Gut Bacteria May Change How Well Tamoxifen Works

The bacteria in your gut may affect whether the common breast cancer medication tamoxifen works for you, suggests a study published in the journal mBio. Scientists could use this information to develop a stool test to determine who should take tamoxifen.

Tamoxifen is used to treat some people who have been diagnosed with estrogen receptor-positive breast cancer or are at high risk of developing breast cancer. But tamoxifen doesn’t reduce the risk of developing breast cancer for nearly 50% of high-risk women who take it.  Up until now, researchers didn’t know why tamoxifen doesn’t work in half of all people who take it.

“Since tamoxifen is taken orally and passes through the gut, this difference in how patients respond may be linked to the gut microbiome — the trillions of bacteria in our intestines, which vary greatly from person to person,” lead study author Yasmine Alam, a Ph.D. candidate in the Department of Biological Chemistry, University of California Irvine, said in a press release.

When a person takes tamoxifen, either as a tablet or liquid, it travels to the stomach and intestines (the gut), where it then gets absorbed into the blood. The drug then travels to the liver, which changes it into a form that’s better at targeting breast cancer. It then gets put back into the blood, where it can move throughout the body.

But sometimes a sugar molecule gets attached to tamoxifen, which sends tamoxifen back to the intestines, where it’s useless against breast cancer.

By studying mice, the researchers discovered that a protein found in some types of bacteria in the gut can remove this sugar molecule from tamoxifen. This allows the drug to move back into the blood.

While more human studies looking at the link between gut bacteria and tamoxifen are needed, the findings in mice may help to explain why tamoxifen works for some people more than others.

If the results are confirmed in people, doctors may be able to create a stool test to screen people with breast cancer who are considering taking tamoxifen. Stool contains the DNA of gut bacteria, which scientists can use to identify it. If a person does have the gut bacteria that helps tamoxifen make its way to the body quickly, the doctor might recommend they take tamoxifen. If they don’t, the doctor might suggest a different medicine.