freeze-thaw cycle comes up often in conversation and rarely with the context attached. Here we lay out the basics in order, then work through the practical considerations.
Updated 2026-02-12. Numbers and descriptions here follow the published literature rather than marketing material.
Bremelanotide is a synthetic cyclic heptapeptide developed under the research code PT-141. The code reflects its position in an internal compound series rather than a chemical classification, and the name bremelanotide was later adopted for regulatory filings. Structurally it belongs to the melanocortin peptide family and shares a core sequence motif with alpha-melanocyte-stimulating hormone. The compound is supplied as an acetate salt in aqueous solution for injection. In reference literature it is indexed under both the code and the generic name, a dual listing that can complicate database searches.
Early work on melanocortin analogs in the 1980s and 1990s produced peptides intended to influence pigmentation and appetite. One of these, melanotan II, was observed to affect sexual desire as an incidental finding in self-administration reports. Researchers then pursued analogs with altered receptor selectivity and improved handling characteristics, and PT-141 emerged from that program in the late 1990s. The development path moved from dermatology and metabolism toward a central nervous system application, a shift that shaped both trial designs and the eventual label.
Regulatory review of bremelanotide concluded in 2019 with approval in the United States for a defined indication in premenopausal women. The reviewed formulation is a single-use prefilled autoinjector given subcutaneously, and its label carries cardiovascular monitoring language tied to blood pressure changes recorded during trials. Availability outside the approving jurisdiction varies, and in several countries the compound remains unapproved or is handled as a prescription-only item. Compounded and research-grade material also circulates, and it differs from the reviewed product in purity, characterization, and chain of custody.
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.
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.
| Property | Value | Notes |
|---|---|---|
| Molecular formula | C50H68N14O10 | Cyclic heptapeptide core |
| Molecular weight | 1025.2 g/mol | Calculated for the free base |
| CAS registry number | 189691-06-3 | Free base; the acetate salt has a separate entry |
| Solubility class | Water-soluble | Freely soluble in aqueous media near neutral pH |
| Typical storage | 2 to 8 °C | Protect from light; avoid repeated freeze-thaw cycles |
Melanocortin receptors form a family of five G-protein-coupled receptors designated MC1 through MC5. Bremelanotide binds most strongly at MC4R and MC1R, with weaker activity reported at MC3R and MC5R. MC4R is expressed in hypothalamic nuclei that coordinate energy balance and aspects of sexual behaviour. The prevailing interpretation is that central MC4R activation, rather than peripheral vascular effects alone, drives the reported changes in desire. This account remains partly inferential, since direct receptor-level measurement in living humans is not practical.
After subcutaneous administration, plasma concentrations rise within roughly thirty minutes and the elimination half-life is short, on the order of two to three hours. Reported physiological responses include transient increases in blood pressure and nausea, which tended to diminish with repeated dosing in trial settings. Because the peptide clears quickly, effects are not expected to persist long after a dose. Absorption from non-injected routes is poorly characterised, and nasal delivery produced variable plasma levels in older work.
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.
Published studies differ in design, population and endpoint definition, so results are not always directly comparable across reports. Some trials used patient-reported measures of desire and distress, while others tracked physiological or behavioural outcomes. Questions that remain open include the durability of effects beyond short follow-up periods, the frequency of transient blood pressure elevation observed after administration, and whether a subtype-selective analogue could separate central effects from pigmentation-related activity. These points are usually framed as unresolved rather than settled in review literature.
Approved use is narrow and jurisdiction-specific. In the United States, the injectable product is authorised for premenopausal women with acquired, generalised hypoactive sexual desire disorder, a diagnosis that requires documented distress. It is not approved for men, for postmenopausal women, or for use alongside hormonal contraceptives under the approved labelling. Outside regulated markets, the same peptide is frequently sold as a research chemical, where identity, purity, and sterility are not independently verified.
Bremelanotide is a synthetic cyclic heptapeptide developed as an analogue of alpha-melanocyte-stimulating hormone, a naturally occurring peptide involved in pigmentation and appetite signalling. Its structure contains seven amino acid residues joined by a lactam bridge that closes the ring between two side chains. The molecular formula is C50H68N14O10 and the nominal molecular mass is near 1025 daltons. Much of the early laboratory literature refers to the same molecule by the development code PT-141.
The compound emerged from a research programme examining melanocortin analogues for effects on skin pigmentation. During early human studies, participants reported spontaneous erections as an unexpected side effect, which redirected development toward sexual function rather than tanning. An intranasal formulation was investigated in clinical trials but did not reach market approval. A subcutaneous injectable version later completed the regulatory process, and the nasal route does not appear in approved labelling.
