PT-141, also called bremelanotide, is a synthetic cyclic heptapeptide built on the message sequence of alpha-melanocyte-stimulating hormone (α-MSH). Its core is the His-D-Phe-Arg-Trp pharmacophore common to melanocortin agonists, closed into a ring by a lactam bridge between an aspartate side chain and a lysine side chain rather than left as a linear chain. That constraint is the design idea: cyclization restricts the conformational freedom of the backbone rather than leaving it free to unfold the way a linear fragment of α-MSH would.

This guide covers what the indexed literature actually establishes about the molecule as a laboratory reagent — its identity, the receptor system it belongs to, the assays in which that system has been characterized, and the substantial parts of the picture that remain unfilled. It is written for researchers handling reference material in vitro. Nothing here is a protocol, a dose, or a statement about what the compound does in any organism.

Identity and characterization

Echo Peptides publishes the following verified identifiers for PT-141:

It is worth putting those numbers next to the ones for melanotan II, the closely related cyclic lactam that Echo Peptides also lists: C₅₀H₆₉N₁₅O₉, 1024.2 g/mol, CAS 121062-08-6. The two share a carbon count and differ by one hydrogen, one nitrogen and one oxygen — the arithmetic signature of a C-terminal primary amide (CONH₂) replaced by a free carboxylic acid (COOH). That single change at the tail of the molecule is the entire formal difference between the two compounds, which is a useful thing to remember when reading structure–activity work: papers on one are frequently informative about the other, and are frequently not interchangeable with it.

For analytical confirmation, PT-141 is tractable by ordinary peptide methods. A forensic laboratory characterized bremelanotide and melanotan II in seized material without reference standards, using liquid chromatography coupled to high-resolution Orbitrap mass spectrometry, and resolved the two on accurate mass of the protonated species, relative isotopic abundance and the accurate masses of collision-induced product ions [1]. That is the shape of a defensible identity check on an unknown vial: elemental composition from accurate mass, structure corroborated by fragmentation, not a single retention time.

Mechanism as studied

PT-141 is described in the melanocortin literature as an agonist at melanocortin receptors, with activity reported at MC3R and MC4R — the two subtypes expressed predominantly in the central nervous system and consequently referred to as the neural melanocortin receptors [2]. The receptor family comprises five rhodopsin-like G protein-coupled receptors. MC4R itself was cloned in 1993 and shown by Northern blot and in situ hybridization to be expressed primarily in brain, with expression notably absent from adrenal cortex, melanocytes and placenta; agonist stimulation of transiently and stably transfected cell lines raised intracellular cAMP, and its response profile across the different melanocortins distinguished it from the previously described receptors [3]. That cAMP readout in a transfected cell is the standard functional assay for this family, and the EC₅₀ values quoted later in this guide are measurements from it.

Numbers for bremelanotide itself exist, from one laboratory. In a study whose subject was a different peptide scaffold — melanocortin pharmacophores grafted onto the sunflower trypsin inhibitor-1 framework — bremelanotide was carried as a comparator and characterized at four human melanocortin receptors, with MT-II run alongside it in the same assays. Binding was measured by competitive displacement of [¹²⁵I]-NDP-α-MSH on membranes prepared from HEK293-6E cells transiently expressing each subtype; function was measured as cAMP accumulation in a HEK293-derived line in which receptor expression was switched on by doxycycline. The reported values are pKᵢ 8.72 and pEC₅₀ 10.02 at hMC1R; pKᵢ 6.72 and pEC₅₀ 8.62 at hMC3R; pKᵢ 7.61 at hMC4R, with pEC₅₀ 8.39 and 9.61 at low and high induced receptor density; and pIC₅₀ 6.71 at hMC5R, the only value reported for that subtype [4]. Two things follow. The highest values in that panel, in both the binding and the cAMP columns, are at MC1R rather than MC4R — which is worth setting against the frequently repeated claim that this compound is primarily an MC4R ligand. And the panel covers four subtypes, from a single laboratory, with the compound serving as a control rather than as the subject.

