PT-141 Research: Structure, Analytical Characterization and Handling

PT-141 is written as Ac-Nle-cyclo[Asp-His-D-Phe-Arg-Trp-Lys]-OH. Key structural features include:

• Seven residues: N-acetyl norleucine followed by a six-residue ring.
• Lactam bridge: an amide bond between the side-chain carboxyl of Asp and the side-chain ε-amine of Lys closes the ring.
• D-phenylalanine at the central position of the His-Phe-Arg-Trp melanocortin pharmacophore.
• Free C-terminal acid, which distinguishes PT-141 from melanotan II (C-terminal amide).
• Molecular formula C₅₀H₆₈N₁₄O₁₀; molecular weight approximately 1025.2 g/mol.

The peptide carries basic Arg and His side chains and is typically supplied as a salt (commonly acetate or trifluoroacetate, depending on the final purification step). The counter-ion contributes to the gross mass of the lyophilized solid, which is why net peptide content can differ from fill weight.

PT-141 is produced by solid-phase peptide synthesis (SPPS), usually with Fmoc chemistry. The linear sequence is assembled on resin, and the Asp and Lys side chains are protected with orthogonal groups so they can be selectively deprotected and coupled to form the lactam ring, either on resin or in solution after cleavage. The crude product is then purified by preparative reversed-phase HPLC and lyophilized.

Synthesis-related impurities worth monitoring in a cyclic peptide like PT-141 include deletion sequences, uncyclized linear precursor, diastereomers arising from racemization (for example at D-Phe or His), dimers formed by intermolecular rather than intramolecular coupling, and oxidation products of tryptophan.

A complete identity and purity package for PT-141 research material typically draws on several orthogonal techniques:

• Reversed-phase HPLC (RP-HPLC) on a C18 column with a water/acetonitrile gradient containing an acidic modifier such as TFA or formic acid, with UV detection around 214–220 nm (peptide bond) and often 280 nm (tryptophan). Purity is reported as main-peak area percentage.
• Mass spectrometry (ESI-MS or MALDI-TOF) to confirm the molecular mass. ESI spectra of PT-141 commonly show multiply charged ions such as [M+2H]²⁺ because of the basic residues.
• LC-MS to assign impurity peaks, for example distinguishing the uncyclized precursor (+18 Da relative to the cyclic product) from deletion sequences.
• Amino acid analysis or quantitative methods to determine net peptide content.
• Water content (Karl Fischer) and counter-ion analysis where a full mass balance is required.

Using more than one method matters because a single HPLC peak can hide co-eluting diastereomers, while mass spectrometry alone cannot distinguish them at all.

A COA for PT-141 should allow a researcher to verify that the material matches the intended compound and to record the lot in laboratory notebooks. Look for:

• Product name, synonyms (PT-141, bremelanotide) and sequence.
• Lot or batch number and date of analysis.
• HPLC purity with the chromatogram attached or available, including method conditions.
• Mass spectrometry result with theoretical and observed mass.
• Fill weight per vial and physical appearance (white to off-white lyophilized solid).
• Recommended storage conditions.

Matching the lot number on the vial label to the COA is a simple but essential step in maintaining traceability for any PT-141 research data set.

Lyophilized PT-141 is best kept sealed, dry and dark at −20 °C. Vials should warm to room temperature before opening so that moisture does not condense on the hygroscopic solid. When preparing stock solutions, sterile water or a dilute aqueous acid generally dissolves the peptide readily because of its basic residues; the concentration in mg/mL follows directly from the fill weight and the solvent volume added. Solutions should be divided into single-use aliquots in low-binding tubes and stored frozen, avoiding repeated freezing and thawing.

Tryptophan is sensitive to light and oxidation, so protecting solutions from light and minimizing headspace air helps preserve integrity over longer experiments. See also preparing lyophilized peptide solutions for laboratory assays.

Analytically verified PT-141 is used as a reference agonist in melanocortin receptor studies, especially at MC3R and MC4R. Because its activity in cAMP and binding assays depends on an intact cyclic structure and correct stereochemistry, the analytical checks above directly affect the reproducibility of receptor pharmacology data. For background on the receptor biology itself, see our overview of PT-141 melanocortin receptor pharmacology.

NuRev supplies PT-141 for laboratory research use only; it is not for human or veterinary use.