Melanotan II (MT-2) Peptide

Melanotan II (MT-2, MT2) is a synthetic cyclic heptapeptide with the sequence Ac-Nle-cyclo[Asp-His-D-Phe-Arg-Trp-Lys]-NH₂, molecular weight 1,024.18 g/mol, and CAS number 121062-08-6. Developed at the University of Arizona by Dr. Victor Hruby in the late 1980s, MT-2 is a non-selective agonist of melanocortin receptors MC1R through MC5R (except MC2R).

MT-2 was originally designed to induce melanogenesis (tanning) as a potential protective strategy against UV-induced skin cancer. However, its broad melanocortin receptor activity produces multiple effects: skin darkening (MC1R), sexual arousal (MC3R/MC4R), appetite suppression (MC4R), and immune modulation (MC1R/MC3R). This multifunctional profile has made MT-2 one of the most widely studied melanocortin peptides in preclinical research.

MT-2 is the parent compound from which PT-141 (Bremelanotide) was derived by converting the C-terminal amide to a free acid — eliminating melanogenesis while preserving sexual function effects.

Melanotan II is a non-selective melanocortin receptor agonist with activity at multiple receptor subtypes:

MC1R (Melanogenesis): The primary tanning receptor on melanocytes. MT-2 binding activates cAMP/PKA → MITF → tyrosinase cascade, increasing eumelanin production. This produces darkening of skin, hair, and nevi without UV exposure. MC1R activation also provides photoprotective and anti-inflammatory effects in skin.

MC3R (Energy Homeostasis): Expressed in hypothalamus and peripheral tissues. MC3R regulates energy partitioning between lean and fat tissue, feeding behavior, and cardiovascular function. MT-2 activates MC3R, contributing to its anti-obesity and cardiovascular effects in animal models.

MC4R (Appetite/Sexual Function): The critical receptor for both appetite suppression (hypothalamic POMC/CART neuron activation) and sexual arousal (PVN oxytocin/dopamine release). MT-2 is a potent MC4R agonist — this is the receptor responsible for the sexual function effects that led to development of PT-141.

MC5R (Exocrine Glands): Regulates sebaceous gland secretion and pheromone production. Less studied but contributes to MT-2 peripheral effects.

The cyclic structure of MT-2 confers resistance to enzymatic degradation compared to linear alpha-MSH, resulting in a much longer duration of action (hours vs minutes).

Melanotan II (MT-2) is a synthetic, conformationally constrained cyclic heptapeptide analog of the endogenous melanocortin α-MSH (α-melanocyte-stimulating hormone). It was designed at the University of Arizona by Victor Hruby, Mac Hadley, and colleagues in the late 1980s as part of a structure-activity program aimed at producing a metabolically stable, high-affinity melanocortin receptor agonist for use as a research probe in pigmentation and energy-balance biology (Al-Obeidi et al., 1989; Hruby et al., 1995).

Structural rationale: Native α-MSH (Ac-Ser-Tyr-Ser-Met-Glu-His-Phe-Arg-Trp-Gly-Lys-Pro-Val-NH₂) is rapidly degraded in biological matrices. The MT-2 design retains only the conserved His-Phe-Arg-Trp pharmacophore plus flanking residues required for receptor docking, with three critical modifications: (1) Met→Nle to eliminate sulfur oxidation, (2) L-Phe→D-Phe to lock the β-turn geometry and resist chymotryptic cleavage, and (3) cyclization through an Asp-Lys lactam bridge to enforce the bioactive conformation predicted by molecular dynamics. These changes collectively yield a peptide with markedly higher proteolytic stability and broad nanomolar agonism across MC receptors (Hadley & Dorr, 2006).

Physicochemical handling notes: The compound is hygroscopic in lyophilized form and should be equilibrated to room temperature before opening. In aqueous solution it is most stable between pH 5 and 7; strongly alkaline conditions accelerate Asp/Asn-type rearrangements and hydrolysis of the lactam bridge. The Trp residue dominates UV absorbance and provides a convenient handle for concentration determination by A280 measurement. MT-2 is supplied strictly as a reference compound for in vitro and non-human in vivo research applications.

Reconstitution solvent: Bacteriostatic water for injection (0.9% benzyl alcohol) is the standard diluent for laboratory research preparations of MT-2. Sterile water without preservative is acceptable for single-use experiments but provides no antimicrobial protection for multi-day studies. Phosphate-buffered saline (pH 7.2–7.4) is also compatible and may be preferred for receptor-binding or in vitro pharmacology experiments where ionic strength matters. MT-2 is highly water-soluble (>10 mg/mL) owing to the basic Arg, His, and Lys residues that confer a net positive charge at physiological pH.

Reconstitution technique: Direct the diluent stream slowly down the inner wall of the vial rather than onto the lyophilized cake. Avoid vigorous shaking, which can introduce micro-bubbles and shear stress at the air-water interface; gentle swirling for 30–60 seconds is typically sufficient. The cyclic, low-molecular-weight (1,024 Da) structure dissolves rapidly into a clear, colorless solution. Any visible particulate, cloudiness, or yellow tint suggests degradation or contamination and the lot should be re-evaluated by HPLC.

