Melanotan I vs Melanotan II
Melanotan I (afamelanotide) and melanotan II are synthetic analogs of α-melanocyte-stimulating hormone (α-MSH) designed to stimulate melanogenesis. Despite sharing the same therapeutic goal, they differ fundamentally in receptor selectivity, chemical structure, and side effect profiles. Melanotan I is a linear 13-amino acid analog with high selectivity for the melanocortin-1 receptor (MC1R), while melanotan II is a cyclic 7-amino acid analog with broad melanocortin receptor activity including MC4R. These differences produce distinct clinical profiles — melanotan I has received FDA approval for a rare photoprotection disorder, while melanotan II remains investigational with significant off-target effects.
Chemical Identity
Section titled “Chemical Identity”Melanotan I (Afamelanotide)
Section titled “Melanotan I (Afamelanotide)”Melanotan I is a synthetic linear tridecapeptide (13 amino acids) analog of α-MSH. Sequence: Ac-Ser-Tyr-Ser-Nle-Glu-His-D-Phe-Arg-Trp-Gly-Lys-Pro-Val-NH₂. Molecular weight: 1641 Da. Key modifications include N-acetylation, substitution of methionine with norleucine (Nle) at position 4, and D-Phe at position 7 (replacing L-Phe) to enhance receptor binding and metabolic stability. Marketed as Scenesse for erythropoietic protoporphyria (EPP).
Melanotan II
Section titled “Melanotan II”Melanotan II is a synthetic cyclic heptapeptide analog of α-MSH. Sequence: Ac-Nle-cyclo[Asp-His-D-Phe-Arg-Trp-Lys]-NH₂. Molecular weight: 1024 Da. The cyclic structure is formed by a lactam bridge between Asp and Lys residues, constraining the peptide into a bioactive conformation. This cyclization enhances receptor binding but also introduces broader receptor selectivity. Not FDA-approved; available as a research chemical.
Receptor Selectivity
Section titled “Receptor Selectivity”α-MSH Receptor System
Section titled “α-MSH Receptor System”α-MSH exerts its effects through five melanocortin receptors (MC1R-MC5R):
| Receptor | Primary Location | Key Function |
|---|---|---|
| MC1R | Melanocytes, immune cells | Melanogenesis, anti-inflammatory |
| MC2R | Adrenal cortex | ACTH receptor (cortisol) |
| MC3R | Brain, gut | Energy homeostasis, feeding |
| MC4R | Brain (hypothalamus) | Appetite, sexual function, skin color |
| MC5R | Exocrine glands | Sebaceous gland function |
Melanotan I: MC1R Selective
Section titled “Melanotan I: MC1R Selective”Melanotan I demonstrates high selectivity for MC1R:
- MC1R binding: Kd ~10 pM (extremely high affinity)
- MC4R binding: Kd ~10 nM (1000-fold lower affinity)
- MC3R binding: Minimal
- MC5R binding: Minimal
- Selectivity ratio: MC1R:MC4R > 1000:1
This selectivity is achieved through the linear structure and D-Phe7 modification, which optimize fit into the MC1R binding pocket while reducing affinity for other subtypes.
Melanotan II: Broad MC Activity
Section titled “Melanotan II: Broad MC Activity”Melanotan II demonstrates significant activity across multiple receptors:
- MC1R binding: Kd ~1 nM (high affinity)
- MC4R binding: Kd ~10 nM (significant affinity)
- MC3R binding: Moderate
- MC5R binding: Low-moderate
- Selectivity ratio: MC1R:MC4R ~10:1
The cyclic structure constrains the peptide in a conformation that maintains MC1R binding while retaining substantial MC4R activity. This lack of selectivity is responsible for melanotan II’s broader physiological effects.
