A practical reference on cyclic heptapeptide: what it is, how it behaves, what the literature reports, and where the honest uncertainties sit.
Reviewed 2026-03-12. Anything still debated is marked as such rather than presented as settled.
Melanotan II binds several melanocortin receptor subtypes rather than a single target. MC1R on melanocytes drives melanin synthesis, while MC3R and MC4R participate in energy balance, appetite, and sexual response pathways. This lack of selectivity explains why reported effects extend beyond skin darkening. Substitutions at positions four and seven, including norleucine and D-phenylalanine, increase potency and resistance to peptidases. Understanding which receptor mediates which effect remains an active area of investigation.
Published human data come mostly from small, short studies rather than large controlled trials. Reported outcomes include increased skin pigmentation and, in some reports, effects on appetite and libido, but sample sizes are small and follow-up is limited. Whether long-term use produces durable pigment changes or adverse effects is not established. Because products sold outside pharmacies are not standardized, the actual content of any given vial is often unknown. Independent testing of such material is uncommon.
Melanotan II is a synthetic cyclic heptapeptide analog derived from the core sequence of alpha-melanocyte-stimulating hormone. Researchers at the University of Arizona synthesized it during the 1980s while studying pigmentation and appetite signaling. The compound is not an approved medicine in any major jurisdiction and appears mainly in laboratory and research-chemical settings. Its structure incorporates a lactam bridge between side chains, which constrains the ring and slows enzymatic breakdown relative to the natural hormone.
Published pharmacokinetic information is limited and comes mainly from small studies rather than registrational trials. Plasma half-life is usually described as short, on the order of tens of minutes, followed by rapid tissue distribution and clearance of the intact peptide. Metabolites and low concentrations of parent compound have been reported in urine, a detail relevant to anti-doping and forensic testing. Whether repeated exposure changes receptor sensitivity or clearance over time remains an open question. Values differ noticeably between analytical assays, so published numbers should be read as approximate rather than definitive.
Melanotan II is a synthetic cyclic heptapeptide with the sequence Ac-Nle-cyclo[Asp-His-D-Phe-Arg-Trp-Lys]-NH2, corresponding to a molecular formula of C50H69N15O9 and a monoisotopic mass near 1024 daltons. It was designed as a structural analogue of alpha-melanocyte-stimulating hormone, a peptide hormone produced by cleavage of proopiomelanocortin. A lactam bridge between the aspartate and lysine side chains closes the ring, and the C-terminal amide removes a free carboxyl group. Both modifications increase resistance to enzymatic degradation compared with the linear parent hormone. Four substitutions distinguish it from afamelanotide, the linear analogue studied under the name melanotan I.
Receptor-binding studies classify melanotan II as a non-selective melanocortin agonist. It interacts with MC1R, MC3R, MC4R and MC5R, with reported affinities in the low nanomolar range and no strong subtype preference. Activation of MC1R on dermal melanocytes shifts pigment synthesis toward eumelanin, the dark polymer deposited in melanosomes and transferred to keratinocytes. Because the same peptide engages MC4R in the hypothalamus, it also appears in animal work on food intake and erectile response, which is why it is discussed in both pigment and metabolic research. Which receptor populations dominate after systemic exposure in humans is not fully established.
| Property | Value | Notes |
|---|---|---|
| Chemical class | Synthetic cyclic heptapeptide | Analog of alpha-melanocyte-stimulating hormone |
| Primary receptor family | Melanocortin receptors MC1R through MC5R | Binding is not subtype-selective |
| Development period | 1980s | Synthesized during pigmentation and appetite research |
| Regulatory status | Not an approved medicine | No major regulator has authorized human use |
| Related compound | Afamelanotide | Linear analog studied for photoprotection |
The peptide was developed during the 1980s by researchers investigating melanocortin signalling and skin pigmentation pathways. Early work focused on analogues of alpha-melanocyte-stimulating hormone that would resist enzymatic breakdown more effectively than the parent molecule. Melanotan-2 emerged from that programme as a shortened, cyclised variant. Reports describing its synthesis and receptor activity later appeared in the scientific literature. Commercial availability grew through unregulated channels rather than through pharmaceutical approval.
Structurally, Melanotan-2 retains the core recognition motif of alpha-melanocyte-stimulating hormone while adding a lactam bridge that links two side chains and constrains the molecule into a ring. This modification lowers susceptibility to enzymatic degradation. The compound acts as an agonist at melanocortin receptors, particularly subtypes associated with melanin production. Because the same receptor family influences several physiological processes, researchers note that its activity is not confined to pigmentation alone. Receptor selectivity continues to be examined in published studies.
Melanotan-2 is a synthetic cyclic heptapeptide designed as an analogue of alpha-melanocyte-stimulating hormone, the naturally occurring peptide involved in pigmentation signalling. Its sequence is conventionally written as Ac-Nle-cyclo[Asp-His-D-Phe-Arg-Trp-Lys]-NH2, with a lactam bridge joining the aspartate side chain to the lysine side chain. The empirical formula is C50H69N15O9 and the monoisotopic mass lies near 1023.5 daltons. N-terminal acetylation and the D-configured phenylalanine both increase resistance to enzymatic breakdown compared with the parent hormone.
