melanotan ii mastery course
Unit 10 of 11

MT-I vs MT-II vs PT-141: family comparison

three peptides, three receptor profiles, three completely different regulatory stories

three peptides, three stories

Melanotan I, Melanotan II, and PT-141 all share the α-MSH lineage. They were designed at the same university, by overlapping research teams, using the same parent hormone as their starting scaffold. But their receptor selectivity profiles, clinical development trajectories, and eventual regulatory outcomes diverged completely from that shared starting point.

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Side-by-Side Comparison Tool

Compare Melanotan I, Melanotan II, and PT-141 across structure, receptor binding, approved indications, and risk profiles. Toggle between dimensions to see where these three peptides converge and diverge.

interactive side-by-side comparison tool

key numbers

three peptides from the same parent hormone, three completely different outcomes.

13 aa
afamelanotide (MT-I) -- linear, MC1R-selective
7 aa
melanotan II -- cyclic, non-selective MC1R-MC5R
7 aa
bremelanotide (PT-141) -- cyclic, MC4R-preferring
2 / 3
approved by regulatory agencies (MT-I and PT-141)
Of the three α-MSH derivatives, only afamelanotide (Scenesse, EMA 2014 / FDA 2019) and bremelanotide (Vyleesi, FDA 2019) have regulatory approval. Melanotan II itself was never approved and pharmaceutical development was abandoned in 2012 due to the non-selective receptor profile. All three share the same parent hormone lineage but have fundamentally different safety and efficacy profiles.

key terms

family comparison vocabulary for this unit. tap to expand.

A afamelanotide approved peptide
Also known as melanotan I or Scenesse. A linear 13-amino-acid NDP-MSH analogue with MC1R selectivity. Approved for erythropoietic protoporphyria (EPP) as a 16 mg subcutaneous implant with 60-day controlled release via a PLGA matrix.
B bremelanotide approved peptide
Also known as PT-141 or Vyleesi. A cyclic 7-amino-acid peptide derived from MT-II, redesigned for MC4R selectivity. FDA-approved for hypoactive sexual desire disorder in premenopausal women. Strict dosing limits: max 1.75 mg per dose, max 8 doses per month.
L lactam bridge structural
A covalent bond that cyclizes MT-II and PT-141 by connecting the side chains of aspartic acid and lysine. This constraint locks the pharmacophore (His-Phe-Arg-Trp) in the bioactive conformation, increasing potency and protease resistance compared to linear α-MSH.
E EPP indication
Erythropoietic protoporphyria. A rare genetic disorder causing extreme photosensitivity and painful phototoxic reactions to sunlight. Afamelanotide (Scenesse) is approved specifically for this condition -- it increases eumelanin production to provide photoprotection independent of UV exposure.
S selectivity pharmacology
The ability of a drug to preferentially activate one receptor subtype over others. MT-I is MC1R-selective (pigmentation). PT-141 is MC4R-preferring (sexual function). MT-II is non-selective across MC1R-MC5R. Selectivity is what separates approved drugs from abandoned ones in this family.

three peptides, three regulatory stories

same parent hormone, completely different trajectories.

afamelanotide (Scenesse)
MC1R-selective linear analogue. approved for EPP (EMA 2014, FDA 2019). delivered as a 16 mg PLGA implant with 60-day release. narrow indication, orphan drug status. minimal off-target effects due to MC1R selectivity.
melanotan II
non-selective cyclic pan-agonist (MC1R-MC5R). unapproved globally, development abandoned 2012. available through gray market. pigmentation plus systemic effects: nausea, BP changes, erectile effects, mole darkening, melanoma concern.
bremelanotide (Vyleesi)
MC4R-preferring cyclic analogue derived from MT-II. FDA-approved 2019 for HSDD in women. on-demand SC injection, max 8 doses/month. 40% nausea rate. targets sexual function, not pigmentation. validates the MT-II lineage for selective applications.