A practical reference on G protein-coupled receptor: what it is, how it behaves, what the literature reports, and where the honest uncertainties sit.
This page was last updated on 2026-08-01 and is reviewed periodically as new material appears.
Certificate of analysis documents usually report purity percentage, molecular weight, and appearance. Researchers verify identity by comparing observed and theoretical mass values. Chain-of-custody records and batch numbers help trace material from source to experiment. Independent verification of supplier claims is considered good practice when material identity is critical to a study. Records should include the storage history of each aliquot, and unexpected deviations in appearance or solubility warrant re-testing before use.
Lyophilized peptide material is generally stored at -20 °C or below to limit degradation, while reconstituted solutions are less stable and are typically kept refrigerated and protected from light. Repeated freeze-thaw cycles can accelerate aggregation and should be minimized. Stability for any specific lot depends on purity, moisture content, and packaging. Handling in a temperature-controlled environment reduces variability across replicates, and exposure to ambient humidity during weighing can introduce error. Aliquotting reduces the number of times a stock container is opened.
Reverse-phase high-performance liquid chromatography is the standard method for assessing purity. Mass spectrometry confirms molecular identity and detects sequence variants or truncation products. Ultraviolet absorbance at 214 or 280 nm is used for quantification, with the choice depending on the peptide sequence. Method validation typically addresses linearity, limit of detection, and precision across a defined concentration range. Impurity profiling may also employ ion-exchange or size-exclusion chromatography, and these techniques complement one another.
Early research on PT-141 grew out of work on melanotan II, a related cyclic peptide studied for pigmentation. Investigators observed that centrally acting melanocortin agonists also influenced sexual behaviour in animal models, and the programme shifted toward that endpoint. A nasal formulation was evaluated in clinical trials but showed inconsistent absorption, and later studies used subcutaneous administration instead. Regulatory approval in the United States followed in 2019 for a defined population of premenopausal women with acquired, generalised hypoactive sexual desire disorder. That approval was specific to that group rather than a broad indication.
Bremelanotide acts as a non-selective agonist at melanocortin receptors, with reported activity at MC1R, MC3R, MC4R and MC5R. The proposed basis for its central effects is activation of MC4R populations in the hypothalamus, a region associated with appetite and reproductive signalling. Because the peptide carries a net positive charge and polar side chains, it does not cross biological membranes freely, which is one reason oral administration is not the standard route. Effects generally appear within an hour of parenteral administration and are described as centrally mediated rather than peripheral.
Bremelanotide, developed under the code PT-141, is a synthetic cyclic heptapeptide analogue of alpha-melanocyte-stimulating hormone. Its structure is Ac-Nle-cyclo[Asp-His-D-Phe-Arg-Trp-Lys]-OH, with a lactam bridge joining the aspartate and lysine side chains. The molecule has the formula C50H68N14O10 and a monoisotopic mass near 1025 daltons. It is commonly prepared as the acetate salt and appears as a white to off-white lyophilised powder in solid form. The free acid is the pharmacologically relevant species, while the counter-ion improves handling and dissolution.
| Property | Value | Notes |
|---|---|---|
| Storage form | Lyophilized powder | Longer stability at -20 °C |
| Reconstitution solvent | Sterile water or buffer | Depends on intended application |
| Purity assessment | Reverse-phase HPLC | Reported as peak area percentage |
| Identity confirmation | Mass spectrometry | Compared against theoretical mass |
| Main stability concern | Repeated freeze-thaw | May promote aggregation |
Routine characterisation of bremelanotide relies on reversed-phase high-performance liquid chromatography with ultraviolet detection near 214 nm, where the peptide backbone absorbs. Mass spectrometry, typically in tandem mode with electrospray ionisation, confirms identity and supports quantification in biological matrices. Additional checks include amino acid analysis, peptide mapping after enzymatic digestion, and confirmation of the lactam bridge, since incomplete cyclisation produces a mass-shifted by-product. Purity values above 95 percent are common in reference-grade material, though reports vary in how strictly related substances are resolved from the parent peak.
