Melanotan II Peptide Research: Melanocortin Receptor Mechanisms, Photoprotection Studies, and Preclinical Data

Melanotan II peptide research centres on a cyclic heptapeptide analogue of alpha-melanocyte-stimulating hormone (alpha-MSH), Ac-Nle-cyclo[Asp-His-D-Phe-Arg-Trp-Lys]-NH2, that is a potent, non-selective agonist at the MC1, MC3, MC4 and MC5 melanocortin receptors. Designed at the University of Arizona in 1989 as a lactam-bridged fragment of alpha-MSH, it became the standard tool compound for two separate research programmes: MC1R-driven melanogenesis and photoprotection in skin, and MC4R-driven control of feeding and energy balance in the hypothalamus. This article summarises the receptor pharmacology, the preclinical evidence in each programme, and the analytical points that matter for a research lot, and it corrects several claims that circulated in the earlier version of this page. The receptor background is covered in more depth in our Melanotan I vs Melanotan II comparison and the PT-141 (bremelanotide) overview.

Disclaimer: For research purposes only. Not for human consumption. Not for diagnostic or therapeutic use. The information presented here is drawn from published scientific literature and is intended solely for educational reference.

Molecular Structure and Design

Native alpha-MSH is the linear tridecapeptide Ac-Ser-Tyr-Ser-Met-Glu-His-Phe-Arg-Trp-Gly-Lys-Pro-Val-NH2. Sawyer and colleagues had shown in 1980 that replacing Met4 with norleucine and Phe7 with D-phenylalanine produces [Nle4, D-Phe7]-alpha-MSH (NDP-MSH), a superpotent and enzymatically resistant analogue that was later developed as Melanotan I and afamelanotide (Sawyer et al., 1980, PNAS 77:5754-5758; PMID 6777774). Al-Obeidi, Hruby and colleagues then used molecular dynamics to design cyclic lactam fragments of the 4-10 core in which the side chains of a Glu or Asp at position 5 and a Lys, Orn, Dab or Dpr at position 10 are joined by an amide bond. In the lizard skin bioassay the Asp5/Lys10 heptapeptide, Ac-[Nle4, Asp5, D-Phe7, Lys10]-alpha-MSH(4-10)-NH2, was about 90 times as potent as alpha-MSH, and it is this compound that was named Melanotan II (Al-Obeidi et al., 1989, J Med Chem 32:2555-2561; PMID 2555512).

Two consequences of that structure matter for everything below. First, the norleucine at position 4 replaces the methionine of alpha-MSH, so MT-II has no methionine and is not subject to methionine sulfoxide formation; its oxidisable residue is the single tryptophan. Second, the sequence contains no asparagine or glutamine, so side-chain deamidation is not a degradation route; the only amide in the molecule is the C-terminal carboxamide. The molecular formula is C50H69N15O9 and the molecular weight is 1024.18 g/mol, as stated on the Melanotan 2 product page. The pilot phase I report by Dorr and colleagues confirms the structure as Ac-Nle4-Asp5-His6-D-Phe7-Arg8-Trp9-Lys10 alpha-MSH(4-10)-NH2 (Dorr et al., 1996, Life Sci 58:1777-1784; PMID 8637402). The historical account of Melanotan I, Melanotan II and their commercial descendants is given by Hadley and Dorr (2006, Peptides 27:921-930; PMID 16412534).

Melanocortin Receptor Pharmacology

Schiöth and colleagues tested MT-II and the related lactam SHU9119 alongside five cyclic disulfide alpha-MSH analogues on cells transiently expressing the human MC1, MC3, MC4 and MC5 receptors. The cyclic lactams showed higher overall affinity for the melanocortin receptors than any of the cyclic disulfide analogues, and substituting D-Phe7 with D-Nal(2′)7, which converts MT-II into SHU9119, shifted the profile toward MC4 selectivity and, at MC3 and MC4, toward antagonism (Schiöth et al., 1997, Peptides 18:1009-1013; PMID 9357059). MT-II itself is a full agonist at all four MSH-responsive subtypes, which is the pharmacological reason it is a useful tool compound and a poor probe of any single receptor. The specific Ki values that circulated in the earlier version of this article, with an n of 3 attached, are not in the 1997 abstract and have been removed; readers who need subtype affinities should take them from the primary binding papers for the receptor construct they are using.

MC1R is the cutaneous melanocortin receptor, expressed on epidermal melanocytes, and it couples through Gs to adenylyl cyclase. The resulting rise in cAMP activates the microphthalmia-associated transcription factor, which drives tyrosinase and the tyrosinase-related proteins and shifts melanin synthesis toward eumelanin. MC4R is the neural receptor that controls feeding, and MC3R and MC5R are expressed in brain and exocrine tissue respectively. Because MT-II activates all of them, any in vivo result with MT-II reflects combined receptor activation unless a selective antagonist or a knockout is used to dissect it, which is exactly how the feeding studies below were designed.

