Scientists test alternative drugs that relieve pain without the dangers of opioids
Last year, Joan Peay slipped on her garage steps and smashed her knee on the welcome mat. Peay, 77, is no stranger to pain. The Tennessee retiree has had 17 surgeries in the last 35 years — knee replacements, hip replacements, back surgery. She even survived a 2012 fungal meningitis outbreak that sickened her and hundreds of others, and killed 64. This knee injury, though, “hurt like the dickens.”
When she asked her longtime doctor for something stronger than ibuprofen to manage the pain, he treated her like a criminal, Peay says. His response was frustrating: “He’s known me for nine years, and I’ve never asked him for pain medicine other than what’s needed after surgery,” she says. She received nothing stronger than over-the-counter remedies. A year after the fall, she still lives in constant pain.
Just five years ago, Peay might have been handed a bottle of opioid painkillers for her knee. After all, opioids — including codeine, morphine and oxycodone — are some of the most powerful tools available to stop pain.
Hitting opioid receptors in the peripheral nervous system keeps pain messages from reaching the brain. But opioids can cause problems by overstimulating the brain’s reward system and binding to receptors in the brain stem and gut.
But an opioid addiction epidemic spreading across the United States has soured some doctors on the drugs. Many are justifiably concerned that patients will get hooked or share their pain pills with friends and family. And even short-term users risk dangerous side effects: The drugs slow breathing and can cause constipation, nausea and vomiting.
A newfound restraint in prescribing opioids is in many cases warranted, but it’s putting people like Peay in a tough spot: Opioids have become harder to get. Even though the drugs are far from perfect, patients have few other options.
Many drugs that have been heralded as improvements over existing opioids are just old opioids repackaged in new ways, says Nora Volkow, director of the National Institute on Drug Abuse. Companies will formulate a pill that is harder to crush, for instance, or mix in another drug that prevents an opioid pill from working if it’s crushed up and snorted for a quick high. Addicts, however, can still sidestep these safeguards. And the newly packaged drugs have the same fundamental risks as the old ones.
The need for new pain medicines is “urgent,” says Volkow.
Scientists have been searching for effective alternatives for years without success. But a better understanding of the way the brain sends and receives specific chemical messages may finally boost progress.
Scientists are designing new, more targeted molecules that might kill pain as well as today’s opioids do — with fewer side effects. Others are exploring the potential of tweaking existing opioid molecules to skip the negative effects. And some researchers are steering clear of opioids entirely, testing molecules in marijuana to ease chronic pain.
Humans recognized the potential power of opioids long before they understood how to control it. Ancient Sumerians cultivated opium-containing poppy plants more than 5,000 years ago, calling their crop the “joy plant.” Other civilizations followed suit, using the plant to treat aches and pains. But the addictive power of opium-derived morphine wasn’t recognized until the 1800s, and scientists have only recently begun to piece together exactly how opioids get such a stronghold on the brain.
Opioids mimic the body’s natural painkillers — molecules like endorphins. Both endorphins and opioids latch on to proteins called opioid receptors on the surface of nerve cells. When an opioid binds to a receptor in the peripheral nervous system, the nerve cells outside the brain, the receptor changes shape and sets in motion a cellular game of telephone that stops pain messages from reaching the brain.
The danger comes because opioid receptors scattered throughout the body and in crucial parts of the brain can cause far-reaching side effects when drugs latch on. For starters, many opioid receptors are located near the base of the brain — the part that controls breathing and heart rate. When a drug like morphine binds to one of these receptors in the brain stem, breathing and heart rate slow down. At low doses, the drug just makes people feel relaxed. At high doses, though, it can be deadly — most opioid overdose deaths occur when a person stops breathing. And high numbers of opioid receptors in the gut — thanks in part to all the nerve endings there — can trigger constipation and sometimes nausea.
No matter how much I say I want to avoid opioids, half of my patients will get some kind of opioid. It’s just unavoidable.
— Christopher Wu
Plus, opioids are highly addictive. These drugs mess with the brain’s reward system, triggering release of dopamine at levels higher than what the brain is used to. Gradually, the opioid receptors in the brain become less sensitive to the drugs, so the body demands higher and higher doses to get the same feel-good benefit. Such tolerance can reset the system so the body’s natural opioids no longer have the same effect either. If a person tries to go without the drugs, withdrawal symptoms like intense sweating and muscle cramps kick in — the body is physically dependent on the drugs. Addiction is a more complex phenomenon than dependence, involving physical cravings so strong that a person will go to extreme lengths to get the next dose. Long-term users of prescription opioids might be dependent on the drugs, but not necessarily addicted. But dependence and addiction often go together.
