Chimeric peptides fusing opioid endomorphins with a ghrelin receptor fragment produced 8-fold stronger pain relief than endomorphins alone with approximately 55% less tolerance development in mice.
55% less toleranceEM-1-DLS showed a tolerance ratio of 2.33-fold vs 5.19-fold for endomorphin-1 alone, representing roughly 55% less acute tolerance development
What the researchers found
Two novel chimeric peptides (EM-1-DLS and EM-2-DLS) combining opioid endomorphin sequences with a ghrelin receptor antagonist fragment produced potent pain relief in mice with significantly reduced tolerance development. EM-1-DLS was approximately 8 times more potent than endomorphin-1 alone and acted through κ-opioid, μ-opioid, and ghrelin (GHS-R1α) receptors simultaneously.
Critically, EM-1-DLS induced an acute tolerance ratio of only 2.33-fold, compared to 5.19-fold for endomorphin-1 alone — a roughly 55% reduction in tolerance development. Cross-tolerance ratios between the chimeric peptides and endomorphins ranged from 0.92 to 1.76, confirming reduced tolerance.
Why it matters
The opioid crisis has made clear the need for effective pain medications that don't rapidly build tolerance, leading to dose escalation and addiction. By fusing opioid peptides with ghrelin receptor-targeting fragments, these chimeric peptides represent a multifunctional approach to analgesia — harnessing multiple pain pathways simultaneously while reducing the tolerance that makes traditional opioids progressively less effective. This 'multitarget peptide' strategy could be a template for designing safer analgesics.
The numbers in context
EM-1-DLS: ~8× more potent than EM-1 · Tolerance ratio: 2.33-fold (vs 5.19 for EM-1) · ~55% reduced tolerance · Cross-tolerance: 0.92–1.76 · Targets: κ-OR, μ-OR, GHS-R1α
How the study worked
Two chimeric peptides were designed by combining endomorphin-1 or endomorphin-2 with the ghrelin receptor antagonist [D-Lys3]-GHRP-6. Receptor binding and functional activity were characterized in vitro. In vivo antinociceptive effects were tested in mice using the tail withdrawal test after intracerebroventricular (brain) injection. Dose-response curves, time-dependency, and acute tolerance development were assessed.
Who was studied
Mice (intracerebroventricular injection pain model)
What this study cannot tell us
This is an animal study with intracerebroventricular (direct brain) injection, which doesn't reflect a practical route of administration for human use. The tail withdrawal test measures only one type of pain (acute thermal). Long-term tolerance, addiction potential, and other side effects were not assessed. Translation from mouse pain models to human chronic pain remains a major challenge.
How to read the evidence
This is a preclinical animal study using intracerebroventricular injection in mice. While the results are mechanistically informative and the tolerance reduction is significant, the route of administration and single pain model limit direct translational relevance. Evidence is preliminary.
When this study was published
Published in 2025, this study represents the current frontier of chimeric peptide design for pain management with reduced opioid tolerance.
The bigger picture
The search for effective, less-addictive painkillers is one of the most urgent challenges in medicine. Traditional opioids target a single receptor and rapidly lose effectiveness through tolerance. The multifunctional peptide approach shown here — simultaneously engaging opioid and ghrelin pathways — represents an innovative strategy in peptide pharmacology. Ghrelin receptors are expressed in pain-modulating brain regions, and their involvement in analgesia is an emerging area of research. This study adds to a growing body of work suggesting that chimeric peptides targeting multiple receptor systems could offer better therapeutic profiles than single-target compounds.
Questions still open
- Can these chimeric peptides be modified for systemic (injectable or oral) administration rather than direct brain injection?
- Would the reduced tolerance development persist over longer treatment periods and in chronic pain models?
- What is the addiction potential of κ-opioid-preferring chimeric peptides compared to traditional μ-opioid agonists?
Common questions
What are endomorphins and why are they combined with a ghrelin fragment?
Why is reduced tolerance so important for pain medication?
Read the original research
Novel chimeric peptides based on endomorphins and ghrelin receptor antagonist produced supraspinal antinociceptive effects with reduced acute tolerance in mice.
Biochimie, 228, 58-70
Citation
Wu, Bing; Cheng, Songxia; Liu, Fuyan; Wei, Jia; Liu, Yongling; Qian, Teng; Ding, Jiali; Xu, Biao; Wei, Jie. (2025). Novel chimeric peptides based on endomorphins and ghrelin receptor antagonist produced supraspinal antinociceptive effects with reduced acute tolerance in mice.. Biochimie, 228, 58-70. https://doi.org/10.1016/j.biochi.2024.08.010