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07
Section 07 · 3 compounds

Melanocortin System & Pigmentation Signaling

The melanocortin pathway: pigmentation, sexual function and photoprotection.

Home/ Sections/ Melanocortin System & Pigmentation Signaling

The melanocortin system links skin pigmentation, appetite and sexual function through a family of related receptors. Two compounds here are FDA-approved for narrow uses, and one is an unapproved analog with documented safety concerns.

Each entry opens with an evidence grade, then covers how the compound works, what human, animal and cell studies show, safety, open questions and regulatory status, with numbered references. Grades in this section: A: 2 · B: 1.

A
Evidence grade A · FDA-approvedWhat this means

Afamelanotide, the drug behind the Melanotan I name, is FDA-approved (2019) and EU-authorized (2014) to increase pain-free light exposure in adults with erythropoietic protoporphyria[1,2,3].

A lab-made, longer-lasting copy of the skin's own tanning hormone, approved as a slow-release implant for adults with a rare inherited light-sensitivity disorder called erythropoietic protoporphyria.

Overview

Melanotan I is the older research name for afamelanotide, a 13-amino-acid copy of alpha-melanocyte-stimulating hormone (alpha-MSH), the hormone that tells pigment cells to make dark melanin. Two amino acid swaps make it far more stable than the natural hormone[1]. It is sold as Scenesse, a small dissolving implant placed under the skin, for adults with erythropoietic protoporphyria (EPP), a rare genetic condition in which visible light causes severe burning pain[1,2].

In two placebo-controlled trials, people with EPP on afamelanotide spent more pain-free time in direct sunlight than those on a dummy implant, though the gains were modest in absolute terms[1,5]. Earlier work in healthy fair-skinned volunteers showed it darkens skin and reduces some markers of UV damage[7]. Uses beyond EPP, including cosmetic tanning, are not approved[1,3].

A tan without the sunburn invitationNormally your skin only ramps up pigment after UV light has already done some damage, like calling the fire department after the smoke alarm goes off. Afamelanotide delivers the "make more pigment" message directly, so the pigment cells build up their dark melanin shield ahead of time, with or without sun.

Mechanism of Action

Afamelanotide binds mainly to the melanocortin 1 receptor (MC1R) on melanocytes, the pigment-making cells of the skin. Activating MC1R shifts melanin production toward eumelanin, the brown-black form that absorbs light and neutralizes some of the reactive molecules light generates. The FDA label states that it increases eumelanin regardless of sun or UV exposure[1]. In EPP, a buildup of protoporphyrin in the skin absorbs visible light and triggers painful reactions; the extra eumelanin is thought to act as a filter, though exactly how much of the benefit comes from pigment versus other MC1R effects has not been settled[1,5].

Its structure is Ac-Ser-Tyr-Ser-Nle-Glu-His-D-Phe-Arg-Trp-Gly-Lys-Pro-Val-NH2. Replacing methionine with norleucine at position 4 and using the mirror-image D-phenylalanine at position 7 protects it from rapid breakdown[1]. The implant releases the peptide slowly from a biodegradable polymer rod. In 12 healthy adults, blood levels peaked at a median of 36 hours and the apparent half-life was about 15 hours, with high variability between people[1].

What the Research Shows

Human studies
  • Phase 3 RCT, United States (CUV039), NEJM 2015 (n=94 adults with EPP, 6 months): median pain-free time in direct sunlight was 69.4 hours with afamelanotide vs 40.8 hours with placebo implant (P=0.04). The FDA's own analysis reported 64.1 vs 40.5 hours[1,5].
  • Phase 3 RCT, Europe (CUV029), NEJM 2015 (n=74, 9 months): median pain-free time outdoors on mostly sunny days was 6.0 vs 0.8 hours (P=0.005), and phototoxic reactions numbered 77 vs 146 (P=0.04). Both trials were funded by the manufacturer, Clinuvel[5].
  • Real-world cohort, JAMA Dermatology 2020 (n=117, median 2 years, single Dutch center): time outside rose by 6.1 hours per week (P<0.001) and quality of life improved 14% (P<0.001). 98% stayed on treatment. The authors noted the placebo-controlled trials showed only a relatively small reduction in symptoms; the registry received manufacturer support[6].
  • Healthy fair-skinned volunteers, 2006 (n=65, about 3 months): melanin density rose 41% in people with the lowest baseline sun tolerance (P<0.0001 vs placebo), sunburn cells after UV exposure fell by more than 50%, and DNA damage markers (thymine dimers) fell 59% (P=0.002)[7].
Animal studies
  • Rats, fertility studies: no effects on fertility at exposures up to 12 times the maximum recommended human amount[1].
  • No carcinogenicity (lifetime cancer) studies have been conducted; genetic toxicity tests (Ames, mouse lymphoma, mouse micronucleus) were negative[1].
What This Could Mean in Practice