Pharmacologically, melanotan-2 behaves as a non-selective agonist across the melanocortin receptor family. Binding at MC1R on dermal melanocytes promotes eumelanin synthesis, which underlies the tanning response described in early human work. Activity at the centrally expressed MC4R receptor is associated with reported effects on appetite and erectile function. Because the peptide does not discriminate strongly among receptor subtypes, attributing any single observed effect to one receptor pathway is generally not possible without selective antagonists or receptor knockout models.
plastid Any of a class of membrane-bound organelles found in the cells of some eukaryotes such as plants and algae which are hypothesized to have evolved from endosymbiotic cyanobacteria; examples include chloroplasts, chromoplasts, and leucoplasts. Plastids retain their own circular chromosomes which replicate independently of the host cell's genome. Many contain photosynthetic pigments which allow them to perform photosynthesis, while others have been retained for their ability to synthesize unique chemical compounds.
=== Analogues === Retigabine is a bioisostere of flupirtine in which the pyridine ring has been replaced with a phenyl ring. Azetukalner (encukalner), pynegabine, and fluoxakalner are also analogues of flupirtine and retigabine.
== Enzyme Structure and Structural studies == Structurally, haloalkane dehalogenases belong to the alpha/beta-hydrolase superfamily. Their active site is buried in a predominantly hydrophobic cavity at the interface of the alpha/beta-hydrolase core domain and the helical cap domain, and is connected to the bulk solvent by access tunnels. The active-site residues that are essential for catalysis are referred to as the catalytic pentad, and comprise a nucleophilic aspartate residue, a basic histidine residue, an aspartic or glutamic acid moiety that serves as a general acid and either two tryptophan residues or a tryptophan-asparagine pair that serve to stabilize the leaving halide ion. The haloalkane dehalogenase family currently includes 14 distinct enzymes with experimentally confirmed dehalogenation activity. An analysis of the sequences and structures of haloalkane dehalogenase and their homologues divided the family into three subfamilies, which differ mainly in the composition of their catalytic pentad and cap domain. As of late 2007, 25 structures have been solved for this class of enzymes, with PDB accession codes PDB: 1B6G, PDB: 1BE0, PDB: 1BEE, PDB: 1BEZ, PDB: 1BN6, PDB: 1BN7, PDB: 1CIJ, PDB: 1CQW, PDB: 1CV2, PDB: 1D07, PDB: 1EDB, PDB: 1EDD, PDB: 1EDE, PDB: 1HDE, PDB: 1K5P, PDB: 1K63, PDB: 1K6E, PDB: 1MJ5, PDB: 2DHC, PDB: 2DHD, PDB: 2DHE, PDB: 2EDA, PDB: 2EDC, PDB: 2PKY, and PDB: 2YXP.
==== Education ==== Even with the acceleration of the transmission of COVID-19 in the city and the increase in the occupation of beds at hospitals, Melo maintained the plan to recommence classes in municipal schools on 22 February 2021. He had only cancelled the beginning of the school year when the city had been put under the black flag category, the highest category for social distancing, due to a high risk of infections. A part of the municipal schools would return to opening at the beginning of May, even if some of the teachers had not been vaccinated, in contrast to what had been happening in neighboring cities such as Esteio and São Leopoldo. Days before, the teachers had called for protests against returning to in-person classes during the middle of the pandemic without vaccination.
=== Disadvantage === As some proteins require chaperonin for proper folding, Komagataella is unable to produce a number of proteins, since it does not contain the appropriate chaperones. The technologies of introducing genes of mammalian chaperonins into the yeast genome and overexpressing existing chaperonins still require improvement.
Sources: en.wikipedia.org
=== Expansion of the human plasma proteome === The human plasma proteome may contain thousands of proteins, however, identifying them presents challenges due to the wide range of concentrations present. Some low abundance proteins may be present in picogram (pg/mL) quantities, while high abundance proteins can be present in milligram (mg/mL) quantities. Many efforts to expand the human plasma proteome overcome this difficulty by coupling some type of high performance liquid chromatography (HPLC) or reverse phase liquid chromatography (RPLC) with high efficiency cation exchange chromatography and subsequent tandem mass spectrometry for protein identification.
A degron can be converted into its active form by a post-translational modification such as phosphorylation of a tyrosine, serine or threonine residue. In this case, the ubiquitin ligase exclusively recognizes the phosphorylated version of the substrate due to stabilization within the binding site. For example, FBW7, the F-box substrate recognition unit of an SCFFBW7ubiquitin ligase, stabilizes a phosphorylated substrate by hydrogen binding its arginine residues to the phosphate, as shown in the figure to the right. In absence of the phosphate, residues of FBW7 repel the substrate.