Despite their risks, opioids are still widely used because they work so well, particularly for moderate to severe short-term pain.
“No matter how much I say I want to avoid opioids, half of my patients will get some kind of opioid. It’s just unavoidable,” says Christopher Wu, an anesthesiologist at Johns Hopkins Medicine.
In the late 1990s and early 2000s, more doctors began doling out the drugs for long-term pain, too. Aggressive marketing campaigns from Purdue Pharma, the maker of OxyContin, promised that the drug was safe — and doctors listened. Opioid overdoses nearly quadrupled between 2000 and 2015, with almost half of those deaths coming from opioids prescribed by a doctor, according to data from the U.S. Centers for Disease Control and Prevention.
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Opioid prescriptions rose in the United States throughout the 1990s and early 2000s. Physicians have begun to back off in the last few years.
Source: IMS Health
Opioid prescriptions have dipped a bit since 2012, thanks in part to stricter prescription laws and prescription registration databases. U.S. doctors wrote about 30 million fewer opioid prescriptions in 2015 than in 2012, data from IMS Health show. But restricting access doesn’t make pain disappear or curb addiction. Some people have turned to more dangerous street alternatives like heroin. And those drugs are sometimes spiked with more potent opioids such as fentanyl (SN: 9/3/16, p. 14) or even carfentanil, a synthetic opioid that’s used to tranquilize elephants. Overdose deaths from fentanyl and heroin have both spiked since 2012, CDC data reveal.
A sharper target
Scientists have been searching for a drug that kills pain as successfully as opioids without the side effects for close to a hundred years, with no luck, says Sam Ananthan, a medicinal chemist at Southern Research in Birmingham, Ala. He is newly optimistic.
“Right now, we have more biological tools, more information regarding the biochemical pathways,” Ananthan says. “Even though prior efforts were not successful, we now have some rational hypotheses.”
Scientists used to think opioid receptors were simple switches: If a molecule latched on, the receptor fired off a specific message. But more recent studies suggest that the same receptor can send multiple missives to different recipients.
The quest for better opioids got a much-needed jolt in 1999, when researchers at Duke University showed that mice lacking a protein called beta-arrestin 2 got more pain relief from morphine than normal mice did. And in a follow-up study, negative effects were less likely. “If we took out beta-arrestin 2, we saw improved pain relief, but less tolerance development,” says Laura Bohn, now a pharmacologist at the Scripps Research Institute in Jupiter, Fla. Bohn and colleagues figured out that mu opioid receptors — the type of opioid receptor targeted by most drugs — send two different streams of messages. One stops pain. The other, which needs beta-arrestin 2, drives many of the negatives of opioids, including the need for more and more drug and the dangerous slowdown of breathing.
Since that work, Bohn’s lab and many others have been trying to create molecules that bind to mu opioid receptors without triggering beta-arrestin 2 activity. The approach, called biased agonism, “has been around some time, but now it’s bearing the fruit,” says Susruta Majumdar, a chemist at Memorial Sloan Kettering Cancer Center in New York City. Scientists have identified dozens of molecules that seem to avoid beta-arrestin 2 in mice. But only a few might make good drugs. One, called PZM21, was described in Nature last year.
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Good without the bad
Morphine, a powerful opioid, is extracted from poppy plants. When it connects with the opioid receptor (blue), it relieves pain, but also causes negative effects.
Scientists have figured out which molecular messages trigger pain relief and which activate problems. The aim is to design new molecules, such as PZM21, that interact with the opioid receptor to instigate only pain relief.
Sources: B.L. Kieffer/Nature 2016; J.D. Violin et al/Trends in Pharmacol. Sci. 2014
Another one has shown promise in humans — a much higher bar. The pharmaceutical company Trevena, headquartered in King of Prussia, Pa., has been working its way through the U.S. Food and Drug Administration’s drug approval process with a molecule called oliceridine. In studies reported in April in San Francisco at the Annual Regional Anesthesiology and Acute Pain Medicine Meeting, oliceridine was as effective as morphine in patients recovering from bunion removal and others who had tummy tuck surgeries. Over the short term, people taking a moderate dose of the drug got pain relief comparable to that of morphine, but reported fewer side effects, such as vomiting and breathing problems.
