Ipamorelin (NNC 26-0161) is a synthetic pentapeptide growth-hormone secretagogue and a selective agonist of the ghrelin / growth hormone secretagogue receptor (GHS-R1a). First described by Raun and colleagues in 1998, it was engineered from the GHRP-1 scaffold to release growth hormone with a selectivity approaching that of endogenous growth-hormone-releasing hormone (GHRH). This monograph summarizes the published scientific literature on its chemistry, mechanism, and documented research findings. It is presented strictly as a research compound for laboratory use.

Background & Discovery

Ipamorelin (research code NNC 26-0161) is a synthetic pentapeptide growth-hormone secretagogue (GHS) first described by Raun and colleagues at Novo Nordisk in a 1998 report in the European Journal of Endocrinology. It emerged from a medicinal-chemistry program built around the growth-hormone-releasing peptide (GHRP) series, and its sequence, Aib-His-D-2-Nal-D-Phe-Lys-NH2, was engineered from the GHRP-1 scaffold to retain potent growth-hormone (GH)-releasing activity while minimizing the off-target pituitary and adrenal effects seen with earlier peptides. Published literature characterizes it as the first GHRP-receptor agonist reported to release GH with a selectivity approaching that of endogenous growth-hormone-releasing hormone (GHRH).

The compound is studied because it functions as a peptidergic agonist at the growth hormone secretagogue receptor subtype 1a (GHS-R1a), the same receptor activated by the endogenous hormone ghrelin. In the original characterization, ipamorelin displayed in-vitro potency and efficacy comparable to GHRP-6 (reported EC50 near 1.3 nmol/l) and released GH dose-dependently in conscious swine, yet did not raise adrenocorticotropic hormone (ACTH) or cortisol to levels significantly beyond those seen after GHRH stimulation, and did not measurably alter follicle-stimulating hormone (FSH), luteinizing hormone (LH), prolactin, or thyroid-stimulating hormone (TSH). This selectivity profile is the principal reason the peptide became a widely used reference tool in GH-axis pharmacology.

Ipamorelin falls into the research category of "GH secretagogues / ghrelin-receptor agonists," a class that also includes the earlier GHRPs (GHRP-2, GHRP-6, hexarelin) and later non-peptide mimetics. Over subsequent decades it was examined across in-vitro pituitary-cell systems, rodent models of bone and body-weight physiology, human pharmacokinetic-pharmacodynamic studies, and a proof-of-concept clinical trial in postoperative ileus, giving it an unusually broad, well-documented literature base for a peptide of its size.

Chemical Identity

Structure & Physicochemical Properties

Ipamorelin is a linear, non-glycosylated pentapeptide with the sequence Aib-His-D-2-Nal-D-Phe-Lys-NH2 (molecular formula C38H49N9O5; average molecular weight approximately 711.9 g/mol; PubChem CID 9831659). Structurally it incorporates several non-proteinogenic and D-configured residues that distinguish it from a simple natural peptide: an N-terminal alpha-aminoisobutyric acid (Aib), a D-2-naphthylalanine (D-2-Nal), and a D-phenylalanine, together with a C-terminal lysine amide. These modifications, particularly the Aib cap, the D-amino acids, and the C-terminal primary amide, are consistent with the design goal of increasing enzymatic stability and receptor selectivity relative to earlier GHRP hexapeptides, and published pharmacokinetic work reported that ipamorelin was moderately resistant to metabolism, with 60-80% of an administered dose recoverable as intact peptide from bile and urine in rats.

As a small, charged peptide (basic imidazole and lysine side chains alongside aromatic naphthyl and phenyl groups), ipamorelin is generally handled as a water-soluble lyophilized powder, most commonly supplied as an acetate salt. Reference-database and pharmacokinetic literature describe good aqueous solubility, a short systemic half-life in vivo (on the order of two hours in human volunteers), and a low systemic plasma clearance relative to other GHRPs. Like most peptides, it is susceptible to hydrolysis, oxidation (the imidazole and aromatic residues), and aggregation in solution, so the physicochemical literature and standard peptide-handling practice emphasize storage as a dry, desiccated solid and reconstitution shortly before experimental use.

Mechanism of Action — as described in the literature

Published literature characterizes ipamorelin as a selective agonist of the growth hormone secretagogue receptor subtype 1a (GHS-R1a), the G-protein-coupled receptor for which ghrelin is the endogenous ligand. GHS-R1a is expressed on somatotroph cells of the anterior pituitary and in hypothalamic nuclei. Agonist binding couples predominantly through Gq/11, activating phospholipase C to generate inositol trisphosphate and diacylglycerol, mobilizing intracellular calcium, and thereby triggering exocytotic release of stored growth hormone from somatotrophs. In this sense ipamorelin is described as a ghrelin mimetic that reproduces the peptidergic GH-releasing arm of the somatotropic axis.