Rosiglitazone (trade name Avandia) is an antidiabetic drug in the thiazolidinedione class. It works as an insulin sensitizer, by binding to the PPAR in fat cells and making the cells more responsive to insulin. It is marketed by the pharmaceutical company GlaxoSmithKline (GSK) as a stand-alone drug or for use in combination with metformin or with glimepiride. First released in 1999, annual sales peaked at approximately $2.5 billion in 2006; however, following a meta-analysis in 2007 that linked the drug's use to an increased risk of heart attack, sales plummeted to just $9.5 million in 2012. The drug's patent expired in 2012. It was patented in 1987 and approved for medical use in 1999. Despite rosiglitazone's effectiveness at decreasing blood sugar in type 2 diabetes mellitus, its use decreased dramatically as studies showed apparent associations with increased risks of heart attacks and death. Adverse effects alleged to be caused by rosiglitazone were the subject of over 13,000 lawsuits against GSK; as of July 2010, GSK had agreed to settlements on more than 11,500 of these suits. Some reviewers recommended rosiglitazone be taken off the market, but an FDA panel disagreed, and it remains available in the US. From November 2011 until November 2013, the US federal government did not allow Avandia to be sold without a prescription from a certified doctor; moreover, patients were required to be informed of the risks associated with its use, and the drug had to be purchased by mail order through specified pharmacies.
== External links == tRNA, the Adaptor Hypothesis and the Wobble Hypothesis Wobble base-pairing between codons and anticodons Genetic Code and Amino Acid Translation Information of Aminoacyl tRNA Synthetases Genomic tRNA Database
=== Relative bioavailability === Relative bioavailability compares the bioavailability between two different dosage forms. Again, the relative AUCs are used to make this comparison and relative doses are used to normalize the calculation.
Sources: en.wikipedia.org
== H == HAART – hairy leukoplakia – half-life – HAM/TSP – Health Care Financing Administration (HCFA) – Health Resources and Services Administration (HRSA) – HELLP syndrome – helper T cells – helper/suppressor ratio (of T cells) – hematocrit – hematotoxic – hemoglobin – hemolysis – hemophilia – hepatic – hepatic steatosis – hepatitis – hepatitis C and HIV coinfection – hepatomegaly – herpes simplex virus 1 (HSV-1) – herpes simplex virus 2 (HSV-2) – herpes varicella zoster virus (VZV) – herpes viruses – highly active antiretroviral therapy (HAART) – histocompatibility testing – histoplasmosis – HIV disease – HIV prevention trials network (HPTN) – HIV set point – HIV vaccine trials network (HVTN) – HIV-1 – HIV-2 – HIV-associated dementia – HIV-related tuberculosis – HLA – Hodgkin's disease – holistic medicine – homology (biology) – hormone – host – host factors – HPTN – HPV – HRSA – HTLV-I – HTLV-I-associated myelopathy/tropical spastic paraparesis (HAM/TSP) – HTLV-II – human growth hormone (HGH) – human immunodeficiency virus type 1 (HIV-1) – human immunodeficiency virus type 2 (HIV-2) – human leukocyte antigens (HLA) – human papilloma virus (HPV) – human T cell lymphotropic virus type I (HTLV-I) – human T cell lymphotropic virus type II (HTLV-II) – humoral immunity – HVTN – hydroxyurea – hypergammaglobulinemia – hyperglycemia – hyperlipidemia – hyperplasia – hyperthermia – hypogonadism – hypothesis – hypoxia
==== Other applications ==== Other MALS applications include nanoparticle sizing, protein aggregation studies, protein-protein interactions, electrophoretic mobility or zeta potential. MALS techniques have been adopted for the study of pharmaceutical drug stability, crystal nucleation and crystallization kinetics and use in nanomedicine.
Fourier-transform ion cyclotron resonance mass spectrometry (also known as Fourier-transform mass spectrometry) is a type of mass spectrometry used for determining the mass-to-charge ratio (m/z) of ions based on the cyclotron frequency of the ions in a fixed magnetic field. The ions are trapped in a Penning trap where they are excited to a larger cyclotron radius by an oscillating electric field perpendicular to the magnetic field. The excitation also results in the ions moving in phase (in a packet). The signal is detected as an image current on a pair of plates which the packet of ions passes close to as they cyclotron. The resulting signal is called a free induction decay (fid), transient or interferogram that consists of a superposition of sine waves. The useful signal is extracted from this data by performing a Fourier transform to give a mass spectrum. Single ions can be investigated in a Penning trap held at a temperature of 4 K. For this the ring electrode is segmented and opposite electrodes are connected to a superconducting coil and the source and the gate of a field-effect transistor. The coil and the parasitic capacitances of the circuit form a LC circuit with a Q of about 50 000. The LC circuit is excited by an external electric pulse. The segmented electrodes couple the motion of the single electron to the LC circuit. Thus the energy in the LC circuit in resonance with the ion slowly oscillates between the many electrons (10000) in the gate of the field effect transistor and the single electron.
