Melanotan-2: Molecular Structure

Molecular Structure and Properties#

Melanotan-2 is a peptide whose molecular structure and properties have been characterized through analytical chemistry and structural biology studies.

Amino Acid Sequence#

Molecular identity and sequence. Melanotan II (MT‑II) is a synthetic cyclic heptapeptide derived from the α‑MSH pharmacophore with the exact sequence Ac‑Nle‑cyclo[Asp‑His‑D‑Phe‑Arg‑Trp‑Lys]‑NH2. The N terminus is acetylated (Ac‑) and the C terminus is amidated (‑NH2). The sequence incorporates norleucine (Nle) in place of Met and D‑phenylalanine (D‑Phe) at the pharmacophore position corresponding to Phe7 of α‑MSH (His‑Phe‑Arg‑Trp) (pqac‑00000000, pqac‑00000001, pqac‑00000002, pqac‑00000003, pqac‑00000004, pqac‑00000005, pqac‑00000007).

Structural features. MT‑II is constrained by a side‑chain‑to‑side‑chain lactam (amide) bridge between the Asp γ‑carboxylate and the Lys ε‑amine, producing a macrocyclic core comprising six residues (i→i+5, Asp5→Lys10) that enforces the His–D‑Phe–Arg–Trp pharmacophore geometry. The D‑Phe substitution and macrocyclization enhance potency and metabolic stability relative to linear α‑MSH analogs. The acetylated N terminus and amidated C terminus further reduce ionization and proteolysis susceptibility (pqac‑00000001, pqac‑00000002, pqac‑00000003, pqac‑00000004, pqac‑00000005, pqac‑00000007).

Molecular weight and formula. Experimental ESI‑MS shows [M+H]+ ≈ 1025 and [M+2H]2+ ≈ 513, consistent with a neutral molecular mass of ~1024.2 Da. One source also lists a molecular formula of C50H69N15O9 for MT‑II (pqac‑00000003, pqac‑00000004, pqac‑00000000).

Physicochemical properties (charge and pI; calculated rationale). Because the Asp and Lys side chains are covalently joined in a neutral lactam and both termini are blocked (Ac‑, ‑NH2), the only ionizable side chains in MT‑II are Arg (guanidinium, pKa ~12.5) and His (imidazole, pKa ~6.0). At pH 7.4, Arg is fully protonated (+1), while His is largely unprotonated (fractional charge ~+0.04), giving an estimated net charge of approximately +1.0 at physiological pH. The isoelectric point is dominated by deprotonation of Arg; a theoretical pI is therefore expected near pH ~12–12.5. These values are calculated from standard peptide pKa assumptions using the verified structural features above; direct experimental pI values were not reported in the retrieved sources (pqac‑00000003, pqac‑00000004, pqac‑00000005, pqac‑00000007).

Charge distribution and ionizable groups. The positive charge at physiological pH is localized primarily on the Arg guanidinium; the His imidazole contributes minimally and is sensitive to microenvironment. Asp and Lys side chains are neutralized by the intramolecular amide of the lactam, and the termini are nonionizable due to acetylation/amidation. The Trp indole, D‑Phe, and Nle side chains are nonionizable in aqueous physiological conditions (pqac‑00000003, pqac‑00000004, pqac‑00000005, pqac‑00000007).

Summary. MT‑II is Ac‑Nle‑c[Asp‑His‑D‑Phe‑Arg‑Trp‑Lys]‑NH2, a six‑residue macrocycle formed by an Asp5–Lys10 side‑chain lactam with D‑Phe7, N‑terminal acetylation, and C‑terminal amidation. It has a measured mass of ~1024.2 Da and, based on its ionizable groups, an estimated net charge of ~+1 at pH 7.4 and a calculated pI near 12–12.5 (pqac‑00000000, pqac‑00000001, pqac‑00000002, pqac‑00000003, pqac‑00000004, pqac‑00000005, pqac‑00000007).