MC1R-Mediated Pigmentation and Photoprotection

The mechanistic case that MC1R activation is photoprotective comes from genetics rather than from MT-II itself. D’Orazio and colleagues showed that ultraviolet light induces MSH expression in keratinocytes but fails to stimulate pigmentation in red/blonde Mc1r e/e mice that lack a functional receptor. Pigmentation in those mice could be rescued by topical forskolin, a cAMP agonist acting downstream of the receptor, and the chemically induced pigmentation protected against UV-induced cutaneous DNA damage and tumorigenesis in a xeroderma pigmentosum complementation group C deficient background (D’Orazio et al., 2006, Nature 443:340-344; PMID 16988713). That experiment establishes the MC1R-cAMP-eumelanin axis as the target; a cyclic MC1R agonist such as MT-II acts on the receptor one step upstream of forskolin.

The pigmentation and photoprotection data in human volunteers that are frequently attributed to MT-II were in fact generated with the linear analogue [Nle4-D-Phe7]-alpha-MSH, that is Melanotan I, not Melanotan II. In the study by Barnetson and colleagues, 65 fair-skinned subjects completed three ten-day cycles of subcutaneous NDP-MSH over three months. Melanin density measured by reflectance spectroscopy increased in every treated subject, with the largest gains in those with the lowest baseline melanin: an average 41 percent increase across eight skin sites in the low minimal erythemal dose skin type versus 12 percent in the high-MED type. Epidermal sunburn cells after 3 MED of UV were reduced by more than 50 percent in the low-MED volunteers and thymine dimer formation in the basal layer by 59 percent (Barnetson et al., 2006, J Invest Dermatol 126:1869-1878; PMID 16763547). The earlier version of this article described that trial as an MT-II study with 79 subjects, a 28 percent melanin increase and a Fontana-Masson biopsy result; none of those details is correct, and the compound was different. No comparable controlled photoprotection dataset exists for MT-II. The “Amaro-Ortiz 2014 PNAS” murine photocarcinogenesis study with a 47 percent reduction in cyclobutane pyrimidine dimers also could not be located and has been removed.

MC4R Pathway Research: Feeding and Energy Balance

MT-II’s second research life began with the agouti obesity problem. Fan and colleagues reasoned that ectopic agouti peptide, a high-affinity MC1R antagonist that also antagonises the hypothalamic MC4R, might cause obesity by blocking central melanocortin signalling. Intracerebroventricular MT-II inhibited feeding in four models of hyperphagia (fasted C57BL/6J mice, ob/ob mice, agouti A(y) mice, and mice injected with neuropeptide Y), co-administration of the antagonist SHU9119 completely blocked the inhibition, and SHU9119 alone enhanced nocturnal and post-fast feeding, showing that melanocortinergic neurons exert tonic inhibition of feeding (Fan et al., 1997, Nature 385:165-168; PMID 8990120). The 37 percent figure, the 1 nmol dose, the Sprague-Dawley rats and the n of 8 previously attached to this study are not in the paper.

The receptor responsible was pinned down genetically. Mice with targeted disruption of MC4R develop maturity-onset obesity with hyperphagia, hyperinsulinaemia and hyperglycaemia, recapitulating the agouti syndrome (Huszar et al., 1997, Cell 88:131-141; PMID 9019399), and Mc4r-null mice do not respond to the anorectic effect of MT-II at all, which shows that alpha-MSH analogues suppress feeding primarily through MC4R rather than MC3R (Marsh et al., 1999, Nat Genet 21:119-122; PMID 9916804). In lean and obese Zucker rats, central MT-II reduced food intake in both genotypes but was more potent in the obese animals, while SHU9119 raised intake only in lean rats, which the authors read as reduced endogenous melanocortin tone in the obese Zucker (Hwa et al., 2001, Am J Physiol Regul Integr Comp Physiol 281:R444-R451; PMID 11448846). The neuropeptide Y summary covers the orexigenic arm of the same arcuate circuit.

The chronic diet-induced obesity data come from Pierroz and colleagues. In C57BL/6J mice with DIO, peripheral MT-II given four times daily produced dose-responsive effects on food intake, body weight, leptin, corticosterone, insulin and free fatty acids; feeding was markedly suppressed for the first four days and returned to control levels by day 5, weight fell over the same four days and then plateaued below control, and after eight days leptin and insulin were substantially lower. Central MT-II for four days suppressed food intake, produced weight loss and increased energy expenditure, and the authors concluded that reduced intake, not raised expenditure, accounted for most of the weight loss (Pierroz et al., 2002, Diabetes 51:1337-1345; PMID 11978628). The rapid tachyphylaxis of the feeding effect is the finding most often left out of secondary summaries. A “Goncalves 2019 Peptides” DEXA study in DIO rats with 12.4 percent fat loss and an 8.7 percent rise in resting metabolic rate, cited in the earlier version of this article, could not be located in any index and has been removed.