Concentration planning: Common working concentrations:

• 10 mg vial + 1 mL BAC water = 10 mg/mL (0.1 mL = 1 mg)
• 10 mg vial + 2 mL BAC water = 5 mg/mL (0.1 mL = 0.5 mg)
• 20 mg vial + 2 mL BAC water = 10 mg/mL (0.1 mL = 1 mg)
• For nanomolar in vitro work, prepare a 1 mM stock (1.024 mg/mL) and serially dilute in assay buffer containing 0.1% BSA to minimize adsorption.

Photoprotection: Tryptophan at position 9 (within the cyclic ring) carries an indole chromophore that absorbs in the UV-A/UV-B range and can undergo photo-oxidation to N-formylkynurenine and kynurenine derivatives. Reconstituted solution should be stored in amber glass, foil-wrapped vials, or in an opaque refrigerated drawer. Brief room-light exposure during pipetting is acceptable, but extended bench exposure (>1 hour) under fluorescent lighting should be avoided.

Adsorption and labware: Like other cationic peptides, MT-2 can adsorb to glass and untreated plastic surfaces at low concentrations (<1 µM). For receptor pharmacology experiments, low-protein-binding polypropylene tubes and pipette tips are recommended. Adding carrier protein (0.1% BSA) or a non-ionic surfactant (0.01% Tween-20) to dilute working solutions further reduces surface losses.

Stability advantage of the cyclic scaffold: The Asp-Lys lactam bridge introduced by Hruby and colleagues constrains the peptide into the bioactive β-turn conformation and dramatically increases resistance to serum and tissue proteases relative to linear α-MSH or ACTH(1-13) fragments (Sawyer et al., 1980; Al-Obeidi et al., 1989). Combined with the N-acetyl cap, the C-terminal carboxamide, the Nle4 substitution that removes oxidation-prone methionine, and the D-Phe7 inversion that disfavors chymotryptic cleavage, MT-2 exhibits a substantially longer functional half-life in vitro than its native counterparts, which is one reason it has remained a widely used reference melanocortin agonist in pharmacology.

Documentation: Label each reconstituted vial with peptide identity, concentration, diluent, date of reconstitution, and initials. Maintain a reconstitution log linked to the certificate of analysis lot number for traceability.

Lyophilized powder: When stored desiccated at -20°C or below, sealed Melanotan II retains >98% purity for 24+ months in HPLC stability studies. At 2-8°C (standard refrigeration), the lyophilized form remains stable for at least 6 months with negligible degradation. Short-term ambient exposure during transit (up to 30 days at <25°C) is generally well-tolerated due to the inherent stability of the cyclic backbone, though prolonged thermal cycling should be avoided to prevent moisture ingress and hydrolysis.

Reconstituted solution: Following reconstitution in bacteriostatic water (0.9% benzyl alcohol), MT-2 should be stored at 2-8°C and used within approximately 28 days, after which preservative efficacy and peptide integrity may decline. Solutions should be protected from direct light because the indole side chain of tryptophan (Trp9) is susceptible to photo-oxidation, generating kynurenine-type degradants that can be detected by characteristic UV/fluorescence shifts. Aliquoting into amber vials or wrapping vials in foil is recommended for extended cold storage.

Freeze-thaw considerations: Repeated freeze-thaw cycles of reconstituted MT-2 are not recommended. Each cycle introduces mechanical and osmotic stress that can promote aggregation at the lactam bridge region and incremental hydrolysis at the amide bonds. If long-term storage of solution is required, single-use aliquots stored at -20°C or -80°C in low-binding polypropylene tubes are preferable to repeated thawing of a single stock.

Structural basis for stability: The 23-membered lactam macrocycle formed between the Asp side chain and the Lys ε-amine confers conformational rigidity that mimics the bioactive β-turn of α-MSH (Hruby et al., 1987). This constraint, combined with the N-terminal acetyl cap and C-terminal primary amide, blocks the canonical recognition sites of aminopeptidases (which require a free α-amine) and carboxypeptidases (which require a free carboxylate). Substitution of Met4 with norleucine (Nle) further eliminates the methionine sulfur, removing a major oxidation liability seen in native α-MSH and ACTH fragments. The D-Phe7 inversion additionally disfavors chymotryptic cleavage at that position.

Quality control indicators: Researchers should monitor stability by analytical RP-HPLC (C18, gradient acetonitrile/0.1% TFA) for purity and ESI-MS for the expected [M+H]+ at m/z 1025.2. Common degradation signals include the appearance of a +16 Da species (Trp oxidation to oxindolylalanine or N-formylkynurenine), ring-opened linear peptide (hydrolysis of the lactam), and aggregation peaks at higher molecular weight. A loss of the characteristic Trp absorbance at 280 nm relative to the peptide bond absorbance at 220 nm is a sensitive indicator of photodamage.

Handling environment: Maintain laboratory humidity below 40% when opening lyophilized vials to limit hygroscopic uptake. Pre-equilibrate sealed vials to room temperature before opening to prevent condensation that can locally hydrolyze peptide near the stopper. For shipment, validated cold-chain or qualified ambient-stable packaging with temperature loggers is preferred for research material crossing extended transit times.