This article is for informational and educational purposes only and does not constitute medical advice. MT-2 is supplied by Wholesale Peps as lyophilized research-grade material for in vitro laboratory use only and is not approved by the FDA for human or veterinary use.
MT-2 (Melanotan II) is a synthetic cyclic heptapeptide analogue of alpha-melanocyte-stimulating hormone (α-MSH), developed at the University of Arizona by Victor Hruby, Mac Hadley, and colleagues as part of a program to create potent, stable melanocortin receptor agonists [1]. The compound is a non-selective agonist at multiple melanocortin receptor subtypes, primarily MC1R and MC4R, with lesser activity at MC3R and MC5R; it does not significantly activate MC2R (the ACTH receptor). Published research in animal models and limited early-phase human studies has examined associations between MT-2 and skin pigmentation (via MC1R), sexual arousal and erectile function (via MC4R), and appetite modulation. Among the most extensively studied synthetic melanocortin analogues in the published peptide pharmacology literature, MT-2 has contributed substantially to understanding of the melanocortin system and served as the structural template for bremelanotide (PT-141), an FDA-approved melanocortin receptor agonist for hypoactive sexual desire disorder. MT-2 itself is not FDA-approved for any indication and is not under active pharmaceutical development as a therapeutic agent as of the review date.
1. Background
1.1 Alpha-MSH and the Melanocortin System
Alpha-melanocyte-stimulating hormone (α-MSH) is a 13-amino acid peptide derived through post-translational processing of pro-opiomelanocortin (POMC), a precursor protein that also gives rise to ACTH, β-MSH, γ-MSH, and β-endorphin. α-MSH is produced by corticotroph cells in the pituitary, neurons in the arcuate nucleus of the hypothalamus, melanocytes in the skin, and other tissues. It plays a central role in regulating skin pigmentation, energy balance, sexual function, and inflammation through interactions with a family of G-protein-coupled receptors designated melanocortin receptors (MCRs) [2].
The critical pharmacophore of α-MSH resides in the central tetrapeptide sequence His-Phe-Arg-Trp (residues 6–9), which is necessary and sufficient for melanocortin receptor binding and activation. This core sequence is conserved across the melanocortin peptide family and represents the primary determinant of receptor affinity. The flanking residues of α-MSH contribute to stability and modulate potency but are not required for receptor activation, making them targets for substitution in synthetic analogues designed for improved pharmacological properties.
1.2 Melanocortin Receptor Subtypes
Five melanocortin receptor subtypes have been identified and cloned (MC1R through MC5R), each with distinct tissue expression patterns and downstream physiological roles [2]:
- MC1R — Expressed on melanocytes and melanoma cells; governs the switch from phaeomelanin (red/yellow) to eumelanin (brown/black) production in response to UV exposure; also expressed on immune cells and in the CNS.
- MC2R — The ACTH receptor, expressed in the adrenal cortex; regulates cortisol synthesis. MT-2 does not meaningfully activate MC2R.
- MC3R — Expressed in hypothalamus, limbic structures, and peripheral tissues; involved in energy homeostasis and nutrient sensing.
- MC4R — Widely expressed in the brain, particularly hypothalamic and limbic circuits; regulates appetite, energy expenditure, sexual arousal, and autonomic function. The most pharmacologically important subtype for MT-2’s CNS effects [5].
- MC5R — Expressed in exocrine glands, skeletal muscle, and immune tissue; roles not fully characterised.
1.3 Development of MT-2 (Melanotan II)
The melanocortin analogue research program at the University of Arizona began in the 1960s and 1970s, with early work by Mac Hadley and colleagues demonstrating the ability of synthetic melanocortin fragments to stimulate pigmentation in animal models. Melanotan-I (MT-I), a linear 13-amino acid analogue incorporating Nle and D-Phe substitutions into the full α-MSH sequence, was an early development from this program. MT-2 emerged from subsequent efforts to identify a shorter, more potent, and more metabolically stable compound by cyclising the minimal active fragment.