Relationship to PT-141 (Bremelanotide)

Bremelanotide is MT-II with the C-terminal amide replaced by a free carboxylic acid, Ac-Nle-cyclo[Asp-His-D-Phe-Arg-Trp-Lys]-OH, and it was developed by Palatin Technologies after the MT-II phase I programme (Hadley and Dorr, 2006; PMID 16412534). The two compounds therefore differ by a single mass unit at the C-terminus and by the loss of one positive charge, and they are routinely encountered together in forensic characterisation of seized peptide products, where LC high-resolution mass spectrometry with accurate-mass MH+ measurement, isotope pattern analysis and fragmentation is used to tell them apart without reference standards (Mestria et al., 2021, Drug Test Anal 13:876-882; PMID 33245851). For a research lot the practical point is that hydrolysis of the MT-II C-terminal amide converts it into bremelanotide, so a “deamidated MT-II” impurity is not a minor variant but a different, separately characterised compound.

Stability, Degradation and Analytical Considerations

Because MT-II carries a lactam bridge rather than a disulfide, it has no sulfur chemistry to scramble, and because it has no Met, Asn or Gln, the degradation routes that dominate for many peptides do not apply. What remains is oxidation of the single tryptophan, which is accelerated by light, peroxides and trace metals, and slow hydrolysis of the C-terminal amide discussed above; the degradation pathways guide covers tryptophan oxidation products and their detection. The specific kinetics previously quoted on this page, a “Verbeken 2018” LC-MS/MS study with 15 percent loss over seven days at 37 °C and an Asn-to-Asp deamidation product, could not be located, and the deamidation product it described is chemically impossible for this sequence. Published analytical work on MT-II includes the LC-HRMS characterisation above and MALDI mass spectrometry imaging of MT-II and its metabolites in tissue for distribution and clearance studies (Chen et al., 2020, Anal Chim Acta 1125:279-287; PMID 32674774).

For a research lot, reversed-phase HPLC purity should be read together with a mass spectrometry identity result confirming the 1024.18 Da parent, since the bremelanotide hydrolysis product is only 1 Da heavier and the tryptophan oxidation products are 4 to 32 Da heavier. Storage of the lyophilised peptide at -20 °C or below, protected from light, and reconstitution into single-use aliquots follow the general rules for tryptophan-containing peptides. The certificate of analysis guide explains how to match the report number and lot on the COA page to the vial in hand.

Key Research Findings

MT-II is a lactam-cyclised alpha-MSH(4-10) analogue about 90 times as potent as alpha-MSH in the lizard skin bioassay, with norleucine in place of methionine and D-Phe at position 7 (Al-Obeidi 1989). It binds MC1, MC3, MC4 and MC5 receptors with higher overall affinity than cyclic disulfide analogues and is not subtype-selective; the D-Nal(2′)7 substitution converts it into the MC3/MC4 antagonist SHU9119 (Schiöth 1997). MC1R signalling through cAMP is required for UV-induced tanning and confers protection against UV DNA damage and tumorigenesis in mice, a result obtained with forskolin in Mc1r-deficient animals rather than with MT-II (D’Orazio 2006); the human pigmentation and thymine-dimer data belong to the linear analogue NDP-MSH, not MT-II (Barnetson 2006). Central MT-II inhibits feeding in fasted, ob/ob, agouti and NPY-injected mice, an effect blocked by SHU9119 and absent in Mc4r-null mice (Fan 1997; Marsh 1999; Huszar 1997). In diet-induced obese mice, peripheral MT-II suppresses feeding for about four days before tolerance develops, and reduced intake rather than raised expenditure explains the weight loss (Pierroz 2002). Bremelanotide is the C-terminal carboxylate of MT-II (Hadley and Dorr 2006; Mestria 2021).

Note on sources. This article was rewritten in September 2026 after a citation audit. The earlier version attributed the Barnetson 2006 pigmentation trial to MT-II when the compound was [Nle4-D-Phe7]-alpha-MSH (Melanotan I) and misreported its size and results; cited an “Amaro-Ortiz 2014 PNAS” photocarcinogenesis study, a “Goncalves 2019 Peptides” body-composition study and a “Verbeken 2018 JPBA” stability study that could not be located; attached Ki values, doses, sample sizes and p-values to Schiöth 1997 and Fan 1997 that are not in those papers; described an Asn-to-Asp deamidation product for a peptide that contains no asparagine; and stated a rodent half-life for MT-II without a source. Every citation above was checked against the indexed abstract on 18 September 2026.

Maple Research Labs submits manufactured batches to an independent analytical laboratory for HPLC testing and publishes the per-batch result and current verification status for Melanotan 2 on the certificates of analysis page. Browse the full research peptide catalog or review the documentation standards.

For research purposes only. Not for human consumption. Not for diagnostic or therapeutic use.

For peer-reviewed research on this topic, visit PubMed.