What Is Melanotan II?
Melanotan II (cyclo-[Nle⁴, D-Phe⁷]-α-MSH; c[Ac-Nle-cyclo[Asp-His-D-Phe-Arg-Trp-Lys]-NH₂]) is a cyclic, lactam-bridged heptapeptide developed by Mac Hadley and Victor Hruby at the University of Arizona. It was designed to be a more potent and metabolically stable analogue of α-MSH, the endogenous melanocortin peptide derived from POMC (pro-opiomelanocortin).
The key structural modifications versus α-MSH: cyclisation via Asp-Lys lactam bridge (increases stability and receptor affinity); substitution of Met⁴→Nle (metabolically stable, no oxidation); D-Phe⁷ (increases potency at MCRs). These changes produce a compound with ~1000× greater potency than α-MSH at MC1R/MC4R with much longer half-life.
The Melanocortin Receptor System
The five melanocortin receptors (MC1R–MC5R) are Gs-coupled GPCRs activated by POMC-derived peptides (α-MSH, β-MSH, γ-MSH, ACTH). Each receptor has distinct tissue distribution and function:
Melanocytes, immune cells, melanogenesis, anti-inflammatory; key target for photoprotective tanning research
Hypothalamus, limbic, energy homeostasis, feeding behaviour, sexual arousal (with MC4R)
CNS (hypothalamic PVN, limbic), sexual arousal, erectile function, energy balance, autonomic regulation
Exocrine glands, sebaceous, lacrimal, salivary secretion; immune modulation
All MCRs signal via Gs → adenylyl cyclase → ↑cAMP → PKA. Melanotan II activates all four (MC1, 3, 4, 5R), which contributes to both its broad research utility and its complex side-effect profile (nausea, spontaneous erection, flushing, from MC3R/MC4R activation; darkening of nevi from MC1R).
MC1R: Melanogenesis & Photoprotection Research
The primary motivation for Melanotan II's development was photoprotective tanning, using pharmacological melanogenesis to protect skin from UV-induced DNA damage without UV exposure.
Melanogenesis Pathway
MC1R activation on melanocytes → ↑cAMP → PKA → MITF (Microphthalmia-associated transcription factor) upregulation → ↑Tyrosinase expression and activity → Melanin synthesis (eumelanin preferentially over pheomelanin).
Eumelanin (dark brown/black) provides substantially better photoprotection than pheomelanin (red/yellow), eumelanin is 5–10× more photoprotective per unit. MC1R activation by MT-II shifts the eumelanin:pheomelanin ratio towards eumelanin synthesis independent of UV irradiation.
Key Preclinical Findings
- In Sinclair swine (a model for photoprotective tanning research), MT-II produced significant pigmentation within 10–14 days without UV exposure
- Pigmented Sinclair swine showed reduced UV-induced erythema, epidermal damage, and DNA photoproduct formation (6-4PP, CPD) versus non-pigmented controls
- In agouti mice (MC1R-deficient model producing only pheomelanin), MT-II could not induce eumelanin, confirming MC1R-dependence of the photoprotective effect
MC4R: Sexual Arousal & Erectile Function Research
The most significant serendipitous discovery in MT-II research was its powerful pro-erectile and pro-arousal effects in human Phase I trials, where spontaneous erections were reported by male subjects receiving IV MT-II for melanogenesis research.
MC4R is expressed in hypothalamic paraventricular nucleus (PVN), limbic structures, and spinal cord neurons. MC4R activation triggers:
- Pro-erectile signalling: Oxytocin release from PVN → spinal cord MC4R neurons → ↑NO-dependent penile blood flow via non-adrenergic/non-cholinergic (NANC) mechanisms
- Central arousal: MC4R activation in limbic areas increases motivation and subjective arousal independent of the peripheral vascular component
- Female arousal: Vaginal congestion and subjective arousal in Phase I trials in women (later developed as PT-141 research rationale)
Phase I/II Sexual Function Data
| Cohort | Route | Erection / Arousal Response | Notes |
|---|---|---|---|
| Healthy males (n=20) | IV 0.01 mg/kg | ~80% spontaneous erections | Dose-finding; nausea dose-limiting |
| ED patients (n=30) | SC 0.025 mg/kg | Significant vs placebo | Phase II; nausea ~25% |
| FSIAD women (n=19) | SC | ↑ Genital arousal (VPP) | Led to PT-141 female HSDD program |
Metabolic Research (MC3R/MC4R)
Melanocortin receptors (MC3R, MC4R) are central regulators of energy homeostasis. MC4R knockout mice are obese; MC4R mutations are the most common monogenic cause of severe human obesity.
