Ipamorelin Peptide Research: GHS-R1a Selectivity, Growth Hormone Kinetics, and Preclinical Bone Data

Ipamorelin peptide research centres on a synthetic pentapeptide that activates the GHS-R1a receptor to stimulate growth hormone release without the ACTH and cortisol elevation produced by GHRP-6 and GHRP-2. In the paper that introduced it, ipamorelin released GH from rat pituitary cells with an EC50 of 1.3 nM, matched GHRP-6 in anaesthetised rats and conscious swine, and left ACTH and cortisol unchanged even at more than 200 times its ED50 for GH release (Raun et al., European Journal of Endocrinology, 1998, 139:552-561). That selectivity is why ipamorelin remains a reference tool compound for investigators modelling GH-axis physiology in animal models, and this article summarises the published pharmacology with the primary source cited for each claim.

Ipamorelin is a synthetic growth hormone secretagogue (GHS) identified at Novo Nordisk within a series of compounds lacking the central Ala-Trp dipeptide of GHRP-1. Unlike earlier secretagogues such as hexarelin and GHRP-6, it stimulates GH release without proportional changes in ACTH, cortisol, prolactin, FSH, LH or TSH in the species tested, which has made it one of the most studied peptides in growth hormone secretagogue research and a standard comparator in the comparison of growth hormone secretagogues.

For research purposes only. Not for human consumption. Not for diagnostic or therapeutic use.

Chemical Properties and Structure of Ipamorelin

Ipamorelin (CAS 170851-70-4) has the molecular formula C₃₈H₄₉N₉O₅ and a molecular weight of 711.85 g/mol. Its sequence is Aib-His-D-2-Nal-D-Phe-Lys-NH₂, as stated in Raun’s paper and on our ipamorelin product page. The N-terminal alpha-aminoisobutyric acid and the two D-configured aromatic residues, D-2-naphthylalanine and D-phenylalanine, block aminopeptidase and endopeptidase attack, and the C-terminal amide removes the free carboxylate. There is no methionine, tryptophan or cysteine in the sequence, so oxidation is a minor concern in storage; the histidine and naphthyl groups are the light-sensitive features. In a pharmacokinetic and pharmacodynamic study in healthy volunteers, the terminal half-life was 2 hours, clearance was 0.078 L/h/kg, the steady-state volume of distribution was 0.22 L/kg, and GH release occurred as a single episode peaking at 0.67 hours before declining exponentially (Gobburu et al., Pharmaceutical Research, 1999, 16:1412-1416). The two-hour figure is sometimes described as a rodent half-life in secondary sources; it was measured in humans.

Mechanism of Action: GHS-R1a Receptor Binding

Ipamorelin is an agonist at the growth hormone secretagogue receptor type 1a (GHS-R1a), the ghrelin receptor, a G protein-coupled receptor expressed in the anterior pituitary and hypothalamus. Receptor activation couples through Gq/11 to phospholipase C, raising inositol trisphosphate and diacylglycerol and mobilising calcium in somatotrophs, which drives exocytosis of GH-containing granules. Raun and colleagues established the receptor pharmacology with antagonists: GHRP antagonists blocked ipamorelin-induced GH release and a GHRH antagonist did not, demonstrating that ipamorelin, like GHRP-6, acts through the GHRP receptor rather than the GHRH receptor (Raun et al., 1998).

The defining property is functional selectivity. In conscious swine, none of the secretagogues tested affected FSH, LH, prolactin or TSH, but GHRP-6 and GHRP-2 both raised plasma ACTH and cortisol. Ipamorelin did not release ACTH or cortisol at levels significantly different from those seen after GHRH, and that held at more than 200-fold the ED50 for GH release. The authors called ipamorelin the first GHRP-receptor agonist with a selectivity for GH release similar to GHRH itself (Raun et al., 1998). A “selectivity index” figure and per-group animal counts that have been attached to this study in secondary accounts are not in the paper’s abstract and are not repeated here; the abstract’s own statement, no ACTH or cortisol response at over 200 times the GH ED50, is the stronger claim.

