PT-141

Overview

Where it comes from and why it was created

The story of PT-141 is really a continuation of the story of Melanotan II. In the 1990s, Hadley and Hruby at the University of Arizona developed Melanotan II as a tanning peptide (an alternative to UV tanning). Phase 1 trials, however, showed that the molecule had a strong unexpected side effect, causing sexual arousal and spontaneous erections in male subjects.

The researchers asked themselves: what if we isolated this “side effect” as the primary indication? Sexual dysfunction is a huge therapeutic market — millions of people, especially women, suffer from hypoactive sexual desire disorder (HSDD), for which virtually no effective pharmacotherapy existed. Viagra and PDE5 inhibitors work peripherally (a vasodilator effect), which works in men for erectile dysfunction, but does not work in women, because female sexual function is more centrally (brain) regulated.

Palatin Technologies (founded in the 1990s specifically for the development of melanocortin agonists) took Melanotan II and began to chemically optimize it for selectivity at MC4R, the receptor that mediates central sexual effects. After hundreds of candidate molecules, they identified PT-141, later named Bremelanotide.

Bifurcation from Melanotan II, key improvements

PT-141 and Melanotan II are chemically related (both cyclic heptapeptides, both with a lactam bridge), but with important differences:

Practical consequence: PT-141 retains the central sexual effects of Melanotan II (via MC4R), but minimizes the side cutaneous and metabolic effects (via weaker activity at MC1R and MC5R).

FDA approval as Vyleesi (2019)

After two decades of development and several Phase 3 trials, Palatin Technologies obtained FDA approval for Bremelanotide in June 2019 under the brand name Vyleesi. Indication: hypoactive sexual desire disorder (HSDD) in premenopausal women.

Vyleesi is a significant regulatory milestone:

1. The first FDA-approved peptide sexual stimulant for women
2. The second drug for female HSDD (after Addyi/flibanserin, 2015), but with a completely different mechanism
3. A demonstration that the melanocortin system is a legitimate therapeutic target for sexual dysfunction

Vyleesi is supplied in a single-use autoinjector with a dose of 1.75 mg subcutaneously. It is applied 45 minutes before planned sexual activity. Maximum 1× per 24 hours, 8× per month.

In the EU, PT-141 is not approved, but some centers use it off-label via special access programs. Molequa supplies PT-141 for research purposes, for laboratory research and replication of clinical experiments.

Male use, off-label

Although Vyleesi is approved only for women, PT-141 is also studied in research contexts in men, particularly for indications:

• Erectile dysfunction (where PDE5 inhibitors do not work well, especially neurogenic forms)
• PSSD (Post-SSRI Sexual Dysfunction), persistent sexual dysfunction after discontinuation of antidepressants
• Anorgasmia
• Low libido in men with normal testosterone

These uses are not formally approved, but several clinical trials are ongoing.

Mechanism of action, central, not peripheral

This is the most important aspect of PT-141, which distinguishes it from other sexual stimulants.

MC4R activation in the hypothalamus

PT-141 crosses the blood-brain barrier and activates MC4R in the hypothalamus, especially in two areas:

1. Medial preoptic area (MPA), the main center for integration of sexual behavior
2. Paraventricular nucleus (PVN), the coordinator of neuroendocrine and autonomic responses

Activation of MC4R in these areas via Gαs → cAMP → PKA leads to:

• Increased dopaminergic signaling in dopaminergic pathways (especially the mesolimbic pathway with the reward system)
• Activation of oxytocinergic neurons
• Inhibition of prolactin (which is a libido suppressor)
• Modulation of serotonin in pathways negatively affecting sexual function

Clinical manifestation

The result of these central changes:

• Increased sexual desire
• Better subjective arousal
• In men: erections via centrally activated parasympathetic pathways
• In women: vaginal lubrication and clitoral arousal via autonomic pathways

Difference from PDE5 inhibitors (Viagra)

This is the key mechanistic difference:

PT-141 thus acts at the level of desire and arousal in the brain, while Viagra acts at the level of vascular response in the genitals. These are two completely different mechanisms and in clinical practice they can complement each other.

