Wakefulness-promoting peptide candidate (OX2R-004)

What this is

OX2R-004 is a computationally designed 18-amino-acid peptide (KGDRYGVAYEHGGAQPFK) intended to act as a selective agonist at the human orexin receptor 2 (OX2R). It is not a natural hormone and not a clinical drug — it is a research candidate whose structure was drafted in silico and is being assessed against a series of computational feasibility checks before any wet-lab synthesis. The orexin system regulates wakefulness, arousal, and appetite, and OX2R agonists are being pursued elsewhere as a way to directly correct the signaling deficit in narcolepsy type 1 rather than just managing symptoms (Hong 2021). The peptide is deliberately cysteine-free, carries no disulfide bond, and was scoped for intranasal CNS delivery rather than systemic injection.

History

OX2R-004 originated from an internal computational design pipeline that uses the active-state cryo-EM structure of OX2R bound to its natural agonist Orexin-B in complex with a Gi protein (PDB 7L1U, 3.2 Å) as the reference scaffold (Hong 2021). The PreIND computational feasibility assessment for this candidate walks it through an eight-gate evaluation pipeline (G1–G8) covering conformational sampling, structural complex analysis, binding energy, molecular dynamics, proteolytic stability, off-target selectivity, and safety patterning. Gates G1–G2 (conformational sampling) passed in prior work; G3–G4 were evaluated using the experimental 7L1U structure as a fallback because the Boltz-2 co-folded prediction of OX2R + OX2R-004 was not returned in time due to HPC queue delays. No peptide agonists of OX2R are currently in clinical development, which is the niche OX2R-004 is being designed into.

What it does

In intent: bind the same orthosteric pocket on OX2R that the natural orexin peptides occupy, and trigger the same downstream signal that promotes wakefulness and arousal. The natural agonist Orexin-B is a 28-residue C-terminally amidated peptide (RSGPPGLQGRLQRLLQASGNHAAGILTM-NH₂) that shows roughly 12-fold selectivity for OX2R over OX1R in binding assays; OX2R-004 was sized down to 18 residues while keeping enough of the binding-pocket contacts to (in theory) reproduce that activation. No functional cell-based assay has yet been run on OX2R-004, so its actual potency, efficacy, and selectivity are predicted rather than measured.

Mechanism

OX2R is a Gαq/Gαi-coupled GPCR whose activation involves substantial transmembrane rearrangement — TM2 helical twist, TM3 rotation, and reorientation of Q134(3.32) — documented in the active-state cryo-EM complex (Hong 2021). The OX2R-004 design strategy is to occupy the same orthosteric peptide-binding pocket described in that structure, contacting the residues that anchor the C-terminus of Orexin-B. In the feasibility assessment, the orthosteric pocket and its 10 key contact residues were verified in the reference structure and the buried surface area threshold was exceeded (G3 "Provisional Pass"), but iPTM and direct contact conservation for the OX2R-004 complex itself are still pending the co-folded prediction. PRODIGY-estimated binding ΔG of −6.8 kcal/mol was computed on the reference OX2R–Orexin-B interface as a proxy, not on the OX2R-004 complex — the underlying Orexin-B Kd (~5–36 nM) implies a true ΔG closer to −10.2 to −11.5 kcal/mol, which is what the design needs to approximate to be credible.

The sequence stored on this card (KGDRYGVAYEHGGAQPFK) is the bare 18-residue backbone with no terminal capping, no amidation, and no fatty-acid conjugation; net charge is approximately +2, GRAVY −1.178, hydrophobic fraction 0.278, and three of the residues are aromatic (Y, Y, F). The peptide contains no cysteines and forms no disulfide bond.

Known effects

No biological effects have been measured for OX2R-004. All current evidence is computational and structural. The peptide has not been synthesized, has not been tested in any binding or functional assay, and has not been administered to any organism.

Evidence

• Human: No human studies. No clinical trials registered for OX2R-004.
• Animal: No animal studies.
• In vitro: No in vitro data on OX2R-004 itself. The reference structure PDB 7L1U (active-state OX2R–Orexin-B–Gi complex) provides the binding-pocket geometry against which the candidate is being designed (Hong 2021).
• In silico: Eight-gate computational feasibility pipeline (PreIND assessment). G1–G2 (conformational sampling) Pass; G3 (structural complex) Provisional Pass via PDB 7L1U fallback; G4 (binding energy) Pending — PRODIGY ΔG of −6.8 kcal/mol was computed on the Orexin-B reference interface, not the OX2R-004 complex; G5 (molecular dynamics) Deferred; G6 (proteolytic stability) Failed — estimated serum half-life ~5 minutes; G7/G8 (selectivity, safety patterning) — high predicted OX1R cross-reactivity, no hemolytic or known-toxin patterns detected.

Safety signals

No empirical safety data exist for OX2R-004 because the peptide has not been synthesized or dosed. The PreIND feasibility assessment notes only computational/pattern-based safety observations: no cysteines or disulfide motifs, low hydrophobic fraction (0.278), no resemblance to known toxin families, and no hemolytic risk pattern detected. None of these substitute for actual toxicology — they are screens that the design did not fail, not evidence that it is safe.

Open questions

This card sits at an early computational stage. The PreIND assessment explicitly flags the following gaps that block IND-enabling work:

• The Boltz-2 co-folded prediction of OX2R + OX2R-004 was never returned (HPC queue delay); without it, iPTM and contact conservation cannot be computed and the binding energy cannot be re-evaluated on the actual candidate interface.
• Molecular dynamics (G5) was deferred — all structural assessments are static snapshots that do not capture conformational flexibility, solvent effects, or entropic penalties.
• Estimated serum half-life of ~5 minutes (G6) is the primary development blocker. This is a known limitation of unmodified linear peptides, not a defect unique to OX2R-004, but it has not been addressed by any modification (PEGylation, lipidation, cyclization, D-amino acid substitution, etc.).
• High predicted cross-reactivity at OX1R. The orthosteric peptide pocket is nearly identical between OX2R and OX1R, and native Orexin-B itself only shows ~12-fold OX2R selectivity in binding — there is no current evidence that OX2R-004 would do better.
• Passive blood-brain-barrier diffusion is not feasible (MW ≈ 1980 Da, GRAVY −1.178, 6/18 charged residues, all violating Lipinski-derived CNS-penetration criteria). The current development route therefore depends on intranasal nose-to-brain delivery, which has not been demonstrated for this peptide in any preclinical species.
• No synthesis. No binding assay. No functional assay. Every claim about activity is currently inferential.

Regulatory status

OX2R-004 is a preclinical research candidate. It has no regulatory status anywhere — not investigational, not approved, not scheduled. It has not entered IND-enabling studies.

Related peptides

• Orexin-B — the 28-residue natural OX2R agonist (RSGPPGLQGRLQRLLQASGNHAAGILTM-NH₂) used as the structural reference for the design (Hong 2021).
• Orexin-A — the other endogenous orexin agonist; binds OX2R with IC50 ≈ 38 nM in binding assays and is the better-characterized of the two natural ligands (per the PreIND benchmarking summary).