Short answer
What is Melanotan II? Melanotan II (MT-II) is a synthetic cyclic lactam heptapeptide with the sequence Ac-Nle-cyclo[Asp-His-D-Phe-Arg-Trp-Lys]-NH2. It is a non-selective agonist at the melanocortin receptors MC1R, MC3R, MC4R, and MC5R, and it is studied in laboratory research as a potent alpha-MSH analog. It is sold as a lyophilized research powder and is not for human use.
What is Melanotan II?
Melanotan II is a heptapeptide derived from alpha-melanocyte-stimulating hormone (alpha-MSH), the endogenous tridecapeptide that activates the melanocortin receptor family. The molecule was designed in the late 1980s by the laboratory of Victor Hruby at the University of Arizona, working with Fahad Al-Obeidi, Ana Maria Castrucci, and Mac Hadley, as part of a program to create conformationally constrained analogs of alpha-MSH with improved potency, stability, and selectivity [1].
The defining structural feature is a 23-membered lactam ring formed by a side-chain-to-side-chain amide bond between an aspartic acid residue and a lysine residue. This cyclization locks the peptide into a stable beta-turn conformation around the core pharmacophore, which is the tetrapeptide sequence His-Phe-Arg-Trp (HFRW) shared by all melanocortin peptides. Because the ring restricts the number of conformations the molecule can adopt, the analog shows markedly higher receptor potency and a longer duration of action than the linear native peptide in bioassays [1].
Melanotan II is often abbreviated MT-II, and it is the parent compound of an entire family of cyclic lactam melanocortin tool molecules. A closely related analog in which the D-Phe residue is replaced with the bulky aromatic residue D-2-naphthylalanine (D-Nal(2')) behaves as an antagonist at MC3R and MC4R while retaining agonist activity at other subtypes; that compound, SHU-9119, is one of the most widely used melanocortin receptor probes in the literature [2].
Chemical characterization: the cyclic lactam scaffold
The sequence Ac-Nle-cyclo[Asp-His-D-Phe-Arg-Trp-Lys]-NH2 consists of seven residues closed into a ring by a lactam bridge between the side chains of aspartate and lysine. The N-terminus is capped with an acetyl group, and the C-terminus carries a carboxamide. The norleucine (Nle) at position 4 replaces the native methionine of alpha-MSH and is used because it resists oxidation, which improves stability during storage and assay work [1].
The design was guided by molecular-dynamics simulations of the alpha-MSH pharmacophore. In the original 1989 paper, the authors synthesized a panel of cyclic analogs of alpha-MSH fragments and measured their activity in melanophore-based bioassays. The 23-membered ring analogs were the most potent, with some compounds roughly 100-fold more potent than native alpha-MSH in the lizard melanophore assay, and they also showed prolonged residual activity after the peptide was washed out of the assay [1].
The work established an important principle for the field: restricting the conformational flexibility of the message sequence can increase both affinity and intrinsic activity at melanocortin receptors, and the resulting cyclic peptides are far more resistant to proteolytic degradation than their linear parents [1][4].
MC receptor research: receptor pharmacology
Melanotan II is studied because it is a broad-spectrum agonist across the melanocortin receptor family. There are five known melanocortin receptors, MC1R through MC5R, all of them G-protein-coupled receptors that are activated by the POMC-derived peptides alpha-MSH, beta-MSH, gamma-MSH, and adrenocorticotropic hormone (ACTH) [3].
Melanotan II activates MC1R, MC3R, MC4R, and MC5R but not MC2R. As an alpha-MSH analog, it carries the HFRW core that is the minimal message sequence recognized by these receptors, and the lactam scaffold constrains that message into its bioactive conformation. In practice, this makes MT-II a useful non-selective tool for probing melanocortin signaling in cell-based assays and in isolated receptor preparations [1][4].
