The melanocortin system is one of the clearer illustrations in peptide pharmacology of why receptor selectivity, rather than potency, is usually the design problem. The compounds in this family are structurally similar. What separates them is which receptors they prefer.
Five receptors
Melanocortin receptors are G-protein-coupled receptors, numbered MC1R through MC5R, and they are distributed very differently.
- MC1R is associated with pigmentation and is expressed in melanocytes.
- MC2R responds to ACTH and is associated with the adrenal cortex.
- MC3R and MC4R are expressed largely in the central nervous system and are studied in energy balance and related signalling.
- MC5R is associated with exocrine tissue.
The endogenous ligands are derived from a single precursor protein, proopiomelanocortin, which is cleaved into alpha-MSH, ACTH and others. One precursor, several products, five receptors with different distributions: that is the system a synthetic analogue has to be selective within.
The common analogues
- MT-1, also written as melanotan-I and afamelanotide, is a linear alpha-MSH analogue with relative preference for MC1R.
- MT-2, melanotan-II, is a cyclic analogue and is markedly less selective, with activity across several subtypes including MC3R and MC4R.
- PT-141, bremelanotide, is a metabolite of MT-2 and is studied for its MC4R activity in particular.
The structural relationship between the last two is worth noting: PT-141 is not a separate discovery so much as a characterised product of MT-2, which is why their activity profiles overlap and why they appear adjacent in every catalog.
Linear and cyclic
MT-1 is linear; MT-2 is cyclic, closed through a lactam bridge. That difference matters for synthesis and for analysis, not only for pharmacology.
- A cyclisation step adds a manufacturing operation and a corresponding failure mode: incompletely cyclised material is a real impurity and has a different mass from the cyclic product.
- Cyclic peptides are generally more resistant to proteolysis than their linear counterparts, which is part of why cyclisation is used.
- A mass spectrometry result distinguishes cyclic from linear material directly, since ring closure involves a loss of water and therefore a mass difference.
For any cyclic compound, that mass check is the one worth confirming on a certificate.
Why selectivity is the research question
Because the five receptors sit in different tissues, a compound active across several of them produces effects in several systems at once. For research design this is the central consideration: a less selective compound is a blunter instrument, and interpreting a result requires knowing which receptors were engaged.
That is why the literature consistently reports selectivity ratios rather than a single potency figure, and why a catalog entry naming only the compound tells you less than one naming its receptor profile.
All compounds are supplied strictly for laboratory research.
Why selectivity is a documentation question too
Receptor selectivity is decided by structure, and structure is what a certificate establishes. A compound sold under a melanocortin name but carrying an unexpected mass is not a selectivity question, it is an identity question, and mass spectrometry and peptide identity is what settles it.
That matters more in this family than most, because several members share a four-residue recognition motif and differ by modifications around it. Two compounds in the class can look similar on a label and behave very differently at a receptor.
What to confirm before ordering
- The full name and any synonym, since several members of this family are sold under more than one designation.
- The identity method on the certificate, and whether it reads a sequence or only a mass.
- The salt form, which changes the mass of powder in the vial without changing the peptide.
- Whether the certificate names the lot printed on the vial you will receive.
Nothing in that list is specific to melanocortins. It is the ordinary pre-order check set out in auditing a peptide supplier, applied to a family where names are used loosely.
Reading published work on this family
Three habits make melanocortin papers easier to assess, and all three are about matching the finding to its conditions. Check which receptor subtype a result applies to, because the family shares a recognition motif rather than a function and a finding at one subtype says nothing about another. Check the species and the preparation, since receptor pharmacology is one of the areas where results transfer least well between systems.
Then check whether the compound studied is the compound you are buying. Analogues in this family frequently differ by a modification that does not change the name in common use, and a paper describing one is not describing the other. Where the mass on your certificate does not match the mass of the compound in the paper, you are reading about a different molecule.
Why this family attracts naming problems
Because the native ligands are processed from a single precursor and the synthetic analogues are built around a shared four-residue core, the category contains several compounds that are genuinely related and easily conflated. Market names compound the problem, since the same material appears under research designations, trade names and descriptive labels.
The defence is the same as elsewhere in this catalogue: work from the certificate rather than the name, and reconcile the mass against the formula before accepting that two sources are describing the same thing. Counterfeit and mislabelled material covers what to do when they do not reconcile.
In short
Selectivity is the whole story in this family, selectivity follows from structure, and structure is what the certificate establishes. Read the mass before reading the name.
For where this sits among the other molecules a catalogue carries, not everything in a peptide catalog is a peptide covers how the classes differ.