PT-141 at a Glance

• PT-141 is bremelanotide a synthetic cyclic heptapeptide analogue of alpha-melanocyte-stimulating hormone (α-MSH) and an agonist at melanocortin receptors, with the formula C₅₀H₆₈N₁₄O₁₀ and a molecular weight of approximately 1,025 Da.
• Its receptor pharmacology is non-selective across the melanocortin family. The approved product’s labelling states that it activates several receptor subtypes, with MC1R and MC4R the most relevant; MC4R is the subtype generally regarded as mediating the central pharmacology studied clinically.
• The mechanism studied is central rather than peripheral receptor activation in hypothalamic circuits, not a vascular or smooth-muscle effect.
• It is chemically derived from Melanotan II differing at the C-terminus: bremelanotide carries a free carboxylic acid where Melanotan II carries a primary amide.
• Bremelanotide was studied in the RECONNECT phase 3 programme two identical randomised, double-blind, placebo-controlled trials in 1,267 premenopausal women with hypoactive sexual desire disorder (HSDD), reported in Obstetrics & Gynecology in 2019.
• An approved prescription product exists. The FDA approved Vyleesi (bremelanotide injection) on 21 June 2019. That product is a regulated, labelled, prescription-only medicine, it is not the same thing as a research-grade vial and nothing here describes its use.
• The documented adverse event profile is dominated by nausea (40.0%) and flushing (20.3%) with transient blood pressure increases and heart rate decreases after each dose and focal hyperpigmentation in approximately 1% of patients receiving up to eight monthly doses.
• Research-grade material sold under the PT-141 designation is research-use-only and carries none of the approved product’s regulatory guarantees.

What Is PT-141?

PT-141 is the development code for bremelanotide catalogued in PubChem as CID 9941379 with the molecular formula C₅₀H₆₈N₁₄O₁₀ and a molecular weight of approximately 1,025 Da.

Structurally it is a cyclic heptapeptide: seven residues, N-terminally acetylated, with a lactam bridge closing the ring between an aspartate and a lysine side chain. Its core pharmacophore is the His-D-Phe-Arg-Trp motif conserved across the melanocortin agonist series, the D-phenylalanine substitution being one of the classical modifications conferring both potency and proteolytic resistance relative to native α-MSH.

Three properties follow from that architecture. It is a constrained macrocycle which is why it survives in plasma far better than a linear peptide of the same length, the ring restricts endopeptidase access and locks the pharmacophore into a bioactive conformation. It contains tryptophan and histidine both oxidation-prone residues, making oxidative degradation the principal chemical liability. And it terminates in a free carboxylic acid the single structural difference from its immediate chemical predecessor and the reason the two compounds are not interchangeable. Unlike several compounds shelved alongside it in research catalogues, bremelanotide is a genuine peptide with a defined sequence, so sequence confirmation is a meaningful item on its certificate of analysis.

How Does PT-141 Work?

The Melanocortin Receptor Family

The melanocortin system comprises five G protein-coupled receptors, MC1R through MC5R canonically Gs-coupled and activated by proopiomelanocortin-derived peptides, α-, β- and γ-MSH and ACTH. Their distribution and physiology are entirely distinct: MC1R in melanocytes, governing pigmentation and cutaneous inflammatory signalling; MC2R in adrenal cortex, the ACTH receptor governing steroidogenesis; MC3R centrally and peripherally, in energy homeostasis and inflammatory regulation; MC4R predominantly central, in energy homeostasis, autonomic outflow and hypothalamically organised behaviours; and MC5R in exocrine tissue.

Designing a selective ligand across this family is genuinely difficult, because the receptors share a conserved binding cleft recognising the same short tetrapeptide message. A 2024 review of recommended tool compounds for the melanocortin receptors exists precisely because selectivity here is so often overstated in the primary literature. Bremelanotide is not an exception to that difficulty; it is an illustration of it.

