Once people realized that opioid drugs could cause addiction and deadly overdoses, they tried to use newer forms of opioids to treat the addiction to its parent. Morphine, about 10 times the strength of opium, was used to curb opium cravings in the early 19th century. Codeine, too, was touted as a nonaddictive drug for pain relief, as was heroin.
Those attempts were doomed to failure because all opioid drugs interact with the brain in the same way. They dock to a specific neural receptor, the mu-opioid receptor, which controls the effects of pleasure, pain relief and need.
Now scientists are trying to create opioid painkillers that give relief from pain without triggering the euphoria, dependence and life-threatening respiratory suppression that causes deadly overdoses.
That wasn't thought possible until 2000, when a scientist named Laura Bohn found out something about a protein called beta-arrestin, which sticks to the opioid receptor when something like morphine activates it. When she gave morphine to mice that couldn't make beta-arrestin, they were still numb to pain, but a lot of the negative side effects of the drug were missing. They didn't build tolerance to the drug. At certain dosages, they had less withdrawal. Their breathing was more regular, and they weren't as constipated as normal mice on morphine.
Before that experiment, scientists thought the mu-opioid receptor was a simple switch that flicked all the effects of opioids on or off together. Now it seems they could be untied. "The hope is you'd have another molecule that looks like morphine and binds to the same receptor, but the way it turns the receptor on is slightly different," says Dr. Aashish Manglik, a researcher at Stanford University School of Medicine who studies opioid receptors.
After Bohn's discovery, a number of people, including a team that includes Manglik, started looking for a drug that could connect to the mu-opioid receptor in a way that avoids the negative effects of beta-arrestin.
To do that, they mapped the receptor's structure in a computer program and started looking for chemicals that would stick to it. "We tried to look for molecules that would still bind to this 3-D structure, but are as far away from morphine and codeine as possible," Manglik says.
The team ran 3 million possibilities through the computer and picked the 23 best candidates to test in a lab. One chemical, PZM21, seems to do what they hoped: Turn the opioid receptor on without using much beta-arrestin. They report their findings in Nature on Wednesday.