Two abstract cyclic peptide ring structures side by side in magenta and purple, representing a comparison of PT-141 and Melanotan II melanocortin agonists.

PT-141 vs Melanotan II: Two Melanocortin Agonists Compared

Key takeaways

PT-141 and Melanotan II share the same Ac-Nle-cyclo[Asp-His-D-Phe-Arg-Trp-Lys] lactam-bridged ring.
The only difference is the C-terminus: free acid (PT-141) versus amide (Melanotan II).
Both are characterized as non-selective agonists of MC1R, MC3R, MC4R, and MC5R.
A 0.98 Da mass difference means identity must be confirmed by mass spectrometry.
For in vitro research use only.

PT-141 and Melanotan II are two cyclic heptapeptide melanocortin-receptor agonists built on the same Ac-Nle-cyclo[Asp-His-D-Phe-Arg-Trp-Lys] ring, and they differ only at the C-terminus: PT-141 ends in a free carboxylic acid, Melanotan II in an amide. This guide compares PT-141 and MT-2 (Melanotan II) by structure and receptor selectivity in vitro.

How do the two structures compare?

Property	PT-141 (bremelanotide)	Melanotan II
Ring system	Asp-Lys lactam, cyclic heptapeptide	Asp-Lys lactam, cyclic heptapeptide
Core motif	His-D-Phe-Arg-Trp	His-D-Phe-Arg-Trp
C-terminus	Free acid (-OH)	Amide (-NH2)
Formula	C50H68N14O10	C50H69N15O9
Molecular weight	1025.18 g/mol	1024.20 g/mol
CAS number	189691-06-3	121062-08-6

How do they compare in receptor selectivity?

Both peptides are characterized in the research literature as non-selective agonists of the Gs-coupled melanocortin receptors MC1R, MC3R, MC4R, and MC5R, and neither is an MC2R ligand in the way ACTH is. Whether a one-atom C-terminal change shifts relative affinity or potency at a given subtype is an empirical question, best answered by profiling both peptides side by side in the same receptor-expressing cell lines. Background on the receptors is in the melanocortin receptor family primer.

How would you design a head-to-head in vitro comparison?

Receptors: stable lines for MC1R, MC3R, MC4R, and MC5R, with the same passage and assay buffer.
Binding: competition against a radiolabeled reference ligand to give Ki for each peptide.
Function: cAMP accumulation to give EC50 and maximal response relative to a reference agonist.
Concentrations: expressed in molarity across a half-log series, with both peptides prepared from stocks of confirmed concentration.

How do you confirm which peptide you have?

The two differ by 0.98 Da, so identity must be confirmed by mass spectrometry and CAS, not by label. The checks are covered in PT-141 name variants and mass-spec verification.

Related research guides

Research-use note

All materials discussed are supplied for in vitro laboratory research use only, and are not for human or animal use.

Frequently asked questions

How does PT-141 differ from Melanotan II?

Both share the same cyclic heptapeptide ring, but PT-141 has a free carboxylic acid C-terminus while Melanotan II has an amide, a 0.98 Da difference. View PT-141

Are they selective for one melanocortin receptor?

Both are characterized as non-selective agonists of MC1R, MC3R, MC4R, and MC5R; relative potency by subtype is determined empirically in receptor-expressing cells. View MT-2 (Melanotan II)

What are their molecular formulas?

PT-141 is C50H68N14O10 (1025.18 g/mol) and Melanotan II is C50H69N15O9 (1024.20 g/mol).

How can the two be told apart in the lab?

By high-resolution mass spectrometry and CAS number: 189691-06-3 for PT-141 and 121062-08-6 for Melanotan II. Verify identity by CAS and mass spec

Is it for human or animal use?

No. Both are supplied for in vitro laboratory research use only.