Melanotan II (MT-II) is a synthetic cyclic heptapeptide analogue of alpha-melanocyte-stimulating hormone (alpha-MSH) and a non-selective melanocortin receptor agonist. Designed through molecular-dynamics-guided medicinal chemistry at the University of Arizona, it engages MC1R, MC3R, MC4R and MC5R while sparing MC2R. Published research has characterized its melanogenic activity in skin bioassays and its central effects on erectile physiology and appetite in animal and human pharmacology studies. This monograph summarizes the peer-reviewed literature on MT-II for laboratory and research use only.

Background & Discovery

Melanotan II (MT-II) is a synthetic cyclic heptapeptide analogue of the endogenous melanocortin alpha-melanocyte-stimulating hormone (alpha-MSH). It emerged from a structure-based medicinal-chemistry program at the University of Arizona in the late 1980s. Al-Obeidi, Castrucci, Hadley and Hruby used molecular-dynamics simulations of the alpha-MSH message sequence to design a conformationally constrained analogue in which a side-chain-to-side-chain lactam bridge (linking an Asp5 residue to a Lys10 residue) locks the biologically active His-D-Phe-Arg-Trp core into a receptor-favorable turn. The resulting compound retained the essential His-Phe-Arg-Trp pharmacophore of alpha-MSH but showed markedly greater potency and a prolonged duration of action in classical amphibian and reptilian skin bioassays, which is why the literature describes it as a "superpotent, prolonged-acting" melanotropin.

The rationale for developing MT-II was originally dermatological: researchers hypothesized that a stable, potent melanocortin agonist able to stimulate MC1R-driven melanogenesis could serve as a pharmacological means of inducing skin pigmentation ("photoprotective tanning") independent of ultraviolet exposure. During early human pharmacology studies, investigators also observed unexpected effects on penile erection and appetite, which redirected substantial research attention toward the central melanocortin receptors MC3R and MC4R. MT-II therefore occupies an important position in melanocortin research as a broadly acting, non-selective probe that engages every melanocortin receptor subtype except the adrenocortical MC2R.

Within the research-chemical and pharmacology literature, MT-II is categorized as a melanocortin receptor agonist and is frequently used as a reference agonist and lead scaffold. Its close structural relatives include the linear analogue NDP-alpha-MSH (afamelanotide/"Melanotan I"), the potent antagonist SHU-9119 (which differs by a single D-Nal(2′) substitution at position 7), and the clinical MC3R/MC4R agonist bremelanotide (PT-141), a metabolite/analogue of MT-II. This body of work is presented here strictly for laboratory and research use; the summaries below describe what published studies report and are not guidance for human use.

Chemical Identity

Structure & Physicochemical Properties

MT-II is a white-to-off-white lyophilized peptide powder. Its cyclic lactam architecture is central to its physicochemical behavior: the intramolecular amide bridge between the Asp5 and Lys10 side chains constrains backbone flexibility, reduces the entropic penalty of receptor binding, and confers substantially greater resistance to proteolytic degradation than linear alpha-MSH. Because the ring is formed by an amide (lactam) rather than a disulfide, the molecule contains no cysteine and no sulfur (formula C50H69N15O9), so it is not susceptible to the oxidation/disulfide-scrambling chemistry that complicates handling of many cyclic peptides. Reported average molecular weight is approximately 1024.2 g/mol for the free base; supplied material is typically an acetate or trifluoroacetate salt, so the measured "as-supplied" mass reflects counterion and residual water content.

As a small, highly polar peptide bearing an acetylated N-terminus, a C-terminal carboxamide, and ionizable His, Arg and Lys residues, MT-II is freely soluble in water and in aqueous buffers and is commonly dissolved in sterile or bacteriostatic water for laboratory use; it is far less soluble in nonpolar organic solvents. In solution the peptide is sensitive to elevated temperature, extremes of pH, and prolonged light exposure (the Trp indole is photolabile), and reference preformulation work on MT-II has examined its aqueous stability profile in the context of developing a chemically defined research peptide. The constrained ring makes MT-II conformationally rigid relative to native alpha-MSH, a feature that both raises melanocortin-receptor affinity and slows enzymatic clearance in the bioassay and pharmacology systems in which it has been characterized.

