Melanotan I 10mg Peptide

Melanotan I is the research-catalogue name for afamelanotide, the tridecapeptide [Nle4, D-Phe7]-alpha-MSH first reported by Sawyer and colleagues at the University of Arizona in 1980. It is native alpha-melanocyte-stimulating hormone with two deliberate changes: methionine at position 4 swapped for norleucine, and L-phenylalanine at position 7 swapped for its D enantiomer. As in the parent hormone, the N terminus is acetylated and the C terminus is amidated.

Neither substitution is cosmetic. Native alpha-MSH is cleaved within minutes by serum peptidases and its Met4 side chain oxidises readily. The modified analogue resisted degradation by serum enzymes and measured 26 times as potent as alpha-MSH in the mouse melanoma adenylate cyclase assay, alongside increased tyrosinase activity in the same cells and prolonged darkening in the frog skin bioassay.

One fact separates this compound from almost everything else in a research catalogue, and it should be stated precisely. Afamelanotide is an approved medicine. Marketed as SCENESSE, it holds a European Union marketing authorisation granted on 22 December 2014 and United States approval granted in October 2019, in both cases as a 16 mg controlled-release subcutaneous implant indicated for prevention of phototoxicity in adults with erythropoietic protoporphyria. The approved article is that implant, manufactured and released under pharmaceutical controls. A lyophilised vial of the free peptide supplied for laboratory work is a different physical article, is not that product, and is not interchangeable with it.

Melanotan I is an agonist at the melanocortin receptor family, of which MC1R on epidermal melanocytes is its highest-affinity site. In COS cells expressing the cloned human subtypes, radioiodinated [Nle4, D-Phe7]-alpha-MSH bound MC1R with a dissociation constant of 85.1 pmol/L, MC3R at 396 pmol/L and MC5R at 5.05 nmol/L. MC1R is Gs-coupled, so occupancy raises intracellular cAMP, activates protein kinase A and phosphorylates CREB at Ser133.

Phosphorylated CREB transactivates MITF, the transcription factor that runs the melanocyte differentiation programme. MITF in turn drives tyrosinase, TYRP1 and DCT, the three enzymes that commit dopaquinone down the eumelanin branch rather than the pheomelanin branch. The measurable output in cell culture is raised tyrosinase activity and darker, more numerous melanosomes, which is precisely what the 1980 characterisation recorded in mouse melanoma cells.

The same cAMP and MITF axis is not purely pigmentary. In melanocyte and keratinocyte systems, MC1R signalling has been associated with increased nucleotide excision repair capacity, reduced oxidative DNA damage and altered cytokine output, which is why the clinical development programme moved into photodermatoses where the endpoint is tolerance of light rather than colour.

MC1R sits at the top of the only signalling route that decides which of the two melanin polymers a melanocyte makes. Occupancy raises cAMP, the cAMP response element binding protein is phosphorylated, MITF transcription rises, and tyrosinase, TYRP1 and DCT are induced together. Tyrosinase catalyses the rate-limiting hydroxylation of tyrosine to DOPA and the oxidation of DOPA to dopaquinone. Where cysteine is available and MC1R signalling is low, dopaquinone is diverted to pheomelanin. Where MC1R signalling is high, DCT and TYRP1 push the same intermediate toward the indole-quinone units of eumelanin.

That distinction matters optically. Pheomelanin is a weak absorber and a photosensitiser in its own right, while eumelanin absorbs broadly from the ultraviolet through the visible, which is what makes it a physical filter rather than merely a colour. The pigment response after melanocortin agonism is therefore reported as melanin density by reflectance spectrophotometry rather than as a subjective shade, and in the solar urticaria study that density rose within seven days and was still elevated at day 60.

Loss-of-function MC1R variants are common in populations of northern European ancestry and shift the resting balance toward pheomelanin. A relevant finding for anyone modelling receptor pharmacology is that the pigment response to the analogue in clinical studies did not require a wild-type receptor genotype: response was recorded across MC1R variant carriers and across skin phenotypes, consistent with a high-affinity agonist driving partially impaired receptors above their signalling threshold.

These two are confused more often than any other pair in melanocortin research, and the usual shorthand, that Melanotan I is MC1R selective and Melanotan II is not, is only half right. Melanotan I is a linear 13-residue peptide. Melanotan II is a cyclic heptapeptide, Ac-Nle-c[Asp-His-D-Phe-Arg-Trp-Lys]-NH2, in which a lactam bridge between the Asp and Lys side chains closes the ring. Both carry the same His-Phe-Arg-Trp message core with the D-Phe substitution. Neither activates MC2R, which responds to ACTH and requires both the ACTH-specific address sequence and the accessory protein MRAP.

