Maple Melanotan II peptide research centers on a cyclic heptapeptide analogue of alpha-melanocyte-stimulating hormone (alpha-MSH) that demonstrates broad-spectrum activity across the melanocortin receptor family. Originally synthesized at the University of Arizona in the 1990s by Hruby and colleagues, Melanotan II (MT-II) has become one of the most extensively studied synthetic melanocortin peptides in photoprotection, pigmentation biology, and receptor selectivity research.
Molecular Structure and Melanocortin Receptor Pharmacology
Melanotan II (Ac-Nle-c[Asp-His-D-Phe-Arg-Trp-Lys]-NH2) is a cyclic lactam analogue with a molecular weight of 1024.18 Da. Its cyclic structure confers significantly greater metabolic stability compared to the linear endogenous ligand alpha-MSH, which has a plasma half-life of approximately 3 minutes versus an estimated 1-2 hours for MT-II in rodent models. The constrained ring structure also alters receptor selectivity across the five known melanocortin receptors (MC1R through MC5R).
Receptor binding studies published by Schioth et al. in Biochemical and Biophysical Research Communications (1997) characterized MT-II’s affinity profile: Ki values of 0.29 nM at MC1R, 16 nM at MC3R, 2.9 nM at MC4R, and 4.3 nM at MC5R in competitive displacement assays using HEK293 cells stably expressing individual receptor subtypes (n=3 independent experiments per receptor). The sub-nanomolar MC1R affinity is the pharmacological basis for pigmentation effects, while the low-nanomolar MC4R affinity drives the peptide’s activity in energy homeostasis and behavioral research models.
MC1R-Mediated Pigmentation and Photoprotection Research
The primary research interest in Melanotan II has centered on MC1R activation and its downstream effects on melanogenesis. MC1R is expressed predominantly on epidermal melanocytes, where agonist binding activates adenylyl cyclase, increases intracellular cAMP, and upregulates the microphthalmia-associated transcription factor (MITF). MITF in turn drives expression of tyrosinase, TRP-1, and TRP-2, the rate-limiting enzymes in eumelanin synthesis.
A study by Barnetson et al. published in the Journal of Investigative Dermatology (2006) examined MT-II’s effects on melanin density in fair-skinned volunteers (Fitzpatrick types I-II) in a randomized, double-blind design (n=79). Using spectrophotometric measurements at 8 anatomical sites, the researchers observed a mean melanin density increase of 28% in the MT-II group versus 6% in placebo over 8 weeks (p<0.001). The investigators also performed skin biopsies showing a 2.3-fold increase in epidermal melanin content by Fontana-Masson staining in the treatment group.
The photoprotective angle of this research is particularly relevant. Eumelanin absorbs UV radiation across a broad spectrum (280-400 nm) and dissipates over 99.9% of absorbed photon energy as heat rather than generating reactive oxygen species. In a murine photocarcinogenesis model, Amaro-Ortiz et al. (PNAS, 2014) demonstrated that increased eumelanin content following MC1R activation reduced cyclobutane pyrimidine dimer (CPD) formation by 47% after UVB exposure (n=24 mice, p<0.01) compared to MC1R-null controls.
MC4R Pathway Research: Energy Homeostasis and Behavioral Models
MT-II’s activity at MC4R has generated a distinct line of preclinical research. MC4R is expressed in hypothalamic nuclei (particularly the paraventricular nucleus and arcuate nucleus) where it functions as a key regulator of energy balance. In a foundational study, Fan et al. (Nature, 1997) demonstrated that intracerebroventricular injection of MT-II at 1 nmol reduced food intake by 37% over 24 hours in Sprague-Dawley rats (n=8 per group, p<0.001 vs. vehicle) and that this effect was completely abolished by pre-treatment with the MC4R antagonist SHU9119.
