Researchers at the National Institutes of Health have found an experimental opioid with an unusual profile in laboratory animals, according to an official NIH announcement. The compound, called DFNZ, produced strong pain relief in preclinical tests while showing minimal signs of respiratory depression, tolerance, withdrawal, or persistent drug-seeking behavior.
The finding comes from a study published in Nature. It focuses on a redesigned member of a long-shelved group of synthetic opioids known as nitazenes. These compounds were known for extreme potency, which made them scientifically interesting and medically risky.
NIH scientists revisited that old chemical class with modern tools. Their goal was to see whether the useful parts of nitazene biology could be separated from some of the dangers that have made opioids so difficult to use safely.
“Developing a highly effective pain medication without these drawbacks would have enormous public health benefits,” said Nora D. Volkow, M.D., director of NIH’s National Institute on Drug Abuse.
A Shelved Opioid Class Gets a Second Look
Nitazenes have a complicated scientific history. They interact with mu-opioid receptors, which are major targets for opioid pain medicines in the brain and peripheral nervous system. Those receptors can quiet pain signals very effectively. They can also drive dangerous side effects when activated in the wrong way or at the wrong dose.
In the 1950s, nitazenes were largely set aside because of their excessive potency. That potency made them difficult candidates for ordinary medication development. The NIH team returned to the class with a different goal. They wanted to study the pharmacology in detail and reshape the compounds toward a safer profile.
Michael Michaelides, Ph.D., a senior author and investigator at the National Institute on Drug Abuse, described the project as a search for a more useful version of a powerful chemical idea. “What we discovered exceeded our expectations,” he said.
The researchers focused on selectivity. Nitazenes are able to engage the opioid receptor strongly, which raised a key question. Could a compound keep powerful pain relief while producing fewer signals associated with overdose risk and addiction potential?
That question matters because opioids remain essential in medicine. Surgery, cancer pain, severe injuries and some chronic pain conditions can require strong analgesics. Safer options could change care for patients who need potent pain relief under medical supervision.
DFNZ Emerges From a Brief Brain Signal
The team’s path to DFNZ began with another formulation called FNZ. Researchers designed FNZ so it could be given to rats and tracked in the brain using a radioactive label. That allowed the scientists to watch the drug’s movement in real time.
They used positron emission tomography, often called PET imaging. This method can show where a labeled compound goes inside a living brain. In the rat studies, FNZ entered the brain for only a short window of time. The signal lasted about five to 10 minutes.
The surprise came afterward. Although the brain signal faded quickly, pain relief lasted for at least two hours. That mismatch suggested another molecule might be doing some of the work.
Many drugs are broken down into metabolites after entering the body. Some metabolites are inactive. Others have powerful effects of their own. The NIH team investigated whether FNZ produced an active by-product that could explain the longer-lasting analgesia.
That search led them to DFNZ. The compound is described as a superagonist because it has extremely high efficacy at the mu-opioid receptor. In simple terms, it can activate the receptor very strongly. The unexpected part was the safety-related pattern seen at therapeutic doses in the animal studies.
Strong Pain Relief Without Breathing Suppression
One of the most dangerous opioid side effects is respiratory depression. When breathing slows too much, oxygen levels can fall. This is a central pathway to fatal overdose with many opioids.
In the NIH study, DFNZ produced strong analgesia in rodents at preclinical therapeutic doses. The researchers reported that these doses produced a moderate and sustained increase in brain oxygen. That pattern stands out because depressed breathing would be expected to lower oxygen availability.
“Its capacity to be administered at therapeutic doses without producing respiratory depression is very important,” Michaelides said.
The researchers also tested repeated dosing. Repeated exposure to many opioids can lead to tolerance, which means a person needs higher doses to get the same effect. It can also lead to physical dependence and withdrawal when the drug is stopped.
In the animal tests reported by NIH, repeated DFNZ doses did not produce tolerance, drug dependency, or meaningful withdrawal effects. Among 14 classic opioid withdrawal signs, the team observed only irritability during handling. That irritability was measured through vocalization in DFNZ-treated rats.
Why the Dopamine Pattern Matters
Addiction risk involves more than pain relief. The brain’s reward circuitry helps determine whether a drug becomes strongly tied to cues, cravings and relapse. Dopamine is one of the major chemical messengers in that circuitry.