MC4R as the Relevant Target

Among the five, MC4R is the subtype of interest for the pharmacology studied in bremelanotide’s clinical programme. It is densely expressed in hypothalamic and brainstem nuclei and is among the best-validated receptors in human neuroendocrinology: loss-of-function variants cause monogenic obesity.

In the early characterisation of PT-141, Molinoff and colleagues (2003) described the compound as a synthetic α-MSH analogue acting as an agonist at melanocortin receptors including MC3R and MC4R which are “expressed primarily in the central nervous system”. The approved labelling is similarly non-committal about a single subtype, stating that bremelanotide nonselectively activates several receptor subtypes with binding most relevant at MC1R and MC4R, and that the precise mechanism by which it improves the labelled indication is unknown. That last statement is worth reproducing rather than paraphrasing away: it is unusual for an approved drug’s label to say the mechanism is unknown, and it is the honest position.

A Central, Not Peripheral, Mechanism

The most important mechanistic distinction in this dossier is that bremelanotide’s pharmacology is central not vascular. Molinoff and colleagues reported that systemic administration in animal models produced increased c-Fos immunoreactivity in the paraventricular nucleus of the hypothalamus a standard marker of neuronal activation, and concluded that central nervous system activation was the basis of the observed effects. That is a mechanistically different proposition from the phosphodiesterase-5 inhibitors, which act peripherally on smooth muscle and vascular tone, and it is reflected throughout the trial record: the compound’s characteristic adverse events, nausea, flushing, headache, autonomic blood pressure and heart rate changes, are those of a centrally acting agent with autonomic reach rather than a peripheral vasodilator.

Derivation from Melanotan II, and Why the Chemistry Diverged

Bremelanotide’s chemical lineage runs directly through Melanotan II (MT-II) catalogued in PubChem as CID 92432 with the formula C₅₀H₆₉N₁₅O₉. Compare that with bremelanotide’s C₅₀H₆₈N₁₄O₁₀: the difference is one nitrogen replaced by one oxygen, the signature of an amide converted to a carboxylic acid. MT-II terminates in a C-terminal primary amide, bremelanotide in the free acid, and bremelanotide is described in the literature as a metabolite of MT-II.

Why that single change mattered enough to found a separate development programme comes down to receptor bias. MT-II is a potent, broadly active melanocortin agonist with substantial MC1R activity, which is why it produces marked pigmentary effects and why it acquired an unregulated market entirely outside clinical development. Removing the C-terminal amide shifts the balance of activity and alters pharmacokinetics; the programme that followed selected the acid.

That divergence is the central reason the two should never be conflated in a research context: they differ by one functional group, have different receptor activity profiles, only one has an approved prescription product, and they are distinguishable only by mass and chromatography, which is exactly why lot-matched analytical documentation is not optional for either.

It is also why the recurring description of PT-141 as a selective MC4R agonist is wrong. The approved labelling describes nonselective activation of several subtypes, so every observed effect must be attributed cautiously, and the hyperpigmentation signal in the trial data is the clearest illustration. Pigmentary change is MC1R pharmacology, and its appearance in the clinical record is direct evidence that MC1R is engaged at clinically relevant exposures in humans. A genuinely MC4R-selective compound would not be expected to produce it.

Research Evidence

Preclinical and Receptor-Level Work

The preclinical dataset is the receptor pharmacology and central-activation work summarised above: agonist activity at melanocortin receptors including MC3R and MC4R, hypothalamic neuronal activation demonstrated by c-Fos immunoreactivity, and structure–activity work within the constrained α-MSH analogue series.

The RECONNECT Phase 3 Programme

The pivotal clinical evidence is the RECONNECT programme: two identical, independently powered, randomised, double-blind, placebo-controlled phase 3 trials, registered as NCT02333071 and NCT02338960, reported together by Kingsberg and colleagues in Obstetrics & Gynecology (2019).

