Met-Enkephalin
Met-enkephalin (YGGFM) is an endogenous opioid pentapeptide with dual roles as a delta/mu-opioid receptor agonist and opioid growth factor (OGF), with research applications in pain modulation, cancer immunotherapy, and immune regulation.
Overview
Met-enkephalin occupies a unique position in peptide biology as both a classical neurotransmitter and a growth-regulatory factor. As a neurotransmitter, it modulates pain perception, mood, and stress responses through delta and mu-opioid receptors in the central and peripheral nervous system. As the opioid growth factor (OGF), it regulates cell proliferation through the OGF receptor (OGFr), a nuclear-associated protein that mediates growth inhibition by upregulating cyclin-dependent kinase inhibitors (p16, p21). This OGF-OGFr axis is tonically active in normal tissues and disrupted in many cancers, making met-enkephalin a subject of cancer immunotherapy research. The connection between met-enkephalin and low-dose naltrexone (LDN) therapy -- where brief opioid receptor blockade upregulates endogenous OGF production -- has generated significant clinical interest.
Mechanism of Action
Met-enkephalin operates through two distinct receptor systems with fundamentally different biological outcomes.
Classical opioid signaling: Met-enkephalin binds delta-opioid receptors (DOR) and mu-opioid receptors (MOR) with Ki values of approximately 1-5 nmol/L and 5-20 nmol/L respectively. Receptor activation couples to Gi/Go proteins, inhibiting adenylyl cyclase, opening GIRK potassium channels, and closing voltage-gated calcium channels. In pain circuits, this reduces neurotransmitter release from nociceptive afferents. In reward circuits, it modulates dopamine release in the nucleus accumbens.
OGF-OGFr growth regulation: Met-enkephalin binds the OGF receptor (OGFr), a ~62 kDa nuclear-associated protein distinct from classical opioid receptors. OGFr is not a G protein-coupled receptor but a nuclear protein that, upon OGF binding, translocates to the nucleus and upregulates cyclin-dependent kinase inhibitors p16^INK4a and p21^WAF1/CIP1. These CDK inhibitors block cyclin D/CDK4 and cyclin E/CDK2 complexes, arresting cells at the G1/S checkpoint (Zagon et al., 2002). This mechanism operates constitutively in normal tissues to maintain growth homeostasis.
TLR4 agonist activity: Recent research has identified met-enkephalin as a Toll-like receptor 4 (TLR4) agonist, activating innate immune responses independently of opioid receptors. TLR4 activation by met-enkephalin enhances macrophage and dendritic cell function, promotes antigen presentation, and stimulates pro-inflammatory cytokine production. This immunostimulatory effect may contribute to the anti-tumor immune responses observed with OGF therapy.
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Research
Immune Regulation
Met-enkephalin enhances natural killer (NK) cell cytotoxicity, stimulates T cell proliferation, and increases macrophage phagocytic activity at low concentrations (10^-12 to 10^-10 mol/L). These immunostimulatory effects are mediated through both classical opioid receptors on immune cells and TLR4 activation. In cancer immunotherapy contexts, OGF administration combines direct anti-proliferative effects (OGFr-mediated) with immune activation (TLR4-mediated), providing a dual anti-cancer mechanism.
Cancer: The OGF-OGFr Axis
The pioneering work of Ian Zagon and Patricia McLaughlin at Penn State University established met-enkephalin as a tonically active growth regulator in cancer. Their research demonstrated that OGFr is expressed in nearly all cancer types studied, and that the OGF-OGFr axis is frequently downregulated in malignancy. Exogenous OGF (met-enkephalin) administration restores growth inhibition in pancreatic cancer, colorectal cancer, head and neck squamous cell carcinoma, ovarian cancer, and melanoma cell lines and xenograft models (Zagon et al., 2008). Critically, OGF does not kill cancer cells through cytotoxicity but arrests proliferation through p16/p21-mediated cell cycle blockade, a cytostatic mechanism that avoids the DNA damage and immunosuppression caused by conventional chemotherapy.
Low-Dose Naltrexone Connection
Low-dose naltrexone (LDN, typically 1.5-4.5 mg nightly) briefly blocks opioid receptors for 4-6 hours, triggering a compensatory upregulation of endogenous OGF (met-enkephalin) production and OGFr expression. When naltrexone clears, the elevated OGF-OGFr system produces enhanced growth inhibition. This paradoxical mechanism -- temporary blockade leading to sustained activation -- explains LDN's therapeutic effects in conditions where OGF-OGFr signaling is beneficial, including cancer, multiple sclerosis, Crohn's disease, and fibromyalgia (Younger et al., 2014).
