What it is
Melanotan II is a synthetic cyclic heptapeptide analog of α-melanocyte-stimulating hormone (α-MSH), sequence Ac-Nle-cyclo[Asp-His-D-Phe-Arg-Trp-Lys]-NH2 (MW 1024.18 Da, CAS 121062-08-6). The lactam bridge between Asp5 and Lys10, the D-Phe7 substitution, and the Nle4-for-Met4 replacement together confer ~33 h plasma half-life in mammals versus minutes for native α-MSH, and give it 100–1000× the potency of α-MSH at MC1R (Hadley 1998; Dorr 1996). It was first synthesized and characterized at Victor Hruby's and Mac Hadley's groups at the University of Arizona in the late 1980s and early 1990s, and the core His-D-Phe-Arg-Trp pharmacophore is preserved in the two descendants that later reached FDA approval — PT-141 (bremelanotide) and setmelanotide.
In plain English
Melanotan II is a lab-made loop-shaped chain of 7 amino acids. It's a souped-up version of a natural body hormone called α-MSH (alpha-MSH), which tells skin cells to make dark pigment. The body's own α-MSH only lasts a few minutes before it's broken down; MT-II's chemistry changes make it last about 33 hours and make it 100 to 1,000 times more powerful. Researchers at the University of Arizona first made it in the late 1980s and early 1990s. Two drugs later derived from MT-II were eventually FDA-approved — PT-141 (for female low sexual desire, 2019) and setmelanotide (for a rare genetic obesity, 2020) — but MT-II itself was never approved.
How it works
- 01
Non-selective agonism across MC1R / MC3R / MC4R / MC5R
Hadley and Hruby's characterization work (Hadley 1998, Life Sci; Cone 2006, Endocr Rev) established MT-II as a high-affinity pan-agonist with Ki values of 0.67 nM at MC1R, 6.6 nM at MC4R, 34 nM at MC3R, and 46 nM at MC5R. It has no meaningful activity at MC2R (the ACTH receptor). All five melanocortin receptors are Gs-coupled class-A GPCRs, so agonism produces adenylyl-cyclase activation, cAMP elevation, and PKA-driven transcriptional changes. Non-selectivity is the core reason MT-II was never advanced as a drug: every on-target indication is coupled to at least one off-target pharmacologic effect from a different MC subtype.
In plain English
It hits four receivers at once — not just the tanning one
Your body has 5 "melanocortin receivers." MT-II strongly triggers 4 of them: MC1R (skin pigment), MC4R (brain — appetite, sex, blood pressure), MC3R (energy balance), and MC5R (oil glands). It doesn't hit MC2R (stress-hormone release). Because it turns on all 4 at once, you can't pick and choose — if you use it to tan (MC1R), you're also going to get the appetite, sex, blood-pressure, and oil-gland effects whether you want them or not. This is the main reason MT-II was dropped as a drug candidate: researchers built cleaner cousin drugs that hit only one receiver.
- 02
MC1R-mediated melanogenesis
At melanocyte MC1R, cAMP elevation activates microphthalmia-associated transcription factor (MITF), upregulating tyrosinase, TRP-1, and TRP-2/DCT (reviewed in D'Orazio 2006, Nature; Abdel-Malek 2014, Photochem Photobiol). The result is preferential eumelanin synthesis and enhanced melanosome transfer to keratinocytes — visible skin darkening without UV exposure. MT-II bypasses the p53-POMC-α-MSH axis that UV normally triggers. Importantly, the same MC1R-cAMP-MITF signaling is implicated in melanocyte proliferation and, under chronic superphysiologic activation, is a theoretical concern for nevus and melanoma biology (Böhm 2025, JEADV).
In plain English
How it tans skin without sun
In your skin, pigment cells (melanocytes) have the MC1R receiver on them. Normally, UV from the sun triggers a signal chain that ends with pigment getting made. MT-II skips the sun step and turns on the same pigment-making machinery directly. The skin gets darker even without any UV. There's a concern with this: the same signal MT-II uses to make pigment can also tell pigment cells to multiply. Chronic over-triggering of this same path is a theoretical worry for mole growth and melanoma risk.
- 03
Central MC4R agonism — appetite, sexual behavior, cardiovascular tone
MT-II is lipophilic enough to cross the blood-brain barrier and activate MC4R in the paraventricular nucleus, lateral hypothalamus, and brainstem. In rodents this produces dose-dependent hypophagia, reduced weight gain in DIO models, increased sympathetic tone (BP and HR elevation), and penile erection / lordosis behavior (Van der Ploeg 2002, PNAS; Wessells 2000, Urology). Co-administration of the MC4R antagonist SHU9119 abolishes the anorectic and erectile effects. The same MC4R pharmacology drove the design of PT-141 (bremelanotide) for sexual dysfunction and of setmelanotide for MC4R-pathway monogenic obesity.
In plain English
It also affects the brain — hunger, sex, blood pressure
MT-II crosses into the brain and turns on the MC4R receiver in several areas. In rodents this causes: less eating, less weight gain, higher blood pressure and heart rate, and unwanted erections / sexual behavior changes. Blocking MC4R specifically stops all of these effects — which proves it's MC4R that causes them. The same MC4R pharmacology is the basis for the approved drugs PT-141 (sex-desire drug) and setmelanotide (obesity drug).
- 04
MC3R, MC5R, and the side-effect tail
MC3R activation in the hypothalamus contributes to energy-expenditure effects that cannot cleanly be separated from MC4R effects in MT-II pharmacology. MC5R activation drives sebaceous gland and exocrine secretion and is a likely contributor to the sebum / acne / darkening-of-moles complaints that appear repeatedly in case reports. Because MT-II hits all four subtypes simultaneously, any attempt to dose for one receptor's effect (e.g., tanning via MC1R) unavoidably produces MC3R/MC4R/MC5R activation at the same exposure. This is the pharmacologic reason the molecule was abandoned as a drug candidate in favor of subtype-selective successors.
In plain English
Other receivers cause side effects (oily skin, darker moles)
MC3R adds to the energy-balance effects in the brain — you can't cleanly separate it from MC4R. MC5R tells oil glands in the skin to produce more oil, which likely explains the acne, oily skin, and rapid darkening of existing moles that MT-II users repeatedly report. Because the drug hits all 4 receivers at the same time, you can't just "dose to tan" without also getting all the other effects. This is exactly why drug companies dropped MT-II and built cleaner drugs.
- 05
Pharmacokinetics and what is NOT known in humans
Lan 1994 (J Pharm Sci) reported the only published preformulation work — 4.6% oral bioavailability in rats, ~33 h half-life, first-order degradation with maximum stability at pH ~5. No human PK study has been published. The Dorr 1996 pilot (n=3, single-blind placebo-controlled, 0.01 mg/kg SC on alternating days for 2 weeks) is the only peer-reviewed human exposure record. There is no published multi-dose safety, PK, or biomarker data in any contemporary adult cohort. All human use outside Dorr 1996 is off-label, unmonitored, and relies on research-chemical-grade material of variable purity.
In plain English
How the body handles it — mostly untested in humans
A 1994 rat study showed MT-II lasts about 33 hours in the body and is poorly absorbed by mouth (under 5%). That's the only real pharmacokinetic data ever published. No published study has ever measured how the drug behaves in the human body. The only real human study (1996, just 3 men, 2 weeks) gave 0.01 mg/kg under the skin on alternating days. Every other human exposure since then — there are thousands of users — has been unmonitored, done with research-chemical-grade material that varies in purity, and is invisible to published medicine.