Sources: en.wikipedia.org
=== Other uses in science and technology === Rapid prototyping, a manufacturing and engineering process Rear projection effect, a film technique Red phosphorus, an allotrope of the element Rendezvous Point in Protocol Independent Multicast, a collection of network layer multicast routing protocols Reversed-phase chromatography, a laboratory technique Route Processor, a general-purpose CPU in some Cisco routers RP, a small rock climbing nut, named after Roland Pauligk RP-1, a rocket propellant RP-3, a British rocket projectile in World War II
=== Synthetic === The availability of synthetic toxin has allowed scientists to further explore the biological activity of the toxin, resulting in the observation that d-ACTX-Ar1a causes repetitive firing and prolongation of the action potential. These actions underlie the clinical symptoms seen following envenomation and further contribute to the understanding of the molecular basis for activity of this potent neurotoxin on voltage-gated sodium channels. Under voltage-clamp conditions in dorsal root ganglion (DRG) neurons it was found that the effects of the synthetic toxin on sodium currents were not significantly different from those previously reported for the native toxin. Neither native nor synthetic d-ACTX-Ar1a had any effect on TTX-resistant sodium currents, but both exerted a potent selective modulation of TTX-sensitive sodium currents consistent with actions on neurotoxin receptor site-3. This includes a slowing of the sodium-channel inactivation, a hyperpolarizing shift in the voltage-dependence of activation and a hyperpolarizing shift in the steady-state sodium-channel inactivation. d-ACTX-Ar1a causes a prolongation of action potential duration, accompanied by spontaneous repetitive firing, but does not depolarize the resting membrane potential. Effects on the autonomic nervous system, including vomiting, profuse sweating, salivation, lachrymation, marked hypertension followed by hypotension, together with effect on the somatic nervous system to cause muscle fasciculation and dyspnea (shortness of breath) are presumably due to excessive transmitter release.
== Structure and variations == The amino acid sequence is reasonably highly conserved among mammalian species, with the native human sequence being YQPPSTNKNTKSQRRKGSTFEERK or Tyr‑Gln‑Pro‑Pro‑Ser‑Thr‑Asn‑Lys‑Asn‑Thr‑Lys‑Ser‑Gln‑Arg‑Arg‑Lys‑Gly‑Ser‑Thr‑Phe‑Glu‑Glu‑Arg‑Lys. However, there are also several other splice variants of IGF-1 aside from MGF. Multiple forms of MGF are used in scientific research such as the native mouse or rat form which have several amino acid changes, or a consensus sequence mainly based on the human sequence but with the arginine changed to histidine at position 23 and sometimes other substitutions, to more closely resemble the rodent version. Since mechano growth factor is rapidly metabolised in vivo, synthetic versions often use two or more unnatural D-amino acids in the central "QRRK" domain at positions 13-16 to improve metabolic stability, the tyrosine residue may also be PEGylated in these peptides.
Middle ear barotrauma (MEBT) is an injury caused by a difference in pressure between the external ear canal and the middle ear. It is common in underwater divers and usually occurs when the diver does not equalise sufficiently during descent or, less commonly, on ascent. Failure to equalise may be due to inexperience or eustachian tube dysfunction, which can have many possible causes. Unequalised ambient pressure increase during descent causes a pressure imbalance between the middle ear air space and the external auiditory canal over the eardrum, referred to by divers as ear squeeze, causing inward stretching, serous effusion and haemorrhage, and eventual rupture. During ascent internal over-pressure is normally passively released through the eustachian tube, but if this does not happen the volume expansion of middle ear gas will cause outward bulging, stretching and eventual rupture of the eardrum known to divers as reverse ear squeeze. This damage causes local pain and hearing loss. Tympanic rupture during a dive can allow water into the middle ear, which can cause severe vertigo from caloric stimulation. This may cause nausea and vomiting underwater, which has a high risk of aspiration of vomit or water, with possible fatal consequences.
Sources: en.wikipedia.org
PT-141 was the internal development code assigned to bremelanotide during preclinical research. Reference sources sometimes index the peptide under the code rather than the generic name. The two terms describe the same molecule.
They are related but distinct cyclic peptides. Melanotan II is an earlier and less selective melanocortin analog. Bremelanotide was developed later with a narrower receptor profile and a defined clinical application.
The reviewed product is given by subcutaneous injection using a single-use autoinjector. Administration occurs under prescription supervision. The device delivers one fixed amount per use rather than a measured or adjustable quantity.
It is frequently described as an MC4 receptor agonist, but binding assays show activity at more than one melanocortin subtype. Selectivity is therefore relative rather than absolute. This matters when interpreting side effects tied to other receptor subtypes.