Key findings
- Varying the lactam ring size across a synthesised series showed a sharp optimum: the 23-membered ring analogues were 100 times as potent as the parent hormone in the lizard skin assay, and both larger and smaller rings were markedly less potent. [1]
- Side-chain topology carries the binding: replacing phenylalanine or tryptophan with alanine gave compounds inactive at micromolar concentrations, replacing arginine gave a hundredfold loss, and replacing histidine changed almost nothing. [2]
- The acetyl-norleucine segment that sits outside the lactam ring has virtually no effect on binding or activation at human melanocortin receptor 4 while being essential for full potency at receptor 5, which is how selective analogues were obtained. [3]
- Replacing the lactam cyclisation with xylene-derived thioether linkers gave affinities spanning low nanomolar to sub-micromolar and produced one compound with marked functional selectivity for human melanocortin receptor 1, unlike the unselective parent. [4]
Identity and structure
- Sequence
- Ac-Nle-c[Asp-His-D-Phe-Arg-Trp-Lys]-NH2, cyclised through a lactam bridge between the aspartate and lysine side chains [5][6]
- Ring size
- The cyclic melanotropins of this series with 23-membered rings were the most potent in the cited bioassays, and changing the ring size in either direction diminished potency [1]
- Parent hormone
- Derived from alpha-melanotropin, Ac-Ser-Tyr-Ser-Met-Glu-His-Phe-Arg-Trp-Gly-Lys-Pro-Val-NH2, by way of the superpotent Nle4 and D-Phe7 linear analogue [1]
- Receptor profile in cited work
- Described as a potent and unselective agonist across the human melanocortin receptors, with selectivity obtained only by modifying the template [3][4]
- Form as supplied
- Sterile lyophilized powder
Mechanism as studied
The cyclisation is the point of the molecule. Seven cyclic lactam analogues of the alpha-MSH 4 to 13 and 4 to 10 fragments were prepared with different bridging residues, giving rings of 20 to 24 members, and their potency in lizard and frog skin bioassays tracked ring size with a clear optimum at 23. The 23 and 24 member rings also showed prolonged residual activity that the smaller rings did not, so conformational constraint rather than composition alone drives the result. [1]
Within that ring the pharmacophore is narrow. Alanine scanning left histidine dispensable, cost a hundredfold at arginine and abolished activity at phenylalanine and tryptophan, and the retro, enantio and retro-enantio analogues all dropped sharply, which the authors read as a crucial role for side-chain topology and a lesser one for the backbone. NMR pointed to phenylalanine and arginine in hydrogen bonds stabilising bent backbone conformations. [2]
Receptor selectivity has been approached from two directions in the cited work: by constraining the ring contents, where bulky beta-substituted prolines at position 6 lost affinity at receptors 3 and 4 faster than at receptor 5, and by replacing the lactam with other linkers, where enhanced sampling simulations connected the linker choice to conformational behaviour and to the resulting receptor 1 affinity. [6][4]
Research findings
Ex vivo
- System
- Frog and lizard skin bioassay preparations, testing seven cyclic lactam analogues of the alpha-MSH 4 to 13 and 4 to 10 fragments with lysine, ornithine, diaminobutyric acid or diaminopropionic acid as the bridging residue
- Measured
- Melanotropic potency of each analogue relative to the parent hormone in both species, as a function of lactam ring size, and residual activity after washout
- Reported
- In the lizard assay relative potencies against the parent hormone ranged from 5 to 100 across the seven analogues, with the two aspartate-bridged lysine analogues highest at 100 and 90. The authors report that the 23-membered ring analogues were 100-fold more potent than the parent hormone and that increasing or decreasing the ring size from 23 diminished potency. Similar ordering held in the frog assay at much lower absolute potency, giving selectivity for lizard over frog melanocyte receptors, and the 23 and 24 member ring analogues showed prolonged residual activity while the 20, 21 and 22 member systems did not. [1]
In vitro
- System
- Cells expressing human melanocortin receptors 3, 4 and 5, testing alanine-substituted analogues and the retro, enantio and retro-enantio analogues of the cyclic peptide prepared by solid phase synthesis
- Measured
- Binding affinity and agonist potency at each receptor for every analogue, and backbone conformation by nuclear magnetic resonance
- Reported
- Replacing histidine with alanine left affinity and potency similar to the parent cyclic peptide, replacing arginine cost a hundredfold, and replacing phenylalanine or tryptophan gave compounds inactive at micromolar concentrations. The retro, enantio and retro-enantio analogues all lost substantial potency, and NMR suggested phenylalanine and arginine participate in hydrogen bonds stabilising bent backbone conformations. [2]
In vitro
- System
- Cells expressing human melanocortin receptors 3, 4 and 5, testing proline-substituted analogues and analogues with the acetyl-norleucine segment outside the lactam ring replaced by flexible, hydrophobic, hydrophilic or proline-like substituents
- Measured
- Binding affinity and agonist potency at each of the three receptors for every analogue
- Reported
- Replacing norleucine at position 4 with proline gave affinity and potency at receptor 4 similar to the parent cyclic peptide but about 400-fold lower potency at receptor 5 and about 20-fold lower at receptor 3. Analogues with other N-terminal substituents behaved similarly at receptor 4 and markedly worse at receptor 5, showing the segment outside the ring is dispensable at receptor 4 and essential at receptor 5. [3]
In vitro
- System
