Melanotan I and Melanotan II: Alpha-MSH Analogs in the Research Literature

Research-use-only context. This article summarizes published third-party scientific literature and laboratory practice. It is not medical advice, not a therapeutic or performance claim, and not a usage guide. US Prime Peptides products are supplied strictly for in vitro laboratory research and are not for human or veterinary use.

This article explains what Melanotan I and Melanotan II are at the level of sequence and ring chemistry, what the receptor-binding and cell literature reports for each, and how their regulatory paths diverged: one became afamelanotide, approved by the FDA as Scenesse in 2019, and the other was never approved anywhere. It closes with what a research certificate should show for each molecule.

Two different molecules that share a name

Melanotan I and Melanotan II are both synthetic analogs of alpha-melanocyte-stimulating hormone (alpha-MSH), but they are structurally distinct compounds: Melanotan I is a linear 13-residue peptide and Melanotan II is a 7-residue cyclic lactam. The shared name comes from the University of Arizona program that designed both in the 1980s, not from any chemical identity.

Native alpha-MSH is Ac-Ser-Tyr-Ser-Met-Glu-His-Phe-Arg-Trp-Gly-Lys-Pro-Val-NH2. Melanotan I keeps all thirteen positions and changes two: norleucine replaces the oxidation-prone methionine at position 4, and D-phenylalanine replaces L-phenylalanine at position 7. The literature calls it [Nle4,D-Phe7]-alpha-MSH or NDP-MSH, and its International Nonproprietary Name is afamelanotide. Melanotan II discards the first three and last three residues, keeps the Nle4 and D-Phe7 changes, and closes the remaining heptapeptide into a ring through a lactam between the side chains of Asp5 and Lys10: Ac-Nle-cyclo[Asp-His-D-Phe-Arg-Trp-Lys]-NH2.

The family tree does not stop at Melanotan II. Bremelanotide, approved by the FDA as Vyleesi in 2019, is the same ring with a C-terminal carboxylate in place of the amide; it has its own PT-141 overview, and all three are compared in Melanotan I vs Melanotan II vs PT-141.

Where the two analogs came from: the Hruby and Hadley program

Both molecules were designed by Victor Hruby and Mac Hadley's group as "superpotent" melanotropins: analogs that activate pigment-cell receptors at far lower concentrations than the native hormone and keep acting after the peptide is washed away. The founding observation for Melanotan I is a 1985 Endocrine Research paper by Hadley and colleagues in Cloudman S91 mouse melanoma cells. The analog raised tyrosinase activity at a minimum effective concentration one hundred times lower than alpha-MSH, and after a 24-hour exposure the response persisted for 72 hours after the peptide was removed, which is why the authors called the activation "irreversible." A 1988 Investigational New Drugs paper by Dorr and colleagues added a mouse toxicology study of the same analog and recorded its slow biodegradation in vivo, a direct consequence of the Nle4 and D-Phe7 substitutions.

Melanotan II appears in a 1989 Journal of Medicinal Chemistry paper by Al-Obeidi, Castrucci, Hadley and Hruby. Starting from the linear analog and molecular-dynamics models of the active conformation, the group synthesized seven cyclic lactam fragments and measured them in the classical frog (Rana pipiens) and lizard (Anolis carolinensis) skin bioassays. The Asp5-Lys10 analogs with a 23-membered ring were about 90 to 100 times as potent as alpha-MSH in lizard skin and showed prolonged residual activity; enlarging or shrinking the ring reduced potency. The 4-10 heptapeptide with that ring is Melanotan II, and the model systems behind it are amphibian and reptile chromatophores and a mouse melanoma line.

Receptor pharmacology in cloned-receptor systems

Radioligand work with the linear analog defined the affinity ladder across melanocortin receptor subtypes. Schioth, Muceniece, Wikberg and Chhajlani, in a 1995 European Journal of Pharmacology paper, expressed the MC1, MC3 and MC5 receptors individually in COS cells and used iodinated [Nle4,D-Phe7]-alpha-MSH as the tracer. It bound a single saturable site with dissociation constants of about 85 pM at MC1, 396 pM at MC3 and 5 nM at MC5, and it became the standard tracer for the receptor family.

Yang and colleagues, in a 1997 Journal of Biological Chemistry paper, mapped the human MC1R residues that matter for both analogs. Mutating the acidic residues Glu94 in transmembrane helix 2 and Asp117 or Asp121 in helix 3 changed the affinity and potency of NDP-MSH, Melanotan II, alpha-MSH and gamma-MSH alike, and mutating aromatic residues in helices 4 to 6 implicated aromatic contacts as well. The shared His-Phe-Arg-Trp core of all four ligands sits in that pocket.

Melanotan II's lack of subtype selectivity is documented, not inferred. Tomassi and colleagues, in a 2022 Journal of Medicinal Chemistry paper, describe MT-II as a potent and unselective agonist of the human melanocortin receptors and used it as the starting point for a series in which the lactam bridge was replaced by xylene-derived thioether linkers. Affinities ran from low nanomolar to sub-micromolar, and one compound showed functional selectivity for hMC1R that the parent ring lacked.