The mechanism is distinct from that of GHRH, which acts at its own receptor through Gs and cyclic AMP. Because the GHS-R1a and GHRH pathways are separate but convergent, the GH-releasing effects of ghrelin-receptor agonists and GHRH are reported to be additive or synergistic, and ipamorelin has frequently been used experimentally alongside GHRH to probe somatotroph reserve. The peptide is also reported to amplify the amplitude of endogenous pulsatile GH secretion rather than creating a wholly non-physiological release pattern; in human pharmacokinetic-pharmacodynamic modeling, a single infusion produced a discrete episode of GH release with a peak near 0.67 h followed by exponential decline.

The defining pharmacological feature reported for ipamorelin is its selectivity within the pituitary. In the original characterization, GH release occurred without a significant rise in ACTH or cortisol beyond that produced by GHRH alone, and without measurable changes in FSH, LH, prolactin, or TSH. Researchers attributed this to a clean interaction with the GH-releasing machinery of the somatotroph, in contrast to earlier GHRPs such as GHRP-2, GHRP-6, and hexarelin, which more readily co-activated the hypothalamic-pituitary-adrenal axis. This selectivity is the mechanistic basis for ipamorelin’s use as a tool compound for isolating GHS-R1a-mediated GH release from confounding corticotropic effects.

Downstream of GH release, the somatotropic axis normally drives hepatic and local production of insulin-like growth factor-I (IGF-I), which mediates many anabolic effects on bone and soft tissue. Notably, several rodent studies of ipamorelin reported skeletal effects (increased longitudinal bone growth, increased bone mineral content, and periosteal bone formation) that were not always accompanied by measurable changes in total circulating IGF-I, prompting authors to consider GH-dependent local or IGF-I-independent mechanisms in bone. This nuance remains an active theme in the mechanistic literature.

Beyond the pituitary, GHS-R1a is expressed in gastrointestinal and other peripheral tissues, and ghrelin-receptor agonism is associated with prokinetic effects on gut motility. This peripheral dimension of the mechanism provided the rationale for evaluating ipamorelin as a ghrelin mimetic in the setting of postoperative ileus, where accelerating the return of gastrointestinal function was the hypothesis under investigation.

Key Published Findings

• Receptor selectivity / GH-axis pharmacology. In the original characterization by Raun et al., ipamorelin released GH in vitro with potency and efficacy comparable to GHRP-6 (EC50 approximately 1.3 nmol/l) and dose-dependently in conscious swine (ED50 approximately 2.3 nmol/kg), yet did not raise ACTH or cortisol beyond levels seen with GHRH and did not alter FSH, LH, prolactin, or TSH, defining it as the first GHRP-receptor agonist with GHRH-like selectivity for GH release.[1]
• Human pharmacokinetics / pharmacodynamics. In healthy male volunteers, Gobburu et al. characterized ipamorelin pharmacokinetics as dose-proportional with a terminal half-life near 2 h, clearance of about 0.078 L/h/kg, and steady-state volume of distribution around 0.22 L/kg; GH release was modeled as a single episode peaking near 0.67 h using an indirect-response model.[2]
• Skeletal growth (longitudinal bone). In adult female rats, Johansen et al. reported that subcutaneous ipamorelin dose-dependently increased longitudinal bone growth rate from 42 to as high as 52 micrometers/day over 15 days without significant changes in total IGF-I, IGF-binding proteins, or serum bone turnover markers.[3]
• Bone mineral content / body composition. Svensson et al. found that 12 weeks of continuous ipamorelin or GHRP-6 infusion increased body weight and total tibial and vertebral bone mineral content (by DXA) in adult female rats; the increase in cortical BMC reflected greater cross-sectional bone area (bone growth) rather than higher volumetric bone mineral density.[4]
• Counteracting glucocorticoid-induced catabolism. Andersen et al. reported that in adult rats co-treated with glucocorticoid, concurrent ipamorelin administration increased maximum tetanic muscle tension and roughly quadrupled the periosteal bone formation rate compared with glucocorticoid alone, indicating partial counteraction of glucocorticoid-induced decreases in muscle strength and bone formation.[5]
• Somatotroph ultrastructure. Jiménez-Reina et al. observed that 21 days of in-vivo ipamorelin (or GHRH) treatment of young female rats increased the volume density of pituitary somatotroph secretory granules in vitro without altering overall somatotroph ultrastructure, consistent with enhanced GH storage/secretory capacity.[6]
• Clinical proof-of-concept (postoperative ileus). In a multicenter, double-blind, placebo-controlled Phase 2 trial in bowel-resection patients, Beck et al. reported that intravenous ipamorelin was well tolerated; median time to first tolerated meal was 25.3 h versus 32.6 h for placebo (p=0.15), and no statistically significant differences were seen in the primary or secondary efficacy endpoints.[7]
• Comparative pharmacokinetics / metabolic stability. In rats, Johansen et al. found ipamorelin had a systemic plasma clearance roughly five-fold lower than GHRP-6, was excreted mainly in urine (versus biliary excretion for GHRP-6), was moderately metabolically resistant (60-80% recovered intact), and showed an estimated intranasal bioavailability of about 20%.[8]