- Cells expressing human melanocortin receptors 3, 4 and 5, testing beta-substituted proline analogues incorporated into the cyclic template at the histidine 6 and D-phenylalanine 7 positions
- Measured
- Binding affinities at the three receptors as substituent bulk increased, and backbone and side-chain conformation by low mode conformational search with an all-atom force field
- Reported
- Affinities generally fell as the steric bulk of the para substituent increased, with a 23-fold loss at receptor 3, 17-fold at receptor 4 and 8-fold at receptor 5, so selectivity for receptor 5 rose. Substituting the D-phenylalanine at position 7 with a phenylproline cost three to five orders of magnitude at all three receptors, and modelling attributed the losses at receptors 3 and 4 to topographical change. [6]
In vitro
- System
- Analogues in which the lactam cyclisation of the cyclic peptide was replaced by different xylene-derived thioether linkers using a chemical linkage to scaffolds strategy
- Measured
- Binding affinities at the human melanocortin receptors, functional selectivity of each analogue, conformational behaviour by enhanced sampling molecular dynamics, and a model of the selective analogue bound to receptor 1
- Reported
- The new peptides showed binding affinities from the low nanomolar to the sub-micromolar range, correlating with the linker used. Unlike the unselective parent, one compound showed remarkable functional selectivity for human melanocortin receptor 1, and the simulations connected the cyclisation strategy to conformational behaviour and to the measured receptor 1 affinity. [4]
Preclinical in vivo
- System
- Frogs, comparing the cyclic peptide against an analogue in which the arginine at position 8 was replaced by lysine and a glycine added at position 10, enlarging the cyclic region
- Measured
- Pigment granule movement within chromatophores and the duration of the melanotropic response for each peptide
- Reported
- The modified analogue acted as a superpotent agonist with melanotropic activity similar to the parent cyclic peptide, and also showed ultraprolonged melanotropic activity, which the authors offer as a tool where sustained activity is wanted. [5]
Preclinical in vivo
- System
- Zebrafish larvae, comparing the cyclic peptide against human alpha-melanocyte stimulating hormone and against naturally occurring variants from several fish species
- Measured
- Degree of melanosome dispersion in the larvae under each peptide
- Reported
- With the human form as standard, the cyclic peptide showed a higher capacity for melanophore dispersion, and the assay separated the fish-derived variants from one another by the degree of dispersion they produced. [7]
Handling for in-vitro work
- Cyclisation in cited synthesis
- The lactam bridge between the aspartate and lysine side chains was formed either in solution or on a solid phase support, and the ring, not the linear sequence, defines the material [1]
- Assay medium in cited work
- Tested in aqueous skin bath preparations and in receptor binding and reporter assays on cultured cells expressing individual human melanocortin receptors [1][2]
- Storage
- Lyophilized at -20 °C, dark and dry; reconstituted aliquots kept cold and used promptly
Open questions
- Ring size, bridging residue and the residues inside the ring each changed potency by one to several orders of magnitude in the cited work, so a listing that gives only a trade name does not fix which analogue was supplied.
- The amphibian and reptilian skin assays that established the potency series and the human receptor assays that established selectivity are different systems, and the cited work reports species differences within the skin assays themselves.
- The parent cyclic peptide is unselective across the human melanocortin receptors in the cited work, so a result at one receptor does not describe its behaviour at the others.
- No cited paper reports an identity or purity method for material sold under this name, so analytical confirmation for a supplied lot is unaddressed here.
Clinical and case literature attached to this name concerns unregulated finished preparations rather than a research material, and is out of scope for this profile.
Lot records
Check the record for the exact material you order. A published paper and a batch certificate answer different questions.
- RV-24-0019-1 ↗Melanotan II · 99.20% HPLC2026-09-16
References
- Al-Obeidi F, Castrucci AM, Hadley ME, et al. Potent and prolonged acting cyclic lactam analogues of alpha-melanotropin: design based on molecular dynamics. Journal of medicinal chemistry. 1989.
- Bednarek MA, Silva MV, Arison B, et al. Structure-function studies on the cyclic peptide MT-II, lactam derivative of alpha-melanotropin. Peptides. 1999.
- Bednarek MA, Macneil T, Kalyani RN, et al. Analogs of MTII, lactam derivatives of alpha-melanotropin, modified at the N-terminus, and their selectivity at human melanocortin receptors 3, 4, and 5. Biochemical and biophysical research communications. 1999.
- Tomassi S, Dimmito MP, Cai M, et al. CLIPSing Melanotan-II to Discover Multiple Functionally Selective hMCR Agonists. Journal of medicinal chemistry. 2022.
- Gao L, Yu Z, Meng D, et al. Analogue of Melanotan II (MTII): A Novel Melanotropin with Superpotent Action on Frog Skin. Protein and peptide letters. 2015.
- Cai M, Cai C, Mayorov AV, et al. Biological and conformational study of beta-substituted prolines in MT-II template: steric effects leading to human MC5 receptor selectivity. The journal of peptide research : official journal of the American Peptide Society. 2004.
- Hong TI, Hwang KS, Choi TI, et al. Zebrafish Bioassay for Screening Therapeutic Candidates Based on Melanotrophic Activity. International journal of molecular sciences. 2021.
Publication records fetched from PubMed on 2026-09-20. Profile text reviewed 2026-09-20.