Melanotan 2 and the Melanocortin Receptor Family: A Research Overview

Melanotan 2 (MT-II) is a synthetic cyclic heptapeptide analog of alpha-melanocyte-stimulating hormone (α-MSH), the endogenous melanocortin-system peptide derived from proopiomelanocortin (POMC). MT-II was developed in the 1980s by Hadley, Hruby, and colleagues at the University of Arizona as part of a structure-activity research program on alpha-MSH analogs, with the design objective of producing a potent, metabolically-stable agonist of the melanocortin receptor family suitable for research applications in pigmentation biology, central energy regulation, and melanocortin pharmacology.

This article is a research-context overview of MT-II. It covers the melanocortin receptor family, the structural design of MT-II, the published animal-research literature, and the role of MT-II as a research probe in melanocortin pharmacology. The framing throughout is research-use only — MT-II is sold for laboratory and animal-research applications, not for any clinical or human-use purpose.

The melanocortin receptor family

The melanocortin system comprises five characterized G-protein-coupled receptors — MC1R, MC2R, MC3R, MC4R, and MC5R — each with distinct tissue distributions and physiological roles:

MC1R is expressed primarily on melanocytes in the skin and in the substantia nigra. Activation drives melanin synthesis (the eumelanin / pheomelanin switch) and is the receptor responsible for the pigmentation response. Variants of MC1R are associated with the red-hair / fair-skin phenotype in humans.
MC2R is the adrenocorticotropic hormone (ACTH) receptor in the adrenal cortex and mediates glucocorticoid synthesis. It is the only melanocortin receptor that is selective for ACTH rather than α-MSH.
MC3R is expressed in the central nervous system (hypothalamus, limbic system) and contributes to energy-homeostasis regulation; the precise role is an active area of research.
MC4R is also central-nervous-system expressed and is the dominant melanocortin receptor in the appetite and energy-balance circuitry. MC4R is the receptor whose loss-of-function variants are the single most common monogenic cause of severe obesity in humans, characterized in Vaisse C, et al. and Yeo GSH, et al.
MC5R is more broadly expressed in peripheral tissues (exocrine glands, immune cells) and has less-well-characterized physiological roles.

All five receptors signal through Gs and cyclic AMP. The endogenous melanocortin agonists are derived from POMC processing in the pituitary and brain: α-MSH (the primary natural agonist of MC1R, MC3R, MC4R, and MC5R), β-MSH, γ-MSH, and ACTH.

A foundational review of the receptor pharmacology is Cone RD, “Anatomy and regulation of the central melanocortin system,” Nature Neuroscience 8(5):571–578, 2005 (DOI: 10.1038/nn1455).

MT-II: structure and design rationale

MT-II is a cyclic seven-residue peptide with the sequence Ac-Nle-cyclo[Asp-His-D-Phe-Arg-Trp-Lys]-OH. The CAS registry number is 121062-08-6; its molecular weight is approximately 1024 Da and its empirical formula is C₅₀H₆₉N₁₅O₉.

The design rationale combines four key modifications of the parent α-MSH core (His-Phe-Arg-Trp, the minimal active sequence of α-MSH):

Acetyl-Nle at the N-terminus to block aminopeptidase degradation and confer increased lipophilicity.
D-Phe substitution at position 7, which both increases receptor-binding affinity and confers proteolytic stability.
Cyclization via a Asp-Lys lactam bridge to constrain the active conformation and improve receptor affinity.
Amide capping at the C-terminus for additional stability.

The result is a cyclic heptapeptide that retains potent agonist activity at MC1R, MC3R, MC4R, and MC5R (but not MC2R, the ACTH-selective receptor), with substantially greater metabolic stability than α-MSH itself. The original design and characterization is in Al-Obeidi F, Castrucci AM, Hadley ME, Hruby VJ, “Potent and prolonged acting cyclic lactam analogues of alpha-melanotropin: design based on molecular dynamics,” Journal of Medicinal Chemistry 32(12):2555–2561, 1989 (DOI: 10.1021/jm00132a010).

MT-II is described as a non-selective melanocortin agonist — it binds and activates the four α-MSH-responsive receptors with broadly comparable affinity. This non-selectivity is the basis for its multiple reported activities in animal research: melanogenesis (MC1R), central energy regulation (MC3R, MC4R), and peripheral effects (MC5R).

Published animal-research findings

Reported activities in published animal-research and cell-culture investigations include:

MC1R-mediated melanogenesis — animal-research and cell-culture studies have reported stimulation of tyrosinase activity and melanin synthesis in melanocytes following MT-II exposure, with the response characterized as eumelanin-biased. This is the basis for MT-II’s role as a research probe in pigmentation biology.
MC4R-mediated central activity — published animal-model investigations of central administration of MT-II have reported effects on food intake and energy expenditure consistent with MC4R agonism. The MC4R-knockout phenotype (obesity) and the MC4R-agonist phenotype (decreased food intake in animal models) provide the physiological context for these findings. See Marsh DJ, et al., “Response of melanocortin-4 receptor-deficient mice to anorectic and orexigenic peptides,” Nature Genetics 21(1):119–122, 1999 (DOI: 10.1038/5070).
MC3R-mediated peripheral effects — the published research on MC3R-mediated activity is less well-characterized than MC1R or MC4R but includes reports of MC3R-relevant effects on energy partitioning in animal models.

