocturnal. Their gut flora are in a different location. Their skin has different absorptive properties than that of humans. Any one of these discrepancies will alter drug metabolism.

” These differences are on a gross level. Medications act on a microscopic level, initiating or interrupting chemical reactions that are far too small for the human eye to observe. “We differ on the cellular level and molecular level and, importantly, that is where disease occurs,” the authors explain. “The cells of chimps are very similar to the cells of humans, but the spatial organisation of the cells is vastly different.”

Even those who favour the animal model admit its unpredictability among their peers. Dr Ralph Heywood, director of Huntingdon Research Center in the United States, says: “The best guess for the correlation of adverse reactions in man and animal toxicity data is somewhere between five and 25%.”


…a game of chance


Dr Herbert Hensel, Director of the Institute of Physiology at Marburg University, goes further: “In the opinion of leading biostatisticians, it is not possible to transfer the probability predictions from animals to humans. At present, therefore, there exists no possibility at all of a scientifically based prediction. In this respect, the situation is even less favourable than a game of chance.”

Even the most widely respected textbook on animal experimentation states: “Uncritical reliance on the results of animal tests can be dangerously misleading and has cost the health and lives of tens of thousands of humans.”(2) The best-known example of this is thalidomide. Mothers who took this drug to relieve morning sickness gave birth to children with shocking deformities, with most lacking developed limbs.

Animal tests had not predicted this.The first recorded case of side effects occurred on Christmas Day 1956, but in 1957 the drug was released anyway.

(1) Vivisection refers to the dissection of, or any cutting or surgery upon, a living animal. More generally, it is used to describe any invasive experiment upon living animals, or any live animal testing, typically for the purpose of physiological or pathological scientific investigation.
(2) Svendsen, Per, “Laboratory Animal Anaesthesia”, in Handbook of Laboratory Animal Science (P. Svendsen and J. Hau, editors), CRC Press, vol. 1, p. 4.

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