Oliceridine is an intravenous opioid, not an oral one. That means it would be administered in the short term in hospitals, during and after surgeries. It’s not a replacement for the pills people can go home with, says Jonathan Violin, Trevena’s cofounder. And it’s not perfect: More side effects cropped up at higher doses. But it’s the first opioid using this targeted approach to get this far in human studies. The company hopes to submit an application for FDA approval by the end of 2017, Violin says.
Avoiding the beta-arrestin 2 pathway isn’t the only approach to targeted opioids — just one of the best studied. Ananthan’s lab is taking a different tack. His team showed that mice lacking a different opioid receptor, the delta receptor, tended not to show negative effects in response to the drugs. Now, the researchers are trying to find molecules that can activate mu opioid receptors while blocking delta receptors.
There may also be a way to direct pain-killing messages specifically to the parts of a person’s body that are feeling pain. In one recent study, scientists described a molecule that bound to opioid receptors only when the area around the receptors was more acidic than normal. Inflammation from pain and injury raises acidity, so this molecule could quash pain where necessary, but wouldn’t bind to receptors elsewhere in the body, reducing the likelihood of side effects. Rats in the study, published in the March 3 Science, didn’t find the new molecule as rewarding as fentanyl, so it may be less addictive. And they were less likely to have constipation and slowed breathing.
Drugs face a long uphill climb from even the most promising animal studies to FDA approval for use in humans. Very few make it that far. It’s too soon to tell whether PZM21 and other molecules being studied in mice will ever end up as treatments for patients.
Unwilling to wait, some people in pain are turning to substances that are already available — without a doctor’s order. And scientists are trying to catch up.
In August 2016, the Drug Enforcement Administration announced that it was cracking down on a supplement called kratom. Officials wanted to put the herb in the same regulatory category as heroin and LSD, labeling it a dangerous substance with no medical value. Members of the public vehemently disagreed. More than 23,000 comments poured in from veterans, cancer survivors, factory workers, lawyers and teachers. Almost all of them said the same thing: Kratom freed them from pain.
Made from the leaves of the tropical plant Mitragyna speciosa, kratom is sold in corner convenience stores and through online retailers. Its pain-killing abilities come mainly from two different molecules in the plant’s leaves: mitragynine and the structurally similar 7-hydroxymitragynine. Both have a structure that’s very different from morphine, but they bind to opioid receptors. That technically makes them opioids, even though they don’t look like morphine or oxycodone, Majumdar says. And that’s what concerned the DEA.
But just like some of the new opioids that scientists are developing, kratom’s active ingredients appear — anecdotally, at least — to deliver pain relief with fewer problems and less risk of tolerance. Some chronic opioid users switch to kratom to wean themselves off of pain pills and ease withdrawal symptoms, says Oliver Grundmann, a medicinal chemist at the University of Florida in Gainesville. Other users have never habitually used opioids but are seeking relief from chronic pain or mental health problems, according to a survey he published online May 10 in Drug and Alcohol Dependence. Grundmann hopes the survey results will help guide research into the substance’s efficacy for specific medical concerns.
The safety and efficacy of kratom is still up for debate. There’s a lack of controlled clinical studies about the leaf’s impact on the body, Grundmann says. Plus, the way kratom is regulated — as a supplement — means that people buying it have no guarantee of what they’re actually getting.
While kratom has its fans, its active compounds aren’t very potent, says Majumdar. He thinks he could make a better drug by modifying these molecules.
Majumdar, Sloan Kettering collaborator András Váradi and colleagues tested a structural cousin of 7-hydroxymitragynine: mitragynine pseudoindoxyl. It binds to mu opioid receptors about 200 times as effectively as mitragynine in mice, the researchers reported in August in the Journal of Medicinal Chemistry. Just like Trevena’s oliceridine, the new molecule does not activate beta-arrestin 2. The pseudoindoxyl version also blocks the delta opioid receptor, further impeding nonpain-related activities.
Majumdar hopes a DEA ban on kratom won’t happen; it would severely restrict access, making research much harder to do. For now, there is no ban — but scientists are wary, he says.
Mix it up
Despite the potential for new, better opioids, other researchers are focused on an altogether different set of pain-killing drugs: the cannabinoids (made famous by marijuana, the dried leaves and other parts of the hemp plant, Cannabis sativa).
The active molecules in marijuana don’t have the same fast-acting pain-quenching abilities that opioids do. “If I go into an emergency room with acute pain, give me morphine,” says Yasmin Hurd, a pharmacologist at Mount Sinai in New York City. But with medical marijuana legal in 29 states plus the District of Columbia, the plant is getting more attention as a potential pain reliever, especially for chronic pain (SN: 6/14/14, p. 16).