Stability and Formulation#

Overview Melanotan-2 (MT‑II; Ac‑Nle‑c[Asp‑His‑D‑Phe‑Arg‑Trp‑Lys]‑NH2) is a cyclic heptapeptide whose aqueous chemical stability has been characterized in preformulation studies. Degradation in solution follows apparent first‑order kinetics, with a modest temperature dependence, a pH‑rate profile showing optimal stability near mildly acidic conditions, and measurable buffer catalysis. Specific degradation products were chromatographically observed but not structurally assigned in the available sources. Practical formulation guidance emerges from these data.

Kinetics and temperature sensitivity

• First‑order decay across studied conditions; at pH 7.0 (0.02 M phosphate), representative Kobs ≈ 6.9×10−3 h−1 at 60 °C; accelerated studies at 50/60/70 °C yielded Arrhenius behavior with activation energy Ea ≈ 7.5 kcal·mol−1 and pre‑exponential factor A ≈ 1.3×10^3 h−1 (linear fit, r2 ≈ 0.98).
• Arrhenius extrapolation to 25 °C gives Kobs ≈ 3.9×10−3 h−1 and t90 ≈ 26.9 h (≈1.1 days) in aqueous phosphate buffer; use extrapolation cautiously for peptides, though here the mechanism appeared consistent over 50–70 °C.

pH stability profile

• Aqueous pH‑rate profile (measured at 60 °C across pH 2.0–9.5) shows maximum stability near pH ≈ 5.0. Empirical rate law indicates base‑catalyzed degradation contributes more than acid‑catalyzed: Kobs ≈ 0.015[H+]−0.102 + 0.047[OH−]0.127; the water term was negligible.
• Practical implication: avoid high pH; mildly acidic formulations near pH ~5 minimize degradation.

Buffer and ionic strength effects

• Phosphate buffer catalyzes degradation (general acid/base catalysis). Increasing phosphate concentration (e.g., 0.02 → 0.10 → 0.50 M) accelerates decay; rate models include buffer‑species terms (e.g., HPO4^2−). Conversely, ionic strength per se had negligible effect (I ≈ 0.15 vs controls).

Degradation pathways and products

• HPLC chromatograms after prolonged storage show multiple degradant peaks; however, specific chemical identities were not assigned in these reports. General peptide pathways applicable to MT‑II—and likely contributors to the observed pH‑ and buffer‑dependent kinetics—include hydrolysis, imide/succinimide formation with subsequent deamidation/isomerization, oxidation, and photodecomposition. Residue‑level assignments (e.g., Asp isomerization, Trp/Nle oxidation) were not delineated in these sources.

Proteolytic and gastric stability (context for formulation)

• In simulated gastric fluid (USP; pepsin, pH ~1.2, 37 °C), MT‑II is relatively stable to HCl; pepsin increases degradation, yet >90% of peptide remains over typical gastric emptying times in reported tests. MT‑II is described as more resistant to enzymatic inactivation than MT‑I in qualitative comparisons.

Formulation considerations

• pH: Target ~5.0 to maximize chemical stability; avoid basic pH where hydroxide catalysis accelerates degradation.
• Buffering: Use the minimum effective phosphate concentration to limit general buffer catalysis; control ionic strength (~0.15 with inert salts).
• Containers/adsorption: Use polypropylene/plastic rather than glass to mitigate adsorption losses; at high pH, measure promptly to avoid precipitation.
• Storage and use period: For aqueous stocks, refrigeration (4 °C) is recommended; prepare fresh weekly for concentrated stocks and use diluted aqueous preparations within ~24 h based on t90 projections and observed long‑term degradants even at 4 °C.
• Lyophilization/light: The available sources did not report lyophilization protocols or photostability studies specific to MT‑II; general peptide risks of photodegradation are noted, but no MT‑II‑specific data were identified here.

Concise summary table

Conclusions MT‑II in aqueous solution degrades with apparent first‑order kinetics and modest Arrhenius temperature dependence (Ea ≈ 7.5 kcal·mol−1). Stability is maximized near pH ~5, with hydroxide‑catalyzed pathways dominating at higher pH and additional general acid/base catalysis from phosphate buffers. Ionic strength shows little direct effect. Multiple degradants are detectable chromatographically during prolonged storage, but residue‑specific mechanisms were not assigned in these reports. Formulations should minimize phosphate concentration, target mildly acidic pH, avoid glass containers, refrigerate aqueous solutions, and limit in‑use time; explicit lyophilization and photostability data for MT‑II were not found in these sources.

