Can a test tell PT-141 from melanotan II?

Yes, with the right method. The two share the same ring and differ only at the C-terminus: bremelanotide has a free acid, melanotan II an amide. Their monoisotopic masses, 1024.52 Da and 1023.54 Da, are less than 1 Da apart. High-resolution mass, MS/MS fragments, or retention time against a bremelanotide reference standard separates them; a low-resolution mass rounded to 1025 does not.

What mass should a bremelanotide certificate show?

PubChem lists bremelanotide, C50H68N14O10, at an average molecular weight of 1025.2 g/mol and a monoisotopic mass of 1024.52 Da, and the FDA label gives 1025.2 for the free base. A useful certificate states the expected and observed values to at least two decimal places, because a whole-number match cannot rule out melanotan II.

Why does the FDA label give two weights for the same amount?

The label states that 1.75 mg of bremelanotide is equivalent to 1.89 mg of bremelanotide acetate. The difference is the acetate counter-ion, which adds weight without adding peptide. The same distinction applies to any vial: a label weight may describe the salt or the peptide, and a good amount result says which one it reports.

Is PT-141 the same as bremelanotide?

Yes. PubChem lists PT-141 as a synonym of bremelanotide, CID 9941379, the active ingredient of the FDA-approved product Vyleesi. A vial labelled PT-141 should therefore match bremelanotide's formula, C50H68N14O10, and its mass. A result matching melanotan II instead means the vial holds a different molecule.

One of the 30 compound testing profiles

PT-141 (bremelanotide)

Bremelanotide (PT-141) is a 7-amino-acid cyclic peptide (C50H68N14O10, average mass 1025.2 g/mol); a test should confirm that mass with enough resolution to rule out melanotan II, which differs only by a C-terminal amide and sits about 1 Da lighter.

Bremelanotide is a synthetic cyclic heptapeptide melanocortin receptor agonist, approved by the US FDA in 2019 as the prescription product Vyleesi (FDA prescribing information).

Test your PT-141 (bremelanotide)

Identity card

Formula: C50H68N14O10
Average mass: 1,025.2 g/mol
Monoisotopic mass: 1,024.52 Da
Length: 7 residues
Modifications: N-terminal acetyl; Norleucine at position 1; D-phenylalanine at position 4; Side-chain lactam ring between Asp2 and Lys7; Free C-terminal acid
Also sold as: Bremelanotide, PT-141, PT141
Sequence: Ac-Nle-cyclo(Asp-His-D-Phe-Arg-Trp-Lys)-OH
Note: The FDA label gives the structure as Ac-Nle-cyclo-(Asp-His-D-Phe-Arg-Trp-Lys-OH) and the molecular weight as 1025.2 for the free base. The marketed drug substance is an acetate salt.

Source: Bremelanotide compound record (CID 9941379), Vyleesi (bremelanotide) prescribing information, 2019.

Compare the mass printed on a certificate with the values above. The mass checker explains charge states and which mass a lab usually reports.

What usually goes wrong with PT-141 (bremelanotide). Documented failure modes, with sources.

Melanotan II in a PT-141 vial, under 1 Da apart

Bremelanotide and melanotan II share the ring Ac-Nle-cyclo(Asp-His-D-Phe-Arg-Trp-Lys). PubChem records bremelanotide with a free C-terminal acid and melanotan II with a C-terminal amide, giving monoisotopic masses of 1024.52 Da and 1023.54 Da (PubChem, bremelanotide; PubChem, melanotan II). Swapping NH2 for OH adds 0.98 Da, so the isotope patterns overlap and an identity line reading "about 1025" cannot separate them. Accurate mass to two decimals, MS/MS fragments covering the C-terminus, or retention time against a bremelanotide reference standard can. The mass checker shows both values.

Acetate salt weight versus peptide weight

The FDA label describes the drug substance as bremelanotide acetate, C50H68N14O10 with one to two molecules of acetic acid, and states that 1.75 mg of bremelanotide is equivalent to 1.89 mg of bremelanotide acetate (FDA prescribing information). A grey-market vial labelled in milligrams rarely says which it means, and synthetic peptides purified by reversed-phase HPLC are often obtained as trifluoroacetate salts instead (Erckes et al., 2025). Only an amount reported as free peptide, with the counter-ion measured separately, compares fairly with a label.