Al-Obeidi, Hadley, Hruby, and colleagues described the synthesis and characterisation of MT-2 in 1989, demonstrating that introducing a cyclic lactam bridge between residues Asp and Lys in a modified heptapeptide fragment of α-MSH produced a compound with substantially greater potency and prolonged duration of action relative to native α-MSH and MT-I in bioassay models [1]. The cyclisation constrains the peptide into a receptor-preferred conformation, reducing entropy costs of binding and conferring resistance to exopeptidases. MT-2 subsequently became a widely used pharmacological tool for studying the melanocortin system and drove the characterisation of MC4R and its CNS roles.
2. Molecular Structure
MT-2 has the sequence Ac-Nle-c[Asp-His-D-Phe-Arg-Trp-Lys]-NH₂, where the “c[...]” notation indicates a cyclic lactam ring formed between the side chains of Asp (position 2) and Lys (position 7). The compound carries an N-terminal acetyl group (Ac-) and a C-terminal amide (-NH₂), both of which contribute to metabolic stability. It is a cyclic heptapeptide with a molecular weight of approximately 1,024 Da.
| Modification | Native α-MSH(4–10) | MT-2 | Rationale |
|---|---|---|---|
| Position 1 (residue 4 of α-MSH) | Met (oxidation-prone) | Nle (Norleucine) | Improved oxidative stability; similar hydrophobicity |
| Position 4 (residue 7 of α-MSH) | L-Phe | D-Phe | Greatly increased receptor affinity and potency; resistance to proteolysis |
| Ring constraint | Linear peptide | Cyclic lactam (Asp–Lys bridge) | Conformational restriction to receptor-preferred geometry; extended half-life |
| N-terminus | Free amine | Acetyl group (Ac-) | Protection from aminopeptidases; reduced charge |
| C-terminus | Free carboxyl | Amide (-NH₂) | Protection from carboxypeptidases; improved membrane permeability |
| Property | Detail |
|---|---|
| Full sequence | Ac-Nle-c[Asp-His-D-Phe-Arg-Trp-Lys]-NH₂ |
| Length | 7 amino acids (cyclic heptapeptide) |
| Molecular weight | ~1,024 Da |
| Structural class | Cyclic lactam; N-acetylated; C-amidated |
| Primary MCR targets | MC1R, MC4R (also MC3R, MC5R at higher concentrations) |
| Origin | University of Arizona (Hruby/Hadley group), 1989 |
| Related approved compound | Bremelanotide (PT-141/Vyleesi) — derived from MT-2 |
3. Mechanism of Action
MT-2 acts as a non-selective agonist at melanocortin receptors, with established binding and activation at MC1R, MC3R, MC4R, and MC5R. The physiological consequences of its two primary targets — MC1R in skin melanocytes and MC4R in hypothalamic and limbic circuits — account for the majority of its studied effects.
4. Key Research Findings
4.1 Melanocortin Receptor Pharmacology
MT-2 has been used extensively as a pharmacological probe for characterising the melanocortin receptor system since the 1990s. Its development contributed directly to the cloning and functional characterisation of MC3R and MC4R, as its potent and prolonged receptor activation in animal models provided a tool for identifying and mapping MCR-expressing neural circuits. Cone (2005) reviewed the central melanocortin system in detail, describing the neural architecture through which MC4R signalling in the hypothalamus regulates energy intake, expenditure, and autonomic outflow [2]. The established receptor binding and activation profile of MT-2 across MCR subtypes is well-characterised in the published pharmacology literature and provides the mechanistic basis for its studied effects.
4.2 Pigmentation
In animal models, MT-2 produces dose-dependent skin darkening through MC1R activation in melanocytes, with effects substantially more potent and longer-lasting than native α-MSH. In the earliest reported human observations, tanning — consistent with MC1R-mediated eumelanin induction — was noted as a pharmacodynamic effect in subjects participating in early MT-2 studies. The pigmentation response to MT-2 occurs through the melanin synthesis pathway and is UV-independent in its initiation, though UV exposure and melanogenesis are not fully decoupled: the relationship between UV-independent melanin deposition and photoprotective function has not been comprehensively characterised for MT-2 in controlled clinical studies.
