PT-141: structure and analysis

What it is

PT-141, also known as bremelanotide, is a synthetic cyclic heptapeptide active at melanocortin receptors. Structurally it is a metabolite of melanotan II: the same seven-residue cyclic scaffold, differing at the C-terminus.

That difference is small in mass terms and significant in identity terms. The two compounds are separate molecules with separate specifications, and they are routinely conflated in this category.

How it is produced

Assembled by solid-phase peptide synthesis: the chain is built one residue at a time on an insoluble resin support, then cleaved from the resin, deprotected and purified — most commonly by reversed-phase high-performance liquid chromatography. The purified material is lyophilized to a dry solid.

The impurity profile that appears on a certificate is a record of where that process was imperfect: deletion sequences from an incomplete coupling, truncated chains, and species retaining a protecting group.

Confirming identity

Two things need confirming here rather than one. Mass spectrometry establishes that cyclisation occurred — the ring-closed product has a lower mass than the linear precursor by a predictable amount — and it establishes which of the two closely related cyclic heptapeptides is present.

The second is the harder measurement. The mass difference between this compound and melanotan II is small enough that a low-resolution instrument cannot settle it, so the resolution of the identity method is a substantive parameter rather than a technicality.

Analytical considerations

Because the compound and its structural parent differ so little in mass, chromatographic resolution shares the identity burden with mass spectrometry. The two species separate on a well-developed reversed-phase method, and that separation is part of what a purity figure for this compound is actually demonstrating.

The uncyclised linear precursor is the other impurity class specific to a cyclic peptide, and it is distinguishable both by mass and by retention.

Cyclisation also makes the molecule a poorer protease substrate than a linear heptapeptide, which contributes to its stability as a dry solid.

What its certificate reports

A certificate for a lot of this material names the lot, names the issuing laboratory, and states for each parameter both the method used and the specification the result was measured against. Identity and purity are the two core results; net peptide content states how much of the fill weight is actually peptide.

A certificate belongs to a lot, not to a product line. When a new lot is released it is tested again, and the certificate published against it is that lot's certificate — a previous lot's result is never carried forward.

Research use only

This page describes structure and analysis. It makes no claim about what this compound does, and none should be inferred from it. Material is supplied for laboratory research use only: it is not a drug, food, dietary supplement, cosmetic or medical device, it is not for human or veterinary consumption, and no dosing, administration or reconstitution guidance is provided for any such use.