Investigated applications

In the published preclinical and clinical literature, effects of PT-141 are documented in the following areas:

• Hypoactive sexual desire disorder (HSDD) in women, the approved indication (Vyleesi, 2019)
• Erectile dysfunction in men, Phase 2 trials (did not reach Phase 3 approval)
• PSSD (Post-SSRI Sexual Dysfunction), developing research in the indication of persistent sexual dysfunction after antidepressants
• Sexual dysfunction from diabetes, preclinical and Phase 2 data
• Sexual dysfunction from multiple sclerosis, preclinical
• Anorgasmia, exploratory clinical experiences
• Hemorrhagic shock, preclinical models (via MC4R-mediated cardiovascular effect)
• Reactive hyperemia, preclinical

Science & Studies

4.1 Key publications

Kingsberg S.A., Clayton A.H., Portman D., et al. (2019). Bremelanotide for the Treatment of Hypoactive Sexual Desire Disorder: Two Randomized Phase 3 Trials. Obstet Gynecol. 134(5):899 to 908., FDA registration study.

Clayton A.H., Althof S.E., Kingsberg S., et al. (2016). Bremelanotide for female sexual dysfunctions in premenopausal women: a randomized, placebo-controlled dose-finding trial. Womens Health (Lond). 12(3):325 to 337., Phase 2b dose-finding.

Diamond L.E., Earle D.C., Heiman J.R., et al. (2006). An effect on the subjective sexual response in premenopausal women with sexual arousal disorder by bremelanotide (PT-141). J Sex Med. 3(4):628 to 638., Early Phase 2 data.

Wessells H., Fuciarelli K., Hansen J., et al. (1998). Synthetic melanotropic peptide initiates erections in men with psychogenic erectile dysfunction. J Urol. 160(2):389 to 393., Foundational article for the erectogenic mechanism.

Pfaus J.G., Shadiack A., Van Soest T., et al. (2007). Selective facilitation of sexual solicitation in the female rat by a melanocortin receptor agonist. Proc Natl Acad Sci USA. 101(27):10201 to 10204., Mechanism in animals.

Simon J.A., Kingsberg S.A., Goldstein I., et al. (2018). Long-term safety and efficacy of bremelanotide for hypoactive sexual desire disorder. Obstet Gynecol. 134(5):909 to 917., Long-term safety.

4.2 Detailed expandable studies

▸ Study 1: Kingsberg 2019, FDA registration study

Citation: Kingsberg S.A., Clayton A.H., Portman D., et al. Bremelanotide for HSDD: Two Randomized Phase 3 Trials (RECONNECT). Obstet Gynecol. 2019;134(5):899 to 908.

What they did: Two parallel Phase 3 studies (RECONNECT 301 and RECONNECT 302). n = 1,247 premenopausal women with HSDD. Randomized double-blind placebo-controlled. Bremelanotide 1.75 mg SC self-administered as needed (before planned sexual activity) vs placebo. Duration: 24 weeks. Primary endpoints: Female Sexual Function Index (FSFI), Female Sexual Distress Scale (FSDS-DAO).

What they found:

• Significant improvement in FSFI desire score vs placebo in both trials
• Significant reduction of FSDS-DAO (sexual distress and disturbance)
• 24.6 % responder rate in Bremelanotide vs 17.1 % in placebo (statistical significance)
• Side effects: nausea (40 %), flushing (20 %), headache (11 %), injection site reaction (12 %)
• 8.8 % discontinued due to nausea

Why it matters: This was the FDA registration study for Vyleesi. After 20 years of development, it finally demonstrated clinically relevant efficacy in a difficult indication (HSDD is notoriously difficult for clinical trials due to high placebo responses and subjective endpoints). Approval in June 2019 was a historical moment for melanocortin therapeutics.

▸ Study 2: Clayton 2016, Phase 2b dose-finding

Citation: Clayton A.H., Althof S.E., Kingsberg S., et al. Bremelanotide for female sexual dysfunctions in premenopausal women. Womens Health (Lond). 2016;12(3):325 to 337.