1,267 premenopausal women with HSDD were randomised to bremelanotide 1.75 mg subcutaneously or placebo over a 24-week double-blind period. The co-primary endpoints were change in the Female Sexual Function Index desire domain (FSFI-D) and change in item 13 of the Female Sexual Distress Scale–Desire/Arousal/Orgasm (FSDS-DAO Q13). The reported results, as scale-point changes relative to placebo, were:

Both trials met both co-primary endpoints with statistical significance. The magnitude of those changes is a separate question from their significance, and the figures are reported here as scale points on validated instruments so that readers can form their own view. Adverse events in 10% or more of treated participants were nausea, flushing and headache, reported as predominantly mild to moderate.

Long-Term Open-Label Data

Participants completing the double-blind period were eligible for an open-label extension, reported by Simon and colleagues (2019). Retention is the most informative feature of that dataset: the LiverTox monograph notes that of 684 patients entering the extension, 272 completed it. For a compound administered on an as-needed basis with a 40% nausea incidence, dropout is itself a tolerability datum. A subsequent prespecified and integrated subgroup analysis examined consistency of the reported effects across demographic and clinical strata.

Cardiovascular Pharmacology: The Ambulatory Monitoring Studies

Because MC4R agonism can raise blood pressure, the cardiovascular pharmacology received dedicated study rather than being left to routine trial monitoring, one of the more methodologically careful parts of the programme.

White and colleagues (2017), in the Journal of Hypertension, reported a randomised, double-blind, placebo-controlled study of three bremelanotide doses (0.75, 1.25 and 1.75 mg) in 397 premenopausal women who were normotensive or had controlled hypertension, using ambulatory blood pressure monitoring alongside pharmacokinetic sampling. Relative to placebo, systolic increases of 2.4 and 3.0 mmHg were observed at 1.25 mg and 3.1 and 3.2 mmHg at 1.75 mg following two doses separated by 24 hours, within the 0–4 hour post-dose interval; peak increases typically lasted under 15 minutes. Diastolic pressure showed similar elevations, and heart rate fell by 4.6 to 4.7 beats per minute at 1.75 mg. Twenty-six participants discontinued for prespecified blood pressure increases, in similar proportions across groups.

That pattern, a small, short-lived pressor response with a concurrent fall in heart rate, is reproduced in the approved labelling, which reports maximal increases of 6 mmHg systolic and 3 mmHg diastolic and a heart rate reduction of up to 5 beats per minute after each dose, usually resolving within 12 hours.

The Documented Adverse Event Profile

Reported plainly as trial data, the approved prescribing information records the following for reactions occurring in more than 4% of treated patients:

Two further findings are recorded as warnings rather than common reactions. Transient blood pressure increase and heart rate decrease occur after each dose, as described above. Focal hyperpigmentation was reported in approximately 1% of patients receiving up to eight monthly doses, at sites including the face, gingiva and breasts, more frequently in patients with darker skin, and with resolution not confirmed in some patients. The LiverTox monograph records mild enzyme elevations in trials and one reported case of acute hepatitis that resolved on withdrawal, with no instances of acute liver failure or chronic liver injury.

The Earlier Development Route

An earlier development route explored intranasal administration and did not reach approval; the programme that did was subcutaneous. Blood pressure effects were a recognised class concern throughout, which is the context in which the ambulatory monitoring work described above was commissioned. Dhillon and Keam’s 2019 review in Drugs is the standard summary of the approval that followed.

What Remains Unknown

The mechanism is not established the approved labelling states so explicitly, and which receptor subtype in which nucleus accounts for the reported endpoint changes remains unresolved. Subtype attribution of adverse effects is incomplete: hyperpigmentation is clearly MC1R-mediated, while nausea, flushing and the autonomic changes are less cleanly assigned. Long-term pigmentary outcomes are not fully characterised the label recording that resolution was not confirmed in some patients. And populations outside the pivotal trials’ inclusion criteria are not characterised by that programme.