Pharmacokinetics#

Summary of available evidence. Quantitative pharmacokinetic (PK) data for MT‑II are published primarily from animal studies; human studies of MT‑II have focused on pharmacodynamic responses and dosing, and do not report serum PK parameters. Rat studies provide IV disposition parameters and an estimate of intestinal bioavailability; human subcutaneous studies report timing of erectile responses but no Cmax/Tmax/AUC or clearance values. Related data for the deaminated MT‑II derivative bremelanotide (PT‑141) exist, but these pertain to a distinct compound and are not directly transferable to MT‑II.

Absorption.

• Intravenous (reference): In rats given MT‑II 0.3 mg/kg IV, biphasic plasma decline was observed; absorption phase is not applicable to IV dosing (serves as 100% bioavailability reference).
• Intestinal (in‑situ jejunal, anesthetized rat): After 6.76 mg/kg in‑situ jejunal dosing, Tmax ≈ 12 h and Cmax ≈ 860 ng/mL over a 60‑min window, with relative bioavailability ≈ 4.6% versus 0.3 mg/kg IV. These data indicate slow, limited enteral absorption.
• Human subcutaneous context: In controlled clinical studies of subcutaneous MT‑II at 0.025–0.157 mg/kg, erections occurred with onset 15–270 min; however, no quantitative plasma PK (Tmax/Cmax/AUC) were reported.

Distribution.

• Rat IV (0.3 mg/kg): The steady‑state volume of distribution Vss was approximately 0.5 ± 0.1 L/kg by HPLC assay (0.2 ± 0.02 L/kg by bioassay), exceeding blood volume and indicating appreciable extravascular distribution.
• No human Vss data for MT‑II were found in retrieved sources.

Metabolism.

• Rat: The disposition profile and assay differences suggest proteolytic degradation contributes to elimination; chemical cyclization and non‑natural residues in MT‑II confer improved protease stability relative to linear analogs.
• Human: Specific biotransformation pathways for MT‑II were not reported in the available clinical literature.

Elimination and clearance.

• Rat IV (0.3 mg/kg): Systemic clearance CL ≈ 0.3 ± 0.1 L·kg−1·h−1 (about 1.5 mL·min−1 in absolute terms), described as low extraction with restrictive clearance.
• Route(s) of excretion were not delineated in the retrieved rat or human reports.

Half‑life.

• Rat IV (0.3 mg/kg): Distribution (alpha) half‑life ~15 min; terminal (beta) half‑life ~1.5 ± 0.5 h by HPLC (bioassay estimated ~0.5 ± 0.1 h; authors judged HPLC‑derived t1/2 more reliable).
• Human: No plasma half‑life values for MT‑II were reported in the retrieved clinical literature.

Bioavailability.

• Absolute bioavailability (IV): 100% by definition for the IV reference in rats.
• Enteral (in‑situ jejunal, rat): Relative bioavailability ≈ 4.6% compared with IV, indicating very low intestinal availability.
• Human subcutaneous and intranasal: No absolute bioavailability values for MT‑II were identified in retrieved human studies; clinical reports document effective SC dosing without PK quantitation.

Embedded data table. The following table consolidates the quantitative values and key gaps.

Contextual note on related peptide. Bremelanotide (PT‑141), a deaminated derivative of MT‑II, shows subcutaneous median Tmax ~0.5 h and mean terminal half‑life ~1.9–2.7 h in humans, but these PK metrics apply to PT‑141, not MT‑II. They are included only to contextualize the absence of human MT‑II PK; direct extrapolation is inappropriate.

Evidence limitations. Human quantitative PK parameters (Cmax, Tmax, AUC, clearance, Vss, bioavailability) for MT‑II were not reported in the retrieved clinical literature; available human studies focused on pharmacodynamic endpoints after SC dosing. Consequently, animal IV and intestinal data presently provide the clearest quantitative PK for MT‑II.

Related Reading#

• Melanotan-2 overview and research guide
• Peptides similar to Melanotan-2
• Melanotan-2 research evidence