PT-141 (Bremelanotide)

Identity, and the amide that was taken off

PubChem CID 9941379 resolves bremelanotide as a cyclic heptapeptide, molecular formula C50H68N14O10, molecular weight 1025.2 g/mol, InChIKey FFHBJDQSGDNCIV-MFVUMRCOSA-N, carrying CAS 189691-06-3 and FDA UNII 6Y24O4F92S. The material in the approved product is the acetate salt, held separately in the FDA Global Substance Registration System under UNII PV2WI7495P and CAS 1607799-13-2, and indexed in PubChem as CID 91971505 at C52H72N14O12 and 1085.2 g/mol. The approved labelling does not fix that stoichiometry: section 11 gives the salt as C50H68N14O10 with one to two acetic acid equivalents. It gives the structure as Ac-Nle-cyclo(Asp-His-D-Phe-Arg-Trp-Lys)-OH: a ring closed by a lactam between the aspartate and lysine side chains, an acetyl group at the amino terminus, and a free carboxylic acid at the other end.

That free acid is the whole structural difference from melanotan II, which terminates in an amide. Hadley and Dorr, writing a history of melanocortin therapeutics in Peptides in 2006 (PMID 16412534), describe PT-141 as an analogue of melanotan II taken forward by Palatin Technologies after early-phase work. The peptide also carries D-phenylalanine as its fourth residue, a substitution shared across this analogue series. Secondary pages describe the compound as the active metabolite of melanotan II rather than as an analogue of it, which is a different assertion about how it comes to exist in a body; that phrasing is logged below as untraced.

Stability has been examined once in the published record. Yuvaraaj and Sharma, reporting forced-degradation work in Analytical Methods in 2026 (PMID 42485063), stressed bremelanotide acetate under acidic, basic, neutral, oxidative, thermal and photolytic conditions to the International Council for Harmonisation protocol, detected eight degradation products by LC-HRMS/MS, and identified deacetylation, peptide-bond hydrolysis, oxidation and epimerisation as the pathways. Degradation was lower under acid than base and marked under oxidative conditions. Sauter and colleagues had earlier given the peptide orally to beagle dogs and reported minimal oral absorption, using a UHPLC-MS/MS assay with a lower limit of quantification of 10 pg/mL (PMID 32353679).

Two authoritative sources rank the receptors differently

The approved labelling states the pharmacology plainly and then declines to state a mechanism. Bremelanotide is described there as a melanocortin receptor agonist that nonselectively activates several receptor subtypes in the order MC1R, MC4R, MC3R, MC5R, MC2R, with binding at MC1R and MC4R the most relevant at therapeutic exposures. The same section records that MC1R is expressed on melanocytes and that binding there leads to melanin expression and increased pigmentation, and it says in as many words that the mechanism behind the effect the drug is approved for is unknown.

A curated pharmacology database ranks the same receptors in a different order. The IUPHAR/BPS Guide to Pharmacology lists bremelanotide as ligand 10408 with inhibition constants against radiolabelled NDP-alpha-MSH on membranes from BHK570 cells expressing each human receptor: 0.25 nM at MC4R, 6.4 nM at MC1R, 17 nM at MC5R and 53 nM at MC3R, all attributed to Conde-Frieboes and colleagues in the Journal of Medicinal Chemistry in 2012 (PMID 22335602). By that measure MC4R comes first and MC3R last, which is not the labelling's sequence. Those IUPHAR figures are displacement affinities from iodinated NDP-alpha-MSH binding on those membranes; the labelling names no assay, no species and no method for its own ordering, so the two cannot be put on a common scale from the retrievable documents. Both are recorded here as published.

Cryo-electron microscopy has resolved the receptor with this ligand in place. Zhang and colleagues reported four structures of full-length MC4R in complex with heterotrimeric Gs in Cell Research in 2021 (PMID 34433901), stimulated by alpha-MSH, by afamelanotide, by bremelanotide and by the small molecule THIQ, and described the conserved binding mode of the peptide agonists. The identifier matters: a second MC4R-Gs structural paper appeared in the same journal in the same year resolving a different pair of ligands, and the two are easily confused. What a structure fixes is geometry, where the peptide sits and which contacts it makes; the behavioural consequence of that geometry is measured in the studies below.

The male programme, and what became of its two efficacy papers

Development began in men. Molinoff and colleagues summarised the early position in the Annals of the New York Academy of Sciences in 2003 (PMID 12851303): erections in rats and nonhuman primates, hypothalamic c-Fos induction after systemic administration, and dose-dependent erectile activity in men. Diamond and colleagues then published intranasal pharmacokinetics in healthy males and in sildenafil-responsive patients (PMID 14963471), reporting median Tmax of 0.50 hours, mean half-life of 1.85 to 2.09 hours, a statistically significant erectile response above 7 mg, and first erection at roughly 30 minutes. Rosen and colleagues repeated the design subcutaneously, with a significant response above 1.0 mg (PMID 14999221). Every one of these endpoints was penile rigidity recorded by RigiScan.