Comparison: Bremelanotide and the Melanocortin Class

Placing bremelanotide against its class relatives clarifies both its receptor position and the regulatory gap between an approved medicine and a research chemical.

Two points follow. First, the melanocortin class is not one pharmacology: approved agents in this family target three different receptor subtypes for three unrelated indications, bremelanotide at MC1R/MC4R, setmelanotide at MC4R for genetic obesity syndromes, and afamelanotide at MC1R for erythropoietic protoporphyria, which is the clearest possible demonstration that “melanocortin agonist” is a family rather than a mechanism. Second, the α-MSH fragment peptides sit outside the receptor story altogether KPV, the C-terminal tripeptide lysine-proline-valine, is studied for anti-inflammatory activity generally not attributed to melanocortin receptor agonism at all, and research catalogues list KPV peptide as an entirely separate research compound. A shared parent hormone does not imply a shared mechanism.

Handling a Cyclic Heptapeptide

Bremelanotide research material is supplied as a lyophilised solid, with two chemical liabilities that follow directly from its structure. Oxidation is the principal degradation route: tryptophan and histidine are among the most oxidation-susceptible residues, so a peptide containing both should be treated as oxygen- and light-sensitive. And macrocyclic constraint helps but does not eliminate hydrolytic liability: the lactam bridge restricts endopeptidase access, but the backbone remains hydrolytically susceptible in solution, and repeated freeze–thaw cycling of reconstituted material is a recognised way to lose defined content.

The standard laboratory reconstitution workflow for a lyophilised peptide applies:

• Bring the vial to room temperature before opening to avoid drawing atmospheric moisture onto cold glass, and disinfect the septum with 70% isopropyl alcohol.
• Add diluent slowly down the inner wall not onto the lyophilised cake, which shears and foams peptide at the liquid interface.
• Swirl gently rather than shaking or vortexing. Mechanical agitation aggregates peptides, and foaming is a visible sign of it.
• Aliquot at the point of reconstitution recording date, diluent and nominal concentration.
• Store protected from light and treat discoloration, particulate or incomplete dissolution as a stability signal rather than a cosmetic one.

For multi-entry aqueous preparations, most laboratory protocols specify a bacteriostatic water meeting the Bacteriostatic Water for Injection, USP monograph, of the kind listed beside research compounds at NextGenPeps, because the benzyl alcohol preservative suppresses microbial growth between entries. Two caveats travel with it. Benzyl alcohol is not inert toward peptides and absorbs in the ultraviolet, so single-entry analytical preparations, particularly anything quantified spectrophotometrically against tryptophan absorbance, are better served by unpreserved sterile water. And it is contraindicated in neonatal use because of benzyl alcohol toxicity.

Regulatory Status: An Approved Product Is Not a Research Vial

Bremelanotide occupies an unusual position: the molecule is the active ingredient of an approved prescription medicine and is also sold as a research chemical. Those two facts are frequently blurred, and the distinction is the whole point.

What is true of the approved product. The NDA 210557 approval letter, dated 21 June 2019 records FDA approval of Vyleesi (bremelanotide injection) for the treatment of premenopausal women with acquired, generalised HSDD, with the applicant named as AMAG Pharmaceuticals. It is supplied as a 1.75 mg/0.3 mL solution in a subcutaneous autoinjector. Its labelling states that no more than one dose should be administered within 24 hours and that more than eight doses per month is not recommended, and it is contraindicated in uncontrolled hypertension or known cardiovascular disease. It is prescription-only, manufactured under pharmaceutical quality systems and dispensed under clinical supervision. Those are label facts, recorded here as regulatory record. They are not guidance, and nothing in this article describes how to use any product.

What is true of research-grade material. A vial of research chemical sold under the PT-141 designation is not Vyleesi. It has no marketing authorisation, no approved labelling, no pharmaceutical manufacturing guarantees, no sterility assurance and no regulatory oversight of identity, purity or content, and the existence of an approved product containing the same active moiety confers exactly nothing on it. Research material is research-use-only not for human or veterinary use, and the existence of an approved medicine makes that distinction sharper rather than softer, because it means an approved, supervised route exists and a research vial is emphatically not it.

