Research Chemical Designation & Liability
Compounds indexed in the MinMaxMuscle archive are strictly for laboratory research and development. They are NOT approved for human consumption. Theoretical dosing data is aggregated from clinical literature and must not be construed as medical instruction. Professional medical consultation is mandatory prior to any research application.
Met-Enkephalin
Immune & Longevity
Molecular Dossier
- Rank Index: 82
- Status: Research Only
- What does this mean?
- Molecular Structure: MW: 573.65 Da | Formula: C₂₇H₃₅N₅O₇S | CAS: 58569-55-4 | Sequence: Tyr-Gly-Gly-Phe-Met
About Met-Enkephalin
Met-enkephalin (methionine enkephalin; YGGFM) is an endogenous pentapeptide opioid discovered in 1975 by Hughes and Kosterlitz as one of the two original enkephalins (the other being Leu-enkephalin). It is derived from the proenkephalin precursor and acts primarily at delta-opioid receptors (DOR) and, with lower affinity, at mu-opioid receptors. Classically studied for antinociceptive (pain-modulating) effects in the central nervous system, interest shifted when researchers discovered that met-enkephalin serves as a key endogenous immunomodulator, directly influencing natural killer (NK) cell function, T-cell proliferation, and macrophage activity. The Opioid Growth Factor (OGF)-OGF receptor (OGFr) axis — where met-enkephalin acts as the endogenous OGF ligand — regulates cell proliferation in both normal tissue and cancer cells. Low Dose Naltrexone (LDN) exerts many of its proposed effects by transiently blocking OGFr, causing rebound upregulation of endogenous met-enkephalin and subsequent immunological and anti-proliferative effects. This makes met-enkephalin central to understanding the proposed mechanisms of LDN in autoimmune disease, cancer, and neurological conditions. Exogenous met-enkephalin administration has been studied in HIV, cancer immunotherapy, and inflammatory conditions. However, it has an extremely short circulating half-life (~2 minutes) due to rapid enzymatic degradation by enkephalinases and peptidases. Stable analogues and administration strategies (intrathecal, intranasal) are subjects of ongoing research. The peptide is not commercially available for human use in most jurisdictions.
Clinical Focus
Endogenous Opioid — Immune Modulation, OGF Axis, Antinociception
Archival Aliases
Methionine enkephalin, MENK, OGF (Opioid Growth Factor), YGGFM
Frequently Asked Questions
What is the relationship between met-enkephalin and Low Dose Naltrexone (LDN)?
LDN (1.5–4.5 mg naltrexone at bedtime) is proposed to work by transiently blocking opioid receptors and the OGF receptor (OGFr) for 4–6 hours, causing a compensatory upregulation of endogenous met-enkephalin production and OGFr sensitivity when the blockade lifts. This rebound increase in endogenous OGF-OGFr signaling is hypothesized to suppress aberrant cell proliferation and modulate immune function. The evidence for this mechanism comes from laboratory work on OGF/OGFr, combined with clinical signals from small LDN trials in Crohn's disease, MS, fibromyalgia, and cancer. It is important to note that the LDN mechanism is not fully established and remains contested.
Why does met-enkephalin have such a short half-life?
Met-enkephalin's plasma half-life of approximately 2 minutes is due to rapid enzymatic degradation by two principal enzymes: neprilysin (neutral endopeptidase, NEP) cleaves the Gly-Phe bond, and aminopeptidase N cleaves the Tyr-Gly bond at the N-terminus. Both enzymes are abundant in plasma, the synaptic cleft, and kidney brush-border membranes. This extreme brevity reflects the peptide's role as a local paracrine/autocrine signaling molecule — it is not designed to circulate. Enkephalinase inhibitors (like thiorphan) can dramatically extend met-enkephalin's in vivo activity and are studied as an alternative approach to opioid-system modulation without the addiction liability of exogenous opioids.
How does met-enkephalin modulate immune function?
Met-enkephalin acts as an endogenous immunomodulator via two distinct mechanisms. Through delta-opioid receptors on lymphocytes, NK cells, and macrophages, it directly enhances NK cell cytotoxicity, promotes T-cell proliferation in response to IL-2, and stimulates macrophage phagocytic activity. Through the OGF-OGFr axis, it acts as a cell proliferation brake — not just in immune cells but in cancer cells, which often overexpress OGFr as part of their growth signaling. The net immune effect is enhanced cytotoxic surveillance (cancer killing) combined with suppression of unchecked immune proliferation, making met-enkephalin of interest in both autoimmune and oncological contexts.
What cancer applications have been studied for met-enkephalin?
Met-enkephalin has been investigated in cancer research via two routes: direct administration and indirect augmentation via LDN. Direct exogenous met-enkephalin was studied in HIV patients and some cancer trials in the 1990s–2000s, showing immune enhancement and some anti-tumor signals in small studies. The OGF-OGFr proliferation-brake mechanism has been studied in pancreatic cancer, colon cancer, and skin cancers — with animal model data showing tumor growth inhibition. LDN trials in cancer, including pancreatic and ovarian cancer, show some early positive signals but remain preliminary. The primary challenge for direct met-enkephalin therapy is its 2-minute half-life, necessitating continuous infusion or stable analogue development for any clinical application.
Clinical Data Matrix
| Parameter | Clinical Value |
|---|---|
| Molecular Weight | MW: 573.65 Da | Formula: C₂₇H₃₅N₅O₇S | CAS: 58569-55-4 | Sequence: Tyr-Gly-Gly-Phe-Met |
| Primary Pathway | Immune & Longevity |
| Research Phase | Research Only |
Clinical Pros
- Highly selective receptor modulation
- Documented efficacy in Phase II trials
- Minimal systemic cross-reactivity
Research Limitations
- Limited long-term human data
- Strict storage and reconstitution requirements
- Potential for acute homeostatic feedback loops
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Data Verified: 2026-05-15