Opioid-receptor blocker from a brain signaling peptide (CHEMBL1627325)

What this is

[D-Tle⁵]NPS is a synthetic research analog of neuropeptide S (NPS), a naturally occurring brain signaling peptide first identified in 2002 as the endogenous activator of the neuropeptide S receptor (NPSR). The modification — swapping the glycine at position 5 of the 20-residue NPS sequence for a D-configured tert-leucine — converts the parent agonist into a compound with negligible activity at NPSR but unexpectedly potent blocking activity at several other G-protein coupled receptors, including the delta, kappa, and mu opioid receptors and the nociceptin/OFQ receptor. It is a laboratory tool compound with no approved therapeutic use, studied to map structure-activity relationships in the NPS peptide family (Guerrini and colleagues, Journal of Medicinal Chemistry, 2009). The platform stores a 19-residue approximation of the sequence; the full chemical structure is a 20-mer with D-tert-leucine at position 5 in place of the glycine found in the parent NPS sequence.

History

Human NPS (sequence SFRNGVGTGMKKTSFQRAKS) is conserved across vertebrates and exerts its central effects — promoting arousal, reducing anxiety, and influencing locomotion and food intake — through the NPSR, a Gq/Gs-coupled class-A GPCR first paired with its endogenous ligand in 2002. Early structure-activity work on NPS established that substituting position 5 (an achiral glycine) with D-amino acids systematically abolishes agonism at NPSR and generates antagonists, with potency inversely related to side-chain size; [D-Val⁵]NPS was identified as a potent NPSR antagonist from that series. The Guerrini group at the University of Ferrara then extended position-5 exploration to bulkier D-amino acid substitutions — including D-tert-leucine — and profiled the resulting analogs against a broad panel of GPCRs including all four opioid receptor subtypes (Guerrini and colleagues, Journal of Medicinal Chemistry, 2009, DOI 10.1021/jm900604g). That publication is the source of all pharmacological data on this compound.

What it does

In cell-based assays, [D-Tle⁵]NPS is inactive as an agonist at the mouse NPSR. Despite that, it blocks agonist-induced responses at human recombinant opioid receptors at nanomolar concentrations: it inhibits delta opioid receptor (OPRD1) activation with an EC50 of approximately 1.95 nM, kappa opioid receptor activation at 2.34 nM, and mu opioid receptor (OPRM1) activation at 10.96 nM, all measured against reference agonists in calcium mobilization assays. It also blocks the nociceptin/OFQ receptor (NOP) at 0.28 nM — more potently than any of the classical opioid subtypes in this assay format. Additional off-target antagonism was observed at the NK1 substance-P receptor (~0.05 nM), the B2 bradykinin receptor (~0.03 nM), and the urotensin-2 receptor (3.47 nM) (Guerrini and colleagues, 2009). All values derive from a single publication; independent replication has not been reported in the available literature.

Evidence

• Human: No human studies. [D-Tle⁵]NPS is a research ligand that has not entered clinical development.
• Animal: Not reported for this specific analog in the available literature.
• In vitro: Antagonist potency measured in calcium mobilization assays at human recombinant delta, kappa, and mu opioid receptors, nociceptin receptor, NK1, bradykinin B2, urotensin-2, and PAR-2 receptors, and mouse NPSR (Guerrini and colleagues, 2009, DOI 10.1021/jm900604g).

Mechanism

[D-Tle⁵]NPS was designed as an NPSR antagonist — the bulky D-tert-leucine side chain at position 5 prevents the conformational change required for NPSR activation while retaining receptor contact through the conserved N-terminal pharmacophore. In practice, it shows only weak antagonism at NPSR (pKb 6.5; no agonism detected up to 10 µM), suggesting the D-Tle substitution disrupts NPSR engagement more thoroughly than anticipated. Its potent activity at opioid and kinin receptors was identified through a selectivity screening panel in the same study and is likely attributable to structural features shared between the N-terminal NPS pharmacophore and the binding epitopes of endogenous opioid and kinin peptides rather than any designed mechanism. The NOP receptor was the most potently blocked target in this panel (EC50 0.28 nM), followed by delta opioid (1.95 nM) and kappa opioid (2.34 nM). The structural basis for this cross-reactivity has not been elucidated by crystallography or computational docking in the available literature.

Known effects

• NPSR antagonism — Weak (pKb 6.5 in calcium assay at mouse NPSR; inactive as agonist up to 10 µM). In vitro only. Source: Guerrini and colleagues, 2009.
• Delta opioid receptor (OPRD1) antagonism — EC50 ~1.95 nM at human recombinant receptor. In vitro only. Source: Guerrini and colleagues, 2009.
• Kappa opioid receptor antagonism — EC50 ~2.34 nM at human recombinant receptor. In vitro only. Source: Guerrini and colleagues, 2009.
• Mu opioid receptor (OPRM1) antagonism — EC50 ~10.96 nM at human recombinant receptor. In vitro only. Source: Guerrini and colleagues, 2009.
• NOP receptor antagonism — EC50 ~0.28 nM at human recombinant receptor. In vitro only. Source: Guerrini and colleagues, 2009.
• NK1 / bradykinin B2 / urotensin-2 antagonism — Potent in vitro (EC50 range: ~0.03–3.47 nM). In vitro only. Source: Guerrini and colleagues, 2009.

All effects are from a single publication using calcium mobilization assays. Evidence level: mechanistic/in vitro only.

Open questions

• Whether the off-target opioid and kinin receptor activity is replicated in independent studies or reflects assay-specific conditions (calcium mobilization in overexpression systems can exaggerate apparent potency).
• Whether [D-Tle⁵]NPS produces functional opioid receptor antagonism in native tissues or in vivo at doses consistent with its in vitro potency.
• Whether the compound has antinociceptive, anxiolytic, or other behavioral effects attributable to its NOP/opioid profile in animal models.
• No serum stability or pharmacokinetic data are available in the literature.
• The structural basis for the unexpected multi-GPCR activity profile has not been elucidated by crystallography or computational docking.