4.3 Sexual Function — Early Human Studies
Wessells and colleagues (1998) conducted a randomised, double-blind, placebo-controlled crossover study examining the effects of subcutaneous MT-2 in men with psychogenic erectile dysfunction, representing one of the earliest peer-reviewed human studies of the compound [3]. MT-2 was associated with a statistically significant increase in erectile events compared with placebo in home monitoring assessments, along with increased sexual desire. The most common adverse effects were nausea, yawning, and spontaneous erections. The study was a small pilot and the authors explicitly noted its limitations in terms of sample size and scope; it should be interpreted as hypothesis-generating rather than as definitive efficacy evidence. Subsequent work on the melanocortin system and sexual function confirmed that MC4R signalling represents a mechanistic pathway for pro-erectile effects that is pharmacologically distinct from PDE5 inhibitors [5].
4.4 Relationship to Bremelanotide (PT-141 / Vyleesi)
Bremelanotide (formerly PT-141) is a cyclic heptapeptide derived from MT-2 by replacing the N-terminal acetyl group with a hydroxyl group on the N-terminus, resulting in a compound with a somewhat modified receptor interaction profile. Bremelanotide was developed by Palatin Technologies and received FDA approval in 2019 under the brand name Vyleesi for the treatment of hypoactive sexual desire disorder (HSDD) in premenopausal women. The approval of bremelanotide establishes proof of concept for melanocortin receptor agonism as a pharmacological approach to sexual dysfunction, and provides a regulatory benchmark for the compound class [4].
However, bremelanotide and MT-2 are distinct compounds. The receptor binding profiles, pharmacokinetics, approved dosing, safety data, and clinical trial populations apply specifically to bremelanotide and cannot be directly extrapolated to MT-2. MT-2 has not been through the Phase 3 regulatory process and does not have an approved indication.
Schematic representation of MT-2 relative activity at melanocortin receptor subtypes. Bar lengths are qualitative and do not represent quantitative binding affinities (Ki). MC2R (ACTH receptor) is not meaningfully activated by MT-2. MC5R is not shown. Actual receptor pharmacology data should be consulted from primary binding studies.
5. Evidence Status
| Claim / Effect | Supporting Evidence | Evidence Level |
|---|---|---|
| Melanocortin receptor binding and activation (MC1R, MC4R) | Extensive in vitro receptor pharmacology; established from decades of published binding and functional assay data | Strong |
| Structural basis for potency (D-Phe, cyclic constraint) | Well-characterised in SAR studies; mechanism of conformational potency enhancement is established | Strong |
| Skin pigmentation in animal models | Consistent preclinical data across multiple species; MC1R mechanism established | Moderate |
| Pro-erectile effects (early human pilot) | Small randomised crossover study (Wessells et al. 1998); statistically significant vs placebo but limited sample size | Moderate |
| MC4R-mediated appetite suppression | Animal model data consistent with MC4R anorexigenic role; not evaluated in controlled human studies specific to MT-2 | Limited |
| Safety and tolerability in humans (systematic) | Adverse effects (nausea, flushing, spontaneous erections, transient BP changes) observed in early studies; no systematic long-term safety data published | Limited |
| Efficacy or safety for any indication (Phase 2/3) | No Phase 2 or Phase 3 trial for MT-2 identified; bremelanotide (a related but distinct compound) is FDA-approved | Limited |
6. Limitations of Current Research
References
- Al-Obeidi FA, Castrucci AM, Hadley ME, Hruby VJ. “Potent and prolonged acting cyclic lactam analogues of α-melanotropin: design based on molecular dynamics.” Journal of Medicinal Chemistry. 1989;32(12):2555–2561.
- Cone RD. “Anatomy and regulation of the central melanocortin system.” Nature Neuroscience. 2005;8(5):571–578. doi:10.1038/nn1455
- 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;160(2):389–393.
- Hadley ME, Dorr RT. “Melanocortin peptide therapeutics: historical milestones, clinical studies and commercialization.” Peptides. 2006;27(4):921–930. doi:10.1016/j.peptides.2005.01.029
- Mountjoy KG, Mortrud MT, Low MJ, Simerly RB, Cone RD. “Localization of the melanocortin-4 receptor (MC4-R) in neuroendocrine and autonomic control circuits in the brain.” Molecular Endocrinology. 1994;8(10):1298–1308. doi:10.1210/mend.8.10.7854347