Melanotan II (MTII)

Non-selective melanocortin agonist developed at University of Arizona (Hadley/Hruby lab, late 1980s) as a sunless-tanning + photoprotection candidate. Clinical development killed by side effects; never FDA-approved. Now lives entirely in the peptide gray market. Two real effects: tanning (over weeks, requires UV exposure) and libido/erection (within hours). Two principal risks: nausea (very common) and darkening/changing existing moles (the dermatologic concern that matters — complicates melanoma surveillance). For Dylan: SKIP — wrong cost-benefit for a brain-and-MMA priority stack. The MC1-selective sister compound afamelanotide (Scenesse) IS FDA-approved (2019) but only for erythropoietic protoporphyria, an orphan indication.

Plain English: Melanotan II is a stripped-down, cyclized version of α-MSH — the body's natural "make more pigment" hormone. By circling the active core into a stable lactam ring, the Arizona team made a peptide that resists enzymatic chewup and hits all four of the relevant melanocortin receptors at once. MC1R on skin melanocytes flips on the eumelanin (dark pigment) production line. MC4R in the hypothalamus drives appetite suppression and turns up sexual arousal pathways. MC3R contributes to energy balance and sexual motivation. MC5R sits on exocrine glands and immune cells. MTII is non-selective — it lights up all of them, which is exactly why the side-effect profile is messy.

Detailed mechanism:

1. Structure. Cyclic heptapeptide: Ac-Nle-c[Asp-His-D-Phe-Arg-Trp-Lys]-NH2. Designed by Al-Obeidi, Hadley, Dorr, and Hruby at the University of Arizona. They identified the minimal active core of α-MSH (the His-Phe-Arg-Trp pharmacophore at positions 6-9), substituted D-Phe7 (resists peptidases + locks turn conformation), and bridged Asp5–Lys10 with an amide bond to lock the bioactive turn. Result: a small peptide ~1000× more potent than native α-MSH at MC1R with a ~33 hour functional half-life vs. native α-MSH's minutes.

2. Receptor profile. Non-selective full agonist at MC1R, MC3R, MC4R, and MC5R. Negligible MC2R activity (which is good — MC2R is the ACTH/cortisol receptor; hitting it would cause adrenal effects). All five MCRs are Gαs-coupled GPCRs; activation drives cAMP → PKA → CREB → downstream transcriptional effects (in the case of MC1R, the MITF-driven melanogenesis cascade upregulating tyrosinase, TRP1, TRP2, DCT).

3. MC1R → tanning. Melanocyte cAMP rises, MITF transcription factor upregulates, tyrosinase expression climbs, and the melanocyte shifts pigment production from pheomelanin (red/yellow, photochemically reactive, weakly photoprotective) toward eumelanin (brown/black, photoprotective). Critical caveat: MTII drives the capacity for melanogenesis. UV exposure (sunlight or tanning bed) is still the primary trigger that converts that capacity into visible tan. Without UV, MTII alone produces only modest pigmentation — generally darkening of existing pigmented areas (moles, freckles, nipples, perineum) rather than uniform skin darkening.

4. MC4R/MC3R → libido + erection + satiety. Central MC4R in the paraventricular nucleus of the hypothalamus and the medial preoptic area drives erectile function via downstream nitric oxide release in cavernosal nerves; MC4R also drives appetite suppression (this is why MC4R loss-of-function mutations cause severe early-onset obesity, and why MC4R agonists like setmelanotide are approved for syndromic obesity). Sexual arousal effect is centrally mediated — i.e., MTII works whether the underlying ED is psychogenic or organic, distinct from PDE5 inhibitors (sildenafil, tadalafil) which only amplify peripheral signaling.

5. MC5R → exocrine + sebaceous + immune. Less well-characterized. Contributes to sebaceous gland activity (oily skin, acne reports in users) and immune modulation.

6. Pharmacokinetics.

   • Route: Subcutaneous injection (oral inactive — peptide destroyed by gut). Intranasal possible but poorly bioavailable for this peptide.
   • Plasma t½: ~33 hours (vs. native α-MSH ~minutes). The cyclization + D-Phe substitution + N-terminal acetyl + C-terminal amide all combine to defeat peptidase chewup.
   • Onset of subjective effects: Libido/erection within 1-3 hours of SC injection. Nausea within 30-90 min. Pigmentation changes visible at 5-7 days of daily loading; full plateau at 4-6 weeks.
   • Distribution: Reaches CNS (this is how it drives MC4R-mediated sexual arousal and satiety), but concentrations are not well-characterized in humans.