Research Use Only. The information presented here is for scientific and educational purposes. These compounds are not intended for human consumption, self-administration, or therapeutic use.
Introduction
PT-141 (also designated bremelanotide) and Melanotan-2 (commonly abbreviated MT-II) are two synthetic cyclic peptides that share a common origin as analogs of alpha-melanocyte-stimulating hormone (alpha-MSH). In laboratory research, both are studied as agonists of the melanocortin receptor family, a group of G-protein-coupled receptors designated MC1R through MC5R. Because the two peptides are structurally almost identical, investigators frequently examine them side by side to understand how a single molecular change can shift activity across individual receptor subtypes.
This comparison focuses on melanocortin receptor selectivity, specifically the relative activity reported at MC1R, MC3R, and MC4R. The distinction is meaningful in preclinical models because each subtype anchors a different line of investigation, from melanogenesis at MC1R to central signaling examined at MC3R and MC4R. Every compound discussed here is offered for research use only, not for human consumption, and the summaries below describe what each peptide has been investigated for in vitro and in animal models rather than any applied outcome.
The Melanocortin Receptor Family
The melanocortin system comprises five receptor subtypes, each with a distinct tissue distribution and research profile. Mapping these subtypes provides the framework for comparing how PT-141 and Melanotan-2 behave in binding and functional assays.
- MC1R: expressed on melanocytes and studied primarily in the context of melanogenesis and pigmentation.
- MC2R: located in the adrenal cortex and responsive to ACTH rather than to alpha-MSH analogs, so it is not a meaningful target for either peptide.
- MC3R: concentrated in the central nervous system and investigated in models of energy homeostasis and inflammatory signaling.
- MC4R: also central, and examined extensively in research on energy balance and feeding behavior.
- MC5R: associated with exocrine gland function in preclinical studies.
Both PT-141 and Melanotan-2 are described in the literature as broadly acting agonists, meaning they engage several of these subtypes rather than one alone. The endogenous ligand they mimic, alpha-MSH, carries a core message sequence (His-Phe-Arg-Trp) that both peptides retain, which explains their overlapping but not identical receptor coverage.
Structural Comparison of PT-141 and Melanotan-2
At the molecular level, PT-141 and Melanotan-2 differ by a single functional group, yet that difference is the primary driver of their divergent receptor profiles. Both are cyclic heptapeptides built on the same lactam-bridged scaffold, and both incorporate D-phenylalanine (D-Phe), a substitution that increases stability and binding affinity relative to native alpha-MSH. The values in the table below reflect commonly reported laboratory attributes and are provided for comparison only.
| Attribute | PT-141 (Bremelanotide) | Melanotan-2 (MT-II) |
|---|---|---|
| Class | Cyclic heptapeptide, melanocortin analog | Cyclic heptapeptide, melanocortin analog |
| Core sequence | Ac-Nle-cyclo[Asp-His-D-Phe-Arg-Trp-Lys]-OH | Ac-Nle-cyclo[Asp-His-D-Phe-Arg-Trp-Lys]-NH2 |
| Molecular formula | C50H68N14O10 | C50H69N15O9 |
| Approx. molecular weight | 1025.2 g/mol | 1024.2 g/mol |
| Sequence length | 7 residues (cyclic) | 7 residues (cyclic) |
| C-terminus | Carboxylic acid (-OH) | Primary amide (-NH2) |
| Structural relationship | Free-acid metabolite of Melanotan-2 | Parent compound |
The single structural distinction sits at the C-terminus: Melanotan-2 terminates in a primary amide group, while PT-141 terminates in a carboxylic acid. PT-141 is, in fact, the free-acid metabolite produced when the terminal amide of Melanotan-2 is cleaved. This apparently minor change alters the charge and hydrogen-bonding character of the molecule, which in turn shifts how tightly it engages each melanocortin receptor subtype.
Receptor Selectivity: MC1R, MC3R, and MC4R
Because PT-141 and Melanotan-2 differ only at the C-terminus, their receptor selectivity overlaps substantially but is not identical. The clearest divergence appears at MC1R. The receptor activities summarized below reflect commonly reported laboratory attributes and are provided for comparison only.
| Receptor | Associated research area | PT-141 reported activity | Melanotan-2 reported activity |
|---|---|---|---|
| MC1R | Melanogenesis, pigmentation | Comparatively weaker agonism | Strong agonism |
| MC3R | Central energy signaling, inflammation | Agonist | Agonist |
| MC4R | Energy balance, feeding behavior | Agonist, principal research focus | Agonist |
MC1R and Melanogenesis
MC1R is the subtype most closely tied to pigment-producing melanocytes. Melanotan-2 is a potent MC1R agonist, which is why it appears frequently in melanogenesis research and in studies of melanin formation. PT-141, having exchanged its terminal amide for a carboxylic acid, shows comparatively weaker MC1R engagement in reported assays, so it is featured less often in pigmentation-focused work.
