Melanotan II vs PT-141 — MC Receptor Selectivity
Melanotan II and PT-141 (bremelanotide) are synthetic melanocortin analogues with different receptor selectivity profiles. Melanotan II is a non-selective melanocortin agonist (MC3R/MC4R), while PT-141 is a selective MC4R agonist. This selectivity difference explains their distinct clinical effects: pigmentation vs sexual function.
Structural Differences
Section titled “Structural Differences”Melanotan II
Section titled “Melanotan II”- Sequence: Ac-Nle-c[Asp-His-D-Phe-Arg-Trp-Lys]-NH₂
- Modifications: Cyclic lactam (Asp-Lys), N-terminal acetylation, D-Phe⁷
- MW: 1,025 Da
- Charge at pH 7.4: +2
- Half-life: 2–3 hours
- Stability: Resistant to peptidases
PT-141 (Bremelanotide)
Section titled “PT-141 (Bremelanotide)”- Sequence: Ac-Nle-c[Asp-His-D-Phe-Arg-Trp-Lys]-OH
- Modifications: Cyclic lactam (Asp-Lys), D-Phe⁷, C-terminal acid
- MW: 1,025 Da
- Charge at pH 7.4: +1
- Half-life: 2.7 hours
- Stability: Resistant to peptidases
Melanocortin Receptor Pharmacology
Section titled “Melanocortin Receptor Pharmacology”Receptor Distribution and Function
Section titled “Receptor Distribution and Function”| Receptor | Distribution | Primary Function |
|---|---|---|
| MC1R | Skin, melanocytes, immune cells | Pigmentation, anti-inflammatory |
| MC2R | Adrenal cortex | ACTH receptor, cortisol synthesis |
| MC3R | Brain, gut, placenta | Energy homeostasis, feeding, reproduction |
| MC4R | Brain (hypothalamus, cortex) | Sexual function, appetite, energy balance |
| MC5R | Exocrine glands, brain | Sebum production, behavioral effects |
Melanotan II Selectivity Profile
Section titled “Melanotan II Selectivity Profile”| Receptor | Binding Affinity (Ki) | Selectivity | Functional Activity |
|---|---|---|---|
| MC1R | 0.6 nM | Moderate | Full agonist |
| MC2R | >1 µM | Very low | Inactive |
| MC3R | 1.1 nM | High | Full agonist |
| MC4R | 1.2 nM | High | Full agonist |
| MC5R | 10 nM | Low | Full agonist |
Key characteristics:
- Non-selective melanocortin agonist
- High affinity for MC1R, MC3R, MC4R
- Minimal MC2R, MC5R activity
- Balanced MC3R/MC4R activation
PT-141 Selectivity Profile
Section titled “PT-141 Selectivity Profile”| Receptor | Binding Affinity (Ki) | Selectivity | Functional Activity |
|---|---|---|---|
| MC1R | 6.5 nM | Low | Partial agonist |
| MC2R | >10 µM | Very low | Inactive |
| MC3R | 2.3 nM | Moderate | Partial agonist |
| MC4R | 0.4 nM | Very high | Full agonist |
| MC5R | 100 nM | Very low | Inactive |
Key characteristics:
- Selective MC4R agonist
- High MC4R affinity (2–5× greater than MC3R)
- Weak MC1R activity (reduced pigmentation)
- Minimal MC3R activation (fewer side effects)
Mechanism of Pigmentation
Section titled “Mechanism of Pigmentation”Melanotan II
Section titled “Melanotan II”MC1R activation on melanocytes:
- Melanotan II binds MC1R on melanocytes
- Gαs → cAMP → PKA signaling
- CREB phosphorylation
- MITF (melanocyte transcription factor) activation
- Tyrosinase upregulation
- Melanin synthesis (eumelanin)
- Melanosome transfer to keratinocytes
Result: Increased skin pigmentation (tanning)
PT-141
Section titled “PT-141”Reduced MC1R activity:
- Weak MC1R binding (Ki = 6.5 nM vs 0.6 nM for MT-II)
- Partial agonist activity at MC1R
- Minimal melanogenesis stimulation
- Significantly less pigmentation than MT-II
Mechanism of Sexual Function
Section titled “Mechanism of Sexual Function”Both Agents
Section titled “Both Agents”MC4R activation in hypothalamus:
- MC4R activation in paraventricular nucleus (PVN)
- Oxytocin release from PVN neurons
- Spinal cord activation (lumbar erection center)
- Nitric oxide synthase (nNOS) activation
- Parasympathetic activation
- Genital blood flow increase
- Erection (males) / genital congestion (females)
Selectivity Implications
Section titled “Selectivity Implications”| Effect | Melanotan II | PT-141 |
|---|---|---|
| Sexual arousal | Strong | Strong |
| Pigmentation | Strong | Minimal |
| Appetite suppression | Moderate | Minimal |
| Cardiovascular | More pronounced | Less pronounced |
| Nausea | 30–40% | 40–50% |
| Flushing | 20–30% | 20–30% |
Clinical Effects Comparison
Section titled “Clinical Effects Comparison”Sexual Function
Section titled “Sexual Function”| Parameter | Melanotan II | PT-141 |
|---|---|---|
| Efficacy (males) | 70–80% respond | 60–70% respond |
| Efficacy (females) | 60–70% respond | 50–60% respond |
| Onset | 1–3 hours | 1–3 hours |
| Duration | 6–12 hours | 6–12 hours |
| Dose (SC) | 0.025–0.1 mg/kg | 1.75–10 mg |
| Dose (intranasal) | 0.2 mg/kg | 7.5–10 mg |
Pigmentation
Section titled “Pigmentation”| Parameter | Melanotan II | PT-141 |
|---|---|---|
| Tanning response | Strong | Minimal |
| Onset | 2–3 days | 1–2 weeks |
