Nociceptin/orphanin FQ
N/OFQ, orphanin FQ, nociceptin, PNOC gene product, FGGFTGARKSARKLANQ
Nociceptin, also called orphanin FQ, is the endogenous ligand of the NOP receptor, identified in 1995 by two groups simultaneously. It is structurally an opioid peptide that is not an opioid: a single amino acid change at the N-terminus abolishes activity at mu, delta and kappa receptors. NOP-targeted drugs remain investigational, and no NOP medicine is approved anywhere.
Mechanism
The PNOC gene yields prepronociceptin, processed to the 17-residue nociceptin plus nocistatin and other fragments. Nociceptin acts selectively at the NOP receptor (OPRL1), the fourth member of the opioid receptor family, which shares roughly 60 per cent homology with the classical opioid receptors yet is not activated by them. The critical structural point is the N-terminal phenylalanine: the classical opioid peptides all begin Tyr-Gly-Gly-Phe, and the tyrosine hydroxyl is essential for mu, delta and kappa binding. Substituting phenylalanine creates a peptide with an opioid architecture and an entirely separate pharmacology.
NOP is Gi/Go-coupled, inhibiting adenylate cyclase, opening GIRK channels and closing N-type calcium channels, so cellular signalling closely resembles classical opioid receptors. The behavioural consequences differ by site. Supraspinally, nociceptin is anti-opioid and produces hyperalgesia by inhibiting descending inhibitory pathways in the periaqueductal grey, which is how it acquired its name; spinally and peripherally, it is analgesic. NOP activation also blunts opioid reward and stress responses, and NOP is highly expressed in cortex, amygdala and hippocampus, supporting roles in anxiety, mood and alcohol drinking.
What the research shows
The NOP-selective clinical story is equivocal and often overstated. LY2940094 (also designated BTRX-246040), a selective NOP antagonist, was tested in an eight-week double-blind placebo-controlled proof-of-concept study in major depressive disorder. The published report is careful about what it found: there was some evidence of an antidepressant effect on the GRID-Hamilton scale and an early shift in emotional stimulus processing, but the predefined proof-of-concept efficacy criterion, a probability of at least 88 per cent that the drug was better than placebo, was not met, reaching 82.9 per cent. It has not advanced to registration.
Cebranopadol, a dual NOP and mu-opioid agonist, was reported by its sponsor to have met the primary endpoints of two phase 3 acute pain trials, in abdominoplasty and bunionectomy. No peer-reviewed primary publication of those trials could be located, so the effect sizes should not be treated as established. The proposed advantage is that NOP co-agonism may limit the reward and respiratory effects of mu activation, though that claim requires abuse-liability and safety data rather than efficacy data to substantiate. Note also the direction problem: agonists are being developed for pain while antagonists are developed for depression, so the system does not have one therapeutic direction.
Evidence assessment
Limited evidence
Downgraded from mixed on audit. The only NOP human trial with a verified peer-reviewed publication explicitly did NOT meet its predefined proof-of-concept efficacy criterion. The phase 3 acute pain results for cebranopadol exist only as sponsor topline announcements with no located peer-reviewed publication, so they cannot support a higher tier. Nothing NOP-targeted is approved, and the peptide itself has never been given to people.
Tiers are applied consistently across the library and re-checked when new trials read out. Read the grading method.
Key studies
Isolation and structure of the endogenous agonist of opioid receptor-like ORL1 receptor Preclinical only
Identified nociceptin as the endogenous ORL1 (NOP) ligand and reported that it lowered pain threshold, giving the peptide its name.
Orphanin FQ: a neuropeptide that activates an opioidlike G protein-coupled receptor Preclinical only
Simultaneous independent identification of the same peptide, named orphanin FQ, establishing the fourth opioid receptor system.
A Selective Nociceptin Receptor Antagonist to Treat Depression: Evidence from Preclinical and Clinical Studies Preclinical only
The predefined proof-of-concept efficacy criterion was NOT met: the probability of the drug being better than placebo reached 82.9 per cent against a prespecified threshold of 88 per cent. The drug was safe and well tolerated and showed an early effect on emotional stimulus processing.
ALLEVIATE-1 and ALLEVIATE-2: cebranopadol for moderate-to-severe acute pain Preclinical only
The sponsor reported that both trials met their primary pain endpoints. No peer-reviewed publication was located, so no effect size is asserted here and the result should not be treated as independently established.
Safety
Nociceptin itself has not been administered to humans therapeutically and has no safety profile. The NOP antagonist studied in depression was reported as safe and well tolerated in an eight-week trial. Cebranopadol retains mu-opioid agonism and therefore genuine opioid risk including respiratory depression and dependence; its phase 3 safety datasets have not been published in peer-reviewed form, so tolerability claims about it should be treated as provisional.
Regulatory status
| Jurisdiction | Status |
|---|---|
| United Kingdom | No UK marketing authorisation for nociceptin or any NOP-targeted medicine. The peptide is an unlicensed substance. |
| United States | Nociceptin has no FDA approval. Cebranopadol remains investigational. No NOP-targeted product is currently approved by the FDA. |
| WADA (sport) | Not individually named on the Prohibited List. Prohibited at all times under section S0 as a non-approved substance. |
Questions
Structurally yes, pharmacologically no. It has the opioid peptide architecture but starts with phenylalanine instead of tyrosine, which abolishes binding at mu, delta and kappa receptors. It acts only at NOP.
Both, depending on site. Given supraspinally it produces hyperalgesia by blocking descending pain inhibition, which is how it was named. Given spinally or peripherally it is analgesic.