For adults with EPP, regulators accept afamelanotide as a way to extend pain-free time in light, with trial gains measured in hours rather than a cure[1,5]. Real-world follow-up suggests most patients choose to stay on it[6]. Evidence for cosmetic tanning or skin cancer prevention in the general population is limited to small early studies of pigment and damage markers, not outcomes like fewer skin cancers[7]. Products sold online as "Melanotan I" are not the regulated implant, and their identity and purity are unknown.

Safety & Side Effects
  • Most common in trials (afamelanotide vs vehicle implant): implant site reactions 21% vs 10%, nausea 19% vs 14%, sore throat 7% vs 5%, cough 6% vs 3%, fatigue 6% vs 3%[1].
  • Skin and moles: it causes generalized darkening and can darken existing moles and freckles. Melanocytic nevus was reported in 4% vs 2%. The label recommends a full-body skin exam twice a year[1].
  • Hypersensitivity: serious reactions including anaphylaxis have been reported[1].
  • Long-term cancer risk: no lifetime animal cancer studies exist, and long-term melanoma risk in humans has not been established[1].
  • Adolescents: pediatric safety is not established. The EU application for teenagers was withdrawn in 2024 after regulators judged the data insufficient[1,4].

Open Questions

  • Whether years of continuous MC1R stimulation changes melanoma or mole risk is not known; the trials lasted 6 to 9 months[1,5].
  • The pivotal trials were small, manufacturer-funded, and used endpoints (hours in sunlight) that patients had to self-record[5].
  • No adequate data exist for children or teenagers, the age when EPP symptoms often matter most[4].
  • Effects in people without EPP (vitiligo, other light disorders, healthy skin) have not led to any approval.

Regulatory Status

FDA-approved as Scenesse (NDA 210797) in October 2019 for adults with EPP and a history of phototoxic reactions; the label was last updated in 2024 and remains adult-only[1,2]. Authorized in the EU since December 2014 under exceptional circumstances; an EU extension to adolescents was withdrawn in April 2024[3,4]. Unregulated injectable "Melanotan I" sold online is not an approved product.

Related Compounds

Melanotan IIPT-141
References (7)
  1. U.S. Food and Drug Administration. SCENESSE (afamelanotide) implant, Prescribing Information (NDA 210797). 2024. accessdata.fda.gov
  2. U.S. Food and Drug Administration. Drugs@FDA: NDA 210797, SCENESSE (afamelanotide). Approval history. accessdata.fda.gov
  3. European Medicines Agency. Scenesse (afamelanotide): EPAR, medicine overview. ema.europa.eu
  4. European Medicines Agency. Scenesse: withdrawal of application for variation to marketing authorisation (adolescents with EPP). 2024. ema.europa.eu
  5. Langendonk JG, Balwani M, Anderson KE, et al. Afamelanotide for erythropoietic protoporphyria. N Engl J Med. 2015;373(1):48-59. DOI: 10.1056/NEJMoa1411481
  6. Wensink D, Wagenmakers MAEM, Barman-Aksözen J, et al. Association of afamelanotide with improved outcomes in patients with erythropoietic protoporphyria in clinical practice. JAMA Dermatol. 2020;156(5). DOI: 10.1001/jamadermatol.2020.0352. PMID: 32186677
  7. Barnetson RS, Ooi TK, Zhuang L, et al. [Nle4-D-Phe7]-alpha-melanocyte-stimulating hormone significantly increased pigmentation and decreased UV damage in fair-skinned Caucasian volunteers. J Invest Dermatol. 2006;126(8):1869-78. PMID: 16763547

Sources last reviewed October 2026.

B
Evidence grade B · Human RCTsWhat this means

Small randomized, placebo-controlled crossover trials in men with erectile dysfunction were published around 2000[1], but development stopped and the compound was never approved anywhere; most later human information comes from case reports of unlicensed use[2,3,4].

An unapproved, lab-made tanning peptide that switches on several melanocortin receptors at once, causing skin darkening, appetite and arousal effects, and a growing list of reported harms.

Overview

Melanotan II is a short, ring-shaped synthetic copy of the core of alpha-melanocyte-stimulating hormone (alpha-MSH). Unlike Melanotan I, it activates several melanocortin receptors, including those in the brain[1]. In small human trials it triggered erections and increased sexual desire, along with frequent nausea and yawning[1].