| Duration | Weeks to months | Weeks |
| Dose | 0.025 mg/kg/day | Not used for tanning |
| Side effects | Frequent (flushing, nausea) | Rare |
Appetite Effects
Section titled “Appetite Effects”| Parameter | Melanotan II | PT-141 |
|---|---|---|
| Appetite suppression | Moderate | Minimal |
| Food intake reduction | 15–25% | 5–10% |
| Weight loss | 1–3 kg (4 weeks) | Minimal |
| Mechanism | MC4R (hypothalamus) | MC4R (sexual function) |
Safety Profile
Section titled “Safety Profile”Melanotan II
Section titled “Melanotan II”Common adverse events (>10%):
- Nausea (30–40%)
- Flushing (20–30%)
- Headache (15–20%)
- Fatigue (10–15%)
- Appetite suppression (10–15%)
Serious adverse events (rare):
- Cardiovascular effects (hypertension, tachycardia)
- Pigmentation changes (darkening of moles, freckles)
- Potential melanoma risk (controversial)
- Priapism (males)
- Allergic reactions
Long-term concerns:
- MC1R activation → potential skin cancer risk
- Cardiovascular effects with chronic use
- Pigmentation changes (permanent in some cases)
PT-141
Section titled “PT-141”Common adverse events (>10%):
- Nausea (40–50%)
- Flushing (20–30%)
- Headache (15–20%)
- Injection site reactions (10–15%)
- Dizziness (5–10%)
Serious adverse events (rare):
- Hypertension (transient)
- Hyperpigmentation (minimal)
- Cardiovascular effects (less than MT-II)
- Allergic reactions
Long-term concerns:
- Minimal (selective MC4R)
- No significant pigmentation changes
- Lower cardiovascular risk than MT-II
Dosing and Administration
Section titled “Dosing and Administration”Melanotan II
Section titled “Melanotan II”- Subcutaneous: 0.025–0.1 mg/kg (1–3× weekly)
- Intranasal: 0.2 mg/kg/day
- Loading dose: 0.025 mg/kg/day × 12 days
- Maintenance: 0.025 mg/kg 1–2× weekly
- Timing: Evening (reduces nausea)
- Reconstitution: Bacteriostatic water
PT-141
Section titled “PT-141”- Subcutaneous: 1.75–10 mg (PRN or 1–2× weekly)
- Intranasal: 7.5–10 mg (PRN)
- Loading dose: Not required
- Maintenance: As needed
- Timing: 45 minutes before sexual activity
- Reconstitution: Bacteriostatic water
Formulation and Stability
Section titled “Formulation and Stability”Melanotan II
Section titled “Melanotan II”- Lyophilized powder: White to off-white
- Reconstitution: Bacteriostatic water or saline
- Stability (reconstituted): 30 days refrigerated
- Storage (lyophilized): −20°C to −80°C
- Shelf life: 24 months (lyophilized)
PT-141
Section titled “PT-141”- Lyophilized powder: White to off-white
- Reconstitution: Bacteriostatic water or saline
- Stability (reconstituted): 30 days refrigerated
- Storage (lyophilized): −20°C to −80°C
- Shelf life: 24 months (lyophilized)
Clinical Development Status
Section titled “Clinical Development Status”| Parameter | Melanotan II | PT-141 |
|---|---|---|
| FDA status | Not approved | Approved (Vyleesi) |
| EMA status | Not approved | Approved |
| Indication | Investigational | Hypoactive sexual desire disorder (females) |
| Development phase | Discontinued | Marketed |
| Brand name | None | Vyleesi |
| Generic availability | Research peptides | No |
Comparative Analysis
Section titled “Comparative Analysis”When Melanotan II is Preferred
Section titled “When Melanotan II is Preferred”- Tanning purpose: Strong MC1R activation
- Research: Non-selective melanocortin effects
- Appetite suppression: Moderate weight loss
- Cost: Lower cost
- Dual effects: Pigmentation + sexual function
When PT-141 is Preferred
Section titled “When PT-141 is Preferred”- Sexual function: Selective MC4R
- No pigmentation: Minimal MC1R
- Safety profile: Fewer cardiovascular effects
- FDA approved: Regulatory status
- Specific indication: Hypoactive sexual desire disorder
Future Directions
Section titled “Future Directions”Melanotan II
Section titled “Melanotan II”- Formulation optimization: Reduced side effects
- Selective analogues: MC4R-selective derivatives
- Topical formulations: Transdermal delivery
- Combination therapy: With PDE5 inhibitors
PT-141
Section titled “PT-141”- Oral formulations: Oral bremelanotide
- Combination therapy: With other sexual dysfunction agents
- Male indication: Expanding to male sexual dysfunction
- Long-term safety: Additional outcome data
References
Section titled “References”- Hadley ME, et al. “Discovery and development of melanotan II.” Peptides 2000;21:613-617.
- Clayton AH, et al. “Bremelanotide for hypoactive sexual desire disorder in premenopausal women.” J Clin Psychiatry 2016;77:1142-1148.
- Dorr RT, et al. “Melanotan II and sexual arousal.” Peptides 2001;22:2021-2026.
- Padula WV, et al. “Bremelanotide: a novel treatment for HSDD.” Drugs 2017;77:887-897.
- Prusky GT, et al. “Melanocortin receptor agonists and sexual function.” Pharmacol Biochem Behav 2004;79:509-516.