Melanotan-2 is a cyclic seven-residue analog of α-melanocyte-stimulating hormone, engineered at the University of Arizona in the late 1980s to be a rigid, protease-resistant version of a hormone that normally survives only minutes in plasma. Three things define it: a lactam ring that is routinely mistaken for a disulfide, a deliberate non-selectivity across four melanocortin receptors, and a family resemblance to two FDA-approved drugs it is constantly confused with. This profile covers the structure, the receptor pharmacology, how it differs from Melanotan I and PT-141, and the handling rules that follow from a tryptophan-containing cyclic peptide. It describes the preclinical and pharmacology record observationally and makes no clinical claims.
Structural identity
- Sequence:
Ac-Nle-cyclo[Asp-His-D-Phe-Arg-Trp-Lys]-NH₂(an N-acetylated, C-amidated cyclic heptapeptide) - Molecular formula: C₅₀H₆₉N₁₅O₉
- Average mass: 1024.18 g/mol
- CAS: 121062-08-6
- Salt form: typically the acetate
- Origin: Designed by Victor Hruby and Mac Hadley at the University of Arizona from a program characterizing α-MSH analogs; the early pharmacology was reported by Dorr and colleagues. Fully synthetic.
The ring is the headline feature. A side-chain amide bond, a lactam, links the aspartate at position 2 to the lysine at position 7, closing the six interior residues into a cycle. The acetylated norleucine sits outside the ring as the N-terminus, and the C-terminus is a primary amide. Norleucine (Nle) is a deliberate substitution for the methionine found in native α-MSH, which removes the only oxidation-prone residue the parent hormone carried.
The lactam ring: why it is not a disulfide
The single most common structural misconception about Melanotan-2 is that its ring is a disulfide. It is not. There is no cysteine anywhere in the sequence, and no sulfur in the ring. The cycle is closed by an amide bond between the Asp side-chain carboxyl and the Lys side-chain amine, the same chemistry as a peptide bond. That distinction has real handling consequences: the reducing agents and thiol-oxidation worries that govern disulfide-bridged peptides (oxytocin, somatostatin) simply do not apply here.
The reason for building a ring at all is rigidity. Cyclizing the message segment locks the pharmacophore into a constrained conformation that the melanocortin receptors bind tightly, and it shields the backbone from peptidases. In the classic in vitro melanotropic assays the cyclic, [Nle⁴,D-Phe⁷]-modified molecule is reported as roughly a thousand-fold more potent than native α-MSH. The D-phenylalanine at position 7 is the key inversion: swapping the natural L-Phe for its D-enantiomer drives most of the potency and stability gain.
What receptors does Melanotan-2 actually hit?
All of them except one. Melanotan-2 is a non-selective agonist at four of the five melanocortin receptors: MC1R, MC3R, MC4R, and MC5R. It does not meaningfully engage MC2R, the adrenal ACTH receptor, which recognizes a different part of the corticotropin sequence rather than the MSH pharmacophore. That broad activity is the molecule's defining property.
Each receptor maps to a different research context. MC1R, on melanocytes, drives eumelanin synthesis, and it is the basis for the melanogenesis endpoints in the preclinical literature, including UV-independent pigment models. MC4R, in the hypothalamus, is associated in the endocrine literature with energy-balance and arousal signaling. MC3R contributes to central energy regulation and MC5R to exocrine-gland function. Hitting all four at once is why the molecule's record is dominated by mixed, multi-system observations rather than a single clean effect.
The melanocortin pharmacophore
Every melanocortin agonist shares a four-residue message sequence: His-Phe-Arg-Trp. It is the minimal core that activates the MC receptors, conserved across α-, β-, and γ-MSH. In Melanotan-2 that sequence sits inside the lactam ring, which is what the cyclization was designed to constrain. Read the structure and the design logic becomes legible: the ring presents His-Phe-Arg-Trp in a fixed geometry, the D-Phe⁷ sharpens the fit, and the Nle⁴ removes the oxidation liability, all without touching the part of the molecule that actually talks to the receptor.
