Melanotan II (MT-II)

Melanotan II is a synthetic cyclic lactam analog of alpha-melanocyte stimulating hormone (α-MSH), developed at the University of Arizona. It was designed with increased potency and prolonged activity compared to the linear native hormone. The peptide is a non-selective melanocortin receptor agonist, activating MC1R, MC3R, MC4R, and MC5R with varying affinities. Originally developed for tanning purposes as a potential photoprotective agent, MT-II was found to have additional effects on sexual function and appetite due to its activity at MC3R and MC4R. Its non-selective nature produces a broader range of effects than more targeted analogs.

Melanotan II acts through multiple melanocortin receptors: **MC1R Activation (Tanning):** The primary mechanism for melanogenesis. MC1R activation on melanocytes triggers a cAMP-dependent cascade that increases tyrosinase activity and melanin production. This shifts the melanin ratio toward eumelanin (brown/black pigment) rather than pheomelanin (red/yellow). **MC4R Activation (Sexual Function):** Activation in the hypothalamus and limbic regions affects sexual arousal and erectile function. This mechanism led to the development of the more selective analog PT-141. **MC3R/MC4R (Appetite/Metabolism):** These receptors in the hypothalamus are involved in energy homeostasis. Activation typically reduces appetite and food intake, explaining MT-II's anorexic effects. **Prolonged Activity:** The cyclic lactam structure and amino acid substitutions (notably D-Phe and Nle) protect against enzymatic degradation, extending the duration of action compared to native α-MSH.