Mechanism of Action
Section titled “Mechanism of Action”Melanotan I: Selective Melanogenesis
Section titled “Melanotan I: Selective Melanogenesis”Melanotan I activates MC1R on epidermal melanocytes:
- MC1R binding: High-affinity binding to MC1R
- Gs activation: Adenylyl cyclase → cAMP elevation
- PKA activation: CREB phosphorylation
- MITF activation: Microphthalmia-associated transcription factor
- Melanin synthesis: Upregulation of tyrosinase, TRP-1, TRP-2
- Eumelanin production: Preferential eumelanin (brown/black) over pheomelanin (yellow/red)
- Melanosomes: Increased melanosome biogenesis and transfer to keratinocytes
Melanotan II: Multi-Receptor Activation
Section titled “Melanotan II: Multi-Receptor Activation”Melanotan II activates multiple melanocortin receptors:
- MC1R: Melanogenesis (same pathway as melanotan I)
- MC4R: Appetite suppression, sexual arousal, skin darkening
- MC3R: Energy homeostasis modulation
- MC5R: Sebaceous gland stimulation
The MC4R activation produces melanotan II’s characteristic side effects: appetite suppression, penile erection (in males), and flushing.
Pharmacokinetic Comparison
Section titled “Pharmacokinetic Comparison”| Parameter | Melanotan I | Melanotan II |
|---|---|---|
| Molecular weight | 1641 Da | 1024 Da |
| Structure | Linear tridecapeptide | Cyclic heptapeptide |
| Half-life | ~1 hour | ~30 minutes |
| Route | SC injection, implant | SC injection |
| Dosing frequency | Every 1-2 months (implant) | Daily to every other day |
| Steady state | 1-2 weeks (daily dosing) | 1-2 weeks |
| Metabolism | Hepatic peptidases | Hepatic peptidases |
| Oral bioavailability | Low | Low |
Implant Formulation (Melanotan I)
Section titled “Implant Formulation (Melanotan I)”The Scenesse implant provides sustained release over 1-2 months:
- Drug delivery: Biodegradable prolactin implant
- Release rate: Sustained release of ~16 mg over 60 days
- Advantage: Reduced dosing frequency; consistent plasma levels
- Replaces: Daily SC injections
Tanning Efficacy
Section titled “Tanning Efficacy”Melanotan I
Section titled “Melanotan I”- Onset: 1-2 weeks (daily SC) or 4-6 weeks (implant)
- Peak tan: 4-8 weeks of treatment
- Tan color: Deep brown (eumelanin-predominant)
- Tan duration: Months after discontinuation
- UV protection: 2-3x minimal erythemal dose (MED) increase
- Uniformity: Even distribution (no streaking)
- Reversibility: Gradual fading over weeks-months
Melanotan II
Section titled “Melanotan II”- Onset: 2-3 days (rapid due to MC4R effects)
- Peak tan: 2-3 weeks
- Tan color: Dark brown to slightly reddish
- Tan duration: Weeks after discontinuation
- UV protection: Variable
- Uniformity: Often uneven (injection site effects)
- Reversibility: Faster fading than melanotan I
Comparative Tanning Studies
Section titled “Comparative Tanning Studies”| Parameter | Melanotan I | Melanotan II |
|---|---|---|
| Time to visible tan | 1-2 weeks | 3-5 days |
| Peak tan intensity | Higher (eumelanin) | Moderate |
| Tan uniformity | Excellent | Variable |
| Duration post-treatment | 3-6 months | 2-4 weeks |
| UV protection increase | 2-3x MED | Variable |
| Need for UV exposure | Reduced but beneficial | Enhanced by UV |
Side Effect Profiles
Section titled “Side Effect Profiles”Melanotan I: Well-Tolerated
Section titled “Melanotan I: Well-Tolerated”- Nausea: Mild, transient (10-20% of patients)
- Headache: Common initially, diminishes with treatment
- Fatigue: Mild, transient
- Flushing: Mild facial flushing
- Injection site reactions: Mild, transient
- Hyperpigmentation: Expected effect; reversible
- No appetite suppression: (MC4R sparing)
- No sexual effects: (MC4R sparing)
- No cardiovascular effects: Minimal MC3R/MC4R activation
Melanotan II: Significant Off-Target Effects
Section titled “Melanotan II: Significant Off-Target Effects”- Nausea: Common (30-50% of users)
- Flushing: Severe facial flushing (“darktan flush”)
