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) →