MT-II (Melanotan II): Melanocortin Receptor Pharmacology and Structure

MT-II derives from α-melanocyte-stimulating hormone, a 13-residue neuropeptide carrying the sequence Ac-Ser-Tyr-Ser-Met-Glu-His-Phe-Arg-Trp-Gly-Lys-Pro-Val-NH2. Three changes produce the analog: methionine at position 4 becomes norleucine, L-phenylalanine at position 7 becomes D-phenylalanine, and a lactam bridge joins an aspartate side chain at position 5 to a lysine side chain at position 10.

Truncation accompanies those substitutions. The parent hormone is cut back to residues 4 through 10, leaving a seven-residue chain whose two side chains are then amide-linked to close a ring. Al-Obeidi and colleagues prepared seven such cyclic lactams in 1989 and bioassayed them in amphibian and reptilian skin preparations, reporting that the aspartate-lysine pairing gave a 23-membered ring roughly 90 to 100 times more potent than α-MSH, and that both enlarging and shrinking the ring away from 23 atoms reduced potency. The 23- and 24-membered analogs also showed prolonged residual activity where the 20-, 21- and 22-membered rings did not.

PubChem records the resulting molecule under CID 92432 with the formula C50H69N15O9, a molecular weight of 1024.2 g/mol and a monoisotopic mass of 1023.5403 Da. Computed descriptors describe a large, strongly polar macrocycle: topological polar surface area 385 Ų, 13 hydrogen bond donors, 11 acceptors, 17 rotatable bonds and an XLogP of 0. The registry number 121062-08-6 covers the peptide itself; commercial solids are usually salts, most often the acetate.

Nomenclature causes recurring confusion. MT-II is a cyclic seven-residue molecule; afamelanotide, sometimes called melanotan-1, is a linear 13-residue α-MSH peptidomimetic carrying the same norleucine and D-phenylalanine substitutions with no cyclization at all. The two differ in mass, formula and registry number, and results reported for one do not transfer to the other.

MT-II presents the His-Phe-Arg-Trp tetrapeptide core that every melanocortin receptor recognizes, held in a constrained ring rather than a flexible chain. Published binding and functional work places the analog at MC1R, MC3R, MC4R and MC5R with nanomolar or better potency, while MC2R, which requires ACTH together with the accessory protein MRAP1, sits outside its range.

Structural work has made that motif concrete. Ma and colleagues reported three cryo-electron microscopy structures of MC1R coupled to heterotrimeric Gs in 2021, resolving a wide-open orthosteric pocket that accommodates the conserved HFRW sequence and identifying a calcium ion as an integral part of the ligand-binding site. The same study described an unexpected contact surface involving the Gβ subunit, a feature not typical of class A receptor complexes.

The calcium finding had emerged a year earlier at a different subtype. Yu and colleagues determined the crystal structure of human MC4R bound to the cyclic antagonist SHU9119 at 2.8 Å resolution and showed that a divalent calcium ion is complexed by residues contributed jointly by the receptor and the peptide. Extracellular calcium raised the affinity of α-MSH at MC4R by 37-fold and its potency by 600-fold, which reframed a decade of binding data collected without controlled divalent ion concentrations.

MC2R is the family exception and the reason MT-II selectivity discussions normally cover four subtypes rather than five. That receptor recognizes ACTH and depends on MRAP1 for trafficking and activation, so α-MSH-derived macrocycles are not part of its pharmacology. Selectivity questions for MT-II therefore concern the spread between MC1R, MC3R, MC4R and MC5R, not the full set.

MT-II potency figures published across the melanocortin subtypes cluster in the subnanomolar to low-nanomolar band, with the widest spread appearing at the human receptors. A 2024 survey of melanocortin tool compounds compiled literature values at mouse MC1R, MC3R, MC4R and MC5R and at their human counterparts, showing how much the number depends on assay format and expression system.

At the mouse receptors the compiled ranges are tight. Weirath and Haskell-Luevano list half-maximal effective concentrations of 0.020 to 0.06 nM at mouse MC1R, 0.011 to 0.18 nM at mouse MC3R, 0.083 to 0.15 nM at mouse MC4R and 0.14 to 0.17 nM at mouse MC5R. Values in that compilation come predominantly from HEK293 lines stably expressing a single recombinant subtype and read out by cyclic AMP accumulation.

