PT-141: Molecular Characterization and Receptor Pharmacology — A Research Reference

The melanocortin peptides share a core motif of four residues — histidine, phenylalanine, arginine, tryptophan — that constitutes the receptor-engaging region of alpha-MSH and its relatives. Two lines of modification defined the field's early chemistry, and both appear in this molecule's lineage.

The first was substitution. Replacing methionine with norleucine at position 4 and inverting the phenylalanine at position 7 to the D configuration produced an analogue of markedly greater potency and far longer duration in vitro than the linear parent — the substitutions removing the most oxidation-prone residue and the most protease-labile stereocentre respectively [1].

The second was cyclization. Constraining the chain into a ring locks the backbone into a defined conformation rather than allowing it to sample many; early cyclic melanotropins closed through a disulfide bridge [4], and the lactam-bridged designs that followed proved more stable and more potent still.

PT-141 combines both. Written out, it is acetyl-norleucine followed by a ring closing between an aspartate and a lysine, enclosing histidine, D-phenylalanine, arginine, and tryptophan, with a free acid at the carboxyl terminus. Its molecular weight is approximately 1,025 daltons. Melanotan II is the same cyclic core carrying an amide at that position instead — hence the roughly one-dalton separation between them.

The Receptor Family

The melanocortin receptors are five class A G-protein-coupled receptors, MC1R through MC5R, cloned as a family in the early 1990s [2]. They couple principally to Gαs; receptor activation stimulates adenylyl cyclase and raises intracellular cyclic AMP. The subtypes differ in tissue distribution and in relative affinity for the natural and synthetic ligands of this class, and that differential affinity — rather than any difference in coupling — is what distinguishes ligands within the family.

A crystal structure of MC4R in its active state, bound to a peptide agonist and coupled to its G protein, subsequently resolved the activation mechanism and the architecture of the ligand-binding pocket [3]. Notably, that work identified a structural feature unusual among class A receptors: a calcium ion participating in the agonist-binding site, forming part of the coordination environment the peptide ligand engages. For structure-activity work in this family it means the ionic composition of an assay buffer is a variable of the binding site itself, not merely of the medium.

Position of this compound

PT-141 is characterized as an agonist across the melanocortin family rather than a subtype-selective one — a property it inherits from the conserved core motif retained in its ring. Its principal value in receptor-pharmacology work is comparative: because it differs from Melanotan II at a single terminal position, the pair isolates the contribution of C-terminal charge to receptor engagement while holding the entire cyclic core constant.

Mass spectrometry confirms the intact mass near 1,025 daltons, and for a cyclic peptide the measurement also reports on ring closure: cyclization eliminates a water molecule, so the ring-closed product is lighter than its linear precursor by a defined increment.

The compound-specific difficulty is the one-dalton separation from Melanotan II. Low-resolution mass measurement cannot reliably distinguish an amide from a free acid across that gap, and a certificate reporting a rounded nominal mass does not settle which compound was supplied. Adequate resolving power, or orthogonal chromatographic comparison against a reference, is what separates them — and given that both compounds are catalogued side by side, it is a check worth making rather than assuming.

Reversed-phase HPLC resolves the main peak from related substances and yields the purity figure. The characteristic related substances are the uncyclized linear peptide and cyclic dimers, where the bridge formed between two chains rather than within one. Racemization at the D-phenylalanine position produces a diastereomer of identical mass, invisible to mass measurement and separable only chromatographically.

Net peptide content accounts for counter-ions and residual water, determining how much peptide a nominal milligram figure represents.

Sealed lyophilized material is stable at ambient temperature for the duration of transit and requires no cold chain in shipping. On receipt, vials are refrigerated and kept out of direct light.

Reconstituted material is held refrigerated and used within the window the receiving facility's protocols specify, and repeated freeze-thaw cycling is avoided.

Light protection matters more here than for many peptides because of the tryptophan residue, which is photosensitive and susceptible to oxidation. Norleucine in place of methionine removes the other major oxidation route, but tryptophan remains, and amber vials or foil wrapping during extended handling is standard practice for sequences containing it.

Cyclic peptides are otherwise well behaved once wet: the ring constrains conformational freedom, reducing the aggregation tendency that troubles long flexible chains, and the molecule dissolves readily without mechanical assistance.

Material supplied by HEEZ Research is research-grade and is not a pharmaceutical product. No manufacturing, labeling, or quality standards for an approved product apply to it. Research material is sold strictly for laboratory research use; it must not be administered to humans or animals. Researchers are responsible for compliance with all institutional and jurisdictional requirements governing research chemicals.

Each batch of research-grade PT-141 is accompanied by an independent certificate of analysis specific to that lot.

PT-141 is a cyclic heptapeptide of approximately 1,025 daltons, closed by an aspartate-to-lysine lactam bridge, carrying a norleucine substitution at the amino terminus, a D-configured phenylalanine within the ring, and a free acid at the carboxyl terminus. It is characterized as an agonist across the MC1R–MC5R family, whose activation mechanism and calcium-containing binding pocket have been resolved structurally at MC4R. Analytical verification is unusually demanding: the one-dalton separation from Melanotan II requires adequate mass resolution or orthogonal chromatographic comparison, alongside routine resolution of linear precursor, dimer, and diastereomer.