Melanotan I, Melanotan II and PT-141: Receptor Selectivity Is the Whole Story

Melanotan I, Melanotan II and PT-141 are routinely lumped together as “the melanotan peptides.” They are related — all three are analogues of α-melanocyte-stimulating hormone — but the differences between them are not cosmetic. They come down to which of the five melanocortin receptors each one actually engages, and in one case the entire difference is a single chemical group at the end of the molecule. This guide puts Melanotan 2 vs PT-141 side by side, with Melanotan I included for context, because the three are constantly confused.

Five receptors, and what each one does

Melanocortin signalling runs through five G-protein-coupled receptors. They sit in different tissues and do entirely different jobs, which is why receptor selectivity determines everything about a given analogue’s profile.

Note the MC2R line. It is a common misconception that a broad melanocortin agonist will stimulate cortisol release; MC2R has a binding requirement that α-MSH-derived analogues do not satisfy.

Melanotan I — the selective analogue

Melanotan I, developed as afamelanotide, is a linear 13-residue analogue: Ac-Ser-Tyr-Ser-Nle-Glu-His-D-Phe-Arg-Trp-Gly-Lys-Pro-Val-NH₂. It keeps close to the natural α-MSH structure and binds predominantly to MC1R.

It is the only one of the three approved as a pigmentation-related therapeutic. The EMA authorised it as Scenesse in January 2015, and the FDA followed in October 2019, in both cases for increasing pain-free light exposure in adults with erythropoietic protoporphyria. It is delivered as a subcutaneous implant with a slow-release depot formulation — a design choice driven by the short half-life of the free peptide.

Melanotan II — the non-selective analogue

Melanotan II took a different structural route: a cyclic lactam heptapeptide, Ac-Nle-cyclo[Asp-His-D-Phe-Arg-Trp-Lys]-NH₂. Cyclisation locks the pharmacophore into a rigid conformation, which raises potency and stability considerably compared with a linear peptide.

The cost of that potency is breadth. Melanotan II is a non-selective agonist across MC1R, MC3R, MC4R and MC5R. In practice that means the pigmentation signal at MC1R and the central signals at MC3R/MC4R are engaged together, which is precisely the property the next compound was developed to separate.

PT-141 — one functional group away

PT-141, or bremelanotide, is an active metabolite of Melanotan II. The chemical difference is small enough to state in one line: where Melanotan II terminates in an amide, bremelanotide terminates in a free hydroxyl — Ac-Nle-cyclo[Asp-His-D-Phe-Arg-Trp-Lys]-OH.

That single change shifts the receptor profile. Bremelanotide still touches MC1 through MC5 (excepting MC2), but functions primarily as an MC3R and MC4R agonist, with much less of the MC1R-driven pigmentation activity that characterises its parent compound.

It is the only one of the three approved for a central indication: the FDA authorised it in June 2019 as Vyleesi, a subcutaneous injection for hypoactive sexual desire disorder in premenopausal women. The mechanistic interest is in MC4R signalling in the hypothalamus rather than anything peripheral or vascular — a different pathway entirely from the PDE5 inhibitor class.

Melanotan 2 vs PT-141 side by side

Melanotan 2 vs PT-141: why one functional group changes so much

This is the part worth internalising, because it generalises well beyond these three molecules. In a cyclic peptide the ring holds the side chains that contact the receptor in a fixed geometry, so the pharmacophore is essentially the same between Melanotan II and bremelanotide. What differs is the electrostatics and hydrogen-bonding capability at the C-terminus — an amide is neutral and donates two hydrogens; a carboxylate is charged at physiological pH.

Receptor subtypes that look similar in a sequence alignment can differ substantially in the charge environment of their binding pocket. A terminal change that is tolerated by one subtype can be penalised by another, and the result is a shift in relative affinity rather than a wholesale loss of activity. That is exactly the pattern here: bremelanotide did not lose MC1R binding, it lost relative preference for it.

Melanotan 2 and PT-141: practical notes for laboratory work

• All three are supplied lyophilised. Store at −20 °C, protected from light, until reconstituted.
• Cyclic peptides are more robust than linear ones, but not indefinitely stable in solution. Melanotan I, being linear, is the least forgiving of the three.
• Use bacteriostatic water when a vial will be entered more than once; the benzyl alcohol preservative is the reason for the distinction.
• Add diluent down the vial wall, swirl gently, never shake. Melanocortin analogues are surface-active and will foam.
• Date every reconstituted vial and avoid freeze–thaw cycling — aliquot once, then draw from aliquots.

Step-by-step method in the peptide reconstitution guide.

Melanotan 2 vs PT-141: frequently asked questions

Related products

References

The primary literature below is indexed on PubMed, and compound records are held at PubChem.

1. Bremelanotide — structure, receptor profile and regulatory history. Ac-Nle-cyclo[Asp-His-D-Phe-Arg-Trp-Lys]-OH; FDA approval as Vyleesi, June 2019.
2. Afamelanotide — Ac-Ser-Tyr-Ser-Nle-Glu-His-D-Phe-Arg-Trp-Gly-Lys-Pro-Val-NH₂; MC1R-predominant agonist; EMA authorisation as Scenesse, January 2015; FDA approval October 2019 for erythropoietic protoporphyria.
3. Melanocortin receptor family — MC1R through MC5R tissue distribution and ligand specificity, including the ACTH-restricted pharmacology of MC2R.