Two later trials used clinical endpoints — one in women, one in men — and both are now flagged. Safarinejad's 2008 study in the Journal of Sexual Medicine, in women with arousal disorder, was retracted at the request of the Editor-in-Chief after the journal re-reviewed all his submissions; the notice states he was asked for his original data and chose not to respond. His 2008 trial in the Journal of Urology, in 342 men who had not responded to sildenafil, reported positive clinical results in 33.5% against 8.5% on placebo and carries an Expression of Concern printed in January 2023 that remains unresolved. PubMed types eighteen of his papers overall as Retracted Publication.

How the male programme ended is documented in a corporate filing rather than in the literature. Palatin's Form 10-K for the year ended 30 June 2008 records that almost 2,000 patients had received at least one dose across nasal, subcutaneous and intravenous formulations, about 1,500 of them multiple doses, that increases in blood pressure were observed in some patients, and that development for sexual dysfunction was discontinued after the FDA raised concerns about the benefit-risk ratio as a first-line erectile dysfunction therapy, primarily because of those increases. No PubMed-indexed paper states this. No registered bremelanotide study targets male sexual dysfunction, and the only two registered studies open to male participants, NCT05709444 in diabetic kidney disease and NCT06565611 in obesity with tirzepatide, concern other indications. The approved labelling carries an explicit limitation of use against that population.

The trials that produced an approval

The programme restarted in women with a dose-finding trial. Clayton and colleagues randomised premenopausal women with sexual dysfunction to placebo or one of three subcutaneous doses over twelve weeks, with 327 in the efficacy analysis (PMID 27181790); for the pooled 1.25 and 1.75 mg arms against placebo they reported changes from baseline of +0.7 against +0.2 satisfying sexual events per month, +3.6 against +1.9 on the Female Sexual Function Index total, and -11.1 against -6.8 on the Female Sexual Distress Scale-Desire/Arousal/Orgasm. Althof and colleagues derived responder thresholds from that same dataset in 2019 (PMID 31277966); whether those definitions were prespecified for the registration trials is not established in the retrievable record.

RECONNECT was two identically designed phase 3 trials, NCT02333071 and NCT02338960, running from January 2015 to mid-2016. Of 1,267 women randomised, 1,247 entered the safety population and 1,202 the efficacy population, receiving 1.75 mg subcutaneously as needed over 24 weeks. Kingsberg and colleagues reported integrated co-primary results of +0.35 on the Female Sexual Function Index desire domain and -0.33 on item 13 of the distress scale against placebo, both at p<0.001 (PMID 31599840). The desire domain runs from 1.2 to 6.0 and the distress item from 0 to 4. Participants were mostly White, 96.6% enrolled at United States sites, with a mean age of 39.

Tolerability accounts for most of what the safety database contains. In the pooled double-blind phase, nausea was recorded in 40.0% against 1.3% on placebo, flushing in 20.3% against 0.3%, injection-site reactions in 13.2% against 8.4% and headache in 11.3% against 1.9%; discontinuation attributable to adverse reactions ran at 18% against 2%. The labelling also reports a single-dose study in which 228 healthy premenopausal women were randomised one to one to 8 mg oral ondansetron or placebo thirty minutes beforehand, with no reduction in the incidence of nausea; the labelling calls that study phase 4 while its registry entry, NCT03973047, types it phase 1, and it has posted no results and produced no publication. Of 856 women eligible for the 52-week open-label extension, 684 enrolled and 272 finished it (PMID 31599847).

A dispute about the size of the effect, conducted in print

Whether those numbers amount to anything has been argued openly. Spielmans, re-analysing the trials from the FDA New Drug Application in the Journal of Sex Research in 2021 (PMID 33678061), reported that 72.72% of protocol-listed outcomes went unreported while fifteen secondary measures appeared that were not in the protocols, and that two figures absent from the primary publication were an odds ratio of 11.98 (95% CI 3.74 to 38.37) for adverse-event-induced discontinuation and an odds ratio of 0.30 (95% CI 0.24 to 0.38) for the combination of completing the trial and electing the open-label extension. His own meta-analysis of the efficacy data matched the published results.

Investigators from the trials replied in the same issue under the title Failure of a Meta-analysis (PMID 33835907). Spielmans and Ellefson returned in 2024 with an examination of the measurement properties of the efficacy instruments (PMID 36809187), reporting that eight of the eleven efficacy outcomes registered on ClinicalTrials.gov had not been published, analysing them, and finding effect sizes ranging from nil to small. Mintzes, Tiefer and Cosgrove, writing in Drug and Therapeutics Bulletin in 2021 (PMID 34642243), argued that the approval rested on the regulatory precedent set by flibanserin rather than on a demonstrated clinically meaningful benefit.

Both camps work from the same trial data and arrive at similar point estimates. Their disagreement is about whether the instruments were validated for this population, whether the reported outcomes were the pre-registered ones, and what a third of a point on a five-point distress item means to the person filling it in. Both positions sit in the peer-reviewed record with replies attached, and this page records them without choosing between them.

