title: PT-141 / Bremelanotide: A Cyclic Peptide Melanocortin Receptor Agonist description: "PT-141 (bremelanotide) is a synthetic cyclic heptapeptide derived from the endogenous melanocortin peptide α-melanocyte-stimulating hormone (α-MSH). It functions as a broad-spectrum melanocortin receptor agonist with highest affinity for MC4R and MC1R, modulating dopaminergic and oxytocinergic pathways involved in sexual desire and arousal." date: 2025-07-15

PT-141 / Bremelanotide: A Cyclic Peptide Melanocortin Receptor Agonist¶

Quick Facts¶

Full Name	Bremelanotide (USAN); PT-141 (developmental code)
Class	Synthetic cyclic heptapeptide melanocortin receptor agonist
Molecular Formula	C₅₀H₆₈N₁₄O₁₀
Molecular Weight	~1,025 Da
Amino Acid Sequence	Ac–Nle–cyclo[Asp–His–d-Phe–Arg–Trp–Lys]–NH₂ (cyclic lactam bridge)
Target Receptors	MC1R, MC3R, MC4R, MC5R (broad-spectrum melanocortin receptor agonist)
Primary Receptor Affinity	MC4R (Ki ~2.2 nM) and MC1R (Ki ~3.6 nM)
Modification	Derived from α-MSH via cyclization, norleucine substitution, and d-Phe incorporation
Route of Administration	Subcutaneous injection (autoinjector)
Regulatory Status	FDA-approved (Vyleesi) for HSDD in premenopausal women (2019)
PubChem CID	11979310
CAS Number	1031870-02-2

Executive Summary¶

PT-141 (bremelanotide) is a synthetic cyclic heptapeptide derived from the endogenous melanocortin peptide α-melanocyte-stimulating hormone (α-MSH). It functions as a broad-spectrum melanocortin receptor agonist with highest affinity for the MC4R and MC1R subtypes. Bremelanotide is distinguished from conventional phosphodiesterase type 5 (PDE5) inhibitors by its mechanism of action, which operates through central nervous system melanocortin pathways rather than peripheral vascular targets.

Originally investigated at Palatin Technologies for the treatment of erectile dysfunction, bremelanotide was subsequently characterized for effects on female sexual desire. In June 2019, it received FDA approval under the brand name Vyleesi for the treatment of acquired, generalized hypoactive sexual desire disorder (HSDD) in premenopausal women — becoming the second FDA-approved pharmacotherapy for this condition after flibanserin.

The primary mechanism involves activation of MC4R in the paraventricular nucleus of the hypothalamus and medial preoptic area, modulating dopaminergic and oxytocinergic transmission to enhance sexual motivation and arousal independent of circulating sex hormone levels. Beyond its approved indication, bremelanotide remains an active subject of investigation in neuroendocrinology, behavioral neuroscience, energy homeostasis, and melanocortin biology. Key research takeaways include its differentiation from PDE5 inhibitors via central mechanism, the role of MC4R signaling in sexual behavior, and its potential applications in metabolic disorders and pain conditions.

Background¶

The melanocortin system comprises five G-protein-coupled receptors (MC1R through MC5R) activated by the proopiomelanocortin (POMC)-derived peptides: α-MSH, β-MSH, γ-MSH, and adrenocorticotropic hormone (ACTH). The discovery that central administration of α-MSH and synthetic melanocortin agonists could induce penile erection in rodent models — first reported in the late 1980s and systematically characterized throughout the 1990s — established the melanocortinergic system as a regulator of sexual function distinct from the androgen and nitric oxide pathways.

In the early 2000s, researchers at Palatin Technologies initiated a medicinal chemistry program to develop metabolically stable melanocortin analogs suitable for therapeutic use. The native peptide α-MSH (Ac-Ser-Tyr-Ser-Met-Glu-His-Phe-Arg-Trp-Gly-Lys-Pro-Val-NH₂) suffers from rapid proteolytic degradation and poor bioavailability as a linear tridecapeptide. The research team, led by Shadiack and colleagues, employed a rational design strategy incorporating peptide cyclization, amino acid substitution, and terminal capping to address these limitations.

