Peptide-based painkillers that mimic a natural brain chemical target the same opioid receptor as morphine but show far fewer side effects and lower addiction potential
Researchers at Tulane University and the Southeast Louisiana Veterans Health Care System report a promising new class of painkillers derived from endomorphin, a natural opioid-like peptide in the brain. Published in Neuropharmacology, the study demonstrates that engineered endomorphin analogs can deliver pain relief as potent as morphine while producing substantially fewer adverse effects, including less respiratory depression, reduced motor impairment, and markedly lower abuse liability.
In animal experiments using rats, the research team compared several metabolically-stable endomorphin analogs with morphine to evaluate analgesic potency, side-effect profiles, tolerance development, and indicators of abuse potential. Like morphine, these peptides act at the mu opioid receptor (MOR), the primary site for opioid analgesia, but their molecular design appears to separate pain relief from many of the harmful consequences commonly associated with traditional opioids.
Opioids such as morphine remain the most effective therapies for severe and chronic pain, but their use is limited by serious side effects. Respiratory depression is the leading cause of fatal opioid overdoses, tolerance and physical dependence promote escalating doses and increased risk, and motor and cognitive impairments can pose major hazards—especially for older adults. Abuse and addiction also have profound public-health consequences.
According to lead investigator James Zadina, a VA senior research career scientist and professor at Tulane University School of Medicine, the endomorphin analogs tested in this study delivered powerful analgesia while minimizing or eliminating many of these drawbacks. “These side effects were absent or reduced with the new drug,” Zadina said. “It’s unprecedented for a peptide to deliver such powerful pain relief with so few side effects.”
Key findings from the rat experiments include:
- Analgesic potency comparable to morphine, with longer-lasting pain relief at equivalent doses.
- Minimal respiratory depression at doses that produced significant respiratory slowing with morphine.
- Little or no impairment of motor coordination, in contrast to clear motor deficits after morphine—an important safety consideration for older patients.
- Substantially lower tolerance development compared with morphine and a lack of spinal glial activation, a morphine-induced inflammatory response linked to tolerance.
- Reduced indicators of reward and abuse potential: the peptides did not produce conditioned place preference, and rats did not increase self-administration behavior for the new compounds as they did for morphine.
The combined results suggest that these endomorphin analogs maintain selective MOR-mediated analgesia while avoiding many of the mechanisms thought to drive opioid side effects and addiction. Reduced glial activation observed with the analogs may help explain their diminished propensity to cause tolerance and hyperalgesia compared with morphine.
Given the promising preclinical evidence, the research team hopes to advance these compounds into human clinical trials within the next two years. If safety and efficacy are confirmed in humans, endomorphin-based analgesics could offer a new therapeutic option for managing severe and chronic pain with a considerably improved safety profile over current opioid treatments.
Source: Keith Brannon – Tulane University
Image credit: Paula Burch-Celentano, Tulane University.
Original research: Abstract for “Endomorphin analog analgesics with reduced abuse liability, respiratory depression, motor impairment, tolerance, and glial activation relative to morphine” by James E. Zadina, Mark R. Nilges, Jenny Morgenweck, Xing Zhang, Laszlo Hackler, and Melita B. Fasold. Published online January 2016 in Neuropharmacology (doi:10.1016/j.neuropharm.2015.12.024)
Abstract (summary)
The study tested metabolically-stable endomorphin analogs that selectively activate the mu opioid receptor. Compared to morphine in rat models, these analogs produced equal or greater antinociception while showing substantially improved safety and tolerability. Specifically, the analogs reduced respiratory depression, motor coordination impairment, tolerance and hyperalgesia, and markers of spinal glial activation (p38/CGRP/P2X7 signaling). Behavioral assays associated with reward and abuse—conditioned place preference and self-administration—indicated markedly lower reward potential for the analogs relative to morphine. These differential effects on glial activation and other pathways suggest a mechanism by which endomorphin analogs achieve potent analgesia with fewer side effects, positioning them as promising candidates for safer opioid therapy.
“Endomorphin analog analgesics with reduced abuse liability, respiratory depression, motor impairment, tolerance, and glial activation relative to morphine” by James E. Zadina, Mark R. Nilges, Jenny Morgenweck, Xing Zhang, Laszlo Hackler, and Melita B. Fasold. Neuropharmacology. Published online January 2016. doi:10.1016/j.neuropharm.2015.12.024