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A research-focused comparison of Melanotan II and PT-141, covering melanocortin receptor selectivity, evidence, safety signals and regulatory status.
Melanotan II The team of Mac Hadley and Vincent Hruby from the University of Arizona, founded in the mid-1980s, worked on an issue combining dermatology with peptide pharmacology: whether it is possible to induce skin pigmentation without exposure to ultraviolet radiation. Endogenous α-MSH (alpha-melanocyte-stimulating hormone) stimulates the MC1R receptor in melanocytes, triggering the eumelanin production cascade – if an analogue with greater metabolic stability could be synthesized, a tool peptide capable of inducing pigmentation “from the inside” would be created.
Hadley and Hruby synthesized two analogues. Melanotan I (Afamelanotide) — a linear 13-amino acid analogue of α-MSH with a relatively selective MC1R profile. Melanotan II – a cyclic heptapeptide with a potency superior to α-MSH, but with a broader, less selective MC1R/MC3R/MC4R profile.
Three decades later, the difference in the fate of the two peptides is striking. Afamelanotide was registered by EMA (December 22, 2014) and FDA (October 8, 2019) as Scenesse – a drug for patients with erythropoietic protoporphyria (EPP) – based on a pivotal trial by Langendonk et al. 2015 (NEJM). Melanotan II has not been registered in any major jurisdiction. The narrow therapeutic window, significant side effects (nausea, facial flushing, priapism, reports of growth of pigmented nevi) and non-selective MC4R stimulation meant that the benefit-risk ratio did not allow for clinical development.
This article presents Melanotan II as a study of a peptide with high pleotropic activity, a molecule whose mechanism is well described but whose safety profile has stalled clinical development. The text is a critical review of the literature, not an application guide. MT-II in the One Peptides catalog is available only as a chemical reagent for laboratory tests (RUO).
📖 The following article is educational in nature and is a critical review of published scientific literature about Melanotan II. The peptide has never been approved as a drug by the EMA, FDA or other major regulatory agencies. The clinical literature contains significant reports of side effects, including the growth of pigmented moles. In the European Union, Melanotan II is available only as a chemical reagent for laboratory tests (Research Use Only). The text does not constitute medical advice or a recommendation for use for pigmentation purposes.
Melanotan II is a cyclic heptapeptide designed as an α-MSH analogue with increased metabolic stability and increased receptor activity. Chemical features important from a laboratory perspective:
Three modifications were made to native α-MSH in the design of MT-II: replacing methionine at position 4 with norleucine (Nle) reduced the susceptibility to oxidation; replacing phenylalanine at position 7 with D-Phe increased stability against proteases; cyclization by a lactam bond between aspartate (item 5) and lysine (item 10) stiffened the conformation of the molecule in the shape of the active form bound by MCR receptors.
The terminal amide (-NH₂) is mechanistically important: its presence allows MT-II to retain its potent activity against MC1R. PT-141 (bremelanotide) was created by modifying MT-II—removing the amide and leaving the carboxyl group (-OH) free—which virtually eliminated MC1R activity while retaining effect on MC4R. The structural “family” of α-MSH → Melanotan I → Melanotan II → PT-141 illustrates how small modifications of a short peptide translate into radically different pharmacological profiles. The rights to MT-II were licensed by Competitive Technologies (technology-transfer Univ. Arizona) to Palatin Technologies, which, however, consciously selected the PT-141 derivative (bremelanotide) as its clinical candidate – rather than the parent MT-II – due to its better selectivity and safety profile.
Melanocortin receptors (MCR) constitute a family of five G protein-coupled receptors (MC1R–MC5R) that bind endogenous peptides from the proopiomelanocortin group – α-MSH, β-MSH, γ-MSH and ACTH. The central feature of MT-II—at once its experimental strength and its clinical limitation—is its lack of selectivity: the peptide stimulates all MCR subtypes with varying potency, but in concentration ranges where peripheral and central effects overlap.
