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title: Ipamorelin: A Pentapeptide Ghrelin Receptor Agonist description: "Ipamorelin is a synthetic pentapeptide growth hormone secretagogue (GHS) that acts as a highly selective agonist of the "

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Ipamorelin: A Pentapeptide Ghrelin Receptor Agonist

Quick Facts

Full Name	Ipamorelin (INN); NNC 26-0161 (developmental code)
Class	Synthetic pentapeptide ghrelin receptor agonist / Growth hormone secretagogue (GHS)
Amino Acid Sequence	Aib–His–d-2-Nal–d-Phe–Lys–NH₂ (modified pentapeptide)
Molecular Formula	C₃₈H₄₉N₉O₅
Molecular Weight	~711 Da
Target Receptor	Growth hormone secretagogue receptor 1a (GHS-R1a)
Mechanism	GHS-R1a agonist; stimulates pulsatile GH release with minimal effect on prolactin, ACTH, or cortisol
Selectivity Profile	Highly selective for GHS-R1a; no significant affinity for GHRH receptor or other pituitary targets
Route of Administration	Subcutaneous injection; intravenous (research use)
PubChem CID	9939673
CAS Number	170851-47-7

Executive Summary

Ipamorelin is a synthetic pentapeptide growth hormone secretagogue (GHS) that acts as a highly selective agonist of the ghrelin receptor (GHS-R1a).

First described by Raun and colleagues in 1998, it was designed as a truncated analog of the hexapeptide GHRP-2 (pralmorelin), retaining full secretory activity while achieving an improved selectivity profile.

Ipamorelin is distinguished from earlier growth hormone-releasing peptides (GHRPs) by its minimal effect on prolactin, adrenocorticotropic hormone (ACTH), and cortisol release. This selectivity makes it a valuable research tool for elucidating the role of the ghrelinergic system in growth hormone regulation, metabolism, and tissue repair.

Ipamorelin is exclusively a research compound and has not received FDA approval for any clinical indication.

Introduction

The discovery of growth hormone secretagogues (GHSs) began with the observation that certain synthetic opioid peptides could stimulate GH release in a manner distinct from GHRH. In the 1980s, Bowers and colleagues synthesized a series of enkephalin-derived peptides-GHRPs-that potently stimulated GH secretion both in vitro and in vivo.

The subsequent identification of the ghrelin/GHS-R1a system in 1999 by Kojima and colleagues provided the molecular basis for understanding how these small peptides exert their effects. Ipamorelin emerged from a structure-activity optimization program aimed at minimizing side effects while maximizing GH secretory potency.

With only five amino acids, it represents one of the smallest and most selective GHS molecules developed to date.

Molecular Characteristics

Ipamorelin has the primary sequence Aib–His–d-2-Nal–d-Phe–Lys–NH₂, where Aib is α-aminoisobutyric acid (a non-coded, achiral amino acid) and d-2-Nal is d-3-(2-naphthyl)alanine.

The presence of d-amino acids and the unnatural Aib residue confers significant resistance to proteolytic degradation, contributing to the peptide's favorable pharmacokinetic profile. The C-terminal amidation is essential for full receptor activation.

The peptide's low molecular weight (~711 Da) places it among the smallest peptide-based GHSs, facilitating subcutaneous absorption. Its compact, partially constrained structure is believed to adopt a β-turn motif critical for high-affinity binding to the GHS-R1a receptor.

The pharmacokinetics of ipamorelin have been characterized in several species, demonstrating a plasma half-life of approximately 1–2 hours in humans following subcutaneous administration, with peak GH levels occurring within 30–60 minutes.

Biological Research Background

GHS-R1a Signaling

The GHS-R1a receptor is a G~q/11~-coupled GPCR highly expressed in the hypothalamus and pituitary, but also detected in peripheral tissues including the pancreas, gut, adipose tissue, and bone.

Ipamorelin binding to GHS-R1a activates phospholipase C (PLC), leading to inositol trisphosphate (IP₃) production, intracellular calcium mobilization, and ultimately GH exocytosis from somatotroph cells.

Unlike GHRH, which acts via the cAMP–PKA pathway, GHSs converge on the same final secretory machinery through a distinct intracellular signaling route, enabling a synergistic GH response when both pathways are engaged.

Selectivity Profile

One of ipamorelin's defining features is its remarkable selectivity. Earlier GHRPs such as hexarelin and GHRP-6 are associated with significant ACTH and cortisol release due to cross-talk at central melanocortin pathways. Ipamorelin, in contrast, triggers a robust GH pulse with minimal (<5% of GH response) stimulation of ACTH, cortisol, or prolactin. This dissociation of GH secretagogue activity from stress-axis activation has made ipamorelin a preferred tool in endocrine research for studying the isolated effects of GH axis stimulation.

Bone and Tissue Research

Preclinical studies have examined ipamorelin's effects on bone metabolism. In animal models of osteoporosis, ghrelin receptor agonists have been shown to stimulate markers of both bone formation (osteocalcin, P1NP) and resorption, but with a net favorable balance. Clinical work by Karsdal and colleagues demonstrated that ipamorelin increased bone formation markers in healthy volunteers, suggesting a potential anabolic effect. Additional research has explored its role in muscle protein synthesis and nitrogen retention, consistent with the known anabolic actions of GH and IGF-1.

Current Research Landscape

Ipamorelin research continues across several fronts:

• Endocrine physiology: The peptide remains a valuable pharmacological probe for dissecting the feedback loops within the somatotropic axis and their interaction with metabolic signals.
• Musculoskeletal research: Studies are investigating whether chronic GHS-R1a agonism can counteract sarcopenia and osteoporosis in aging populations, though long-term data remain limited.
• Metabolic studies: Ghrelin receptor agonists influence appetite regulation, energy expenditure, and glucose homeostasis.

