Your gut bacteria could neutralize your cancer medication.

Nature looks at another way personalized treatments could save lives – by taking into account how different people’s gut bacteria affect drugs differently:

Bacteria living in the human body will eat any nutrient that comes their way, whether it’s food from the host’s diet or a drug that the person is taking. But this dietary flexibility can become problematic if the microbes metabolize a drug into useless or toxic compounds.

Computational biologist Leah Guthrie at the Albert Einstein College of Medicine in New York City discussed data on a chemotherapy drug called irinotecan, which causes severe diarrhoea in some patients. Previous research in mice found that bacterial enzymes called β-glucuronidases can modify the chemical structure of irinotecan and other drugs. Normally, the liver detoxifies these treatments by adding a chemical group called glucuronidate. But the bacterial enzyme removes the group, turning the drug into a toxic compound.

Guthrie and her colleagues collected faecal samples from 20 healthy people. They treated the samples with irinotecan, and measured the compounds produced by bacteria in the samples as they interacted with the drug. The team found that 4 of the samples contained high levels of the toxic form of irinotecan, but found no significant differences between the bacterial species present in any of the samples.

When the researchers analysed the proteins produced in the faecal samples, they found that those from people with high bacterial metabolisms contained strains that made more β-glucuronidases. These people also had increased levels of proteins that transport sugar into cells, which suggests they would be more likely to absorb the toxic compound and develop gastrointestinal problems.

Researchers have identified dozens of examples of gut bacteria that seem to modify therapeutic drugs, including some that treat Parkinson’s disease and anxiety, says Emily Balskus, a biochemist at Harvard University in Cambridge, Massachusetts. She says that bacterial interference could also help to explain why animal models don’t always predict drug toxicity in humans, because animals contain different microbes.

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