The lyophilised solid is relatively stable when kept dry, protected from light and held at reduced temperature, commonly minus 20 degrees Celsius or lower for long-term storage. In solution the peptide is more vulnerable: tryptophan oxidation, hydrolysis of the lactam bridge and aggregation all become relevant over time, and the rate depends on pH, buffer composition and concentration. Repeated freeze-thaw cycles are generally avoided because they promote aggregation. Aqueous working solutions are typically prepared fresh or split into single-use aliquots to limit degradation before analysis.
The peptide contains seven amino acids arranged in a ring, closed by a lactam bridge between a side-chain acid and an amine. This cyclic constraint holds the backbone in a defined conformation and increases resistance to enzymatic breakdown relative to linear analogs. N-terminal acetylation and a C-terminal amide further protect the molecule from exopeptidases. The result is a compound with a comparatively long circulation time for a small peptide. Structural modification of the bridge alters receptor affinity, which is one reason analogs in this family differ in their subtype preferences.
Whether the behavioral effect originates centrally, peripherally, or through both remains an active question. Animal experiments using receptor antagonists and site-specific injections point toward hypothalamic melanocortin circuits as a key locus, but translating those findings to humans is not straightforward. Blood pressure changes observed in trials suggest a vascular component that may be peripherally mediated. The relationship between receptor occupancy and reported effect has not been mapped in humans, and no validated biomarker predicts response. This gap makes it difficult to explain individual variability on pharmacological grounds alone.
Bremelanotide acts as an agonist at melanocortin receptors, a family of five G-protein-coupled receptors labeled MC1 through MC5. Binding studies indicate activity at several of these subtypes rather than strict selectivity for one. Signalling proceeds mainly through Gs-mediated activation of adenylyl cyclase, raising intracellular cyclic AMP. The MC4 receptor, expressed in hypothalamic and limbic circuits, is widely regarded as the subtype most relevant to sexual response. Because the molecule is not subtype-selective, effects at other melanocortin receptors are expected and are used to explain some observed side effects.
PT-141 is a synthetic cyclic heptapeptide whose development code is bremelanotide. It belongs to the class of melanocortin receptor agonists and acts by mimicking endogenous peptide hormones. The compound originated from research on melanotan II, where investigators exploring derivatives found distinct pharmacological features. Unlike the parent compound, PT-141 showed effects on pathways related to sexual desire and function in early studies, prompting its development as a separate candidate. Its molecular design aimed to separate receptor activity from pigmentary effects.
The molecular backbone consists of seven amino acid residues joined into a ring through a disulfide bridge. This cyclic conformation is critical for receptor binding. The sequence includes an acetylated N-terminus and an amidated C-terminus, modifications that improve resistance to enzymatic breakdown. One residue is in the D-configuration, a feature that further stabilizes the peptide against protease activity. Together, the ring structure and specific stereochemistry determine selectivity among melanocortin receptor subtypes.
Across literature and commercial listings, the substance appears under several names, including bremelanotide, PT-141, and various brand designations. A CAS registry number provides a unique identifier for the compound, aiding retrieval from chemical databases. Synonym lists may also contain early development codes and descriptive chemical names. When consulting sources, researchers need to confirm that the cited names refer to the same molecular structure to avoid confusion. Naming consistency matters especially when comparing analytical data across publications.
PT-141 initiates cellular signaling by binding to specific subtypes within the melanocortin receptor family. These receptors belong to the G protein-coupled receptor superfamily, and activation raises intracellular cyclic adenosine monophosphate levels. This cascade ultimately influences neuronal circuits in the central nervous system that are associated with sexual desire and arousal. Research indicates the compound's action concentrates in hypothalamic regions rather than peripheral tissues, which helps explain some observed pharmacological features. The selectivity of receptor binding underlies its functional differences.
Compared with the related compound melanotan II, PT-141 shows markedly weaker activation of receptors tied to pigmentation. This difference stems from subtle structural variations that alter affinity distribution across receptor subtypes. Investigators propose that such selectivity produces a different side effect profile in specific applications. However, downstream consequences of prolonged receptor activation remain uncertain in the literature. Published studies do not fully agree on the duration of signaling pathway activity and the mechanisms of desensitization.