Mechanism, measured in thirty-one women and in hamsters

Thurston and colleagues ran the only published human mechanism study, an investigator-sponsored trial at Imperial College funded in part by AMAG Pharmaceuticals. Forty premenopausal women with the diagnosis were randomised to a two-way crossover of 1.75 mg subcutaneously against placebo, with 31 completing both visits and functional imaging at 45 and 240 minutes (PMID 36189794). Significantly more participants reported increased sexual desire in the 24 hours after the active agent than after placebo (p=0.007). Imaging showed enhanced right cerebellar and supplementary motor area activity, deactivation of secondary somatosensory cortex during erotic stimuli, and increased amygdala-insula connectivity.

An animal result points away from the mechanism most often quoted downstream. Borland and colleagues, working in female Syrian hamsters and publishing in Neuropharmacology in 2025 (PMID 39793696), found melanocortin 3 and 4 receptor mRNA predominantly in ventral tegmental dopamine neurons, while in the nucleus accumbens MC4R sat in interneurons and rarely alongside dopamine D1 or D2 receptors. Neither dose changed melanocortin receptor mRNA in that circuit, and although sexual experience produced a conditioned place preference, treatment did not enhance it. Their stated conclusion is that the compound does not act on the ventral tegmental-accumbens reward pathway.

The founding rodent result is narrower than it is usually rendered. Pfaus and colleagues reported in the Proceedings of the National Academy of Sciences in 2004 (PMID 15226502) that PT-141 selectively facilitated solicitational behaviour in female rats without affecting lordosis, pacing, other sexual behaviours, generalised motor activation or the perception of sexual reward. The full text, free at PMC454387, sets out the design: forty ovariectomised, hormone-primed females were assigned at random to 0, 50, 100 or 200 micrograms per kilogram, dissolved in saline and injected subcutaneously five minutes before each behavioural test. The paper carries a printed disclosure that two authors held stock in the sponsor, that the sponsor funded the work, and that the technicians running the tests were blinded to dose.

Regulatory position, and what is registered

Bremelanotide holds NDA 210557, approved on 21 June 2019, with one supplement approved in October 2020 and a labelling revision dated November 2025. The indication covers premenopausal women with acquired, generalised hypoactive sexual desire disorder, and the labelling states in its own limitations of use that the drug is not indicated in postmenopausal women, not indicated in men, and not indicated to enhance sexual performance. It is contraindicated in uncontrolled hypertension and in known cardiovascular disease. The regulatory position is, in other words, unusually specific about who was studied.

Searching the European Medicines Agency's complete downloadable medicines report, which covers authorised, refused and withdrawn products, returns no occurrence of either the substance name or the approved product name across 20,745 indexed strings. Health Canada's active-ingredient interface returns an empty result for the substance. The text of the World Anti-Doping Agency Prohibited List 2026 contains no occurrence of bremelanotide, melanotan or melanocortin, while Bromantan and BPC-157 both appear, which confirms the extraction was working; section S0 reaches substances with no current approval from any governmental regulatory health authority, which does not describe this one. Outside the United States, then, the record is thin.

Registration coverage is incomplete. ClinicalTrials.gov lists ten studies with bremelanotide as an intervention, four of which have results posted. The six that do not include a 193-participant phase 3 bridging trial in South Korea that completed in May 2023 and has no PubMed record, a 16-participant phase 2 study in diabetic kidney disease completed in April 2024, a 108-participant phase 2 obesity trial combining the peptide with tirzepatide whose primary completion date passed in February 2025 and which remains active and not recruiting, and a ten-participant lactation pharmacokinetic study completed in November 2025. Material circulating outside the regulated supply has been examined once: Mestria and colleagues characterised melanotan II and bremelanotide in eight seized samples by accurate-mass measurement, working without reference standards (PMID 33245851).

What is not known

Long-term safety beyond the 52-week open-label extension is not characterised, and the extension itself is uncontrolled and lost most of its enrolment: 684 of 856 eligible women entered and 272 finished (PMID 31599847). Whether the reduction in reported distress translates into any durable change is unmeasured, since no trial followed participants after discontinuation. Efficacy has been tested only in premenopausal women; the postmenopausal and male populations are excluded by the labelling and by the absence of trials, and no registered study targets male sexual dysfunction, the only two registered studies open to male participants (NCT05709444 in diabetic kidney disease, NCT06565611 in obesity with tirzepatide) concerning other indications. Reproductive risk has not been resolved: embryofetal toxicity was elevated three- to eightfold above controls across all treated groups in a dog study without dose dependence, developmental delays appeared in first-generation mice, and no developmental no-observed-effect level was set in either species, while the human record comprises seven pregnancies. Mechanism is unsettled at the level of the primary sources, with the labelling declaring it unknown, one human imaging study describing cortical and connectivity changes, and one hamster study excluding the reward circuit most often invoked. Two authoritative sources disagree about which melanocortin receptor the compound engages most strongly. Six of the ten registered trials, including a completed 193-participant phase 3 trial in South Korea and a phase 2 obesity trial whose primary completion date passed in February 2025 and which remains active, not recruiting, have posted no results and produced no publication. Nothing in the published record addresses material obtained outside the regulated supply chain, its content, or its sterility.

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