Much of the excitement about stem cells with regards to their clinical application is regenerative medicine. iPS cells especially can be used to produce all sorts of cells for transplantation therapies including ongoing treatments for degenerative diseases of the brain, heart and eyes. Another benefit of stem cells and often shadowed by their regenerative medicine potential is the reduction of animal experiments for drug development. A new study by the laboratory of CiRA Professor Wataru Fujibuchi suggests, however, questions this premise.

Animal experiments have long been a part of drug development. The purpose of these tests is not only to confirm that the drug has the desired effect against the disease, but to also confirm that it does not have any toxic effects on the body. Indeed, a long list of drugs have been recalled even after approval because of toxicity to the liver, heart or other organs. Moreover, primates, pigs and other expensive and highly intelligent animals are used, because their physiology most closely resembles humans.

Since their discovery, scientists have considered pluripotent stem cells such as iPS cells as a substitute for living animals to investigate drug toxicity.

"Initially, we don't know which organs are susceptible to drug toxicity, which is why we need to test on animals. Pluripotent stem cells can in principle become any cells in the body, so they provide a first test to determine if toxicology is a concern," explained Fujibuchi.

However, he added, this is only a theory.

"There is extensive research on cancer stem cells and drug sensitivity. These studies suggest that stemness confers multidrug resistance. But we don't know if the same is true for non-cancer stem cells like iPS cells."

While "stemness" is a key property of stem cells, the quality of the stemness differs between stem cell types. All pluripotent stem cells are believed to resemble cells in the embryo. However, mouse pluripotent stem cells have a stemness that represents an earlier embryonic stage than human pluripotent stem cells. Furthermore, the reprogramming method that produces iPS cells seems to confer different levels of stemness.

The study examined the toxicity of eight common drugs including aspirin and ibuprofen on two types of human pluripotent stem cells: iPS cells and embryonic stem cells. The eight drugs were selected based on their range of liver toxicity risk, and the two pluripotent stem cell types showed similar responses to them.

However, the cells were more sensitive to the drugs based on databases for drug toxicity on other cell types.

"It was interesting. The rank of the toxicity for a drug did not change very much, but in all cases pluripotent stem cells had more drug sensitivity," observed Dr. Yulia Panina, the first author of the study.

By modulating the metabolism of the iPS cells so that their pluripotency shifted to an earlier embryonic stage, Panina and her colleagues found they could increase the drug sensitivity even more, suggesting the stemness of pluripotent stem cells correlates with drug resistance in ways quite different from cancer stem cells.

Moreover, the toxicology ranking of the eight drugs differed when tested on pluripotent stem cells and on live animals, suggesting that toxicology studies of cells in a dish and live animals do not always agree.

Although some of these findings may suggest that pluripotent stem cells are not an absolute substitute for animal experiments, Fujibuchi is more optimistic.

"At present there just isn't much information on the reliability of pluripotent stem cells for drug toxicology screening. As we gather more data and apply advanced AI methods to moderate in-vitro versus in-vivo differences, we will learn to use these cells effectively and reduce the number of animals for drug experiments," he said.

• doi: 10.2131/jts.46.131.