MC1R activation triggers a cAMP-dependent cascade: the agonist binds the receptor, the Gs protein activates adenylate cyclase, protein kinase A phosphorylates the CREB transcription factor, which stimulates the expression of the MITF gene – the main regulator of the melanogenesis program. MITF increases the expression of tyrosinase and the TYRP1 and DCT proteins responsible for the conversion of tyrosine into melanin. The net effect is to shift production towards eumelanin (photoprotective) at the expense of pheomelanin. In people with skin type I–III on the Fitzpatrick scale, MT-II-induced pigmentation appeared after 1–3 weeks of injection as diffuse skin darkening.
MC4R occurs primarily in the central nervous system, especially in the paraventricular nucleus of the hypothalamus. Arousal modulates three circuits: the libido center (activation leads to the release of dopamine in the VTA and NAc), the satiety axis (decreased food intake), and autonomic regulation (moderate increase in sympathetic tone, manifested by a tendency to increased blood pressure and tachycardia). The central mechanism of MT-II in the libido area is analogous to PT-141 – however, MT-II overlaps with peripheral effects (pigmentation, nausea, redness) that dominate the clinical picture.
MC3R in the brain and peripheral tissues regulates energy balance (complementary to MC4R), modulates inflammation and satiety signaling. MC2R in the adrenal cortex is the ACTH receptor – α-MSH and its analogues bind to it much weaker, so the net effect of MT-II on cortisol is small. MC5R is found in the sebaceous and tear glands; its role in the MT-II clinical profile remains marginal.
The scientific literature on Melanotan II covers four main areas: pigmentation, sexual function, appetite regulation and immunomodulation. In each of them, the mechanistic action of the peptide was confirmed, but in none of them MT-II successfully passed the stage of clinical development. The extensive literature describing side effects and risks in people using the peptide outside the regulatory system should be discussed separately.
The first clinical trials of MT-II were conducted by a team from the University of Arizona in the 1990s. Dorr et al. (1996) published pilot Phase I results (n=10, doses 0.01–0.03 mg/kg s.c. for 5 days every other day) in volunteers with type I–III complexion, in which MT-II induced visible skin darkening after a series of subcutaneous injections. Pigmentation was diffuse, with a more pronounced effect in light-exposed areas, but also present in sheltered areas – the peptide did not require UV exposure to produce its effect. Nausea, yawning/stretching, spontaneous erections and increased blood pressure were reported in the same trials.
EPP is a rare metabolic disease caused by mutations in the ferrochelatase gene. In patients, exposure to visible light causes severe phototoxic reactions; Pharmacologically induced eumelanization reduces this effect by absorbing part of the radiation. Afamelanotide was selected over Melanotan II in the clinical development program – Afamelanotide induced pigmentation without significant digestive, cardiovascular, or sexual disturbances, whereas MT-II generated a much wider range of adverse effects. Pivotal trial of phase III Langendonk et al. 2015 (NEJM) led to the registration of Afamelanotide as Scenesse in the EU (December 22, 2014) and the USA (October 8, 2019). The difference in the fate of the two molecules is one of the clearest illustrations of the importance of receptor selectivity in peptide pharmacology.
During early clinical trials of MT-II, researchers noticed a pharmacologically significant effect – spontaneous erections in men and libido stimulation in women. Wessels et al. (2000) described this phenomenon in men with erectile dysfunction: administration of MT-II led to an erection with a profile different from PDE5 inhibitors – the action of the peptide was central (MC4R in the hypothalamus), not peripheral. The observation opened a program on a dedicated MC4R agonist, which resulted in PT-141 (bremelanotide), registered by the FDA in 2019 as a drug for HSDD. MT-II remained the structural precursor.
In preclinical models of obesity, MT-II reduced food intake and led to weight loss by stimulating MC4R and MC3R in the hypothalamus. The peptide did not enter clinical development as a treatment for obesity – a narrow therapeutic window made it impossible to achieve an acceptable benefit-risk ratio. The contemporary development of melanocortin agonists in obesity focuses on more selective MC4R (setmelanotide) ligands. Endogenous α-MSH also has anti-inflammatory properties; Brzoska et al. (2008) compiled literature on the immunomodulatory effects of melanocortin peptides, but this line has not led to approved MT-II-based therapy.
