PT-141 (bremelanotide) is Melanotan II with one change: a carboxylic acid at the C-terminus where Melanotan II has an amide. Both are seven-residue rings cut down from alpha-melanocyte-stimulating hormone, a 13-residue peptide, and a third relative, afamelanotide (Melanotan I), keeps the full linear chain with two substitutions. Bremelanotide (Vyleesi, 2019) and afamelanotide (Scenesse, 2019) are the active ingredients of FDA-approved products; Melanotan II has never been approved.

The parent: alpha-MSH

Pro-opiomelanocortin (POMC, UniProt P01189) is a 267-residue precursor that is cut into several hormones. Residues 138 to 150 become alpha-MSH, a 13-residue peptide with an acetylated serine at one end and an amidated valine at the other:

Ac-SYSMEHFRWGKPV-NH2

Its receptors are the melanocortin receptors, a family of five G protein-coupled receptors named MC1R to MC5R. The core of the sequence, His-Phe-Arg-Trp (residues 6 to 9), is the part every analogue keeps. The methionine at position 4 is the residue chemists first replaced, because it oxidizes and because a 1980 study found that racemization at positions 4 and 7 during exposure to heat and alkali prolonged the peptide's activity in a frog-skin assay. Every analogue below carries norleucine at position 4 and D-phenylalanine at position 7.

Melanotan I: afamelanotide

Afamelanotide is [Nle4, D-Phe7]-alpha-MSH: the full 13-residue sequence with norleucine replacing methionine at position 4 and D-phenylalanine replacing L-phenylalanine at position 7. Sawyer and colleagues published it in 1980 as an analogue "resistant to enzymatic degradation by serum enzymes." Norleucine is methionine with the sulfur swapped for a methylene, so it has the same shape and no oxidizable atom.

PubChem CID 16197727 gives C78H111N21O19 and an average molecular weight of 1646.8; CAS 75921-69-6. Drugs@FDA lists Scenesse (afamelanotide), NDA 210797, Clinuvel, approved October 8, 2019, as an implant, marketing status Prescription. The Scenesse label calls it "a synthetic tridecapeptide and a structural analog of α-melanocyte stimulating hormone." Melanotan I in our catalog is a lyophilized vial at that molecular weight; the approved implant and a research vial are different articles.

Melanotan II: the cyclic lactam

Melanotan II was designed by Al-Obeidi and colleagues in 1989 as a cyclic fragment of alpha-MSH. They kept residues 4 to 10, replaced Met4 with norleucine and Phe7 with D-phenylalanine as before, then changed Glu5 to aspartate and Gly10 to lysine and joined the two new side chains to each other. The bond between the aspartate side-chain carboxyl and the lysine side-chain amine is an amide, and an amide that closes a ring is called a lactam. The sequence is written:

Ac-Nle-cyclo[Asp-His-D-Phe-Arg-Trp-Lys]-NH2

Seven residues, one acetyl cap, one C-terminal amide, one lactam bridge. PubChem CID 92432 gives C50H69N15O9 and 1024.2; CAS 121062-08-6. In the 1989 paper the cyclic peptides were assayed in frog and lizard skin preparations. Our Melanotan II listing is a lyophilized vial of this molecule.

PT-141: bremelanotide

Bremelanotide is Melanotan II with one change at the C-terminus: the lysine's alpha-carboxyl is a free acid, not an amide. The Vyleesi label writes it as Ac-Nle-cyclo-(Asp-His-D-Phe-Arg-Trp-Lys-OH), formula C50H68N14O10, molecular weight 1025.2 (free base), supplied as an acetate salt. PubChem CID 9941379 agrees; CAS 189691-06-3. The ring, the norleucine, the D-phenylalanine and the acetyl group are all identical to Melanotan II. The mass differs by one dalton, which is why a certificate for either molecule needs a method that can tell an amide from an acid, not just a nominal-mass match; see reading a chromatogram.

Drugs@FDA lists Vyleesi (bremelanotide acetate), NDA 210557, Cosette, approved June 21, 2019, marketing status Prescription, and the label describes the drug as a melanocortin receptor agonist. The bremelanotide we list is a lyophilized research vial, and FDA's 2026 warning letters to research-peptide sellers name "PT-141" as an unapproved new drug, and Import Alert 66-41's Red List carries a "PT 141 Peptide" entry: an approved product exists, and a research vial is not it.

The family at a glance

What FDA has said about Melanotan II

Melanotan II was placed in FDA's 503A Category 2 on September 29, 2023. On April 22, 2026 FDA's page moved it to "nominated but withdrawn" because the nominators withdrew it; it is now in no category and is not on the 503A bulks list. FDA's text stays on the page:

Compounded drugs containing Melanotan II may pose risk for immunogenicity for certain routes of administration due to the potential for aggregation or peptide-related impurities. Published case reports discuss serious adverse events including melanoma, posterior reversible encephalopathy syndrome, sympathomimetic toxidrome and priapism.FDA, bulk drug substances page, content current April 22, 2026

Import Alert 66-41's Red List also names a Canadian firm for "Melanotan (I and II)"; the import alert piece quotes the entry, and our research use only piece covers the doctrine behind it.

Regulatory status as of September 21, 2026

Absence from the WADA list is not clearance; S0 and the "similar biological effect" clauses are catch-alls, and the list's own text says so. Absence from Drugs@FDA for Melanotan II is exactly what it sounds like: no product, no label, no approval. For the same question across 27 peptides, see approved, investigational, or neither.

Sources

• UniProt P01189, Pro-opiomelanocortin (human), UniProt Consortium
• Afamelanotide, CID 16197727, PubChem
• Melanotan-II, CID 92432, PubChem
• Bremelanotide, CID 9941379, PubChem
• Sawyer TK et al. 4-Norleucine, 7-D-phenylalanine-alpha-melanocyte-stimulating hormone. Proc Natl Acad Sci USA, 1980
• Al-Obeidi F et al. Potent and prolonged acting cyclic lactam analogues of alpha-melanotropin. J Med Chem, 1989
• Drugs@FDA: Scenesse, NDA 210797, FDA
• Drugs@FDA: Vyleesi, NDA 210557, FDA
• Vyleesi (bremelanotide) prescribing information, DailyMed, National Library of Medicine
• FDA, Certain bulk drug substances for use in compounding that may present significant safety risks, content current April 22, 2026
• FDA, Import Alert 66-41
• WADA, The 2026 Prohibited List, valid 1 January 2026