Endogenous opioid peptides are core molecules in the body's pain regulation and neural signal transduction. Met-Enkephalin-Arg-Phe (CAS 73024-95-0), a natural heptapeptide derived from proenkephalin A, exhibits 8 times greater analgesic activity and stronger receptor affinity compared to classic methionine enkephalins due to its C-terminal two-amino acid extension structure. It is a key component of the central and peripheral pain regulation network. The raw material is a white lyophilized powder with good water solubility and a purity ≥97%. It combines low addictiveness with the advantages of multi-target neuromodulation, making it suitable for neuroscience research, analgesic drug development, and receptor mechanism exploration.
Molecular Archives of Heptapeptide Extensions
Met-Enkephalin-Arg-Phe, chemically known as Tyr-Gly-Gly-Phe-Met-Arg-Phe, has the molecular formula C₄₂H₅₆N₁₀O₉S, a molecular weight of 877.02, and CAS number 73024-95-0. Belonging to the endogenous opioid peptide family, it is a C-terminal extension derivative of methionine enkephalin, with a core structure of a pentapeptide backbone superimposed with the Arg⁶-Phe⁷ two-amino acid sequence. The raw material is a white or off-white lyophilized powder with a purity ≥97%. It exhibits good water solubility, dissolving directly in water or buffer solutions. It is stable when stored at room temperature away from light. The lyophilized formulation has a shelf life of over 24 months. Quality can be controlled across multiple batches, making it suitable for high-purity applications in research and in vitro experiments.
The molecule consists of an N-terminal active core region and a C-terminal regulatory region. The N-terminal pentapeptide sequence is identical to that of Met-Enkephalin, a key structure for opioid receptor binding. The C-terminus is additionally linked with arginine and phenylalanine, forming a unique heptapeptide extension conformation. This structural modification is the core reason for its high receptor affinity and strong analgesic activity. The phenolic hydroxyl group of Tyr¹, the benzene ring of Phe⁴, and the thioether bond of Met⁵ together constitute the pharmacophore for opioid receptor binding, exhibiting a spatial arrangement highly similar to the active groups of classic opioid drugs such as morphine, providing a structural basis for receptor recognition and activation.
The C-terminal Arg⁶-Phe⁷ extension possesses dual functions: Firstly, the positively charged guanidinium group of Arg⁶ can form ionic bonds with negatively charged amino acids in the extracellular domain of opioid receptors, significantly enhancing receptor binding affinity, making Met-Enkephalin-Arg-Phe's affinity for μ receptors far superior to Met-Enkephalin. Secondly, the hydrophobic benzene ring of Phe⁷ participates in the transmembrane hydrophobic interaction of the receptor, stabilizing the receptor-ligand complex conformation and prolonging the duration of action. Simultaneously, the C-terminal extension structure resists degradation by enkephalins such as aminopeptidase and carboxypeptidase, improving metabolic stability. Compared to pentapeptide enkephalin, its in vivo half-life is extended by 2–3 times, reducing the need for frequent dosing.
The molecular spatial conformation is compact and linearly folded. The N-terminus and C-terminus form a flexible linker region through peptide bonds, allowing the conformation to be adjusted according to the receptor binding interface, adapting to the binding pockets of μ, δ, and κ opioid receptors, thus exhibiting multi-receptor targeting characteristics. Without the chiral isomer racemic issue, it requires no complex resolution after synthesis, and its high-purity preparation process is mature, enabling large-scale production via solid-phase peptide synthesis. It also boasts low impurity content, meeting the quality control standards for scientific research and drug development.
The unique heptapeptide extended configuration, high receptor affinity pharmacophore, optimized metabolic stability, and favorable physicochemical properties collectively constitute the core competitiveness of Met-Enkephalin-Arg-Phe, making it a key member of the endogenous opioid peptide family with stronger activity and more complex functions, providing an ideal molecular template for pain mechanism research and the development of novel analgesics.
Opioid receptor activation and neural signal regulation
Met-Enkephalin-Arg-Phe's core mechanism of action relies on a dual mechanism of high-affinity opioid receptor activation and neurotransmitter release regulation. By binding to and activating μ, δ, and κ opioid receptors, it initiates Gi/Go protein-mediated signaling pathways, inhibiting neuronal excitatory and nociceptive neurotransmitter release, while simultaneously regulating inflammatory factors and neuroimmune responses. This achieves potent analgesia, neuroprotection, and mood regulation, unlike exogenous opioids such as morphine, which have advantages such as low addictiveness, fewer side effects, and strong endogenous adaptability.