Research Applications

• Investigated in vitro in rat pituitary and somatotroph cell systems as a selective GHS-R1a agonist for characterizing peptidergic growth-hormone release.
• Used as a pharmacological reference/tool compound for distinguishing GHS-R1a-mediated GH secretion from corticotropic (ACTH/cortisol) and other pituitary effects.
• Examined in rodent models of longitudinal (endochondral) bone growth and skeletal development.
• Investigated in adult female rat models of bone mineral content, cortical bone geometry, and bone remodeling.
• Studied in rat models of glucocorticoid-induced catabolism to probe effects on skeletal muscle strength and periosteal bone formation.
• Applied as a probe of somatotroph ultrastructure and secretory-granule dynamics following chronic in-vivo exposure.
• Characterized in human pharmacokinetic-pharmacodynamic studies of GH secretory dynamics and dose-proportionality.
• Evaluated in clinical research on ghrelin-receptor agonism and gastrointestinal motility in the context of postoperative ileus.
• Used in comparative pharmacology against GHRP-2, GHRP-6, and hexarelin to benchmark GH-axis selectivity and metabolic stability.

Related & Comparator Compounds

Ipamorelin is one member of the growth hormone secretagogue / ghrelin-receptor agonist class, and the literature repeatedly contrasts it with structurally related peptides. It was derived from the GHRP-1 series and is most directly compared with the earlier hexapeptides GHRP-6, GHRP-2, and hexarelin; the key distinction reported is selectivity, since those earlier peptides more readily co-stimulate ACTH, cortisol, and prolactin, whereas ipamorelin’s GH release was characterized as selective and GHRH-like. Its endogenous counterpart is ghrelin, the natural GHS-R1a ligand, which differs in that native ghrelin carries an octanoyl acyl modification and additional metabolic/appetite signaling roles. The orally active non-peptide mimetic ibutamoren (MK-677) targets the same receptor but is a small molecule with a much longer duration of action. Ipamorelin is mechanistically separate from GHRH-receptor agonists such as sermorelin, tesamorelin, and CJC-1295, which act through the GHRH receptor and cyclic-AMP signaling; because the two receptor systems converge on the somatotroph, GHS-R agonists and GHRH analogs are frequently studied together for additive GH release. Macimorelin, a ghrelin-receptor agonist developed as a GH-stimulation diagnostic, is another comparator within the broader secretagogue landscape.

Handling, Reconstitution & Storage

In a research setting, ipamorelin is typically supplied as a lyophilized (freeze-dried) powder, commonly as the acetate salt, and standard peptide-handling practice is to store the sealed, desiccated solid frozen (for example at -20 °C or colder) and protected from light and moisture, under which conditions small peptides are generally stable for extended periods. Published and reference-database handling guidance describes reconstitution in sterile or bacteriostatic water (or an appropriate buffer) shortly before use, gentle dissolution without vigorous vortexing, and division into single-use aliquots to avoid repeated freeze-thaw cycles that can promote degradation and aggregation. Reconstituted solutions are usually kept refrigerated and used within a short window because peptides in solution are more susceptible to hydrolysis and oxidation than the dry solid. All such handling is described strictly in the context of laboratory research use; this information does not constitute human dosing or administration guidance.

Analytical & Quality Considerations

Analytical characterization of ipamorelin for research use typically centers on reversed-phase HPLC to establish chromatographic purity (commonly reported at greater than 98%) and mass spectrometry (ESI-MS) to confirm identity against the expected average molecular weight of roughly 711.9 g/mol (protonated [M+H]+ near 712), with the C38H49N9O5 composition and Aib-His-D-2-Nal-D-Phe-Lys-NH2 sequence as the identity anchors. Because these peptides are usually isolated as acetate or trifluoroacetate salts, a rigorous certificate of analysis distinguishes gross weight from net peptide content and reports the counter-ion, along with water/moisture content, residual solvents, and, where relevant, endotoxin levels. Amino-acid analysis or sequencing can corroborate composition, and the presence of D-amino acids and the non-standard Aib residue makes orthogonal identity confirmation valuable. Independent, third-party COA verification matters because nominal label identity and true purity, counter-ion load, and net peptide content can diverge substantially between suppliers, and downstream experimental reproducibility depends on knowing the actual quantity and quality of active peptide present.

