Pain-relief research peptide (YPWFGLM)

What this is

CHEMBL2158955 is a synthetic seven-residue peptide (YPWFGLM) designed as a chimera of two naturally occurring neuropeptides: endomorphin-1 and the C-terminal tail of substance P. It was synthesized and characterised as a research ligand for the mu-opioid receptor (OPRM1), the primary molecular target of endogenous pain-relief peptides and opioid analgesics. The ChEMBL database records a binding affinity of Ki = 1.6 nM at OPRM1, placing it in the high-potency range for mu-opioid ligands (Varamini and colleagues, Bioorganic & Medicinal Chemistry, 2012).

History

Endomorphin-1 (Tyr-Pro-Trp-Phe-NH₂) was identified in 1997 by Zadina and colleagues as the most selective endogenous agonist for the mu-opioid receptor — notable for its sub-nanomolar affinity and exceptional selectivity over delta- and kappa-opioid receptor subtypes. Despite this pharmacological profile, endomorphin-1 suffers from rapid enzymatic degradation in plasma and poor central nervous system penetration, limiting its usefulness as a drug lead.

Researchers later recognised that conjugating C-terminal fragments of substance P (an 11-residue nociceptive neuropeptide, RPKPQQFFGLM-NH₂) to opioid peptides could produce chimeric compounds with enhanced analgesic properties and, crucially, reduced tendency to develop tolerance on repeated dosing — a limitation of classical opioids including morphine. CHEMBL2158955 represents one such hybrid: the endomorphin-1 tetrapeptide (YPWF) fused at its C-terminus with the SP7-11 fragment (FGLM), linked through the shared terminal phenylalanine residue, producing the seven-residue YPWFGLM sequence. This chemistry was explored systematically by Varamini and colleagues (2012).

What it does

CHEMBL2158955 binds the mu-opioid receptor (OPRM1) with high affinity (Ki = 1.6 nM; Varamini et al., 2012). Activation of OPRM1 inhibits adenylyl cyclase via Gi/Go coupling, suppresses neuronal excitability, and reduces the transmission of nociceptive signals — the same pathway engaged by endogenous endorphins and exogenous analgesics such as morphine.

The substance P-derived FGLM tail appended to the endomorphin-1 core was expected to engage the neurokinin-1 (NK1) receptor, which co-localises extensively with OPRM1 in pain-processing regions of the spinal cord and brain. This dual-receptor rationale underpins the chimeric design: simultaneous activation of MOR and NK1 has been associated with sustained analgesic efficacy and reduced development of opioid tolerance in preclinical models, a property not seen with mu-opioid agonists alone.

Evidence

• Human: No human studies reported for CHEMBL2158955.
• Animal: No in vivo data reported specifically for CHEMBL2158955. The parent compound endomorphin-1 and closely related lipo-endomorphin-1 derivatives from the same research group demonstrated antinociceptive activity in rat neuropathic pain models (Varamini et al., PLoS ONE, 2012).
• In vitro: Ki = 1.6 nM at OPRM1 (radioligand binding assay; Varamini et al., Bioorganic & Medicinal Chemistry, 2012), registered in ChEMBL as compound CHEMBL2158955.

Known effects

• OPRM1 binding — Ki = 1.6 nM (in vitro; Varamini et al., 2012)
• Dual MOR/NK1 targeting — Mechanistic only; the chimeric design was intended to activate both OPRM1 and the NK1 receptor, based on the established rationale for opioid/substance P hybrids in the field

Mechanism

The N-terminal YPWF segment of CHEMBL2158955 is structurally identical to endomorphin-1, the endogenous tetrapeptide with the highest known selectivity for OPRM1 among mammalian opioid peptides. Endomorphin-1 engages the orthosteric binding pocket of OPRM1 and couples through Gαi/Gαo proteins to inhibit cAMP production, activate inwardly-rectifying potassium channels, and suppress voltage-gated calcium channels — collectively dampening the firing of pain-transmitting neurons.

The C-terminal FGLM extension derives from the functionally active C-terminal tail of substance P (SP7-11, residues 7–11 of RPKPQQFFGLM-NH₂). Substance P is the primary endogenous ligand for the NK1 neurokinin receptor, which is co-expressed with OPRM1 in laminae I and II of the spinal dorsal horn and in supraspinal pain centres. The design hypothesis is that the SP-derived tail of the hybrid reaches the NK1 receptor at sites of MOR/NK1 co-localisation, producing concurrent signalling that counters the upregulation of substance P and NK1 that occurs with prolonged opioid exposure — the mechanistic basis of tolerance. The shared phenylalanine at the junction of the two parent sequences serves as the structural hinge of the chimera.

Open questions

• Whether CHEMBL2158955 achieves functionally meaningful NK1 receptor activation in addition to OPRM1 binding has not been published
• Metabolic stability, plasma half-life, and blood-brain barrier permeability for this specific sequence have not been reported
• No selectivity data versus delta- or kappa-opioid receptor subtypes have been published for CHEMBL2158955
• No in vivo analgesia or tolerance studies for this compound appear in the literature