Cancer patients with high levels of good gut bacteria appear more likely to respond to immunotherapy, potentially opening up a new way to optimize the use of modern medicines that are highly effective but only work in some people.
The finding, reported in two scientific papers on Thursday, suggests patients may in future be told to actively nurture their good bugs when taking so-called PD-1 drugs like Merck & Co's Keytruda or Bristol-Myers Squibb's Opdivo.
The twin publications in the journal Science are the latest examples of the importance of the microbiome - the vast community of microbes living inside us - which has been linked to everything from digestive disorders to depression.
"You can change your microbiome, it's really not that difficult, so we think these findings open up huge new opportunities," said Jennifer Wargo of the MD Anderson Cancer Center in Texas, one of the study authors.
Options for manipulating the microbiome including changes in diet, avoiding antibiotics, taking probiotics or - less appetizingly - receiving a fecal transplant, either as a capsule or by enema.
Good bacteria seem to help in cancer by priming immune cells and smoothing the path for PD-1 drugs that work by taking the brakes off the immune system.
Such immunotherapy drugs are revolutionizing cancer care, but only around 20 to 30 percent of patients respond, prompting a race by scientists and drug companies to find better ways to identify those who will benefit.
The latest microbiome work in humans builds on initial research in mice in 2015, which first found a connection between good bacteria and immunotherapy drug responses.
Now a team at the Gustave Roussy Cancer Campus in France has studied more than 200 patients taking PD-1 drugs for lung, kidney and bladder cancer. They found those on antibiotics, due to routine problems like urinary or dental infections, had worse survival prospects.
Wargo's group, meanwhile, looked at melanoma patients and discovered that responders to immunotherapy had more diverse gut bacteria.
The Texas team now plan to run a clinical trial to test the benefits of combining immunotherapy with microbiome modulation in cancer patients.
Some biotech companies are already exploring this interface between the microbiome and cancer treatment, including Vedanta Biosciences, an affiliate of PureTech Health, which is doing pre-clinical research in the field.