What they did: n = 397 premenopausal women with HSDD and/or female sexual arousal disorder (FSAD). Randomization: Bremelanotide 0.75, 1.25 or 1.75 mg SC vs placebo. Duration: 12 weeks. Assessment: Sexual Encounter Profile (SEP), FSFI, FSDS-R.

What they found:

• Dose-dependent improvement of sexual endpoints
• The 1.75 mg dose had the best efficacy vs side effects ratio, this dose was chosen for Phase 3
• 0.75 mg was subtherapeutic
• Side effects: predominantly mild to moderate, dose-dependent

Why it matters: The study defined the optimal dose for the clinical program. Identification of 1.75 mg as the sweet spot between efficacy and tolerability was critical for success in Phase 3. From a research perspective, it is a reference for dosing protocols.

▸ Study 3: Diamond 2006, early Phase 2

Citation: Diamond L.E., Earle D.C., Heiman J.R., et al. An effect on the subjective sexual response in premenopausal women with sexual arousal disorder by bremelanotide (PT-141). J Sex Med. 2006;3(4):628 to 638.

What they did: n = 18 premenopausal women with sexual arousal disorder. Double-blind placebo-controlled study. PT-141 20 mg intranasally (the then-tested route of administration) vs placebo. Assessment: subjective sexual responses, physiological markers (vaginal photoplethysmograph), safety.

What they found:

• Significant improvement in sexual desire vs placebo
• Improvement in subjective arousal and genital congestion
• No serious side effects
• Mild nausea in some subjects
• Mild increase in blood pressure in the hours after administration

Why it matters: This was one of the first clinical studies of PT-141 in women. It validated the concept that an MC4R agonist can be effective in women (unlike Viagra). The route of administration was later changed from intranasal to subcutaneous (for safety reasons — intranasal PT-141 caused higher blood pressures).

▸ Study 4: Wessells 1998, foundational erectogenic study

Citation: Wessells H., Fuciarelli K., Hansen J., et al. Synthetic melanotropic peptide initiates erections in men with psychogenic erectile dysfunction: double-blind, placebo controlled crossover study. J Urol. 1998;160(2):389 to 393.

What they did: n = 10 men with psychogenic erectile dysfunction. Double-blind placebo-controlled crossover study with Melanotan II (more precisely the predecessor of PT-141) 0.025 mg/kg SC vs placebo. Assessment: RigiScan tumescence, duration of erection, side effects.

What they found:

• Erectile response in 80 % of subjects in the active group vs 20 % placebo
• Mean time to erection: 45 minutes
• Mean duration of erection: 90 minutes
• Subjectively satisfactory penetrative capability in 70 %
• Nausea in 50 %, flushing in 30 %

Why it matters: This was the foundational article for the entire field of melanocortin erectogenesis and sexual stimulation. Wessells and colleagues showed that the melanocortin mechanism acts via central pathways and works even when the peripheral PDE5-mediated pathway (Viagra) fails. This was the spark that led to the development of PT-141 as a more selective derivative.

▸ Study 5: Pfaus 2007, animal mechanism

Citation: Pfaus J.G., 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. 2007;101(27):10201 to 10204.

What they did: Female rats in various endocrine states (proestrus, diestrus, ovariectomized) received PT-141 or placebo. Assessment: proceptive sexual behavior (solicitation, hops, darts — these are specific movements by which the female signals interest in the male), receptive behavior (lordosis), motor activity.

What they found:

• PT-141 selectively increased proceptive (solicitation) behavior, i.e., the female’s initiative for sexual contact
• Receptive behavior (lordosis, passive receptivity) did not change
• c-fos activation in the medial preoptic area (MPA) of the hypothalamus, a key center of sexual behavior
• The effect did not depend on endocrine status and worked also in ovariectomized females (without estrogen)

Why it matters: This is a mechanistic study that explains why PT-141 works in women. “Proceptive behavior” is the translation of the concept of sexual desire and initiative from an animal model — precisely what PT-141 stimulates in human subjects. Central action in the MPA of the hypothalamus provides the anatomical basis for the clinical effect.

▸ Study 6: Simon 2018, long-term safety

Citation: Simon J.A., Kingsberg S.A., Goldstein I., et al. Long-term safety and efficacy of bremelanotide. Obstet Gynecol. 2018;134(5):909 to 917.

