Ipamorelin: The Selective Ghrelin Receptor Agonist in the Research Literature

Research-use-only context. This article summarizes published third-party scientific literature and laboratory practice. It is not medical advice, not a therapeutic or performance claim, and not a usage guide. US Prime Peptides products are supplied strictly for in vitro laboratory research and are not for human or veterinary use.

This article explains what ipamorelin is as a molecule, which receptor it acts on and how that receptor was discovered, what the 1998 characterization paper and later cell, animal and volunteer studies report about growth hormone secretion, how the Novo Nordisk and Helsinn development programs ended, and what a research certificate should show for a pentapeptide of this kind.

Ipamorelin is a five-residue ghrelin receptor agonist

Ipamorelin is the pentapeptide Aib-His-D-2-Nal-D-Phe-Lys-NH2, where Aib is alpha-aminoisobutyric acid and D-2-Nal is D-3-(2-naphthyl)alanine. It has the formula C38H49N9O5, a molecular weight of about 711.9 Da, and no natural counterpart; it is a wholly synthetic compound built to mimic the growth hormone-releasing peptides (GHRPs) with fewer residues. Three of its five positions are non-standard or D-configured, which is why it is not cleaved by the common proteases that inactivate natural peptides.

Raun and colleagues at Novo Nordisk described its origin in a 1998 European Journal of Endocrinology paper titled "Ipamorelin, the first selective growth hormone secretagogue." The compound came out of a chemistry program that systematically deleted the central Ala-Trp dipeptide from GHRP-1 (Ala-His-D-2-Nal-Ala-Trp-D-Phe-Lys-NH2) and replaced the N-terminal alanine with Aib. The result keeps the two D-aromatic residues and the C-terminal lysine amide that the receptor requires, which is the pharmacophore the anti-doping structure-activity work discussed below later confirmed.

The hexapeptides that preceded it are covered in the GHRP-2 and GHRP-6 overview. Ipamorelin is often paired in the literature and in blends with a GHRH analog that acts on a different receptor entirely; that distinction is the subject of CJC-1295 vs ipamorelin.

The receptor was found before its natural ligand

Ipamorelin acts on the growth hormone secretagogue receptor type 1a (GHS-R1a), now called the ghrelin receptor, a G-protein-coupled receptor that was cloned three years before anyone knew what hormone it normally binds. Howard and colleagues at Merck, in a 1996 Science paper, used synthetic secretagogues as the bait to clone the receptor from swine and human pituitary and from the arcuate, ventromedial and infundibular hypothalamus, and argued from its pharmacology that the synthetic molecules must be mimicking an undiscovered hormone. Kojima and colleagues supplied that hormone in a 1999 Nature paper, purifying a 28-residue peptide from rat stomach with an n-octanoyl group on serine 3 and naming it ghrelin; the acyl group was essential for activity at the receptor.

The receptor's structure is now known at near-atomic resolution. Liu and colleagues, in a 2021 Nature Communications paper, reported cryo-electron microscopy structures of the human ghrelin receptor coupled to Gi, bound either to ghrelin or to the non-peptide agonist ibutamoren, and identified a salt bridge and an aromatic cluster near the binding pocket as motifs important for activation. Ipamorelin's two D-aromatic residues are generally modeled into that aromatic cluster.

Structure-activity data for the peptide class come from an unexpected source: anti-doping laboratories. Ferro and colleagues, in a 2017 Drug Testing and Analysis paper, used a radio-competition assay against labeled ghrelin at GHS-R1a to map which positions in the common GHRP core tolerate change, and confirmed that intact ipamorelin and its active metabolites recovered from excretion studies still bound the receptor. Fowkes and colleagues, in a 2018 European Journal of Medicinal Chemistry paper, built peptidomimetic secretagogue derivatives as positron emission tomography ligands for imaging the same receptor.

What the 1998 characterization paper measured

The defining data on ipamorelin come from Raun and colleagues' 1998 paper, which compared it head-to-head with GHRP-6 and GHRP-2 across in vitro and in vivo systems. In primary rat pituitary cells, ipamorelin released growth hormone with a half-maximal concentration of about 1.3 nM and a maximal response about 85 percent of that of GHRP-6 (2.2 nM, 100 percent). Profiling with GHRP and GHRH antagonists showed that the response ran through the GHRP-type receptor and not through the GHRH receptor. In pentobarbital-anesthetized rats and in conscious swine, potency and maximal response were again comparable to GHRP-6, while GHRP-2 was more potent but less efficacious.

