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Research Catalog

Compound monograph

Melanotan II

Melanotan II is a synthetic cyclic heptapeptide analog of alpha-melanocyte-stimulating hormone, closed by a lactam bridge between an aspartate and a lysine side chain.

Published: 2026-09-13Literature/identifier verification: 2026-08-23
Melanotan-IIMT-IIMT-2Cyclic melanocortin analogs

01

Identity & nomenclature

Melanotan II is a synthetic cyclic heptapeptide analog of alpha-melanocyte-stimulating hormone, closed by a lactam bridge between an aspartate and a lysine side chain.

The unqualified word melanotan is not a usable identity field. Melanotan II is this cyclic lactam analog; Melanotan I is afamelanotide, a linear thirteen-residue peptide with a different formula and mass. The two are distinct molecules that share a naming convention and nothing else.

Declared aliases and development codes: Melanotan-II, MT-II, MT-2.

02

Molecular properties

Molecular formulaC50H69N15O9
Molecular mass1024.18 Da
CAS Registry Number121062-08-6
PubChem CID92432
UNIIUPF5CJ93X7
Three-letter sequenceAc–Nle–Asp–His–D-Phe–Arg–Trp–Lys–NH2

A linear one-letter string cannot describe this molecule and none is published here. The peptide is cyclized through a lactam bridge between the aspartate side-chain carboxyl and the lysine side-chain amine, and carries a norleucine substitution and a D-phenylalanine. Written as a plain seven-residue sequence it would denote a different, linear compound.

03

Structural characteristics

Structure-activity work on the lactam scaffold reports that replacing the phenylalanine or the tryptophan with alanine abolished measurable activity at human MC3, MC4 and MC5 receptors, while replacing the histidine did not, and that substituting the arginine reduced potency roughly a hundredfold. Retro, enantio and retro-enantio analogs all lost substantial potency, which the source literature attributes primarily to side-chain topology rather than backbone arrangement. Separate work replacing the lactam cyclization with xylene-derived thioether linkers reports that the linker chemistry itself correlates with receptor affinity, indicating the ring is a determinant of the pharmacology rather than a passive constraint.

  • Side-chain to side-chain lactam bridge, Asp to Lys
  • Norleucine substitution
  • D-phenylalanine substitution

Terminal features: N-terminal acetylation, C-terminal amide.

04

Molecular targets & mechanisms

Melanocortin receptors

MC1R · MC2R · MC3R · MC4R · MC5R · MCR

A family of five G protein-coupled receptors, MC1R through MC5R. ACTH recognition and signaling at MC2R depend on the accessory protein MRAP. Native hormones, peptide analogs and nonpeptide ligands differ in receptor-subtype recognition and functional direction; binding affinity, agonism and antagonism must be interpreted separately in the experimental system studied.

05

Analytical considerations

The identity problem specific to this compound is its relationship to PT-141, which differs only in that the C-terminal amide is a free acid. The monoisotopic masses are 1023.54027 for Melanotan II and 1024.52428 for PT-141, a difference of 0.98402 Da. That is close to, but not, one dalton — and the consequence is sharper than the round number suggests. The first carbon-13 isotope peak of Melanotan II falls at 1024.54363, only 0.0194 Da from the monoisotopic ion of PT-141. Separating those two requires a resolving power near 53,000, and the requirement is the same at the doubly charged state. A nominal-mass measurement, or a high-resolution measurement at routine settings, cannot distinguish PT-141 from the isotope envelope of Melanotan II. A certificate reporting a mass near 1024 together with an HPLC purity figure does not establish which of the two compounds is present.

06

Verified bibliography

  1. Structure-function studies on the cyclic peptide MT-II, lactam derivative of alpha-melanotropin.

    Bednarek MA, Silva MV, Arison B, MacNeil T, Kalyani RN, Huang RR, Weinberg DH

    Peptides · 1999 · Journal Article

    current
    PMID 10447101DOI 10.1016/s0196-9781(99)00048-0Melanotan IIMC1R
  2. CLIPSing Melanotan-II to Discover Multiple Functionally Selective hMCR Agonists.

    Tomassi S, Dimmito MP, Cai M, D'Aniello A, Del Bene A, Messere A, Liu Z, Zhu T, Hruby VJ, Stefanucci A, Cosconati S, Mollica A, Di Maro S

    Journal of medicinal chemistry · 2022-02-21 · Journal Article

    current
    PMID 35188390DOI 10.1021/acs.jmedchem.1c01848Melanotan IIMC1R
  3. Goldilocks-Inspired Design of Mid-Size Macrocycles for Selective Targeting of Human Melanocortin Receptors.

    Merlino F, Jia L, Boccino I, Carotenuto A, Tammaro F, Santoro F, Thompson PE, Grieco P

    Journal of medicinal chemistry · 2026-08-13 · Journal Article

    current
    PMID 42340853PMC13492293DOI 10.1021/acs.jmedchem.6c01074Melanotan IIPT-141 (Bremelanotide)MC1R
  4. Dissection of the anti-inflammatory effect of the core and C-terminal (KPV) alpha-melanocyte-stimulating hormone peptides.

    Getting SJ, Schiöth HB, Perretti M

    The Journal of pharmacology and experimental therapeutics · 2003-05-15 · Journal Article

    current
    PMID 12750433DOI 10.1124/jpet.103.051623alpha-MSHSHU9119KPVMelanotan IIMC1R

10

Methodology & citation verification

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