Melanotan II (MT-II) and Melanotan I (MT-I) are synthetic analogues of α-melanocyte-stimulating hormone (α-MSH) developed through research into melanocortin signaling and melanogenic activity. Although both compounds are derived from the α-MSH framework, they are chemically distinct peptides with important differences in sequence, structure, receptor pharmacology, research history, and regulatory status.
The most basic difference between them would be their structures. Melanotan I (MT-I), also known as NDP-MSH and associated with the pharmaceutical name afamelanotide, is a linear 13-amino-acid α-MSH analogue, while Melanotan II is a cyclic heptapeptide containing a lactam bridge between the side chains of Asp5 and Lys10. For a closer look, see our overview of the Melanotan II peptide.
What is Melanotan I?
Melanotan I (MT-I) is the synthetic α-MSH analogue [Nle⁴, D-Phe⁷]-α-MSH, in which norleucine replaces methionine at position 4, and D-phenylalanine replaces phenylalanine at position 7. These substitutions were developed as part of efforts to produce potent and more stable melanocortin analogues.
The other alternative name for Melanotan I is NDP-MSH, while the generic name for the above peptide is Afamelanotide. According to the PubChem database, afamelanotide is a 13-amino-acid peptide with the molecular formula C₇₈H₁₁₁N₂₁O₁₉ and a molecular weight of approximately 1,646.8 g/mol.
Importantly, Melanotan I is a linear peptide, not a cyclic peptide. This distinction is essential when comparing it with MT-II.
Afamelanotide subsequently entered pharmaceutical development. The FDA prescribing information identifies SCENESSE (afamelanotide) as a melanocortin-1 receptor (MC1R) agonist indicated to increase pain-free light exposure in adults with a history of phototoxic reactions from erythropoietic protoporphyria (EPP). FDA prescribing information states that SCENESSE received its initial U.S. approval in 2019.
This regulatory status applies to the approved pharmaceutical product and its specific indication. It should not be interpreted as FDA approval of products marketed simply as Melanotan I.
What is Melanotan II?
Melanotan II has a cyclic heptapeptide structure formed by a lactam bridge within the peptide.
The human study conducted by Dorr et al. (1996) characterized MT-II as a lactam-linked peptide with the sequence notation Ac-Nle⁴-Asp⁵-His⁶-D-Phe⁷-Arg⁸-Trp⁹-Lys¹⁰-NH₂. This compound was investigated in a small pilot Phase I study using three healthy male subjects.
Dorr et al. (1996) investigated MT-II as a potent cyclic melanotropic peptide and reported melanotropic activity in their pilot human study. The study also reported pigmentation responses in two participants and pharmacological effects including nausea, somnolence, fatigue, and intermittent spontaneous penile erections. Because the study involved only three volunteers, these findings represent early clinical pharmacology rather than evidence establishing the safety or clinical usefulness of MT-II.
The PubChem record for Melanotan II lists the compound as C₅₀H₆₉N₁₅O₉ with a molecular weight of approximately 1,024.2 g/mol.
Melanotan II vs. Melanotan I: Key Structural Differences
| Feature | Melanotan I / NDP-MSH | Melanotan II |
| Relationship to α-MSH | Synthetic analogue | Synthetic analogue |
| Peptide length | 13 amino acids | 7 residues |
| Overall structure | Linear | Cyclic |
| Important modifications | Nle⁴ and D-Phe⁷ | Nle⁴ and D-Phe⁷ with lactam cyclization |
| Molecular formula | C₇₈H₁₁₁N₂₁O₁₉ | C₅₀H₆₉N₁₅O₉ |
| Approx. molecular weight | 1,646.8 g/mol | 1,024.2 g/mol |
| Common research terminology | NDP-MSH / MT-I | MT-II |
This difference is thus not just limited to peptide length. While Melanotan I retains the long peptide α-MSH backbone, Melanotan II uses a short cyclic peptide structure. These differences are pertinent when scientists study conformation, receptor binding, stability, and activity of the peptides.
How Do Their Melanocortin Receptor Profiles Differ?
Both MT-I/NDP-MSH and MT-II interact with melanocortin receptor systems, but their pharmacological profiles should not be assumed to be identical.
Melanocortin receptors include MC1R, MC2R, MC3R, MC4R, and MC5R, and synthetic α-MSH analogues have been investigated for their interactions with several of these receptor subtypes. Luevano et al. (1996) research on NDP-MSH and related melanocortin analogues has demonstrated that structural modifications can influence receptor affinity, potency, and selectivity.
