A new treatment for endometriosis — a painful, chronic condition that around 190 million people live with — might be on the horizon.

The disease is caused by cells of the inner lining of the uterus growing in parts of the body where they don’t belong. The resulting lesions are typically treated with surgery, hormone therapy or a combination of the two. These options come with various complications, however, and are often ineffective in the long term.

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Now researchers have tweaked niclosamide, a drug used to treat parasitic infections such as tapeworms, to target cells implicated in the disease, and tested it in mice. The study is published in Advanced Healthcare Materials¹.

If replicated in people, the results could provide a “breakthrough” in endometriosis treatment, says co-author Kanako Hayashi, a reproductive biologist at Washington State University in Pullman.

The study is a “beautiful first step” towards non-surgical, non-hormone-based therapies, says Elise Courtois, a molecular biologist at the Jackson Laboratory in Farmington, Connecticut.

When treatment misses the mark

Endometriosis can affect all those born with a uterus, and is most commonly diagnosed in girls and women between puberty and menopause.

People with the condition often experience severe abdominal and pelvic pain, that typically worsens during menstruation, as well as problems with fertility. The condition is underdiagnosed and undertreated, in part because it can be definitively diagnosed only with surgery to identify lesions outside the uterus. Other factors include the normalization of menstrual pain and the historical gender gap in health care.

Current treatments are far from adequate. Nearly half of people who undergo surgery to remove lesions — the gold-standard treatment — experience recurrence within five years. And hormone therapies are disruptive; they often prevent pregnancy, and side effects are wide-ranging and include depression, hot flushes and osteoporosis.

So researchers are looking for alternatives. In 2016, Hayashi worked on a study which found that niclosamide’s anti-inflammatory effects helped to attenuate the growth of endometrial tissue that had been implanted into mice².

But there was a problem. Niclosamide isn’t well-absorbed in the body because of its low solubility in water. This means people must take high doses for it to work, and this often causes nausea, appetite loss and stomach cramps. In fact, the US Food and Drug Administration has approved niclosamide for use for only seven days at a time. That isn’t practical for treating a chronic condition.