Key Research Findings: Growth Hormone Release Kinetics

The potency data are precise and worth stating as published. In primary rat pituitary cells, ipamorelin released GH with an EC50 of 1.3 ± 0.4 nmol/L and an Emax of 85 ± 5 percent of the GHRP-6 maximum, against 2.2 ± 0.3 nmol/L and 100 percent for GHRP-6. In pentobarbital-anaesthetised rats the ED50 was 80 ± 42 nmol/kg with an Emax of 1545 ± 250 ng GH/mL, against 115 ± 36 nmol/kg and 1167 ± 120 ng/mL for GHRP-6. In conscious swine the ED50 was 2.3 nmol/kg with an Emax of 65 ng GH/mL plasma, against 3.9 nmol/kg and 74 ng/mL for GHRP-6; GHRP-2 was more potent (ED50 0.6 nmol/kg) but less efficacious (Emax 56 ng/mL) (Raun et al., 1998). In humans, the concentration producing half-maximal GH stimulation was 214 nmol/L in an indirect-response model, with GH release described as a zero-order burst over a finite duration rather than a sustained elevation (Gobburu et al., 1999).

Repeated administration has been examined in rats. Johansen and colleagues gave ipamorelin to adult female rats three times daily for 15 days and then tested pituitary responsiveness to a provocative intravenous challenge: the GH response to ipamorelin itself was marginally reduced (P less than 0.03), the response to GHRH was unchanged, and pituitary GH content was unaffected (Johansen et al., Growth Hormone and IGF Research, 1999, 9:106-113). That is the published evidence on tolerance, and it is modest desensitisation at the GHS receptor, not its absence. A “Hansen 1999 European Journal of Endocrinology” study previously cited here for 15-day ipamorelin tolerance in rats is a pharmacological characterisation of NN703, an oral secretagogue derived from ipamorelin, and does not report those data (Hansen et al., European Journal of Endocrinology, 1999, 141:180-189). Claims of a fixed fold-increase for ipamorelin over GHRH(1-29) in pituitary cells, and of a 2.5-fold synergy with CJC-1295, could not be traced to a primary study; the GHRP and GHRH synergy literature is reviewed with its sources in the CJC-1295 and ipamorelin combination post.

Preclinical Evidence: Bone and Skeletal Muscle Research

Three rat studies define the skeletal evidence, and none of them is the ovariectomy or fracture-healing study that secondary sources describe. Johansen and colleagues measured longitudinal bone growth rate in adult female rats by tetracycline labelling of the proximal tibial metaphysis: 15 days of ipamorelin increased growth rate dose-dependently from 42 µm/day in vehicle-treated animals to 44, 50 and 52 µm/day (P less than 0.0001), with a pronounced dose-dependent increase in body weight gain, no change in total IGF-I, IGF-binding proteins or serum markers of bone formation and resorption, and no change in osteoclast numbers (Johansen et al., 1999).

Svensson and colleagues gave 13-week-old female Sprague-Dawley rats ipamorelin, GHRP-6, GH or vehicle by continuous osmotic minipump for 12 weeks and followed them by DXA every four weeks. All three treatments increased body weight and total tibial and vertebral bone mineral content, but bone mineral content corrected for body weight was unaffected; tibial areal density rose while volumetric density measured by peripheral quantitative computed tomography and by Archimedes’ principle was unchanged, and ash weight rose with no change in mineral concentration. The conclusion was that the secretagogues increased bone mineral content by increasing bone dimensions, not by densifying existing bone (Svensson et al., Journal of Endocrinology, 2000, 165:569-577). Andersen and colleagues then asked whether ipamorelin could counteract glucocorticoid catabolism: in 8-month-old female rats given methylprednisolone for three months, adding ipamorelin significantly increased maximum tetanic tension of the calf muscles and raised the periosteal bone formation rate four-fold compared with glucocorticoid alone (Andersen et al., Growth Hormone and IGF Research, 2001, 11:266-272). The 14.7 percent tibial bone mineral density figure, the 12-week ovariectomy design and the fracture callus data previously attributed to these authors do not appear in any of these papers.