From a pharmacokinetic standpoint, the peptide is usually delivered by injection because oral bioavailability is very low; proteases in the digestive tract degrade it rapidly. After subcutaneous administration, plasma concentrations reach a peak within roughly one hour. Its elimination half-life is relatively short, with most reports placing it in the range of a few hours. Nasal formulations have also been examined, though absorption varies widely between individuals. Metabolism proceeds mainly through peptidase cleavage, and the resulting products are excreted by the kidneys.
== Selected articles == McCartney, Fiona; Jannin, Vincent; Chevrier, Stéphanie; Boulghobra, Hakime; Hristov, Delyan R.; Ritter, Nicolas; Miolane, Cédric; Chavant, Yann; Demarne, Frédéric; Brayden, David J. (2019). "Labrasol® is an efficacious intestinal permeation enhancer across rat intestine: Ex vivo and in vivo rat studies". Journal of Controlled Release. 310: 115–126. doi:10.1016/j.jconrel.2019.08.008. PMID 31401199. Hristov, Delyan; McCartney, Fiona; Beirne, Jason; Mahon, Eugene; Reid, Stephanie; Bhattacharjee, Sourav; Penarier, Geraldine; Werner, Ulrich; Bazile, Didier; Brayden, David J. (2020). "Silica-coated nanoparticles with a core of zinc, l-Arginine, and a peptide designed for oral delivery". ACS Applied Materials & Interfaces. 12 (1): 1257–1269. Bibcode:2020AAMI...12.1257H. doi:10.1021/acsami.9b16104. PMID 31802658. Twarog, Caroline; Liu, Kai; O'Brien, Peter J.; Dawson, Kenneth A.; Fattal, Elias; Illel, Brigitte; Brayden, David J. (2020). "A head-to-head Caco-2 assay comparison of the mechanisms of action of the intestinal permeation enhancers: SNAC and sodium caprate (C10)". European Journal of Pharmaceutics and Biopharmaceutics. 152: 95–107. doi:10.1016/j.ejpb.2020.04.023. PMID 32387703. Khandelia, Rumi; Hodgkinson, Tom; Crean, Daniel; Brougham, Dermot F.; Scholz, Dimitri; Ibrahim, Hossam; Quinn, Susan J.; Rodriguez, Brian J.; Kennedy, Oran D.; O’Byrne, John M.; Brayden, David J. (2024). "Reproducible synthesis of biocompatible albumin nanoparticles designed for intra-articular administration of celecoxib to treat osteoarthritis".
=== Gas chromatography === GC is a method involving the separation of different analytes within a sample of mixed gases. The separated gases can be detected multiple ways, but one of the most powerful detection methods for gas chromatography is mass spectrometry. After the gases separate, they enter the mass spectrometer and are analyzed. This combination not only separates the analytes, but gives structural information about each one. The GC sample must be volatile, or able to enter the gas phase, while also being thermally stable so that it does not break down as it is heated to enter the gas phase. Mass spectrometry ionization techniques requiring the sample to be in the gas phase have similar concerns. Electron ionization (EI) in mass spectrometry requires samples that are small molecules, volatile, and thermally stable, similar to that of gas chromatography. This ensures that as long as GC is performed on the sample before entering the mass spectrometer, the sample will be prepared for ionization by EI. Chemical ionization (CI) is another method that requires samples to be in the gas phase. This is so that the sample can react with a reagent gas to form an ion that can be analyzed by the mass spectrometer. CI has many of the same requirements in sample preparation as EI, such as volatility and thermal stability of the sample. GC is useful for sample preparation for this technique as well. One advantage of CI is that larger molecules separated by GC can be analyzed by this ionization method.