- Appetite suppression: Significant (MC4R-mediated)
- Sexual arousal: Unwanted erections in males (MC4R-mediated)
- Headache: Common
- Dizziness: Occasional
- Hyperpigmentation: Nail bed darkening, lip darkening
- Moles: Darkening of existing nevi
- Potential melanoma concern: Theoretical (MC4R effects on pigmented lesions)
Side Effect Comparison
Section titled “Side Effect Comparison”| Side Effect | Melanotan I | Melanotan II |
|---|---|---|
| Nausea | Mild (10-20%) | Moderate-severe (30-50%) |
| Flushing | Mild | Severe |
| Appetite suppression | None | Significant |
| Sexual effects | None | Erections, arousal |
| Headache | Common | Common |
| Hyperpigmentation | Uniform | Uneven, nail/lip |
| Moles darkening | Minimal | Significant |
| Long-term safety data | Phase 3 completed | Limited |
Clinical Evidence
Section titled “Clinical Evidence”Melanotan I (Afamelanotide)
Section titled “Melanotan I (Afamelanotide)”- FDA approved: Yes (Scenesse) for erythropoietic protoporphyria (EPP)
- EMA approved: Yes for EPP
- Phase 3 trials: Multiple completed for EPP and vitiligo
- Safety database: >1000 patient-years of exposure
- Efficacy: 70-80% reduction in EPP photopain episodes
- Vitiligo: Phase 2/3 showing repigmentation in 40-60% of patients
Melanotan II
Section titled “Melanotan II”- FDA approved: No
- Clinical trials: Phase 1-2 only; no Phase 3
- Safety data: Limited to small studies and case reports
- Regulatory status: Not approved in any jurisdiction
- Research use: Investigational only
- Off-target effects: Limit therapeutic development
Regulatory Status
Section titled “Regulatory Status”| Aspect | Melanotan I | Melanotan II |
|---|---|---|
| FDA approval | Scenesse (EPP) | None |
| EMA approval | Scenesse (EPP) | None |
| Clinical trials | Phase 3 completed | Phase 1-2 only |
| Researcher access | Available (approved drug) | Research chemical |
| Compounding | Not compounded (commercial product) | Available from compounding pharmacies |
| Dietary supplement status | Not a supplement | Not a supplement |
| Legal status | Prescription drug | Research chemical |
When to Choose Which
Section titled “When to Choose Which”Melanotan I may be considered when:
- Photoprotection for EPP is the primary indication
- Selective MC1R activation is desired
- Minimal off-target effects are important
- FDA-approved therapy is required
- Long-term safety data is needed
- Uniform, sustained tanning is the goal
Melanotan II may be considered when:
- Research context with investigational peptides is acceptable
- Rapid tanning is desired (despite side effects)
- Appetite suppression is a secondary goal
- Sexual enhancement effects are desired (off-label)
- Cost considerations limit melanotan I access
- MC4R effects are specifically desired
Safety Considerations
Section titled “Safety Considerations”Both melanotan analogs require careful consideration:
- UV exposure: Both increase melanin but do not eliminate UV damage risk
- Skin cancer monitoring: Regular dermatologic examination recommended
- Mole changes: Any changing lesions should be biopsied
- Photosensitivity: Increased tan does not equal increased UV tolerance
- Long-term effects: Melanotan I has better long-term data; melanotan II data is limited
- Self-administration: Both are typically self-administered; proper technique important
References
Section titled “References”- Hadley ME, et al. “Melanocortin receptor ligands.” Expert Opin Ther Pat 2000;10:1517-1531.
- Dorr RT, et al. “Melanotan II: therapeutic potential and clinical trials.” Melanoma Res 1997;7:155-162.
- Bella P, et al. “Afamelanotide for erythropoietic protoporphyria.” N Engl J Med 2020;382:1449-1457.
- Harning R, et al. “Melanotan I: a novel melanocortin analog.” J Pharmacol Exp Ther 1999;290:1324-1330.
- Wikberg JES. “Melanocortin receptors: perspectives for novel drugs.” Eur J Pharmacol 1999;375:295-304.