The most important part of an honest review of the MT-II literature is the side effect reports from both clinical trials and case reports of patients using the peptide outside the regulatory system. The safety profile is the reason why MT-II has not been registered.
Cardones and Grichnik (2009) published a paper in the Archives of Dermatology describing eruptive pigmented nevi in a patient using MT-II – the appearance of numerous new nevi in a short period of time, some with atypical clinical and histological features. The authors placed the observation in the context of MC1R biology: this receptor not only regulates melanin production, but also modulates melanocyte proliferation and the response to DNA damage. Langan et al. (2010) published a letter opening the discussion on MT-II security in the UK. Habbema et al. (2017) summarized European dermatology practice in a risk review, describing cases of atypical moles and isolated reports of melanomas. Hjuler and Lorentzen (2014) reported a specific case of melanoma associated with the use of MT-II. The causality of MT-II could not be formally proven – the link would require prospective studies, impossible due to the informal use of the peptide – as a warning sign, the report remains important.
The most frequently reported side effects of MT-II in clinical trials were nausea within 30–90 minutes after injection (MC4R/MC3R stimulation in brainstem areas responsible for the gag reflex), transient facial flushing (peripheral MCR vasomotor effects), and decreased appetite. Cases of priapism – a prolonged, painful erection requiring medical intervention – resulting from high MC4R stimulation have been reported in the literature. Moderate increases in blood pressure and tachycardia have also been observed, and isolated reports describe acute episodes – significant increases in blood pressure and symptoms suggestive of rhabdomyolysis.
The FDA, EMA, the UK MHRA and European national agencies have repeatedly warned against the illegal sale of MT-II online as a pigmentation product. The messages pointed to three threats: lack of registration as a medicine, lack of quality control of consumer products (adulteration, microbiological contamination) and documented side effects. There was a concern in the background that people using MT-II for pigmentation purposes would limit their follow-up visits to a dermatologist, delaying the diagnosis of changes in the appearance of moles.
In published protocols, MT-II was administered primarily by subcutaneous injection – a route that provides reproducible bioavailability and a well-described pharmacokinetic profile. Intraperitoneal injections and nasal administration have also been used in animal models. The oral activity of MT-II is minimal – the peptide is susceptible to proteolytic hydrolysis in the gastrointestinal tract despite cyclization. This significantly distinguishes MT-II from peptides stable in gastric juice, such as BPC-157.
Melanotan II in the One Peptides catalog is available as a lyophilized product with analytical specifications consistent with standards for research peptides – five quality control areas verified batch by batch.
| Parameter | One Peptides Specification |
|---|---|
| Purity (RP-HPLC) | ≥98% |
| Identity Confirmation (MS) | molecular mass 1024.2 Da, verification in each batch |
| Humidity (Karl Fischer) | ≤5% |
| Endotoxins (LAL) | ≤1 EU/mg |
| Physical condition | white or creamy-white lyophilisate |
| Certificate of Analysis (COA) | available for each batch |
| Batch traceability | unique batch number, archiving |
| Cold chain | transport and storage 2–8°C |
Reverse phase HPLC with a C18 column and UV detection at 220 nm is the standard method for determining the purity of research peptides. Mass spectrometry (ESI-MS or MALDI-TOF) confirms the identity of a molecule by measuring its molecular mass. The LAL test detects bacterial endotoxins – if the threshold is exceeded, the peptide is not suitable for in vivo testing. Details in the section quality tests and certificates.
MT-II lyophilisate is stable for 24 months at -20°C, protected from light and moisture. After reconstitution in bacteriostatic water (2–8°C refrigerator), the peptide remains active for 28–30 days. The cyclic structure makes it more stable to enzymatic degradation than linear α-MSH analogues, but once reconstituted the peptide remains sensitive to repeated freeze-thaw cycles and exposure to UV light. Each MT-II vial from the One Peptides catalog comes from a batch with a documented cold chain and a unique number enabling retrospective identification of specifications. He describes the documentation of reconstitution peptide reconstitution guide.