What they did: n = 684 women who completed Phase 3 trials (RECONNECT) and continued in an open-label extension. PT-141 1.75 mg SC self-administered for 52 weeks. Assessment: sustained efficacy, cumulative safety, blood pressure, heart rate, skin changes.

What they found:

• Efficacy was maintained during 52 weeks of follow-up
• No new safety signals vs Phase 3 data
• Nausea decreased with repeated use (initial desensitization)
• Mild increase in blood pressure after each dose, but no cumulative effect
• No pigmentation changes or new nevi, a key difference from Melanotan II
• No case of cardiac ischemic incident (cardiovascular concern was the main one for regulators)

Why it matters: The study was critical for FDA approval, demonstrating that long-term use is safe and clinically effective. The absence of cutaneous changes (in contrast to Melanotan II) confirmed the advantages of MC4R selectivity. After publication, FDA approved Vyleesi in June 2019.

▸ Study 7: King 2016, safety review article

Citation: King S.H., Mayorov A.V., Balse-Srinivasan P., et al. Melanocortin receptor agonists, structure-activity relationships, and applications in the treatment of obesity. Curr Top Med Chem. 2016;7(11):1098 to 1106.

What they did: Review article covering the pharmacology and safety of melanocortin agonists — PT-141, Melanotan II, Setmelanotide, other candidates. Discussion: receptor selectivity, clinical data, safety profiles, regulatory perspectives.

What they found (summary):

• MC4R-selective agonists (PT-141, Setmelanotide) have a better safety profile than non-selective ones (Melanotan II)
• Main side effects of all melanocortin agonists: nausea, flushing, mild increase in blood pressure
• Pigmentation changes are tied to MC1R activity; selective MC4R agonists do not have them
• Setmelanotide (Imcivree) was later approved by the FDA for rare genetic obesities (2020)

Why it matters: Review context allows understanding of why PT-141 was clinically more successful than Melanotan II. Selectivity is key for clinical tolerance in melanocortin pharmacology. This principle led to the development of even more selective molecules (Setmelanotide for obesity, further generations in the Palatin pipeline).

Storage

Lyophilizate (dry powder before reconstitution)

• 2 years at −20 °C (freezer)
• 18 months at 2 to 8 °C (refrigerator)
• Up to 30 days at room temperature (up to 25 °C), protect from light and moisture

After reconstitution (peptide in solution with bacteriostatic water)

• Up to 30 days at 2 to 8 °C, protected from light
• The cyclic structure makes PT-141 relatively stable in solution, more stable than most linear peptides

Practical storage rules

• Allow the vial to warm to room temperature (15 to 20 min) before opening.
• Avoid light completely — PT-141 contains tryptophan and histidine, which are sensitive to UV degradation. Use a dark box in the refrigerator.
• Avoid contact with strong oxidizing agents.
• Do not shake! Although the cyclic form is more robust, mechanical stress can disrupt the conformation.
• The solution should remain clear to very slightly yellowish. Darker coloring indicates oxidation, do not use.

Reconstitution

3-step visual

1. Reconstitute — add bacteriostatic water down the wall of the vial
2. Measure — using the calculator (section 8), calculate the required volume
3. Store — refrigerator 2 to 8 °C, protect from light

Detailed protocol

What you will need:

• Vial of PT-141 (5 mg lyophilizate)
• 2 to 2.5 ml of bacteriostatic water (contains 0.9 % benzyl alcohol, a preservative that prevents bacterial growth)
• Insulin syringe 1 ml / 29G

Procedure:

1. Allow the PT-141 vial to reach room temperature (15 to 20 min). Cold vial + warm water = condensation, which disrupts peptide stability.
2. Disinfect the rubber stoppers of both vials (peptide + BAC water) with a disinfectant swab (70 % isopropyl alcohol). Allow the alcohol to evaporate.
3. Draw the required volume of BAC water with an insulin syringe. The standard for a 5 mg vial is 2.5 ml → resulting concentration 2 mg/ml = 2000 µg/ml. This provides a 1.75 mg dose in a 0.875 ml volume, paralleling Vyleesi.
4. Inject water slowly down the wall of the vial. Never directly onto the lyophilizate.
5. Give the vial 1 to 2 minutes of rest. PT-141 dissolves relatively quickly thanks to its compact cyclic structure.
6. Gently swirl the vial in circular motions (NEVER shake!) for 30 to 60 seconds until all powder is dissolved. The solution should be completely clear or very slightly yellowish (due to tryptophan).
7. Store in the refrigerator at 2 to 8 °C, in a dark box.

