PT-141 (Bremelanotide): Research Applications and Quality Assessment

PT-141 (Bremelanotide): Research Applications and Quality Assessment

Bremelanotide, also known as PT-141, is a synthetic melanocortin receptor agonist with a unique mechanism of action, primarily investigated for its potential effects on sexual dysfunction. Unlike other treatments targeting vascular mechanisms, PT-141 directly influences the nervous system, impacting sexual desire. This profile provides a detailed overview of PT-141, covering its molecular structure, mechanism of action, research applications, crucial quality markers, common impurities, storage requirements, and practical guidance for researchers seeking to utilize this peptide.

Molecular Structure and Properties

PT-141 is a cyclic heptapeptide analog of alpha-melanocyte-stimulating hormone (?-MSH). Its chemical formula is C₅₀H₆₈N₁₄O₁₀, and its molecular weight is approximately 1025.2 Da. The amino acid sequence is Ac-Nle-cyclo[Asp-His-D-Phe-Arg-Trp-Lys]-NH₂. The cyclization occurs between the Asp residue's side chain carboxyl group and the Lys residue's side chain amino group, forming a lactam bridge.

The presence of non-natural amino acids, such as Norleucine (Nle) and D-Phenylalanine (D-Phe), contributes to PT-141's enhanced stability and receptor selectivity compared to its parent molecule, ?-MSH. These modifications improve its resistance to enzymatic degradation and influence its binding affinity to melanocortin receptors.

Mechanism of Action

PT-141 exerts its effects by binding to and activating melanocortin receptors, primarily MC1R and MC4R. While MC1R is associated with pigmentation and inflammation, MC4R is primarily involved in regulating sexual function, appetite, and energy homeostasis. The activation of MC4R in the central nervous system, particularly in the hypothalamus, is believed to be the primary mechanism through which PT-141 influences sexual desire and arousal.

Unlike PDE5 inhibitors (e.g., sildenafil), which act on the peripheral vasculature to improve blood flow, PT-141 works centrally, modulating neuronal pathways involved in sexual motivation. This central mechanism makes it a potentially attractive alternative for individuals who do not respond to or cannot tolerate PDE5 inhibitors.

It's important to note that PT-141's selectivity for MC4R over other melanocortin receptors is not absolute. Activation of MC3R, for instance, could potentially contribute to some of its observed effects. Further research is needed to fully elucidate the specific roles of different melanocortin receptors in mediating PT-141's actions.

Research Applications

PT-141 is primarily investigated for its potential therapeutic applications in treating:

• Hypoactive Sexual Desire Disorder (HSDD) in premenopausal women: This is the area where PT-141 has received the most attention and regulatory approval (as Vyleesi).
• Erectile Dysfunction (ED) in men: While less extensively studied than in women with HSDD, research suggests potential benefits for men with ED, particularly those who don't respond well to PDE5 inhibitors.
• Other potential applications: Preliminary research explores its potential role in treating other conditions related to melanocortin receptor signaling, such as obesity and inflammatory disorders. These areas are still in early stages of investigation.

Researchers are also investigating the pharmacokinetic and pharmacodynamic properties of PT-141, including its absorption, distribution, metabolism, and excretion. Studies are exploring different routes of administration (e.g., subcutaneous injection, nasal spray) to optimize its bioavailability and efficacy.

Quality Markers to Look For

Ensuring the quality of PT-141 is paramount for reliable research outcomes. Key quality markers include:

Peptide Purity

Purity refers to the percentage of the peptide that is actually the desired sequence. It is typically assessed using High-Performance Liquid Chromatography (HPLC). A purity level of 98% or higher is generally considered acceptable for research purposes. Lower purity can lead to inaccurate results due to the presence of unwanted peptides or impurities.

Practical Tip: Request a Certificate of Analysis (CoA) from the supplier that includes an HPLC chromatogram. Examine the chromatogram for any significant peaks besides the main PT-141 peak, which would indicate the presence of impurities.

Peptide Identity

Identity confirmation verifies that the peptide is indeed PT-141 and not another compound. Mass Spectrometry (MS) is the gold standard for confirming peptide identity. The measured molecular weight should match the theoretical molecular weight of PT-141 (1025.2 Da) within a reasonable tolerance (typically ± 1 Da).

Practical Tip: Ensure the CoA includes MS data confirming the peptide's identity. Look for a clear molecular ion peak at m/z = 1025.2 ± 1 Da. Some suppliers also provide tandem mass spectrometry (MS/MS) data for further confirmation of the amino acid sequence.

Peptide Content

Peptide content refers to the actual amount of peptide present in the vial, taking into account factors like water content and residual solvents. This is often expressed as a percentage. A low peptide content means you are paying for less peptide than you think.