Human receptor figures scatter far more widely: 0.032 to 0.30 nM at MC1R, 0.20 to 34 nM at MC3R, 0.011 to 8.6 nM at MC4R and 2.8 to 34 nM at MC5R. Three orders of magnitude separate the extremes at MC3R and MC4R. Anyone comparing a single quoted value against another laboratory's number is comparing assay conditions as much as molecules, which is exactly why the survey was assembled as a reference set.

Aggregated affinity data in the IUPHAR/BPS Guide to Pharmacology follow the same rank ordering, listing MT-II as a full agonist with pIC50 values of 9.4 at MC1 and 9.0 at MC5, and pKi values of 8.3 at MC3 and 8.2 to 8.8 at MC4. MC1R sits highest in essentially every dataset; the practical point is that the separation between subtypes is under two log units, which is not enough to call the molecule selective.

MT-II binds all four responsive melanocortin subtypes with high potency and discriminates poorly among them, which is precisely what made its macrocyclic scaffold useful to medicinal chemists. Ring size, ring chemistry and the identity of the position-7 aromatic residue have each been varied against the same template to build subtype-preferring ligands.

The most consequential single change is at position 7. Replacing D-phenylalanine with the bulkier D-2-naphthylalanine, while keeping the identical lactam cyclization, converts the agonist scaffold into SHU9119, a compound that behaves as an antagonist at MC3R and MC4R. One aromatic substitution flips functional direction without touching the ring, which is why SHU9119 appears alongside MT-II in almost every melanocortin structure-activity paper as the paired control.

Ring dimension has been revisited with modern chemistry. Merlino and colleagues designed and synthesized 14 macrocyclic peptide analogs spanning 19- to 23-membered rings, built on the MT-II, SHU-9119 and PG-901 scaffolds, and profiled them at human MC1R, MC3R, MC4R and MC5R. Two compounds, FM648 and FM636, emerged as potent and selective hMC4R antagonists, and a third, FM635, showed agonist activity with pronounced selectivity for the same subtype. Contracting the ring below the 23 atoms that Al-Obeidi identified is therefore not simply destructive; it redistributes subtype preference.

Alternative macrocyclization chemistries have also been tested against this template. Todorovic and colleagues condensed a 5-hydroxypyrroloindoline with either a cysteine thiol or a tryptophan indole to form tryptathionine and 2,2'-bis-indole staples, motifs borrowed from amatoxins and staurosporine, and applied the chemistry to α-MSH and to melanotan-II. Both classes of stapled peptide retained nanomolar inhibition constants, with one construct reaching a sub-nanomolar value, showing that the lactam is one solution to conformational constraint rather than the only one.

MT-II research now sits inside a structural literature that did not exist a decade ago. Cryo-electron microscopy and crystallography have resolved all five human melanocortin receptors in complex with heterotrimeric Gs, in ligand-bound and unliganded states, giving the constrained macrocycle a concrete binding pocket rather than an inferred pharmacophore model.

Subtype discrimination was addressed directly in 2023. Feng and colleagues reported three cryo-electron microscopy structures, MC3R-Gs bound to γ-MSH and MC5R-Gs bound to either α-MSH or the synthetic agonist PG-901. Both endogenous peptides adopt a U-shaped conformation, penetrate a wide-open orthosteric pocket and form extensive shared contacts through the HFRW motif. Selectivity was traced to the peptide C-terminus: that of γ-MSH occupies an MC3R-specific complementary groove, whereas the α-MSH C-terminus contributes negligible contact. PG-901 matches α-MSH potency at shorter length by cyclizing to mimic an intramolecular salt bridge, the same logic that produced MT-II.

Basal receptor behaviour was mapped in 2025. Feng and colleagues determined unliganded structures of MC1R, MC2R, MC3R, MC4R and MC5R with Gs at global resolutions of 2.98, 3.01, 2.75, 3.12 and 2.86 Å, and found binding poses and Gs interactions closely resembling agonist-bound states. Extracellular helix ends differed, with variable shifts of transmembrane helix 3 and outward displacement of helix 4. The study also reported that zinc, but not calcium, positively regulated MC4R activity in a concentration-dependent fashion, adding a second divalent ion to the picture.