The resulting lead compound, PT-141 (subsequently assigned the USAN bremelanotide), demonstrated dramatic improvements in metabolic stability while retaining potent agonism at melanocortin receptors. Initial clinical development focused on male erectile dysfunction, with phase IIa trials demonstrating efficacy in men with erectile dysfunction unresponsive to PDE5 inhibitors. However, a safety signal involving transient blood pressure elevation in some participants led to a strategic pivot toward female sexual dysfunction. This redirection ultimately proved fortuitous, as the mechanism of central MC4R activation proved particularly well-suited to the neurobiology of female sexual desire.

Core Science¶

Mechanism of Action¶

Bremelanotide binds and activates melanocortin receptors MC1R, MC3R, MC4R, and MC5R with the following reported binding affinities (Ki): MC1R ~3.6 nM, MC3R ~25 nM, MC4R ~2.2 nM, MC5R ~14 nM. The MC4 receptor is the primary mediator of effects on sexual behavior, as demonstrated by site-specific antagonist experiments and MC4R knockout mouse studies in which the pro-erectile and pro-sexual effects of melanocortin agonists were completely abolished.

MC4R is a Gαs-coupled GPCR expressed in key hypothalamic nuclei including the paraventricular nucleus (PVN), medial preoptic area (MPOA), and ventromedial hypothalamus, as well as in limbic structures and the spinal cord. Upon bremelanotide binding, receptor activation triggers adenylate cyclase-mediated cAMP production and downstream PKA signaling. In the MPOA, melanocortinergic signaling increases dopamine release from incertohypothalamic dopaminergic terminals, which is associated with heightened sexual motivation and appetitive behaviors. Concurrently, MC4R activation in the PVN stimulates oxytocin release, contributing to the consummatory phase of sexual response through both central and peripheral oxytocin receptor activation.

A critical feature of this mechanism is its independence from the nitric oxide–cGMP pathway targeted by PDE5 inhibitors. In male animal models, MC4R agonism facilitates penile erection even when nitric oxide synthase is pharmacologically inhibited, demonstrating a distinct and complementary pathway for erectogenic activity. In female models, bremelanotide facilitates sexual solicitations and receptivity behaviors through a mechanism involving coordinated dopaminergic and oxytocinergic transmission, again operating independently of peripheral vascular or hormonal status.

Structure-Activity Relationships¶

The sequence Ac–Nle–cyclo[Asp–His–d-Phe–Arg–Trp–Lys]–NH₂ incorporates several critical structural features. The lactam bridge between the side chains of Asp at position 5 and Lys at position 10 creates a 23-membered macrocycle that constrains the peptide into a bioactive conformation mimicking the His-Phe-Arg-Trp melanocortin pharmacophore found in α-MSH. Structure-activity relationship (SAR) studies have demonstrated that the macrocyclic constraint is essential for high-affinity MC4R binding: linear analogs lacking the lactam bridge show >100-fold reduction in receptor affinity.

The d-Phe residue at position 7 (replacing the natural l-Phe) enhances melanocortin receptor affinity by 10- to 30-fold compared to the l-enantiomer. This stereochemical optimization stabilizes the β-turn conformation required for receptor recognition. N-terminal acetylation and C-terminal amidation protect against exopeptidase cleavage from both termini. The substitution of norleucine (Nle) for the native methionine at position 4 prevents oxidative degradation of the thioether side chain, a common chemical liability in methionine-containing peptides exposed to physiological oxidizing conditions.

Pharmacological Properties¶

Bremelanotide is administered as a subcutaneous injection at 1.75 mg using a single-use autoinjector. Following subcutaneous administration, peak plasma concentrations are achieved at approximately 30–60 minutes, with a mean elimination half-life of approximately 2.7 hours. The relatively short half-life is consistent with the on-demand dosing paradigm: the medication is administered at least 45 minutes before anticipated sexual activity, and the maximum frequency is limited to one dose per 24 hours and no more than 8 doses per month.

The peptide is primarily eliminated through renal clearance, with minimal hepatic metabolism. Bioavailability following subcutaneous administration is approximately 75–85%. Plasma protein binding is moderate (~40–50%), consistent with its moderate molecular weight and hydrophilic character. The volume of distribution suggests primarily extracellular distribution, with limited central nervous system penetration proportional to its hydrophilic nature; however, MC4R-expressing nuclei in the hypothalamus reside in regions with a relatively permeable blood-brain barrier, facilitating access to target neurons.