Doctors in states where marijuana is legal write fewer prescriptions for opioid painkillers, a 2016 study in Health Affairs showed. Those states also had about a 25 percent lower rate of opioid overdose deaths compared with states that didn’t legalize marijuana, according to a 2014 study in JAMA Internal Medicine. When marijuana becomes legally available, some people might choose it instead of opioids.
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Pump up the volume
Small molecules that latch on to the body’s cannabinoid receptors can make other docking sites on the receptor change shape to fit the body’s natural cannabinoids. The better fit may amp up the body’s ability to send pain-dulling messages to the brain.
Source: R.A. Ross/Trends in Pharmacol. Sci. 2007
There might be some merit to that choice. There are plenty of cannabinoid receptors in parts of the brain that process pain messages. But unlike opioid receptors, few exist in the brain stem. That means cannabinoids are far less likely to influence breathing than opioids, says Joseph Cheer, a neurobiologist at the University of Maryland School of Medicine in Baltimore. Fatal overdoses are nearly unheard of.
As with kratom, though, there’s a glut of anecdotal evidence suggesting marijuana’s power to cure everything from pain to anxiety to ulcers — but not many controlled clinical trials to back up the assertions (SN Online: 1/12/17). The knowledge gap is made even wider by the fact that marijuana has wildly different effects depending on how it’s ingested and the relative ratios of certain active molecules in each strain of the plant.There might be some merit to that choice. There are plenty of cannabinoid receptors in parts of the brain that process pain messages. But unlike opioid receptors, few exist in the brain stem. That means cannabinoids are far less likely to influence breathing than opioids, says Joseph Cheer, a neurobiologist at the University of Maryland School of Medicine in Baltimore. Fatal overdoses are nearly unheard of.
“People think they know how marijuana affects the brain,” Hurd says. In reality, “there’s been very little evidence-based structural scientific studies done with marijuana.”
Aron Lichtman, a pharmacologist at Virginia Commonwealth University in Richmond, agrees. “There’s definitely medicine in that plant — that’s been proven,” he says. “The challenge is that it may not work for everybody and every type of pain.”
Scientists who are serious about figuring out marijuana are breaking it down, looking at the plant’s active molecules — cannabinoids — one by one. Cannabidiol, or CBD, has garnered particular attention. Because of the way it indirectly interacts with cannabinoid receptors, it doesn’t give people the high that’s characteristic of tetrahydrocannabinol, or THC, the mind-altering chemical in marijuana. That makes CBD less rewarding and better suited to longer-term use. The molecule can influence signals sent by a number of other receptors in the brain, many involved in pain and inflammation.
But THC might have merit, too. It’s already used in a couple of FDA-approved drugs to treat nausea and vomiting from chemo-therapy. There’s some evidence that those medications might also help relieve pain, though Lichtman calls those studies a “mixed bag.”
Alone, cannabinoids might be fairly weak painkillers. But combined with opioids, he’s shown, they can amplify the pain relief and reduce the opioid dose needed in mice.
Drugs that might amp up the power of the body’s natural cannabinoids are another option. That’s what Ruth Ross of the University of Toronto is studying. A few years ago, her team identified a region on a cannabinoid receptor called CB1 that has an interesting property: Small molecules that bind to it act like volume knobs for the body’s natural cannabinoids, called endocannabinoids. When a molecule of the right shape locks on to CB1, it makes endocannabinoids naturally present in the body more likely to latch on. That boosts pain relief in a targeted way — when endocannabinoids are already being released by the body, such as after injury or stress.
“You magnify the already existing effects of the compound,” Ross says. Her team has identified and patented several of these volume-knob molecules, and is working on improving them.
“For various reasons they wouldn’t be good as drugs,” she says. They have too many effects on the body beyond their intended one. But she’s making slight tweaks to their chemical structures to try to reduce those off-target effects, with the hope that one day the molecules could be studied in patients.
Safer opioids or alternative painkillers would help people deal with their pain without risking addiction or death. Peay has gotten to know people — as a member of social media groups for those living with chronic pain — who are experiencing the crushing results of poorly managed pain. People lose their jobs, she says, or move to Colorado just to get access to legal marijuana. As for her? “I still have my sense of humor, and that helps me get through all the pain.” But she’s holding out for something better.
This article appears in the June 10, 2017, issue of Science News with the headline, "Beyond today's opioids: Scientists search for better pain drugs."
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