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Very few students in medical research dream that one day they will give a mouse a tumor or a rabbit heart disease. But animal experiments like this have become fundamental to modern medicine. For regulatory bodies around the world, treatments that can save human lives must be proven safe and effective in animals with similar conditions before they can be brought to market.
So how is this ethical bargain holding up? A paper in PLOS Biology by Benjamin Ineichen of the University of Zurich and his colleagues shows that it is not enough. Of the 367 biomedical treatments tested on animals in thousands of studies, a surprisingly low proportion – just 5% – eventually received approval from the Food and Drug Administration, the US regulatory agency for drugs, for use in humans.
To arrive at this figure, Dr. Ineichen and his team analyzed 122 research reviews that assessed how well the results of animal studies applied to humans. These reviews covered more than 50 diseases, from diabetes to lung cancer, and included treatments ranging from anticoagulants to non-pharmaceutical interventions such as exercise and green tea.
Overall, the researchers found that half of the treatments tested on animals had results encouraging enough to warrant subsequent trials on humans, but only one in 20 eventually made it to market. Some rate of failure is inevitable, says Dr. Ineichen. For one thing, lab animals are not, after all, exact models for humans. In addition, factors other than clinical success can also influence a treatment’s fate, including lack of commercial interest. “Even if every drug that works in animals also works in humans, we’d only see a 25% success rate,” says Joseph Garner of Stanford University, who was not involved in the study. Once you take that into account, he says, “a number of 5% is nowhere near shocking.”
Still, there are ways to make the process more efficient. Most obviously, not all animal studies are conducted with the same rigor, says Dr. Ineichen. Many mouse tests are done on young, male animals whose immune systems have not been tested, while potential human patients can be both males and females of all ages, with a number of comorbidities. Some animal studies are not randomized, and still others are not blinded — procedures necessary to produce accurate results. Dr. Ineichen’s analysis also found that of the tests whose results were most frequently replicated (an indicator of how well the study is done), 86% produced similar results in humans.
To further strengthen the evidence base, he recommends making animal experiments more closely resemble the human trials that might one day take place. This means, among other things, ensuring greater diversity among individuals – and not just feeding genetically identical individuals the same diet, while keeping them in the same cages at the same temperature. After all, patients in clinical trials will rarely be so friendly. There are already signs that such an approach will work. A 2017 study found that wild mice with a natural microbiome were more accurate models for humans than laboratory animals with artificially controlled microbiomes. Similar breakthroughs have been achieved since then.
More sophisticated animal replacements could also help reduce waste. So-called “organs-on-a-chip,” which are battery-sized devices containing human cells, or organoids, which are replica organs created from tissues grown in the lab, can mimic the body’s response to a therapy. Computer simulations have helped confirm the feasibility of therapies before they are tested on animals, and an artificial-intelligence model trained on existing studies is able to predict how a chemical will affect humans, without involving any lab mice. Such methods are promising, but have not yet reliably demonstrated whole-body effects like animal models. For now, at least, animal experiments are here to stay.
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© 2024, The Economist Newspaper Limited. All rights reserved. From The Economist, published under licence. Original content can be found at www.economist.com.
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