PT-141, also known by its nonproprietary name bremelanotide, is one of the most searched research peptides, and also one of the most misunderstood. It is a synthetic cyclic heptapeptide that acts at melanocortin receptors, a family of G protein-coupled receptors involved in pigmentation, energy balance and central signaling. This overview covers what PT-141 is chemically, which receptors it engages, how it differs from its parent compound, and where the published evidence stops. It is written for laboratory readers. PT-141 supplied by Karma Research Peptides is for research use only and is not for human or veterinary consumption.
What PT-141 Is
PT-141 is a cyclic peptide closely related to Melanotan II. Both are built around the core melanocortin pharmacophore, the His-Phe-Arg-Trp sequence shared by the endogenous melanocortins (alpha-, beta- and gamma-MSH and ACTH). In PT-141 the phenylalanine is in the D-configuration and the ring is closed by a lactam bridge, which constrains the peptide’s shape and makes it more resistant to enzymatic breakdown than linear alpha-MSH.
The key structural difference from Melanotan II is at the C-terminus: PT-141 carries a free carboxylic acid where Melanotan II is amidated. That small change is enough to make the two separate research compounds, and papers or catalog listings that use the names interchangeably should be read with caution. For background on how peptide bonds and modifications like these shape a molecule, see our guide to peptide bond chemistry.
Receptor Profile: MC3R, MC4R and Beyond
There are five melanocortin receptors (MC1R through MC5R). PT-141 is generally described in the literature as a non-selective agonist with its most studied activity at MC3R and MC4R, both of which are expressed in the central nervous system, including hypothalamic regions.
Activity at MC1R, the receptor that controls melanin production in melanocytes, explains why pigmentation changes appear in the safety literature for this compound class. Researchers designing receptor assays should keep this cross-reactivity in mind: a readout attributed to MC4R may be partly driven by other subtypes unless selective antagonists or receptor-specific cell lines are used as controls.
Regulatory and Development History
Bremelanotide went through formal clinical development and was approved by the FDA in 2019 as a prescription drug product, Vyleesi, for a specific indication. An earlier intranasal formulation was dropped during development after blood pressure effects were reported. That history matters for researchers in two ways. First, it means a substantial body of pharmacology and safety data exists for the approved product. Second, the approved drug and research-use-only material are different things. RUO PT-141 is not the approved product, is not made to drug manufacturing standards, and nothing in the clinical record transfers automatically to a given research lot.
Study Design Considerations
- Name the compound precisely. Record “PT-141 (bremelanotide)” rather than a general “melanocortin agonist,” and do not substitute Melanotan II mid-study.
- Control for receptor subtype. Use subtype-specific cell systems or antagonists when attributing an effect to MC3R or MC4R.
- Account for central versus peripheral readouts. Many reported effects of this compound class are mediated centrally, so in vitro peripheral models may not capture them.
- Document the lot. Record lot number, labeled content and the supporting documentation for every vial used. Our COA review criteria explain what to look for.
- Store it correctly. Lyophilized peptides should be kept cold, dry and protected from light; see how to store lyophilized peptides.
Evidence Limits
Most mechanistic understanding of PT-141 comes from receptor pharmacology and from the clinical program for the approved drug product. Those findings describe that product, under its approved labeling, in its approved population. They do not establish safety or effectiveness for any other use, and they say nothing about the identity or purity of an unrelated research lot. Preclinical and in vitro observations are hypotheses for further study, not evidence of benefit.
Research Use Only
PT-141 from Karma Research Peptides is supplied strictly for laboratory research. It is not for human or veterinary consumption, and this article does not provide dosing, administration or clinical guidance. For more on what that label means in practice, read Research Use Only meaning for lab buyers.
Conclusion
PT-141 is best understood as a constrained melanocortin agonist with broad receptor activity and a well-documented development history. For researchers, the practical points are precise naming, subtype-aware controls and careful lot records. Browse related compounds in our overview of neuropeptides in research, or contact Karma Research Peptides for product documentation.