The distinction between selectivity and potency matters for experimental design. Because MT-II engages four of the five receptor subtypes, a study that observes an MT-II-dependent response cannot attribute that response to a single receptor without additional experiments, for example using subtype-selective antagonists such as SHU-9119 or receptor-specific cell lines [2][4].
Research history
The history of Melanotan II tracks the development of synthetic melanocortin pharmacology. The 1989 report by Al-Obeidi and colleagues introduced the cyclic lactam approach and reported the potent, long-acting analogs that became the template for the whole family [1]. In 1995, the same research program reported that replacing D-Phe7 with a bulky aromatic residue converted the agonist scaffold into antagonists at MC3R and MC4R, demonstrating that the cyclic lactam ring could tolerate systematic modification at the message sequence without destroying receptor recognition [2].
Melanotan II itself went on to become a benchmark reference compound in melanocortin research. It has been used to study the physiological roles of the melanocortin system across energy homeostasis, cardiovascular control, and neuroendocrine circuits. A comprehensive review of the field by Cone and colleagues synthesizes the cloning, pharmacology, and physiology of the five receptors and positions synthetic agonists such as MT-II as key reagents for dissecting the system [3].
More recently, the design principles established with MT-II have been revisited in reviews of cyclic peptide design, where the compound is cited as a canonical example of how conformational constraint can be used to engineer peptide tool molecules with improved pharmacology [4].
What research shows about MT-II as a research tool
When Melanotan II is used as a reagent, the published literature supports several reproducible findings that researchers rely on:
For laboratories working on melanocortin receptor biology, MT-II is therefore most valuable as a broad-spectrum positive control and as a starting point for structure-activity comparisons rather than as a subtype-selective probe.
Melanotan II compared with related research peptides
Melanotan II sits in a family of melanocortin-pathway research peptides, and it is important to distinguish the members:
Choosing between MT-II and PT-141 in a melanocortin project generally comes down to whether the experiment calls for a broad-spectrum agonist (MT-II) or a compound with a more MC3R/MC4R-centric profile and a distinct body of clinical-research background (PT-141).
For a broader introduction to how peptides like these are classified and used in the laboratory, see our what are peptides guide.
How to evaluate research-grade Melanotan II
When sourcing Melanotan II for laboratory work, the quality markers are the same as for any research peptide:
For a detailed look at what a peptide COA should contain, see our peptide COA guide, and for a comparison of the analytical methods used to verify purity, our article on HPLC versus mass spectrometry for peptide purity.
Storage and handling
Lyophilized Melanotan II is stable when stored correctly. Standard laboratory handling guidance applies:
For a step-by-step protocol, see our peptide reconstitution and laboratory handling guide, and for general storage recommendations, our how to store peptides guide.
Summary
Melanotan II is a well-characterized cyclic lactam heptapeptide and a non-selective agonist at MC1R, MC3R, MC4R, and MC5R. It was developed through molecular-dynamics-guided design of alpha-MSH analogs and remains a benchmark reagent in melanocortin receptor research. Because it is a broad-spectrum agonist, it is best used as a positive control and structural template rather than a subtype-selective probe. Research-grade material should be verified for HPLC purity, mass-spectrometry identity, and batch-specific COA, and handled according to standard lyophilized-peptide protocols.
FAQ
What is Melanotan II?
Melanotan II is a synthetic cyclic lactam heptapeptide (Ac-Nle-cyclo[Asp-His-D-Phe-Arg-Trp-Lys]-NH2) studied as a non-selective melanocortin receptor agonist.
Which melanocortin receptors does Melanotan II activate?
It activates MC1R, MC3R, MC4R, and MC5R but not the ACTH receptor MC2R.
What makes Melanotan II cyclic?
A lactam bridge between the side chains of aspartate and lysine closes the seven-residue peptide into a 23-membered ring that constrains the bioactive conformation.
What purity should research-grade Melanotan II have?
At least 98-99% by HPLC, with identity confirmed by mass spectrometry and a batch-specific COA.