Ipamorelin is a synthetic pentapeptide that acts as an agonist at the growth hormone secretagogue receptor type 1a (GHS-R1a), the receptor for the endogenous peptide ghrelin. Laboratory studies use it to examine ghrelin receptor signalling and the regulation of growth hormone secretion in cultured pituitary cells and animal models, and it is notable for having been described as unusually selective among compounds of its class. This article summarises its structure, mechanism, published research areas and the analytical and storage standards needed for reproducible work. It is written for laboratory researchers and offers no guidance on human use.
Wondering how ipamorelin compares with a GHRH analogue? See our side-by-side guide to ipamorelin vs tesamorelin, covering receptors, structure and UK rules.
What is ipamorelin?
Ipamorelin is classified as a synthetic growth hormone secretagogue and a pentapeptide. Its sequence is Aib-His-D-2-Nal-D-Phe-Lys-NH2, where Aib is alpha-aminoisobutyric acid and D-2-Nal is D-2-naphthylalanine. Three of the five residues are non-natural, which gives the molecule resistance to proteolytic cleavage, and the C-terminus is amidated. The molecular formula is C38H49N9O5 and the molecular weight is approximately 712 Da, which makes it a small molecule by peptide standards and far simpler to characterise than larger analogues.
The compound belongs to a lineage of synthetic secretagogues that began in the late 1970s and early 1980s, when Cyril Bowers and colleagues described enkephalin-derived peptides that released growth hormone from pituitary cells through a mechanism distinct from that of growth hormone-releasing hormone. The best known of these early compounds is growth hormone-releasing peptide 6 (GHRP-6). The receptor they acted on remained unidentified for years. It was cloned in 1996, and in 1999 Kojima and colleagues identified ghrelin, a 28-residue acylated peptide from the stomach, as its natural ligand, which explained why synthetic secretagogues worked.
Ipamorelin was developed by Novo Nordisk in the 1990s, and its pharmacological characterisation was reported by Raun and colleagues in 1998 in the European Journal of Endocrinology under the title describing it as the first selective growth hormone secretagogue. It has not been authorised as a medicine in the United Kingdom, and clinical development did not lead to a marketed product.
Researchers will encounter the compound sold as ipamorelin acetate, and sometimes under shorthand such as “ipa”. The salt form affects the stated gross weight, so the certificate of analysis should name it. Regulatory context matters for any UK buyer: the compound appears on the World Anti-Doping Agency prohibited list, and the Medicines and Healthcare products Regulatory Agency (MHRA) has issued warnings about unlicensed products marketed for human use. It is relevant here only as a laboratory research reagent.
Mechanism of action
Ipamorelin acts on GHS-R1a, a class A G protein-coupled receptor expressed in the anterior pituitary, in the hypothalamus and in several other tissues. The receptor couples mainly to the G protein Gq/11. Agonist binding activates phospholipase C, which cleaves a membrane phospholipid into inositol trisphosphate and diacylglycerol. Inositol trisphosphate releases calcium from intracellular stores, and diacylglycerol activates protein kinase C. The resulting rise in intracellular calcium promotes exocytosis of stored growth hormone from the secretory granules of somatotroph cells. This is a separate route from the cyclic AMP pathway used by the growth hormone-releasing hormone receptor, and the difference is the reason the two classes of compound can be studied together.
GHS-R1a has a notable property: it shows substantial constitutive activity, meaning it signals even without a ligand. This complicates interpretation of agonist experiments, because the baseline is not zero, and inverse agonists and antagonists are used alongside agonists to define the receptor’s contribution. Researchers therefore usually test ipamorelin in cells expressing the receptor and compare responses with untransfected cells and with a known antagonist.
A second mechanistic point concerns selectivity. Several older secretagogues also raised adrenocorticotropic hormone (ACTH) and cortisol in the animal models used, which was interpreted as activity beyond the somatotroph. Ipamorelin was reported not to do so at the exposures tested, which is why it was described as selective. That claim comes from specific models and exposures and should be checked against the primary paper and not assumed to hold in other systems.
