Fold №55 — Semax Head-to-Tail Macrocyclization

cyclo[Succ-MEHFPG-k-P] · MC4R · REFINED

In silico prediction only. All structural, binding, and property data are computational estimates from Boltz-2. No wet-lab validation has been performed. This is not medical advice.

Mechanism of Action

Semax (MEHFPGP) is a synthetic ACTH(4-10)-derived heptapeptide with established nootropic, neuroprotective, and anti-inflammatory activity in rodent and clinical models. Its biological mechanism is multi-modal: downstream neurotrophin upregulation (BDNF, NGF), monoaminergic modulation (serotonin, dopamine in striatum), immune pathway regulation, and copper chelation via the His imidazole + Met-1 α-amine coordinate system. Its ACTH lineage grounds the hypothesis that MC4R is a relevant receptor target — the His-Phe dipeptide motif is the established minimum pharmacophore for melanocortin activity across the ACTH/MSH family — but direct MC4R binding data for native Semax have not been published in the retrieved literature.

MC4R (UniProt P32245) is a Gs-coupled GPCR broadly expressed in hypothalamic and limbic circuits, with characterized roles in energy homeostasis, cognition, reward, and neuroprotection. Cyclic melanocortin analogues (MT-II, SHU9119) that constrain the His-Phe pharmacophore into a type-II β-turn via lactam bridges have achieved nanomolar MC4R potency — this is the chemical precedent the current design draws upon.

Important context: recent evidence (Liu et al., 2025) implicates the μ-opioid receptor/USP18 axis as an additional primary mediator of Semax's effects. This means that a highly MC4R-selective macrocyclic analogue may be a powerful probe tool while underreproducing Semax's full biological phenotype.

Performance Applications

If the predicted MC4R engagement is validated, the macrocyclic analogue would be of interest in the following research contexts:

• Cognitive enhancement and memory consolidation — MC4R signaling in hippocampal and prefrontal circuits is implicated in synaptic plasticity and memory encoding; a protease-stable, high-affinity MC4R agonist would allow longer-duration receptor engagement than linear Semax.
• Neuroprotection — MC4R agonism activates downstream CREB/BDNF pathways that parallel Semax's documented neurotrophin effects; the macrocycle could serve as a research tool to dissect MC4R-dependent vs. MC4R-independent components of Semax's neuroprotection.
• Receptor pharmacology probe — The macrocycle's selectivity profile (gain: MC4R potency / protease resistance; loss: copper chelation) makes it useful for mechanistic dissection of Semax's receptor vs. metal-chelation biology.

⚠️ BBB penetration is predicted at 0.0 by heuristic scoring — CNS bioavailability of the macrocyclic form is uncertain and requires direct experimental assessment before any CNS application claims can be made.

Modification Rationale

Linear Semax pays a steep entropic cost upon MC4R binding: the flexible backbone must adopt a specific β-turn geometry around the His-Phe pharmacophore, and without pre-organization, a significant fraction of the binding energy is spent ordering the peptide rather than forming receptor contacts. The modification strategy addresses this directly:

D-Lys insertion (position 6.5, between Gly-6 and Pro-7): Provides an orthogonal ε-amine handle for ring closure without displacing any native residue from the pharmacophore register. D-stereochemistry positions the ε-amine geometrically for lactam closure without introducing steric clash into the HFPGP turn.

Succinyl spacer (Met-1 α-N → D-Lys ε-N): Provides ~5 atoms of bridge length to span the N-to-tail distance of the 7-residue sequence without inducing ring strain. Succinyl (4-carbon dicarboxylate) is a well-precedented spacer in macrocyclic peptide chemistry for this approximate sequence length.

Lactam bridge: Forms an amide bond between the succinyl terminal carbonyl and the D-Lys ε-amine, closing the macrocycle. This eliminates the free N-terminus (aminopeptidase cleavage site) and the conformational flexibility of the backbone simultaneously.

C-terminal Pro-7 carboxylate left free: Preserves the PGP C-terminal element, which has documented independent transcriptional activity (Dmitrieva et al., 2010; Medvedeva et al., 2017) and should not be buried or bridged.

Tradeoff acknowledged: The succinyl linker caps the Met-1 α-amino group, which participates in Cu²⁺ coordination alongside His imidazole (Sciacca et al., 2022). This design choice sacrifices the copper-chelation function in exchange for macrocyclic constraint — appropriate if the target application is MC4R pharmacology, but a genuine loss of one of Semax's characterized neuroprotective mechanisms.

This fold is architecturally distinct from all prior Semax distillations in this lab:

• Fold #1 (Ac-Met-1): N-terminal cap, linear backbone, REFINED — blocked aminopeptidase but left backbone flexible
• Fold #24 (4F-Phe-4): Single aromatic substitution, linear backbone, DISCARDED — insufficient differentiation
• Fold #49 (Nπ-Me-His-3): His tautomer lock, linear backbone, REFINED — improved imidazole geometry without backbone constraint

Fold №55 completes the logical progression: where folds #1 and #49 made point modifications on a flexible scaffold, the macrocyclization directly addresses the entropy problem those edits could not solve. The ipTM advantage (0.83 vs. pTM-only metrics from prior linear folds) reflects this structural step change.

Predicted Properties (Favourable Changes from Native Semax)

Suggested Next Steps

Further variants to distill:

1. Shorter spacer variant — Replace succinyl (4C) with malonyl (3C) or glutaryl (5C) to probe ring strain sensitivity and optimize macrocycle geometry; pLDDT and ipTM sensitivity to spacer length would be informative.
2. L-Lys insertion control — Distill the same macrocycle with L-Lys at position 6.5 (vs. D-Lys here) to quantify the stereocontrol contribution to predicted interface quality.
3. Copper chelation rescue variant — Replace the succinyl bridge with a linker that does not cap the Met-1 α-N (e.g., a side-chain-to-side-chain lactam between Glu-2 and a C-terminal Lys) to recover chelation while retaining macrocyclic constraint.
4. Nπ-Me-His-3 macrocycle — Combine the His tautomer lock from fold #49 with the macrocyclic scaffold of fold #55; this compound would simultaneously constrain backbone conformation and imidazole geometry.
5. Selectivity panel — Predict the macrocycle against MC1R and MC3R to assess selectivity within the melanocortin family.

Validation experiments (wet lab):

1. Synthesis and characterization — Fmoc SPPS with D-Lys(Mtt) at position 6.5, on-resin succinylation of α-N(Met-1), global deprotection, and lactam cyclization in dilute solution; confirm ring closure by HRMS and ROESY NMR.
2. MC4R binding assay — Competitive radioligand binding (¹²⁵I-NDP-α-MSH displacement) and cAMP functional assay (Gs activation) to directly measure Ki and EC50; compare to linear Semax.
3. Proteolytic stability — Incubation in human plasma and brain homogenate; HPLC half-life comparison vs. linear Semax to confirm the predicted stability benefit.
4. BBB permeability — PAMPA-BBB or Caco-2 assay; if passive permeability is lost (consistent with heuristic prediction), explore CNS delivery strategies (prodrug, nanoparticle) or reframe as a peripheral MC4R tool.
5. Copper chelation — ITC or UV-Vis Cu²⁺ titration to confirm whether the succinyl cap abolishes chelation activity as predicted.
6. In vivo cognition — Morris water maze or novel object recognition in rodents, comparing macrocycle vs. linear Semax at equimolar doses to determine whether the MC4R-optimized scaffold translates to behavioural benefit.