MT-II has also been used as the parent research compound from which the selective MC4R agonist bremelanotide (PT-141) was developed via N-terminal modification and acetylation changes. Bremelanotide / PT-141 is available from most research-supply vendors as a separate research compound.

MT-II vs. PT-141 vs. α-MSH

For research applications in the melanocortin pathway, the relevant compound choice depends on the research question:

α-MSH is the natural agonist; metabolically unstable; useful only for short-duration in-vitro work.
MT-II is the non-selective stable analog; broadly active at MC1R, MC3R, MC4R, MC5R; useful for melanogenesis research and for general melanocortin-pathway probing.
PT-141 (bremelanotide) is the more MC4R-selective analog derived from MT-II; useful for research selectively probing the MC4R pathway.

A research design comparing the pharmacological profiles of these three compounds is a standard way of dissecting the melanocortin-receptor contributions to a given biological response.

Reading an MT-II CoA

A research-grade MT-II CoA should document the molecular weight (≈1024 Da, consistent with the cyclic heptapeptide structure), HPLC purity ≥99.0%, mass spectrometry confirmation of the [M+H]⁺ ion at m/z ≈ 1025 (single charge) or the [M+2H]²⁺ at m/z ≈ 513 (double charge), and net peptide content. The CoA should also identify the cyclic-lactam linkage by describing the synthesis route or by including 2D structural confirmation. The acetylated N-terminus and the amidated C-terminus should both be documented.

For a step-by-step guide on reading a peptide CoA, see article on how to read a peptide Certificate of Analysis.

Storage notes

MT-II is supplied as a lyophilized powder. Storage protocols:

Lyophilized vial: -20°C, dry, protected from light. The cyclic heptapeptide structure is relatively robust; long-term stability is good under proper storage.
Reconstituted: 2-8°C in sterile bacteriostatic water. Reconstituted MT-II has typical refrigerated stability of several weeks.
For long-term reconstituted storage: aliquot, freeze at -20°C, thaw each aliquot once.

For the full reconstitution-and-storage protocol, see the lyophilized vs. reconstituted peptide storage article.

Summary

Melanotan 2 (MT-II) is a synthetic cyclic heptapeptide non-selective agonist of the melanocortin receptor family (MC1R, MC3R, MC4R, MC5R). It was developed in the 1980s as a metabolically-stable α-MSH analog and has been the workhorse research probe in melanocortin pharmacology for over three decades. The published animal-research literature spans pigmentation biology (MC1R), central energy regulation (MC3R, MC4R), and peripheral melanocortin effects (MC5R). MT-II is also the parent compound from which the MC4R-selective analog bremelanotide (PT-141) was derived. As a research-use compound, MT-II is supplied lyophilized and is intended for laboratory and animal-research investigation of the melanocortin pathway.

Selected sources

Cone RD. “Anatomy and regulation of the central melanocortin system.” Nature Neuroscience 8(5):571–578, 2005. DOI: 10.1038/nn1455.
Al-Obeidi F, Castrucci AM, Hadley ME, Hruby VJ. “Potent and prolonged acting cyclic lactam analogues of alpha-melanotropin: design based on molecular dynamics.” Journal of Medicinal Chemistry 32(12):2555–2561, 1989. DOI: 10.1021/jm00132a010.
Marsh DJ, Hollopeter G, Huszar D, Laufer R, Yagaloff KA, Fisher SL, Burn P, Palmiter RD. “Response of melanocortin-4 receptor-deficient mice to anorectic and orexigenic peptides.” Nature Genetics 21(1):119–122, 1999. DOI: 10.1038/5070.
Hadley ME, Dorr RT. “Melanocortin peptide therapeutics: historical milestones, clinical studies and commercialization.” Peptides 27(4):921–930, 2006. DOI: 10.1016/j.peptides.2005.01.029.
Wessells H, Levine N, Hadley ME, Dorr R, Hruby V. “Melanocortin receptor agonists, penile erection, and sexual motivation: human studies with Melanotan II.” International Journal of Impotence Research 12 Suppl 4:S74–S79, 2000. DOI: 10.1038/sj.ijir.3900582.

Research Use Only — Disclaimer

The research peptides discussed on this page are described for laboratory and research purposes only. They are intended exclusively for in vitro experimentation and for use in animal studies under appropriate institutional oversight. They are not drugs, dietary supplements, cosmetics, or food additives. They are not for human consumption, not for veterinary use in companion animals, and not for any therapeutic, diagnostic, preventive, or palliative purpose.

Nothing on this page constitutes medical advice. References to historical clinical-research studies on MT-II are included as part of the published research record and do not constitute a recommendation, claim, or representation that the compound is safe, effective, or appropriate for any human use.

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