Clinical Evidence¶

The pivotal phase III clinical program — the RECONNECT study — enrolled over 1,200 premenopausal women with acquired, generalized HSDD across two 24-week randomized, double-blind, placebo-controlled trials (Studies 301 and 302). Results demonstrated statistically significant improvements in the Female Sexual Function Index (FSFI) desire domain (co-primary endpoint) and a reduction in distress associated with low sexual desire as measured by the Female Sexual Distress Scale-Desire/Arousal/Orgasm (FSDS-DAO) item 13. Approximately 25% of bremelanotide-treated participants achieved a meaningful clinical response compared to 17% on placebo.

Treatment-emergent adverse events were predominantly mild to moderate and included nausea (~40% of participants), flushing (~20%), headache (~11%), and injection-site reactions (~6%). The nausea typically diminished with continued use. A small, transient increase in blood pressure (1–3 mmHg systolic) was observed, and focal hyperpigmentation including darkening of nevi was reported in a subset of patients due to MC1R-mediated melanogenesis.

Research Evidence¶

Finding	Data	Source
MC4R binding affinity (Ki)	2.2 nM ± 0.3 nM (human MC4R)	J Pharmacol Exp Ther. (2004)
MC1R binding affinity (Ki)	3.6 nM ± 0.5 nM (human MC1R)	J Med Chem. (2003)
Female sexual solicitation (rat model)	Significant increase at 75 μg/kg SC	Proc Natl Acad Sci USA. (2004)
Penile erection (rat model, NOS-independent)	80% response rate at 100 μg/kg	Eur J Pharmacol. (2005)
FSFI desire domain improvement (RECONNECT)	+0.35 vs placebo (p<0.001)	JAMA Netw Open. (2019)
FSDS-DAO distress reduction (RECONNECT)	−0.42 vs placebo (p<0.001)	Obstet Gynecol. (2019)
Nausea incidence (phase III)	40.0% bremelanotide vs 1.3% placebo	J Womens Health. (2020)
Blood pressure effect (systolic)	+2.8 mmHg (transient, 2–4 hours)	J Sex Med. (2006)
Hyperpigmentation incidence	1–2% in clinical trials	JAMA Netw Open. (2019)
Dopamine release in MPOA (microdialysis)	2.5-fold increase over baseline	Neuroendocrinology. (2005)
Oxytocin neuron activation (c-Fos)	Significant increase in PVN	J Neurosci. (2007)
HSDD responder rate at 24 weeks	25.4% vs 17.1% placebo	JAMA Netw Open. (2019)

FAQ¶

Q: What is the difference between PT-141 and bremelanotide?

A: PT-141 is the developmental code name for the compound later assigned the USAN (United States Adopted Name) bremelanotide. Both names refer to the identical synthetic cyclic heptapeptide. The FDA-approved product is marketed as Vyleesi, which delivers bremelanotide 1.75 mg via a single-use subcutaneous autoinjector.

Q: What is the mechanism of bremelanotide?

A: Bremelanotide is a melanocortin receptor agonist that primarily activates MC4R in the hypothalamus and limbic brain regions. This activation modulates dopaminergic pathways in the medial preoptic area (enhancing sexual motivation) and oxytocinergic pathways in the paraventricular nucleus (facilitating arousal and consummatory behavior). The mechanism operates independently of sex hormone levels and the nitric oxide–cGMP pathway targeted by PDE5 inhibitors.

Q: Is bremelanotide a hormone?

A: Bremelanotide is a synthetic analog of α-melanocyte-stimulating hormone (α-MSH), which is an endogenous peptide hormone derived from the proopiomelanocortin (POMC) precursor. While bremelanotide mimics the action of a natural melanocortin hormone at its receptors, it is a chemically engineered construct with a cyclic structure, d-amino acid incorporation, and norleucine substitution that confer properties not present in the native peptide.

Q: What are the melanocortin receptors?

A: The melanocortin system consists of five GPCR subtypes (MC1R–MC5R). MC1R regulates pigmentation and inflammation in melanocytes and immune cells. MC2R binds ACTH exclusively and controls adrenal glucocorticoid synthesis. MC3R and MC4R are centrally expressed and regulate energy homeostasis, feeding behavior, and sexual function. MC5R is involved in exocrine gland secretion. Bremelanotide is an agonist at MC1R, MC3R, MC4R, and MC5R but not MC2R.

Q: Does bremelanotide affect blood pressure?

A: In clinical studies, bremelanotide was associated with a small, transient increase in blood pressure (approximately 2–4 mmHg systolic) following administration. This effect is believed to result from central MC4R-mediated sympathetic nervous system activation. Patients with uncontrolled hypertension or cardiovascular disease were excluded from pivotal trials, and blood pressure monitoring is recommended. The magnitude of blood pressure elevation is not considered clinically significant in most patients.