In cell-based work, the usual readouts are calcium flux, inositol phosphate accumulation and growth hormone released into the medium of primary pituitary cultures. In these systems, “dosage” means molar concentration in the medium, which depends on net peptide content, adsorption to plasticware and protease activity in any serum used. Because ipamorelin is resistant to many peptidases, its effective concentration is more stable than that of a native peptide, though it should still be verified.
What the research shows
The literature on ipamorelin is smaller than that on the ghrelin receptor as a whole. Four areas are summarised below in general terms. Readers should consult the primary papers for methods, concentrations and statistics, because commercial summaries often overstate findings.
The first area is the original characterisation. Raun and colleagues compared ipamorelin with GHRP-6 and related compounds in cultured rat pituitary cells, and in anaesthetised rats and conscious swine. They reported that the compound released growth hormone in a dose-related manner with a potency comparable to the comparators, and that, unlike some of them, it did not produce significant rises in ACTH or cortisol at the exposures tested. Exposures in the animal experiments were expressed per kilogram of body weight and differ with species and route, so the values should be taken from the paper. The work can be located through a PubMed search for the ipamorelin characterisation study.
The second area is the identification of the receptor and its natural ligand. The cloning of the receptor in 1996 and the isolation of ghrelin in 1999 by Kojima and colleagues established the physiology behind synthetic secretagogues. These papers describe receptor pharmacology in transfected cells and in rodent tissues, and they are the foundation for work on ipamorelin.
The third area is the interaction between the secretagogue and growth hormone-releasing hormone pathways. Work with GHRP-6 and related peptides showed that combining a secretagogue with a releasing-hormone analogue produced a larger growth hormone response than either alone in cultured pituitary cells and in animal studies. Proposed explanations include convergence of the cyclic AMP and calcium pathways and effects on somatostatin tone. The topic can be explored through a PubMed search on synergistic growth hormone release by GHRH and GH-releasing peptides.
The fourth area is the broader secretagogue literature in animal models, including studies of the ghrelin system in feeding, gastrointestinal motility and the regulation of the growth hormone axis. Where ipamorelin features in these papers, it is typically one of several comparators. These are controlled experiments under ethical approval, with exposures expressed per kilogram, and they carry no implication for use outside the laboratory.
A recurring limitation runs through this body of work. Studies use different species, routes and vehicles, results from native ghrelin and from other secretagogues are sometimes extended to ipamorelin without direct testing, and the exact material used is not always characterised analytically. Reproducibility therefore depends on knowing exactly what was in the vial.
Research applications
In the laboratory, ipamorelin is used in several contexts. The most common is ghrelin receptor pharmacology, where it serves as a selective agonist at GHS-R1a to characterise signalling, benchmark novel ligands and validate calcium and inositol phosphate assays. Its small size and stability make it convenient in assays that run for hours.
A second context is the study of selectivity. Because ipamorelin was described as selective for growth hormone release, it is used as a reference when comparing newer secretagogues in terms of effects on other pituitary hormones, with ACTH and prolactin measured in parallel.
A third context is neuroendocrine physiology in animal models, where secretagogues are used to probe the dynamics of the growth hormone axis, somatostatin tone and the interaction with releasing-hormone signalling. These experiments require institutional ethical review and, in the United Kingdom, authorisation under the Animals (Scientific Procedures) Act 1986.
A fourth context is analytical and anti-doping science. Growth hormone secretagogues appear on the World Anti-Doping Agency prohibited list, and reference samples of defined identity and purity are used to develop and validate liquid chromatography and mass spectrometry methods for their detection. For this purpose, accurate mass and quantified purity are essential, because a method is only as reliable as the reference material.
Across these settings, the sensible meaning of exposure is a defined concentration or per-kilogram quantity, justified by prior literature and verified by analytical characterisation of the material actually used.
