What Has Published Research Reported About Melanotan II?

What is melanotan II?

Melanotan II (MT-II, also written MTII) is a synthetic cyclic peptide built from a fragment of the hormone alpha-melanocyte-stimulating hormone (alpha-MSH). The title of a 1995 Journal of Medicinal Chemistry paper writes the parent structure as Ac-Nle4-cyclo[Asp5, D-Phe7, Lys10] alpha-MSH-(4-10)-NH2 (source 3). Read literally, that is residues 4 to 10 of alpha-MSH, which makes seven residues, with an acetylated amino terminus, a carboxyl-terminal amide, substitutions at positions 4, 5, 7 and 10, and a ring closed between the side chains at positions 5 and 10. A 2006 review by Hadley and Dorr described melanotan II as a cyclic truncated peptide, set it beside the linear melanotan I, and noted that both had been patented and tested clinically (source 4).

PubChem lists the molecular formula as C50H69N15O9 with an average molecular weight of 1024.2 (source 13). From that formula, a monoisotopic mass of 1023.54 can be calculated, which is a calculated figure and not one reported in a cited paper. This Steadfast Research Group profile covers the receptor, cell, rodent and analytical literature. Clinical publications exist (source 4) and are outside its scope. General background is in the overview of research peptides, and a related cyclic analog is profiled in What Has Published Research Reported About PT-141?.

How did melanotan II emerge from alpha-MSH research?

The line of work began with changes to the natural hormone. In 1980, Sawyer, Hruby, Hadley and colleagues described 4-norleucine, 7-D-phenylalanine alpha-MSH, which the authors reported to be about 26 times as potent as alpha-MSH in an adenylate cyclase assay and resistant to breakdown by serum enzymes. They also reported much higher activity in a frog skin bioassay, in activation of adenylate cyclase from mouse melanoma, and in stimulation of tyrosinase in mouse melanoma cells (source 1). The two substitutions, norleucine at position 4 and D-phenylalanine at position 7, are still visible in the 1995 title (source 3).

A 1989 paper by Al-Obeidi, Castrucci, Hadley and colleagues belongs to the next step. Its title described potent and prolonged-acting cyclic lactam analogs of alpha-melanotropin designed on the basis of molecular dynamics (source 2). A lactam is an amide bond formed between side chains, and the cyclo[Asp5, Lys10] notation in the 1995 title (source 3) describes that kind of ring. In the 2006 review, melanotan II was one of two analogs followed from the laboratory into clinical studies and commercialization (source 4).

Which melanocortin receptors does melanotan II act on?

Cone’s 2006 review described the melanocortin system as a set of hormonal, neuropeptidergic and paracrine pathways with five G protein-coupled receptors, peptide agonists derived from proopiomelanocortin, and the antagonists agouti and agouti-related protein. It listed the functions regulated as pigmentation, adrenocortical steroid production, energy homeostasis, natriuresis, erectile responses and exocrine gland secretion (source 5).

Within that system, melanotan II was described as a relatively nonselective agonist. Molden and colleagues, writing in Molecular Endocrinology in 2015, grouped it with other MC4R agonists on that basis. In N2A cells expressing a tagged MC4R, they reported that melanotan II was a more potent agonist than alpha-MSH, with an EC50 of about 0.5 nM against about 14 nM for alpha-MSH (source 6). That is a cell-line result in one expression system, and EC50 values depend on the assay.

The same peptide scaffold produced antagonists. In the 1995 paper, Hruby and colleagues reported that replacing D-Phe at position 7 with bulky aromatic residues gave analogs with high antagonist potency and selectivity at specific melanocortin receptors. The D-2-naphthylalanine analog, SHU9119, was reported as a potent MC4 antagonist (pA2 9.3) and a weaker MC3 antagonist (pA2 8.3) that still acted as an agonist at MC1 and MC5. The D-p-iodophenylalanine analog was reported as an MC1 agonist and an MC3 and MC4 antagonist, whereas the D-p-chloro and D-p-fluoro analogs lacked antagonist activity (source 3).

What did rodent studies report?

Much of the animal literature delivered melanotan II directly into the brain. Fan, Boston, Kesterson, Hruby and Cone published a 1997 Nature paper on melanocortinergic neurons in feeding and the agouti obesity syndrome (source 7). Later papers cite that report for a dose-related suppression of food intake after melanotan II was delivered into the cerebral ventricles of rodents.

