What Has Published Research Reported About Ipamorelin?

What is ipamorelin, and how was it first described?

Ipamorelin is a synthetic five-residue peptide first characterized in a 1998 paper in the European Journal of Endocrinology by Raun, Hansen, Johansen and colleagues, under the title “Ipamorelin, the first selective growth hormone secretagogue” (source 3). The authors reported its sequence as Aib-His-D-2-Nal-D-Phe-Lys-NH2. Two of those residues are non-standard building blocks: Aib is alpha-aminoisobutyric acid, and D-2-Nal is the D form of 2-naphthylalanine. The C-terminus is amidated.

The paper placed ipamorelin in the family of growth hormone releasing peptides, or GHRPs, a group of small synthetic peptides that stimulate growth hormone (GH) release from the pituitary. One of the older members of that group, hexarelin, was built from GHRP-6 by adding a methyl group to its D-tryptophan residue. Its central claim, spelled out in the title, was selectivity: in the animal models studied, ipamorelin released GH without the accompanying stress-axis hormone release that the authors measured with older GHRPs (source 3).

This profile summarizes what the published record reports about ipamorelin, from receptor pharmacology through animal and clinical studies. Like every compound profile on the Steadfast Research Group blog, it covers literature only, written for researchers reviewing that literature, and it says nothing about research-grade material beyond the closing note on identity verification.

Which receptor does ipamorelin act on?

The receptor story ran in an unusual order. GHRPs were made and studied before anyone knew what they bound to. In 1996, Howard and colleagues reported in Science the cloning of a G protein-coupled receptor expressed in the pituitary and in the arcuate ventromedial and infundibular hypothalamus of swine and humans, and showed it was the target of growth hormone secretagogues (source 1). The authors argued that the receptor defined a neuroendocrine pathway for pulsatile GH release and that the synthetic secretagogues were mimicking a hormone that had not yet been discovered.

That hormone was identified three years later. In 1999, Kojima and colleagues reported in Nature the purification from rat stomach of a 28-amino-acid peptide carrying an n-octanoyl group on serine 3, which bound the secretagogue receptor and released GH both in vivo and in vitro (source 2). They named it ghrelin. The 1998 ipamorelin paper therefore predates ghrelin by a year, which is why the early literature describes ipamorelin as acting at a “GHRP receptor” and later literature describes it as a ghrelin receptor agonist or ghrelin mimetic (sources 6 and 7).

Growth hormone secretagogue (GHS)

Any compound that stimulates GH release. The term covers peptides such as ipamorelin and GHRP-6 as well as non-peptide molecules (source 1). One non-peptide example is covered in the MK-677 profile.

GHS receptor (ghrelin receptor)

The G protein-coupled receptor cloned in 1996 as the target of growth hormone secretagogues and later shown to bind ghrelin (sources 1 and 2).

Ghrelin

The 28-amino-acid acylated stomach peptide identified in 1999 as the receptor’s endogenous ligand (source 2).

EC₅₀ and ED₅₀

The concentration (in cell assays) or administered amount (in animals) that produced half of the maximal measured response. E_max is that maximal response.

What did the 1998 characterization study measure?

Raun and colleagues tested ipamorelin in three systems of increasing complexity (source 3). In primary rat pituitary cells, ipamorelin released GH with an EC₅₀ of 1.3 ± 0.4 nmol/L and an E_max of 85 ± 5 percent. In pentobarbital-anaesthetised rats, the reported ED₅₀ was 80 ± 42 nmol/kg with a maximal GH level of 1,545 ± 250 ng/mL. In conscious swine, the ED₅₀ was 2.3 ± 0.03 nmol/kg and the maximal plasma GH level was 65 ± 0.2 ng/mL.

The large gap between the rat and swine figures is a reminder that potency values from one species and one preparation do not transfer directly to another. The swine model was the one the authors used for the selectivity experiments described next.

Why is ipamorelin described as a selective secretagogue?

The study’s defining result came from the swine experiments. The authors reported that GHRP-6 and GHRP-2 both increased plasma ACTH and cortisol, the hormones of the hypothalamic-pituitary-adrenal stress axis. Ipamorelin did not release ACTH or cortisol at significant levels, and that absence held even at doses more than 200-fold higher than its ED₅₀ for GH release (source 3).

On that basis, Raun and colleagues described ipamorelin as the first GHRP-receptor agonist with a selectivity for GH release similar to that of growth hormone-releasing hormone (GHRH), the hypothalamic peptide that normally drives GH secretion (source 3). That word “selective” has followed the compound through every later paper. It is worth reading precisely: the claim was a measured absence of ACTH and cortisol elevation in a specific animal model, not a general statement about every possible off-target effect.

What did human pharmacokinetic modeling report?

