PT-141, Bremelanotide and Melanotan II: Telling Them Apart on Paper

PT-141 and Melanotan II are separated by a single atom-level change at one end of the molecule, and their masses differ by about one unit. That is an uncomfortably small margin for anyone reconciling a certificate of analysis, and it is the reason this pair causes more identity confusion than any other in the melanocortin group. Add the nonproprietary name bremelanotide, the development code PT-141, and the fact that Melanotan I is an entirely different length of peptide despite the shared prefix, and the naming alone is enough to derail a comparison. This article is about telling the three apart on paper.

Identity data for PT-141

PT-141 and bremelanotide are the same thing

PT-141 is a development code. Bremelanotide is the international nonproprietary name assigned later. Neither replaced the other in general usage, so supplier listings, analytical documents and literature references all continue to use whichever convention they started with. A certificate headed PT-141 and a reference entry headed bremelanotide are describing the same structure, and CAS 189691-06-3 is the field that establishes it. This is the same situation as SS-31 and elamipretide: a code name and an INN coexisting, not a correction.

The one-unit problem with Melanotan II

Here are the two side by side.

The sequences are otherwise identical. The difference is the C-terminus: Melanotan II carries a terminal amide, PT-141 carries a free acid. In formula terms that is one nitrogen exchanged for one oxygen, and in mass terms it is a difference of roughly one unit on a molecule of a thousand.

Two practical consequences follow. First, a mass determination reported to the nearest whole number does not reliably separate these two compounds; the resolution has to be good enough that a one-unit difference is unambiguous rather than within the rounding. Second, incomplete amidation in a Melanotan II synthesis produces the free-acid form as a related impurity, which is to say it produces PT-141, and the reverse holds. That is not a hypothetical contamination pathway; it is the ordinary failure mode of the chemistry. A certificate that resolves the target peak from a peak one unit away is doing real work. One that reports a rounded mass and a purity figure is telling you less than it appears to.

Both are cyclic peptides, closed through a lactam bridge between the aspartate and lysine side chains. The linear precursor has the same residues and a different mass, and it resolves differently on reverse phase, so the chromatogram carries information about whether cyclization went to completion as well as about purity in the general sense.

Melanotan I is not a shorter Melanotan II

Melanotan I, nonproprietary name afamelanotide, shares a prefix and nothing else structurally useful. It is a linear thirteen-residue peptide, Ac-Ser-Tyr-Ser-Nle-Glu-His-D-Phe-Arg-Trp-Gly-Lys-Pro-Val-NH2, formula C78H111N21O19, mass 1646.8, CAS 75921-69-6. It is not cyclic and it is over 600 g/mol heavier. The only thing the numbering communicates is the order in which the two entered the literature. Treating them as a size pair, the way one might treat a 5 mg and a 10 mg vial of the same compound, is a category error, and it shows up in sourcing decisions more often than it should.

Counter-ions, and the limits of the paperwork

These are basic peptides isolated as salts, typically acetate or trifluoroacetate, and the counter-ion contributes mass without contributing peptide. Where net peptide content is reported, it answers a different question than purity does: purity is the proportion of peptide material that is the target sequence, net peptide content is the proportion of vial contents that is peptide at all. Comparing one supplier purity figure against another supplier net peptide figure produces a gap that exists only in the comparison.

Identification matters more than usual for this group, because these are three white powders whose names overlap and whose documents do not. Certificates we have published are on the certificates of analysis page, and the right one is found by the cap colour and the crimp colour, not by reading the label. Not every product and not every batch has a published certificate; where testing has been done it is by HPLC together with purity testing. The purity guarantee covers what is promised and what is not claimed.

All three, stocked in Canada

PT-141 10mg, Melanotan II 10mg and Melanotan I 10mg are stocked in Canada and grouped under melanocortin peptides. Current pricing is on the Canadian price index.

The practical upshot

PT-141 and bremelanotide are one compound under two conventions. PT-141 and Melanotan II differ only in the C-terminus, about one mass unit apart, and each is the characteristic related impurity of the other, so rounded mass data does not separate them. Melanotan I shares the prefix and is a different peptide entirely, linear and 1646.8 g/mol. Check CAS numbers rather than names, and check the cap and crimp colours before assuming a certificate belongs to the vial in front of you.

All compounds referenced here are supplied by ExoLabz for laboratory research use only. They are not for human or veterinary use, are not authorised by Health Canada as a drug or natural health product, and nothing on this page is guidance on preparation, administration or use.