Peptide MT2, better known as Melanotan II, started life in an Arizona chemistry lab as a tanning project. It didn’t stay one. Within a few years the same molecule had turned up in appetite work, erection physiology and, later, mouse models of autism. That’s what happens when you build a key that fits almost every melanocortin lock. Here’s what the primary papers actually measured, with links so you can check us.
Key findings
- MT2 is a cyclic lactam heptapeptide, Ac-Nle4-Asp5-His6-D-Phe7-Arg8-Trp9-Lys10 α-MSH4-10-NH2 (Dorr 1996).
- Closing the ring into a 23-membered lactam made these analogs about 90 to 100 times more potent than α-MSH in lizard skin assays (Al-Obeidi 1989).
- In a 3-volunteer pilot study, two men showed darker skin a week after the last exposure (Dorr 1996).
- In diet-induced obese rats, final body fat tracked the amount of MT2 given, not prior food restriction (Seeley 2005).
- Case reports tie unregulated MT2 use to rhabdomyolysis and to melanoma, and tested online vials held 43 to 88% of the labeled amount.
Where MT2 came from
The starting point was α-MSH, the 13-residue hormone that tells melanocytes to make pigment. It’s potent but short-lived, so Victor Hruby, Mac Hadley and their colleagues at the University of Arizona went looking for something sturdier.
Their trick was a lactam bridge. In 1989 they tied the side chain of an aspartic or glutamic acid at position 5 to a lysine (or a shorter cousin) at position 10, then tested seven cyclic versions on frog and lizard skin [1]. Ring size mattered a lot. The 23-membered rings were roughly 100 times more potent than α-MSH in the lizard assay, and both 23 and 24-membered rings kept working long after washout. Shrink the ring to 20, 21 or 22 atoms and that lingering activity disappeared.
The Asp5/Lys10 version trimmed to residues 4 to 10 came in at 90 times α-MSH. That compact molecule is MT2. Funny detail: it was judged on lizard skin long before anyone put it near a person.
Structure and what it binds
Dorr’s group spelled out the sequence in their 1996 paper: Ac-Nle4-Asp5-His6-D-Phe7-Arg8-Trp9-Lys10 α-MSH4-10-NH2 [2]. Three design choices stand out. Norleucine replaces methionine, which is prone to oxidation. D-Phe at position 7 swaps in a mirror-image residue that proteases don’t handle well. And the lactam ring locks the His-Phe-Arg-Trp core into a turn.
That core is the shared message of every melanocortin peptide. So MT2 isn’t picky. Papers routinely describe it as a non-selective melanocortin receptor agonist [3], and one research group used it specifically as an MC4R agonist to push on oxytocin signaling [4]. MC1R sits on melanocytes. MC4R lives mostly in the brain and is linked to appetite and energy balance. Hit both at once and you get the odd mix of effects below.
A note on chemistry claims you’ll see online, like a precise synthesis yield. We couldn’t trace those to a paper we could read, so they’re gone from this version.
Pigmentation: the original target
The first human data came from a tiny pilot. Dorr and colleagues ran a single-blind, placebo-controlled study in 3 healthy male volunteers, alternating MT2 and saline days over two weeks [2]. Two of the three had more pigment on the face, upper body and buttocks a week after the last exposure, measured by reflectance and by eye.
They also logged side effects. Mild nausea showed up at most levels tested. One man had grade II sleepiness and fatigue at the highest level. And there was a stretching and yawning pattern that seemed to line up with spontaneous erections, which nobody had set out to study.
Three people is a very small study. Read it as a signal, not a verdict.
The unplanned finding: erection physiology
That yawning observation sent the Arizona team down a new path. In a double-blind, placebo-controlled crossover study of 20 men with erectile dysfunction, MT2 produced erections in 17 of them without sexual stimulation, with a mean of 41 minutes at RigiScan tip rigidity above 80% [3]. Nausea and yawning were common, and 12.9% had severe nausea at the higher level tested.
Older versions of this article put the rigidity time at 38 minutes. The review abstract we checked says 41, so we’ve used that.
Where does the effect come from? Giuliano’s group worked it out in anesthetized rats [5]. MT2 delivered into the paraventricular nucleus of the hypothalamus triggered erectile events. Given systemically, it also boosted responses to cavernous nerve stimulation, and that boost vanished once the lumbar sympathetic chain was removed. MT2 placed directly in penile tissue did nothing. So the action is neural, both central and spinal, not local. This line of work eventually led to bremelanotide, the PT-141 analog of MT2 (we have a PT-141 compound page if you want that side of the story).