Frequently Asked Research Questions

Q. What is ipamorelin?
A. Ipamorelin (NNC 26-0161) is a synthetic pentapeptide growth-hormone secretagogue with the sequence Aib-His-D-2-Nal-D-Phe-Lys-NH2. Published literature characterizes it as a selective agonist of the ghrelin/growth hormone secretagogue receptor (GHS-R1a) that stimulates growth-hormone release. It is described here strictly as a research compound.

Q. How does ipamorelin differ from GHRP-6 and other GHRPs?
A. In the original 1998 characterization, ipamorelin released GH with potency and efficacy similar to GHRP-6 but, unlike GHRP-6, GHRP-2, and hexarelin, did not significantly elevate ACTH or cortisol beyond GHRH levels and did not alter prolactin, FSH, LH, or TSH. This selectivity is the main way the literature distinguishes it.

Q. Is ipamorelin the same as GHRH (or sermorelin/CJC-1295)?
A. No. GHRH and its analogs (sermorelin, tesamorelin, CJC-1295) act at the GHRH receptor through cyclic-AMP signaling, whereas ipamorelin acts at the separate GHS-R1a (ghrelin) receptor. The two pathways converge on the pituitary somatotroph, and researchers report their GH-releasing effects can be additive.

Q. What have human studies of ipamorelin reported?
A. Human pharmacokinetic-pharmacodynamic work described dose-proportional kinetics with a roughly 2-hour half-life and a discrete GH-release episode. A Phase 2 postoperative-ileus trial found intravenous ipamorelin was well tolerated but did not reach statistical significance on its primary or secondary efficacy endpoints versus placebo.

Q. What is its molecular weight and formula?
A. Reference databases list the molecular formula as C38H49N9O5 with an average molecular weight of approximately 711.9 g/mol, CAS 170851-70-4, and PubChem CID 9831659.

Q. How is ipamorelin stored and handled in research settings?
A. It is typically supplied as a lyophilized powder (often the acetate salt), stored frozen, desiccated, and protected from light. Standard practice is to reconstitute in sterile or bacteriostatic water shortly before use and aliquot to avoid repeated freeze-thaw cycles. This is laboratory handling information only, not human dosing guidance.

Peer-Reviewed References

1. Raun K, Hansen BS, Johansen NL, Thøgersen H, Madsen K, Ankersen M, Andersen PH. Ipamorelin, the first selective growth hormone secretagogue. European Journal of Endocrinology. 1998. PubMed →
2. Gobburu JV, Agersø H, Jusko WJ, Ynddal L. Pharmacokinetic-pharmacodynamic modeling of ipamorelin, a growth hormone releasing peptide, in human volunteers. Pharmaceutical Research. 1999. PubMed →
3. Johansen PB, Nowak J, Skjærbæk C, Flyvbjerg A, Andreassen TT, Wilken M, Ørskov H. Ipamorelin, a new growth-hormone-releasing peptide, induces longitudinal bone growth in rats. Growth Hormone & IGF Research. 1999. PubMed →
4. Svensson J, Lall S, Dickson SL, Bengtsson BA, Rømer J, Ahnfelt-Rønne I, Ohlsson C, Jansson JO. The GH secretagogues ipamorelin and GH-releasing peptide-6 increase bone mineral content in adult female rats. Journal of Endocrinology. 2000. PubMed →
5. Andersen NB, Malmlöf K, Johansen PB, Andreassen TT, Ørtoft G, Oxlund H. The growth hormone secretagogue ipamorelin counteracts glucocorticoid-induced decrease in bone formation of adult rats. Growth Hormone & IGF Research. 2001. PubMed →
6. Jiménez-Reina L, Cañete R, de la Torre MJ, Bernal G. Influence of chronic treatment with the growth hormone secretagogue Ipamorelin, in young female rats: somatotroph response in vitro. Histology and Histopathology. 2002. PubMed →
7. Beck DE, Sweeney WB, McCarter MD; Ipamorelin 201 Study Group. Prospective, randomized, controlled, proof-of-concept study of the ghrelin-receptor agonist ipamorelin for the management of postoperative ileus in bowel resection patients. International Journal of Colorectal Disease. 2014. PubMed →
8. Johansen PB, Hansen KT, Andersen JV, Johansen NL. Pharmacokinetic evaluation of ipamorelin and other peptidyl growth hormone secretagogues with emphasis on nasal absorption. Xenobiotica. 1998. PubMed →

For laboratory and research use only. Not for human or veterinary use, diagnosis, or treatment. This overview summarizes published scientific literature for informational and educational purposes and is not medical advice; no claims are made regarding safety or efficacy in humans.

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