Melanotan-2

Chemical identity

Identity here is settled, which is not always the case. PubChem carries a single compound record under the name Melanotan II: CID 92432, CAS 121062-08-6, molecular formula C50H69N15O9, molecular weight 1024.2 g/mol, InChIKey JDKLPDJLXHXHNV-MFVUMRCOSA-N. The FDA Global Substance Registration System files the same molecule as UNII UPF5CJ93X7, with the same CAS number, the same formula and a computed mass of 1024.18. IUPHAR lists it as ligand 1323, ChEMBL as CHEMBL430239. Both records carry the peptide shorthand Ac-Nle-cyclo[Asp-His-D-Phe-Arg-Trp-Lys]-NH2. The two registries agree on formula and registry number. Neither records a marketing authorisation for the substance in any jurisdiction.

The molecule is residues four to ten of alpha-melanocyte-stimulating hormone, closed into a ring. Al-Obeidi and colleagues at the University of Arizona described the design in 1989: norleucine at position four, D-phenylalanine at position seven, and a lactam bridge between the aspartate side chain at position five and the lysine side chain at position ten. That bridge produces a twenty-three-membered ring. The same paper compared analogues built with twenty, twenty-one, twenty-two, twenty-three and twenty-four-membered rings in the skin bioassays used, and reported that the smaller rings were less potent and that activity persisting after washout was confined to the twenty-three and twenty-four-membered rings.

Bremelanotide is a structurally adjacent molecule and is routinely conflated with this one. PubChem CID 9941379 gives bremelanotide as C50H68N14O10, molecular weight 1025.2: the same acetylated seven-residue lactam with the C-terminal carboxamide replaced by a free carboxylic acid. One functional group separates the two structures. Afamelanotide, sometimes called Melanotan-I, is a different molecule again, a linear thirteen-residue analogue rather than a cyclic heptapeptide. Findings reported for any one of the three do not transfer to the others, and much of the secondary material on this compound does not keep them apart.

The controlled human record is three studies and twenty-three men

Dorr and colleagues published the first human data in 1996, a pilot Phase 1 study in three healthy male volunteers at the University of Arizona. The design was single-blind and alternating-day, saline against peptide, given subcutaneously across a dose range of 0.01 to 0.03 mg/kg, with two subjects reaching the top of that range and one 0.025 mg/kg. At 0.03 mg/kg one of two subjects recorded Grade II somnolence and fatigue by WHO criteria, and mild nausea was reported at most levels. Two of three subjects showed increased pigmentation of the face, upper body and buttock by quantitative reflectance and by visual perception one week after dosing ended.

Wessells and colleagues then ran two placebo-controlled crossover studies using RigiScan monitoring across six-hour windows. The 1998 study enrolled ten men with erectile dysfunction of no known organic cause: eight of ten developed clinically apparent erections, and mean duration of tip rigidity above eighty per cent was 38.0 minutes against 3.0 minutes on placebo (p=0.0045). The 2000 study enrolled ten men with organic risk factors; subjectively reported erections followed twelve of nineteen active injections against one of twenty-one placebo doses, with mean tip rigidity above eighty per cent lasting 45.3 minutes against 1.9 (P=0.047). Four of nineteen active injections were associated with severe nausea.

Pooling those two trials, a 2000 review by the same group reported erection in seventeen of twenty men in the absence of sexual stimulation, increased sexual desire after thirteen of nineteen active doses against four of twenty-one placebo doses (P<0.01), and severe nausea in 12.9 per cent of subjects at 0.025 mg/kg. That is the whole controlled human record: twenty-three men, all male, one institution, publications spanning 1996 to 2000, observation windows measured in hours and dosing periods of at most two weeks. A PubMed search for human pharmacokinetics returned nothing. The published pharmacokinetic work is in rats, mice and rabbits.

What the potency measurements actually say

Haskell-Luevano and colleagues measured binding and cyclic AMP accumulation at the cloned human MC1 receptor in 1994. Alpha-MSH gave a binding IC50 of 6.5 plus or minus 0.9 nM and a cAMP EC50 of 2.0 plus or minus 0.6 nM. Melanotan-2 gave a binding IC50 of 0.57 plus or minus 0.08 nM and a cAMP EC50 of 0.20 plus or minus 0.05 nM. Against the parent hormone that is roughly an elevenfold difference in binding and a tenfold difference in second-messenger potency, at the receptor governing skin pigmentation. Those are the only direct human MC1R figures located for this peptide.