The conserved melanocortin pharmacophore includes the His-Phe-Arg-Trp region, while other structural features influence how individual analogues interact with receptors. Structural research has specifically examined melanocortin peptide agonists, including NDP-MSH and MT-II, in relation to MC1R.
These results provide insight into why comparison between MT-I and MT-II has scientific significance. Changes in peptide structure can influence receptor binding, selectivity, stability, and pharmacological activity, depending on the specific modification. This is covered further in our guide to the main neuropeptide families.
What Does the Research Show?
Melanotan I and Melanotan II have different research histories.
In a study, Dorr et al. (2000) investigated NDP-MSH and reported increased pigmentation and changes associated with eumelanin production in volunteers. This work contributed to research into synthetic melanocortin analogues and preceded the later pharmaceutical development of afamelanotide.
MT-II was investigated in humans in the earlier Dorr et al. (1996) pilot Phase I study. Two of the three participants demonstrated increased pigmentation, while several pharmacological effects were reported. The very small sample size means the findings should be interpreted within the context of an early-stage experimental study rather than generalized to broader populations.
Is Melanotan II the Same as Afamelanotide?
No. Melanotan II and afamelanotide are different peptide compounds.
Afamelanotide is the pharmaceutical development associated with the NDP-MSH/Melanotan I analogue and is the active ingredient in SCENESSE. The FDA-approved product has a specific indication for adults with EPP.
Melanotan II has a different peptide sequence and a cyclic structure and does not have the same FDA-approved status. Therefore, the regulatory status of afamelanotide should not be transferred to MT-II.
Melanotan II for Laboratory Research
Neuro Peptide lists Melanotan II 10 mg Peptide as a research peptide. The company’s product information identifies the material as intended for laboratory research purposes rather than human or veterinary use.
For researchers evaluating MT-II, important considerations include compound identity, peptide sequence, analytical characterization, batch documentation, purity information, and suitability for the intended research protocol. Do not treat a research material as equivalent to an approved pharmaceutical merely because both compounds are derived from the α-MSH framework. Batch results for our own vials are published on current lot certificates.
FAQs
Q1. What is the main difference between Melanotan II and Melanotan I?
The primary difference is their molecular structure. Melanotan I/NDP-MSH is a linear 13-amino-acid α-MSH analogue, while Melanotan II is a shorter cyclic heptapeptide containing a lactam bridge.
Q2. Is Melanotan I the same as afamelanotide?
Melanotan I is also known as NDP-MSH, and afamelanotide is the pharmaceutical name associated with this peptide. Afamelanotide is the active ingredient of SCENESSE, an FDA-approved product for a specific EPP indication.
Q3. Is Melanotan II a cyclic peptide?
Yes. Melanotan II is a cyclic heptapeptide. Dorr et al. (1996) specifically described it as a lactam-bridged cyclic heptapeptide analogue of α-MSH.
Q4. Is Melanotan I cyclic?
No. Melanotan I/NDP-MSH is a linear 13-amino-acid peptide analogue. Do not confuse it with the cyclic structure of Melanotan II.
Q5. What are the molecular weights of Melanotan I and Melanotan II?
Afamelanotide/Melanotan I has a molecular weight of approximately 1,646.8 g/mol, while Melanotan II has a molecular weight of approximately 1,024.2 g/mol, according to their respective PubChem records.
Q6. Is Melanotan II FDA approved?
No. Melanotan II does not have the FDA-approved status of afamelanotide/SCENESSE. The FDA approval discussed in relation to melanocortin peptides applies specifically to SCENESSE (afamelanotide) and its labeled EPP indication.
Conclusion
Melanotan I and Melanotan II share an α-MSH-derived foundation but are structurally distinct research compounds. Melanotan I/NDP-MSH is a linear 13-amino-acid analogue, whereas Melanotan II is a shorter lactam-bridged cyclic heptapeptide. Their different structures provide a basis for investigating differences in receptor interactions, conformational behavior, and biological activity.
Afamelanotide has progressed into an FDA-approved pharmaceutical product for a specific EPP indication, while Melanotan II remains a research compound without FDA approval. For laboratory researchers, understanding the exact molecular identity, structure, analytical characterization, and regulatory context of each peptide is essential when interpreting research findings or evaluating research materials. We go into more detail in our guide to what to look for when buying Melanotan II.
