Opiorphin

What is Opiorphin?

Opiorphin is a five-amino-acid endogenous peptide (sequence: Gln-Arg-Phe-Ser-Arg, single-letter QRFSR) discovered in human saliva in 2006 by Catherine Rougeot, Vinciane Wisner-Lynch, and colleagues at the Institut Pasteur in Paris. The discovery paper, published in PNAS, identified opiorphin as the human homolog of the rat sialorphin peptide (Gln-His-Asn-Pro-Arg) that the Rougeot laboratory had reported three years earlier — both peptides are processed from the prolactin-induced protein (PIP, encoded by the BPLF1/PROL1 gene) precursor and both function as endogenous dual ectopeptidase inhibitors. Mechanistically, opiorphin blocks two distinct membrane-bound peptidases simultaneously: neutral endopeptidase (NEP, also called neprilysin or membrane metalloendopeptidase, encoded by MME), and aminopeptidase N (APN, also called CD13, encoded by ANPEP). Both peptidases participate in the rapid proteolytic degradation of endogenous Met- and Leu-enkephalin, the brain's intrinsic mu- and delta-opioid receptor agonists. By inhibiting both enkephalinases, opiorphin extends the half-life and signaling duration of endogenous enkephalins at their receptors — producing naloxone-reversible analgesia indirectly through potentiation of endogenous opioid tone rather than direct mu/delta-receptor binding. The resulting pharmacology is remarkable: in rodent pain models, opiorphin produces analgesia of magnitude comparable to morphine (Wisner et al. 2006 PNAS), but with a markedly different side-effect profile. The 2010 Rougeot, Robert, Menz, Bisson, and Messaoudi PNAS paper reported that systemically active opiorphin lacks physiological dependence, lacks tolerance development, and lacks abuse liability in laboratory mice — a side-effect profile that has driven sustained translational interest. Beyond analgesia, opiorphin has documented antidepressant-like effects mediated through delta-opioid receptors (Javelot et al. 2010 J Physiol Pharmacol), urogenital smooth-muscle effects (Tong, Chiosso, Rougeot, Burnett 2008 Eur J Pharmacol — the opiorphin gene PROL1 functions in erectile physiology), and effects on colonic motility and visceral nociception. Drug development around opiorphin has produced STR-324, a stabilized synthetic analog from Pharmaleads (later acquired by Helsinn) that has advanced through Phase 2 clinical trials for postoperative pain — among the more promising endogenous-opioid-pathway drug candidates of the past two decades, though it has not yet reached approval as of 2026.

What Opiorphin Is Investigated For

Opiorphin sits at one of the more pharmacologically interesting points in modern analgesic drug development. The mechanism — dual inhibition of neutral endopeptidase and aminopeptidase N to potentiate endogenous Met- and Leu-enkephalin signaling at mu/delta-opioid receptors — is a refined version of the older 'enkephalinase inhibitor' drug-development class, which had produced candidates like RB-101 and PL37/PL265 over earlier decades. What distinguishes opiorphin is that it is itself an endogenous human peptide (rather than a synthetic enkephalinase inhibitor), it produces morphine-comparable analgesia in standard rodent pain models, and the 2010 Rougeot et al. PNAS paper reported that systemically active opiorphin lacks physiological dependence, tolerance development, and abuse liability in laboratory mice — a side-effect profile that, if it translates to humans, would distinguish opiorphin-pathway drugs from classical mu-opioid agonists like morphine and oxycodone. Drug development has been led by Pharmaleads (a French biotech founded around the dual-enkephalinase-inhibitor concept by Bernard Roques and colleagues) and continued under Helsinn ownership. The most advanced candidate, STR-324 (a stabilized opiorphin analog), has progressed through Phase 1 and Phase 2 clinical trials for postoperative pain (Sitbon 2016 Anesthesiology; Roques 2017 Pain Medicine on neuropathic pain) — among the more promising endogenous-opioid-pathway candidates of the past two decades. Beyond analgesia, opiorphin has documented antidepressant-like activity through delta-opioid potentiation, urogenital smooth-muscle effects, and colonic motility modulation. The honest framing is that opiorphin is a pharmacologically distinctive endogenous peptide with an active translational program, an interesting non-addictive analgesic profile in preclinical models, and a clinical chapter that has not yet reached completion — STR-324 remains the principal candidate to watch for whether the opiorphin pharmacology translates to approved human therapeutics. As of 2026, no opiorphin or opiorphin-analog product has reached approval anywhere in the world.