Mechanism of Action — as described in the literature

MT-II is a non-selective agonist of the melanocortin receptor family, a group of class-A (rhodopsin-like) G-protein-coupled receptors comprising MC1R through MC5R. It engages MC1R, MC3R, MC4R and MC5R with high affinity while sparing MC2R (the adrenocorticotropic-hormone-specific receptor). The pharmacophore responsible for this broad activity is the conserved His-Phe-Arg-Trp "message" sequence of alpha-MSH; in MT-II a D-phenylalanine at position 7 and the Asp5-Lys10 lactam ring pre-organize this motif, which underlies both its high potency and its prolonged action as first demonstrated in frog and lizard skin melanotropic bioassays by the original Arizona design group.

At the signaling level, agonist occupancy of melanocortin receptors classically couples through Gs to activate adenylyl cyclase, raising intracellular cyclic AMP and activating protein kinase A. In MC1R-expressing melanocytes this cAMP/PKA cascade drives CREB-dependent up-regulation of microphthalmia-associated transcription factor (MITF) and downstream melanogenic enzymes such as tyrosinase, promoting a shift toward eumelanin synthesis. This is the mechanistic basis for the melanogenic ("tanning") activity attributed to MT-II in cell, amphibian, and human studies.

The behavioral and metabolic effects of MT-II are mediated primarily through the central melanocortin receptors MC3R and MC4R, which are densely expressed in hypothalamic and limbic circuits that regulate energy homeostasis and sexual function. Central administration of MT-II in rodents suppresses food intake and, when co-administered with the orexigenic peptide neuropeptide Y (NPY), blunts NPY-driven hyperphagia and fat accumulation, consistent with MC4R acting as a satiety/anti-adipogenic node that opposes NPY/AgRP signaling. These observations position MT-II as a pharmacological tool for interrogating the leptin-melanocortin axis.

MT-II’s effect on penile erection is attributed largely to central MC4R activation rather than a direct peripheral vascular action. In rabbit studies, MT-II increased intracavernosal pressure through a centrally initiated pathway, and this pro-erectile response was abolished by inhibition of nitric-oxide synthase, implicating downstream neuronal nitric-oxide release as a required effector. This central melanocortin-to-nitric-oxide mechanism distinguishes melanocortin agonists from phosphodiesterase-5 inhibitors, which act peripherally on the nitric-oxide/cGMP pathway.

Because MT-II activates several receptor subtypes with overlapping but distinct signaling and tissue distributions, structure-activity work has used the MT-II lactam scaffold to dissect subtype selectivity. Systematic modification of the core (for example, substitutions at position 7 that convert agonists such as MT-II into antagonists such as SHU-9119, and further lactam derivatives yielding subtype-selective ligands like the MC3R-preferring antagonist PG-106) has clarified which structural elements govern MC3R- versus MC4R- versus MC5R-directed activity. This medicinal-chemistry lineage ultimately informed development of more selective clinical melanocortin ligands.

Key Published Findings

• Molecular design and melanotropic potency: Al-Obeidi and colleagues used molecular-dynamics-guided design to build a side-chain lactam-bridged cyclic analogue of alpha-MSH(4-10); in frog and lizard skin melanotropic bioassays the cyclic lactam analogues were superpotent and prolonged-acting relative to native alpha-MSH, establishing the MT-II scaffold.[1]
• Melanogenesis in humans (Phase I): In a pilot single-blind, placebo-controlled Phase I study in normal male volunteers, low subcutaneous doses of MT-II produced measurable increases in skin pigmentation by quantitative reflectance and visual assessment, demonstrating melanogenic activity in humans; nausea, yawning/stretching, and spontaneous penile erection were noted.[2]
• Erectile response in psychogenic ED: In a double-blind, placebo-controlled crossover study of ten men with psychogenic erectile dysfunction, subcutaneous MT-II produced clinically apparent erections in 8 of 10 subjects, with mean RigiScan tip rigidity >80% lasting about 38 minutes versus 3 minutes for placebo.[3]
• Sexual motivation in men: A summary of human studies reported that MT-II elicited penile erection in 17 of 20 men across psychogenic and organic ED cohorts and increased self-reported sexual desire more often than placebo, with nausea and yawning as common adverse effects.[4]
• Central melanocortin-nitric oxide erectile pathway: In rabbits, activation of central melanocortin receptors by MT-II increased intracavernosal pressure, and the response was abolished by nitric-oxide synthase inhibition, indicating a centrally initiated, nitric-oxide-dependent pro-erectile mechanism.[5]
• Feeding, adiposity, and NPY antagonism: Seven-day central infusion of MT-II in male rats produced transient anorexia and, when co-infused with neuropeptide Y, counteracted NPY-driven hyperphagia and fat accumulation and lowered insulin and leptin, without reversing NPY’s suppression of the gonadotropic and somatotropic axes.[8]
• Structure-activity and subtype selectivity: Further structure-activity studies of lactam derivatives of MT-II and SHU-9119 characterized activity and selectivity at human MC3R, MC4R and MC5R and yielded new subtype-selective tools, including PG-106, an MC3R-selective antagonist useful for dissecting melanocortin receptor pharmacology.[6]