It was never approved by any regulator. It circulates widely as an unlicensed "tanning injection"[2,3]. The FDA notes published case reports of serious adverse events, including melanoma, a brain swelling syndrome (PRES), a stimulant-like toxic reaction, and priapism[4]. Its chemical successor, bremelanotide (PT-141), did reach approval for a different use[5,6].

A master key instead of a single keyThe body uses a family of melanocortin locks: one mainly for skin pigment, others for appetite, arousal and more. Melanotan I is cut to fit mostly the pigment lock. Melanotan II is closer to a master key: it opens the pigment lock but also the brain's appetite and arousal locks, which is why its effects, and side effects, spread well beyond the skin.

Mechanism of Action

Melanotan II is a cyclic heptapeptide (seven amino acids, most of them closed into a ring) built around the core alpha-MSH sequence His-D-Phe-Arg-Trp, which it shares with bremelanotide[6]. It is a non-selective melanocortin receptor agonist[1]. Activating MC1R on melanocytes raises eumelanin production and darkens skin, including existing moles[3,4]. Activating MC3R and MC4R in the brain affects feeding and sexual function: in mice, injecting it into the brain suppressed feeding in four different overeating models, an effect completely blocked by a melanocortin receptor antagonist[7]. Central melanocortin signaling is also the leading explanation for the erections seen in human trials[1].

Its molecular formula is C50H72N15O9[5]. Bremelanotide (PT-141), approved as Vyleesi, has the same ring sequence but ends in a free acid instead of an amide (C50H68N14O10)[5,6]. Case authors have proposed that excess sympathetic nervous system activity and blood vessel constriction could explain some reported harms, such as kidney infarction, but this mechanism is speculative[2].

What the Research Shows

Human studies
  • Randomized, double-blind, placebo-controlled crossover studies, 2000 (n=20 men with psychogenic or organic erectile dysfunction): erections occurred without sexual stimulation in 17 of 20 men, with a mean of 41 minutes of tip rigidity above 80% over 6 hours. Increased desire was reported after 68% of active vs 19% of placebo administrations (P<0.01). Nausea and yawning were common, and severe nausea occurred in 12.9% at the higher level tested[1].
  • Case report, Dermatology 2014 (n=1): a 20-year-old woman with fair skin developed cutaneous melanoma on the buttock about three months after a 3 to 4 week course of self-injected melanotan II combined with sunbeds. A case report cannot show cause and effect[3].
  • Case report and review, CEN Case Reports 2020 (n=1): a previously healthy 45-year-old man had a kidney infarction affecting about half of one kidney after six months of self-injection; no embolic source was found, and he was left with new high blood pressure and slightly reduced kidney function. The review also cites reports of rhabdomyolysis with kidney failure and of eruptive moles[2].
  • Poison center data, Sweden 2008 to 2019: 215 melanotan II cases were recorded, indicating wide unlicensed use[2].
Animal studies
  • Mice, feeding models (Nature 1997): brain injection suppressed feeding in fasted, leptin-deficient (ob/ob), agouti obese and neuropeptide Y-stimulated mice; a melanocortin antagonist blocked the effect, helping establish the brain melanocortin system as a brake on appetite[7].
What This Could Mean in Practice

Because melanotan II was never approved and its formal human testing stopped at small early trials, there is no regulator-accepted use[1,4]. The early data show it reliably affects skin, arousal and appetite pathways, but they cannot tell anyone whether it is safe over months or years. The case literature describes melanoma, mole changes, kidney injury, priapism and toxic reactions in people using unregulated products[2,3,4]. Whether those events are caused by the peptide, contaminants, or sun exposure cannot be sorted out from case reports.

Safety & Side Effects
  • Common in trials: nausea and yawning; severe nausea in about 13% at the higher level tested[1].
  • Skin: generalized darkening and darkening or eruption of moles; at least one published melanoma case, which makes changing moles hard to monitor[2,3].
  • Serious events in case reports cited by FDA: melanoma, posterior reversible encephalopathy syndrome, sympathomimetic toxidrome, and priapism[4].
  • Kidney: renal infarction and rhabdomyolysis with kidney failure have been reported[2].
  • Product quality: all products in circulation are unlicensed; the FDA also flags a risk of immune reactions to compounded versions[4].

Open Questions

  • No controlled trial has measured long-term safety, including melanoma risk.
  • The size of any real tanning, appetite or sexual benefit has never been tested in modern, adequately sized trials.
  • Reported harms come from case reports with unverified products, so how often they happen is unknown[2,3].
  • Interactions with sun exposure and sunbed use, which many users combine with it, are unstudied[3].