History & Discovery

Opiorphin's discovery emerged from a multi-year research program at the Institut Pasteur in Paris led by Catherine Rougeot, who had spent the early 2000s characterizing endogenous regulators of pain and stress signaling. The first major finding was the 2003 PNAS paper from Rougeot, Messaoudi, Hermitte, Rigault, Caer, Le Coadic, Wisner-Lynch, and Caron describing sialorphin (Gln-His-Asn-Pro-Arg), a five-amino-acid rat peptide expressed in submandibular salivary glands and released under sympathetic control. Sialorphin was characterized as a natural inhibitor of rat membrane-bound neutral endopeptidase (NEP, neprilysin) with potent analgesic activity in standard pain assays — establishing the dual-ectopeptidase-inhibitor framework that would extend to opiorphin three years later. The human homolog identification came in 2006. Vinciane Wisner, Evelyne Dufour, Manel Messaoudi, Abderrazzak Nejdi, Annick Marcel, Marie-Noelle Ungeheuer, and Catherine Rougeot at the Institut Pasteur reported in PNAS the identification of opiorphin (Gln-Arg-Phe-Ser-Arg) in human saliva — the human peptide is sequence-distinct from rat sialorphin (QHNPR vs QRFSR) but operates through a similar dual-ectopeptidase-inhibitor mechanism, blocking both NEP and aminopeptidase N (APN, CD13). The 2006 paper named the peptide 'opiorphin' (combining 'opi' for opioid and 'orphan' or 'orphin' connoting the previously uncharacterized status of an endogenous human enkephalinase inhibitor) and reported morphine-comparable analgesic potency in rodent pain models. The discovery framed opiorphin as the human paradigm of an endogenous dual ectopeptidase inhibitor — a refined and naturally-occurring version of the older synthetic enkephalinase inhibitor drug-development class that included compounds like RB-101 and PL37/PL265 from Bernard Roques and colleagues' programs. The pivotal preclinical paper for the differentiated side-effect profile came in 2010. Rougeot, Robert, Menz, Bisson, and Messaoudi published in PNAS that systemically active human opiorphin produces analgesia without physiological dependence, without tolerance development, and without abuse liability in laboratory mice — an unusually clean side-effect profile for an opioid-pathway analgesic. The mechanistic explanation centers on the fact that opiorphin potentiates endogenous enkephalin signaling rather than directly activating mu-opioid receptors, preserving the spatial and temporal regulation of opioid signaling that is normally provided by enkephalin synthesis and release. The 2010 Javelot et al. J Physiol Pharmacol paper extended the pharmacology to delta-opioid-receptor-mediated antidepressant-like effects. The urogenital biology developed as a parallel story. Tong, Chiosso, Rougeot, and Burnett published in 2008 (Eur J Pharmacol) that the opiorphin gene (PROL1) and its homologues function in erectile physiology. Davies, Rougeot, and Burnett (2009 J Sex Med) extended the work to opiorphin family roles in urogenital smooth-muscle biology, with subsequent papers documenting opiorphin-induced experimental priapism through polyamine-pathway activation and (Calenda 2014 J Sex Med) opiorphin as a master regulator of the hypoxic response in corporal smooth muscle cells. The 2009 Rougeot and colleagues' colonic motility and nociception paper extended the GI biology. Drug development has been led by Pharmaleads, the French biotech founded around the dual-enkephalinase-inhibitor concept by Bernard Roques and colleagues, later acquired by Helsinn. The most advanced candidate, STR-324, is a stabilized synthetic analog of opiorphin that overcomes the parent peptide's very short plasma half-life (caused by rapid aminopeptidase-mediated N-terminal degradation). STR-324 has progressed through Phase 1 and Phase 2 clinical trials for postoperative pain (Sitbon, Van Elstraete, Hamdi, Yaiche, Mazoit, Naline, Rougeot, and Benhamou, Anesthesiology 2016) and neuropathic pain (Roques 2017 Pain Medicine). The compound represents one of the more promising endogenous-opioid-pathway analgesic candidates of the past two decades. As of 2026, STR-324 has not reached registrational Phase 3 completion or regulatory approval, but the program remains active. The 2023 systematic review on enhancing opiorphin's metabolic stability and the 2023 meta-analysis on opiorphin as a biomarker of orofacial conditions represent the contemporary state of the field — active translational interest combined with continued mechanistic and biomarker characterization.