The structural basis of activation has been resolved for MC4R. A cryo-electron microscopy structure of the human MC4R–Gs signaling complex bound to the cyclic peptide agonist setmelanotide described the activation mechanism, and its authors report that their findings indicate calcium is required for agonist, but not antagonist, efficacy [5]. On that account the divalent cation is not an inert buffer component, which is worth attending to when designing binding or functional assays with cyclic melanocortin agonists. Note the scope — that structure was solved with setmelanotide, not with bremelanotide. Bremelanotide has one of its own: a separate report of four high-resolution structures of full-length MC4R in complex with heterotrimeric Gs described the receptor stimulated by α-MSH, by afamelanotide, by bremelanotide and by the small molecule THIQ, and its authors report a conserved binding mode across the peptidic agonists [6].

What the cyclic lactam scaffold contributes

The scaffold PT-141 sits on has been examined far more thoroughly than PT-141 itself. The class originated with a set of α-MSH(4–13) and α-MSH(4–10) cyclic lactam analogs designed from molecular dynamics, in which the side-chain bridge was formed between Asp or Glu at position 5 and Lys, Orn, Dab or Dpr at position 10. Tested in frog (Rana pipiens) and lizard (Anolis carolinensis) skin bioassays, the 23-membered ring analogs were roughly 100-fold more potent than α-MSH in the lizard assay, and both increasing and decreasing the ring size from 23 members reduced potency; the 23- and 24-membered rings also showed prolonged residual activity in both assays [7]. Ring size, in other words, is not incidental packaging.

Subtype selectivity across the family is the harder problem, and the honest summary is that the parent lactams are not very selective. Binding of the cyclic lactam MSH(4–10) analogs MT-II and SHU-9119, alongside cyclic disulfide analogs, was measured on cells transiently expressing human MC1, MC3, MC4 and MC5 receptors; substituting D-Phe⁷ with D-Nal(2')⁷ shifted selectivity toward MC4R in both the lactam and disulfide series, while the lactams as a class showed higher overall affinity across the receptors than any of the disulfide analogs tested [8]. Later work modified position 6 of MT-II and SHU-9119 and characterized the analogs at human MC3R, MC4R and MC5R, yielding among other compounds a selective MC4R agonist reported at roughly 100-fold selectivity over MC3R (IC₅₀ 0.58 nM versus 55 nM) [9]. More recently, replacing the MT-II lactam cyclization with xylene-derived thioether linkers produced peptides with binding affinities spanning the low nanomolar to sub-micromolar range and, in one case, functional selectivity toward hMC1R that the parent MT-II — characterized in that work as potent and unselective — does not show [10]. Each of these results is about a congener. None of them is a measurement on PT-141.

Model systems in which it has been examined

Three kinds of system carry most of the melanocortin receptor pharmacology relevant here.

Heterologous expression systems. Transiently or stably transfected mammalian cell lines expressing a single cloned human receptor subtype are where binding affinities and cAMP potencies for melanocortin analogs are generated [3, 8, 9]. Their strength is that one subtype is interrogated at a time; their limitation is that receptor density, coupling partners and accessory proteins are artifacts of the expression system rather than properties of any tissue.

Neuronal cell lines with endogenous receptor. The hypothalamic GT1-1 line expresses functional MC4R endogenously. In that line, high-affinity binding sites for NDP-α-MSH were demonstrated (Kᵢ ≈ 1.1 × 10⁻¹⁰ M), α-MSH stimulated cAMP production with an EC₅₀ of about 2.2 × 10⁻⁸ M, and that cAMP response was inhibited by agouti-related protein, the endogenous antagonist [11]. This is the closest thing in routine use to a native neuronal context for MC4R signaling, and it is still a cell line.

Classical pigment-cell bioassays. The frog and lizard skin assays used to rank the original cyclic lactams [7] measure melanosome dispersion in amphibian and reptilian melanophores. They remain the historical basis for potency claims about this scaffold, and they are not mammalian receptor assays.

