PT-141 (Bremelanotide): Cyclic Structure, CAS Records, Salt Form and Expected Mass

Yes: PT-141 and bremelanotide are the same molecule. PT-141 is the research code and bremelanotide is the international nonproprietary name (INN) for the cyclic heptapeptide Ac-Nle-cyclo[Asp-His-D-Phe-Arg-Trp-Lys]-OH, formula C50H68N14O10, average molecular weight 1025.17 g/mol, CAS 189691-06-3. PT-141 is not oxytocin: oxytocin is a nine-residue disulfide peptide, C43H66N12O12S2, 1007.19 g/mol, CAS 50-56-6, and the two share no amino acid. Pepta Labs lists PT-141 as a supplier-stated 10 mg of lyophilized powder per sealed glass vial from $39.76; this page sets out its registry records, salt form and the masses that separate it from look-alike names.

Is PT-141 the same as bremelanotide?

Yes. PT-141 is the same compound as bremelanotide: PT-141 is the research code and bremelanotide the INN for the cyclic heptapeptide Ac-Nle-cyclo[Asp-His-D-Phe-Arg-Trp-Lys]-OH, C50H68N14O10, 1025.17 Da, CAS 189691-06-3, and both names point to one registry structure. PubChem CID 9941379 is titled Bremelanotide and lists "PT 141" and "PT-141 free base" among its synonyms, and the FDA Global Substance Registration System (GSRS) records the same structure under UNII 6Y24O4F92S. PT-141, PT141 and PT 141 are spellings of one code. The acetate salt has its own PubChem record and CAS number, covered below.

How is the PT-141 sequence written?

Ac-Nle-cyclo[Asp-His-D-Phe-Arg-Trp-Lys]-OH reads from the N-terminus to the C-terminus. "Ac-" is an acetyl cap on the norleucine amine, the six residues inside "cyclo[...]" form the ring, "D-Phe" marks the one D-configured residue, and "-OH" marks a free C-terminal carboxylic acid. Norleucine sits outside the ring. The peptide sequence notation guide explains each of these symbols.

The formula checks by hand. The seven free amino acids sum to C48H80N14O16. Six backbone peptide bonds and the side-chain lactam each remove one water, seven in all, leaving C48H66N14O9, and the N-acetyl cap adds C2H2O, giving C50H68N14O10.

PT-141 and Melanotan-II differ only at the C-terminus: PT-141 ends in a free carboxylic acid (-OH) and Melanotan-II in a primary amide (-NH2). They share the same seven residues, the same D-Phe4 and the same Asp2-Lys7 lactam ring. That one change moves the formula from C50H68N14O10 to C50H69N15O9, the average molecular weight from 1025.17 to 1024.18 g/mol and the CAS number from 189691-06-3 to 121062-08-6. The Melanotan-II guide gives that compound's full record.

Is PT-141 the same as oxytocin?

No. PT-141 and oxytocin are different molecules with different formulas, different ring chemistry and no amino acid in common. Oxytocin (PubChem CID 439302, CAS 50-56-6, UNII 1JQS135EYN) is the nine-residue peptide amide H-Cys-Tyr-Ile-Gln-Asn-Cys-Pro-Leu-Gly-NH2, closed by a disulfide bond between Cys1 and Cys6. PT-141 has seven residues, no cysteine and no sulfur, and its ring is a lactam. Some listings and search results pair the two names; the registry records show two unrelated structures. Pepta Labs does not list oxytocin.

The separating number is the mass. The monoisotopic masses differ by 18.09 Da, so an electrospray spectrum of PT-141 shows its protonated ion at m/z 1025.53 and one of oxytocin at m/z 1007.44. One caution applies: a PT-141 ion that has lost one water in the source would sit at a calculated m/z 1007.52, only 0.08 away from oxytocin's [M+H]⁺. A peak near 1007.5 beside a main ion at 1025.5 is consistent with PT-141; a spectrum whose main ion is 1007.4 with nothing at 1025.5 is not PT-141.

What CAS numbers cover PT-141 and bremelanotide acetate?

CAS 189691-06-3 identifies the free PT-141 peptide, and PubChem records bremelanotide acetate separately under CAS 1607799-13-2. The Pepta Labs listing shows the free-peptide number, 189691-06-3; the acetate number is a registry value that the listing does not display. Both records describe the same peptide chain. The acetate record adds one acetic acid to the formula, and PubChem lists "PT-141" among its synonyms too. A PT-141 listing that shows CAS 121062-08-6 (Melanotan-II) or 75921-69-6 (Melanotan-I) is mislabelled.