Research Applications

• Investigated as a reference agonist in amphibian and reptilian skin melanotropic bioassays (frog and lizard skin) that model melanocortin-driven melanogenesis.
• Investigated in rodent models of energy balance and appetite, where central MC3R/MC4R activation modulates food intake, body weight, and adiposity.
• Examined in rabbit and rodent models of erectile physiology to probe central melanocortin control of penile erection and its nitric-oxide dependence.
• Used in vitro at cloned human melanocortin receptors (MC1R, MC3R, MC4R, MC5R) to characterize agonist potency, efficacy, and subtype-selectivity profiles.
• Employed as a pharmacological probe of MC1R-driven melanogenesis and pigmentation signaling (cAMP/PKA/MITF/tyrosinase pathway) in melanocyte systems.
• Studied within human clinical-pharmacology literature on melanocortin-mediated skin pigmentation and sexual response, cited as reference scientific data.
• Used as a lead scaffold in medicinal-chemistry structure-activity studies that generated subtype-selective melanocortin ligands and the clinical analogue bremelanotide (PT-141).
• Applied as a tool compound in studies of the leptin-melanocortin axis, including melanocortin regulation of insulin sensitivity and leptin signaling.

Related & Comparator Compounds

The melanocortin agonist family provides the closest comparators. NDP-alpha-MSH (afamelanotide, historically "Melanotan I") is a linear, superpotent alpha-MSH analogue developed alongside MT-II; the literature distinguishes it as a linear peptide advanced primarily for skin pigmentation, whereas MT-II is a cyclic, truncated peptide additionally characterized for central (erectile and appetite) effects. Bremelanotide (PT-141) is a C-terminally modified MT-II analogue (a des-amino, free-acid metabolite/derivative) with relatively greater MC4R/MC3R directionality and reduced melanogenic emphasis, developed as an injectable clinical agent for sexual dysfunction; reviews describe it as the direct commercial successor to the MT-II program. SHU-9119 is a structurally near-identical lactam peptide that differs by a single position-7 substitution (D-Nal(2′) for D-Phe), converting the agonist into a potent MC3R/MC4R antagonist, and is frequently paired with MT-II in mechanistic studies. Setmelanotide is a later, MC4R-selective cyclic agonist used clinically for rare melanocortin-pathway obesity, and it is contrasted with MT-II precisely because MT-II is non-selective across MC1R/MC3R/MC4R/MC5R. Native ligands alpha-, beta- and gamma-MSH and ACTH, along with the endogenous antagonists agouti and AgRP, complete the comparator landscape that MT-II is routinely benchmarked against.

Handling, Reconstitution & Storage

In published and reference research-handling descriptions, MT-II is supplied as a lyophilized powder and is typically stored desiccated and protected from light, with long-term storage at -20 C (or colder) and short-term working storage refrigerated. For laboratory reconstitution it is dissolved in sterile water or bacteriostatic water given its high aqueous solubility; once in solution the peptide is described as less stable than in the dry state and is commonly aliquoted to minimize repeated freeze-thaw cycles, kept cold, and shielded from light to protect the photolabile tryptophan residue. These points reflect general research-peptide handling practice and physicochemical characterization of MT-II and are provided for laboratory context only, not as instructions for administration to humans or animals.

Analytical & Quality Considerations

Analytical characterization of MT-II typically combines reversed-phase HPLC for purity (commonly with UV detection near 218 nm for the peptide bond and around 280 nm for the Trp chromophore) with mass spectrometry for identity; electrospray ionization readily generates a doubly charged [M+2H]2+ ion near m/z 513, and the intact monoisotopic/average mass (~1024 Da free base) confirms the expected sequence and lactam cyclization. Orthogonal identity checks include amino-acid analysis and MS/MS sequencing, while net-peptide content, counterion (acetate vs trifluoroacetate), residual solvent, water content, and endotoxin are routinely reported on a certificate of analysis. The importance of independent, third-party QC is underscored by analytical surveys of illicitly marketed MT-II: Breindahl and colleagues used LC-UV-MS/MS to show that vials sold online contained only 4.32-8.84 mg of peptide despite "10 mg" labeling and carried unidentified impurities of roughly 4-5%, illustrating why documented HPLC purity, confirmed identity, and an authoritative COA are essential for interpretable, reproducible research.