The claim in the paper's title rests on the swine data. None of the secretagogues tested changed FSH, LH, prolactin or TSH. GHRP-6 and GHRP-2, however, both raised plasma ACTH and cortisol, whereas ipamorelin did not raise either above the levels seen after GHRH itself, even at amounts more than 200 times its half-maximal value for growth hormone release. That dissociation of the growth hormone response from the ACTH and cortisol response, in pigs, is what "selective" means in the ipamorelin literature; it is a statement about pituitary hormone specificity in one species, not a claim about anything downstream.

Hansen and colleagues, also from Novo Nordisk, showed in a 2001 Bioorganic and Medicinal Chemistry Letters paper that reintroducing ipamorelin's C-terminal pharmacophores into the orally available small molecule NN703 produced hybrids with unexpectedly high potency in the rat pituitary cell assay and in pigs, which indicates how the two chemical series were related within the company's program.

Human volunteer data and the development history

Ipamorelin has been studied in people in two settings, and neither led to an approved product. Gobburu, Agerso, Jusko and Ynddal published a pharmacokinetic-pharmacodynamic model in a 1999 Pharmaceutical Research paper based on healthy male volunteers given one of five amounts over fifteen minutes, eight per level. Exposure was proportional to the amount given, the terminal half-life was about two hours, and clearance was 0.078 liters per hour per kilogram. Growth hormone rose in a single episode that peaked at about 0.67 hours and then declined exponentially; an indirect-response model with a release phase of finite duration described the data, with a half-maximal concentration of 214 nmol/L.

The second program belonged to Helsinn, which licensed the compound for postoperative ileus. Venkova and colleagues, in a 2009 J Pharmacol Exp Ther paper, reported that ipamorelin shortened the time to first bowel movement in a rat model of surgical ileus. Beck and colleagues then ran a multicenter, double-blind, placebo-controlled phase 2 proof-of-concept study, published in 2014 in the International Journal of Colorectal Disease: 117 adults were enrolled after bowel resection, the key endpoint was time to tolerance of a standard solid meal, and the median was 25.3 hours with ipamorelin versus 32.6 hours with placebo (p = 0.15). No secondary endpoint separated from placebo either, and the program did not continue.

Ipamorelin is therefore not an approved drug anywhere; it has no approved indication, no brand name, and no product monograph. It is also listed under section S2 of the World Anti-Doping Agency Prohibited List as a growth hormone secretagogue. A research vial carries none of the clinical history above with it; the only data that apply to a given lot are the data on its certificate.

Other model systems in the recent literature

Because the compound is commercially available as a chemical, it continues to appear as a ghrelin receptor tool in species far from the original pig and rat work. Gouda and Ganesh, in a 2024 Animal Reproduction Science paper, used ipamorelin acetate as a ghrelin agonist in the tilapia Oreochromis mossambicus and reported changes in pituitary luteinizing hormone and in testicular androgen receptor expression, which they interpreted as ghrelin-axis effects on the fish reproductive axis. Studies of this kind are useful mainly as evidence that the receptor pharmacology is conserved across vertebrates; they say nothing about mammals and are not replications of the 1998 findings.

What the literature does not contain is as important as what it does. There are no published cell-culture studies showing effects of ipamorelin in tissues that lack the ghrelin receptor, no human studies outside the two programs described above, and no long-term exposure data in any species. Claims that go beyond pituitary growth hormone secretion in the cited models are not supported by the primary literature.

What a certificate should show for ipamorelin

Ipamorelin is small enough that mass spectrometry sees it as a single protonated ion near m/z 712.9, with a doubly charged ion near 356.9, and the identity line on the certificate should quote the observed value against that expectation. What mass spectrometry cannot do is confirm the stereochemistry of the two D-residues or distinguish Aib from any other 86-Da residue, so a sequence with the correct mass is a necessary but not sufficient identity check; this limitation is explained in LC-MS identity confirmation. Reverse-phase HPLC retention is strongly influenced by the two naphthyl and phenyl side chains, so the main peak elutes late and well separated from polar impurities, and purity should be reported as area percent with the chromatogram attached.

