Research Cluster

Tanning, Pigmentation & Melanocortin Peptides

The melanocortin system is the most mechanistically unified cluster on this site. Every compound documented here acts on the same receptor family—five G protein-coupled receptors (MC1R through MC5R) with distinct tissue expression patterns and physiological roles. MC1R governs pigmentation and inflammatory modulation in skin. MC2R is the adrenal ACTH receptor controlling cortisol production. MC3R and MC4R regulate energy homeostasis and appetite in the hypothalamus. MC5R is expressed in exocrine glands. The same ancestral hormone—POMC and its cleavage products—speaks to all five receptors, producing radically different outcomes depending on which receptor is engaged, where, and at what dose.

This receptor selectivity map is the essential context for understanding why compounds in this cluster have such different approved indications. Afamelanotide (MC1R) is approved for a rare photodermatosis. Setmelanotide (MC4R) is approved for genetic obesity. Bremelanotide (MC3R/MC4R) is approved for hypoactive sexual desire disorder. ACTH (MC2R) is approved for adrenal diagnostic testing. Same receptor family, four entirely different approved applications.

The tanning application—the reason Melanotan I and II were originally developed and the reason most community users know this cluster—has produced one approved drug (afamelanotide for EPP, a rare disease) and one Phase I compound that was never approved (Melanotan II) with a safety profile that warrants specific documentation. The cosmetic tanning use of afamelanotide is not what the drug was approved for. That distinction is made precisely for every compound in this cluster.

Melanocortin Receptor Map—Reading the Cluster