Signaling readouts have grown more granular alongside the structures. Dai and colleagues examined SHU9119 and MBP10 at wild-type human MC4R and at six naturally occurring constitutively active variants, reporting that both peptides produced no or negligible Gαs-cyclic AMP agonism yet stimulated ERK1/2 phosphorylation. Mechanisms diverged: SHU9119 acted through phosphatidylinositol 3-kinase, MBP10 through both PI3K and β-arrestin. The authors classify SHU9119 as a biased ligand and MBP10 as a biased allosteric modulator, meaning a compound scored inactive on a cyclic AMP assay may be active on another branch.

MT-II identification in published analytical work rests on three orthogonal measurements: chromatographic retention on a reversed-phase column, ultraviolet absorbance from its tryptophan and histidine residues, and accurate-mass detection of the doubly protonated molecular ion near m/z 513. Method papers combine all three because no single one is specific enough alone.

Breindahl and colleagues developed and validated exactly that combination, using liquid chromatography with ultraviolet detection at 218 nm and tandem mass spectrometry after collision-induced fragmentation of the doubly charged precursor at m/z 513, with identity confirmed by retention time plus the relative abundance ratios of five qualifier fragment ions. Applied to vials from three online vendors, the method found unknown impurities of 4.1 to 5.9 percent in material from two of them and total content between 4.32 and 8.84 mg where every vendor declared 10 mg. Label declaration and measured content are separate quantities.

Distribution work has used imaging mass spectrometry. Chen and colleagues took the molecule as a proof-of-concept cyclic peptide and paired matrix-assisted laser desorption/ionization imaging with droplet-based liquid microjunction surface sampling coupled to high-resolution LC-MS, mapping distribution in tissue while profiling structural information on parent and metabolite species.

Degradation chemistry for this peptide class has been characterized on the linear analog. Chawathe and Sharma subjected afamelanotide to acidic, basic, neutral, oxidative, photolytic and 60 °C thermal stress under ICH Q1A(R2) and Q5C, separated fourteen distinct degradation products by gradient reversed-phase HPLC on a Zorbax SB C18 column (300 Å, 4.6 × 150 mm, 3.5 µm) and characterized them by UHPLC coupled to high-resolution tandem mass spectrometry. Truncation, methylation, deacetylation and oxidation were among the pathways identified.

MT-II is supplied as a lyophilized solid, most often the acetate salt, and its computed descriptors explain that format. PubChem lists a topological polar surface area of 385 Ų, thirteen hydrogen bond donors, eleven acceptors and a computed XLogP of 0, describing a strongly polar macrocycle that dissolves in water and dilute aqueous acid.

Weighed mass and net peptide are different quantities for any solid-phase product. Preparative reversed-phase purification leaves a counterion, commonly trifluoroacetate or acetate, and lyophilized peptides retain bound water. Lot documentation therefore separates gravimetric mass from peptide content, and a molar concentration calculated from weighed mass alone will read high.

Identity confirmation follows the pattern set in the published methods. Electrospray ionization is checked against a monoisotopic mass of 1023.5403 Da, with the doubly protonated ion near m/z 513 serving as the working precursor rather than the singly charged species near m/z 1024.5, because the 2+ envelope dominates for a peptide of this size. Chromatographic purity is measured by reversed-phase HPLC with ultraviolet detection in the 214 to 220 nm region, and the tryptophan residue supplies a second absorbance near 280 nm useful as a confirmatory channel.

Stability expectations follow from the chemistry. The lactam macrocycle removes the free termini that exopeptidases attack, which is the structural basis for the reported retention of full activity after 60 minutes of peptidase exposure, but tryptophan, histidine and methionine-free though the sequence is, the indole and imidazole side chains remain oxidation-sensitive. Forced-degradation work on the linear analog afamelanotide identified truncation, methylation, deacetylation and oxidation as active pathways across acid, base, peroxide, ultraviolet and 60 °C thermal stress. Lyophilized material is normally held frozen, desiccated and shielded from light, with aqueous stocks divided into single-use aliquots so that repeated freeze-thaw cycling is avoided.

Material described here is offered strictly for in vitro laboratory research and analytical reference. It is not a drug, not a food, not a cosmetic, and is not intended for human or veterinary use.