=== Bipolar disorder === Endoxifen is used to treat manic or mixed episodes associated with bipolar I disorder in India. It has been found that the endoxifen improves manic symptoms as well as mixed episode symptoms of patients with bipolar I disorder and has been considered an effective and well-tolerated treatment for this condition. Bipolar disorder is associated with overactive protein kinase C (PKC) intracellular signaling. To date, there have been three phases of clinical trials. And, in the phase III trials, endoxifen reduced the total Young Mania Rating Scale (YMRS) score from 33.1 to 17.8. A significant (p < 0.001) improvement in Montgomery–Åsberg Depression Rating Scale (MADRS) score was observed for endoxifen (4.8 to 2.5). The endoxifen is well-tolerated by the subjects as depicted in the changes in Clinical Global Impression-Severity of Illness scores.
Cinoxacin is a quinolone antibiotic that has been discontinued in the U.K. as well the United States, both as a branded drug or a generic. The marketing authorization of cinoxacin has been suspended throughout the EU. Cinoxacin was an older synthetic antimicrobial related to the quinolone class of antibiotics with activity similar to oxolinic acid and nalidixic acid. It was commonly used thirty years ago to treat urinary tract infections in adults. There are reports that cinoxacin had also been used to treat initial and recurrent urinary tract infections and bacterial prostatitis in dogs. however this veterinary use was never approved by the United States Food and Drug Administration (FDA). In complicated UTI, the older gyrase-inhibitors such as cinoxacin are no longer indicated.
Sources: en.wikipedia.org
During World War II the Germans experimented with the idea of the schnorchel (snorkel) from captured Dutch submarines but did not see the need for them until rather late in the war. The schnorchel is a retractable pipe that supplies air to the diesel engines while submerged at periscope depth, allowing the boat to cruise and recharge its batteries while maintaining a degree of stealth. Especially as first implemented however, it turned out to be far from a perfect solution. There were problems with the device's valve sticking shut or closing as it dunked in rough weather. Since the system used the entire pressure hull as a buffer, the diesels would instantaneously suck huge volumes of air from the boat's compartments, and the crew often suffered painful ear injuries. Speed was limited to 8 knots (15 km/h), lest the device snap from stress. The schnorchel also created noise that made the boat easier to detect with sonar, yet more difficult for the on-board sonar to detect signals from other vessels. Finally, allied radar eventually became sufficiently advanced that the schnorchel mast could be detected beyond visual range. In clear weather, diesel exhausts can be seen on the surface to a distance of about three miles, while "periscope feather" (the wave created by the snorkel or periscope moving through the water) is visible from far off in calm sea conditions. Modern radar is also capable of detecting a snorkel in calm sea conditions.
Proteomics permits the quantitative analysis and detection of changes to proteins or protein biomarkers. Protein biomarkers detect a variety of biological changes, such as protein-protein interactions, post-translational modifications and immunological responses. Protein biomarkers are widely used in diagnostics due to their direct involvement in cellular functions and pathways. Protein biomarkers can provide direct information about the functional state of cells and tissues, offering insights into disease mechanisms. Techniques such as mass spectrometry, immunohistochemistry, ELISA, and flow cytometry are employed to detect protein biomarkers, which can indicate protein presence and quantification. For example, in about 20% of breast cancers, the cancer cells have an overexpression of the HER2 gene, leading to more aggressive tumor growth. HER2 status can be determined using IHC and FISH. HER2-positive breast cancers are treated with targeted therapies that specifically target the HER2 protein, inhibiting the growth of cancer cells.
== Role in nuclear waste == Neptunium accumulates in commercial household ionization-chamber smoke detectors from decay of the (typically) 0.2 microgram of americium-241 initially present as a source of ionizing radiation. With a half-life of 432 years, the americium-241 in an ionization smoke detector includes about 3% neptunium after 20 years, and about 15% after 100 years. Under oxidizing conditions, neptunium-237 is the most mobile actinide in the deep geological repository environment of the Yucca Mountain project in Nevada. This makes it and its predecessors such as americium-241 candidates of interest for destruction by nuclear transmutation. Due to its long half-life, neptunium will become the major contributor of the total radiotoxicity at Yucca Mountain in 10,000 years. As it is unclear what happens to the non-reprocessed spent fuel containment in that long time span, an extraction and transmutation of neptunium after spent fuel reprocessing could help to minimize the contamination of the environment if the nuclear waste could be mobilized after several thousand years.
Sources: en.wikipedia.org
No regulator in a major market has approved it for human use. It appears in research settings and in products marketed outside pharmacy channels. Legal status for personal possession varies by country.
Afamelanotide is a linear analog that has received approval in some jurisdictions for a specific photoprotection indication. Melanotan II is cyclic and less selective across melanocortin receptors. The two are related but are not interchangeable in regulatory or clinical terms.
A compound that activates several receptor subtypes may produce effects beyond pigmentation. Selectivity influences which tissues respond and shapes the side-effect profile described in reports. Designing subtype-selective analogs is a stated goal of melanocortin research.
No. It is a synthetic analogue carrying four amino acid changes, a lactam ring and an amidated C-terminus. The natural hormone is a linear thirteen-amino-acid peptide processed from proopiomelanocortin.