Pancreatic Cancer
Pancreatic cancer has been the most extensively studied cancer type in OGF research. Zagon et al. (2008) demonstrated that continuous OGF administration inhibits pancreatic cancer growth in xenograft models by 40-60% without toxicity. Phase I/II clinical trials (NCT01035866) administered OGF at 250 microg/kg IV weekly to patients with advanced pancreatic cancer, demonstrating safety and preliminary efficacy signals including stable disease and improved survival compared to historical controls.
Multiple Sclerosis
The OGF-OGFr axis regulates oligodendrocyte proliferation and differentiation. McLaughlin & Bhatt (2014) showed that OGF modulates experimental autoimmune encephalomyelitis (EAE), an animal model of multiple sclerosis, reducing disease severity when administered during the remission phase. LDN clinical trials in multiple sclerosis have shown improvements in quality of life metrics, though large randomized controlled trials remain limited.
OGF-OGFr axis and cell proliferation
Opioid growth factor (chemically [Met5]-enkephalin) and its nuclear-associated receptor OGFr form a tonic inhibitory axis that delays the G1/S transition of the cell cycle in homeostatic and neoplastic tissue, providing the biological basis for its growth-inhibitory effects on epithelial cancers.
- OGF inhibits growth of human pancreatic cancer cells in culture and xenografts
- Receptor internalization is clathrin-mediated and required for downregulation of proliferation
Immunotherapy and regulatory T-cell modulation
As MENK, the peptide binds opioid receptors on immune cells to enhance cytotoxic T-lymphocyte and dendritic-cell activity while reducing CD4+Foxp3+ regulatory T cells, remodeling the tumor immune microenvironment.
- Increases CD8+ T-cell activation and IFN-gamma secretion in tumor-bearing mice
- Reduces Treg proportions and improves lymphocyte subpopulations in cancer-patient blood samples
Safety Profile
Met-enkephalin has an excellent safety profile in clinical studies. As an endogenous peptide operating within established physiological systems, it produces minimal adverse effects at therapeutic doses. Phase I clinical trials of OGF (250 microg/kg IV) reported no dose-limiting toxicities, with the most common side effects being mild injection site reactions and transient lightheadedness. Unlike mu-opioid agonists, met-enkephalin at growth-regulatory doses does not produce respiratory depression, constipation, or euphoria. The cytostatic (not cytotoxic) mechanism of OGFr-mediated growth inhibition avoids the myelosuppression, nausea, and immunosuppression associated with conventional chemotherapy. Long-term safety of chronic OGF administration has been demonstrated in preclinical studies over 6-12 months without cumulative toxicity.
Pharmacokinetic Profile
Met-Enkephalin — Pharmacokinetic Curve
Molecular Structure
- Formula
- C27H35N5O7S
- Weight
- 573.7 Da
- CAS
- 58569-55-4
- PubChem CID
- 443362
- Exact Mass
- 350.1630 Da
- LogP
- 2.4
- TPSA
- 54.6 Ų
- H-Bond Donors
- 1
- H-Bond Acceptors
- 4
- Rotatable Bonds
- 2
- Complexity
- 658
Identifiers (SMILES, InChI)
InChI=1S/C21H22N2O3/c1-12-16-10-23-8-7-14-13-5-3-4-6-18(13)22-20(14)19(23)9-15(16)17(11-26-12)21(24)25-2/h3-6,10-12,15,19,22H,7-9H2,1-2H3/t12-,15+,19+/m1/s1
BXTHVTLKWJZGAA-YFROIQMUSA-NResearch Indications
Primary Research Areas
OGF ([Met5]-enkephalin) has been tested as a biotherapy in chemotherapy-refractory advanced pancreatic cancer, with phase I and open-label phase II data suggesting tolerability and possible survival benefit; still investigational.
MENK acts on opioid receptors of immune cells to activate cytotoxic T cells and dendritic cells while inhibiting regulatory T cells, an immune-based antitumor mechanism under preclinical and early clinical study.
Research Protocols
subcutaneous Injection
Subcutaneous administration produces peak levels at 15-30 minutes with approximately 50-70% bioavailability relative to IV dosing.
| Goal | Dose | Frequency | Duration |
|---|---|---|---|
| 250 microg/kg OGF IV weekly | See literature | Once weekly | 8 weeks |
| 10^-12 to 10^-10 mol/L | 1.5-4.5 mg | Per protocol | — |
intravenous Injection
Following intravenous administration at 250 microg/kg, peak plasma concentrations of 50-100 ng/mL are achieved immediately.
| Goal | Dose | Frequency | Duration |
|---|---|---|---|
| General Research Protocol | 1.5-4.5 mg | Per protocol | — |
oral
Low-Dose Naltrexone (Indirect OGF Enhancement) Clinical LDN protocols use 1.5-4.5 mg naltrexone orally at bedtime. Absorption Met-enkephalin is not orally bioavailable.
| Goal | Dose | Frequency | Duration |
|---|---|---|---|
| 10 mg/kg IP daily | 10 mg, 1.5-4.5 mg | Daily | 8 weeks(Route: Oral) |
Interactions
Peptide Interactions
The most pharmacologically validated combination. Brief naltrexone blockade upregulates OGF production, then exogenous OGF supplementation during the unblocked period provides additional growth inhibition. This approach is theoretically applicable to any condition where OGF-OGFr signaling is bene...