What the volunteer studies in the literature actually measured

A few small published volunteer studies exist for both analogs, and they should be read as pharmacology papers from a university program, not as evidence for any use of a research vial. Dorr and colleagues reported a pilot phase I study of Melanotan II in a 1996 Life Sciences paper. Three volunteers received graded amounts in an alternating placebo design over two weeks; the authors recorded increased pigmentation by quantitative reflectance in two of the three, along with somnolence, nausea, a stretching-and-yawning complex and spontaneous penile erections that tracked the amount given. That last observation is the origin of the bremelanotide program, as Hadley recounts in a 2005 Peptides paper.

For the linear analog, Dorr and colleagues published a 2000 Photochemistry and Photobiology study in seven volunteers of skin types III and IV. Pigmentation was measured by computerized light reflectance, and forearm shave biopsies were analyzed by HPLC for eumelanin (as the permanganate oxidation product PTCA) and pheomelanin (as the hydroiodic acid digestion product AHP). One week after exposure ended, forearm eumelanin had risen about 98 percent over baseline while pheomelanin did not change. The paper is as useful for its melanin assay methods as for its result.

Afamelanotide: from research analog to Scenesse

Melanotan I is the only alpha-MSH analog that became an approved drug, under the name afamelanotide, and the approved product is a controlled-release implant, not a vial of powder. Clinuvel developed the linear analog for erythropoietic protoporphyria (EPP), a rare photodermatosis. Langendonk and colleagues reported the two pivotal randomized, double-blind, placebo-controlled trials in a 2015 New England Journal of Medicine paper: 74 participants in the European Union and 94 in the United States received implants containing 16 mg of afamelanotide or placebo every 60 days, and the primary endpoint, hours of direct sunlight exposure without pain, was longer in the afamelanotide groups in both studies.

The European Medicines Agency approved afamelanotide in 2014, and the FDA approved it as Scenesse in October 2019 for adults with EPP. Minder, Barman-Aksoezen and Schneider-Yin reviewed the pharmacokinetic history in a 2017 Clinical Pharmacokinetics paper. Spichty and colleagues, in a 2013 Journal of Pharmaceutical and Biomedical Analysis paper, built a cell-based bioassay for neutralizing antibodies using a B16 mouse melanoma line expressing human MC1R; half-maximal concentrations for cAMP formation were about 38.8 nM for alpha-MSH and 10.9 nM for afamelanotide. That design, a cAMP readout in an MC1R-expressing line, is the standard functional test for any research lot of this sequence.

A research vial labeled Melanotan I or afamelanotide is not Scenesse. It is not the implant formulation, it has not passed the release controls of the approved product, and its certificate speaks only to the identity and purity of the powder.

Melanotan II: never approved, and the unregulated-use literature

Melanotan II has never been approved by the FDA, the EMA, the Australian TGA or any other regulator, and the literature on it since the 1990s is dominated by reports about products sold online rather than by new pharmacology. Evans-Brown and colleagues flagged the trend in a 2009 BMJ letter on use of Melanotan I and II in the UK general population. Habbema and colleagues reviewed the problem in a 2017 International Journal of Dermatology paper, collecting case reports of changes in existing moles and new dysplastic nevi, including four reports of melanoma arising in existing moles during or shortly after exposure; they note that a causal link is not established and that multiple national health organizations have issued safety warnings about both melanotans.

Gilhooley, Daly and McKenna took a qualitative approach in a 2021 Dermatology paper, analyzing 623 entries from 205 participants in UK and Irish online forums; the themes included misinformation, preparation practices, sunbed use alongside the peptide, and contaminated or mislabeled material. The FDA, the TGA and the UK MHRA have each published consumer warnings about melanotan products sold on the internet. For a laboratory, the lesson is provenance: the compound in these reports is an uncharacterized street product, and only a lot-matched certificate from an independent laboratory separates a research sample from that description.

What a certificate should show for each molecule

For Melanotan I, identity by mass spectrometry should show a protonated mass near 1,647 Da, usually observed as the doubly and triply charged ions, and HPLC purity should be reported as area percent with the chromatogram attached. The norleucine substitution removes the methionine oxidation peak that complicates native alpha-MSH. The D-phenylalanine at position 7 cannot be distinguished from L-phenylalanine by mass spectrometry, a limitation discussed in LC-MS identity confirmation; only chiral amino acid analysis confirms it, and standard panels do not include it.

For Melanotan II, the useful mass check is the ring. Lactam formation releases one molecule of water, so the cyclic peptide is 18 Da lighter than its uncyclized precursor: a protonated mass near 1,025 Da is consistent with the closed ring, and a peak 18 Da higher indicates uncyclized material that should also appear as a separate HPLC peak. Net peptide content, endotoxin by LAL, sterility and ICP-MS metals complete the panel. US Prime Peptides posts the lot-matched certificate for Melanotan I 10 mg and for Melanotan II 10 mg on each product page before purchase; both ship as lyophilized powder synthesized and lyophilized in the USA. The fragment that keeps only the last three residues of alpha-MSH, and none of the receptor-binding core, is covered in the KPV overview.