Melanotan I vs Melanotan-2 vs PT-141
Three compounds get lumped together under the "melanotan" label, and they are structurally distinct molecules, not dose variants of one drug. This is the comparison most people are actually searching for.
| Melanotan I (afamelanotide) | Melanotan-2 (MT-2) | PT-141 (bremelanotide) | |
|---|---|---|---|
| Topology | Linear 13-mer | Cyclic heptapeptide (lactam) | Cyclic heptapeptide |
| Receptor profile | MC1R-predominant | Non-selective MC1/3/4/5R | MC4R-preferring |
| Relationship to MT-2 | Linear sibling, shared [Nle⁴,D-Phe⁷] lineage | — | Des-amido metabolite (C-terminal carboxyl) |
| Regulatory status | Approved as Scenesse (EPP) | None | Approved as Vyleesi (HSDD) |
The relationships are worth stating precisely. Melanotan I is the linear molecule [Nle⁴,D-Phe⁷]-α-MSH, also called NDP-MSH, and it is the one approved as afamelanotide (Scenesse) for erythropoietic protoporphyria. PT-141, bremelanotide, is a metabolite of Melanotan-2 in which the C-terminal amide has been hydrolyzed to a carboxylic acid; that single change shifts its selectivity toward MC4R, and it is marketed as Vyleesi. All three trace back to the same Arizona α-MSH program.
Published research scope
The Melanotan-2 literature is predominantly preclinical, with a small amount of early human pharmacology. The orientation points:
- Origin pharmacology (Arizona, late 1980s–1990s). Hruby and Hadley's group designed the cyclic [Nle⁴,D-Phe⁷] analogs as superpotent melanotropins; Dorr and colleagues reported the early pharmacology and a phase 1 pilot.
- Melanogenesis models. MC1R-driven eumelanin synthesis in cultured melanocytes and rodent models, frequently UV-independent, is the most-studied endpoint.
- Central melanocortin signaling. MC4R and MC3R work on energy balance and behavior in rodent models, independent of the pigment axis.
Controlled human data are limited and the compound never advanced to an approved product, unlike its two narrower cousins. Researchers should treat it as a non-selective melanocortin probe and design controls for the multi-receptor activity accordingly. As with every compound in this library, the findings above are summarized observationally from the published record and are not treatment guidance.
Stability and handling
Two structural facts set the handling rules. First, the tryptophan at the back of the ring is photolabile: the indole side chain absorbs in the near-UV and degrades under light and oxygen, so light protection is the primary storage concern. Second, the absence of both cysteine and methionine means there is no disulfide to scramble and no methionine to oxidize, which removes two of the degradation pathways that constrain many peptides.
- Lyophilized, sealed: stable for the standard peptide window (roughly 24 months refrigerated, 36+ months at −20°C) with light protection.
- Reconstituted in bacteriostatic water: 2–8°C, protected from light, used within a few weeks. Aliquot and freeze for longer holds.
Among compounds in the HelixCore catalog, Melanotan-2 sits at the top of the photosensitivity hierarchy precisely because of that tryptophan, a point made in the storage and stability article. Amber vials or foil-wrapping during refrigerated storage, and not leaving reconstituted material under bench lighting between withdrawals, are the practical countermeasures. The general mechanics (diluent against the glass wall, swirl rather than shake) are in the reconstitution guide.
Common analytical and handling notes
On reverse-phase HPLC Melanotan-2 elutes as a single peak; the cyclic structure and the aromatic residues give it reasonable retention under a standard water/acetonitrile gradient. The most diagnostic check is mass: the observed mass on ESI-MS should match 1024.18 g/mol for the cyclic, acetylated, amidated molecule. A few mass shifts are worth recognizing: a roughly 18 Da increase points to hydrolytic opening of the lactam ring, while a roughly 1 Da increase points to des-amidation of the C-terminal amide to a carboxylic acid, the change that produces the PT-141-type species. The CoA reading cues are in our CoA guide, and every lot we release publishes its report in the open CoA library.
Common errors to avoid:
- Calling the ring a disulfide. It is a lactam (an amide bond). No cysteine, no sulfur, no reducing-agent sensitivity.
- Confusing it with Melanotan I. Melanotan I (afamelanotide) is linear and MC1R-predominant; Melanotan-2 is cyclic and non-selective.
- Skipping light protection. The tryptophan makes it one of the more photolabile peptides in the catalog. Amber or foil, always.
- Quoting the 33-hour half-life. It is an unreliable figure. The plasma half-life is short.
- Designing single-receptor experiments. Melanotan-2 hits MC1R, MC3R, MC4R, and MC5R together. Controls have to account for all four.
Where to find primary literature
PubMed indexes the record under "melanotan II" and "MT-II"; the foundational design and pharmacology work appears under the Hruby and Hadley names and the Dorr phase 1 report. For the two related molecules, search "afamelanotide" (the linear Melanotan I) and "bremelanotide" or "PT-141" (the MC4R-preferring metabolite). The melanocortin-receptor reviews are the entry point for the His-Phe-Arg-Trp pharmacophore. HelixCore supplies Melanotan-2 as a 10 mg lyophilized vial, paired with bacteriostatic water for reconstitution.