Net peptide content is worth reading closely for a lysine-containing pentapeptide with one free basic amine and the histidine imidazole: as an acetate or trifluoroacetate salt, counter-ions can account for a meaningful share of the vial mass, a point covered in TFA salt vs acetate salt. Endotoxin by LAL, sterility and ICP-MS metals complete the panel. US Prime Peptides posts the lot-matched independent certificate for Ipamorelin 5 mg and Ipamorelin 10 mg on each product page before purchase. The two-component CJC-1295 / Ipamorelin 5 mg / 5 mg vial requires a certificate that identifies and quantifies each peptide separately, since the two are different sizes, act on different receptors and elute at different times.

Frequently Asked Questions

What is ipamorelin?

Ipamorelin is a synthetic pentapeptide, Aib-His-D-2-Nal-D-Phe-Lys-NH2, with a molecular weight of about 712 Da. It was identified at Novo Nordisk in the 1990s as a growth hormone secretagogue acting on the ghrelin receptor (GHS-R1a), and the 1998 characterization paper reported that it released growth hormone in rat pituitary cells, rats and pigs without raising ACTH or cortisol in pigs.

Is ipamorelin approved by the FDA?

No. Ipamorelin is not an approved drug in the United States or anywhere else. Novo Nordisk's original program did not produce a product, and Helsinn's phase 2 study in postoperative ileus, published in 2014, found no significant difference from placebo. It is also listed on the WADA Prohibited List.

What receptor does ipamorelin act on?

Ipamorelin is an agonist of the growth hormone secretagogue receptor 1a, the ghrelin receptor, which Howard and colleagues cloned in 1996 before its natural ligand ghrelin was identified by Kojima and colleagues in 1999. It does not act on the GHRH receptor, which is why it is pharmacologically distinct from sermorelin, TH9507 and CJC-1295.

Why is ipamorelin called a selective secretagogue?

In the 1998 Raun paper, GHRP-6 and GHRP-2 raised ACTH and cortisol in pigs while ipamorelin did not, even at amounts far above those needed for maximal growth hormone release. "Selective" refers to that pituitary hormone specificity in swine, not to any effect outside the pituitary.

Citations

1. Howard AD, Feighner SD, Cully DF, et al. A receptor in pituitary and hypothalamus that functions in growth hormone release. Science. 1996;273:974-977. PMID 8688086
2. Raun K, Hansen BS, Johansen NL, et al. Ipamorelin, the first selective growth hormone secretagogue. Eur J Endocrinol. 1998;139:552-561. PMID 9849822
3. Gobburu JV, Agerso H, Jusko WJ, Ynddal L. Pharmacokinetic-pharmacodynamic modeling of ipamorelin, a growth hormone releasing peptide, in human volunteers. Pharm Res. 1999;16:1412-1416. PMID 10496658
4. Kojima M, Hosoda H, Date Y, et al. Ghrelin is a growth-hormone-releasing acylated peptide from stomach. Nature. 1999;402:656-660. PMID 10604470
5. Hansen TK, Ankersen M, Raun K, Hansen BS. Highly potent growth hormone secretagogues: hybrids of NN703 and ipamorelin. Bioorg Med Chem Lett. 2001;11:1915-1918. PMID 11459660
6. Venkova K, Mann W, Nelson R, Greenwood-Van Meerveld B. Efficacy of ipamorelin, a novel ghrelin mimetic, in a rodent model of postoperative ileus. J Pharmacol Exp Ther. 2009;329:1110-1116. PMID 19289567
7. Beck DE, Sweeney WB, McCarter MD, Beart R; Ipamorelin 201 Study Group. Prospective, randomized, controlled, proof-of-concept study of the ghrelin mimetic ipamorelin for the management of postoperative ileus in bowel resection patients. Int J Colorectal Dis. 2014;29:1527-1534. PMID 25331030
8. Ferro P, Krotov G, Zvereva I, Rodchenkov G, Segura J. Structure-activity relationship for peptidic growth hormone secretagogues. Drug Test Anal. 2017;9:87-95. PMID 26811125
9. Fowkes MM, Lalonde T, Yu L, et al. Peptidomimetic growth hormone secretagogue derivatives for positron emission tomography imaging of the ghrelin receptor. Eur J Med Chem. 2018;157:1500-1511. PMID 30282322
10. Liu H, Sun D, Myasnikov A, et al. Structural basis of human ghrelin receptor signaling by ghrelin and the synthetic agonist ibutamoren. Nat Commun. 2021;12:6410. PMID 34737341
11. Gouda M, Ganesh CB. The influence of ghrelin agonist ipamorelin acetate on the hypothalamic-pituitary-testicular axis in a cichlid fish, Oreochromis mossambicus. Anim Reprod Sci. 2024;268:107550. PMID 38996787

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