Regulatory Status

Not approved by the FDA or any other major regulator. On FDA's list of compounding bulk substances with safety concerns, melanotan II appears under nominations that were withdrawn after it had been placed in category 2, with FDA citing immunogenicity risk and published reports of serious adverse events[4]. Its derivative bremelanotide is approved separately as Vyleesi[6].

Related Compounds

Melanotan IPT-141
References (7)
  1. Wessells H, Levine N, Hadley ME, et al. Melanocortin receptor agonists, penile erection, and sexual motivation: human studies with Melanotan II. Int J Impot Res. 2000;12(Suppl 4):S74-S79. DOI: 10.1038/sj.ijir.3900582
  2. Peters B, Hadimeri H, Wahlberg R, et al. Melanotan II: a possible cause of renal infarction: review of the literature and case report. CEN Case Rep. 2020;9(2):159-161. PMID: 31953620
  3. Hjuler KF, Lorentzen HF. Melanoma associated with the use of melanotan-II. Dermatology. 2014;228(1):34-36. DOI: 10.1159/000356389
  4. U.S. Food and Drug Administration. Certain bulk drug substances for use in compounding that may present significant safety risks. Updated 2026. fda.gov
  5. National Center for Biotechnology Information. PubChem Compound Summary: melanotan-II (CID 92432). pubchem.ncbi.nlm.nih.gov
  6. U.S. Food and Drug Administration. VYLEESI (bremelanotide injection), Prescribing Information (NDA 210557). 2020. accessdata.fda.gov
  7. Fan W, Boston BA, Kesterson RA, et al. Role of melanocortinergic neurons in feeding and the agouti obesity syndrome. Nature. 1997;385:165-168. DOI: 10.1038/385165a0

Sources last reviewed October 2026.

A
Evidence grade A · FDA-approvedWhat this means

Bremelanotide (Vyleesi) was FDA-approved in June 2019 for acquired, generalized hypoactive sexual desire disorder in premenopausal women, based on two phase 3 randomized trials[1,2].

A brain-acting peptide, FDA-approved as an as-needed injection for premenopausal women with persistent, distressing low sexual desire; its benefits in trials were real but small.

Overview

PT-141 is the development code for bremelanotide, a ring-shaped seven-amino-acid peptide closely related to Melanotan II[1,6]. It activates melanocortin receptors, chiefly MC4R in the brain, and works on desire circuits rather than blood flow, unlike erectile dysfunction pills[1,5]. It is sold as Vyleesi, a self-injected autoinjector used before anticipated sexual activity[1].

In two 24-week phase 3 trials (RECONNECT) involving more than 1,200 premenopausal women, bremelanotide improved desire scores and reduced distress about low desire more than placebo, but the differences were modest and the number of satisfying sexual events did not improve[1,5]. Nausea affected 40% of users[1]. It is not approved for men or postmenopausal women[1].

Turning up the motivation dial, not the plumbingDrugs like sildenafil widen blood vessels so the body can respond once desire is already there. Bremelanotide works one step earlier, nudging brain circuits that govern sexual interest and motivation. It is less like fixing a pipe and more like turning up the volume on a signal that has gone quiet.

Mechanism of Action

Bremelanotide is a melanocortin receptor agonist that activates several receptor subtypes; at therapeutic exposures the most relevant are MC4R and MC1R[1]. MC4R is expressed in brain regions involved in sexual motivation, including the medial preoptic area, and reviews describe bremelanotide as acting on presynaptic MC4R to promote dopamine release there[5]. The FDA label is candid that the mechanism by which it improves low desire is unknown[1]. MC1R activation on pigment cells explains the patches of skin darkening some users develop[1].

Its sequence is Ac-Nle-cyclo(Asp-His-D-Phe-Arg-Trp-Lys)-OH, molecular formula C50H68N14O10[1]. Melanotan II has the same sequence but ends in an amide group rather than a free acid (C50H72N15O9)[1,6]. After injection under the skin it is essentially 100% bioavailable, peaks in about 1 hour, and has a half-life of about 2.7 hours[1]. Earlier intranasal versions raised blood pressure concerns, and later trials added close blood pressure monitoring[5].