How It Works

Opiorphin is a tiny five-amino-acid peptide that your salivary glands release into your saliva. Instead of activating opioid receptors directly the way morphine does, opiorphin works by blocking two enzymes that destroy your brain's natural opioids (the enkephalins). Think of it like blocking the trash collectors that haul away your endogenous painkillers — without those enzymes operating, your enkephalins hang around longer and keep activating mu and delta opioid receptors. The net effect is morphine-comparable pain relief in rodent studies, but without the addiction, tolerance, or withdrawal that come with morphine and other classical opioids. The reason is that opiorphin only amplifies the opioid signaling your body is already doing — it doesn't flood the system with supra-physiological levels of mu-opioid activation the way morphine or fentanyl do. A French company has developed a stabilized version of opiorphin called STR-324 that's currently being tested in clinical trials for postoperative pain, but no opiorphin-based drug has been approved yet.

Opiorphin is a five-amino-acid C-terminally non-amidated peptide with the sequence Gln-Arg-Phe-Ser-Arg (single-letter QRFSR), encoded as part of the prolactin-induced protein (PIP) precursor and processed by proteolytic cleavage. The peptide is concentrated in human salivary glands and is detectable in saliva at low-nanomolar concentrations. Tissue and plasma quantification have been characterized by Brkljacic and colleagues (2012 J Pharm Biomed Anal, validated LC-MS/MS quantification method) and subsequent bioanalytical studies. Plasma opiorphin is short-lived because of rapid aminopeptidase-mediated N-terminal degradation — an ironic vulnerability given that the peptide's mechanism is aminopeptidase inhibition. The mechanism is dual ectopeptidase inhibition. Opiorphin simultaneously inhibits two membrane-bound peptidases that participate in enkephalin degradation: neutral endopeptidase (NEP, also called neprilysin or membrane metalloendopeptidase, encoded by the MME gene), and aminopeptidase N (APN, also called CD13, encoded by the ANPEP gene). Both enzymes cleave Met- and Leu-enkephalin at distinct positions: NEP attacks the central peptide bond, APN attacks the N-terminal tyrosine. Inhibiting either enzyme alone produces partial preservation of enkephalin signaling; inhibiting both simultaneously (the dual-inhibitor strategy) produces substantially extended enkephalin half-life and sustained mu/delta-opioid receptor activation. Opiorphin's IC50 values for NEP and APN are in the low-micromolar range, with structure-activity-relationship studies (Bogeas, Halimi, Roques 2012 Bioorg Med Chem) defining the residues responsible for dual-inhibitor activity. The rat homolog sialorphin (Gln-His-Asn-Pro-Arg, identified by the Rougeot laboratory in 2003) operates through a similar but not identical dual-inhibitor mechanism — opiorphin and sialorphin are distinct peptides with related pharmacology rather than direct cross-species homologs. The analgesic phenotype follows from the mechanism. Opiorphin administration in rats produces naloxone-reversible analgesia in standard pain models (tail flick, paw pressure, formalin, neuropathic pain) of magnitude comparable to morphine — Wisner, Dufour, Messaoudi, Nejdi, Marcel, Ungeheuer, and Rougeot established this in the 2006 PNAS discovery paper. The 2010 Rougeot, Robert, Menz, Bisson, and Messaoudi PNAS paper extended the work to systemically active opiorphin and reported that the analgesia was achieved without physiological dependence, tolerance development, or abuse liability — the differentiated side-effect profile that has driven translational interest. The mechanistic explanation is that potentiating endogenous enkephalin signaling preserves the spatial and temporal regulation of opioid signaling that is normally provided by enkephalin synthesis and release, in contrast to direct mu-agonists which saturate opioid receptors at supraphysiological levels and engage reward and dependence circuits. Beyond analgesia, opiorphin has documented effects across multiple physiological systems. Antidepressant-like activity through delta-opioid receptor potentiation has been characterized by Javelot, Messaoudi, Garnier, and Rougeot (2010 J Physiol Pharmacol) in rodent depression models. Urogenital smooth-muscle effects have been characterized by Tong, Chiosso, Rougeot, and Burnett (2008 Eur J Pharmacol — opiorphin gene PROL1 functions in erectile physiology) and Davies, Rougeot, and Burnett (2009 J Sex Med — opiorphin family in urogenital smooth muscle biology), with subsequent work documenting opiorphin-induced experimental priapism through polyamine-pathway activation. Effects on colonic motility and visceral nociception have been characterized by Sitbon, Rougeot, and colleagues. The 2014 Calenda, Tong, Kanika, and Davies J Sex Med paper showed that opiorphin is a master regulator of the hypoxic response in corporal smooth muscle cells, expanding the urogenital biology. Drug development has produced STR-324, a stabilized synthetic opiorphin analog from Pharmaleads (the French biotech founded around the dual-enkephalinase-inhibitor concept by Bernard Roques, later acquired by Helsinn). STR-324 has progressed through Phase 1 and Phase 2 clinical trials for postoperative pain (Sitbon 2016 Anesthesiology — STR-324 causes analgesia in postoperative pain by activating endogenous opioid receptor-dependent pathways) and neuropathic pain (Roques 2017 — STR-324 decreases sensory hypersensitivity in rat neuropathic pain). The compound represents one of the more advanced endogenous-opioid-pathway analgesic candidates of the past two decades, but as of 2026 has not reached registrational Phase 3 completion or approval. Subsequent work (2023 review on enhancing opiorphin's metabolic stability; 2023 meta-analysis on opiorphin as a biomarker of orofacial conditions) has continued to characterize the peptide's pharmacology and translational potential.

