Leu-Enkephalin

An endogenous opioid pentapeptide (Tyr-Gly-Gly-Phe-Leu) isolated from brain in 1975 that preferentially activates delta-opioid receptors, is degraded within minutes by peptidases, and circulates today as a laboratory reference reagent rather than a therapeutic.

Also referenced as: Leu5-Enkephalin, [Leu5]-Enkephalin, Leucine Enkephalin, Leu-Enkephalin Acetate, Tyr-Gly-Gly-Phe-Leu, YGGFL

Also appears in: Other

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What is Leu-Enkephalin?

Leu-enkephalin is a linear pentapeptide with the sequence Tyr-Gly-Gly-Phe-Leu (YGGFL), a free acid C-terminus, a molecular weight near 555.6 Da, and CAS number 58822-25-6. It was isolated from pig brain alongside its one-residue variant Met-enkephalin (YGGFM) by Hughes, Kosterlitz, and colleagues, who reported both sequences in December 1975 as the first endogenous substances in the brain with potent opiate agonist activity (Hughes et al., Nature, 1975, 258(5536):577–580). That paper is the origin point of the entire endogenous opioid field: it established that the nervous system makes its own morphine-like ligands. It did not itself define a receptor class — the multiple-receptor framework, including the delta receptor described below, was built on these ligands over the following two years, and it is still the map opioid pharmacology uses.

Its presence in the research-compound market is almost entirely reagent-driven rather than therapeutic. Leu-enkephalin is the canonical endogenous delta-opioid ligand and therefore a standard positive control in receptor-binding, cAMP, and beta-arrestin assays, and it is also a routine analytical standard — the singly protonated ion at m/z 556.2771 (and the deprotonated ion at m/z 554.2615) is the conventional lock-mass calibrant for quadrupole time-of-flight mass spectrometry, a high-volume use with no pharmacological content at all. Consumer-facing interest, where it exists, borrows the “natural endorphin” narrative rather than any clinical record.

How it works

• Delta-preferring opioid agonist. Enkephalin’s relative potency in the mouse vas deferens versus the guinea-pig ileum was the observation that defined the delta receptor as distinct from mu (Lord et al., Nature, 1977, 267(5611):495–499). The receptor was later cloned independently by two groups (Evans et al., Science, 1992, 258(5090):1952–1955; Kieffer et al., Proceedings of the National Academy of Sciences, 1992, 89(24):12048–12052), and the cloned receptors were profiled against a panel of opioid ligands including the enkephalins (Raynor et al., Molecular Pharmacology, 1994, 45(2):330–334). Leu-enkephalin also binds mu receptors with lower affinity, so it is delta-preferring, not delta-selective — a distinction that matters when interpreting any effect attributed to it (Cassell et al., Molecules, 2019, 24(24):4542).
• Gi/o-coupled inhibitory signaling. Delta-receptor activation inhibits adenylyl cyclase and lowers cAMP, opens inwardly rectifying potassium channels, and closes voltage-gated calcium channels, reducing neurotransmitter release and dampening nociceptive transmission at spinal and supraspinal sites (García-Domínguez, Biomolecules, 2024, 14(8):926). Leu-enkephalin analogs are routinely characterized on both cAMP inhibition and beta-arrestin-2 recruitment, because the two arms can be dissociated by single-position substitutions (Cassell et al., Molecules, 2019, 24(24):4542).
• Two separate biosynthetic routes. Human proenkephalin is a 267-amino-acid precursor carrying six interspersed Met-enkephalin sequences — four released as the free pentapeptide, two as the C-terminally extended peptides Met-enkephalin-Arg-Gly-Leu and Met-enkephalin-Arg-Phe — against a single Leu-enkephalin sequence (Comb et al., Nature, 1982, 295(5851):663–666). Leu-enkephalin is additionally generated from prodynorphin products, which carry YGGFL at their N-terminus — a dynorphin-derived pathway of Leu-enkephalin production was demonstrated in rat substantia nigra (Zamir et al., Nature, 1984, 307(5952):643–645). Met- and Leu-enkephalin are therefore not simply two outputs of one gene.
• Signal termination by peptidases is the defining pharmacological fact. Neprilysin (neutral endopeptidase 24.11, the original “enkephalinase”) cleaves the Gly3–Phe4 bond, and aminopeptidase N removes the N-terminal tyrosine that is required for receptor binding; angiotensin-converting enzyme contributes as well (Roques et al., Pharmacological Reviews, 1993, 45(1):87–146). The enkephalin-degrading activity in brain was characterized shortly after the peptides themselves (Malfroy et al., Nature, 1978, 276(5687):523–526). Measured in vitro in pooled human plasma, Leu-enkephalin degradation proceeded with a half-life of roughly 11.8 minutes under aminopeptidase-dominated conditions and was prolonged to about 31 minutes by the aminopeptidase inhibitor bacitracin (Mosnaim et al., Pharmacology, 2003, 67(1):6–13). In vivo the peptide is cleared faster still, and being hydrophilic it does not passively cross the blood-brain barrier.