MT-II administration in rodent obesity models:
- Reduces food intake (anorexigenic effect via MC4R in hypothalamic PVN)
- Increases energy expenditure (thermogenesis via brown adipose tissue; MC4R → SNS activation)
- Improves insulin sensitivity independent of weight loss
- Reduces body weight in DIO (diet-induced obesity) mouse models at doses producing sexual effects
The metabolic and sexual effects of MT-II cannot be easily separated because both are MC4R-mediated, this drove the development of more selective MC4R agonists (and the spin-off into PT-141 for selective sexual dysfunction research).
Melanotan II vs PT-141 Comparison
| Property | Melanotan II | PT-141 (Bremelanotide) |
|---|---|---|
| Structure | Cyclic (lactam bridge) | Linear (ring-opened) |
| MC1R activity | Strong | Reduced |
| MC4R activity | Strong | Strong (selective) |
| Tanning effect | Strong (MC1R) | Minimal |
| Sexual effect | Strong (with tanning) | Strong (isolated) |
| FDA approval | No | Yes (Vyleesi, 2019) |
| BP elevation | Moderate transient | Moderate transient |
| Nausea | Common (20–40%) | Common (40%) |
For researchers studying the melanocortin system as a whole, including both pigmentation and sexual/metabolic circuits, MT-II remains the broader pharmacological tool. For isolated sexual dysfunction research, PT-141 provides cleaner MC4R-focused data with less MC1R confound.
Research Protocols
Melanogenesis (Skin Model)
- Model: Sinclair swine, human skin organ culture, B16 melanoma cells (in vitro)
- Dose (in vitro): 0.1–10 nM × 72h
- Endpoints: Melanin content (spectrophotometry), tyrosinase activity, MITF protein
- Controls: α-MSH, vehicle, MC1R antagonist (Ac-HfrRWKNHtyr)
Erectile Function (Rodent)
- Model: Rat IPN (non-contact erection test)
- Dose: 25–100 µg/kg SC or ICV
- Endpoints: Ex copula erection score, intracavernous pressure (ICP)
- Controls: Saline, MC4R antagonist (HS024), sildenafil comparison arm
Obesity / Metabolic
- Model: DIO C57BL/6 mice or Zucker fa/fa rats
- Dose: 0.1–0.5 mg/kg IP × 14–28 days
- Endpoints: Body weight, food intake, GTT, DEXA body comp
- Controls: Vehicle, MC4R KO control, pair-fed group
FAQ
What is Melanotan II?
A synthetic cyclic α-MSH analogue developed at the University of Arizona. It is a broad-spectrum melanocortin agonist (MC1R, MC3R, MC4R, MC5R) studied for photoprotective tanning (MC1R), sexual dysfunction (MC4R), and metabolic regulation.
How does Melanotan II differ from PT-141?
PT-141 (bremelanotide) is a hydrolysis metabolite of MT-II with reduced MC1R activity. PT-141 is more selective for MC3R/MC4R, producing sexual arousal with less tanning. MT-II has broader receptor activation. PT-141 is FDA-approved; MT-II is not.
Is the tanning effect UV-independent?
Yes, MC1R activation by MT-II drives melanogenesis (eumelanin synthesis) independently of UV irradiation. This "sunless tanning" mechanism was the original research rationale. However, UV co-exposure significantly amplifies the melanogenic response.
Why was MT-II not developed as a drug?
The lack of receptor selectivity (activating MC1R, MC3R, MC4R, MC5R simultaneously) made it difficult to develop for any single indication without off-target effects. The spin-off compound PT-141 was selectively developed for sexual dysfunction and achieved FDA approval, making further MT-II clinical development redundant.
Melanotan II for Research
Lyophilised Melanotan II ≥98% purity (HPLC), cyclic lactam form, with full COA and mass spec verification.
View PT-141 (Bremelanotide) →