Alternative volumes for different resulting concentrations

Rule: For PT-141, we recommend a 2.5 ml volume to parallel the clinical Vyleesi dose. At 2 mg/ml concentration, a 1.75 mg dose = 87.5 IU on an insulin syringe (round to 88 IU).

Combination tips — Frequently combined peptides and molecules

PT-141 is in the research literature used predominantly alone (for the sexual indication), but some combinations are described in exploratory protocols.

Melanotan II, alternative, not combination

This is an alternative, not a combination. Melanotan II is a non-selective MC agonist (stronger tanning effect, more side effects), PT-141 is MC4R-selective (weaker tanning effect, cleaner sexual effect). For research focused only on the sexual pathway, PT-141 is the cleaner molecule.

Melanotan I (Afamelanotide), for parallel cutaneous indication

If research wants to combine controlled tanning (via Afamelanotide, an MC1R agonist) with an independent sexual effect (via PT-141, an MC4R agonist), this is a mechanistically valid combination. The selectivity of each molecule minimizes the mutual overlap of effects.

Oxytocin, complementary sexual axis

Oxytocin is a peptide acting in parasympathetic and central social pathways. During sexual activity, oxytocin mediates bonding and the orgasmic response. PT-141 mediates initial desire and arousal. Together they could cover a broader spectrum of sexual function. A hypothetical research combination, clinical data are lacking.

PDE5 inhibitors (Viagra, Cialis), for men

In erectile dysfunction where PDE5 inhibitors alone are insufficient (for example in psychogenic or neurogenic etiology), combination with PT-141 can cover both central and peripheral components. Caution: the combination may amplify vasoactive effects (increase in blood pressure from PT-141, vasodilation from PDE5), requires caution in cardiovascular patients.

Kisspeptin, hormonal axis

Kisspeptin regulates GnRH and thus testosterone/estrogen. PT-141 acts independently of sex hormones. For research in patients with low libido due to hormonal imbalances, the combination can address both levels.

No combinations with antidepressants (SSRI/SNRI)

In SSRI/SNRI-induced sexual dysfunction (PSSD-like), PT-141 is studied as monotherapy. Combination with active antidepressants may be complex — SSRIs increase serotonin, which may weaken the MC4R effect. This is an active research area.

FAQ — Frequently asked questions

What is the difference between PT-141 and Melanotan II? Both are cyclic heptapeptide analogs of α-MSH, but with different selectivity:

PT-141 is the “cleaner” molecule for the sexual indication only; Melanotan II is the choice if you also want a tanning effect.

Is PT-141 approved by regulators? Yes, FDA approved Bremelanotide under the brand Vyleesi in June 2019 for HSDD in premenopausal women. Vyleesi is supplied in an autoinjector with a 1.75 mg dose for subcutaneous self-administered use. It is not approved in the EU — some centers use it off-label via special access programs.

Does PT-141 work in men? Yes, but off-label. The original clinical development included male indications (erectile dysfunction, particularly psychogenic and neurogenic), but the Phase 3 program for men was not completed and FDA approval is only for women. In a research context, PT-141 is actively studied in men for:

• Psychogenic erectile dysfunction
• ED that does not respond to PDE5 inhibitors (Viagra, Cialis)
• PSSD (Post-SSRI Sexual Dysfunction)
• Low libido with normal testosterone

What is the difference between PT-141 and Viagra? Completely different mechanisms:

PT-141 and Viagra are not competitors, they are complementary molecules. PT-141 addresses “desire and arousal”, Viagra addresses “performance”.