The NIH team found that DFNZ increased slow-acting dopamine release in reward-related brain regions. The compound did not produce the rapid dopamine bursts that are associated with strong drug-cue learning. That difference may help explain why the animals showed a less persistent drug-seeking pattern in the study.
For general readers, the timing of dopamine release is important. A sharp burst can help the brain link a drug to an environment, a cue, or a routine. Those learned associations can become powerful drivers of craving.
DFNZ’s slower dopamine pattern suggests a different reward signal in the animals tested. The compound still had rewarding effects, since rats self-administered it. The notable feature was what happened when the drug was removed.
“DFNZ has an unprecedented pharmacology for an opioid,” Michaelides said. His statement captures the central scientific point. DFNZ activated a receptor linked to strong analgesia, while its broader behavioral and breathing effects looked unusual in these preclinical experiments.
What the Rat Studies Showed
The NIH research relied on laboratory animals, so the results should be read as early-stage drug science. Rodent studies can reveal important biological patterns. Human safety and effectiveness require further testing under strict regulatory review.
In the pain experiments, DFNZ produced sustained analgesia. The researchers first saw an odd pattern with FNZ, which left the brain quickly while pain relief continued. That led them to the metabolite work that identified DFNZ as the likely driver of the prolonged effect.
The team then studied behaviors linked to reward and drug-seeking. Rats were trained to press a lever for a dose of the pain-relieving drug. The animals readily self-administered DFNZ, which showed that the compound has some rewarding effect.
The next part of the experiment was especially important. When DFNZ was replaced with saline, the animals stopped the drug-seeking behavior. NIH noted that this immediate change differs from patterns commonly observed with other opioids such as heroin, morphine and fentanyl. With those drugs, animals often keep seeking the drug even after it has been removed.
The researchers also looked at withdrawal-related signs. DFNZ-treated rats showed minimal withdrawal indicators under the conditions tested. This does not prove the same outcome in humans. It does give scientists a concrete reason to study the compound further.
A Possible Path Toward Human Trials
The next step is additional preclinical work. The NIH team plans to pursue studies that could support an application for regulatory approval to test DFNZ in humans. That process is designed to evaluate risks before a drug reaches clinical volunteers or patients.
Several questions remain. Researchers will need to examine dosing, duration of action, metabolism, safety margins and effects across more models. They will also need to clarify how DFNZ behaves in conditions that resemble human pain more closely.
Drug development often changes direction as evidence grows. A compound can look promising in animals and still fail in human trials. Differences in metabolism, brain chemistry, dose response and side effects can emerge later.
Still, the DFNZ findings are scientifically meaningful because they challenge a common assumption in opioid pharmacology. High-efficacy mu-opioid receptor drugs have often been viewed as poor candidates for safer analgesics. The NIH study suggests that receptor efficacy alone may give an incomplete picture of risk.
The broader lesson is about mechanism. DFNZ’s profile appears to involve how the receptor is engaged, how the compound moves through the body and how reward circuits respond over time. That combination gives researchers a fresh map for designing future pain medicines.
Potential Uses in Pain and Opioid Use Disorder
The NIH team believes several patient groups could eventually benefit if DFNZ proves safe and effective in humans. The source material points to surgical settings, cancer-related pain and chronic pain as areas with a high need for effective treatment.
The compound may also have potential in opioid use disorder. Current opioid agonist medications can help stabilize patients and reduce harms. Some also carry a risk of respiratory depression. The authors argue that DFNZ should be explored as a possible treatment candidate because of its preclinical profile.
That idea remains early. Any use in opioid use disorder would require careful human trials. Researchers would need to evaluate whether the compound reduces craving, withdrawal, or illicit opioid use. They would also need to test whether its reward profile remains favorable in people.
For pain care, the attraction is clear. Doctors need medications that can relieve severe pain while reducing the risks that surround opioid prescribing. A drug that keeps strong analgesia and lowers respiratory or addiction-related liabilities would be a major advance if human studies confirm the animal results.
For now, DFNZ is best understood as a promising experimental compound. The discovery gives scientists a sharper view of how opioid receptors might be targeted. It also opens a possible route toward medicines that treat pain with a wider safety margin.
