PT-141: Molecular Structure

Molecular Structure and Properties#

PT-141 (bremelanotide) is a rationally designed cyclic heptapeptide engineered to activate melanocortin receptors in the central nervous system. Its molecular design draws from the structure-activity relationships established through decades of melanocortin peptide research, incorporating specific modifications that enhance metabolic stability, receptor selectivity, and pharmacokinetic properties compared to the endogenous melanocortin ligand alpha-melanocyte stimulating hormone (alpha-MSH).

Primary Structure#

Amino Acid Sequence#

The complete sequence of PT-141 in abbreviated notation:

Ac-Nle-c[Asp-His-D-Phe-Arg-Trp-Lys]-OH

In full notation: Acetyl-norleucine-cyclo[aspartate-histidine-D-phenylalanine-arginine-tryptophan-lysine]-carboxyl

Sequence Analysis#

The N-terminus is protected with an acetyl (Ac) group, and the C-terminus retains its free carboxyl. The cyclization occurs through a lactam bond between the side chain carboxyl of Asp (position 2) and the side chain amino group of Lys (position 7), creating a 23-membered macrocyclic ring.

The Melanocortin Core Pharmacophore#

The critical pharmacophore for melanocortin receptor binding is the tetrapeptide sequence His-D-Phe-Arg-Trp (positions 3-6). This sequence was identified through systematic structure-activity relationship studies of alpha-MSH and represents the minimum sequence required for melanocortin receptor activation. Key features include:

• D-Phenylalanine: The D-configuration at position 4 is essential. Substitution with L-Phe dramatically reduces receptor binding affinity. The D-isomer positions the aromatic ring optimally for interaction with the hydrophobic pocket in the melanocortin receptor transmembrane domain.
• Tryptophan: The indole side chain makes critical contacts with the receptor. The bulky aromatic system is required for high-affinity binding.
• Arginine: The guanidinium group provides electrostatic interactions with acidic residues in the receptor binding site.
• Histidine: The imidazole ring contributes to the spatial positioning of the pharmacophore and participates in hydrogen bonding.

Physicochemical Properties#

Key Structural Modifications#

PT-141 incorporates several deliberate modifications compared to the native alpha-MSH sequence:

1. Norleucine substitution: The methionine at position 4 of alpha-MSH is replaced by norleucine (2-aminohexanoic acid). This prevents methionine sulfoxide formation (oxidation), which would reduce biological activity and create manufacturing and storage challenges.

2. Cyclization: The lactam bridge constrains the peptide backbone, reducing conformational flexibility. This pre-organizes the pharmacophore for receptor binding, increasing binding affinity and reducing the entropic penalty upon binding. The cyclic structure also provides significant resistance to exopeptidases.

3. D-amino acid incorporation: D-Phe at position 4 provides resistance to endopeptidases that require L-amino acids for substrate recognition. It also optimizes the spatial orientation of the aromatic side chain for receptor binding.

4. Truncation: Compared to alpha-MSH (13 amino acids) and Melanotan-2 (10 amino acids), PT-141's 7-residue sequence represents the minimal active fragment, reducing manufacturing complexity and immunogenicity.

Pharmacokinetic Properties#

PT-141's pharmacokinetic profile has been characterized in human clinical trials, making it one of the most thoroughly studied peptides in this regard.

Absorption#

Following subcutaneous injection of the approved 1.75 mg dose:

• Bioavailability: Approximately 100%
• Tmax (time to peak): Approximately 1 hour
• Cmax: Approximately 80-100 ng/mL

Distribution#

• Volume of distribution: Approximately 20.6 L (indicating distribution beyond the plasma compartment)
• Protein binding: Approximately 21% bound to plasma proteins

Metabolism and Elimination#

• Half-life: Approximately 2.7 hours
• Clearance: Primarily through hydrolysis by general peptidases
• Renal excretion: Approximately 64.8% of the dose excreted in urine (primarily as metabolites)
• Fecal excretion: Approximately 22.8% of the dose

The relatively short half-life supports on-demand dosing, with drug levels declining to sub-therapeutic concentrations within 12 hours of administration.

Comparison with Related Peptides#

PT-141 vs Alpha-MSH#

PT-141 vs Melanotan-2#

The sole structural difference between PT-141 and Melanotan-2 is the C-terminal modification: PT-141 has a free carboxyl (-OH) while Melanotan-2 has an amide (-NH2). This single change alters the receptor binding profile, reducing MC1R-mediated melanogenesis while preserving MC4R-mediated sexual function effects.

Receptor Binding Profile#

PT-141 binds to multiple melanocortin receptor subtypes with varying affinities:

The therapeutic effects of PT-141 are mediated primarily through MC4R activation in the hypothalamus. The MC1R activity accounts for the observed side effect of focal hyperpigmentation in some patients.

Stability and Formulation#

Commercial Formulation (Vyleesi)#

The approved formulation is a single-dose prefilled auto-injector containing 1.75 mg bremelanotide in an aqueous solution for subcutaneous injection.

Research-Grade Material#

Lyophilized PT-141 for research use:

• Stability: Stable as lyophilized powder at -20C for extended periods
• Reconstitution: Dissolves readily in sterile water or bacteriostatic water
• Solution stability: Reconstituted solutions stable at 2-8C for weeks
• pH: Most stable at pH 4-6

Degradation Pathways#

The primary degradation pathways for PT-141 include:

• Hydrolysis: Cleavage of peptide bonds or the lactam bridge
• Deamidation: Of asparagine or glutamine residues (though PT-141 lacks Asn/Gln in the ring)
• Oxidation: Of tryptophan residues under light exposure
• The norleucine substitution eliminates the major oxidation pathway (methionine sulfoxide formation) that affects alpha-MSH

Analytical Methods#

Methods for characterizing PT-141 include:

• Mass spectrometry (ESI-MS, MALDI-TOF): Confirms molecular weight of 1025.18 Da
• HPLC: Reverse-phase HPLC for purity assessment
• Circular dichroism: Characterizes secondary structure in solution
• NMR spectroscopy: Detailed structural analysis of the cyclic peptide conformation
• Chiral analysis: Confirms D-configuration at the Phe position

Evidence Gaps#

• High-resolution crystal structure of PT-141 bound to MC4R not yet published
• Detailed binding kinetics at individual MCR subtypes in human tissues
• Full characterization of active metabolites and their receptor profiles
• Structure-activity relationships for MC4R vs MC1R selectivity optimization
• Whether additional structural modifications could further reduce MC1R activity while preserving MC4R efficacy

Related Reading#

• PT-141 overview and research guide
• Peptides similar to PT-141
• PT-141 research evidence