What does cyclo mean in the PT-141 sequence?

It means the bracketed residues form a ring, closed by a bond between the aspartate and lysine side chains. The acetyl-norleucine sits outside the ring. A COA listing the residues without the cyclo notation has not described the molecule.

Is a cyclic peptide more stable than a linear one?

Generally yes. Closing a ring removes backbone flexibility and eliminates one degradation route, so cyclic peptides tend to be more robust than linear peptides of similar size.

Can mass spectrometry verify PT-141 completely?

Not completely. It confirms the cyclic form, which is 18 daltons lighter than the linear counterpart because cyclisation releases water. It cannot detect racemisation at the D-phenylalanine, since D and L residues share a mass.

Identification·2 min read

PT-141 (Bremelanotide) — a cyclic peptide, and why that matters

Bremelanotide is cyclised head-to-side-chain, not linear. The ring changes its stability, its analysis, and what a COA can and cannot confirm.

PT-141, or bremelanotide, is a cyclic seven-residue melanocortin peptide, CAS 189691-06-3, molecular weight 1025.18 g/mol. The defining structural feature is the ring: it is cyclised rather than linear, which changes its stability profile and limits what routine mass spectrometry can confirm about it.

By the Bench Grade Research Team · Last updated October 7, 2026

PT-141 identity data

Identity
Property	Value
CAS Registry Number	189691-06-3
Molecular formula	C50H68N14O10
Molecular weight	1025.18 g/mol
Sequence	Ac-Nle-cyclo[Asp-His-D-Phe-Arg-Trp-Lys]-OH
Structure	Cyclic, with an N-terminal acetyl and a norleucine

Reading that sequence notation

The brackets matter. `cyclo[Asp-His-D-Phe-Arg-Trp-Lys]` means those six residues form a ring, closed by a bond between the aspartate side chain and the lysine side chain. The N-terminal acetyl-norleucine sits outside the ring. A COA that lists the residues without the cyclo notation has not described the molecule.

Why cyclisation is a stability advantage

A linear peptide has two exposed termini and a flexible backbone, both of which make it easier to degrade. Closing a ring removes that flexibility and eliminates one route of attack. Cyclic peptides are generally more robust than linear ones of similar size, which is one reason the structure was chosen.

What mass spectrometry cannot tell you here

Cyclisation releases a water molecule, so the cyclic form is 18 daltons lighter than its linear counterpart. Mass spectrometry can confirm that difference. What it cannot confirm is the D-phenylalanine: D and L residues have identical masses, so a peptide that racemised at that position would report the correct mass. That requires chiral analysis, which routine COAs do not include.

What the published work examined

Four studies in our research library, from 2004 to 2019, are clinical investigations of this compound. They are linked below and describe research findings rather than any use we recommend or endorse.

References

Primary literature for the compounds discussed above. Links open the record on PubMed.

Bremelanotide for the Treatment of Hypoactive Sexual Desire Disorder: Two Randomized Phase 3 Trials Kingsberg SA, et al. Obstet Gynecol. 2019. PMID 31599840 → · summary
Long-Term Safety and Efficacy of Bremelanotide for Hypoactive Sexual Desire Disorder Simon JA, et al. Obstet Gynecol. 2019. PMID 31599847 → · summary
Double-blind, placebo-controlled evaluation of the safety, pharmacokinetic properties and pharmacodynamic effects of intranasal PT-141, a melanocortin receptor agonist, in healthy males and patients with mild-to-moderate erectile dysfunction Diamond LE, et al. Int J Impot Res. 2004. PMID 14963471 → · summary
Evaluation of the safety, pharmacokinetics and pharmacodynamic effects of subcutaneously administered PT-141, a melanocortin receptor agonist, in healthy male subjects and in patients with an inadequate response to Viagra Rosen RC, et al. Int J Impot Res. 2004. PMID 14999221 → · summary

Primary sources

Frequently asked questions

What does cyclo mean in the PT-141 sequence?: It means the bracketed residues form a ring, closed by a bond between the aspartate and lysine side chains. The acetyl-norleucine sits outside the ring. A COA listing the residues without the cyclo notation has not described the molecule.
Is a cyclic peptide more stable than a linear one?: Generally yes. Closing a ring removes backbone flexibility and eliminates one degradation route, so cyclic peptides tend to be more robust than linear peptides of similar size.
Can mass spectrometry verify PT-141 completely?: Not completely. It confirms the cyclic form, which is 18 daltons lighter than the linear counterpart because cyclisation releases water. It cannot detect racemisation at the D-phenylalanine, since D and L residues share a mass.

Compounds discussed in this guide

Who publishes this guide

Who sells it: Bench Grade Peptides LLC, a US supplier of lyophilized synthetic peptides for laboratory research use only. It works with US manufacturing partners rather than running a facility of its own. Is Bench Grade Peptides legit? →
Certificates of Analysis: 139 third-party Certificates of Analysis covering 39 of 44 live products, published per lot in a public library anyone can read before ordering. A very recent lot can appear a few days after it enters the catalogue. COA library →
Ships from: Florida, by UPS, to US addresses only (no international shipping). The registered address in Dover, Delaware is the legal address only. Shipping →
Use: For laboratory research use only — not for human or veterinary use. Not a drug, supplement or medical device. Compliance and research use →

All products discussed in this guide are supplied by Bench Grade Peptides for laboratory research use only. Not for human or veterinary use.