Melanotan-1: Molecular Structure

Molecular Structure and Properties#

Melanotan-1 is a peptide whose molecular structure and properties have been characterized through analytical chemistry and structural biology studies.

Amino Acid Sequence#

Overview Afamelanotide (Melanotan‑1; NDP‑α‑MSH) is a linear, 13‑residue α‑MSH analogue engineered for enhanced potency and stability at melanocortin receptors, particularly MC1R. Its sequence retains the core HFRW pharmacophore and incorporates specific substitutions and terminal caps to resist proteolysis and oxidation.

Amino‑acid sequence and chemical modifications • Primary structure: Ac‑Ser‑Tyr‑Ser‑Nle‑Glu‑His‑D‑Phe‑Arg‑Trp‑Gly‑Lys‑Pro‑Val‑NH2. This explicitly includes N‑terminal acetylation (Ac‑) and C‑terminal amidation (‑NH2). Relative to native α‑MSH (Ac‑SYSMEHFRWGKPV‑NH2), Met4→norleucine (Nle4) and Phe7→D‑Phe7 are substituted while preserving the HFRW core motif (His‑Phe‑Arg‑Trp).

Physicochemical properties • Ionizable residues and charge determinants: With both termini neutralized by acetylation and amidation, the ionizable side chains are Glu (acidic), His (titratable, pKa ~6), Arg (basic), Lys (basic), and Tyr (phenolic, pKa ~10). These groups govern charge across pH; terminal capping removes the otherwise titratable α‑NH3+ and α‑COO− groups. • Net charge at physiological pH: At pH ~7.4, Arg and Lys are protonated (+1 each), Glu is deprotonated (−1), His is partly protonated (often near neutral on average), and Tyr is largely neutral. This yields an expected net charge of approximately +1 at pH 7.4. • Isoelectric point (pI): With termini blocked, the pI is set by the balance of Lys/Arg protonation versus deprotonation of Glu and the phenolic Tyr. The pI is therefore predicted to be high, near ~10.1–10.3 (calculated from sequence and standard peptide pKa considerations). • Approximate molecular mass: From the evidenced sequence (Nle and D‑Phe included, plus acetyl cap and C‑terminal amide), the molecular mass is approximately 1.65 kDa (approximate, derived from composition). • Solubility considerations: Solubility is typically minimized at the pI; away from the pI (e.g., neutral/slightly acidic buffers), the peptide bears a net positive charge and tends to be more soluble. Buffer selection should consider peptide pI and avoid reactive excipients with susceptible residues (e.g., Tris with Tyr).

Structural features and receptor‑binding determinants • Pharmacophore: The conserved HFRW tetrapeptide is the minimal pharmacophore for nanomolar melanocortin agonism and is preserved in afamelanotide. • Role of substitutions: D‑Phe7 alters side‑chain topology and stabilizes bioactive conformers, increasing potency and influencing receptor subtype selectivity and agonist/antagonist balance. Nle4 improves chemical stability (removing the oxidation‑prone Met thioether) and contributes to affinity/selectivity. • Terminal caps and stability: N‑terminal acetylation and C‑terminal amidation enhance resistance to exopeptidases and are standard in α‑MSH analogues, supporting prolonged biological activity. • Conformation: Solution studies indicate broadly similar backbones among α‑MSH analogues, with side‑chain orientation (especially Trp9 and D‑Phe7) dominating receptor recognition; conformational constraints in related analogues further enhance potency/selectivity, underscoring the role of reduced conformational entropy.

Quick reference summary

Stability and Formulation#

Overview Afamelanotide (Melanotan‑1; MT‑I) shows moderate solution stability at acidic–neutral pH and is increasingly unstable under alkaline conditions. Its chemical degradation in aqueous solution is dominated by base‑catalyzed processes; lowering pH and temperature improves stability. Solid‑state performance during PLGA‑implant fabrication and moderate sterilizing irradiation appears compatible with peptide integrity.

pH stability

• Quantitative solution kinetics indicate apparent first‑order degradation with strong base catalysis. At 25 °C, t90 ≈ 60 days at pH 2.5, ≈ 40 days at pH 7.3, and ≈ 117 hours at pH 8.7, demonstrating rapid loss under alkaline conditions. A fitted rate expression Kobs = 0.936·[OH−]^0.298 with a negligible neutral hydrolysis term supports hydroxide‑driven degradation as the principal pathway in solution (25 °C). Ionic strength effects were negligible, and phosphate buffer up to 0.5 M did not measurably alter the rate (suggesting minimal general acid/base catalysis under the tested conditions).

Temperature sensitivity

• An Arrhenius activation energy of ~15.8 kcal·mol−1 was reported for MT‑I solution degradation. This magnitude implies meaningful acceleration of degradation with temperature elevation and, conversely, substantial stabilization with refrigeration or freezing. While specific 4 °C t90 values were not reported in the retrieved MT‑I sources, the Arrhenius relationship indicates that reduced temperatures will extend shelf life relative to the 25 °C values above.

Degradation pathways

• The kinetic pH‑rate behavior for MT‑I indicates base‑catalyzed chemical degradation dominates in aqueous solution; the specific molecular events were not directly identified in the retrieved excerpts. By analogy to peptide chemistry, base‑catalyzed routes may include backbone hydrolysis at susceptible amide bonds and potential side‑chain reactions; however, such mechanisms were not experimentally assigned in the MT‑I texts gathered here. No direct evidence for oxidation, deamidation, diketopiperazine formation, aggregation, or light sensitivity of MT‑I was captured in the retrieved excerpts. Notably, the norleucine substitution (Nle) at position 4 reduces methionine oxidation liability conceptually, but this was not experimentally confirmed in the retrieved material.