What the Research Shows

Human studies
  • Two phase 3 RCTs (RECONNECT), label data 2019 (n=1,267 randomized, 24 weeks, premenopausal women with HSDD): desire domain scores rose +0.5 to +0.6 vs +0.2 on placebo (P=0.0002 and P<0.0001), and distress about low desire fell -0.7 vs -0.4 (P<0.0001 and P=0.0053). Satisfying sexual events did not differ from placebo[1].
  • Open-label extension, label data (n=684, 52 weeks): no new safety signal beyond one case of acute hepatitis with very high liver enzymes that resolved after stopping; a role for bremelanotide could not be excluded[1].
  • Phase 2b dose-finding RCT, 2016 (n=397 randomized, 12 weeks, premenopausal women with desire and/or arousal disorders): satisfying sexual events rose +0.7 vs +0.2 per month (P=0.018), sexual function scores +3.6 vs +1.9 (P=0.0017), and distress scores -11.1 vs -6.8 (P=0.0014). The lowest level tested did not beat placebo[3].
  • Trial populations: participants were aged 19 to 56 (mean 39) and 86% were White, so evidence in other groups is thinner[1].
Animal studies
  • Rats and mice, 2-year studies: no significant increase in tumors[1].
  • Pregnancy studies: in dogs, post-implantation pregnancy loss rose about 3 to 8 times over controls; in mice, developmental delays appeared in offspring at high exposures[1].
What This Could Mean in Practice

For premenopausal women with acquired, generalized low sexual desire that causes distress and is not explained by another condition, relationship problems or medication, bremelanotide is an approved option[1]. Trial gains averaged a fraction of a point on desire and distress scales, without more satisfying sexual events, and reviewers note a large placebo response[1,5]. Nausea is common, especially early on[1]. Benefits in men, postmenopausal women, or as a performance enhancer have not been established, and the label excludes those uses[1].

Safety & Side Effects
  • Most common in phase 3 (bremelanotide vs placebo): nausea 40.0% vs 1.3%, flushing 20.3% vs 0.3%, injection-site reactions 13.2% vs 8.4%, headache 11.3% vs 1.9%, vomiting 4.8% vs 0.2%. About 8% stopped because of nausea[1].
  • Blood pressure: transient rises of up to 6 mmHg systolic and 3 mmHg diastolic, peaking 2 to 4 hours after injection, with heart rate falling up to 5 beats per minute. Contraindicated in uncontrolled hypertension or known cardiovascular disease[1].
  • Skin darkening: focal hyperpigmentation (face, gums, breasts) in 1% in trials, but 38% with eight consecutive daily injections in a separate study; more likely in darker skin and not always reversible[1].
  • Drug interactions: it can slow stomach emptying and sharply reduce levels of oral naltrexone[1].
  • Pregnancy: animal studies showed pregnancy loss and developmental effects; human pregnancy data are limited to seven trial pregnancies[1].

Open Questions

  • Whether the modest score changes translate into meaningful improvement for most women; satisfying events did not increase[1].
  • Long-term safety beyond about 18 months of intermittent use, including liver effects, is not well characterized[1].
  • Efficacy in men and postmenopausal women has not been established in approval-level trials[1].
  • The exact brain mechanism behind the desire effect remains unproven[1,5].

Regulatory Status

FDA-approved prescription drug (Vyleesi, NDA 210557) since June 21, 2019, limited to premenopausal women with acquired, generalized HSDD and explicitly not indicated for postmenopausal women, men, or to enhance sexual performance[1,2]. Originally developed by Palatin Technologies and licensed to AMAG for North America[4].

Related Compounds

Melanotan IIKisspeptinOxytocin
References (6)
  1. U.S. Food and Drug Administration. VYLEESI (bremelanotide injection), Prescribing Information (NDA 210557). 2020. accessdata.fda.gov
  2. U.S. Food and Drug Administration. Drugs@FDA: NDA 210557, VYLEESI (bremelanotide). Approval history. accessdata.fda.gov
  3. Clayton AH, Althof SE, Kingsberg S, et al. Bremelanotide for female sexual dysfunctions in premenopausal women: a randomized, placebo-controlled dose-finding trial. Womens Health (Lond). 2016;12(3):325-337. DOI: 10.2217/whe-2016-0018
  4. Dhillon S, Keam SJ. Bremelanotide: first approval. Drugs. 2019;79(14):1599-1606. DOI: 10.1007/s40265-019-01187-w
  5. Edinoff AN, Sanders NM, Lewis KB, et al. Bremelanotide for treatment of female hypoactive sexual desire. Neurol Int. 2022;14(1):75-88. DOI: 10.3390/neurolint14010006
  6. National Center for Biotechnology Information. PubChem Compound Summary: melanotan-II (CID 92432). pubchem.ncbi.nlm.nih.gov

Sources last reviewed October 2026.

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