One is linear, selective and a licensed medicine under another name. The other is cyclic, non-selective and never licensed. The structural and regulatory differences explained.
Melanotan I and Melanotan II are both synthetic analogues of alpha-melanocyte-stimulating hormone. They were made in the same Arizona laboratory and they share a name. In structure, receptor selectivity and regulatory history they are different molecules.
| Property | Melanotan I | Melanotan II |
|---|---|---|
| Other names | Afamelanotide, [Nle4, D-Phe7]-alpha-MSH | MT-2, MT-II |
| Structure | Linear, 13 residues | Cyclic, 7 residues, lactam bridge |
| Molecular weight | About 1647 Da | About 1024 Da |
| Receptor profile | MC1R-preferring | MC1R, MC3R, MC4R, MC5R |
| Licensed anywhere | Yes, as Scenesse (implant) | No |
Melanotan I keeps the full length of the natural hormone and makes two substitutions: norleucine for methionine at position 4 and D-phenylalanine for L-phenylalanine at position 7. These two changes make it far more stable than alpha-MSH while leaving the linear chain intact.
Melanotan II goes further. It cuts the chain down to the core residues that bind the receptor and closes them into a ring with a lactam bridge. A shorter, rigid ring binds more tightly and survives longer, and it also loses the selectivity that the longer chain carries.
This is the difference that matters pharmacologically. Melanotan I acts mainly at MC1R, the receptor on melanocytes. Melanotan II acts at MC1R and also at MC3R, MC4R and MC5R, which are found in the brain and in other tissues. The reported effects of the two peptides in animal and early human studies differ accordingly, and so do the adverse events reported for each. The full receptor table is on the Melanotan II identity page.
Afamelanotide, the pharmaceutical name for Melanotan I, was taken through clinical development by Clinuvel and holds a marketing authorisation in the EU and the UK as Scenesse, a subcutaneous implant indicated for adults with erythropoietic protoporphyria. That is a specific product, strength and indication, and it describes nothing about a research vial.
Melanotan II was studied in the 1990s and was not taken to authorisation. It has no licensed product anywhere. Its metabolite bremelanotide was developed as a separate medicine.
The phrase appears in the search data, usually with "reddit" attached. For a laboratory, neither is better. They are two reagents with different receptor profiles, and the choice depends on which receptor question is being asked. For anyone considering them for personal use, both are unlicensed for that purpose in the UK, and the MHRA has warned specifically about melanotan products.
The catalogue item is the Melanotan II 10mg research vial, with a batch certificate in the COA library. Melanotan I is not currently listed. Reports of adverse events associated with unlicensed Melanotan II products are summarised in Melanotan II adverse effects.
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What Melanotan II is, how it relates to alpha-MSH, which melanocortin receptors it binds, and why it has never been a licensed medicine anywhere.
Research DocumentationThe adverse events reported in early studies and in case reports of unlicensed Melanotan II use, with the evidence tier of each, and what cannot be concluded from them.
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