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Antibiotic Shows Promise As Treatment For Irritable Bowel Syndrome

Irritable bowel syndrome  makes life miserable for something like 15 percent of young adults – tens of millions of people. But IBS doesn't get much attention. A pair of new studies helps remedy that.

The industry-sponsored research shows that two weeks of treatment with an antibiotic called rifaximin quelled the symptoms of moderate IBS in 41 percent of the patients who took it. The studies, involving a total of 1,260 patients, appear in this week's New England Journal of Medicine.

Improvement in 4 out of 10 people seems pretty good, considering there aren't very good treatments for the bloating, cramps and diarrhea IBS can cause.  And some of the patients for whom it works are rapturous (more on that below). But the enthusiasm has to be tempered by the fact that 30 percent of patients who got a placebo also reported "adequate relief" of their IBS symptoms.

A NEJM editorial sniffs that a 10 percent advantage over placebo "is in the lower spectrum of what is considered to be clinically relevant" – that is, improvement that's meaningful to patients and doctors. Also, these studies didn't include people with the variety of IBS marked by constipation.

But lead study author Dr. Mark Pimentel of Cedars Sinai Medical Center in Los Angeles told Shots that the patients who do benefit would disagree.

Pimentel points out that once these patients finished their 14-day course of rifaximin, "they continued to stay better through the entire 12 weeks of the study. That suggests we weren't just treating symptoms, we actually touched on a cause of IBS."

And what might that cause be? Pimentel's convinced it's an overgrowth of bacteria in the small intestine. Thus the rationale for antibiotic treatment.

Earlier studies didn't work. In fact, the antibiotics that were tried added their side effects to patients' misery. But rifaximin stays in the gut; it doesn't get absorbed into the bloodstream, which apparently gives it a low side-effect profile.

The patients who've benefited from rifaximin therapy are a grateful bunch.

"To my delight, after…a 14-day treatment, I felt substantially better – I would say 60 to 70 percent better," a Southern California woman named Amy McMahon told Shots. After a second course of rifaximin some months later, she's been almost free of IBS symptoms for about three years.

Diana Shoolman, a 48-year-old real estate broker in Miami Beach, says she "went from being absolutely fine to violently ill after I graduated college at 22."

She suffered for years with disabling bowel symptoms before discovering Pimentel's antibiotic therapy. She says the "constant feeling that you never knew if you were going to get sick was eliminated within five days of taking the drug – for the first time in 20 years. For me it was amazing."

Shoolman's family foundation has invested modest amounts in Pimentel's ongoing research.

Pimentel says the latest studies are just the beginning of a new way of looking at IBS. He believes 70 to 80 percent of cases are triggered by an episode of food poisoning – perhaps years before IBS symptoms appear. The toxins from food poisoning bacteria such as E. coli, Salmonella and Shigella, he thinks, damage intestinal nerves. That slows down the passage of intestinal contents and fosters the growth of bacteria in the small intestine.

Whatever the mechanism, Salix Pharmaceuticals, which markets rifaximin under the name Xifaxan, is poised to capitalize on the new research.

It's an enormous market. But unfortunately, there's no way to tell right now just which IBS patients are likely to benefit from antibiotic treatment. Clearly, from the new results, most won't.

Rifaximin is already approved for treatment of travelers' diarrhea and hepatic encephalopathy, a brain condition in patients with liver failure. Salix has petitioned the FDA to allow the antibiotic to be marketed for IBS. A decision is expected in March.

It's not clear how much IBS treatment with Xifaxan would cost. Drugstore.com lists it at nearly $22 a pill -- or $910 for the 14-day treatment course.

Copyright 2020 NPR. To see more, visit https://www.npr.org.

Since he joined NPR in 2000, Knox has covered a broad range of issues and events in public health, medicine, and science. His reports can be heard on NPR's Morning Edition, All Things Considered, Weekend Edition, Talk of the Nation, and newscasts.