Maple Research comparison of melanocortin system peptides: Melanotan I (afamelanotide), Melanotan II, and PT-141 (bremelanotide). Covers the five melanocortin receptors (MC1R-MC5R), structural differences between linear and cyclic analogues, receptor selectivity profiles, and the relationship between these compounds.
The Melanocortin Receptor System
The melanocortin system consists of five G protein-coupled receptors (MC1R through MC5R) that are activated by endogenous peptides derived from proopiomelanocortin (POMC) processing, primarily alpha-MSH. Each receptor has a distinct tissue distribution and functional profile:
| Receptor | Primary Location | Function | Key Agonist |
|---|---|---|---|
| MC1R | Melanocytes (skin, hair follicles) | Melanogenesis (eumelanin production), UV-protective pigmentation | MT-I (afamelanotide) |
| MC2R | Adrenal cortex | Cortisol synthesis (ACTH receptor) | ACTH only |
| MC3R | Hypothalamus, GI tract | Energy homeostasis, feeding behavior | Gamma-MSH |
| MC4R | Hypothalamus, brainstem, spinal cord | Appetite regulation, energy expenditure, autonomic/sexual function | PT-141 (bremelanotide) |
| MC5R | Exocrine glands, adipose tissue | Sebaceous gland regulation, immune function | Alpha-MSH |
Three Compounds, One System
Melanotan I, Melanotan II, and PT-141 are structurally related but pharmacologically distinct. PT-141 is actually a metabolite of Melanotan II, discovered when researchers observed that MT-II’s melanocortin effects included CNS-mediated signaling through MC4R that was independent of its pigmentation effects via MC1R.
| Parameter | Melanotan I (Afamelanotide) | Melanotan II | PT-141 (Bremelanotide) |
|---|---|---|---|
| Structure | Linear 13-amino acid peptide | Cyclic 7-amino acid peptide (lactam bridge) | Cyclic 7-amino acid peptide (free acid C-terminus) |
| Derived From | Alpha-MSH analogue (linear) | Alpha-MSH core with cyclization | Active metabolite of Melanotan II |
| Receptor Selectivity | MC1R-preferring (relatively selective) | Non-selective (MC1R, MC3R, MC4R, MC5R) | MC4R-preferring (MC3R, MC4R) |
| Primary Research Activity | Melanogenesis (pigmentation) | Melanogenesis + MC4R CNS effects | MC4R CNS-mediated signaling |
| Pigmentation Effect | Strong (EMA-approved for this purpose) | Strong | Weak to moderate |
| CNS MC4R Activity | Minimal (poor BBB penetration, MC1R preference) | Significant | Primary mechanism |
| Regulatory Status | EMA approved (Scenesse, 2014, erythropoietic protoporphyria) | Research only | FDA approved (Vyleesi, 2019) |
| Key Structural Feature | Linear (no lactam bridge) | Lactam bridge (Asp5-Lys10), amide C-terminus | Lactam bridge, free acid C-terminus (from MT-II metabolism) |
Linear vs Cyclic: Why Structure Determines Selectivity
The structural difference between MT-I (linear) and MT-II/PT-141 (cyclic) directly determines receptor selectivity:
Melanotan I retains the linear peptide structure of alpha-MSH, which gives it relatively selective MC1R binding. The linear conformation presents the pharmacophore (His-Phe-Arg-Trp core) in an orientation that preferentially engages MC1R’s binding pocket. This MC1R selectivity is why MT-I’s primary effect is pigmentation without significant CNS signaling.
Melanotan II and PT-141 contain a lactam bridge that constrains the peptide into a cyclic conformation. This cyclization locks the His-Phe-Arg-Trp pharmacophore into a more rigid orientation that engages multiple melanocortin receptor subtypes, particularly MC4R. The cyclic structure increases metabolic stability (resistance to peptidases) and alters receptor selectivity from MC1R-preferring to broadly active (MT-II) or MC4R-preferring (PT-141).
Cardiovascular and Autonomic Considerations
MC4R activation in the central nervous system engages autonomic signaling pathways, which has implications for cardiovascular parameters in research settings. PT-141 (bremelanotide) produces transient increases in blood pressure and decreases in heart rate in preclinical and clinical studies. This autonomic activation is a direct consequence of central MC4R signaling, not a peripheral vascular effect, and is unique to the MC4R-active compounds (MT-II and PT-141) but not MC1R-selective compounds (MT-I).
Research Selection Guide
| Research Focus | Recommended | Rationale |
|---|---|---|
| MC1R/melanogenesis pathway | Melanotan I | MC1R-preferring, minimal MC4R confounding |
| MC4R CNS signaling | PT-141 | MC4R-preferring, FDA-approved reference |
| Broad melanocortin system | Melanotan II | Non-selective, activates MC1R + MC3R + MC4R + MC5R |
| Pigmentation without CNS effects | Melanotan I | Linear structure, poor BBB penetration |
| Autonomic/cardiovascular MC4R effects | PT-141 | Cleanest MC4R signal, clinical BP/HR data available |
| Structure-activity relationships | All three | Linear vs cyclic comparison, free acid vs amide C-terminus |
Research Products
Available at Maple Research Labs
Third-party COA verification | Same-day Canadian shipping
Deep-Dive Research Pages
Melanotan (Melanocortin System) Research | MT-I vs MT-II, full MC receptor pharmacology
PT-141 (Bremelanotide) Research | MC4R mechanism, CNS signaling, FDA approval data
Frequently Asked Questions
What is the difference between Melanotan 1 and Melanotan 2?
Melanotan I is a linear 13-amino acid peptide that preferentially activates MC1R (the melanogenesis receptor), producing pigmentation effects with minimal CNS signaling. Melanotan II is a cyclic 7-amino acid peptide with a lactam bridge that activates multiple melanocortin receptors non-selectively (MC1R, MC3R, MC4R, MC5R), producing both pigmentation and CNS-mediated effects via MC4R. The structural difference (linear vs cyclic) is what determines their different receptor selectivity profiles.
What is the relationship between Melanotan II and PT-141?
PT-141 (bremelanotide) is an active metabolite of Melanotan II. When MT-II is processed enzymatically, its C-terminal amide is converted to a free acid, producing PT-141. This metabolite retains the cyclic structure and MC4R activity but has shifted receptor preference toward MC4R and away from MC1R. PT-141 was developed as a standalone compound after researchers recognized that MT-II’s CNS-mediated effects via MC4R were pharmacologically distinct from its pigmentation effects via MC1R.
Where can I buy melanocortin peptides for research in Canada?
Maple Research Labs supplies Melanotan 1, Melanotan 2, and PT-141 within Canada, with third-party purity verification via HPLC and mass spectrometry. All products include batch-specific COAs and ship same-day from Canadian facilities. These compounds are supplied exclusively for in-vitro and preclinical research use.