Gastrointestinal Motility Research

Because GHS-R1a is the ghrelin receptor, ipamorelin has also been studied as a prokinetic. In a rat model of postoperative ileus, a single intravenous administration of ipamorelin or GHRP-6 shortened the time to the first bowel movement, and repeated administration over two days significantly increased cumulative faecal output, food intake and body weight gain (Venkova et al., Journal of Pharmacology and Experimental Therapeutics, 2009, 329:1110-1116). In a related rat model of surgically induced gastroparesis, ipamorelin accelerated gastric emptying, reducing the fraction of a radiolabelled meal remaining in the stomach from 78 percent in vehicle-treated animals to 52 percent, close to the 44 percent of non-operated controls (Greenwood-Van Meerveld et al., Journal of Experimental Pharmacology, 2012, 4:149-155). A multicentre phase 2 trial in bowel-resection patients followed (Beck et al., International Journal of Colorectal Disease, 2014, 29:1527-1534). This is the largest body of ipamorelin research outside the GH axis and is often omitted from secretagogue-focused summaries.

Ipamorelin Versus Other Growth Hormone Secretagogues

Against GHRP-6, the head-to-head data are Raun’s: comparable potency and efficacy for GH release in cells, rats and swine, with GHRP-6 raising ACTH and cortisol and ipamorelin not. Against GHRP-2, ipamorelin is less potent but more efficacious in swine and, again, does not release cortisol. Hexarelin has the highest acute GH-releasing potency of the peptide secretagogues and shares the cortisol and prolactin liability of GHRP-6; a specific claim about ipamorelin lacking cardiac receptor binding could not be sourced and is not repeated. MK-677 (ibutamoren) is an orally active non-peptide GHS-R1a agonist with a much longer duration of action, producing sustained rather than episodic GH elevation; the contrast with ipamorelin’s single two-hour GH burst is the reason ipamorelin is preferred when the experimental question concerns pulsatile GH physiology. The ipamorelin versus tesamorelin post covers the GHRH-receptor side of the same comparison.

Analytical Verification and Quality Control

Reversed-phase HPLC with UV detection near 220 nm is the standard purity method for ipamorelin, and mass spectrometry should confirm the [M+H]+ ion near m/z 712.4. The certificate should state the measured purity of the specific batch rather than a nominal specification. Maple Research Labs publishes independent third-party Certificates of Analysis for tested batches, documenting HPLC purity and mass spectrometric identity so researchers can review the analytical record for a lot before committing it to an experiment. Where net peptide content matters, amino acid analysis provides the orthogonal measurement that HPLC area percent does not, and residual trifluoroacetate explains why a 5 mg vial of a lysine-containing pentapeptide contains less than 5 mg of peptide.

Research Applications and Future Directions

Current preclinical directions include GH-axis modelling in aged animals, body composition in diet-induced obesity models, glucocorticoid-catabolism models following Andersen’s design, gastrointestinal motility, and co-administration with GHRH analogues such as CJC-1295 with or without DAC. The clean selectivity profile continues to make ipamorelin the preferred tool compound when a study needs GHS-R1a activation without the confounding adrenal and lactotroph responses of less selective secretagogues.

For Canadian researchers sourcing ipamorelin for research applications, a domestic supplier with published batch testing removes the customs delays and uncertainty of cross-border procurement, particularly following the closure of major US-based suppliers in 2026.

Research Summary

Ipamorelin is the pentapeptide Aib-His-D-2-Nal-D-Phe-Lys-NH2, a GHS-R1a agonist with an EC50 of 1.3 nM in rat pituitary cells, an ED50 of 80 nmol/kg in anaesthetised rats and 2.3 nmol/kg in conscious swine, and no ACTH or cortisol response at more than 200 times the GH ED50 (Raun 1998). Its terminal half-life in humans is 2 hours and it produces a single GH burst peaking at about 40 minutes (Gobburu 1999). In rats it increases longitudinal bone growth rate and body weight over 15 days with marginal desensitisation of the pituitary response (Johansen 1999), increases bone mineral content over 12 weeks by enlarging bone dimensions rather than raising volumetric density (Svensson 2000), and counteracts glucocorticoid-induced loss of muscle strength and periosteal bone formation (Andersen 2001). As a ghrelin-receptor agonist it accelerates gastric emptying and colonic transit in rat models of postoperative ileus (Venkova 2009; Greenwood-Van Meerveld 2012).

Disclaimer: This article is for informational and research purposes only. It is not intended as medical advice and does not constitute a recommendation for human use. All peptides discussed are intended for laboratory research use only. Not for human consumption. Not for diagnostic or therapeutic use.

Maple Research Labs is a Canadian supplier of research-grade peptides that publishes independent third-party test results for tested batches. Browse our full catalog or review our documentation standards.

For peer-reviewed research on this topic, visit PubMed.