Melanotan I vs Melanotan II

One binds a single melanocortin receptor; the other binds the family. Why selectivity is the whole distinction, and what it means for a study design.

Selective against non-selective

Both are synthetic analogues of alpha-melanocyte stimulating hormone, and both were developed from the same research programme. The difference is receptor breadth.

Melanotan I, also called afamelanotide, is a linear analogue with a profile weighted towards MC1R — the receptor on melanocytes that governs the switch from phaeomelanin to eumelanin synthesis. Its activity is largely confined to that pathway.

Melanotan II is a cyclic lactam analogue that binds across the melanocortin receptor family: MC1R, but also the central MC3R and MC4R and the exocrine MC5R. Its research profile is correspondingly broader and harder to attribute.

Where they differ

Structure. Melanotan I is linear; Melanotan II is cyclised through a lactam bridge, which constrains conformation and increases affinity.
Receptor profile. MC1R-weighted against activity across MC1R, MC3R, MC4R and MC5R.
Central activity. Minimal for Melanotan I. Present and significant for Melanotan II, through MC3R and MC4R.
Attribution. An effect seen with Melanotan I can reasonably be assigned to MC1R. An effect seen with Melanotan II cannot be assigned to any one receptor without a selective comparator.
Clinical development. Afamelanotide has been through formal clinical development for a specific photodermatosis. Melanotan II has not; its most developed descendant is PT-141, refined for MC4R.
Presentation. Melanotan II is also supplied as a metered intranasal preparation; Melanotan I is vial only.

Which suits which question

For work on pigmentation specifically, Melanotan I is the cleaner instrument and the one whose results can be attributed. Using a pan-melanocortin agonist to study a single receptor pathway builds an attribution problem into the design from the outset.

For work on central melanocortin signalling — appetite, arousal, MC3R and MC4R pharmacology — Melanotan II is the relevant compound, and its breadth is the reason. Where the target is MC4R specifically, PT-141 narrows it further and is usually the better choice. Using the pair together, one selective and one not, is a standard way to separate MC1R effects from the rest.

How both are supplied

Both are lyophilised in sealed vials. Each batch is specified at 99% purity or better by HPLC with identity confirmed by mass spectrometry and released against a batch-matched Certificate of Analysis, by the process on the lab testing page. For Melanotan II, confirming that the lactam bridge is intact is the part of identity testing that matters: a linear peptide of identical composition is a different molecule with a different receptor profile. Both sit in the skin and cosmetic group.

Research use only

Both compounds are laboratory reference materials. Neither is a medicine, a cosmetic, a supplement, or for human or veterinary use, and no dosing guidance or administration protocol is provided for either. The full terms are in the research use policy. Every guide is indexed in the research library.