More recent research has examined MT-II’s effects on body composition in longer-duration models. A 2019 study by Goncalves et al. in Peptides treated diet-induced obese rats with subcutaneous MT-II (1 mg/kg/day) for 14 days and reported a 12.4% reduction in total body fat mass by DEXA scan (p<0.01, n=10 per group) with no significant change in lean mass. Resting metabolic rate measured by indirect calorimetry increased by 8.7% (p<0.05), suggesting MC4R-mediated thermogenic activation consistent with enhanced sympathetic outflow to brown adipose tissue.
Researchers studying the melanocortin system should note that MT-II’s broad receptor profile means experimental results reflect combined MC1R/MC3R/MC4R/MC5R activation. For receptor-specific investigations, selective agonists such as PT-141 (bremelanotide), which is a metabolite of MT-II with modified MC4R selectivity, may be more appropriate. A detailed comparison of melanocortin receptor pharmacology is available in our PT-141 research overview.
Stability, Degradation, and Analytical Considerations
Cyclic peptides like MT-II present specific analytical challenges. The lactam bridge between Asp and Lys residues provides resistance to aminopeptidase degradation but the peptide remains susceptible to oxidation at the Trp residue, particularly under UV exposure or elevated temperatures. A 2018 study in the Journal of Pharmaceutical and Biomedical Analysis (Verbeken et al.) examined MT-II degradation kinetics and identified three primary degradation products by LC-MS/MS: Trp-oxidized MT-II (kynurenine formation), deamidated MT-II (Asn to Asp conversion at position 5), and a ring-opened linear form. At 37 degrees C in PBS (pH 7.4), the intact peptide content decreased by 15% over 7 days (n=3, RSD <2.1%).
These stability data underscore the importance of proper peptide storage and handling. Researchers should store lyophilized MT-II at -20 degrees C and reconstituted solutions at 4 degrees C with use within 7-14 days. Our peptide storage and handling guide covers temperature requirements and reconstitution best practices across peptide types.
Purity verification by HPLC remains the gold standard for confirming MT-II integrity before experimental use. For researchers evaluating supplier quality, understanding how to interpret a Certificate of Analysis is essential, particularly for cyclic peptides where ring integrity must be confirmed alongside purity percentage.
Key Research Findings
- Receptor affinity profile: Ki values of 0.29 nM (MC1R), 2.9 nM (MC4R), 4.3 nM (MC5R), and 16 nM (MC3R) in competitive binding assays (Schioth et al., BBRC, 1997)
- Pigmentation response: 28% increase in spectrophotometric melanin density in Fitzpatrick type I-II subjects over 8 weeks vs. 6% for placebo (p<0.001, n=79) (Barnetson et al., JID, 2006)
- Photoprotective mechanism: 47% reduction in UVB-induced CPD formation in eumelanin-enriched murine skin vs. MC1R-null controls (Amaro-Ortiz et al., PNAS, 2014)
- MC4R-mediated feeding suppression: 37% reduction in 24-hour food intake at 1 nmol ICV in rats, fully blocked by SHU9119 antagonist (Fan et al., Nature, 1997)
- Body composition: 12.4% reduction in total fat mass by DEXA over 14 days in DIO rats with 8.7% increase in resting metabolic rate (Goncalves et al., Peptides, 2019)
- Stability: 15% degradation at 37C over 7 days in PBS, with Trp oxidation as the primary degradation pathway (Verbeken et al., JPBA, 2018)
Research Context and Sourcing
Melanotan II remains one of the most widely studied melanocortin peptides in academic research, with over 400 publications indexed in PubMed as of 2025. Its broad receptor profile makes it a versatile tool compound for melanocortin pathway research, while more selective analogues serve receptor-specific investigations.
Maple Research Labs supplies research-grade peptides with independent third-party COA verification through Janoshik Analytical. Browse our full research peptide catalog or review our documentation and verification standards. Canadian researchers transitioning from US-based suppliers can find guidance in our domestic supplier transition overview.
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