== H == habitat – HACEK organism – halobacteria – haploid – Hardy–Weinberg principle – heart – Hela cell – helper T cell – Hepadnaviridae – hepatitis B – herbivore – heredity – hereditary disease – hermaphrodite – herpetology – Hershey–Chase experiment – heterochromatin – heterotroph – heterozygote – Hfr cell – hibernation – hierarchy of life – Hill reaction – His tag – histone – homeobox – homeostasis – homologous recombination – homology – homoplasy – homozygote – homunculus – horizontal gene transfer – hormone – host – household gene – human – Human Genome Project – humoral immunity – hybrid (biology) – hybridization – hydrolysis – hydrolytic enzyme – hygiene – hyperpolarization (biology)
He found that he could extract some pigments (such as orange carotenes and yellow xanthophylls) from leaves with non-polar solvents, but others (such as chlorophyll) required polar solvents. He reasoned that chlorophyll was held to the plant tissue by adsorption, and that stronger solvents were necessary to overcome the adsorption. To test this, he applied dissolved pigments to filter paper, allowed the solvent to evaporate, then applied different solvents to see which could extract the pigments from the filter paper. He found the same pattern as from leaf extractions: carotene could be extracted from filter paper using non-polar solvents, but chlorophyll required polar solvents. Tsvet's work saw little use until the 1930s.
Sources: en.wikipedia.org
Reticulocytes: Increased (normal range: 0.5–1.5% of the RBC listed above). The body to mass-produces red blood cells (reticulocytes being young erythrocytes) even as the spleen filters spherocytes out. This is known as reticulocytosis. Unconjugated bilirubin: Increased (normal range: 0.2–1.2 mg/dL). This is caused by heme released into the hepatosplenic circulation by macrophages that have phagocytosed erythrocytes. The unconjugated bilirubin is not soluble in water (blood), so it binds to albumin, and is processed in the liver. Haptoglobin (free): Decreased (normal range: 41–165 mg/dL). This is caused by hemoglobin binding to haptoglobin, thus making it no longer "free". Lactate dehydrogenase (LDH): Increased (normal range: 110–295 U/L in children). This is due to extravascular hemolysis. Peripheral blood smear: Directly shows spherocytes on microscope. Eosin-5-maleimide binding test: Positive (reduced mean fluorescence), as the test will demonstrate a reduced ability of the eosin-5-maleimide dye to bind to erythrocyte plasma membrane proteins. The process relies upon flow cytometry. Gold standard test that produces results at low cost within ~2 hours. In chronic cases, patients who have taken iron supplementation, have heterozygous hemochromatosis, or received numerous blood transfusions, iron overload may cause additional health issues. Measuring iron stores is sometimes considered part of the diagnostic approach to hereditary spherocytosis in older patients presenting with heart muscle damage of unknown etiology or liver disease without apparent cause.
CE capillary is coupled directly to an electrospray ionization source with a sheathless interface system. The electric contact for ESI is realized by using capillary coated with conductive metal. Because no sheath liquid is used, the system has high sensitivity, low flow rates and minimum background. However, these interface designs, all have challenges including low mechanical robustness, poor reproducibility. The latest sheathless interface design features porous ESI emitter through chemical etching. This design effectively provides robust interfacing with mass spectrometry and addresses the reproducibility challenges associated with previous designs. This porous emitter interface has been explored to couple of CITP/CZE (or transient ITP) which greatly improves sample loading capacity of CE and enabled ultrasensitive detection of trace analytes. High reproducibility, robustness and sensitivity were achieved in sheathless transient capillary isatochophoresis (CITP)/capillary zone electrophoresis (CZE) -MS interface, where conductive liquid was used. Conductive liquid contacts with the metal-coated outer surface of the emitter completing the circuit, but at the same time it does not mix with separation liquid and therefore there is no sample dilution.
