Chemists redesign fentanyl to preserve pain relief and reduce respiratory risk

Scripps Research scientists have re-engineered fentanyl’s core structure to retain potent analgesia while limiting respiratory depression in preclinical tests – a strategy that could inform safer opioid therapies, curb overdose deaths

Fentanyl ranks among the most potent analgesics in modern medicine, yet its clinical value has long sat alongside a stark hazard profile. The drug can suppress respiration to a dangerous extent and carries a high risk of dependence. Those safety concerns have restricted its use, even as clinicians continue to rely on it to manage severe acute pain in anaesthesia, trauma care and oncology. At the same time, its relatively straightforward and inexpensive manufacture has enabled extensive illicit production.

Researchers at Scripps Research, San Diego, California, USA, have now reported a structural redesign of fentanyl that aimed to separate its analgesic potency from its most perilous adverse effects. The team modified the drug’s molecular core and described a compound that preserved strong pain relief in experimental systems while markedly reducing respiratory depression.

“For decades, the pharmaceutical industry has been constrained by the assumption that major structural changes to opioids would eliminate their analgesic properties,” said Dr. Kim D. Janda, who is the ‘Ely R. Callaway Jr.’ professor of chemistry at Scripps Research and senior author of the study.

“Our research has identified a different possibility – that fundamental structural redesign can preserve pain relief while improve safety,” he said.

Synthetic opioids such as fentanyl occupy a paradoxical role in healthcare. When first introduced, they were promoted as powerful analgesics but with limited addiction potential, a claim that has later proved to be tragically off the mark. Despite that history, such drugs have become indispensable in clearly defined clinical contexts. The central scientific challenge has been to retain analgesic function while reducing the risks of respiratory depression, overdose and dependence.

To pursue that aim, the Scripps team applied a medicinal chemistry approach known as bioisosteric replacement. In essence, chemists substitute one part of a molecule with a distinct structural fragment that mimics certain chemical features but alters others. The technique can preserve biological activity while modifying other pharmacological properties.

Rather than alter peripheral elements of the fentanyl molecule, the researchers replaced its central ring system with a radically different three-dimensional architecture. They introduced a structure known as 2-azaspiro[3.3]heptane, a spirocyclic scaffold that consists of two four-membered rings connected at a single shared atom. This geometry contrasts sharply with fentanyl’s original core and imposes a distinct spatial arrangement on the molecule.

“Rather than tweak small parts of the molecule, we replaced the entire central structure with something that looks completely different in three-dimensional space,” said Dr. Arran Stewart, research associate in the Janda laboratory and first author of the paper.

Despite that pronounced structural shift, the modified compound retained strong analgesic activity in preclinical assays. The investigators attributed this effect to preserved binding affinity at the opioid receptor. Opioid drugs act primarily at the mu opioid receptor – a protein embedded in the membranes of certain nerve cells. A positively charged region of the drug forms an electrostatic interaction with a negatively charged amino acid residue within the receptor’s binding pocket. This interaction anchors the molecule in place and allows the receptor to adopt an active configuration.

The redesigned compound maintained this crucial electrostatic anchor while alter many of the surrounding molecular contacts. As a result, it achieved sufficient receptor activation to produce analgesia, even though its overall binding pattern differed from that of fentanyl.

A notable feature of the analogue concerned intracellular signalling. Conventional opioids can activate multiple downstream pathways after receptor engagement. Among them is recruitment of beta-arrestin, a signalling protein that some researchers have linked to respiratory depression and other adverse effects. In the present study, the modified compound showed no detectable recruitment of the beta-arrestin pathway under the experimental conditions tested.

In animal models, respiratory depression occurred only at very high doses and proved transient, with breathing return to baseline by 30 minutes post administration. The compound also exhibited a short half-life of around 27 minutes, which indicates rapid clearance from the body. Such a pharmacokinetic profile could offer advantages in tightly controlled medical settings, where clinicians often require a potent but short-acting analgesic.

The authors have framed the work as part of a broader effort to mitigate the harms associated with synthetic opioids. By re-engineering the fentanyl scaffold, they have created what they describe as a chemical addendum that could serve as a platform for further refinement. The team has also indicated plans to integrate these structural insights into the development of opioid vaccines that aim to prompt the immune system to recognise and neutralise fentanyl molecules before they reach the brain.

“Finding ways to preserve analgesic properties of synthetic opioids without [the associated] perils of respiratory depression could help to derisk the toxicity associated with synthetic opioid use while provide a conduit for pain management,” said Janda.

The work remains at a preclinical stage and extensive evaluation will be required to establish safety, efficacy and addiction liability in humans. Nevertheless, the study challenges a long-held assumption in opioid pharmacology – that significant structural modification inevitably destroys analgesic potency.

For further reading please visit: 10.1021/acsmedchemlett.5c00672