Evidence Snapshot

Research Gaps & Open Questions

What the current literature has not yet settled about Opiorphin:

• 01Whether STR-324 will progress through registrational Phase 3 trials and reach approval for postoperative pain, neuropathic pain, or other indications — the most advanced opiorphin-pathway candidate has not yet completed late-stage development.
• 02Whether the preclinical non-addictive analgesic profile (Rougeot 2010 PNAS) translates to humans at registrational scale — the human safety database for opiorphin-pathway compounds remains limited.
• 03Whether dual NEP/APN inhibition produces clinically significant off-target effects through accumulation of other peptidase substrates (substance P, bradykinin, natriuretic peptides), particularly in cardiovascularly vulnerable populations.
• 04Whether opiorphin's antidepressant-like delta-opioid effects observed in rodent models translate to clinical antidepressant efficacy in humans — the delta-opioid antidepressant hypothesis has been pursued for decades without an approved drug.
• 05The clinical relevance of salivary opiorphin as a biomarker for orofacial pain, stress, and other conditions — observational associations are accumulating but the diagnostic threshold and clinical actionability are not established.
• 06The role of opiorphin in human urogenital physiology, particularly in erectile dysfunction — Tong/Burnett/Davies and colleagues have characterized rodent pharmacology, but human translation remains preliminary.
• 07Whether the dual-enkephalinase-inhibitor approach (opiorphin-derived or PL37/PL265-class synthetic compounds) will ultimately produce a non-addictive analgesic class that displaces some clinical mu-agonist use — a longstanding goal of opioid-sparing pain medicine.

Forms & Administration

Native opiorphin is not formulated or approved as a clinical therapeutic. Research applications use synthetic opiorphin (Gln-Arg-Phe-Ser-Arg) for in vitro NEP/APN inhibition assays, ex vivo enkephalin-potentiation studies, and parenteral administration in animal pain models. The clinically relevant candidate is STR-324, the stabilized opiorphin analog from Pharmaleads/Helsinn, administered intravenously in postoperative pain trials and characterized in early-phase clinical pharmacology. Compounded opiorphin in peptide-marketplace channels has no validated clinical use.

Common Questions

Who Opiorphin Is NOT For

Drug & Supplement Interactions

There is no validated human drug-interaction profile for opiorphin because no opiorphin product has been clinically approved. Theoretical interactions follow from documented signaling. Mu- and delta-opioid agonists (morphine, oxycodone, fentanyl, tapentadol) would have additive analgesic and respiratory-depressant effects with opiorphin's enkephalin-potentiation mechanism. Opioid antagonists (naloxone, naltrexone, methylnaltrexone) would block the downstream effect. Central nervous system depressants (benzodiazepines, gabapentinoids, alcohol) could amplify opioid-pathway respiratory depression and sedation. Neprilysin-pathway-active drugs are the most distinctive interaction concern: sacubitril (in sacubitril/valsartan / Entresto for heart failure) inhibits the same NEP that opiorphin targets, with theoretical additive effects on natriuretic peptide accumulation, bradykinin accumulation (relevant to angioedema risk), and substance P signaling. Aminopeptidase N-targeted drugs (some experimental cancer agents) could similarly produce additive enzyme inhibition. None of these interactions has been characterized in controlled human studies for opiorphin specifically; they are mechanistic predictions from the dual-NEP/APN-inhibitor pharmacology.

Safety Profile

Legal Status

Regulatory status changes over time. Verify current local rules with a qualified professional.

Myths & Misconceptions