A CAS number names a structure, not a production run. The counter-ion, acetate or trifluoroacetate, comes from purification and any salt exchange, so a document that names a salt form gives it separately from the CAS number, and the acetate record's one-to-one formula does not fix how much counter-ion a given run carries. Counter-ion content is not among the tested attributes, which are HPLC purity and mass-spectrometry identity. Each acetate adds 60.05 to the formula weight and each trifluoroacetate adds 114.02, which is why salt form matters when a weighed mass is converted to moles of peptide. The TFA vs acetate guide works through that arithmetic, and the CAS numbers and molecular weights reference lists the other catalog compounds.

Which mass should a PT-141 mass spectrum show?

A PT-141 mass spectrum in positive electrospray mode should show [M+H]⁺ at a calculated m/z 1025.53 and [M+2H]²⁺ at m/z 513.27. Those values are the monoisotopic mass of C50H68N14O10, 1024.524 Da, plus one or two protons. The observed value for the sampled production run is the number printed on the report.

All values are calculated from the formulas, not measured. Read the lowest-mass peak of the isotope cluster, not the second one. Melanotan-II's carbon-13 isotope peak falls at m/z 1025.55, within 0.02 of PT-141's [M+H]⁺, so a cluster that starts at 1024.55 is Melanotan-II and one that starts at 1025.53 is PT-141. A report that quotes only the average molecular weight, 1025.17, without an observed m/z has not shown identity. The expected mass table gives the same calculation for every verified catalog compound, and HPLC vs mass spec explains why purity and identity are separate tests.

How is PT-141 supplied and documented?

Pepta Labs lists PT-141 as a single size: a supplier-stated 10 mg of lyophilized powder in a sealed glass vial, in single-vial, 3-vial and 5-vial packs. Vial labels identify the compound, quantity and research-use designation. They do not carry lot or batch numbers.

• What is tested: Independent third-party HPLC purity analysis and mass-spectrometry identity confirmation, commissioned through the purchasing group on a sampled production run from the source. Pepta Labs does not test in-house.
• How to get the report: Sent by email on request, before you order if you want to see it first. Use the report request form.
• What is not claimed: Sterility, endotoxin or LAL, potency, pharmaceutical-grade, cGMP, contaminant-panel and vial fill-quantity testing are not claimed.

Sealed vials are stored at 2-8 C on arrival or -20 C for long-term holding, dry and away from light; the peptide storage guide has the details. On US status, Drugs@FDA lists bremelanotide acetate under application NDA 210557, a record for the sponsor's finished product that does not cover research-grade PT-141, and PT-141 is not named in either table of FDA's April 22, 2026 category page (see FDA Category 2 peptides 2026 and peptide legal status 2026). New to the category? What are research peptides covers the basics.

Frequently asked questions

Is PT-141 the same as bremelanotide?

Yes. PT-141 is the research code and bremelanotide the INN for the same cyclic heptapeptide, Ac-Nle-cyclo[Asp-His-D-Phe-Arg-Trp-Lys]-OH: C50H68N14O10, 1025.17 g/mol, CAS 189691-06-3, PubChem CID 9941379, UNII 6Y24O4F92S.

Is PT-141 the same as oxytocin?

No. Oxytocin is a nine-residue disulfide peptide amide, C43H66N12O12S2, 1007.19 g/mol, CAS 50-56-6. PT-141 has seven residues, a lactam ring and no sulfur, and the two share no amino acid. Their calculated [M+H]⁺ ions are m/z 1025.53 for PT-141 and 1007.44 for oxytocin.

How is PT-141 different from Melanotan-II?

Only at the C-terminus: a free carboxylic acid in PT-141 and a primary amide in Melanotan-II. The formulas are C50H68N14O10 and C50H69N15O9, the average molecular weights 1025.17 and 1024.18 g/mol, and the CAS numbers 189691-06-3 and 121062-08-6.

Why might a report say acetate or TFA when the CAS is for the free peptide?

The CAS number 189691-06-3 identifies the peptide structure. Acetate or TFA names the counter-ion left by purification, a property of the salt form rather than of the peptide chain, so it is written separately from the CAS number. The tested attributes are HPLC purity and mass-spectrometry identity, not counter-ion content. PubChem records bremelanotide acetate under its own CAS number, 1607799-13-2, which the Pepta Labs listing does not display.

Which mass should a PT-141 mass spectrum show?

[M+H]⁺ at a calculated m/z 1025.53 and [M+2H]²⁺ at m/z 513.27, from a monoisotopic mass of 1024.524 Da. A cluster starting at m/z 1024.55 points to Melanotan-II, and a main ion at 1007.44 points to oxytocin.

All products are supplied for laboratory research use only and are not for human or animal use.