Frequently Asked Questions

What is the difference between Melanotan I and Melanotan II?

Melanotan I is a linear 13-residue analog of alpha-MSH with two substitutions (Nle4, D-Phe7) and is the same molecule as afamelanotide. Melanotan II is a 7-residue cyclic lactam that keeps the His-D-Phe-Arg-Trp core and closes a ring between Asp5 and Lys10. They differ in size, shape, receptor selectivity and regulatory status.

Is Melanotan I the same as afamelanotide or Scenesse?

Melanotan I and afamelanotide are the same sequence, [Nle4,D-Phe7]-alpha-MSH. Scenesse is the controlled-release implant containing afamelanotide, approved by the EMA in 2014 and by the FDA in 2019. A research vial of lyophilized Melanotan I is not Scenesse.

Is Melanotan II approved by the FDA?

No. Melanotan II has never been approved by the FDA, the EMA or the Australian TGA, and those agencies have issued public warnings about melanotan products sold online. The only approved compound derived from the Melanotan II ring is bremelanotide, approved by the FDA as Vyleesi in 2019.

Which melanocortin receptors do Melanotan I and II bind?

In cloned-receptor binding studies the linear analog bound MC1R with picomolar affinity and MC3R and MC5R with lower affinity. Melanotan II is described in the medicinal chemistry literature as a potent, non-selective agonist across the human melanocortin receptors, which is why selective analogs have been built from it.

Citations

1. Hadley ME, Abdel Malek ZA, Marwan MM, et al. [Nle4, D-Phe7]-alpha-MSH: a superpotent melanotropin that "irreversibly" activates melanoma tyrosinase. Endocr Res. 1985;11:157-170. PMID 3009169
2. Dorr RT, Dawson BV, al-Obeidi F, et al. Toxicologic studies of a superpotent alpha-melanotropin, [Nle4, D-Phe7]alpha-MSH. Invest New Drugs. 1988;6:251-258. PMID 2852652
3. Al-Obeidi F, Castrucci AM, Hadley ME, Hruby VJ. Potent and prolonged acting cyclic lactam analogues of alpha-melanotropin: design based on molecular dynamics. J Med Chem. 1989;32:2555-2561. PMID 2555512
4. Schioth HB, Muceniece R, Wikberg JE, Chhajlani V. Characterisation of melanocortin receptor subtypes by radioligand binding analysis. Eur J Pharmacol. 1995;288:311-317. PMID 7774675
5. Dorr RT, Lines R, Levine N, et al. Evaluation of melanotan-II, a superpotent cyclic melanotropic peptide in a pilot phase-I clinical study. Life Sci. 1996;58:1777-1784. PMID 8637402
6. Yang YK, Dickinson C, Haskell-Luevano C, Gantz I. Molecular basis for the interaction of [Nle4,D-Phe7]melanocyte stimulating hormone with the human melanocortin-1 receptor. J Biol Chem. 1997;272:23000-23010. PMID 9287296
7. Dorr RT, Dvorakova K, Brooks C, et al. Increased eumelanin expression and tanning is induced by a superpotent melanotropin [Nle4-D-Phe7]-alpha-MSH in humans. Photochem Photobiol. 2000;72:526-532. PMID 11045725
8. Hadley ME. Discovery that a melanocortin regulates sexual functions in male and female humans. Peptides. 2005;26:1687-1689. PMID 15996790
9. Evans-Brown M, Dawson RT, Chandler M, McVeigh J. Use of melanotan I and II in the general population. BMJ. 2009;338:b566. PMID 19224885
10. Spichty R, Balimann M, Barman J, Minder EI. A bioassay for the detection of neutralizing antibodies against the alpha-melanocyte stimulating hormone analog afamelanotide in patients with erythropoietic protoporphyria. J Pharm Biomed Anal. 2013;75:192-198. PMID 23277150
11. Langendonk JG, Balwani M, Anderson KE, et al. Afamelanotide for erythropoietic protoporphyria. N Engl J Med. 2015;373:48-59. PMID 26132941
12. Minder EI, Barman-Aksoezen J, Schneider-Yin X. Pharmacokinetics and pharmacodynamics of afamelanotide and its clinical use in treating dermatologic disorders. Clin Pharmacokinet. 2017;56:815-823. PMID 28063031
13. Habbema L, Halk AB, Neumann M, Bergman W. Risks of unregulated use of alpha-melanocyte-stimulating hormone analogues: a review. Int J Dermatol. 2017;56:975-980. PMID 28266027
14. Gilhooley E, Daly S, McKenna D. Melanotan II user experience: a qualitative study of online discussion forums. Dermatology. 2021;237:995-999. PMID 34464955
15. Tomassi S, Dimmito MP, Cai M, et al. CLIPSing melanotan-II to discover multiple functionally selective hMCR agonists. J Med Chem. 2022;65:4007-4017. PMID 35188390

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