PT-141: Melanocortin Selectivity and the Lactam Bridge

PT-141 is a cyclic heptapeptide and the most receptor-selective member of the melanocortin group commonly held in research inventories. It is also the one most often misfiled, because it is structurally close to melanotan-II and behaves very differently at the receptor.

Where it comes from structurally

The melanocortin peptides descend from α-MSH, a linear 13-residue sequence. Melanotan-II is a cyclic analog of that sequence, closed through a lactam bridge, and PT-141 is derived from melanotan-II by removing the C-terminal amide group and leaving a free carboxylic acid in its place.

That single change is the whole story. It is a small edit by mass and a large one by pharmacology, and it is the reason the two compounds cannot be treated as variants of each other in an experimental design. PT-141 is also referred to in the literature by its international non-proprietary name, PT-141.

Receptor selectivity, which is the point

There are five melanocortin receptor subtypes. MC1R is the pigmentation-associated receptor; MC3R and MC4R are the centrally expressed pair; MC2R responds to ACTH rather than to α-MSH analogs; MC5R is the least characterised.

Melanotan-II is a broad agonist across MC1R, MC3R, MC4R and MC5R. PT-141 retains activity at MC3R and MC4R while showing markedly reduced potency at MC1R. For anyone using these compounds as receptor tools, that is the entire reason to hold both: a comparison between them isolates the MC1R-dependent component of whatever is being measured, which no single compound can do on its own.

Our overview of melanocortin receptor subtypes and selectivity sets out the family in more detail. The indexed literature is at PubMed.

Choosing the readout

The melanocortin receptors are class A GPCRs that couple to Gαs. Activation drives adenylate cyclase and raises intracellular cAMP, so a cAMP accumulation assay is the appropriate default measurement.

This is worth stating because the wrong instrument produces a confident null. A calcium flux assay, which is correct for a Gαq-coupled receptor such as KISS1R — see kisspeptin-10 and KISS1R signalling — will read a genuinely active melanocortin agonist as inactive. Selecting the assay from the receptor’s coupling rather than from habit removes a whole category of wasted work.

Subtype selectivity also has to be designed for rather than assumed. A cell line expressing several melanocortin receptors reports a composite response, and a selectivity claim needs each subtype measured separately in a line expressing one.

The lactam bridge, and what it means analytically

PT-141 is cyclised through a lactam bridge rather than a disulfide, which matters more than the distinction first suggests.

A disulfide is reversible: reducing agents open it, and thiol-disulfide exchange runs whenever a free thiol is present. A lactam is an amide bond and is not reduced by dithiothreitol or β-mercaptoethanol, so PT-141 is unusually robust to buffer components that would compromise a disulfide-bridged peptide. The general case is covered in cyclic versus linear peptides.

The practical consequence for identity checking runs the other way. Because there is no reduced form to look for, the informative mass comparison is against the cyclic sequence itself, and the impurity worth checking is the uncyclised linear precursor — which differs from the intended product by the mass of one water molecule, about 18 daltons. That is a larger gap than the one-dalton amide-versus-acid difference elsewhere in peptide chemistry, so it resolves cleanly, but only if the certificate reports both the calculated and the observed mass. Our guides to reading a certificate of analysis and how purity and identity are verified cover what each method resolves.

Distinguishing it from melanotan-II on paper

Because the two differ by the loss of an amide group, they are close in mass and close on a reversed-phase column. A retention time consistent with expectation is therefore weak evidence of which one is in the vial, particularly for a laboratory holding both.

Mass discriminates them, and the sequence on the certificate should be read rather than skimmed. Where both compounds are in inventory, label aliquots with the compound and the batch at the point of reconstitution rather than afterwards — see inventory, labels and records. Mixing up two melanocortin agonists with different subtype profiles produces results that look like biology and are not.

Handling

PT-141 ships as a lyophilised powder and is stable sealed, cold and dry. Let a vial reach room temperature before opening it: a cold vial opened in a warm room draws moisture onto the cake, and water drives hydrolysis and deamidation during storage. See storage and stability.

Cyclic peptides of this size dissolve readily in aqueous buffer, so the usual solubility difficulties are less pronounced here than with longer or more hydrophobic sequences. The residual risks are the ordinary ones: adsorption losses at low working concentrations, which flatten the bottom of a dose-response curve, and freeze-thaw cycling of a reconstituted stock. Aliquot once into single-use volumes — see aliquoting reconstituted peptides.

The milligram figure on the vial is gross weight; counter-ions and residual water are part of that mass and not part of the peptide, so molarity calculated from the label is overstated. See net peptide content explained.

Products

PT-141 10mg, listed with melanotan-II and melanotan-I under melanocortin peptides. Batch certificates are published on the certificates page.

ExoLabz supplies compounds for laboratory research use only. Nothing on this page is medical advice or a suggestion of human or veterinary use. These compounds hold no Canadian market authorisation for human use.