Quick answer: PT-141 (bremelanotide) and melanotan II share the same cyclic melanocortin core and an acetylated norleucine start. PT-141 ends in a free acid and MT-II ends in an amide, a mass difference of about one dalton. Both are described as non-selective melanocortin receptor agonists. A purity percentage cannot tell them apart, so the identity line on the certificate matters. All materials are for laboratory research use only.
Two cyclic melanocortin peptides that differ by one end group
PT-141 (bremelanotide) and melanotan II (MT-II) show up together in nearly every melanocortin catalog, and many buyers assume they are two names for one material. They are not. They are close chemical relatives, and the difference between them is small enough to be missed on a product label and large enough to matter when a protocol names one of them.
This guide compares the two on structure, receptor pharmacology as described in the literature, the history that links them, and the analytical checks a certificate should support. It is background for laboratory research use only. Nothing here describes an effect or a use in people or animals. Single-compound background is in the PT-141 research overview and the melanotan II research overview.
Where the two molecules come from
Both peptides descend from alpha-melanocyte-stimulating hormone (alpha-MSH), a 13-residue melanocortin that is cleaved from the precursor proopiomelanocortin. Native alpha-MSH is cleared quickly and is cut apart by proteases, so it is a poor laboratory tool. In the 1980s a group working in Arizona set out to build analogs that were more stable and longer lasting. One result was a cyclic lactam: a short core of the hormone, a norleucine in place of the oxidation-prone methionine, a D-phenylalanine, and a ring closed through a side-chain amide bond between an aspartic acid and a lysine. That compound is melanotan II. The medicinal chemistry that produced the series is described in the Hruby group papers, including the 1995 J Med Chem paper in the reference list.
PT-141 was identified later as a metabolite-like relative of MT-II. It keeps the same cyclic core and the same acetylated norleucine at the front, but it ends in a free carboxylic acid instead of the C-terminal amide found in MT-II. The history of the program, and how a research tool was developed into a clinical-stage candidate, is covered in the 2006 review by Hadley and Dorr.
For a buyer, the key point is that PT-141 is not a different family of peptide. It is MT-II with one end group changed, which is exactly why the two are easy to confuse and why identity checks are worth the effort.
Side-by-side comparison
| Feature | PT-141 (bremelanotide) | Melanotan II (MT-II) |
|---|---|---|
| Peptide family | Cyclic lactam melanocortin analog | Cyclic lactam melanocortin analog |
| Parent molecule | Alpha-MSH | Alpha-MSH |
| Ring | Asp to Lys side-chain amide bridge | Asp to Lys side-chain amide bridge |
| N-terminus | Acetyl-norleucine | Acetyl-norleucine |
| C-terminus | Free carboxylic acid | Amide |
| Approximate mass | About 1,025 Da | About 1,024 Da |
| Receptors described | MC1R, MC3R, MC4R, MC5R (non-selective) | MC1R, MC3R, MC4R, MC5R (non-selective) |
| Key identity check | Measured mass matches the acid form | Measured mass matches the amide form |
| Listed by V8 | Yes, for research use only | Yes, as MT-2, for research use only |
Structure: the amide versus the acid
Both molecules are cyclic heptapeptides built around the sequence Nle, Asp, His, D-Phe, Arg, Trp, Lys, with the ring closed between the Asp side chain and the Lys side chain. Both have an acetyl group on the N-terminal norleucine. The only difference in the formula is at the C-terminus of the lysine. MT-II carries an amide (-NH2). PT-141 carries a free acid (-OH).
That swap changes the monoisotopic mass by only about one dalton, because replacing NH2 with OH adds roughly 0.98 Da. Both peptides sit near 1,025 Da in total, and a basic purity run by HPLC would show each as a single clean peak if made well. The two will also elute close together. A purity percentage therefore cannot say which of the two a vial contains. Only a measured mass with enough resolution, or a retention-time comparison against a reference standard, can do that.
The ring is the second structural detail worth checking. The lactam bridge is what gives these peptides their stability compared with linear alpha-MSH. An open-chain, uncyclized version of the same sequence is a different species with a different mass and different behavior in assays. If a certificate does not mention cyclization at all, the reader should ask how identity was confirmed.
Receptor pharmacology in the published work
Humans and rodents have five melanocortin receptors, MC1R through MC5R. Alpha-MSH and its analogs act on several of them. MT-II is described in the literature as a non-selective agonist across MC1R, MC3R, MC4R and MC5R, with high potency in cell-based assays. PT-141 is also described as a non-selective agonist that acts on MC1R, MC3R, MC4R and MC5R, and papers discussing it tend to emphasize its activity at MC3R and MC4R, the receptors found in the central nervous system.
