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How gut microbes are joining the fight against cancer

Bertrand Routy earned a lamentable reputation with Parisian oncologists in 2015. A doctoral student at the nearby Gustave Roussy cancer centre, Routy had to go from hospital to hospital collecting stool samples from people who had undergone cancer treatments. The doctors were merciless. “They made fun of me,” Routy says. “My nickname was Mr Caca.”

But the taunting stopped after Routy and his colleagues published evidence that certain gut bacteria seem to boost people’s response to treatment1. Now, those physicians are eager to analyse faecal samples from their patients in the hope of predicting who is likely to respond to anticancer drugs. “It was an eye-opener for a lot of people who couldn’t see the clinical relevance of gut microbes,” says Routy, who is now at the University of Montreal Health Centre in Canada.

Cancer has been a late bloomer in the microbiome revolution that has surged through biomedicine. Over the past few decades, scientists have linked the gut’s composition of microbes to dozens of seemingly unrelated conditions — from depression to obesity. Cancer has some provocative connections as well: inflammation is a contributing factor to some tumours and a few types of cancer have infectious origins. But with the explosive growth of a new class of drug — cancer immunotherapies — scientists have been taking a closer look at how the gut microbiome might interact with treatment and how these interactions might be harnessed.

After preliminary findings in mice and humans revealed that gut bacteria can sway responses to such drugs, scientists started trying to decipher the mechanisms involved. And researchers are launching a handful of clinical trials that will test whether the gut microbiome can be manipulated to improve outcomes.

Some proponents say that strategies to mould the microbiome could be game-changing in cancer treatment. “It’s a smart place to be,” says Jennifer Wargo, a surgeon–scientist at MD Anderson Cancer Center in Houston, Texas. But others are worried that the move to the clinic is premature. William Hanage, an epidemiologist at the Harvard T. H. Chan School of Public Health in Boston, Massachusetts, calls the idea “phenomenally interesting”, but adds: “I have some anxiety about the notion that only beneficial effects are possible.”

….Wargo is planning a similar trial. Together with the Parker Institute for Cancer Immunotherapy in San Francisco, California, and the biotech company Seres Therapeutics in Cambridge, Massachusetts, she expects to test whether faecal transplants can reshape the gut microbiome of non-responders in a beneficial way.

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