In radioligand binding, [Nle4, D-Phe7]-alpha-MSH is not subtype-exclusive. Its dissociation constants of 85.1 pmol/L at MC1R, 396 pmol/L at MC3R and 5.05 nmol/L at MC5R put MC1R ahead by roughly fivefold over MC3R, not by orders of magnitude. Describing the linear peptide as a clean MC1R ligand overstates what the binding data support.

The real separation is conformational. Mutagenesis and peptide-scanning work on the melanocortin receptors showed that selectivity for MC3R and MC4R is directed by how the core sequence is presented rather than by any single side-chain contact, and that constraining the core changes which subtype reads it best. Closing the cyclic lactam does exactly that constraining, which is why the cyclic analogue produces MC3R and MC4R-associated responses, including erectile and appetite effects, at exposures that also pigment, and why its active metabolite bremelanotide was developed as a central melanocortin agonist rather than a pigmentary one.

Pharmacokinetics widens the gap further. The linear peptide clears from human plasma with a beta-phase half-life of roughly 0.8 to 1.7 hours after subcutaneous administration, which is why its approved presentation is a controlled-release implant rather than a bolus. A short-lived linear agonist delivered slowly into skin favours the cutaneous receptor pool, and a conformationally constrained analogue with meaningful central exposure does not. Selectivity in practice is a product of structure, route and exposure profile, not of a single binding constant.

Erythropoietic protoporphyria follows from partial ferrochelatase deficiency. Protoporphyrin IX accumulates in erythrocytes instead of being metallated to heme, and circulating protoporphyrin IX in dermal capillaries absorbs strongly at its Soret band near 400 to 410 nm. Photoexcitation generates singlet oxygen and reactive intermediates in the vessel wall, producing burning pain within minutes of light exposure and often without visible erythema. Ordinary sunscreens, formulated against ultraviolet, do little against violet-blue visible light.

The rationale for a melanocortin agonist here is optical rather than metabolic: raise epidermal eumelanin and the same incident light delivers less energy to the porphyrin. Two multicentre, randomised, double-blind, placebo-controlled trials of the 16 mg implant tested that, one in the European Union with 74 patients receiving five implants at 60-day intervals and one in the United States with 94 patients receiving three. The primary endpoint in both was hours of direct sun exposure without pain. Both favoured the active arm, and both are registered on ClinicalTrials.gov as NCT00979745 and NCT01605136.

The absolute hour counts are not comparable between the two studies because exposure was recorded differently, so the meaningful comparison in each is against its own placebo arm. Later real-world data run in the same direction: in a 29-patient United States cohort, median symptom-free exposure rose from 12.5 to 120 minutes while erythrocyte and plasma protoporphyrin concentrations and liver biochemistries were unchanged. That dissociation is the clearest statement of what the peptide does and does not do in this disease model.

Vitiligo repigmentation depends on melanocyte precursors in the hair follicle bulge migrating into depigmented epidermis, proliferating and differentiating. Narrowband UV-B at 311 nm drives that process but slowly, and it does so least well where the cosmetic contrast is greatest, in darker phototypes. A melanocortin agonist supplies the differentiation signal directly, which is the basis for combining the two.

The pivotal randomised comparison, run across four United States centres by Lim, Grimes, Hamzavi and colleagues, enrolled 55 adults with nonsegmental vitiligo affecting 15 to 50 percent of body surface area, phototypes III to VI. Twenty-eight received monthly 16 mg implants for four months on top of narrowband UV-B, and 27 received phototherapy alone. Mean repigmentation at day 168 was 48.64 percent against 33.26 percent, superiority was already detectable at day 56, and median time to first repigmentation was 41.0 against 61.0 days on the face and 46.0 against 69.0 days on the upper extremities.

The subgroup result is the informative one: the difference reached significance in phototypes IV to VI and did not in phototype III. That is the pattern expected of an agent whose contribution is melanogenic capacity rather than immunosuppression, and it also explains the cosmetic objection raised during the trial, that combination-treated skin darkened overall. Earlier open-label work by Grimes and colleagues in a small phototype IV to VI series set up the same question, and a larger Phase 3 comparison of the implant plus narrowband UV-B against phototherapy alone (NCT06109649, 200 planned participants) is listed as active and not recruiting.