Formulation considerations

• Aqueous solutions: Based on the observed kinetics, acidic to near‑neutral pH is preferred; avoid alkaline pH (>8) to limit rapid degradation. Given the ~15.8 kcal·mol−1 activation energy, refrigeration is expected to extend solution stability; ionic strength adjustments and phosphate up to 0.5 M had little effect on MT‑I degradation rate in the studied range.
• Solid/implant formulations: For PLGA implants, MT‑I showed no detectable degradation during fabrication and retained full bioactivity after gamma irradiation up to 2.5 Mrad; bioactivity remained acceptable even with certain higher doses, suggesting compatibility with common sterilization conditions. Release studies were conducted in pH 7.4 buffer at 37 °C, with ~3% released at 24 h in vitro.
• Comparative context from MT‑II: Although not MT‑I, preformulation work on the closely related analog MT‑II found optimal stability near pH ~5, with increased degradation from phosphate general acid/base catalysis; recommended strategies included low phosphate concentrations and refrigeration for aqueous solutions. MT‑II’s t90 at 25 °C was much shorter (~27 h), reinforcing that melanotropic peptides can be sensitive in solution and benefit from mildly acidic pH and cold storage (contextual guidance; not a substitute for MT‑I data).

Storage and handling recommendations (evidence‑based and inferred)

• Prefer acidic to neutral pH for solution storage; avoid alkaline conditions. Refrigeration is expected to extend shelf life materially given Ea ~15.8 kcal·mol−1. If long‑term storage is required, lyophilization is a reasonable strategy by general peptide formulation principles, though direct MT‑I lyophile data were not found in the retrieved texts. For implant products, PLGA fabrication and moderate gamma sterilization appear compatible with MT‑I integrity.

Limitations

• The retrieved sources provide robust pH‑rate and temperature‑dependence for MT‑I in solution and compatibility data for PLGA implants, but do not directly map specific chemical degradation pathways (oxidation, deamidation, aggregation) for MT‑I. Consequently, mechanistic attributions remain inferential and should be validated with targeted forced‑degradation and analytical studies.

Embedded summary table

Pharmacokinetics#

We summarize the pharmacokinetics of Melanotan‑1 (afamelanotide; NDP‑MSH) in humans, including absorption, distribution, metabolism, elimination, terminal half‑life, and bioavailability by route. Where available, we provide quantitative parameters; otherwise, we report qualitative findings. A concise table is embedded.

Key findings by route

• Intravenous (IV): After 0.16 mg/kg IV in healthy volunteers, terminal (β‑phase) half‑life averaged about 1.07 ± 0.88 h (individual 0.48–2.08 h). Systemic clearance was ≈0.41 ± 0.13 L·kg−1·h−1, and apparent volume of distribution about 0.54 L/kg (individual IV Vd 0.38–0.87 L/kg). Urinary recovery of intact peptide over 24 h was low (~3.36% in one subject), consistent with extensive metabolism rather than renal excretion of intact drug. Cmax is not applicable for IV bolus; Tmax not applicable. Oral dosing produced no detectable plasma levels.

• Subcutaneous (SC) injection: Absorption was rapid (detectable within minutes), with absorption half‑life 0.07–0.79 h. The terminal (β) half‑life after SC dosing averaged ~1.30 ± 0.46 h. Absolute bioavailability of SC versus IV was essentially complete (~100%). Reported SC apparent Vd ranged ~0.21–0.39 L/kg; SC clearance ~0.12–0.19 L·kg−1·h−1. Urinary recovery of intact peptide was ≤~3.9% over 24 h. Tmax and Cmax were not reported in primary human SC studies.

• Subcutaneous controlled‑release implant (SCENESSE 16 mg): Implant provides controlled systemic exposure over days. Mean Cmax about 3.7 ± 1.3 ng/mL, with AUC0–inf ~138.9 ± 42.6 h·ng/mL; in most subjects the last measurable concentration occurred by ~96 h, and plasma levels were often undetectable by day 14. An apparent half‑life of ~15 h has been reported for the implant. Quantitative Vd, clearance, and absolute bioavailability for the implant are not reported in the retrieved sources.

Distribution

• Volumes of distribution from IV/SC injection studies indicate limited distribution beyond extracellular fluid (Vd ~0.2–0.9 L/kg). Protein binding percentages were not reported; reviews note possible receptor/plasma protein interactions without quantitation.

Metabolism and elimination

• Afamelanotide is a 13‑mer peptide designed for enhanced stability vs endogenous α‑MSH yet is still cleared primarily by proteolytic degradation. Minimal intact peptide appears in urine (≤~3–4% in 24 h), implying metabolism to small peptides/amino acids prior to elimination. Specific enzymes/pathways are not fully defined in humans. For the PLG implant, the polymer matrix hydrolyzes to lactic and glycolic acids, which are further metabolized to CO2 and water.

Bioavailability

• Oral: Not measurable; no detectable plasma levels after 0.16 mg/kg PO in volunteers.
• SC injection: Approximately complete (near 100%) absolute bioavailability relative to IV.
• SC implant: Absolute bioavailability vs IV not reported; systemic exposure evidenced by measurable Cmax/AUC for several days.

Additional notes

• Inter‑subject variability in plasma concentrations is noted in reviews; PK after implant shows low concentrations with many subjects measurable up to ~96 h and often undetectable by 14 days.

Summary table

Evidence gaps

• Human data for Tmax and Cmax after single SC injection are not reported in the primary Ugwu study excerpt. Protein binding and detailed metabolic pathways are not quantified in the retrieved sources. Regulatory summaries (e.g., EMA EPAR) likely contain additional details but were not available in the present excerpts.

Related Reading#

• Melanotan-1 overview and research guide
• Peptides similar to Melanotan-1
• Melanotan-1 research evidence