Frequently Asked Research Questions

Q. What is Melanotan II and how is it classified?
A. Melanotan II (MT-II) is a synthetic cyclic heptapeptide analogue of alpha-melanocyte-stimulating hormone (alpha-MSH). It is classified as a non-selective melanocortin receptor agonist that activates MC1R, MC3R, MC4R and MC5R while sparing MC2R. It is intended for laboratory and research use only.

Q. How is MT-II structurally different from native alpha-MSH?
A. MT-II retains the His-D-Phe-Arg-Trp message sequence of alpha-MSH but is truncated and cyclized through a side-chain Asp5-to-Lys10 lactam bridge, with an N-acetyl group and norleucine replacing Met4. This constraint makes it more resistant to proteolysis and, in classical skin bioassays, superpotent and longer-acting than alpha-MSH.

Q. Is the MT-II ring a disulfide bond?
A. No. The molecular formula C50H69N15O9 contains no sulfur, and the ring is an amide (lactam) bridge between the Asp5 and Lys10 side chains rather than a cysteine disulfide. This is why MT-II is not subject to disulfide-scrambling or oxidation chemistry.

Q. Which receptors and pathways mediate MT-II’s reported effects in research models?
A. Melanogenic activity is attributed to MC1R signaling through the cAMP/PKA/MITF/tyrosinase pathway in melanocytes, while appetite and erectile effects are attributed mainly to central MC3R/MC4R activation; in rabbit studies the pro-erectile response was nitric-oxide-synthase-dependent.

Q. Why does third-party analytical verification matter for MT-II?
A. An LC-UV-MS/MS survey of illicitly sold MT-II found vials containing well below the labeled amount of peptide plus unidentified impurities, so documented HPLC purity, confirmed mass-spectrometric identity, and an independent certificate of analysis are important for reproducible research.

Peer-Reviewed References

1. Al-Obeidi F, Castrucci AM, Hadley ME, Hruby VJ. Potent and prolonged acting cyclic lactam analogues of alpha-melanotropin: design based on molecular dynamics. Journal of Medicinal Chemistry. 1989. PubMed →
2. Dorr RT, Lines R, Levine N, Brooks C, Xiang L, Hruby VJ, Hadley ME. Evaluation of melanotan-II, a superpotent cyclic melanotropic peptide in a pilot phase-I clinical study. Life Sciences. 1996. PubMed →
3. Wessells H, Fuciarelli K, Hansen J, Hadley ME, Hruby VJ, Dorr R, Levine N. Synthetic melanotropic peptide initiates erections in men with psychogenic erectile dysfunction: double-blind, placebo controlled crossover study. Journal of Urology. 1998. PubMed →
4. Wessells H, Levine N, Hadley ME, Dorr R, Hruby V. Melanocortin receptor agonists, penile erection, and sexual motivation: human studies with Melanotan II. International Journal of Impotence Research. 2000. PubMed →
5. Vemulapalli R, Kurowski S, Salisbury B, Parker E, Davis H. Activation of central melanocortin receptors by MT-II increases cavernosal pressure in rabbits by the neuronal release of NO. British Journal of Pharmacology. 2001. PubMed →
6. Grieco P, Cai M, Han G, Trivedi D, Campiglia P, Novellino E, Hruby VJ. Further structure-activity studies of lactam derivatives of MT-II and SHU-9119: their activity and selectivity at human melanocortin receptors 3, 4, and 5. Peptides. 2007. PubMed →
7. Hadley ME, Dorr RT. Melanocortin peptide therapeutics: historical milestones, clinical studies and commercialization. Peptides. 2006. PubMed →
8. Raposinho PD, White RB, Aubert ML. The melanocortin agonist Melanotan-II reduces the orexigenic and adipogenic effects of neuropeptide Y (NPY) but does not affect the NPY-driven suppressive effects on the gonadotropic and somatotropic axes in the male rat. Journal of Neuroendocrinology. 2003. PubMed →
9. Breindahl T, Evans-Brown M, Hindersson P, McVeigh J, Bellis M, Stensballe A, Kimergard A. Identification and characterization by LC-UV-MS/MS of melanotan II skin-tanning products sold illegally on the Internet. Drug Testing and Analysis. 2015. PubMed →

For laboratory and research use only. Not for human or veterinary use, diagnosis, or treatment. This overview summarizes published scientific literature for informational and educational purposes and is not medical advice; no claims are made regarding safety or efficacy in humans.

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