Al-Obeidi 1989 measured relative melanotropic potency in the classical amphibian and reptile skin bioassays instead. Setting alpha-MSH at one, the analogue that became Melanotan-2 scored ninety in the lizard skin bioassay. The paper states that similar results were obtained in the frog skin bioassay but that the analogues were much less potent there, and reports selectivity for lizard over frog melanocyte receptors. Measured potency therefore depends on the assay used, and the two retrievable comparisons differ by roughly an order of magnitude before species or route is considered. Reviews and case reports describe it as a non-selective agonist across MC1, MC3, MC4 and MC5 receptors, but no published panel giving measured affinities at all four human receptors for this peptide was retrieved here.

Feeding suppression in rodents, and what happened at four weeks

Fan and colleagues reported the foundational feeding result in Nature in 1997. Intracerebroventricular administration inhibited feeding in four models of hyperphagia, being fasted C57BL/6J mice, ob/ob mice, A(y) mice and mice injected with neuropeptide Y, and co-administration of the melanocortin antagonist SHU9119 completely blocked that inhibition. The paper's subject was the agouti obesity syndrome rather than the peptide; the agonist served as a tool for showing that melanocortinergic neurons exert tonic inhibition of feeding. The route used was injection into the cerebral ventricle, which limits what the result establishes about peripheral administration.

Jonsson and colleagues infused the peptide intracerebroventricularly in rats for four weeks and reported the shape of the response over time. Treated animals ate less and lost considerably more weight than controls during the first week. During the second and third weeks they gained weight, and by the end of the treatment period weight gain was similar to that of the control group. Food conversion ratio was lowered relative to controls, but that effect was greatest in week one and had attenuated to control level by the close of the study.

Trivedi and colleagues gave the peptide intravenously to rats, found it suppressed food intake, then showed by autoradiography that radiolabelled peptide did not accumulate in brain parenchyma. The only labelled structures were circumventricular organs sitting outside the blood-brain barrier, and direct LC-MS-MS measurement confirmed negligible brain penetration. Hatziieremia and colleagues later gave 1 mg/kg intraperitoneally to mice and measured 11.4 plus or minus 5.5 ng/g in brain homogenate at thirty minutes, low against plasma at the same time point. The intracerebroventricular and subcutaneous routes are therefore not interchangeable for this endpoint.

The clinical literature after 2009 is case reports

Cardones and Grichnik published the first of these reports in 2009. A forty-year-old man with a history of melanoma and multiple dysplastic naevi self-administered a synthetic alpha-MSH peptide and developed crops of new pigmented naevi, many with atypical clinical and histopathological features, while pre-existing naevi darkened and acquired growth features; the lesions lightened progressively after use stopped. Reports of the same kind followed from Ireland, Australia, Sweden, Germany, Spain and Denmark. Habbema and colleagues, reviewing the field in 2017, counted four case reports of melanoma emerging from existing moles during or shortly after melanotan use and stated that conclusive evidence linking the two was lacking.

Nelson and colleagues reported a thirty-nine-year-old man who injected 6 mg subcutaneously and presented two hours later with a heart rate peaking at 146 beats per minute, mydriasis, diaphoresis and diffuse tremor; creatine kinase rose from 1760 to 17,773 IU/L over twelve hours and creatinine reached 2.25 mg/dL. Mass spectrometry confirmed the injected material against a purchased reference standard. Eijmael and colleagues published a closely similar sympathomimetic presentation in 2022, a twenty-seven-year-old man with no relevant medical history who reached a Dutch emergency department two hours after subcutaneous self-administration and was treated with lorazepam, potassium supplementation and intravenous fluid resuscitation. Single reports also exist of low-flow priapism managed with cavernosal aspiration, irrigation and intracavernosal phenylephrine, of posterior reversible encephalopathy syndrome, and of renal infarction.

Bonchev followed a patient who self-administered over sixty-four days and recorded brown pigmentation of the attached gingiva in both arches and of both buccal mucosae; buccal pigmentation had nearly disappeared one month after stopping, while gingival pigmentation persisted at three months at reduced intensity. Separately, a twenty-two-year-old woman who used a melanotan-labelled nasal spray was reported with a mucosal malignant melanoma of the anterior maxilla, confirmed histologically and treated by surgical resection followed by immunotherapy. None of this material carries a denominator. No cohort study, case-control study or registry analysis of melanotan users was located, so the case series establishes that events occurred and nothing about how often.