Thymosin alpha-1 enhances T cell and NK cell function through mechanisms distinct from met-enkephalin's opioid receptor and TLR4 pathways. Combined immunostimulation could provide broader anti-tumor immune activation. Both peptides have favorable safety profiles.
Both enkephalins are co-released from proenkephalin-expressing neurons in an approximate 4:1 met:leu ratio. Met-enkephalin's broader receptor profile (DOR + MOR + OGFr + TLR4) and leu-enkephalin's higher DOR selectivity provide complementary signaling.
What to Expect
What to Expect
Rapid onset expected; half-life of ~2 minutes (plasma) indicates fast-acting pharmacokinetics
When naltrexone clears, the elevated met-enkephalin provides enhanced growth-regulatory and immunomodulatory signaling for the remaining 18-20 hours.
Protocol: 250 microg/kg OGF IV weekly for up to 8 weeks, with dose escalation in the Phase I component from 50-250 microg/kg.
Long-term safety of chronic OGF administration has been demonstrated in preclinical studies over 6-12 months without cumulative toxicity.
Continued use as directed
Quality Indicators
What to look for
- Human clinical trials conducted
- Well-established safety profile
- Multiple peer-reviewed studies available
- Oral administration available
Caution
- Injection site reactions reported
Frequently Asked Questions
References (16)
- [4]
- [8]
- [9]Liu et al -- Met-enkephalin as a TLR4 agonist: implications for innate immunity and cancer immunotherapy Front Immunol (2023)
- [10]Toljan & Vrooman -- Low-dose naltrexone: mechanism of action and therapeutic potential Int Immunopharmacol (2022)
- [7]
- [1]Hughes et al *Nature* Nature (1975)
- [2]Zagon et al *Brain Res Rev* Brain Res Rev (2002)
- [3]Zagon et al *Int J Oncol* Int J Oncol (2008)
- [5]McLaughlin & Bhatt *Exp Biol Med* Exp Biol Med (2014)
- [6]Younger et al *Pain Med* Pain Med (2014)
- [11]Smith JP, Conter RL, Bingaman SI, et al. Treatment of advanced pancreatic cancer with opioid growth factor: phase I Anti-Cancer Drugs (2004)
→ Phase I dose-escalation established a maximum tolerated dose of 250 mcg/kg IV over 30 minutes (hypotension dose-limiting); mean survival exceeded 8.5 months and two patients showed resolution of liver metastases.
- [12]Smith JP, Bingaman SI, Mauger DT, Harvey HH, Demers LM, Zagon IS. Opioid growth factor improves clinical benefit and survival in patients with advanced pancreatic cancer Open Access Journal of Clinical Trials (2010)
→ Prospective open-label phase II trial of weekly OGF 250 mcg/kg IV in 24 chemotherapy-refractory advanced pancreatic cancer patients reported improved clinical benefit response and survival relative to historical controls.
- [13]Zhao D, Plotnikoff N, Griffin N, Song T, Shan F. Methionine enkephalin, its role in immunoregulation and cancer therapy International Immunopharmacology (2016)
→ Review summarizing MENK as an immunoregulatory peptide that activates immune cells and inhibits regulatory T cells (Tregs) by binding opioid receptors on immune and tumor cells, supporting its evaluation as a cancer biotherapy.
- [14]Li W, Chen W, Herberman RB, et al. Immunotherapy of cancer via mediation of cytotoxic T lymphocytes by methionine enkephalin (MENK) Cancer Letters (2014)
→ MENK up-regulated CD8+ T cells, induced activation markers, increased cytotoxic activity against S180 tumor cells and boosted IFN-gamma secretion, describing a CTL-mediated antitumor mechanism.
- [15]Li X, Meng Y, Plotnikoff NP, et al. Methionine enkephalin (MENK) inhibits tumor growth through regulating CD4+Foxp3+ regulatory T cells (Tregs) in mice Cancer Biology & Therapy (2015)
→ MENK suppressed tumor growth in mice at least in part by reducing the proportion and suppressive function of CD4+Foxp3+ regulatory T cells.
- [16]Wang Q, Gao X, Yuan Z, et al. Methionine enkephalin (MENK) improves lymphocyte subpopulations in human peripheral blood of 50 cancer patients by inhibiting regulatory T cells (Tregs) Human Vaccines & Immunotherapeutics (2014)
→ In peripheral blood from 50 cancer patients, MENK ex vivo improved lymphocyte subpopulations and reduced Treg proportions, supporting an immunorestorative effect in humans.
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