Few students go into medical research dreaming that one day a mouse with a tumor or heart disease will be infected by a rabbit. But animal testing of this kind has become central to modern medicine. For regulators around the world, treatments that could save human lives must prove safe and effective in animals with similar conditions before they can be marketed.
So how does this ethical agreement hold up? Not enough, suggests a paper published in PLOS Biology by Benjamin Ineichen of the University of Zurich and colleagues. Of 367 biomedical therapies tested in animals across thousands of studies, a surprisingly low proportion (just 5%) ultimately gained approval from the Food and Drug Administration, the US drug regulatory agency, for use in humans.
To arrive at this figure, Dr. Ineichen and his team analyzed 122 research studies that evaluated the efficacy of results from animal studies for application in humans. These studies covered more than 50 diseases, from diabetes mellitus to lung cancer, and involved therapies ranging from blood thinners to nonpharmaceutical interventions such as exercise and green tea.
In all, the researchers found that half of the therapies tested in animals had results encouraging enough to warrant further testing in humans, but only one in 20 ultimately made it to market. There is inevitably a certain failure rate, says Dr. Ineichen. For one thing, lab animals, after all, are not exact models for humans. What’s more, factors other than clinical success, including lack of commercial interest, can also influence the fate of a therapy. “Even if every drug that works in animals also worked in humans, we would only see a 25% success rate,” says Joseph Garner of Stanford University, who was not involved in this study. Once you factor that in, he says, “the 5% figure is not nearly as surprising.”
There are ways to make the process more efficient, though. The most obvious, Dr. Ineichen says, is that animal studies are not all conducted with equal rigor. Many mouse trials are conducted on young, male animals with untested immune systems, while potential human patients can be both male and female, of all ages, with multiple concurrent diseases. Some animal studies are not randomized, and others are not blinded — procedures considered essential to generating accurate results. Dr. Ineichen’s analysis also found that of the trials whose results were most often reproduced (an indicator of how well a study is conducted), 86% led to similar results in humans.
To further strengthen the evidence base, he recommends designing animal experiments that more closely resemble human trials that may one day be followed. This means, among other things, ensuring greater diversity among individuals and not giving genetically similar individuals identical diets, while keeping them in identical cages at identical temperatures. After all, patients in clinical trials will rarely be so compliant. There are already signs that this approach will work. A 2017 study found that wild mice with natural microbiomes were more accurate models for humans than laboratory animals with artificially controlled microbiomes. Similar successes have since been achieved.
More sophisticated animal substitutes can also help reduce waste. So-called “organs on a chip,” which are battery-sized devices lined with human cells, or organoids, which are replicas of organs built from lab-grown tissue, can mimic a body’s response to a therapy. Computer simulations have helped confirm the feasibility of therapies before they are tested in animals, and an AI model trained on existing studies has been shown to predict how a chemical will affect humans — without having to involve a lab rat. These methods are promising, but they have not yet reliably demonstrated whole-body effects the way animal models can. For now, at least, animal testing is here to stay.
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