Solar urticaria is a rare immediate photosensitivity with a highly variable action spectrum: some patients weal to ultraviolet B, others to ultraviolet A, others well into visible wavelengths. That variability makes it a demanding test of any photoprotective strategy, because a filter has to work across whichever band a given patient responds to. Five patients received a single 16 mg implant in winter and were phototested with monochromated light from 300 to 600 nm at days 0, 30 and 60. Weal area fell significantly across responding wavelengths at day 60 (P=0.049) and minimal urticarial doses rose by more than twofold in responders, tracking the rise in spectrophotometric melanin density.

The same logic has been carried into other light-triggered conditions. Registered studies include Phase 3 work in polymorphic light eruption (NCT00472901 and NCT04704713), Phase 2 studies in xeroderma pigmentosum complementation groups C and V (NCT05159752 and NCT05370235), variegate porphyria (NCT05854784), actinic keratoses and squamous cell carcinoma in transplant recipients (NCT00829192), and a Phase 1 study of DNA repair capacity in ten healthy volunteers (NCT05368857).

Two registered studies sit outside photobiology entirely and are worth knowing about because they probe the non-pigmentary arm of melanocortin signalling: a Phase 2 safety study in acute arterial ischaemic stroke (NCT04962503) and a Phase 2 study of the peptide as an adjunct to photodynamic therapy with porfimer sodium (NCT04425746). Both are small, and neither has produced the kind of controlled efficacy data the porphyria programme has.

Human pharmacokinetics for the free peptide were characterised by Ugwu and colleagues in 1997. After subcutaneous administration, absorption half-lives ranged from 0.07 to 0.79 hours and beta-phase elimination half-lives from 0.8 to 1.7 hours, with clearance of 0.12 to 0.19 L/kg/h. This is a molecule that arrives quickly and leaves quickly, and the pigment response it produces long outlasts its presence in plasma because that response is transcriptional.

Route matters more than usual here. In the human pharmacology work summarised by Minder and colleagues, daily subcutaneous administration at 0.08 and 0.21 mg/kg in saline produced sustained pigmentation with essentially complete bioavailability, while oral and transdermal delivery produced no measurable pigmentary response at all. A 13-residue peptide with a blocked N terminus is not orally available, and the molecule is far too large for meaningful passive skin permeation.

The controlled-release implant exists to solve that arithmetic. A biodegradable poly(D,L-lactide-co-glycolide) rod releases the peptide over days, so a single article placed at 60-day intervals holds the melanogenic signal above threshold with far lower total exposure than repeated bolus administration would require. This is the most important practical difference between the approved product and free peptide in a vial: the approved product is a formulation, and the formulation does a large part of the pharmacological work.

Long-term follow-up is unusually good for a peptide of this class. Cohorts have been followed out to 25 years after early exposure, and continuous treatment in the porphyria population has been reported out to eight years, with adverse events dominated by nausea, transient facial flushing and hyperpigmentation, including reports of oral mucosal pigmentation.

Lyophilised material is stored at minus 20 degrees Celsius or below, desiccated and protected from light. Two residues drive the handling requirements. Trp9 is photolabile and oxidisable, which is the reason for amber or foil-protected vials and for minimising bench exposure. The other is an absence: replacing Met4 with norleucine removed the oxidisable thioether that generates the methionine sulfoxide degradation peak in native alpha-MSH preparations, so the analogue has one fewer degradation route than its parent and a cleaner chromatogram.

Repeated freeze-thaw cycling is the usual cause of assay drift in stock solutions, so aliquoting at first thaw rather than returning a single vial to the freezer is standard laboratory practice. In solution, peptides carrying Asn, Gln or free Cys degrade fastest through deamidation or disulfide scrambling. This sequence has none of them, and its practical stability limits are set instead by oxidation, photodegradation and adsorption to container surfaces at low concentrations.

Analytical identity rests on three checks. Reversed-phase HPLC at 214 nm resolves the main peak from deletion and oxidation impurities. Electrospray mass spectrometry gives a mass consistent with C78H111N21O19 and an observed molecular ion near 1646 Da, the figure quoted on most certificates of analysis. Chiral or enzymatic confirmation of the D configuration at position 7 is the check almost nobody runs, and it is the one that actually distinguishes this compound from an L-Phe7 sequence that would be neither protease-resistant nor unusually potent.

Peptides of this type are isolated as acetate salts, so gross vial mass and net peptide content are not the same quantity. A certificate reporting net peptide content, counterion identity and water content by Karl Fischer tells a laboratory what it actually has. One reporting only HPLC area percent does not.