PT-141 (Bremelanotide)

Quick summary

PT-141, generically known as bremelanotide, is a synthetic cyclic peptide that activates melanocortin receptors in the brain. It is approved by the FDA under the brand name Vyleesi for hypoactive sexual desire disorder (HSDD)in premenopausal women. It is used on demand, administered as a subcutaneous injection about 45 minutes before anticipated sexual activity.^[3]

PT-141 acts centrally on melanocortin receptors in the hypothalamus, which distinguishes its mechanism from phosphodiesterase type-5 (PDE5) inhibitors such as sildenafil that act on peripheral vascular tissue. The most common adverse effect is nausea, which affects a substantial proportion of patients in trials.^[2],[3]

Outside the United States, bremelanotide is not approved by the EMA, MHRA, TGA, or Health Canada for any indication as of the last review date.

Discovery and development

Bremelanotide was derived from the melanocortin peptide Melanotan II (MTII), a synthetic analog of alpha-melanocyte-stimulating hormone (α-MSH). Melanotan II was originally investigated as a tanning agent but was observed to cause spontaneous erections in male clinical volunteers, prompting interest in its potential for sexual dysfunction.

PT-141 was developed as a modified cyclic analog with an improved safety and selectivity profile relative to Melanotan II. Early clinical studies at Palatin Technologies used an intranasal formulation. The programme was subsequently reformulated for subcutaneous injection and advanced by AMAG Pharmaceuticals through Phase 3.^[1]

The FDA approved bremelanotide injection (Vyleesi) for acquired, generalised HSDD in premenopausal women in June 2019, based primarily on the two RECONNECT Phase 3 trials. It became the second FDA-approved pharmacological treatment for HSDD in premenopausal women, following flibanserin (Addyi, approved 2015).^[3]

Structure

Bremelanotide is a cyclic heptapeptide derived from α-MSH. Its molecular formula is C₅₀H₆₈N₁₄O₁₀, with a molecular weight of approximately 1,025 daltons. The cyclic structure, formed by a lactam bridge between lysine and aspartate residues, confers conformational constraint that improves receptor binding selectivity and metabolic stability relative to linear analogs of α-MSH.

The Vyleesi formulation is supplied as 1.75 mg bremelanotide in 0.4 mL solution for subcutaneous injection, delivered via an autoinjector pen to the abdomen or thigh.^[3]

Mechanism of action

Bremelanotide is a non-selective melanocortin receptor agonist with activity at MC1R, MC3R, and MC4R. The sexual-desire effects observed clinically are attributed primarily to MC4R and MC3R activation in central nervous system pathways, particularly in the hypothalamus and limbic system.^[4],[5]

• MC4R (hypothalamus, brain): activation of MC4R in the mediobasal hypothalamus is associated with pro-sexual signalling in animal models. MC4R-knockout rodents show impaired sexual behaviour, and MC4R-selective agonists facilitate sexual solicitation in female rats at doses that do not produce significant systemic effects.^[5]
• MC3R: contributes to energy balance and may play a supporting role in the pro-sexual signalling circuit, though its relative contribution in humans is less established.
• MC1R (skin melanocytes): activation of MC1R causes skin pigmentation (flushing and transient hyperpigmentation), which is an on-target adverse effect of bremelanotide and was the original intended use of Melanotan II.

Crucially, bremelanotide acts centrally on desire and arousal pathways rather than on genital vasculature, distinguishing it from PDE5 inhibitors. Its efficacy requires engagement of the central nervous system, and it is ineffective in the absence of sexual stimulation or interpersonal context in the trial setting.

Pharmacokinetics in summary

• Route: subcutaneous injection (abdomen or thigh), on-demand.
• Time to peak (Tmax): approximately 1 hour after subcutaneous administration.^[6]
• Half-life (t½): approximately 2.7 hours.^[3]
• Protein binding: approximately 21 %.
• Metabolism: primarily through peptide hydrolysis; no significant hepatic CYP450 involvement. Metabolites are not pharmacologically active.
• Elimination: excreted in urine and faeces; renal excretion of intact drug is limited. The prescribing information contraindicates use in patients with severe renal or hepatic impairment.

Clinical evidence

The FDA approval of Vyleesi rested on two Phase 3 randomised, double-blind, placebo-controlled trials in the RECONNECT programme (NCT02338960), which enrolled premenopausal women with acquired, generalised HSDD.^[2]

The effect sizes on desire and distress scores, while statistically significant, were modest in absolute terms. The FDA review noted that the clinical meaningfulness was a subject of discussion at the advisory committee stage. The drug was approved on the basis that even a modest improvement in desire and reduction in distress represented a meaningful benefit for patients with a condition that lacks many treatment options.^[2]

Safety overview

The most common adverse effect of bremelanotide is nausea, reported by approximately 40 % of patients across the RECONNECT trials, compared with about 1 % on placebo.^[2] Anti-emetic pre-treatment is recommended in the prescribing information.

Other notable adverse effects include:

• Flushing: related to MC1R activation on skin vasculature.
• Headache: reported in a significant minority of treated patients.
• Injection-site reactions: local bruising and pain at the injection site.
• Transient blood pressure increase: bremelanotide causes a transient increase in blood pressure (approximately 6 mmHg systolic and 3 mmHg diastolic in trials, peaking around 12 hours post-dose and returning to baseline within 12 hours). The drug is contraindicated in patients with uncontrolled hypertension or established cardiovascular disease.^[3]
• Focal hyperpigmentation: with repeated use, some patients develop facial, gingival, or breast hyperpigmentation from MC1R activation.

The prescribing information includes a warning to limit use to no more than one dose within 24 hours and no more than approximately 8 uses per month to minimise hyperpigmentation risk.^[3]

Approved indications and regulatory status

Bremelanotide is approved in the United States only among the five jurisdictions covered by this hub:

• United States (FDA): Vyleesi, 1.75 mg subcutaneous injection, approved June 2019 for acquired, generalised HSDD in premenopausal women. Not approved for use in men, postmenopausal women, or other sexual dysfunction diagnoses.^[3]
• European Union (EMA): no marketing authorisation as of the last review date.
• United Kingdom (MHRA): not authorised.
• Australia (TGA): not registered.
• Canada (Health Canada): not approved.

Off-label and research use of PT-141 outside the US falls into a regulatory grey area in most jurisdictions. The compound should not be conflated with approved medicines, and the safety and quality of non-pharmaceutical-grade PT-141 peptides available through research chemical suppliers has not been verified by any regulatory agency.