PT-141 (bremelanotide) is a cyclic melanocortin peptide derived from Melanotan II through metabolic deacetylation. Unlike its parent compound, PT-141 has been primarily studied for its activity at central melanocortin-3 and melanocortin-4 receptors (MC3R/MC4R), with research focusing on CNS-mediated physiological responses in preclinical and clinical models.
Chemistry and Relationship to Melanotan II
PT-141 (Ac-Nle-cyclo[Asp-His-D-Phe-Arg-Trp-Lys]-OH) differs from Melanotan II by the absence of the C-terminal amide group—a modification that occurs naturally through enzymatic hydrolysis. This seemingly minor structural change significantly alters the receptor selectivity profile: while MT-II activates all five melanocortin receptor subtypes roughly equally, PT-141 shows reduced affinity for MC1R (the melanogenesis receptor) with relatively preserved activity at MC3R and MC4R.
The molecular weight of PT-141 is 1025.2 Da. Like MT-II, it retains the cyclic lactam bridge between Asp and Lys side chains that constrains the backbone into a β-turn conformation. This cyclization provides resistance to exopeptidases and endopeptidases, yielding a plasma half-life of approximately 120 minutes in animal models—considerably longer than linear melanocortin peptides.
Melanocortin-4 Receptor Signaling
MC4R is a Gαs-coupled receptor predominantly expressed in the paraventricular nucleus (PVN), ventromedial hypothalamus, and other CNS regions. Upon activation by PT-141, MC4R stimulates adenylyl cyclase, increasing intracellular cAMP and activating protein kinase A (PKA). This leads to phosphorylation of CREB and downstream gene transcription changes.
MC4R also signals through Gαq-coupled pathways involving phospholipase C (PLC) activation, inositol trisphosphate (IP3) generation, and intracellular calcium release. PT-141 has been characterized as a biased agonist at MC4R in some studies, preferentially activating certain signaling pathways over others—a property that may explain its distinct pharmacological profile compared to other melanocortin agonists.
Research using MC4R knockout mice and selective MC4R antagonists (e.g., HS024) has demonstrated that MC4R activation is both necessary and sufficient for PT-141’s central effects in rodent models, confirming the receptor assignment.
Preclinical Research Models
PT-141 has been studied across multiple animal models. In rodent models, intracerebroventricular (ICV) and subcutaneous administration have been used to characterize dose-response relationships, onset kinetics, and duration of action. Typical research doses range from 0.01-1 mg/kg subcutaneously in rats, with central effects observed within 15-30 minutes and lasting 4-6 hours.
Electrophysiological studies have examined PT-141’s effects on hypothalamic neuron firing patterns, identifying dose-dependent changes in activity within the medial preoptic area (MPOA) and PVN. These studies utilize extracellular recording techniques with concurrent peptide microinjection to map the neural circuits engaged by melanocortin signaling.
Pharmacokinetics
Subcutaneous administration of PT-141 in animal models produces rapid absorption with Tmax of 30-45 minutes. The peptide undergoes renal clearance as the primary elimination route, with a terminal half-life of approximately 2.5 hours. Bioavailability via subcutaneous injection is approximately 100% in rodent models, reflecting the peptide’s resistance to first-pass hepatic metabolism.
PT-141 distributes across the blood-brain barrier, a property unusual for peptides of its size. This CNS penetration is thought to be facilitated by receptor-mediated transcytosis via melanocortin receptors expressed on brain endothelial cells, though passive diffusion through circumventricular organs (which lack a complete blood-brain barrier) may also contribute.
Analytical Methods
PT-141 purity and identity are assessed using reversed-phase HPLC (C18 column, acetonitrile/water gradient with 0.1% TFA) and electrospray ionization mass spectrometry (expected [M+H]⁺ = 1025.2). Researchers should verify the absence of the amidated form (MT-II) as a synthetic impurity, which can be resolved by HPLC due to the slight polarity difference. Quantification in biological matrices requires LC-MS/MS with limits of detection typically at 0.1-1 ng/mL.
Frequently Asked Questions
How does PT-141 differ pharmacologically from Melanotan II?
PT-141 lacks the C-terminal amide of MT-II, reducing its MC1R affinity (melanogenesis receptor) while preserving MC3R/MC4R activity. This means PT-141 is studied primarily for CNS-mediated effects through hypothalamic melanocortin circuits rather than for pigmentation-related research. The two peptides share the same cyclic core structure and similar plasma half-lives.
What receptor antagonists are used as controls in PT-141 research?
SHU9119 is a non-selective MC3R/MC4R antagonist commonly used to block PT-141 effects. HS024 is a more selective MC4R antagonist. Agouti-related protein (AgRP) fragments, which are endogenous MC3R/MC4R inverse agonists, are used for physiological context. These antagonists help confirm receptor specificity in behavioral and electrophysiological studies.
What is the recommended storage for PT-141?
Lyophilized PT-141 should be stored at -20°C, desiccated, and protected from light. Reconstituted solutions in sterile water or bacteriostatic water should be stored at 2-8°C and used within 4 weeks. Avoid repeated freeze-thaw cycles, which accelerate degradation. Verify purity by HPLC before use in experiments.