A 1998 study by Grill and colleagues tested melanotan II and SHU9119 delivered to the fourth ventricle, the ventricle adjacent to the caudal brainstem. The authors reported that the dose-response curves for melanotan II on food intake, body weight and water consumption did not differ significantly from those obtained with lateral ventricle placement. Over the 24 hours after injection, melanotan II lowered intake and body weight, while SHU9119 produced the opposite change. For melanotan II, no effect was seen beyond that period, whereas a single SHU9119 injection continued to augment daily food intake (source 8).

In 2005, Heijboer and colleagues infused melanotan II into the lateral ventricle of male C57Bl/6 mice for 24 hours and measured insulin sensitivity with hyperinsulinaemic-euglycaemic clamps. Basal endogenous glucose production and glucose disposal were higher in the treated mice, glucose disposal during hyperinsulinaemia was higher, and muscle Glut4 mRNA rose. The authors concluded that central MC3 and MC4 receptor stimulation appeared to affect insulin sensitivity in a tissue-specific way that was separate from its effects on feeding and body weight (source 9).

How have analytical laboratories characterized melanotan II material?

Several analytical papers examined melanotan II sold outside regulated channels. In 2015, Breindahl and colleagues validated an LC-UV method, with detection at 218 nm, and an MS/MS method that fragmented the doubly charged [M+2H]2+ precursor ion at m/z 513, and applied them to vials bought from three online shops. Each shop claimed 10 mg of melanotan II per vial. The measured amounts ranged from 4.32 to 8.84 mg, vials from two shops contained unknown impurities of 4.1 to 5.9 percent, and the impurities from the third shop were below the quantification limit (source 10). The precursor mass fits the formula in source 13, since a peptide of about 1024 daltons carrying two protons gives a mass-to-charge ratio near 513. That is a calculation, not a figure from the paper.

A 2024 forensic paper by Deville and colleagues described the difficulty of identifying the powder in two vials labeled melanotan II. A library search of the HPLC-DAD data returned no match, mass spectrometry was complicated by a molecular weight of about 1024 Da and multiple charging, and comparison with a reference standard confirmed the identity and indicated that the material was only about 30 percent pure (source 11). Langan, Nie and Rhodes, in a 2010 review, noted that the UK Medicines and Healthcare products Regulatory Authority had raised awareness of the potential impurity of unregulated melanotropic peptide products bought online (source 12).

These papers describe particular products from particular sources, and they do not generalize to other material. For a research laboratory they illustrate why identity and purity are confirmed by independent methods, and why a result belongs to one lot. The guides to reading a peptide COA, mass spectrometry verification and HPLC purity testing cover those checks.

What terms recur in the melanotan II literature?

Alpha-MSH

The natural melanocortin peptide hormone from which melanotan II was derived (sources 3 and 5).

Lactam bridge

A side-chain amide bond that closes a peptide ring, written as cyclo[Asp5, Lys10] in the parent structure (sources 2 and 3).

MC1R through MC5R

The five melanocortin receptor subtypes of the melanocortin system (source 5).

EC50

The concentration giving half of the maximal response in a functional assay, reported as about 0.5 nM for melanotan II at MC4R in one cell study (source 6).

pA2

A measure of antagonist potency, reported as 9.3 at MC4 and 8.3 at MC3 for SHU9119 (source 3).

Hyperinsulinaemic-euglycaemic clamp

A laboratory technique for measuring insulin sensitivity by holding glucose steady while insulin is infused, used in the 2005 mouse study (source 9).

What remains unresolved in the melanotan II literature?

Several points stay open. The sources reviewed did not give a complete, directly comparable set of binding constants for melanotan II across MC1R to MC5R, so this profile prints only the one potency figure it could attribute to a specific paper (source 6). The 1997 feeding paper is described here through later citations, and its own methods and figures should be read directly by anyone relying on them (source 7). Most animal findings above used delivery into the brain ventricles, so they say little about other routes or tissues.

Independence is also limited. The authors of the 2006 review disclosed financial ties to companies that commercialized the analogs (source 4), and the 2010 review authors disclosed ties to the maker of a related melanocortin analog (source 12). The analytical papers (sources 10 and 11) report on unregulated products, so their purity figures describe those products and not research material in general.

How can a laboratory confirm the identity of melanotan II?

Identity can be checked against the formula C50H69N15O9 and an average molecular weight of about 1024.2 (source 13), and a doubly charged ion near m/z 513 was the precursor used in the 2015 method (source 10). A related cyclic analog described on this site, PT-141, ends in a free acid and not an amide, so the two calculate about 1 dalton apart. This is a calculated difference, not a figure from a cited paper, and a mass spectrometer can resolve it. Purity and peptide content should be reported separately, and a Certificate of Analysis should state which lot it covers.