Tryptophan oxidation and light

Bremelanotide contains one tryptophan. Trp oxidation is known to occur easily, including during sample handling, and gives oxidised forms such as kynurenine that LC-MS resolves by mass (König et al., 2023). The storage section of the FDA label tells users to protect the approved product from light (FDA prescribing information). Keep the sealed vial dark until it reaches the partner laboratory.

Loss of the D-phenylalanine configuration

Position 4 is D-phenylalanine. Fmoc deprotection during synthesis can racemize residues and create diastereomeric impurities (D'Hondt et al., 2014). A product with L-Phe at position 4 has the same formula and mass as bremelanotide, so mass spectrometry cannot see it. Only chromatography against an authentic reference standard shows a diastereomer as a separate peak.

Which tests matter most

HRMSAccurate mass is what separates bremelanotide at 1024.52 Da from melanotan II at 1023.54 Da with confidence.
LC-MSMS/MS fragments covering the C-terminus confirm the free acid, and the run detects oxidised Trp forms.
HPLC-UVRetention time against a bremelanotide standard reveals same-mass diastereomers and gives the purity figure.
Counter-ion contentThe approved substance is an acetate salt with one to two acetates, so salt weight and peptide weight differ.

Public Tested.ph results

No public PT-141 (bremelanotide) records yet

Each public PT-141 (bremelanotide) record will appear here with its identity, HPLC purity and measured amount against the label, once the first vial is tested.

FIG 13Amount against the label

Questions about testing PT-141 (bremelanotide)

Can a test tell PT-141 from melanotan II?: Yes, with the right method. The two share the same ring and differ only at the C-terminus: bremelanotide has a free acid, melanotan II an amide. Their monoisotopic masses, 1024.52 Da and 1023.54 Da, are less than 1 Da apart. High-resolution mass, MS/MS fragments, or retention time against a bremelanotide reference standard separates them; a low-resolution mass rounded to 1025 does not.
What mass should a bremelanotide certificate show?: PubChem lists bremelanotide, C50H68N14O10, at an average molecular weight of 1025.2 g/mol and a monoisotopic mass of 1024.52 Da, and the FDA label gives 1025.2 for the free base. A useful certificate states the expected and observed values to at least two decimal places, because a whole-number match cannot rule out melanotan II.
Why does the FDA label give two weights for the same amount?: The label states that 1.75 mg of bremelanotide is equivalent to 1.89 mg of bremelanotide acetate. The difference is the acetate counter-ion, which adds weight without adding peptide. The same distinction applies to any vial: a label weight may describe the salt or the peptide, and a good amount result says which one it reports.
Is PT-141 the same as bremelanotide?: Yes. PubChem lists PT-141 as a synonym of bremelanotide, CID 9941379, the active ingredient of the FDA-approved product Vyleesi. A vial labelled PT-141 should therefore match bremelanotide's formula, C50H68N14O10, and its mass. A result matching melanotan II instead means the vial holds a different molecule.

Sources

Bremelanotide compound record (CID 9941379), PubChem, US National Library of Medicine
melanotan-II compound record (CID 92432), PubChem, US National Library of Medicine
Vyleesi (bremelanotide) prescribing information, 2019, US Food and Drug Administration
König S et al. Oxidation products of tryptophan and proline in adipokinetic hormones. Life (Basel). 2023;13(12):2315, Life (PubMed)
D'Hondt M et al. Related impurities in peptide medicines. J Pharm Biomed Anal. 2014;101:2-30, Journal of Pharmaceutical and Biomedical Analysis (PubMed)
Erckes V et al. Towards a consensus for the analysis and exchange of TFA as a counterion in synthetic peptides. Pharmaceuticals. 2025;18(8):1163, Pharmaceuticals (PMC)

Last reviewed 6 Oct 2026.