Ipamorelin's GH-mediated lipolytic effects are of interest in the context of metabolic syndrome. - Gastrointestinal motility: Given ghrelin's role as a prokinetic hormone, GHS-R1a agonists are being explored for gastroparesis and postoperative ileus, though ipamorelin is less studied in this context than other ghrelin mimetics.

Related Research

Tesamorelin Research Profile

GHRH analog for comparative endocrine research.

Peptide Signaling Pathways

GPCR signaling mechanisms of ghrelin mimetics.

Metabolic Research

Endocrine and metabolic peptide research.

Frequently Asked Questions

+ **How does ipamorelin differ from other growth hormone secretagogues?**

Ipamorelin is distinguished by its high selectivity for GHS-R1a and minimal activation of the stress axis. Unlike GHRP-6, hexarelin, or other earlier secretagogues, ipamorelin causes negligible increases in ACTH, cortisol, and prolactin. It is also one of the smallest GHSs, comprising only five amino acids.

+ **Does ipamorelin work through the same mechanism as GHRH?**

No. Ipamorelin acts on the GHS-R1a receptor (the ghrelin receptor), which is distinct from the GHRH receptor. GHS-R1a signals primarily through the phospholipase C–IP₃–calcium pathway, whereas GHRH signals through the adenylyl cyclase–cAMP–PKA pathway. Both converge on GH release from pituitary somatotrophs and can produce additive or synergistic effects when combined.

+ **Is ipamorelin FDA-approved?**

No. Ipamorelin is not FDA-approved for any clinical indication. It is a research compound used in preclinical and early-phase clinical studies to investigate GH axis physiology and potential therapeutic applications.

+ **What is the significance of the modified amino acids in ipamorelin?**

The inclusion of Aib (α-aminoisobutyric acid) and d-amino acids (d-2-Nal, d-Phe) confers resistance to proteolytic cleavage by endo- and exopeptidases, significantly improving the peptide's in vivo stability compared to all-L-amino-acid peptides. This is a common strategy in peptide drug design to enhance pharmacokinetic properties.

+ **What are the reported effects of ipamorelin on bone?**

In preclinical and early clinical studies, ipamorelin has been shown to stimulate bone formation markers such as osteocalcin and P1NP. The effect is thought to be mediated through GH–IGF-1 axis activation, which promotes osteoblast differentiation and activity. However, large-scale clinical trials for bone indications have not been conducted.

+ **Can ipamorelin be taken orally?**

While ipamorelin is more resistant to proteolysis than linear all-L-peptides, its oral bioavailability remains negligible. It is administered by subcutaneous or intravenous injection in research settings. Oral formulation approaches (e.g., permeation enhancers) have not been clinically validated for this peptide.

+ **How long does the GH response to ipamorelin last?**

Following subcutaneous administration, peak GH levels are typically achieved within 30–60 minutes, with GH levels returning to baseline by approximately 2–3 hours post-injection. The total GH secretory pulse magnitude is dose-dependent and has been characterized in several human pharmacokinetic/pharmacodynamic studies.

+ **Does ipamorelin affect appetite?**

Ghrelin is known as the "hunger hormone" and stimulates appetite via GHS-R1a activation in the hypothalamus. While ipamorelin is a potent GHS-R1a agonist, its effect on appetite in human studies appears to be less pronounced than that of acylated ghrelin itself, possibly due to differences in pharmacokinetics or CNS penetration.

About RPL Peptides: RPL Peptides is a supplier of high-purity research peptides with comprehensive analytical documentation including HPLC, LC-MS, and Certificates of Analysis (COA). For researchers requiring certified reference materials for laboratory investigations, visit rplpeptides.com or explore detailed molecular data at the RPL Peptides Data Center.

References

- Raun K, Hansen BS, Johansen NL, et al. Ipamorelin, the first selective growth hormone secretagogue. Eur J Endocrinol. 1998;139(5):552-561. doi:10.1530/eje.0.1390552 - Karsdal MA, Byrjalsen I, Riis BJ, et al. Effect of the growth hormone secretagogue ipamorelin on markers of bone turnover in healthy volunteers. Bone. 2011;49(4):702-708. doi:10.1016/j.bone.2011.06.010 - Svensson J, Lönn L, Jansson JO, et al. Two-month treatment of obese subjects with the oral growth hormone secretagogue ipamorelin. Obes Res. 2000;8(2):186-192. doi:10.1038/oby.2000.19 - Hansen TK, Møller J, Khalaf N, et al. Intravenous growth hormone secretagogue ipamorelin stimulates GH secretion in healthy humans. Clin Endocrinol (Oxf). 1999;51(2):185-191. doi:10.1046/j.1365-2265.1999.00739.x - Jørgensen JO, Møller N, Wolthers T, et al. Growth hormone secretagogues: a new class of compounds. Growth Horm IGF Res. 1998;8 Suppl A:21-26. doi:10.1016/s1096-6374(98)80005-3 - Bowers CY. Growth hormone-releasing peptide (GHRP). Cell Mol Life Sci. 1998;54(12):1316-1329. doi:10.1007/s000180050257 - Kojima M, Hosoda H, Date Y, et al. Ghrelin is a growth-hormone-releasing acylated peptide from stomach. Nature. 1999;402(6762):656-660. doi:10.1038/45230 - Nass R, Pezzoli SS, Chapman IM, et al. GH secretagogues and bone. Endocr Rev. 2011;32(3):376-397. doi:10.1210/er.2010-0007