PT-141: structure, nomenclature and handling

The melanocortin peptides share a common core motif of four residues — histidine, phenylalanine, arginine and tryptophan — found in alpha-MSH and the related endogenous melanocortin sequences. That tetrapeptide is the region the melanocortin receptors engage, and synthetic analogues in this family are generally built by stabilising it rather than replacing it.

PT-141 is a cyclic heptapeptide constructed on that principle. Written out, it is acetyl-norleucine followed by a ring closing between aspartate and lysine, enclosing histidine, D-phenylalanine, arginine and tryptophan. The ring is a lactam bridge: an amide bond formed between the side-chain carboxyl of the aspartate and the side-chain amine of the lysine.

Two modifications sit alongside the cyclization. Norleucine replaces methionine at the amino terminus, removing the sulphur-containing side chain that is the most oxidation-prone residue in the natural sequence. The phenylalanine is present in the D configuration rather than the L, a substitution that both alters the ring's preferred conformation and resists cleavage by proteases, which act on L-amino acid sequences.

Its molecular weight is approximately 1,025 daltons. It is characterized against the melanocortin receptor family, five G-protein-coupled receptors designated MC1R through MC5R that differ in distribution and in their affinity for the ligands of this class.

PT-141 is a development code. The international non-proprietary name for the same compound is bremelanotide, and both appear across catalogues along with the spellings PT141 and PT-141 acetate.

The relationship to melanotan II is the point most worth understanding, because the two are structurally adjacent. Melanotan II is the same cyclic core with an amidated carboxyl terminus; PT-141 carries a free acid at that position instead. That single difference — an amide replaced by a hydroxyl — separates the two molecules by roughly one dalton, with melanotan II near 1,024 and PT-141 near 1,025.

A one-dalton separation matters for identity work: low-resolution mass spectrometry cannot reliably distinguish the two, and a certificate reporting a rounded nominal mass does not settle which was supplied. Adequate resolving power, or an orthogonal chromatographic comparison against a reference, is what separates them. Registry numbers reflect that they are distinct compounds — bremelanotide under CAS 189691-06-3, melanotan II under CAS 121062-08-6.

As with most synthetic peptides in this class, the material is typically isolated as an acetate salt, which affects the mass weighed out without altering identity.

Mass spectrometry confirms the intact molecular mass. For a cyclic peptide the measurement carries an additional piece of information: cyclization eliminates a water molecule, so the ring-closed product is lighter than its linear precursor by a defined increment. An observed mass matching the linear form indicates incomplete cyclization rather than a different sequence.

High-performance liquid chromatography resolves the main peak from related substances and yields the purity figure. The characteristic related substances here are the uncyclized linear peptide and cyclic dimers, where the ring formed between the wrong pair of termini. Both are visible on a trace and invisible in a headline percentage.

Racemization is the third consideration. The D-phenylalanine position can invert during synthesis to yield the L-form diastereomer, a compound of identical mass that mass spectrometry cannot detect at all. It is separable chromatographically, which is one of the clearer illustrations of why the two methods are run together rather than either being treated as sufficient.

HEEZ Research supplies this compound in a single format: a 10 mg lyophilized vial. There is no second strength in the current catalogue, so the format decision that arises with two-size products does not apply here.

The material is supplied as a lyophilized powder. Freeze-drying removes water and leaves the peptide as a solid, markedly more stable than the same peptide in solution, and stable enough at ambient temperature to ship without refrigeration in transit.

Reconstitution converts the dried cake into a working solution, and it is a standard laboratory step rather than a product specification. Which solvent is used, and at what final concentration, is determined by the protocol in force at the receiving facility.

What HEEZ specifies is the contents of the vial: lyophilized PT-141 in a single 10 mg format, stated on the label and on the batch certificate of analysis. Where a protocol is expressed in molar rather than mass terms, the additional figure it needs is the molecular weight, which the certificate records for the batch supplied.

Sealed lyophilized material is ambient-stable for the duration of transit, so no cold chain or insulated packaging is required for shipping. On receipt the vial goes to refrigeration and is 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. Solutions are treated as short-lived relative to the lyophilized form.

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 for solutions in extended handling is standard practice for sequences containing it.

The certificate records what an independent facility measured for one specific batch. Its value lies in being batch-specific, so the first check is that the lot number on the document matches the vial in hand.

The identity section compares observed molecular mass against a stated theoretical mass. Given the one-dalton separation from melanotan II, the useful thing to look for is whether the reported figure carries enough decimal places to distinguish the two, and whether the theoretical mass quoted is the free-acid form.

The purity figure is the HPLC main-peak area as a percentage of total peak area, and the attached chromatogram shows how that figure was arrived at. On a cyclic peptide it is worth checking whether impurity peaks sit close to the main peak, which suggests diastereomer or uncyclized material, or well away from it, which suggests unrelated synthesis by-products.

Peptide content, or net peptide content, is sometimes reported separately. It accounts for counter-ions and residual water, and it is the figure determining how much actual peptide a nominal 10 mg represents.