Regulatory status also differs by jurisdiction. The US approval says nothing about position elsewhere, and laboratories outside the United States should verify local status directly rather than inferring it.

Where to Source Research-Grade PT-141

Because bremelanotide differs from Melanotan II by a single functional group, the analytical burden on a supplier is higher here than for most research peptides, and identity confirmation is not a formality. A defensible purchase record contains:

• A lot-matched certificate of analysis tied to the specific lot shipped rather than a representative batch.
• Mass spectrometric confirmation consistent with the expected 1,025 Da for the free acid, the single most important item, because it is what distinguishes bremelanotide from its amide predecessor.
• Sequence confirmation plus confirmation of the cyclic rather than linear form.
• HPLC or UPLC purity from a named third-party laboratory, with a stated method, mobile phase, gradient, detection wavelength, and a test date. Area-percent purity with no method attached is uninterpretable.
• Net peptide content reported separately from chromatographic purity, because counter-ion and residual water can account for a substantial fraction of vial mass.
• Residual solvent and trifluoroacetate data TFA being a routine residue of reverse-phase purification with biological activity of its own in cell-based systems.
• Endotoxin and sterility data where the work requires it, and explicit research-use-only labelling with no protocol, dosing guidance, human-use framing or therapeutic claim in the listing.

Red flags are specific here. A listing that does not distinguish bremelanotide from Melanotan II has failed the one analytical test that matters most. A listing that cites the approved product’s label as evidence of the research material’s safety or quality has committed the central category error described above. A certificate with no named laboratory, lot reference or test date is decoration rather than evidence. And any listing supplying a human protocol, an administration schedule or a claim of clinical benefit has left the research-use-only frame entirely, which, for a molecule with an approved prescription product, is not merely non-compliant but actively misleading.

A catalogue that lists PT-141 as a research compound can be assessed against exactly these criteria: lot-matched third-party analysis, mass confirmation consistent with the free acid, sequence and cyclisation confirmation, a stated HPLC method, net peptide content reported separately, and research-use-only labelling with no protocol content attached.

Frequently Asked Questions

Is PT-141 the same as bremelanotide?

Yes. PT-141 is the development code and bremelanotide the international non-proprietary name for the same cyclic heptapeptide; Vyleesi is the brand name of the approved prescription product containing it. A research vial labelled PT-141 contains the same active moiety as the approved product but is not that product and carries none of its regulatory guarantees.

Is PT-141 FDA approved?

The molecule is the active ingredient of an FDA-approved prescription medicine: Vyleesi was approved on 21 June 2019 under NDA 210557 for premenopausal women with acquired, generalised HSDD. Research-grade material sold under the PT-141 designation is not an approved product, no marketing authorisation, no approved labelling, no pharmaceutical manufacturing oversight, and is research-use-only.

Which melanocortin receptor does it act on?

Not one. The approved labelling states that bremelanotide nonselectively activates several subtypes, with MC1R and MC4R the most relevant, and early characterisation described agonist activity at receptors including MC3R and MC4R. MC4R is generally regarded as the subtype relevant to the central pharmacology studied clinically, but the label states that the precise mechanism underlying the labelled indication is unknown.

How is it related to Melanotan II?

Bremelanotide differs at the C-terminus: it carries a free carboxylic acid where Melanotan II carries a primary amide, the two formulae differing by one nitrogen replaced by one oxygen. Bremelanotide is described in the literature as a metabolite of Melanotan II. The two are not interchangeable, different receptor activity profiles, different pharmacokinetics, and only one with an approved prescription product.

What does the phase 3 trial record actually report?

