PT-141: the melanocortin agonist melanotan II left behind

PT-141, known by the research name bremelanotide, is a cyclic heptapeptide that has become a standard reference ligand in melanocortin signalling work. It descends chemically from melanotan II: it is the deaminated, C-terminal free-acid analogue of that older compound, which means one amide group is removed and the ring is closed through a lactam bridge. Those two structural facts, the cyclisation and the free-acid terminus, are what give the molecule its receptor profile and its stability in laboratory buffers.

Where it came from

The melanocortin story starts with alpha-melanocyte-stimulating hormone, a short peptide that acts on a family of five G-protein-coupled receptors labelled MC1R through MC5R. Early work on melanotan analogues was aimed at skin-pigmentation research, because MC1R drives melanin production. Investigators then noticed a second, unexpected signal in animal models: activation of central melanocortin receptors changed sexual-behaviour readouts independent of any pigment effect.

PT-141 emerged from that line of work as the metabolite that kept the behavioural signalling activity while shedding some of the pigmentary potency. The lineage is worth stating precisely. Melanotan II is the parent; PT-141 is its major active metabolite, formed when the C-terminal amide is hydrolysed to a carboxylic acid. Because of that relationship, the two compounds are frequently studied side by side to separate the contributions of different receptor subtypes.

How it works

PT-141 is a non-selective agonist across several melanocortin receptors, with characterised activity at MC1R, MC3R and MC4R. The subtype of most interest in central signalling research is MC4R, which is expressed densely in hypothalamic nuclei. When an agonist binds MC4R, the receptor couples to Gs, adenylyl cyclase is activated, and intracellular cyclic AMP rises. That second-messenger step is the readout most cell assays measure.

Two properties make the compound attractive as a tool. First, the cyclic backbone resists exopeptidase cleavage better than a simple linear peptide, so it survives longer in assay conditions. Second, its agonist activity is measurable at more than one receptor subtype, which lets researchers use it as a broad melanocortin probe and then layer in subtype-selective antagonists to work out which receptor carries a given effect. That subtractive approach, agonise broadly then block selectively, is common in receptor pharmacology.

It helps to know where these receptors sit. MC1R is found mostly on pigment cells and immune cells; MC3R and MC4R are concentrated in the central nervous system, with MC4R the dominant subtype in appetite and autonomic circuits and MC3R more enigmatic. Because a broad agonist touches all of these at once, the same molecule can produce a pigment-related signal in a skin-cell assay and a neuronal signal in a hypothalamic slice. Reading the literature carefully means checking which tissue and which subtype a given result actually came from, rather than assuming one number describes the whole molecule.

The chemistry, spelled out

The chemistry drives everything else. PT-141 has the molecular formula C50H68N14O10 and a molecular weight near 1025 g/mol, with the CAS registry number 189691-06-3. The fourteen nitrogen atoms reflect a peptide dense in basic and aromatic residues, and the cyclic lactam constrains the backbone into a single dominant conformation. That rigidity is useful: a constrained peptide presents a consistent face to its receptor, which sharpens the link between structure and measured potency. Groups comparing lots usually confirm identity by mass spectrometry and purity by reversed-phase HPLC, because salt form and residual solvent can shift the apparent mass away from the free-base value.

What the literature examines

Published work clusters into a few areas. One is straightforward receptor pharmacology: binding affinity and functional cyclic-AMP potency at cloned human and rodent melanocortin receptors, usually reported as EC50 values in transfected cell lines. A second area is central signalling in rodent models, where researchers map which brain regions respond and which neurotransmitter systems sit downstream. A third, smaller area is structure-activity work that compares PT-141 with melanotan II and with newer constrained analogues to understand how ring size and terminus chemistry tune subtype preference.

A note on experimental design follows from all of this. Because the compound is a broad agonist, a well-built melanocortin experiment rarely uses it alone. The typical pattern pairs PT-141 as the activating ligand with a subtype-selective antagonist such as an MC4R blocker, so that any change which disappears when the antagonist is present can be attributed to that receptor. Vehicle controls and a concentration series are standard, since melanocortin responses are concentration-dependent in the assay sense and can plateau or even reverse at high occupancy.

Honest limitations

Melanocortin receptors are promiscuous, so a compound that is useful precisely because it hits several subtypes is also hard to interpret when an effect could arise from more than one receptor. Data generated in rodent models do not transfer cleanly to other species, and much of the behavioural literature uses readouts that are model-specific. Formulation is another practical caveat: as a peptide the compound is sensitive to repeated freeze-thaw and to oxidation, so reconstituted stock handled carelessly can lose potency before an experiment even begins. None of the published signalling work should be read as describing an outcome outside the laboratory context in which it was measured.

For laboratory research use only. Not for human or veterinary use.

Verified molecule

• Research name: bremelanotide (PT-141)
• Class: cyclic heptapeptide; melanocortin-receptor agonist
• Molecular formula: C50H68N14O10
• Molecular weight: 1025.2 g/mol
• CAS number: 189691-06-3
• Relationship: deaminated, C-terminal free-acid metabolite of melanotan II

Common questions

Is PT-141 the same as melanotan II?

No. They are chemically related but distinct. PT-141 is the deaminated, C-terminal free-acid metabolite of melanotan II. In practice researchers keep them as separate reference compounds and often run them together to compare receptor-subtype activity.

Which receptors does PT-141 act on?

Published pharmacology characterises it as an agonist at MC1R, MC3R and MC4R. MC4R is the subtype most studied in central signalling assays, where agonist binding raises intracellular cyclic AMP.

Why study it as a cyclic peptide rather than a linear one?

The lactam ring locks the backbone into a stable conformation and slows enzymatic breakdown in assay buffers. That combination makes measured potency more reproducible and lets the molecule serve as a consistent probe of melanocortin signalling.

References

1. Diamond LE, et al. An effect on subjective sexual response by bremelanotide (PT-141), a melanocortin receptor agonist. Journal of Sexual Medicine, 2006.
2. Molinoff PB, Shadiack AM, Earle D, Diamond LE, Quon CY. PT-141: a melanocortin agonist studied for sexual function. Annals of the New York Academy of Sciences, 2003.
3. Pfaus JG, Shadiack A, Van Soest T, Tse M, Molinoff P. Selective facilitation of sexual solicitation in the female rat by a melanocortin receptor agonist. Proceedings of the National Academy of Sciences, 2004.