What forensic analysis found in the vials

Breindahl and colleagues bought vials from three online shops and analysed them by liquid chromatography with ultraviolet detection and tandem mass spectrometry, validating the methods against guidelines used for active substances in authorised medicines. Identity was confirmed by retention time and five qualifying fragment ions. Every shop's label claimed 10 mg per vial. Measured content ranged from 4.32 to 8.84 mg. Vials from two of the three shops contained unidentified impurities at 4.1 to 5.9 per cent, and impurities from the third fell below the limit of quantification. Nothing in that study identified what the impurities were.

Deville and Charlier, in a Belgian forensic toxicology laboratory, reported two vials of pharmaceutical appearance submitted for identification under a Melanotan II label. The compound was absent from their spectral library, its ultraviolet spectrum was not available in the literature, and its mass sat above the range their instruments were configured to scan. Once a reference standard was obtained, identification was confirmed and the purity of the powder determined at 30 per cent. Venhuis and colleagues had earlier found melanotan II among falsified biopharmaceutical injectables reported by twelve European countries during Operation Pangea 7, a single 2014 enforcement week.

Labelling claims about origin were contested in the enforcement record as well as content claims. The Federal Register notice recording the permanent debarment of the principal of a Tennessee company that sold the peptide states that the company's website advertised the product as one hundred per cent United States made, whereas some of what it sold was manufactured in and imported from China. The same notice records that the peptide was marketed, sold and shipped by that company to customers in the United States and abroad, and that the shipments were not confined to one jurisdiction.

Regulatory position and the one registered trial

The Food and Drug Administration issued a warning letter to a Tennessee company selling the peptide on or about 30 August 2007, stating that, on the basis of statements on that company's website, the product constituted a new drug that could not be introduced into interstate commerce without an approved application. Correspondence through late 2007 reiterated that position and added that unapproved new drugs do not qualify for export. On 28 August 2015 the United States District Court for the Middle District of Tennessee entered judgment against the company's principal on two counts of conspiracy under 18 U.S.C. 371, and a permanent debarment order took effect on 14 November 2016 at 81 FR 79501.

British regulators have published their own counts under freedom of information. In December 2021 the Medicines and Healthcare products Regulatory Agency reported thirteen suspected adverse drug reaction reports associated with melanotan II received through the Yellow Card scheme between 1 January 2011 and 9 December 2021. In April 2024 it reported sixteen such reports between 1 January 2012 and 31 December 2022, stated that it had made no recommendations and taken no specific action in response to them, and noted that it had repeatedly acted to remove melanotan products from the market for over ten years. The same responses set out the classification rule: injectable products are treated as medicines, nasal sprays only where medicinal claims are made.

Two melanocortin agonists are approved medicines and neither is this one. Bremelanotide was approved by the FDA on 21 June 2019 for acquired, generalised hypoactive sexual desire disorder in premenopausal women, and afamelanotide holds approvals in erythropoietic protoporphyria. Melanotan-2 is not approved for human use in any jurisdiction. A ClinicalTrials.gov intervention search returned exactly one registered study: NCT07437560, a randomised, quadruple-masked Phase 2 trial of the peptide as an adjunct to narrowband UV-B phototherapy in sixty adults with stable non-segmental vitiligo, industry-sponsored, recruiting at a single hospital site in Shenzhen, with an actual start of 2 February 2026 and estimated primary completion of 14 February 2027.

What is not known

The published record does not establish what happens beyond a few weeks of exposure. Human dosing in the three controlled studies ran for at most two weeks in three subjects and for single sessions in the other twenty, with outcome windows of six hours. No human pharmacokinetic study exists, so absorption, distribution, clearance and terminal half-life in people are uncharacterised; the circulating half-life figures have no measured basis. No human body-composition, endocrine, cardiovascular, hepatic or renal safety dataset exists. No study has measured whether the induced pigmentation alters erythema threshold, UV damage or skin cancer incidence in either direction, which leaves both the protective claim and the carcinogenic concern unresolved by direct evidence. The melanocytic and systemic harms in the literature are individual case reports and small series with no denominator: no cohort, case-control or registry study of users was located, and no prevalence estimate specific to this compound was found, so incidence cannot be calculated from what is published. Receptor selectivity is asserted qualitatively across MC1, MC3, MC4 and MC5 without a retrievable panel of measured human affinities. Nothing addresses use in women, in adolescents, during pregnancy or lactation, or in people with existing dysplastic naevi, which is the group the case reports repeatedly involve. Melanotan-2 is not approved for human use in any jurisdiction, and forensic analyses of material bought online found a uniform 10 mg label against a measured low of 4.32 mg, unidentified impurities, and vials submitted to a forensic laboratory whose powder was determined at 30 per cent purity.

Questions