Practical Tip: Ask the supplier about the method used to determine peptide content. Amino acid analysis (AAA) is a highly accurate method for determining the actual amount of peptide present. Alternatively, quantitative UV spectrophotometry can be used, but it requires accurate knowledge of the peptide's extinction coefficient.

Water Content

Peptides are hygroscopic, meaning they readily absorb water from the atmosphere. Excessive water content can affect the peptide's stability and accurate weighing. The water content is typically determined using Karl Fischer titration. A water content of less than 10% is generally considered acceptable.

Practical Tip: Check the CoA for the water content. If the water content is high, consider lyophilizing (freeze-drying) the peptide upon arrival to remove excess water.

Counterion Content

Peptides are often synthesized as salts (e.g., acetate salt, trifluoroacetate salt) to improve their solubility and stability. The counterion content should be specified on the CoA. Trifluoroacetate (TFA) is a common counterion, but it can sometimes interfere with certain biological assays. If TFA is a concern, consider purchasing PT-141 with an acetate counterion.

Practical Tip: Be aware of the counterion and its potential impact on your experiments. If necessary, consider using a TFA scavenger during your assays.

Endotoxin Levels

Endotoxins, particularly lipopolysaccharides (LPS), are bacterial toxins that can contaminate peptides during synthesis or handling. Even trace amounts of endotoxins can trigger an immune response and confound experimental results, especially in cell-based assays. Endotoxin levels are typically measured using the Limulus Amebocyte Lysate (LAL) assay and are expressed in Endotoxin Units per milligram (EU/mg). For cell culture experiments, an endotoxin level of less than 10 EU/mg is generally recommended.

Practical Tip: Request endotoxin testing if you plan to use PT-141 in cell-based assays or *in vivo* studies. Choose a supplier that offers low-endotoxin peptides.

Common Impurities

Peptide synthesis is not a perfect process, and several impurities can arise during synthesis and purification. Common impurities include:

• Deletion sequences: Peptides missing one or more amino acids.
• Truncated sequences: Peptides with incomplete amino acid chains.
• Diastereomers: Peptides with incorrect stereochemistry at one or more chiral centers.
• Acetylated or modified peptides: Peptides with unintended modifications.
• Residual solvents: Organic solvents used during synthesis and purification.

These impurities can affect the peptide's activity, stability, and toxicity. High-quality suppliers employ rigorous purification and analytical techniques to minimize the presence of these impurities.

Storage Requirements

Proper storage is crucial for maintaining the integrity and stability of PT-141. Follow these guidelines:

• Lyophilized peptides: Store at -20°C or -80°C in a tightly sealed container. Protect from moisture and light.
• Reconstituted peptides: Store at 2-8°C for short-term storage (days) or aliquot and freeze at -20°C or -80°C for long-term storage (months). Avoid repeated freeze-thaw cycles.
• Solvents: Use sterile, endotoxin-free solvents for reconstitution, such as sterile water or phosphate-buffered saline (PBS).
• Container: Store reconstituted peptides in sterile, low-binding microcentrifuge tubes.

Practical Tip: Always reconstitute PT-141 immediately before use to minimize degradation. Avoid storing reconstituted peptides for extended periods, even at low temperatures.

Sourcing Considerations

Choosing a reliable supplier is essential for obtaining high-quality PT-141. Consider the following factors:

• Reputation and experience: Select a supplier with a proven track record of producing high-quality peptides.
• Quality control: Ensure the supplier employs rigorous quality control procedures and provides comprehensive Certificates of Analysis.
• Analytical capabilities: The supplier should have access to state-of-the-art analytical equipment, such as HPLC, MS, and AAA.
• Customer support: Choose a supplier that offers excellent customer support and is responsive to inquiries.
• Price: While price is a factor, prioritize quality over cost. Extremely low prices may indicate compromised quality.

Practical Tip: Obtain samples from multiple suppliers and compare their quality based on the criteria outlined above. Perform your own independent analysis to verify the supplier's claims.

Comparison Table: Quality Markers

Key Takeaways

• PT-141 (Bremelanotide) is a melanocortin receptor agonist primarily researched for sexual dysfunction.
• It works centrally, influencing neuronal pathways involved in sexual motivation, unlike PDE5 inhibitors.
• Key quality markers include purity (? 98%), confirmed identity by MS, low water content (? 10%), and low endotoxin levels (? 10 EU/mg for cell culture).
• Common impurities include deletion sequences, truncated sequences, and residual solvents.
• Store lyophilized peptides at -20°C or -80°C and reconstituted peptides at 2-8°C for short-term storage or -20°C/-80°C for long-term storage, avoiding repeated freeze-thaw cycles.
• Choose a reputable supplier with robust quality control measures and comprehensive Certificates of Analysis.