Melanotan 2: Mechanism of Action
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Mechanism of Action
Melanocortin Receptor Binding
MT-II is a non-selective agonist at four of five melanocortin receptors (MCRs), all members of the GPCR superfamily:
| Receptor | Primary Location | Function When Activated | Affinity |
|---|---|---|---|
| MC1R | Melanocytes (skin) | Eumelanin synthesis → tanning/photoprotection | High |
| MC3R | Hypothalamus, NAcc | Energy homeostasis, feeding behavior | Ki ~1.3 nM |
| MC4R | Hypothalamus (PVN), spinal cord | Erectile function, appetite suppression, thermogenesis | Ki ~1.1 nM |
| MC5R | Exocrine glands, lymphocytes | Sebum production, immune modulation | Moderate |
| MC2R | Adrenal cortex | ACTH receptor — NO MT-II binding | None |
Primary Signaling: cAMP-PKA Pathway
Upon MCR binding, MT-II activates Gs-coupled adenylate cyclase → increased intracellular cAMP → PKA activation:[2]
- Melanogenesis (MC1R): PKA → CREB phosphorylation → MITF transcription → tyrosinase upregulation → eumelanin production
- Erectile function (MC4R/CNS): Hypothalamic PVN activation → dopaminergic/oxytocinergic downstream → neuronal NO release → intracavernosal pressure increase[5]
- Appetite suppression (MC3R/MC4R): Hypothalamic MCR activation → reduced food intake + increased thermogenesis[6]
- Social behavior (MC4R): Selective nucleus accumbens activation → oxytocin-dependent social learning[7]
Off-Target: Mast Cell Activation
MT-II cross-reacts with MRGPRB2/MRGPRX2 receptors on mast cells, causing pseudo-allergic histamine release → H1 receptor activation → hypothermia (in mice). Subcutaneous dosing reduces histamine release by 63% vs. intraperitoneal.[8]
vs. Related Compounds
| Compound | Structure | BBB | Key Difference |
|---|---|---|---|
| MT-II | Cyclic heptapeptide | Yes | Non-selective MCR agonist; tanning + erectogenic + anorexigenic |
| Melanotan I (Afamelanotide) | Linear [Nle⁴,D-Phe⁷]-α-MSH | No | Tanning only (peripheral MC1R); no CNS effects; TGA/EMA approved for EPP |
| Bremelanotide (PT-141) | Deaminated MT-II metabolite | Yes | FDA-approved (Vyleesi) for HSDD; reduced tanning activity |
| α-MSH (native) | Linear tridecapeptide | Limited | Short half-life; rapidly degraded; weak potency |
References
- Dorr RT, Lines R, Levine N, et al. Evaluation of melanotan-II, a superpotent cyclic melanotropic peptide in a pilot phase-I clinical study. Life Sciences. 1996;58(20):1777-1784.
- Hadley ME, Dorr RT. Melanocortin peptide therapeutics: Historical milestones, clinical studies and commercialization. Peptides. 2006;27(4):921-930.
- Wessells H, Fuciarelli K, Hansen J, et al. Synthetic melanotropic peptide initiates erections in men with psychogenic erectile dysfunction: Double-blind, placebo controlled crossover study. The Journal of Urology. 1998;160(2):389-393.
- FDA Warning Letters regarding unauthorized marketing of Melanotan products.
- Vemulapalli R, Kurowski S, Salisbury B, et al. Activation of central melanocortin receptors by MT-II increases cavernosal pressure in rabbits by the neuronal release of NO. British Journal of Pharmacology. 2001;134(8):1705-1710.
- Côté I, et al. Activation of the central melanocortin system chronically reduces body mass without the necessity of long-term caloric restriction. Canadian Journal of Physiology and Pharmacology. 2017.
- Ford CL, McDonough AA, Horie K, Young LJ. Melanocortin agonism in a social context selectively activates nucleus accumbens in an oxytocin-dependent manner. Neuropharmacology. 2024;247:109848.
- Jain S, Panyutin A, Liu N, et al. Melanotan II causes hypothermia in mice by activation of mast cells and stimulation of histamine 1 receptors. American Journal of Physiology-Endocrinology and Metabolism. 2018;315(3):E357-E366.
- Wessells H, Levine N, Hadley ME, Dorr RT, Hruby VJ. Effect of an alpha-melanocyte stimulating hormone analog on penile erection and sexual desire in men with organic erectile dysfunction. Urology. 2000;56(4):641-646.
- Eliason NL, Martin L, Low MJ, Sharpe AL. Melanocortin receptor agonist melanotan-II microinjected in the nucleus accumbens decreases appetitive and consumptive responding for food. Neuropeptides. 2022;96:102289.
- Minakova E, Lang J, Medel-Matus JS, et al. Melanotan-II reverses autistic features in a maternal immune activation mouse model of autism. PLoS ONE. 2019;14(1):e0210389.
- Ter Laak MP, et al. Melanotan II promotes peripheral nerve regeneration in a rat sciatic nerve crush model. 2003.
- Evans-Brown M, Dawson RT, Chandler MD, McVeigh J. Use of melanotan I and II in the general population. BMJ. 2009;338:b566.
- Wu JC, Tsai HE, Hsiao YH, et al. Topical MTII Therapy Suppresses Melanoma Through PTEN Upregulation and Cyclooxygenase II Inhibition. International Journal of Molecular Sciences. 2020;21(2):681.
- Dreyer BA, Amer T, Fraser M. Melanotan-induced priapism: a hard-earned tan. BMJ Case Reports. 2019;12(2):e227644.
- Nelson ME, Bryant SM, Aks SE. Melanotan II injection resulting in systemic toxicity and rhabdomyolysis. Clinical Toxicology. 2012;50(10):1169-1173.
- Peters B, Hadimeri H, Wahlberg R, Afghahi H. Melanotan II: a possible cause of renal infarction. CEN Case Reports. 2020;9(2):159-161.
- Sivyer GW. Dermatological changes with melanotan II use in a FAMMM patient. Dermatology Practical & Conceptual. 2012.
- Ryakhovsky VV, Khachiyan GA, Kosovova NF, et al. The first preparative solution phase synthesis of melanotan II. Beilstein Journal of Organic Chemistry. 2008;4:39.
- Hjuler KF, Lorentzen HF. Melanoma associated with the use of melanotan-II. Dermatology. 2014;228(1):34-36.
- Giuliano F, Clement P, Droupy S, et al. Melanotan-II: Investigation of the inducer and facilitator effects on penile erection in anaesthetized rat. Neuroscience. 2006;138(1):293-301.
- Li G, Zhang Y, Wilsey JT, Scarpace PJ. Unabated anorexic and enhanced thermogenic responses to melanotan II in diet-induced obese rats. Journal of Endocrinology. 2004;182(1):123-132.
- King SH, et al. Melanocortin Receptors, Melanotropic Peptides and Penile Erection. Current Topics in Medicinal Chemistry. 2007;7(11):1111-1119.
- Wessells H, Levine N, Hadley ME, Dorr RT, Hruby VJ. Melanocortin receptor agonists, penile erection, and sexual motivation: human studies with Melanotan II. International Journal of Impotence Research. 2000;12(Suppl 4):S74-S79.
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