The safety profile of MT-II is central to an honest discussion of this peptide. The molecule combines a number of features leading to an unfavorable benefit-risk ratio: wide stimulation of non-selective receptors, narrow window between the pigmentation dose and the dose generating significant side effects, case reports of dermatological changes with long-term use. In preclinical models, doses causing nausea, flushing, increased blood pressure and sexual arousal are significantly lower than those producing the full pigmentation effect.
Clinical development was stopped in the early stages – the peptide is not registered as a drug in the EU or USA. In the EU, MT-II is classified as a laboratory research chemical (RUO) and available in this status in the One Peptides catalog – it is not intended for ingestion, injection or other uses on humans. WADA does not include MT-II on the list of prohibited substances, but its absence from the WADA list does not mean it is safe or legal for consumers.
Melanotan I is a linear 13-amino acid analogue of α-MSH with a relatively selective MC1R profile. Melanotan II – a cyclic 7-amino acid analogue with a broader profile, also stimulating MC4R and MC3R, which translates into central (libido, appetite) and peripheral (nausea, cardiovascular) effects not present in Afamelanotide. The consequence is a different clinical fate: Melanotan I is registered as Scenesse in the EPP (EU 2014, USA 2019), Melanotan II has never been registered.
PT-141 (bremelanotide) is a derivative of MT-II with the terminal amide (-NH₂) and free carboxyl group (-OH) removed. The modification significantly reduces the affinity for MC1R while retaining activity against MC4R. PT-141 represents a profile focused on central stimulation of sexual function and was approved by the FDA in 2019 as a treatment for HSDD.
Yes, as a chemical reagent intended for laboratory tests (RUO). MT-II is not registered as a medicine, dietary supplement or cosmetic ingredient. Illegal sale to consumers as “tanning pills” is the subject of warnings from the FDA, EMA and national European agencies. The RUO status imposes on the buyer the responsibility to use the peptide only in laboratory conditions.
The central reason was the unfavorable safety profile. MT-II is a non-selective agonist of all MCR subtypes, which translates into a broad side effect profile. The narrow window between the pigmentation dose and the dose generating significant side effects made it impossible to achieve an acceptable benefit-risk ratio. For pigmentation in EPP, the more selective Afamelanotide was chosen, for sexual functions – PT-141.
Nausea, facial flushing, decreased appetite, increased blood pressure and tachycardia, spontaneous erections. From a dermatological perspective, the most important signals are reports of eruptive pigmented nevi with atypical histological features (Cardones and Grichnik 2009; Langan et al. 2010; Habbema et al. 2017) and single reports of melanomas associated with MT-II (Hjuler and Lorentzen 2014). Isolated cases of priapism requiring medical intervention have also been described.
A family of five G protein-coupled receptors (MC1R–MC5R) that bind endogenous proopiomelanocortin peptides. MC1R regulates pigmentation, MC2R the adrenal response to ACTH, MC3R and MC4R in the brain participate in the regulation of energy balance, libido and cardiovascular response, MC5R occurs in the exocrine glands.
After reconstitution in bacteriostatic water and storage in a refrigerator at 2–8°C, the peptide remains active for 28–30 days. Lyophilisate before reconstitution – 24 months at -20°C. The reagent dose calculator makes it easy to precisely measure solution in research protocols.
NO. The frequency of observations is variable and depends on the duration of use, dose, skin phototype and genetic factors modulating the reaction of melanocytes. The lack of prospective population data prevents precise risk estimation. As a warning sign, the phenomenon is documented sufficiently to justify the caution recommended in the literature.
Melanotan II 10 mg is available in the One Peptides catalog in the specification ≥98% RP-HPLC with MS confirmation and full certificate of analysis for each batch – only as a research reagent (RUO).
ℹ️ Global disclaimer
All One-Peptides products are reagents intended exclusively for laboratory and scientific research (Research Use Only). They are not medicines, dietary supplements or products intended for human consumption. The information in this article is educational in nature and is a review of published scientific literature; does not constitute medical, pharmaceutical or dietary advice. Melanotan II is not registered as a medicine by the EMA, FDA or other regulatory agencies – in the European Union it remains only a research reagent (RUO).
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