Research status

There are no human trials of Leu-enkephalin itself. A ClinicalTrials.gov search on interventions containing “enkephalin” returns studies of Met-enkephalin (opioid growth factor), the enkephalin gene-therapy vector NP2, and the Russian analog dalargin — but no registered study in which leucine enkephalin is the administered agent. Everything below is adjacent evidence, and it is presented as adjacent evidence rather than as support for the peptide.

The closest human data come from a stabilized analog, and they are old and small. Onofrio and Yaksh reported that an intrathecally delivered delta-receptor ligand — DADLE, [D-Ala2, D-Leu5]-enkephalin — produced analgesia in patients (The Lancet, 1983, 1(8338):1386–1387). DADLE is a synthetic, peptidase-resistant molecule delivered directly into the intrathecal space; the result speaks to delta-receptor pharmacology, not to Leu-enkephalin as an administrable compound.

The two most rigorous attempts to exploit this receptor clinically failed.

• Pain. A Phase 2 randomized trial of the delta agonists ADL5859 and ADL5747 in knee osteoarthritis enrolled 408 participants and completed in June 2010 (NCT00979953). The posted results show least-squares mean change from baseline in the average pain numeric rating scale at week 2 of −2.20 for placebo, −2.24 for controlled-release oxycodone, −1.77 for ADL5859, and −1.57 for ADL5747 — both delta agonists performed numerically worse than placebo, and the active comparator was indistinguishable from it. The program did not advance to Phase 3.
• Depression. A randomized, placebo-controlled pilot of the delta agonist AZD2327 in anxious major depressive disorder randomized 22 subjects to 3 mg twice daily or placebo for four weeks and found no significant drug effect on either the Hamilton Depression Rating Scale (p = 0.39) or the Hamilton Anxiety Rating Scale (p = 0.15), though the anxiety measure carried the larger effect size (Richards et al., Psychopharmacology, 2016, 233(6):1119–1130).

Raising endogenous enkephalin locally reached Phase 2 and stopped. NP2, a replication-defective herpes simplex vector expressing human preproenkephalin, was injected intradermally in ten subjects with intractable cancer pain in a dose-escalation Phase 1 study; it was well tolerated with no agent-related serious adverse events, and the middle- and high-dose cohorts reported pain reduction while the low-dose cohort did not (Fink et al., Annals of Neurology, 2011, 70(2):207–212). The follow-on Phase 2 (NCT01291901, 33 participants, sponsored by Diamyd Inc) completed in November 2013. No results were posted to ClinicalTrials.gov and no peer-reviewed report of that Phase 2 has appeared; the program was not carried forward.

The one commercially successful path went through the enzyme, not the peptide. Racecadotril, an orally administered prodrug of the enkephalinase inhibitor thiorphan, is authorized in several European countries for acute diarrhea — it works by prolonging the action of endogenous enkephalins rather than by supplying them. It has never been approved in the United States. An endogenous enkephalinase inhibitor, opiorphin, was later identified in human saliva and shown to produce opioid-dependent antinociception in rats (Wisner et al., Proceedings of the National Academy of Sciences, 2006, 103(47):17979–17984).