What is the half-life of PT-141? 2 to 4 hours in plasma. A short molecule, no albumin binding. The cyclic structure provides mild metabolic stability. In the clinical protocol, it is applied 45 minutes before planned sexual activity — this corresponds to peak plasma concentration and central activation.

What is the recommended dosing? From published clinical protocols:

• Vyleesi (FDA-approved): 1.75 mg SC, as needed, 45 min before sexual activity
• Maximum 1× per 24 hours, 8× per month
• Phase 2 dose-finding: 0.75 to 1.75 mg, optimum at 1.75 mg
• Research protocols in men: similar doses

Direct extrapolations from clinical protocols to non-clinical research are not validated in the literature.

Does PT-141 work orally? Not effectively. The peptide is broken down in the stomach. The cyclic structure provides mild resistance, but oral bioavailability is not documented. Subcutaneous administration is the standard. Previous intranasal administration was abandoned due to higher blood pressures.

Are side effects known? Yes, they are well characterized from RECONNECT Phase 3 trials (n > 1200):

Common (>10 %):

• Nausea (40 %), the strongest side effect, dose-dependent
• Flushing (20 %), sensation of heat/redness in the hour after administration
• Headache (11 %)
• Injection site reaction (12 %), mild redness, pain

Less common (1 to 10 %):

• Mild increase in blood pressure, typically 4 to 6 mmHg during 6 hours after administration
• Dizziness
• Skin darkening (rare, in contrast to Melanotan II ~5 %)
• Pulmonary discomfort

Rare but important:

• Persistent skin darkening — given MC4R selectivity, less common than with MT-II
• Headache may persist for several hours

Nausea is the main reason for discontinuation from clinical trials — 8.8 % in Phase 3.

Who should NOT take PT-141 (clinically)? Clinically (via Vyleesi approval), it is contraindicated in:

• Uncontrolled hypertension or known cardiovascular disease
• Severe renal or hepatic disease
• Pregnancy (category X)
• Prior allergic reaction to PT-141
• Concurrent use with alpha-adrenergic agonists (Sudafed)

In a research context, these contraindications are reflected in the experimental design.

Does PT-141 cause skin darkening like Melanotan II? Very rarely and more weakly, due to MC4R selectivity and weak activity at MC1R (the receptor for pigmentation). In RECONNECT Phase 3 trials, skin darkening occurred only in ~5 % of subjects vs ~50 to 80 % with Melanotan II. This difference is a key advantage of PT-141 for the sexual indication — it does not want cutaneous effects.

Does PT-141 cause nausea? Yes, this is the main side effect. In Phase 3 trials, nausea occurred in 40 % of subjects. The mechanism is likely via MC4R activation in the area postrema (nausea center) and modulation of serotonin pathways. Nausea is dose-dependent and typically decreases with repeated use (Simon 2018). In some patients, however, it is a reason for discontinuation (~9 % in Phase 3).

What is the WADA status? PT-141 belongs to category S2 (Peptide Hormones, Growth Factors, Related Substances and Mimetics) on the WADA Prohibited List 2026 as a melanocortin peptide analog. Prohibited for professional athletes including out-of-competition periods.

Can PT-141 be combined with antidepressants? Clinical data are limited. SSRIs and SNRIs increase serotonin, which can weaken the MC4R-mediated sexual effect. From a research perspective, PT-141 is actively investigated as a treatment for SSRI-induced sexual dysfunction (PSSD) — a paradox in which PT-141 may help patients who have sexual dysfunction because of SSRIs. Clinical trials are ongoing.

What is the difference between Molequa PT-141 and Vyleesi? The active substance is identical, Bremelanotide acetate. Differences:

For research, the flexibility of Molequa lyophilizate is more advantageous. For clinical use, Vyleesi is the approved formulation (available by prescription in the US).

What is the purity of this batch? The current batch 2026-04-P: ≥ 99.1 % HPLC. A full CoA with HPLC chromatogram, MS spectrum (confirmation of MW 1,025.16 Da) and related impurity profile is available for download or upon request. For PT-141 we specifically check the cyclization integrity of the lactam bridge — the open-chain form has dramatically lower activity and in quality synthesis should be below 5 %.