Melanotan II (MT-2)

Melanotan II is a cyclic heptapeptide analogue of α-MSH, developed at the University of Arizona. Cyclisation locks the peptide into a rigid conformation, which both protects it from enzymatic attack and raises its affinity across the melanocortin receptor family.

Where Melanotan I is largely MC1R-selective, MT-2 binds MC1R, MC3R, MC4R and MC5R. It is therefore a broad-spectrum melanocortin tool rather than a targeted one.

The breadth of receptor activity is why MT-2 appears in research areas that otherwise have little to do with one another:

• MC1R — pigmentation. Melanogenesis and melanin density, the same subject as MT-1.

• MC3R and MC4R — energy balance. These receptors are central to appetite and energy homeostasis research.

• MC4R — sexual function. The observation that led to PT-141 being developed from an MT-2 metabolite.

• MC5R — exocrine function, including sebaceous gland activity.

A compound acting at four receptors produces effects that cannot be assigned to any one of them without further work. This is the central methodological caveat in the MT-2 literature, and it is why selective ligands are used alongside it — the comparison, not the compound alone, is what isolates a mechanism.

Melanotan I is the MC1R-selective comparator and PT-141 is the MC3R/MC4R-focused metabolite. Using all three is the standard approach to separating melanocortin subtype contributions.