Pain-signal blocker (CHEMBL2112245)

What this is

CHEMBL2112245 is a synthetic cyclic peptide designed in the laboratory to block the receptors that respond to substance P — a signalling molecule involved in pain, nausea, and inflammation. It was not isolated from nature but was engineered as part of a research programme at the University of Pennsylvania that explored whether the structural scaffold of somatostatin (a hormone that regulates several secretory processes) could be repurposed to create antagonists at a completely different class of receptors, the tachykinin receptors TACR1 (NK1) and TACR2 (NK2). The compound has been studied only as a biochemical tool; it has not entered clinical development.

The stored sequence CKNFFTFTS is a nine-residue linear approximation. The actual compound is a 12-residue cyclic peptide (molecular weight ~1547 Da) in which a disulfide bond between the terminal cysteine residues creates a ring structure; the sequence also incorporates D-tryptophan and a para-fluorophenylalanine residue. These structural features — the ring, the D-amino acid, and the fluorinated side chain — are absent from the raw stored sequence and are essential to the compound's receptor-binding geometry (Liu and colleagues, J Med Chem 2000).

What it does

CHEMBL2112245 binds to and inhibits two closely related G-protein-coupled receptors: TACR1 (the neurokinin 1 receptor, NK1R), whose main endogenous activator is substance P, and TACR2 (the neurokinin 2 receptor, NK2R), whose preferred natural ligand is neurokinin A. In biochemical binding assays it displaces radiolabelled substance P from cells expressing the human receptors, demonstrating competitive antagonist behaviour. Its affinity for NK2R is approximately five-fold greater than for NK1R (Liu and colleagues, J Med Chem 2000).

Substance P and neurokinin A are released at peripheral and central nerve endings where they amplify pain signals, trigger smooth-muscle contraction in airways and gut, promote neurogenic inflammation, and activate vomiting circuits in the brainstem. Blocking these receptors with small peptides or non-peptide compounds has been validated clinically: NK1R antagonists aprepitant (approved 2003), fosaprepitant (2008), rolapitant (2015), and netupitant are all licensed for chemotherapy-induced nausea and vomiting (CINV), and broader therapeutic roles in pain, depression, and inflammatory disease have been investigated (Garcia-Recio and Gascón, BioMed Res Int 2015; Yin and colleagues, Nat Commun 2018).

Mechanism

TACR1 and TACR2 are class A GPCRs. NK1R signals primarily via Gq/G11 (activating phospholipase C and releasing intracellular calcium) and, to a lesser degree, via Gs (stimulating adenylyl cyclase). NK2R couples to both Gs and Gq/G11. Crystal structures of NK1R bound to clinical antagonists reveal a hydrophobic orthosteric pocket nestled between transmembrane helices III–VI; several residues are critical contact points, including histidine H197 (the "histidine-lock" that reshapes the helix V–VI interface), phenylalanine F264, and methionine M291 (Yin and colleagues, Nat Commun 2018).

Mutagenesis data from the CHEMBL2112245 bioassay directly implicate the equivalent histidine residue (H265A substitution) in binding: replacing it with alanine reduced the compound's IC50 at NK1R from ~260 nM to ~9600 nM — a roughly 38-fold loss of affinity — confirming that this histidine is a key anchor for the cyclic peptide's binding mode (Liu and colleagues, J Med Chem 2000).

The design rationale drew on earlier work in the same laboratory showing that a somatostatin-like cyclic scaffold (β-D-glucose or cyclic hexapeptide) can present side chains that mimic the β-turn pharmacophore of substance P — the so-called pseudosymmetry concept, where a single cyclic template can bind different receptor classes by presenting overlapping but distinct sets of residues to each binding pocket. Compounds from this series had previously achieved IC50 values of 22–150 nM at human NK1R (Hirschmann and colleagues, J Med Chem 1996; reviewed in Hirschmann and colleagues, Acc Chem Res 2009).

Evidence

• Human: No human studies. CHEMBL2112245 is a research ligand characterised only in cell-based assays.
• Animal: No in vivo animal studies reported in the source publication.
• In vitro: Binding assays in CHO and COS cells expressing human TACR1 and TACR2. IC50 = 250 nM at human NK1R (TACR1) and IC50 = 46 nM at human NK2R (TACR2). Site-directed mutagenesis established that H265 of NK1R is critical for binding (Liu and colleagues, J Med Chem 2000).

Known effects

• NK1R (TACR1) antagonism — In vitro only; IC50 250 nM (human, cell-based radioligand binding assay)
• NK2R (TACR2) antagonism — In vitro only; IC50 46 nM (human, cell-based radioligand binding assay)

Open questions

• No selectivity data versus NK3R (TACR3) reported; cross-reactivity unknown.
• Metabolic stability and serum half-life not characterised; cyclic disulfide peptides are generally more protease-resistant than linear peptides, but specific data for this compound are not available.
• No functional (agonism/antagonism) assay data published beyond binding displacement; intrinsic efficacy at Gq or Gs pathways is uncharacterised.
• No in vivo pharmacology reported.