Direct in-vitro work with bremelanotide as the subject rather than as a comparator is sparse. One report examined bremelanotide in cultured human glioblastoma-derived cell lines, where it reduced survivin expression and viability at concentrations described as non-toxic to normal human cells, and both effects were abolished by an antagonist of melanocortin receptors 3 and 4; forced overexpression of survivin prevented the cell death [12]. The antagonist reversal is the informative part — it argues the effect ran through MC3R/MC4R rather than off-target chemistry. It is a single cell-culture study in one laboratory, and it should be read as a hypothesis about a signaling route, not as a finding about tumors.

What the evidence does not establish

Several things a reader might reasonably expect to find here are absent, and saying so is more useful than filling the gap with adjacent citations.

  • An independently replicated subtype panel. The affinity and potency values above come from one laboratory, in a paper where bremelanotide was a comparator rather than the subject [4]. No second panel run under different conditions was retrievable. Rank-order selectivity claims about this compound rest on that single dataset — which does not put MC4R first in binding — so check them against it rather than against repetition.
  • Transfer from congener to compound. The lactam SAR summarized above [7, 8, 9, 10] characterizes analogs that differ from PT-141 by defined substitutions, including at the C-terminus. Potency and selectivity in this series change substantially with single-residue and single-linker changes — that is the finding of the SAR work — which is precisely why those results cannot be assigned to PT-141 by resemblance.
  • Anything about organisms. No claim in this guide concerns behavior, physiology or outcomes in humans or animals, and none should be inferred from it. The bulk of bremelanotide-specific publication is clinical and lies outside the scope of a research-reagent guide.
  • Replication of the cell-culture result. The tumor cell line work [12] is, as far as the indexed record shows, unreplicated. A single positive report with a pharmacological control is a lead, not a fact.
  • Signaling beyond cAMP. The melanocortin receptors are described canonically as Gs-coupled and are assayed as such [3]. Whether PT-141 engages β-arrestin recruitment, Gq coupling or other transducers differently from other agonists at this scaffold is not something the retrievable literature answers for this compound.

Handling, stability and material verification

No compound-specific stability study for bremelanotide — forced degradation, solid-state shelf life, reconstituted half-life under defined buffer conditions — appears in the indexed literature. What can be said rests on general peptide and protein formulation science, and should be labeled as such. Therapeutic peptides and proteins in liquid dosage forms commonly show instability during handling as a result of chemical degradation, which is why solid formulations, freeze-drying among them, are used to maintain stability during storage and transport [13]. That principle explains why the material ships lyophilized; it does not license a numerical shelf-life claim for this peptide, and none is made here. Echo Peptides specifies refrigerated storage at 2–8 °C for the lyophilized powder.

Structurally, PT-141 has no disulfide bond — its ring is a lactam — so the thiol-disulfide exchange chemistry that dominates degradation of cysteine-containing peptides does not apply. It does carry a tryptophan residue, and the general susceptibility of that residue to oxidation is worth measuring rather than assuming for any given lot.

The most concrete handling lesson in this literature is about the material itself rather than the molecule. When vials of melanotan II sold through online shops were analyzed by validated LC-UV and LC-MS/MS methods, total peptide content ranged from 4.32 to 8.84 mg against a labeled 10 mg, and unknown impurities of 4.1–5.9% were quantified in vials from two of three suppliers [14]. That study concerns a different compound from a different supply channel, and it is not a statement about any particular vendor's product — but it is a direct demonstration that stated mass on a label and actual peptide in a vial are independent quantities. If a calculation depends on the mass, weigh or assay it; if an identity depends on the label, confirm it by mass spectrometry [1].

Open questions

For a compound this widely referenced, the primary characterization record is thinner than its reputation implies. A replication of the four-receptor panel [4] in a second laboratory and a second expression system would turn one dataset into a settled rank order. Transducer profiling would establish whether the scaffold shows any bias. And a straightforward solid-state and solution stability study — the kind routinely performed for formulated peptides [13] — would replace the inference in the section above with data.

Until those exist, the defensible position is narrow: PT-141 is a well-defined cyclic lactam α-MSH analog with confirmable identity by mass spectrometry, belonging to a ligand class whose receptor pharmacology is characterized in transfected cells and one hypothalamic cell line, and whose own subtype profile rests, in the retrievable primary record, on a single comparator panel [4].