In the central nervous system, Met-Enkephalin-Arg-Phe crosses the blood-brain barrier and targets presynaptic/postsynaptic membrane opioid receptors in pain-regulating brain regions such as the dorsal horn of the spinal cord, the periaqueductal gray matter of the midbrain, and the amygdala. Activation of presynaptic μ receptors inhibits adenylate cyclase activity via Gi protein, reducing intracellular cAMP concentration, closing voltage-gated Ca²⁺ channels, and decreasing the release of nociceptive neurotransmitters such as glutamate, substance P, and calcitonin gene-related peptide, thus blocking the ascending transmission of pain signals. Activation of postsynaptic receptors opens G protein-coupled inwardly rectifying K⁺ channels, promoting K⁺ efflux, neuronal hyperpolarization, and inhibiting the relay and transmission of pain signals, achieving a central analgesic effect.
In peripheral tissues, Met-Enkephalin-Arg-Phe is released from the adrenal medulla, gastrointestinal nerve endings, and other sites, activating opioid receptors on the surface of peripheral sensory nerve endings and immune cells. On the one hand, it inhibits the sensitivity of peripheral nociceptors, reducing the generation of local pain signals; on the other hand, it regulates the function of immune cells such as macrophages and T cells, inhibits the release of pro-inflammatory factors such as TNF-α and IL-1β, and alleviates the local inflammatory response in neuropathic pain and inflammatory pain, forming a central-peripheral synergistic analgesic network. Its inhibitory effect on chronic pain and neuropathic pain is significantly superior to that of classic enkephalins.
Met-Enkephalin-Arg-Phe's analgesic activity is dose-dependent and receptor-specific. Intraventricular administration in mice resulted in an ED₅₀ of 38.5 nmol/mouse, with an analgesic potency 8 times that of Met-Enkephalin. Furthermore, its effect can be completely reversed by the non-selective opioid receptor antagonist naloxone, confirming that its action is strictly dependent on opioid receptor activation. At low doses, it preferentially activates δ receptors, exerting analgesic and anti-anxiety effects without significant addictiveness. At high doses, it activates μ receptors, enhancing the analgesic effect but accompanied by mild tolerance, unlike the strong addictiveness and respiratory depression side effects of morphine.
The entire mechanism of action is progressive, from high-affinity receptor binding, activation of the Gi protein signaling pathway, regulation of ion channels, and inhibition of neurotransmitter release, to the regulation of peripheral inflammatory responses and the blocking of central pain transmission, forming a complete pain regulation system. Leveraging its four major advantages-potent analgesia, low addictiveness, multi-receptor regulation, and endogenous safety adaptation-Met-Enkephalin-Arg-Phe can precisely regulate acute pain and alleviate chronic neuropathic pain, providing a novel endogenous adaptation strategy for pain treatment.
Localization of neural modulation and multi-system effects
In the field of basic neuroscience research, Met-Enkephalin-Arg-Phes serve as specific opioid receptor probes for studying pain transmission pathways, receptor distribution and localization, and neurotransmitter interactions. By fluorescently or isotopically labeling MERFs, their distribution and metabolic trajectories in central and peripheral tissues can be tracked, clarifying the expression and function of μ/δ/κ receptors in pain-regulating brain regions. They are also used to construct animal models of pain, assess changes in pain thresholds, and elucidate the regulatory role of the endogenous opioid peptide system in pain homeostasis, providing precise tools for pain mechanism research.
In the field of analgesic drug development, Met-Enkephalin-Arg-Phes serve as lead compounds and structural templates for developing low-addiction, highly effective analgesics. Based on the heptapeptide structure of Met-Enkephalin-Arg-Phe, C-terminal modification, N-terminal alteration, or scaffold optimization are performed to design highly stable and receptor-selective analogs, avoiding the addictive and respiratory depressant side effects of drugs like morphine. Simultaneously, it is used in the design of multi-target analgesic drugs, fusing MERF pharmacophores with serotonin and norepinephrine reuptake inhibitors to develop combination drugs with both analgesic and anti-anxiety/antidepressant effects, meeting the treatment needs of chronic pain combined with mood disorders.
In the field of receptor mechanism exploration, it is used to study the conformation and signaling pathways of opioid receptors, elucidating conformational changes after receptor activation, G protein coupling patterns, and downstream signaling molecule interaction mechanisms. Point mutation technology is used to modify the amino acid sequence of Met-Enkephalin-Arg-Phe to identify key receptor binding sites and reveal the molecular basis of the selectivity differences between μ/δ receptors. It is also used to study the heterodimerization of opioid receptors with other GPCRs, exploring new mechanisms of multi-receptor interaction in pain regulation, providing support for the discovery of new drug targets.