Appetite, body fat and the brain
Because MT2 hits MC4R, obesity researchers picked it up early. The most interesting paper here is Seeley’s 2005 study in diet-induced obese rats [6]. Before the 28-day MT2 infusion, rats were slimmed down by different amounts of food restriction. You’d expect the leaner rats to end leaner. They didn’t. Final body fat was set by how much MT2 each group got, and rats coming off restriction actually ate more calories than ad libitum rats on MT2.
The authors’ read: melanocortin signaling defends a body fat set point, and food intake just adjusts to hit it. That’s a cleaner idea than “MT2 suppresses appetite,” and it’s the one we’d keep in mind.
Then there’s the autism work. Minakova and colleagues used a maternal immune activation mouse model, where male offspring show poor social behavior, fewer vocalizations and more repetitive behavior [4]. Seven days of continuous MT2 rescued the social metrics in those mice. Normal mice didn’t change socially, though they lost weight. One mouse model, one lab. Interesting, not settled.
Safety signals from the real world
MT2 never became an approved drug, but it did leak onto the internet. That gave us a set of case reports nobody planned.
| Report | What was described |
|---|---|
| Nelson 2012 [7] | A 39-year-old man took 6 mg of an online product (confirmed MT2 by mass spec). Tachycardia, tremor, rhabdomyolysis (CPK peaked at 17,773 IU/L) and kidney dysfunction followed. |
| Hjuler 2014 [8] | Melanoma found in a 20-year-old woman who had used MT2 alongside sunbeds three months earlier. |
| Breindahl 2015 [9] | Vials from three online sellers labeled 10 mg held 4.32 to 8.84 mg, and two sellers had 4.1 to 5.9% unknown impurities. |
A case report can’t prove cause. Still, the Breindahl data make a point every lab should care about. If the label can be off by half, your results can be too. Identity and purity testing by an independent lab isn’t optional for this peptide.
Handling in the lab
We won’t repeat shelf-life numbers we can’t source. What’s well grounded is general peptide practice: keep the lyophilized powder cold, dry and dark, avoid repeated freeze and thaw once it’s in solution, and check identity by LC-MS. The Breindahl team showed MT2 gives a clean doubly charged ion at m/z 513, which makes confirmation fairly straightforward [9]. Our Melanotan 2 compound page collects the basic data in one spot.
MT2 is a research chemical. It isn’t approved for human use anywhere we know of, and the studies above are reported here as history, not as a template.
References
- Al-Obeidi F, Castrucci AM, Hadley ME, Hruby VJ. Potent and prolonged acting cyclic lactam analogues of alpha-melanotropin: design based on molecular dynamics. J Med Chem. 1989. PMID 2555512
- Dorr RT, Lines R, Levine N, et al. Evaluation of melanotan-II, a superpotent cyclic melanotropic peptide in a pilot phase-I clinical study. Life Sci. 1996. PMID 8637402
- Wessells H, Levine N, Hadley ME, Dorr R, Hruby V. Melanocortin receptor agonists, penile erection, and sexual motivation: human studies with Melanotan II. Int J Impot Res. 2000. PMID 11035391
- Minakova E, Lang J, Medel-Matus JS, et al. Melanotan-II reverses autistic features in a maternal immune activation mouse model of autism. PLoS One. 2019. PMID 30629642
- Giuliano F, Clément P, Droupy S, et al. Melanotan-II: investigation of the inducer and facilitator effects on penile erection in anaesthetized rat. Neuroscience. 2006. PMID 16360286
- Seeley RJ, Burklow ML, Wilmer KA, et al. The effect of the melanocortin agonist, MT-II, on the defended level of body adiposity. Endocrinology. 2005. PMID 15932932
- Nelson ME, Bryant SM, Aks SE. Melanotan II injection resulting in systemic toxicity and rhabdomyolysis. Clin Toxicol (Phila). 2012. PMID 23121206
- Hjuler KF, Lorentzen HF. Melanoma associated with the use of melanotan-II. Dermatology. 2014. PMID 24355990
- Breindahl T, Evans-Brown M, Hindersson P, et al. Identification and characterization by LC-UV-MS/MS of melanotan II skin-tanning products sold illegally on the Internet. Drug Test Anal. 2015. PMID 24771717
Disclaimer: This content is intended for research purposes only and is not meant to constitute medical advice.
This educational material discusses laboratory research concepts only. Loti Labs products are intended strictly for research and development use and are not for human consumption.