Two identical randomised, double-blind, placebo-controlled trials randomised 1,267 premenopausal women with HSDD to bremelanotide 1.75 mg subcutaneously or placebo over 24 weeks. Both met both co-primary endpoints: change in the FSFI desire domain (+0.30 and +0.42 scale points versus placebo) and change in FSDS-DAO item 13 (−0.37 and −0.29 scale points), with p values below .005 in all four comparisons.

What adverse events were reported?

In the trial programme: nausea in 40.0% of treated patients, flushing in 20.3%, injection site reactions in 13.2%, headache in 11.3% and vomiting in 4.8%, with transient blood pressure increase and concurrent heart rate decrease after each dose. Focal hyperpigmentation was reported in approximately 1% of patients receiving up to eight monthly doses, more frequently in those with darker skin, with resolution not confirmed in some cases. These are trial and label data, not a safety assessment of research material.

Why does hyperpigmentation appear at all, and why does the chemistry need such careful documentation?

Pigmentary change is MC1R pharmacology, and its appearance in human trial data is direct evidence that MC1R is engaged at clinically relevant exposures, the clearest single argument against describing bremelanotide as a selective MC4R agonist. The documentation point is related: bremelanotide and Melanotan II differ by one functional group and are otherwise identical in length, sequence and cyclisation, so distinguishing them requires mass spectrometry and chromatography rather than a label.

The Bottom Line

Bremelanotide is one of the few compounds in the research-peptide market with a genuine, published, regulator-reviewed clinical dossier, which makes precision about what that dossier says more important, not less.

What the record establishes: bremelanotide is a cyclic heptapeptide α-MSH analogue, chemically one functional group removed from Melanotan II, acting as a non-selective melanocortin receptor agonist with MC1R and MC4R the most relevant subtypes. Its studied mechanism is central demonstrated preclinically by hypothalamic neuronal activation rather than any peripheral vascular effect. Two identical phase 3 trials in 1,267 participants met both co-primary endpoints with statistically significant but numerically small changes on validated instruments. Its adverse event profile is dominated by nausea at 40%, with a transient pressor response quantified by dedicated ambulatory monitoring and a low-frequency, incompletely resolving pigmentary signal. An approved prescription product exists, and its own labelling states that the mechanism of the labelled effect is unknown.

What the record does not establish is any equivalence between a research vial and that approved product. The approval is a fact about a regulated medicine manufactured to pharmaceutical standards and prescribed under clinical supervision. It is not a warrant for anything sold as a research chemical, and the trial and label data reproduced above are regulatory record rather than guidance. For a laboratory working on melanocortin receptor pharmacology, bremelanotide is a well-documented reference ligand with an unusually rich human dataset and one specific analytical requirement: prove it is the acid and not the amide, from a lot-matched certificate, every time.

By [AUTHOR NAME PLACEHOLDER], [CREDENTIALS PLACEHOLDER]. Fact-checked by [FACT-CHECKER NAME PLACEHOLDER].

Research Use Only Disclaimer

Research-grade PT-141 (bremelanotide) and all other compounds discussed in this article are intended for laboratory research use only and are not for human or veterinary use. Research material is not a pharmaceutical product, holds no marketing authorisation, and is not approved by the U.S. Food and Drug Administration or any comparable authority for the diagnosis, treatment, cure or prevention of any disease in any species.

The indication, dosage form, strength, contraindications, warnings and adverse event incidences of the approved prescription product containing bremelanotide are reported here solely as regulatory and trial record. They are not dosing guidance, not a protocol, not a recommendation and not a therapeutic claim, and they do not apply to research material. Clinical trial results are reported as the original investigators and the approved labelling state them, and do not transfer to any other preparation, population, route or context.

Nothing here is medical, veterinary or pharmaceutical advice. Readers with clinical questions should consult a qualified healthcare professional; prescription medicines should be used only as directed by a prescriber. Bacteriostatic Water for Injection, USP contains benzyl alcohol and is contraindicated in neonates. Readers are responsible for compliance with all applicable laws, institutional review requirements, anti-doping regulations and biosafety rules in their jurisdiction.

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