The honest summary is that the two peptides overlap heavily. Reported potency values vary with the assay system, the cell line, the species of receptor and the lab, so a single table of numbers would be misleading and this guide does not give one. When a design depends on receptor selectivity, the right move is to read the primary assay papers and run the experiment with matched controls, not to rely on a supplier's description.
Because both compounds touch MC1R, pigment-cell models are a common context for MT-II work, while PT-141 literature leans toward central melanocortin signaling. That difference in emphasis comes from how each program developed, not from a clean difference in receptor binding. The comparison page for an older and better-known pair, KPV vs. alpha-MSH, shows how much melanocortin signaling varies by fragment.
What a certificate should show
For both compounds, start with HPLC purity, then read the identity statement closely. A good report names the exact compound (bremelanotide or melanotan II, not a generic melanocortin label), shows a measured mass next to the expected mass, and carries a lot identifier and test date that match the vial. Because the two differ by about one dalton, a report that only lists approximate mass, or only a purity number, cannot separate them.
Ask whether the ring is confirmed. A cyclic peptide and its open form differ by the mass of one water molecule, so the cyclic form should be what the report matches. Buyers working with cells should also look for an endotoxin result and a sterility statement. For background on reading these documents, see understanding HPLC purity and how to read a COA. Compound-specific pages are PT-141 purity testing and melanotan II purity testing.
V8 Peptides publishes certificates for only some products. Where one exists, it reports HPLC purity and identity, endotoxin and PCR sterility. This article describes general buyer practice and is not a statement about any particular lot. Product listings are at PT-141 and MT-2 (melanotan II). Sourcing checklists are in where to buy PT-141 and where to buy melanotan II. A plain-language explainer of the nickname is in what is PT-141.
Choosing between them for a research design
Start from the question. If the design reproduces published work on one named compound, use that compound, because swapping in its relative changes the experiment even if the receptor profile looks similar on paper. If the design compares the two, handle them as separate reagents with separate certificates, separate lot numbers and separate controls.
Handle both as sealed lyophilized powders: cold, dry and away from light, with the date each vial is opened written down. This guide does not give volumes or concentrations, because those belong in a lab's own validated protocol. In the lab notebook, record the exact compound name, lot, supplier, certificate date and storage conditions for every vial. Two vials sold under one name by different suppliers are not guaranteed to match, and in this pair the name is the thing most often blurred.
Frequently asked questions
Are PT-141 and melanotan II the same peptide?
No. They share the same cyclic core and N-terminal acetyl-norleucine. PT-141 ends in a free carboxylic acid and MT-II ends in an amide.
How big is the difference between them?
About one dalton in mass, from swapping an amide for an acid at the C-terminus. That is too small for a purity percentage to reveal.
Can HPLC purity tell me which one is in the vial?
Not on its own. Identity needs a measured mass with enough resolution, or a retention-time comparison with a reference standard.
Do they act on the same receptors?
The literature describes both as non-selective agonists across MC1R, MC3R, MC4R and MC5R. Reported potencies depend on the assay system.
Why is the ring important?
The lactam bridge is what stabilizes these peptides compared with alpha-MSH. An open-chain form is a different species with a different mass.
Where does melanotan II come from?
It was developed in the 1980s as a stable cyclic analog of alpha-MSH. The Hruby group papers describe the chemistry.
Is PT-141 a different class of molecule?
No. It is a close relative of MT-II with one end group changed, which is why the names are easily confused.
Does V8 sell both?
Yes, both are listed for laboratory research use only. MT-II is listed under the product name MT-2.
Does this article describe a use in people?
No. It summarizes structure, published preclinical pharmacology and analytical practice. Neither product is sold for human or animal consumption.
What should I record for each vial?
The exact compound name, lot identifier, certificate date, supplier, storage conditions and the date the vial was opened.
References
- Hadley ME, Dorr RT. Melanocortin peptide therapeutics: historical milestones, clinical studies and commercialization. Peptides. 2006;27(4):921-930.
- Hruby VJ, Lu D, Sharma SD, et al. Cyclic lactam alpha-melanotropin analogues of Ac-Nle4-cyclo[Asp5, D-Phe7,Lys10] alpha-melanocyte-stimulating hormone-(4-10)-NH2 with bulky aromatic amino acids at position 7 show high antagonist potency and selectivity at specific melanocortin receptors. J Med Chem. 1995;38(18):3454-3461.