Source comparison
MT-II vs MT-I: Receptor Selectivity and Clinical Implications
Melanotan I (afamelanotide) is a 13-amino-acid peptide with high selectivity for melanocortin-1 receptors. It binds MC1R with nanomolar affinity (Ki ≈ 0.3nM) but shows 1000-fold lower affinity for MC3R and MC4R. This selectivity profile produces robust melanog
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- Melanotan I (afamelanotide) is a 13-amino-acid peptide with high selectivity for melanocortin-1 receptors. It binds MC1R with nanomolar affinity (Ki ≈ 0.3nM) but shows 1000-fold lower affinity for MC3R and MC4R. This selectivity profile produces robust melanogenesis with minimal systemic effects. Afamelanotide is FDA-approved (under the brand name Scenesse) for photoprotection in erythropoietic protoporphyria, a rare disorder where patients experience severe phototoxicity upon sun exposure. The approved dosing regimen is a 16mg subcutaneous implant placed every 60 days, which maintains therapeutic plasma levels of 5–10ng/mL.
- Melanotan II is a truncated 7-amino-acid cyclic peptide with reduced receptor selectivity. It activates MC1R, MC3R, MC4R, and MC5R with roughly equal affinity. MC1R activation drives melanogenesis. MC3R and MC4R activation in the hypothalamus suppress appetite and increase energy expenditure. This is why MT-II users commonly report reduced food intake and mild nausea during the loading phase. MC4R activation also mediates erectile function in males, which is why spontaneous erections are a frequently reported side effect at doses above 0.5mg. MT-II's half-life is approximately 33 minutes after subcutaneous injection, requiring daily dosing to maintain receptor occupancy.
- The dosing difference between these peptides is dramatic. MT-I requires 16mg every 60 days to maintain pigmentation, while MT-II achieves comparable results at 0.5–1mg daily. This 30-fold dose difference reflects both MT-II's reduced molecular weight (allowing greater tissue penetration) and its multi-receptor activation, which amplifies downstream signaling. Users typically follow a loading protocol of 0.25–1mg daily for 7–14 days until desired pigmentation is reached, then switch to a maintenance dose of 0.25–0.5mg 2–3 times weekly.
- At Real Peptides, we manufacture research-grade peptides through small-batch synthesis with amino acid sequencing verified by mass spectrometry. Every batch includes a certificate of analysis confirming purity above 98% and correct molecular weight. Our peptide tools are designed for researchers who need reliable, reproducible compounds for melanocortin pathway studies.