Relistor, also known as methylnaltrexone bromide, can help patients with this problem.
Development and use
People who take opioids for pain may experience severe constipation.
In 1978, colleagues at the University of Chicago started looking for ways to help patients who would not take morphine for pain, because it would cause unbearable constipation.
They wanted to find a medication to treat the constipation, without reducing the painkilling effects of the opioid.
The opioid would need to target the receptors associated with pain relief but without causing the side effects.
The lead researcher, Dr. Goldberg, noticed that medications were already available that acted on the opioid receptors of the digestive system, without crossing into the brain, such as loperamide.
The team started screening compounds, in the search for a drug that would relieve constipation without entering the brain and neutralizing morphine's painkilling effects.
A compound called N-methyl-naltrexone (MNTX), created by Boerhringer Ingelheim, a German pharmaceutical company, looked promising.
In 2005, following a number of tests, two pharmaceutical companies signed an agreement to develop the drug and commercialize methylnaltrexone.
The new drug would be used to treat opioid-induced side effects, including constipation and post-operative ileus. In this condition, a part of the intestine becomes paralyzed, so that food and drink cannot be pushed forward. Post-operative ileus can happen after abdominal surgery.
Relistor was approved for the treatment of opioid-induced constipation in 2008 by the United States Food and Drug Administration (FDA).
How Relistor works
Opioids, such as morphine, stop pain by numbing the receptors in the brain, but they also numb the receptors in the digestive tract.
Numbness in the digestive system means that the muscles do not move undigested food and stools forward.
The slower the food moves through the digestive tract, the more the colon will absorb water from the food. The stools, or feces, become dry and hard.
Meanwhile, contractions occur in the middle of the small intestine, but these contractions do not push the food forward. Digestive secretions are reduced, the urge to defecate is less, and constipation results.
Relistor, or methylnaltrexone bromide, blocks the numbing in the intestines. It binds to the receptors in the gut, and it blocks morphine's effects on them, so that they will no longer be numb. This means that the muscles in the gut can react normally again.
Relistor does not block the opioids' painkilling effects. It does not cross the brain barrier, and so it does not enter the brain. Morphine's painkilling effects inside the brain continue to work.
If methylnaltrexone bromide entered the brain, it would cancel out morphine's painkilling effects, and the patient would be in pain again.
Studies have suggested that 30 percent of patients had a bowel movement within 30 minutes of having a dose of Relistor. Sixty percent of patients in one study had a bowel movement within 4 hours of the first injection.
Another study says that at least 50 percent of people taking methylnaltrexone will experience laxation within 24 hours, during the first 2 weeks of using it. Pain in these patients remained under control, and they did not have serious adverse effects.
One study shows that side effects are "unlikely," and that fewer than 1 percent of patients experience diarrhea that is serious enough to warrant stopping the treatment.
Abdominal pain affected 17 to 30 percent of people taking the drug, compared with 10 percent to 13 percent of people who were taking a placebo.
It is not recommended for people who have a gastrointestinal blockage.