PT-141 vs Melanotan 2
Research Comparison

Research Overview

PT-141 vs Melanotan 2 compares one cyclic peptide scaffold in two forms. Both are Ac-Nle-cyclo[Asp-His-D-Phe-Arg-Trp-Lys], a lactam-bridged analog of residues 4 to 10 of alpha-melanocyte-stimulating hormone (alpha-MSH). Melanotan II ends in a C-terminal amide; PT-141, also called bremelanotide, ends in a free carboxylic acid (PubChem CIDs 92432 and 9941379). Melanotan II came first (Al-Obeidi et al., 1989); PT-141 followed as a new melanotan II analog (Hadley and Dorr, 2006).

The two act across the same receptor family but took different regulatory paths. Bremelanotide was FDA approved in June 2019 as Vyleesi, for one indication in premenopausal women; melanotan II is not an approved drug, and FDA cites immunogenicity risk and case reports. The catalog carries both as 10MG lyophilized powders: PT-141 and Melanotan 2.

Structure and Class

PubChem gives both the same seven stereocenters, the same 23-membered lactam ring and the same IUPAC name up to its last term: carboxamide for melanotan II, carboxylic acid for bremelanotide. The FDA label of the drug product likewise describes a free acid at the carboxyl terminus. In formula terms, one NH2 becomes OH: C50H69N15O9 becomes C50H68N14O10.

That swap is what a C-terminal deamidation would do, and a review describes PT-141 as a melanotan II metabolite lacking the C-terminal amide group (Gong, 2014). A 2026 review is more cautious, calling it a derivative that may represent a melanotan II metabolite in humans (Feng et al., 2026). None of the papers cited on this page measured that conversion.

The product pages list 1025.18 g/mol (PT-141) and 1024.18 g/mol (Melanotan 2), matching PubChem, and the monoisotopic masses differ by 0.984 Da. The free acid also adds an ionizable group, so at neutral pH PT-141 carries one more negative charge than melanotan II, which matters for buffer choice and chromatography.

Side by Side

Research Context: PT-141

PT-141 went through a full clinical program: phase 1 and 2 trials as a new melanotan II analog (Hadley and Dorr, 2006), then a sponsored phase 3 program and a New Drug Application filed by a North American licensee (Dhillon and Keam, 2019). Drugs@FDA records the approval on June 21, 2019. The label describes nonselective activation in the order MC1R, MC4R, MC3R, MC5R, MC2R and notes that MC1R is expressed on melanocytes, where binding leads to melanin expression and increased pigmentation.

Pooled safety data from that program span 43 completed studies and about 3,500 participants. In the integrated double-blind phase 3 period, the most common adverse events were nausea (40.0 percent vs 1.3 percent with placebo), flushing and headache. Focal hyperpigmentation was rare under the labeled regimen but occurred in more than a third of participants given it daily for up to 16 consecutive days, and ambulatory monitoring found small, transient blood pressure increases (Clayton et al., 2022). Model: human clinical trials of the drug product, not the research material sold here.

Research Context: Melanotan 2

In the 1989 design study, the Asp5 to Lys10 lactam analog that corresponds to MT-II was about 90 times as potent as alpha-MSH in a lizard skin bioassay, and analogs with this 23-membered ring showed prolonged activity (Al-Obeidi et al., 1989). A 1995 paper reported that MC1R, MC3R, MC4R and MC5R all recognize it, while MC2R primarily recognizes only ACTH (Hruby et al., 1995).

In a 2026 study in HEK293 cells expressing one human receptor each, MT-II served as the reference agonist, with pEC50 values of 10.3 at MC1R, 10.1 at MC3R, 10.2 at MC4R and 9.1 at MC5R (Merlino et al., 2026). Models: frog and lizard skin, cultured cells.

A pilot phase 1 study in three healthy men reported mild nausea at most levels tested, somnolence and fatigue in one participant, a stretching and yawning complex, and increased skin pigmentation measured by reflectance in two participants (Dorr et al., 1996). Melanotan I and II were patented and tested clinically (Hadley and Dorr, 2006), yet Drugs@FDA lists no melanotan product (openFDA search, October 2026).

