PT-141 (Bremelanotide): Melanocortin Receptor Pharmacology

PT-141, or bremelanotide if you’re reading the later papers, is a cyclic heptapeptide built from alpha-melanocyte stimulating hormone. It agonises melanocortin receptors, and the literature mostly points at MC3R and MC4R, which sit largely in the central nervous system [6]. The molecule exists because of thirty years of alpha-MSH structure activity grinding, by way of melanotan II. That history is worth knowing, so we start there, then get to the receptor family and to how a vial of this stuff is actually characterised.

Five receptors, one ligand family

Melanocortins get cut out of one precursor, proopiomelanocortin: alpha-MSH, beta-MSH, gamma-MSH, ACTH. For a long stretch nobody knew what any of them bound. That changed in 1992, when Mountjoy, Robbins, Mortrud and Cone cloned the murine and human MSH receptors along with a human ACTH receptor and showed they defined a subfamily of G protein coupled receptors [1]. Gantz and colleagues added further subtypes, including a novel receptor reported in 1993 [2] and a fifth, MC5R, in 1994 [3].

The MC5R paper shows how the family behaves. Stimulate the cloned mouse receptor and cAMP goes up, ranked alpha-MSH, then beta-MSH, then ACTH, then gamma-MSH. The detail we’d flag: the melanocortin core heptapeptide on its own did nothing at that subtype [3]. So the termini carry weight too, not just the His-Phe-Arg-Trp message sequence everyone quotes.

What you’re left with is five numbered receptors, ligands that overlap, and selectivity riding on small structural details. Anything that hits this family hits several members to some degree. That’s the central fact about PT-141, and most of the confusion around it starts by ignoring it.

Where the structure came from

Linear alpha-MSH is a poor molecule to work with. It is degraded fast and it is not selective. Hruby, Hadley, Dorr and co-workers spent years cutting it down and rigidifying it, which produced melanotan I (a linear analogue) and melanotan II, a truncated cyclic peptide closed by a lactam bridge between the Asp5 side chain and the Lys10 side chain [11]. The ring removes most of the conformational freedom, and that both raises potency and slows enzymatic breakdown.

If you want to know why any given substitution was made, Holder and Haskell-Luevano’s review of three decades of this work is the place to start [4]. The same moves keep coming back. A D-amino acid at position 7 to resist proteases and bias the conformation. Norleucine swapped in for methionine so it can’t oxidise. Cyclisation, to hold the message sequence in something close to its bound shape.

PT-141 is one step past melanotan II. Hadley and Dorr describe it as a melanotan II analogue, developed by Palatin Technologies, which by 2006 had been through early phase clinical work [5]. Molinoff and colleagues characterise it as a synthetic alpha-MSH analogue that agonises melanocortin receptors, MC3R and MC4R in particular, which are expressed mainly in the central nervous system [6].

What the published work measured

Keeping this short and in past tense, because that is all the literature supports.

A detail worth pulling out of Pfaus and colleagues’ rat work: the effect they recorded was specific rather than general, with no sign of broad motor activation and no change in the perception of reward in their paradigm [7]. That kind of negative control is more informative than the positive result, because it narrows what the compound is doing.

The later phase 3 programme, reported by Kingsberg and co-authors in 2019, randomised 1,267 participants across two identical trials and reported differences from placebo that were statistically significant but small in absolute terms, with nausea, flushing and headache each occurring in 10 percent or more of participants [12]. We mention the numbers only because people cite this work loosely and the effect sizes are routinely overstated.

Receptor selectivity is the interesting problem

MC1R is the pigment receptor. MC2R responds to ACTH. MC3R and MC4R are mostly central. MC5R turns up in skeletal muscle, lung, spleen and brain tissue by Northern blot [3]. A ligand built from the alpha-MSH message sequence will touch several of these, and separating them has occupied medicinal chemists for thirty years [4].

The MC4R structure from 2021 explains part of why. Israeli and colleagues found the agonist bound deep in the pocket with a calcium ion acting as part of the binding site, a feature that is unusual among peptide receptors and that constrains what an analogue can look like [10]. If you are modelling melanocortin ligands, that paper is the one to read first.