The most detailed human pharmacology publication is a 1999 paper in Pharmaceutical Research by Gobburu, Agersø, Jusko and Ynddal (source 4). Adult male volunteers, eight per dose level, received 15-minute intravenous infusions at five escalating dose levels, and the investigators modelled the relationship between ipamorelin concentrations and the GH response.

The authors reported dose-proportional pharmacokinetics, a short terminal half-life of 2 hours, a clearance of 0.078 L/h/kg and a steady-state volume of distribution of 0.22 L/kg (source 4). GH rose in a single episode that peaked at about 0.67 hours and then declined to negligible levels at every dose level studied. To capture that pattern, they used an indirect response model with a zero-order GH release rate over a finite period, and estimated a half-maximal stimulation concentration (SC₅₀) of 214 nmol/L and a maximal GH production rate of 694 mIU/L/h (source 4). The single, time-limited GH episode was a notable finding: concentrations of ipamorelin outlasted the GH pulse it triggered.

What did rat bone studies observe?

Skeletal measures were another early focus in rodents. In a 2000 study in the Journal of Endocrinology, Svensson and colleagues gave 13-week-old female Sprague-Dawley rats either ipamorelin or GHRP-6 by continuous subcutaneous delivery from osmotic minipumps for 12 weeks (source 5). Both secretagogues increased total tibial and vertebral bone mineral content (BMC), measured in vivo by dual-energy X-ray absorptiometry (DXA).

The authors then used peripheral quantitative computed tomography (pQCT) to look inside that result. The increase in cortical BMC came from a larger cross-sectional bone area, while cortical volumetric bone mineral density was unchanged (source 5). In other words, the bones were bigger rather than denser. The authors attributed the higher BMC to increased bone growth with larger bone dimensions, a distinction that a DXA reading alone would not have revealed.

Why was ipamorelin studied in gastrointestinal motility models?

A second research line studied ipamorelin as a ghrelin mimetic in models of gut motility (sources 6 and 7). In a 2009 study in the Journal of Pharmacology and Experimental Therapeutics, Venkova and colleagues used a rat model of postoperative ileus, a temporary loss of coordinated gut movement after abdominal surgery, created in fasted male rats by laparotomy and intestinal manipulation (source 6).

The authors reported that a single administration of ipamorelin shortened the time to the first bowel movement, and that repeated administration increased cumulative fecal pellet output, food intake and body-weight gain compared with vehicle (source 6). In isolated gastric smooth muscle from operated animals, contractile responses were markedly inhibited, and both ipamorelin and ghrelin reversed that inhibition (source 6).

A follow-up study by Greenwood-Van Meerveld, Tyler, Mohammadi and Pietra, published in the Journal of Experimental Pharmacology in 2012, focused on gastric emptying in the same kind of surgical model (source 7). Surgery left 78 ± 5 percent of a test meal in the stomach of vehicle controls. After intravenous ipamorelin, 52 ± 11 percent remained, a significant acceleration (P < 0.05), against 44 ± 6 percent in non-surgical controls. The authors concluded that the effect ran through stimulation of gastric contractility by a ghrelin receptor-mediated mechanism involving cholinergic excitatory neurons (source 7).

What did the clinical proof-of-concept study report?

The rodent ileus work led to a phase 2 clinical study published in 2014 in the International Journal of Colorectal Disease by Beck, Sweeney, McCarter and the Ipamorelin 201 Study Group (source 8). It was a prospective, randomized, placebo-controlled proof-of-concept trial in adults undergoing small or large bowel resection by open or laparoscopic surgery, with intravenous ipamorelin or placebo given in the postoperative period.

The primary comparison was time to the first tolerated meal. The investigators reported a median of 25.3 hours in the ipamorelin group and 32.6 hours in the placebo group, a difference of 7.3 hours that was not statistically significant (p = 0.15) (source 8). For a research profile, that result is the key point of the clinical record: the effect seen in rat models did not produce a statistically significant difference in this human trial.

How do the published ipamorelin studies compare?

What remains unresolved in the ipamorelin literature?

The published record is small and mostly two decades old. The selectivity finding rests largely on the swine experiments in the original 1998 paper (source 3). The human pharmacology data come from intravenous infusions in adult men only (source 4). The bone study used female rats only and found a size effect rather than a density effect (source 5), and the gastrointestinal findings in rats were not matched by a statistically significant result in the one published clinical trial (sources 6 to 8). All of the human studies were conducted by investigators under regulatory oversight with defined populations, and they are summarized here strictly as published literature. None of the figures in this post should be read as applying beyond the conditions of the studies that produced them.

For laboratories, the chemistry in source 3 has a practical side. Ipamorelin is defined by a specific five-residue sequence that includes two non-standard residues and a C-terminal amide, so confirming identity against that expected structure by mass spectrometry matters alongside an HPLC purity figure. The CJC-1295 / Ipamorelin blend page sets out the identity and Certificate of Analysis details for the related catalog material.