— Research note —
Melanotan II
Cyclic heptapeptide alpha-MSH analog developed at the University of Arizona, investigated as a non-selective melanocortin receptor agonist.
Melanotan II is a cyclic heptapeptide alpha-melanocyte-stimulating hormone (alpha-MSH) analog developed in the 1980s by Mac Hadley, Victor Hruby, and colleagues at the University of Arizona. The molecule was designed within a structure-activity exploration of alpha-MSH analogs with the goals of enhanced potency, enzymatic stability, and bioactivity following non-parenteral administration. The cyclization through a lactam bridge between aspartate and lysine residues produced a conformationally constrained molecule with broad agonist activity at multiple melanocortin receptor subtypes including MC1R, MC3R, MC4R, and MC5R.
The peptide has been studied as a research tool across multiple physiological systems. The MC1R activity drives pigmentary effects in skin, the original research focus that gave the molecule its name. The MC4R activity engages central nervous system circuits regulating appetite, energy balance, and sexual response. The broad receptor profile distinguishes Melanotan II from more selective melanocortin analogs such as the MC1R-selective afamelanotide (Melanotan I) and the MC4R-preferring bremelanotide (PT-141), both of which derive from related medicinal chemistry traditions.
Academic research has used Melanotan II as a probe in melanocortin receptor pharmacology, central appetite regulation, and the characterization of melanocortin system contributions to sexual response. The compound has not advanced to formal pharmaceutical approval, although it has been the subject of substantial research and has informed the development of more selective melanocortin analogs that have subsequently reached clinical research and approval.
Melanotan II is supplied here for laboratory research use only and is not intended for human consumption. The pigmentary and appetite-related effects observed in preclinical research are reported here to characterize the pharmacological profile of the molecule, not as endorsement of any application outside research settings.
Mechanism
Melanotan II engages all five melanocortin receptor subtypes (MC1R through MC5R) as a non-selective agonist, with affinity for each receptor in the nanomolar to low-nanomolar range. All five receptors are class A G-protein coupled receptors coupled to Gas with downstream elevation of intracellular cAMP.
MC1R activation on melanocytes drives eumelanin synthesis through tyrosinase upregulation, producing the pigmentary effects characteristic of melanocortin signaling. MC4R activation in hypothalamic neurons engages central circuits regulating appetite, with effects observed on food intake in preclinical feeding studies. The same MC4R activation engages central sexual response circuits, producing the effects on sexual desire and erectile response that have been characterized in preclinical and early clinical research. MC3R and MC5R activations contribute additional effects on energy metabolism and on exocrine gland function, respectively. The cyclic lactam structure of the molecule confers conformational constraint that supports high-affinity binding at multiple receptor subtypes and provides resistance to proteolytic cleavage.
Research history
The University of Arizona melanocortin research program led by Mac Hadley and Victor Hruby began in the 1970s with structure-activity studies of alpha-MSH and its analogs. Melanotan II was disclosed in publications from the late 1980s as an optimized non-selective agonist with enhanced potency and stability relative to the native hormone.
Early clinical research at the University of Arizona examined pigmentary effects and characterized side effect profiles including effects on appetite and sexual response. The observation that Melanotan II produced effects on erectile function in male research subjects led to the subsequent development of PT-141 (bremelanotide) as a metabolite-derived MC4R-preferring analog. Parallel development of MC1R-selective afamelanotide (Melanotan I) by Australian and European groups proceeded toward photoprotection applications.
Melanotan II itself did not advance to formal pharmaceutical development through the typical regulatory channels, although it has been the subject of substantial research and has been widely distributed in research-chemical channels. Academic research has continued to use the molecule as a probe in melanocortin pharmacology, with publications examining receptor binding profiles, in vivo pharmacodynamics, and structure-activity relationships. The compound has informed the broader medicinal chemistry of melanocortin agonists, which has subsequently produced the approved drugs setmelanotide (MC4R-selective) and bremelanotide.
References
- Al-Obeidi F, et al. 1989. Design of a new class of superpotent cyclic alpha-melanotropins based on quenched dynamic simulations. J Am Chem Soc.
- Hadley ME, Dorr RT. 2006. Melanocortin peptide therapeutics: historical milestones, clinical studies and commercialization. Peptides. PMID: 16413095
- Dorr RT, et al. 1996. Evaluation of melanotan-II, a superpotent cyclic melanotropic peptide in a pilot phase-I clinical study. Life Sci. PMID: 8786705
- Wessells H, et al. 1998. Synthetic melanotropic peptide initiates erections in men with psychogenic erectile dysfunction. Urology. PMID: 9690675
- Sawyer TK, et al. 1980. 4-Norleucine, 7-D-phenylalanine-alpha-melanocyte-stimulating hormone: a highly potent alpha-melanotropin with ultralong biological activity. Proc Natl Acad Sci U S A. PMID: 6253995
- Hadley ME. 2005. Discovery that a melanocortin regulates sexual functions in male and female humans. Peptides. PMID: 15990202
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Information presented in third-person scientific context. Research use only. Not medical advice; not for human consumption.