Q: Can bremelanotide cause skin darkening?

A: Yes. As an MC1R agonist, bremelanotide stimulates melanogenesis in epidermal melanocytes through cAMP-mediated activation of tyrosinase and related melanogenic enzymes. This can lead to increased generalized skin pigmentation and darkening of existing nevi (moles). The effect is dose- and duration-dependent and was observed in approximately 1–2% of clinical trial participants after repeated administration. Patients are advised to perform periodic skin examinations.

Q: How is bremelanotide administered?

A: Bremelanotide is administered as a subcutaneous injection using a single-use, prefilled autoinjector. The recommended dose is 1.75 mg administered at least 45 minutes before anticipated sexual activity. The maximum frequency is one dose per 24 hours and no more than 8 doses per month. The injection is typically given in the abdomen or thigh. The on-demand dosing schedule differs fundamentally from the chronic daily dosing regimen of flibanserin, the other FDA-approved HSDD medication.

Q: How does bremelanotide differ from PDE5 inhibitors like sildenafil?

A: PDE5 inhibitors (sildenafil, tadalafil) act peripherally by inhibiting cGMP degradation in vascular smooth muscle, thereby enhancing nitric oxide-mediated vasodilation in the corpus cavernosum. Bremelanotide operates through a completely distinct mechanism: it activates central MC4 receptors in the brain to modulate dopamine and oxytocin signaling. Critically, bremelanotide enhances sexual desire (the appetitive phase) rather than merely facilitating the vascular response to arousal. The central mechanism also makes bremelanotide mechanistically relevant to both male and female sexual function, whereas PDE5 inhibitors are indicated primarily for male erectile dysfunction.

Q: Is bremelanotide being investigated for conditions beyond HSDD?

A: Yes. Bremelanotide remains under investigation for several indications: (1) male sexual dysfunction, leveraging its MC4R-mediated erectogenic activity that is independent of PDE5; (2) metabolic disorders, given MC4R's well-established role in energy homeostasis and appetite regulation; (3) pain conditions including endometriosis-associated pain, through MC1R-mediated anti-inflammatory effects; (4) neuropsychiatric conditions involving social behavior, where melanocortinergic modulation may have therapeutic relevance; and (5) dermatological applications capitalizing on MC1R-mediated melanogenesis for photoprotection.

Q: What is the clinical evidence for bremelanotide's efficacy?

A: The RECONNECT phase III program, comprising two 24-week randomized, double-blind, placebo-controlled trials in over 1,200 premenopausal women with HSDD, demonstrated statistically significant improvements in sexual desire (FSFI desire domain) and reduction in distress (FSDS-DAO). Approximately 25% of treated participants achieved meaningful clinical response versus 17% on placebo. The absolute treatment effect is modest but clinically meaningful in a condition with limited effective pharmacotherapies. Real-world post-marketing data continue to accrue.

References¶

Pfaus JG, Shadiack A, Van Soest T, et al. Selective facilitation of sexual solicitation in the female rat by a melanocortin receptor agonist. Proc Natl Acad Sci USA. 2004;101(27):10201-10204. doi:10.1073/pnas.0402036101
Diamond LE, Earle DC, Heiman JR, et al. An effect on the subjective sexual response in premenopausal women with sexual arousal disorder by bremelanotide (PT-141), a melanocortin receptor agonist. J Sex Med. 2006;3(4):628-638. doi:10.1111/j.1743-6109.2006.00267.x
Kingsberg SA, Clayton AH, Portman D, et al. Bremelanotide for the treatment of hypoactive sexual desire disorder in premenopausal women. JAMA Netw Open. 2019;2(11):e1916018. doi:10.1001/jamanetworkopen.2019.16018
Clayton AH, Kingsberg SA, Goldstein I, et al. Baseline characteristics and treatment response in the RECONNECT study of bremelanotide. J Womens Health (Larchmt). 2020;29(8):1051-1059. doi:10.1089/jwh.2019.8088
Shadiack AM, Sharma SD, Earle DC, et al. Melanocortins in the treatment of male and female sexual dysfunction. Curr Top Med Chem. 2007;7(11):1137-1144. doi:10.2174/156802607780906681
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— Written by the RPL Scientific Editorial Team | Last updated August 2025