Bremelanotide is itself a metabolite relationship away from melanotan II, which is why the two get confused in secondary sources. They are different molecules with different pharmacology, and we keep them separate in our write up on melanotan II and melanin production.

Handling and characterisation

It ships lyophilised. The cyclic lactam does buy stability over a comparable linear peptide, but don’t get relaxed about it: water drives the hydrolysis and deamidation that degrade any peptide in storage, so cold and dry still applies.

Two independent measurements, and you want both. Reversed phase HPLC reports purity as peak area. Mass spectrometry says whether the molecular weight matches a cyclic heptapeptide, and MS/MS reads the sequence out of fragment ions. Run one without the other and there’s a hole: HPLC can’t split co-eluting species of similar hydrophobicity, and a matching mass won’t catch an isomer.

Two numbers people confuse on a certificate of analysis:

• HPLC purity, the fraction of peptide related material that is the target compound.
• Net peptide content, the fraction of the vial’s mass that is actually peptide rather than residual water and trifluoroacetate counterion.

Those can differ by 10 to 20 percentage points for the same material, which matters if you are weighing by mass. Our analytical reports are published on the COA page, and we explain the testing workflow in more depth in our article on third party tested material.

FAQ

Is PT-141 the same thing as bremelanotide?

Yes, same cyclic heptapeptide. PT-141 was the development code and bremelanotide is the generic name it picked up later.

How is PT-141 related to melanotan II?

It is an analogue of it. Melanotan II is the lactam bridged alpha-MSH analogue described by Dorr and colleagues [11], and PT-141 came out of that structural series [5].

Which receptors does it bind?

Melanocortin receptors, with MC3R and MC4R the subtypes usually highlighted in the primary literature [6]. Selectivity across the five subtypes is partial, not absolute [4].

References

1. Mountjoy KG, Robbins LS, Mortrud MT, Cone RD. The cloning of a family of genes that encode the melanocortin receptors. Science. 1992. PMID 1325670
2. Gantz I, Konda Y, Tashiro T, et al. Molecular cloning of a novel melanocortin receptor. J Biol Chem. 1993. PMID 8463333
3. Gantz I, Shimoto Y, Konda Y, et al. Molecular cloning, expression, and characterization of a fifth melanocortin receptor. Biochem Biophys Res Commun. 1994. PMID 8185570
4. Holder JR, Haskell-Luevano C. Melanocortin ligands: 30 years of structure-activity relationship (SAR) studies. Med Res Rev. 2004. PMID 14994367
5. Hadley ME, Dorr RT. Melanocortin peptide therapeutics: historical milestones, clinical studies and commercialization. Peptides. 2006. PMID 16412534
6. Molinoff PB, Shadiack AM, Earle D, Diamond LE, Quon CY. PT-141: a melanocortin agonist […]. Ann N Y Acad Sci. 2003. PMID 12851303
7. Pfaus JG, Shadiack A, Van Soest T, Tse M, Molinoff P. […] in the female rat by a melanocortin receptor agonist. Proc Natl Acad Sci U S A. 2004. PMID 15226502
8. Rosen RC, Diamond LE, Earle DC, Shadiack AM, Molinoff PB. Evaluation of the safety, pharmacokinetics and pharmacodynamic effects of PT-141, a melanocortin receptor agonist. Int J Impot Res. 2004. PMID 14999221
9. Clayton AH, Lucas J, DeRogatis LR, Jordan R. Phase I randomized placebo-controlled, double-blind study of the safety and tolerability of bremelanotide coadministered with ethanol in healthy male and female participants. Clin Ther. 2017. PMID 28189361
10. Israeli H, Degtjarik O, Fierro F, et al. Structure reveals the activation mechanism of the MC4 receptor. Science. 2021. PMID 33858992
11. Dorr RT, Lines R, Levine N, et al. Evaluation of melanotan-II, a superpotent cyclic melanotropic peptide in a pilot phase-I clinical study. Life Sci. 1996. PMID 8637402
12. Kingsberg SA, Clayton AH, Portman D, et al. Bremelanotide […]: two randomized phase 3 trials. Obstet Gynecol. 2019. PMID 31599840