PT-141, also known as bremelanotide, is a cyclic heptapeptide: Ac-Nle-Asp-His-DPhe-Arg-Trp-Lys-NH2, closed into a ring through a lactam bridge. Molecular formula C50H68N14O10, molecular weight 1025.2 g/mol.
It is a metabolite of Melanotan II. Where MT-II carries a C-terminal amide, PT-141 has a free carboxyl group in its place, and that single alteration substantially changes the receptor selectivity profile — enough that the two are studied for different endpoints despite their close structural relationship.
Research context
PT-141 is examined as a melanocortin receptor agonist with activity weighted toward MC4R and MC1R. The MC4R strand is the more studied: this receptor is expressed in the hypothalamus and is investigated in central pathways relating to sexual arousal and appetite regulation.
The distinction from MT-II matters to experimental design. MT-II’s stronger MC1R activity produces pronounced pigmentation effects in model systems, which is a confound in any behavioural work; PT-141’s shifted selectivity was the reason it was separated out and pursued independently.
Structurally, the cyclisation and the D-phenylalanine at position four are both deliberate — the ring constrains the molecule into an active conformation, and the D-isomer resists peptidase cleavage.
For a fuller treatment of the sequence, published models and technical data, see the PT-141 research guide.
Specification
CAS number 189691-06-3. Supplied as a white lyophilized powder, soluble in water. This batch assayed at 99.60% purity, verified by HPLC and LC-MS at ILS Lab.
Handling
Reconstitute with bacteriostatic or sterile water, introducing the diluent against the vial wall and swirling gently rather than shaking. Store lyophilized at −20°C protected from light; refrigerate at 2–8°C once reconstituted.
Ships with a batch-specific Certificate of Analysis. Cross-check the batch ID printed on your vial against the CoA before use — see the guide to reading a CoA if you are unfamiliar with the format.
Frequently asked questions about PT-141
What is the difference between PT-141 and Melanotan II?
One functional group. MT-II carries a C-terminal amide; PT-141 has a free carboxyl in its place. PT-141 is in fact a metabolite of MT-II. That single change shifts the melanocortin selectivity profile enough that the two are studied for different endpoints — PT-141 weighted toward MC4R, MT-II broadly active including strong MC1R engagement.
Which should I use for behavioural research?
PT-141, generally. MT-II’s strong MC1R activity produces pronounced pigmentation effects in model systems, and that is a confound in any study measuring behavioural endpoints — you cannot cleanly attribute an observation when a visible physiological change is running alongside it. PT-141’s shifted selectivity is precisely why it was separated out and pursued independently.
Why is PT-141 cyclic?
Conformational constraint and stability. The lactam bridge holds the seven-residue backbone in an active conformation rather than letting it sample many shapes in solution, which improves receptor binding. The D-phenylalanine at position four does separate work — D-amino acids resist peptidase cleavage, so a short peptide survives substantially longer than an all-L sequence would.
How stable is PT-141 in solution?
More stable than a linear peptide of comparable length, for the reasons above — cyclisation and the D-residue both work against degradation. Reconstitute with bacteriostatic or sterile water, swirling gently rather than shaking. Store lyophilized at −20°C protected from light; refrigerate at 2–8°C once reconstituted and avoid repeated freeze-thaw cycles.
Research use only. Not for human or veterinary use, not for use in diagnostic procedures, and not evaluated by the U.S. Food and Drug Administration.