MC3R and MC4R Signaling
MC3R and MC4R are concentrated in the central nervous system and are studied in models of energy homeostasis. Research on PT-141 tends to emphasize MC4R activity, whereas Melanotan-2 acts broadly across MC1R, MC3R, and MC4R. That broader coverage is why Melanotan-2 is often characterized as a non-selective melanocortin agonist, while PT-141 is described as relatively more oriented toward the MC3R and MC4R axis. For researchers comparing the two, this shift in the balance of receptor engagement, rather than any wholesale change in target, is the defining variable.
Research Handling, Purity, and Sourcing
Both peptides are supplied as lyophilized (freeze-dried) powders and are typically reconstituted with bacteriostatic water before laboratory use. As an illustrative example, reconstituting a 10 mg vial with 2 mL of bacteriostatic water yields a working concentration of 5 mg/mL. Melanocortin peptides are generally light-sensitive, so protecting solutions from prolonged light, keeping them cold, and minimizing freeze-thaw cycles all help preserve molecular integrity. Step-by-step handling is covered in the bacteriostatic water reconstitution guide.
Because PT-141 and Melanotan-2 are so close in mass and formula, confirming identity and purity is an important part of any comparative study. A certificate of analysis documents peptide content and purity, and understanding how to read a peptide COA helps verify that a vial matches its stated molecular profile. Selecting a vetted research peptide supplier that publishes third-party certificates supports reproducibility across experiments and keeps comparisons meaningful.
Frequently Asked Questions
What is the main difference between PT-141 and Melanotan-2?
PT-141 and Melanotan-2 share the same cyclic heptapeptide backbone and differ only at the C-terminus, where Melanotan-2 carries a primary amide and PT-141 carries a carboxylic acid. PT-141 is the free-acid metabolite of Melanotan-2. In receptor assays this small change reduces PT-141 activity at MC1R while largely preserving activity at MC3R and MC4R.
Are PT-141 and Melanotan-2 selective for a single melanocortin receptor?
Neither peptide is strictly selective. Both are broadly acting melanocortin agonists. Melanotan-2 engages MC1R, MC3R, and MC4R strongly, whereas PT-141 is comparatively weaker at MC1R and is studied more for its MC3R and MC4R activity.
Why is Melanotan-2 linked with melanogenesis research?
Melanogenesis is driven largely through MC1R on melanocytes. Because Melanotan-2 is a potent MC1R agonist, it is a common tool compound in laboratory studies of pigment formation. PT-141, with weaker MC1R engagement, appears less often in that context.
How do the molecular weights of PT-141 and Melanotan-2 compare?
The two are nearly identical in mass. PT-141 has an approximate molecular weight of 1025.2 g/mol (C50H68N14O10), and Melanotan-2 is close behind at roughly 1024.2 g/mol (C50H69N15O9). The small difference comes from replacing a terminal amide with a carboxylic acid.
How are these melanocortin peptides handled in the laboratory?
Both are stored as lyophilized powders and reconstituted with bacteriostatic water for research. They are treated as light-sensitive and kept cold with minimal freeze-thaw cycling. All handling is conducted strictly on a research use only basis.
Can PT-141 and Melanotan-2 be studied side by side?
Yes. Their close structural relationship makes them a natural pair for comparative receptor-selectivity research, since observed differences in MC1R, MC3R, and MC4R activity can be attributed largely to the single C-terminal modification.
References and Further Reading
- Hadley ME and Hruby VJ (University of Arizona), foundational development of the melanotan class of cyclic melanocortin analogs. PubMed: melanotan melanocortin
- Hruby VJ and colleagues, structure-activity studies of alpha-MSH analogs and constrained cyclic melanocortin peptides. PubMed: alpha-MSH analog structure activity
- Overview of melanocortin receptor subtypes (MC1R to MC5R) and their pharmacology and distribution. PubMed: melanocortin receptor subtypes
- Bremelanotide as a melanocortin receptor agonist in preclinical research. PubMed: bremelanotide melanocortin receptor
- MC4R signaling and energy homeostasis in animal models. PubMed: MC4R energy homeostasis
- MC1R and melanogenesis in melanocyte research. PubMed: MC1R melanogenesis
- Receptor selectivity and functional profiling among melanocortin peptide analogs. PubMed: melanocortin receptor selectivity
