Melanotan II: The Cyclic Peptide Behind PT-141, Not Scenesse

Melanotan II is a cyclic seven-amino-acid analog of alpha-melanocyte-stimulating hormone (α-MSH). University of Arizona chemists built it to darken skin through the melanocortin-1 receptor (MC1R). It also activates MC4R, which is why early volunteers got erections, and why Palatin later carved PT-141 (bremelanotide, Vyleesi) out of the same ring.

TL;DR

• MT-II is a lactam-bridged heptapeptide, Ac-Nle-cyclo[Asp-His-D-Phe-Arg-Trp-Lys]-NH2, about 90 to 100 times as potent as native α-MSH in lizard-skin assays (Al-Obeidi et al., 1989).
• It is not selective. MC1R tans melanocytes. MC4R in the brain sits on sexual-arousal and appetite circuits.
• A 1996 three-person Phase I showed darkening after five low subcutaneous doses, plus nausea, yawning, and erections (Dorr et al., 1996).
• Two small crossover studies found erections versus placebo in men with psychogenic or organic erectile dysfunction (Wessells et al., 1998, 2000).
• Approved cousins exist. Afamelanotide (Scenesse) is the linear 13-mer. Bremelanotide (Vyleesi) is the C-terminal acid of MT-II. Melanotan II itself is not an approved drug.

Educational only. Not medical advice, and not a reason to treat a research vial as Scenesse or Vyleesi.

What is Melanotan II, chemically?

Native α-MSH is 13 amino acids and falls apart fast in blood. The Arizona group locked a seven-residue core into a 23-membered lactam ring between Asp5 and Lys10, swapped in norleucine and D-phenylalanine, and amidated the C-terminus (Al-Obeidi et al., 1989).

That ring is the whole point. The cyclic analog holds the His-D-Phe-Arg-Trp motif in a shape melanocyte receptors like. The lizard-skin assay called Ac-[Nle4,Asp5,D-Phe7,Lys10]-α-MSH4-10-NH2 about 90 to 100 times as potent as native α-MSH.

For how signaling peptides are built, start with Peptides 101. The same His-Phe-Arg-Trp core shows up in setmelanotide and in the α-MSH tail peptide KPV.

Why does one peptide tan skin and also affect sexual function?

Melanocortin receptors are a family. MT-II is a broad agonist, not a precision key.

MC1R on melanocytes raises cyclic AMP, turns on MITF, and shifts pigment toward eumelanin. That is the tanning claim.

MC3R and MC4R in the hypothalamus sit on appetite and sexual-function circuits. Hadley and Dorr later wrote that MT-II was being studied for erectile dysfunction because those central receptors fired in the tanning pilots (Hadley and Dorr, 2006).

Nausea, stretching, and yawning showed up in the same sessions. Those are the peptide hitting more than one receptor at once.

What did the Arizona human studies actually measure?

The human file on MT-II is real. It is also small.

In the Phase I, two of three men had darker face, upper body, and buttock a week after five every-other-day injections stopped. Mild nausea showed up at most dose levels (Dorr et al., 1996).

The erection signal was not a forum rumor. Wessells, Hadley, Hruby, and Dorr took it into placebo-controlled RigiScan studies. In psychogenic ED, 8 of 10 men developed clinically apparent erections on MT-II (Wessells et al., 1998).

In men with organic risk factors, desire scores also rose versus placebo, and 4 of 19 injections brought severe nausea (Wessells et al., 2000). Three people and ten people are pharmacology, not modern Phase 3.

How is Melanotan II different from Scenesse and Vyleesi?

Listings treat Melanotan I, Melanotan II, and PT-141 as three names for a tan. They are three different sequences.

Afamelanotide is Melanotan I in the clinic. FDA approved Scenesse to increase pain-free light exposure in adults with erythropoietic protoporphyria (EPP), a rare phototoxic disease (FDA, NDA 210797). It is an implant placed by a clinician, not a tanning pen.

Bremelanotide is MT-II with the amide swapped for a carboxylic acid. Palatin took that analog into hypoactive sexual desire disorder (HSDD) after the Arizona data. FDA approved Vyleesi on June 21, 2019 for acquired, generalized HSDD in premenopausal women (FDA, NDA 210557).

We covered that drug as a clinician piece: Bremelanotide (Vyleesi / PT-141). Hadley’s 2006 review already named the split: melanotan I for tanning studies, melanotan II for erectile-function studies, and PT-141 as the analog heading toward commercialization (Hadley and Dorr, 2006).

What should enthusiasts actually check?

Identity is a sequence question. A vial labeled MT-2, melanotan, or “tanning peptide” is not Scenesse and it is not Vyleesi. Read the COA the way you would for any research peptide: how to read a peptide COA.

Unlicensed tanning injections are a known dermatology problem. Hjuler and Lorentzen reported a 20-year-old woman who injected MT-II for three to four weeks to boost sunbed tanning and presented with a gluteal melanoma. That is a case report, not proof of causation, and the authors flagged the sunbed coincidence (Hjuler and Lorentzen, 2014).

Darkening of existing moles is expected MC1R pharmacology, and it makes skin exams harder. Licensed melanocortin drugs, when prescribed, live in pharmacy and telehealth channels, which is what the provider directory and price index compare.

Handling education for lyophilized peptides lives on the reconstitution guide and the calculator. Expert voices on this receptor family are collected on experts.

Bottom line: Melanotan II is a real, well-characterized cyclic α-MSH analog with small 1990s human studies behind the tan and the erection signal. The approved drugs that came out of that chemistry are Scenesse and Vyleesi. A research vial is neither.

Educational only. Not medical advice. Not a recommendation to use Melanotan II, afamelanotide, bremelanotide, or any research-grade melanocortin peptide.