The neuroprotection and ischemia-tolerance literature is largely about the receptor and about analogs, not about Leu-enkephalin. DADLE induced a hibernation-like state and reduced infarct volume in experimental stroke (Borlongan et al., BMC Biology, 2009, 7:31); ischemic preconditioning in the intact rat heart was shown to be delta1-mediated rather than mu- or kappa-mediated (Schultz et al., Circulation, 1998, 97(13):1282–1289); and delta-receptor signaling in hypoxic and ischemic stress has been reviewed at length (Chao & Xia, Progress in Neurobiology, 2010, 90(4):439–470). Leu-enkephalin itself is used as a tool compound in cardiac ischemia work, and its metabolic instability is explicitly the limitation motivating analog design (Cassell et al., Molecules, 2019, 24(24):4542). Rodent work on non-opioid Leu-enkephalin analogs administered neonatally after antenatal hypoxia exists but is confined to a small number of reports from a single group (for example, Simankova et al., Bulletin of Experimental Biology and Medicine, 2017, 163(5):594–598).

Regulatory position. There is no FDA-approved Leu-enkephalin product for any indication, and a DailyMed search for “enkephalin” returns no labeled drug products at all.

Common dosage forms

• Lyophilized research powder — supplied as the acetate or trifluoroacetate salt in milligram quantities (commonly 1, 5, 10, or 25 mg), white to off-white, stored frozen and protected from light, for reconstitution in water, ethanol, or DMSO for in vitro receptor and cell assays.
• Analytical reference standard — high-purity material supplied with a certificate of analysis for method development, method validation, and quality control, and as the lock-mass calibrant solution for accurate-mass LC-MS, typically diluted to nanogram-per-microliter concentrations in acetonitrile/water.
• Labeled variants — tritiated, stable-isotope-labeled, and fluorophore-conjugated versions used as radioligands and tracers in binding and metabolism studies.
• Not available in any administrable format. There is no sterile injectable Leu-enkephalin, no compounded pharmacy preparation, no oral or topical product, and no established human route of administration. The human analgesia literature discussed above used a different, peptidase-resistant molecule delivered intrathecally in a hospital setting.

Key considerations

• No approval, anywhere in the record checked. Leu-enkephalin has no FDA-approved product and no DailyMed label. It exists commercially as a laboratory reagent and analytical standard, and material sold for research use is not manufactured, tested, or labeled to the standards that apply to a drug product.
• The pharmacokinetics are the reason it is not a drug, and they are not a solvable formulation problem. Cleavage by neprilysin at Gly3–Phe4 and by aminopeptidase N at the essential N-terminal tyrosine destroys the molecule within minutes, and its hydrophilicity keeps it out of the central nervous system after systemic dosing. Effects reported after direct central administration in animals cannot be assumed to transfer to any peripheral route. This constraint is the entire reason the field pivoted to stabilized analogs and to enkephalinase inhibitors.
• The evidence gap is total at the human level for this specific peptide. Every piece of clinical evidence in the delta-opioid space involves something else: a D-amino-acid analog, a non-peptide agonist, a gene-therapy vector, or an enzyme inhibitor. Two of the best-conducted of those programs — ADL5859/ADL5747 in osteoarthritis and AZD2327 in anxious depression — returned negative results.
• Class safety signals should not be dismissed as irrelevant. Delta-receptor agonists carry a documented convulsant liability: the reference agonist SNC80 produces convulsions in rodents, and the effect has been localized to delta receptors on parvalbumin-expressing interneurons (Walicki et al., Behavioural Pharmacology, 2026, 37(4):139–149). Leu-enkephalin’s residual mu-receptor activity also means opioid-typical effects cannot be assumed absent. No controlled human safety data exist for this peptide, so there is no safety profile to cite — which is different from a clean one.
• Naming confusions are frequent and consequential. Met-enkephalin (YGGFM) differs at a single residue and is a distinct compound; it is also the peptide marketed in the oncology literature as opioid growth factor (OGF), which has its own Phase 1 dose-escalation data in unresectable hepatocellular cancer (Kimchi et al., Cancer Investigation, 2025, 43(3):224–235) — none of which applies to Leu-enkephalin. Dalargin (Tyr-D-Ala-Gly-Phe-Leu-Arg) is a stabilized hexapeptide built from the Leu-enkephalin scaffold and registered as a drug in Russia, with a peripherally restricted profile that deliberately differs from the parent peptide. DADLE and DPDPE are synthetic delta-selective analogs that carry most of the animal literature. And the bare word “enkephalin” is ambiguous, referring variously to either pentapeptide or to extended proenkephalin-derived peptides such as Met-enkephalin-Arg-Phe. Identity should be confirmed by sequence rather than by label.