PT-141 vs Melanotan II
PT-141 (bremelanotide) and Melanotan II share the same cyclic heptapeptide core and differ only at the C-terminus: a free acid versus an amide. Bremelanotide is FDA-approved as Vyleesi; Melanotan II is unapproved.
Melanotan II is a synthetic cyclic analogue of alpha-melanocyte-stimulating hormone (α-MSH), developed in academic research on skin pigmentation. PT-141, later named bremelanotide, has the same cyclic core but ends in a free carboxylic acid instead of an amide. It is often described as a metabolite of Melanotan II.
Despite this close structural relationship, the two have very different regulatory positions. Bremelanotide was developed through clinical trials and approved by the FDA in 2019 as Vyleesi for hypoactive sexual desire disorder in premenopausal women. Melanotan II has never been approved by any major regulator, and products sold under that name have no approved label, specification or quality standard.
Side-by-side
| Attribute | PT-141 (bremelanotide) | Melanotan II |
|---|---|---|
| Parent hormone | α-MSH analogue | α-MSH analogue |
| Core structure | Ac-Nle-cyclo[Asp-His-D-Phe-Arg-Trp-Lys] | Ac-Nle-cyclo[Asp-His-D-Phe-Arg-Trp-Lys] |
| C-terminus | Free acid (-OH) | Amide (-NH<sub>2</sub>) |
| Cyclization | Lactam bridge between Asp and Lys side chains | Lactam bridge between Asp and Lys side chains |
| Receptor activity | Melanocortin agonist developed for MC4R/MC3R-mediated effects; retains some MC1R activity | Nonselective melanocortin agonist (MC1R, MC3R, MC4R, MC5R) |
| Research areas | Sexual desire and arousal (clinical); earlier studied in erectile dysfunction | Skin pigmentation, appetite and sexual function in animal and early human studies |
| Evidence level | Approved | Early-clinical |
| FDA status | Approved (Vyleesi) for HSDD in premenopausal women | Not approved by the FDA for any use |
| Analytical pitfalls | Distinguish acid from amide (about 1 Da apart); Trp oxidation | Distinguish amide from acid; Trp oxidation; confirm cyclization |
One atom group apart
The only covalent difference between the two molecules is the C-terminal group. Melanotan II ends in a carboxamide and bremelanotide in a carboxylic acid, a mass difference of about one dalton. That small difference is easy to miss in low-resolution mass spectra, so a certificate of analysis should report a measured mass precise enough to separate the two, and HPLC retention should be compared against a reference standard where one is available. The sequence analyzer can show both expected masses.
Receptor selectivity
Melanocortin receptors MC1R to MC5R mediate different effects: MC1R controls melanin production in melanocytes, while MC3R and MC4R in the central nervous system are involved in energy balance and sexual behavior. Melanotan II activates several subtypes broadly, which explains why it was studied for both tanning and sexual function. Bremelanotide was developed for its central melanocortin effects, but it still has MC1R activity, and focal hyperpigmentation is listed among the effects in its prescribing information.
The regulatory gap
Bremelanotide passed through phase 3 trials and has an approved label, a manufacturing specification and post-marketing oversight. Melanotan II has none of these. Material sold under that name is unapproved, and regulators in several countries have issued warnings about it. Because the molecules are so similar, the approval of bremelanotide is sometimes wrongly taken to apply to Melanotan II. It does not: approval covers a specific molecule, formulation and indication.
Frequently asked questions
Is PT-141 the same as Melanotan II?
No, though they are closely related. Both share the cyclic heptapeptide core Ac-Nle-cyclo[Asp-His-D-Phe-Arg-Trp-Lys]. PT-141 (bremelanotide) ends in a free carboxylic acid, while Melanotan II ends in an amide. That difference affects receptor activity and corresponds to a mass difference of about one dalton.
Is PT-141 FDA approved?
Yes. Bremelanotide was approved by the FDA in 2019 as Vyleesi for hypoactive sexual desire disorder in premenopausal women. That approval covers only the specified indication and formulation and does not extend to Melanotan II or to research-grade material.
How can PT-141 and Melanotan II be told apart analytically?
High-resolution mass spectrometry can separate the roughly one-dalton difference between the C-terminal acid and amide, provided the isotope pattern is read carefully. HPLC comparison against a characterized reference standard adds confidence, since the two molecules can have similar but distinct retention times.
Full entries
- PT-141 (bremelanotide)
PT-141, or bremelanotide, is a cyclic heptapeptide melanocortin receptor agonist derived from melanotan II. It was FDA-approved in 2019 as Vyleesi for hypoactive sexual desire disorder in premenopausal women.
- Melanotan II
Melanotan II is a synthetic cyclic heptapeptide analogue of alpha-melanocyte-stimulating hormone that activates several melanocortin receptors. It is not approved as a medicine anywhere, and regulators have warned about unlicensed tanning products.
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