In the field of neuroimmunological regulation research, it is used as a neuroimmunological regulator to explore the interaction between the opioid system and the immune system. Met-Enkephalin-Arg-Phe can inhibit the release of pro-inflammatory factors from immune cells and regulate the activation and differentiation of T cells and macrophages, making it useful for studying the neuro-immune imbalance mechanisms in chronic pain, neuroinflammation, and autoimmune diseases. It can also be used to assess the impact of analgesic drugs on immune function, guiding rational drug use in clinical practice, reducing the immunosuppressive side effects of long-term analgesia, and meeting the treatment needs of patients with chronic pain and immune abnormalities.
Furthermore, it can be applied to research areas such as mood regulation, drug addiction mechanisms, and gastrointestinal function regulation, exploring the mechanisms of action of MERF in anxiety, depression, opioid withdrawal, and gastrointestinal motility regulation. The raw materials meet research-grade quality control standards, and the high-purity products can meet the needs of in vitro, cell, and animal experiments. The synthesis process is mature, and it can be supplied in batches, providing stable and high-quality core research raw materials for the fields of neuroscience and drug development.
Stabilization modification and conversion of low-addiction analgesics
Long-acting stabilization modifications have become the mainstream research focus. Addressing the shortcomings of Met-Enkephalin-Arg-Phes, such as easy enzymatic degradation and short half-life in vivo, techniques like C-terminal amidation, D-type amino acid substitution, PEGylation, and cyclization are employed to improve metabolic stability and extend the duration of action in vivo. For example, C-terminal amidation can resist carboxypeptidase degradation, extending the half-life to 4–6 hours; PEGylation can reduce renal clearance, improve bioavailability, and reduce dosing frequency, meeting the needs of long-term chronic pain treatment.
Receptor selectivity optimization continues to advance. Through site-directed amino acid mutations, conformational constraints, and pharmacophore optimization, highly delta-receptor selective Met-Enkephalin-Arg-Phe analogs are being developed. Highly delta-selective analogs exhibit strong analgesic activity and lack μ-receptor-mediated addiction and respiratory depression side effects, making them suitable for long-term treatment of chronic neuropathic pain and cancer pain. Simultaneously, μ/delta dual-receptor agonists are being designed to balance analgesic efficacy and addiction risk, providing new options for the treatment of moderate to severe pain.
Delivery system development has become a hot topic. Addressing the challenges of Met-Enkephalin-Arg-Phe's difficulty in crossing the blood-brain barrier and its weak central effect with peripheral administration, delivery systems such as liposomes, nanoparticles, blood-brain barrier-penetrating peptide conjugates, and in-situ gels are being developed. Liposome encapsulation improves water solubility and stability, prolonging circulation time; blood-brain barrier-penetrating peptide conjugates can efficiently target brain regions that regulate central pain, enhancing central analgesia, reducing peripheral side effects, and meeting the needs of central pain treatment.
Clinical translational research is deepening. Phase I/II clinical trials based on Met-Enkephalin-Arg-Phe and its analogues are gradually being conducted to evaluate their safety and efficacy in indications such as postoperative pain, cancer pain, and postherpetic neuralgia. Preliminary data show that MERF analogues have significant analgesic effects, low addictiveness, and no serious adverse reactions, demonstrating potential for clinical application. Simultaneously, non-injectable delivery routes such as nasal sprays and transdermal patches are being explored to improve patient adherence and promote clinical implementation.
Continuous exploration of multi-target combination therapy strategies is underway. Based on the opioid receptor regulatory properties of Met-Enkephalin-Arg-Phe, these strategies combine MERF with nonsteroidal anti-inflammatory drugs, antidepressants, calcium channel blockers, and other medications to construct a multi-target analgesic system of "opioid-anti-inflammatory-neuromodulation." This synergistic effect reduces the dosage of single drugs, lowers the incidence of side effects, and adapts to the treatment needs of complex chronic pain, providing a novel strategy for pain management.
Conclusion
Met-Enkephalin-Arg-Phe, with its unique heptapeptide extended molecular structure, high-affinity opioid receptor activation mechanism, potent and low-addiction analgesic activity, and endogenous biocompatibility, has become a key member of the endogenous opioid peptide family, possessing both scientific research value and translational potential. It precisely targets μ/δ/κ opioid receptors, inhibits the release of noxious neurotransmitters, regulates neuroimmune responses, and achieves synergistic central-peripheral analgesia, balancing highly effective analgesia with low addiction risk.
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References
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