A 2017 dermatology review of unregulated melanotan use summarized case reports of changes in existing moles, newly emerging dysplastic nevi and four melanomas arising during or shortly after use, noted that conclusive evidence of a causal link is lacking, and reported that multiple national health organizations have issued safety warnings (Habbema et al., 2017).

An FDA page on bulk drug substances that may present significant safety risks lists Melanotan II among substances once in category 2 of its compounding interim policies whose nominations were withdrawn. It says compounded Melanotan II may pose a risk of immunogenicity for certain routes and cites published case reports of serious adverse events, including melanoma, posterior reversible encephalopathy syndrome, sympathomimetic toxidrome and priapism.

Choosing Between Them for a Research Question

Choose PT-141 when a design needs an anchor in published clinical pharmacology: a drug label, a pooled adverse event record and a characterized degradation profile. Choose melanotan II when the question concerns the original lactam scaffold and its structure-activity history, or needs the agonist a 2026 four-receptor study used as its reference (Merlino et al., 2026).

The pair is also a matched set: same ring, same stereochemistry, one C-terminal group apart. Testing both in one assay isolates what the amide-to-acid change does to receptor activity, charge and stability. No paper cited here ran that head-to-head; the receptor figures above come from different methods and are not a potency ranking. The molar masses differ by 0.1 percent, so equal masses give nearly equal molar amounts, about 0.98 µmol per milligram of free peptide.

Handling and Storage

Both product pages list the same storage: -20°C for the lyophilized powder as shipped, 2 to 8°C after reconstitution, protected from light, with freeze/thaw cycles avoided. Let a sealed vial reach room temperature before opening so moisture does not condense on the powder. In forced-degradation work on bremelanotide acetate, the peptide degraded less under acidic than basic conditions, showed significant susceptibility to oxidation, and broke down under heat and light; eight degradation products traced to deacetylation, peptide-bond hydrolysis, oxidation and epimerization (Yuvaraaj and Sharma, 2026). Melanotan II shares the oxidation-prone tryptophan and histidine, so the same precautions apply.

Identity checks need more than nominal mass. From the PubChem masses, the doubly charged ions fall near m/z 512.78 (melanotan II) and 513.27 (PT-141), both m/z 513 at unit resolution, and the melanotan II carbon-13 isotope peak lands about 0.01 m/z from the PT-141 ion. Since hydrolysis of the melanotan II amide would yield the PT-141 structure, check aged melanotan II solutions for that shift.

A forensic laboratory characterized both peptides in seized samples by high-resolution Orbitrap mass spectrometry, using accurate mass, isotopic patterns and relative isotopic abundance (Mestria et al., 2021). Keep separate, labeled stocks and base molar amounts on the measured content in each lot's certificate in the COA portal.

Questions Researchers Ask

PT 141 vs MT2: what is the actual difference?

One C-terminal group. MT2 (melanotan II) ends in an amide and PT 141 in a free carboxylic acid, while the seven residues, the ring and the stereochemistry match (PubChem CIDs 92432 and 9941379). The swap shifts the monoisotopic mass by 0.984 Da and adds a negative charge at neutral pH. The PT-141 research guide and the Melanotan 2 research guide cover each in depth.

Is PT-141 a metabolite of melanotan II?

Some reviews call it a melanotan II metabolite lacking the C-terminal amide (Gong, 2014), while a 2026 review says only that it may represent a metabolite in humans (Feng et al., 2026). Structurally, it is melanotan II after C-terminal deamidation. The catalog PT-141 is a separate lyophilized peptide with its own lot certificate.

Melanotan 2 vs PT-141: do they act on the same receptors?

Largely, yes. Melanotan II is recognized by MC1R, MC3R, MC4R and MC5R (Hruby et al., 1995), and the bremelanotide label lists MC1R, MC4R, MC3R, MC5R and MC2R in falling order of potency. Neither is selective, and no paper cited here compared them directly. The melanocortin peptides guide maps what each receptor does.

Bremelanotide vs melanotan II: which one is an approved drug?

Only bremelanotide, as Vyleesi, FDA approved in June 2019 for one indication in premenopausal women (Dhillon and Keam, 2019). The research-grade PT-141 sold here is not that drug product. Drugs@FDA lists no melanotan product, and an FDA page on compounding bulk substances cites immunogenicity risk and serious adverse event case reports for Melanotan II.