Purity, storage and handling
A pentapeptide with three non-natural residues is straightforward in length but not trivial in chemistry. The hindered Aib residue can couple slowly, the D-amino acids must be incorporated without racemisation, and the bulky naphthylalanine residue can lead to side reactions during cleavage. Reversed-phase high-performance liquid chromatography (HPLC) with UV detection at 214 or 220 nm is the standard purity method, and a main peak of at least 98 per cent is commonly expected for receptor pharmacology work. The naphthylalanine residue absorbs strongly at longer wavelengths, so a second wavelength such as 280 nm helps to identify the peptide and to detect impurities lacking the aromatic group. A stated purity is meaningful only if the method and chromatogram are available.
HPLC must be paired with mass spectrometry. The expected mass is about 712 Da. A mass about 16 Da higher suggests oxidation, a mass lower by the weight of a residue indicates a deletion product, and a species of the right mass but different retention time may be a diastereomer, which would be a significant problem for a peptide whose activity depends on its D-amino acids. A clean peak at the wrong mass is a clean peak of the wrong compound.
A batch-specific certificate of analysis matters because purity and peptide content vary between synthesis runs. It should carry a lot number matching the vial, the HPLC trace or purity value, the observed and theoretical masses, the test date and ideally the salt form and net peptide content. Lyophilised powders contain counterions and residual water, so the weighed mass is not the mass of peptide, and ignoring this introduces systematic errors into any concentration-response comparison.
Because racemisation and degradation can alter a peptide whose activity depends on stereochemistry, batch-level verification matters more here than with a simple linear sequence. UK laboratories sourcing Ipamorelin should expect a batch-specific certificate of analysis that states the salt form and the mass found, with lyophilised storage at -20°C, and the same documentation standard applies when weighing any best peptides supplier uk claim, since a supplier’s analytical record rather than its search ranking shows whether the vial matches its stated specification.
Lyophilised ipamorelin is generally stored at -20°C in a sealed container with desiccant, and many laboratories use -80°C for long-term archives. Vials should reach room temperature before opening so that condensation does not form on the cold powder. After reconstitution, stability falls. Standard practice is to dissolve the peptide in sterile water or a suitable buffer, divide the solution into single-use aliquots and freeze them to avoid repeated freeze-thaw cycles. Refrigerated storage at 2 to 8°C is sometimes used for solutions consumed within days, but the acceptable period should be validated for the assay concerned, preferably by re-running HPLC on stored solution.
Light sensitivity is a secondary but genuine concern. The naphthylalanine, phenylalanine and histidine side chains can undergo photo-oxidation, so amber vials or dark storage are advisable, and prolonged exposure to bright laboratory lighting should be avoided. Peptides also adsorb to glass and some plastics at low concentrations, so low-bind tubes are preferred for dilute working solutions.
Labelling is a final indicator of quality. Clear naming, sequence, molecular weight, lot number, salt form and an unambiguous research-only statement suggest a supplier that understands its market, while any wording that implies use in humans is a warning sign and raises regulatory concerns in the United Kingdom.
Frequently asked questions
What is ipamorelin used for in laboratory research?
It is used as a selective agonist of the ghrelin receptor GHS-R1a in cell-based pharmacology, in studies of growth hormone secretion in cultured pituitary cells and approved animal models, and as a reference material in anti-doping analytical methods. It is a research reagent and not an authorised medicine in the United Kingdom.
What is the molecular weight and sequence of ipamorelin?
Ipamorelin is a pentapeptide with the sequence Aib-His-D-2-Nal-D-Phe-Lys-NH2 and a molecular weight of about 712 Da. It contains three non-natural residues. The certificate of analysis should show a mass spectrometry result matching this value.
How should ipamorelin be stored for research?
Keep lyophilised peptide at -20°C, or -80°C for long-term archiving, in a sealed, dry, light-protected container. Allow vials to reach room temperature before opening. After reconstitution, aliquot and freeze the solution to avoid repeated freeze-thaw cycles and prolonged storage in solution.
How can I verify the purity of an ipamorelin batch?
Request a batch-specific certificate of analysis showing HPLC purity and a mass spectrometry result near 712 Da, with the salt form and a lot number matching the vial. For critical work, independent testing by an accredited analytical laboratory confirms identity, purity and net peptide content.
Ipamorelin is supplied for in-vitro laboratory research only and is not intended for human or veterinary use.