To bypass these effects, it is standard clinical practice to coadminister (with levodopa) a peripheral DOPA decarboxylase inhibitor (DDCI) such as carbidopa (medicines containing carbidopa, either alone or in combination with levodopa, are branded as Lodosyn (Aton Pharma) Sinemet (Merck Sharp & Dohme Limited), Pharmacopa (Jazz Pharmaceuticals), Atamet (UCB), Syndopa and Stalevo (Orion Corporation) or with a benserazide (combination medicines are branded Madopar or Prolopa), to prevent the peripheral synthesis of dopamine from levodopa). However, when consumed as a botanical extract, for example from M pruriens supplements, a peripheral DOPA decarboxylase inhibitor is not present. Inbrija (previously known as CVT-301) is an inhaled powder formulation of levodopa indicated for the intermittent treatment of "off episodes" in patients with Parkinson's disease currently taking carbidopa/levodopa. It was approved by the US Food and Drug Administration (FDA) on 21 December 2018, and is marketed by Acorda Therapeutics. Coadministration of pyridoxine without a DDCI accelerates the peripheral decarboxylation of levodopa to such an extent that it negates the effects of levodopa administration, a phenomenon that historically caused great confusion. In addition, levodopa, co-administered with a peripheral DDCI, is efficacious for the short-term treatment of restless leg syndrome. The two types of response seen with administration of levodopa are:
Sources: en.wikipedia.org
== Further reading == Aziz, Nusrate; Asadullah, M Niaz (2017). "Military spending, armed conflict and economic growth in developing countries in the post–Cold War era" (PDF). Journal of Economic Studies. 44 (1): 47–68. doi:10.1108/JES-01-2015-0021. Bartel, Fritz (2022). The Triumph of Broken Promises: The End of the Cold War and the Rise of Neoliberalism. Harvard University Press. ISBN 9780674976788. Henriksen, Thomas H. (2017). Cycles in US Foreign Policy Since the Cold War. Palgrave Macmillan. Jones, Bruce D.; Stedman, Stephen John (2017). "Civil Wars & the Post–Cold War International Order". Dædalus. 146 (4): 33–44. Menon, Rajan; Rumer, Eugene B., eds. (2015). Conflict in Ukraine: The Unwinding of the Post–Cold War Order. MIT Press. Peterson, James W. (2017). Russian-American relations in the post–Cold War world. Oxford UP. Sakwa, Richard (2017). Russia against the Rest: The Post–Cold War Crisis of World Order. Cambridge UP. p. 362. Wood, Luke B. (2017). "The politics of identity and security in post–Cold War Western and Central Europe". European Politics and Society. 18 (4): 552–556. doi:10.1080/23745118.2017.1376444.
In biochemistry and molecular biology, a binding site is a region on a macromolecule such as a protein that binds to another molecule with specificity. The binding partner of the macromolecule is often referred to as a ligand. Ligands may include other proteins (resulting in a protein–protein interaction), enzyme substrates, second messengers, hormones, or allosteric modulators. The binding event is often, but not always, accompanied by a conformational change that alters the protein's function. Binding to protein binding sites is most often reversible (transient and non-covalent), but can also be covalent reversible or irreversible.
=== Blood pool labeling === When 99mTc is combined with a tin compound, it binds to red blood cells and can therefore be used to map circulatory system disorders. It is commonly used to detect gastrointestinal bleeding sites as well as ejection fraction, heart wall motion abnormalities, abnormal shunting, and to perform ventriculography.
The two studies where inconclusive for other outcome measures (including all-cause and cardiovascular mortality, non‐fatal myocardial infarction, non‐fatal stroke, congestive heart failure, serious and non‐serious adverse events (increased “non‐serious side effects” where concluded only for acarbose), hypoglycaemia, measures of blood glucose control, blood lipids, body weight, and blood pressure).
Sources: en.wikipedia.org
Storage at -20 °C or below is standard for long-term stability. Desiccant and sealed containers limit moisture exposure. Solutions are prepared only when needed.
Reverse-phase HPLC separates components by hydrophobicity and reports purity as a percentage of total peak area. It does not by itself confirm molecular identity. Mass spectrometry is used alongside it for that purpose.
They link a specific lot to its analytical results and storage history. This traceability supports reproducibility when results differ between experiments. Records also help identify when re-testing is warranted.
PT-141 was the development code used for bremelanotide during its preclinical and early clinical programme. The peptide is now generally referred to by its international nonproprietary name.