Ipamorelin: a selective ghrelin-receptor tool peptide

Ipamorelin is a synthetic pentapeptide studied as a selective agonist of the growth hormone secretagogue receptor, the same receptor engaged by the endogenous peptide ghrelin. It emerged in the late 1990s from a research program aimed at building small, well-behaved compounds that could switch on pituitary growth hormone release without pulling on the rest of the endocrine axis. Among the first generation of secretagogues it stood out because the published record described a comparatively clean signal. This note collects the verified chemistry alongside the peer-reviewed literature, and it is written strictly as a laboratory reference.

What the molecule is

The compound is a five-residue peptide with the structure Aib-His-D-2-Nal-D-Phe-Lys-NH2. Two of those building blocks are worth flagging. Aib is alpha-aminoisobutyric acid, a non-standard residue that resists enzymatic breakdown and nudges the backbone into a preferred shape. D-2-Nal and D-Phe are D-configured aromatic residues rather than the L-forms found in ordinary proteins. Because of these features the structure cannot be written as a plain one-letter amino-acid string, which is why chemical suppliers list it by full residue notation and by its molecular formula rather than a conventional sequence code.

That design was deliberate. Swapping in D-amino acids and a constrained residue like Aib gives a peptide that holds together long enough in a research setting to produce a measurable signal, while keeping the molecule small and synthetically tractable. The C-terminal amide, written as NH2, caps the chain and removes the free carboxyl group that would otherwise sit at that end. Each of these choices trades a little synthetic complexity for a more predictable, longer-lived research reagent.

Molecular identity

Ipamorelin carries the molecular formula C38H49N9O5 and an average molecular weight near 711.9 g/mol. Its CAS registry number is 170851-70-4. The compound is usually supplied as a lyophilized powder, most often as an acetate salt, which shifts the measured weight of a given vial slightly above the free-peptide figure. Independent identity checks in a laboratory typically rely on mass spectrometry to confirm the intact mass and on reversed-phase chromatography to gauge purity. Those two methods together are enough to distinguish the intended pentapeptide from common synthesis by-products.

Verified molecule

• Class: synthetic pentapeptide, growth hormone secretagogue
• Structure: Aib-His-D-2-Nal-D-Phe-Lys-NH2 (contains non-standard Aib and D-configured residues)
• Molecular formula: C38H49N9O5
• Molecular weight: approximately 711.9 g/mol
• CAS number: 170851-70-4
• Target: growth hormone secretagogue receptor (GHS-R1a), the ghrelin receptor

How it works

The receptor at the center of the story is GHS-R1a, a G-protein-coupled receptor expressed on pituitary somatotrophs and in parts of the brain. Its natural ligand is ghrelin, a stomach-derived peptide. When an agonist binds GHS-R1a, the receptor couples through the Gq pathway, phospholipase C activity rises, and intracellular calcium climbs, which prompts somatotrophs to release stored growth hormone. Ipamorelin mimics that activation. In the cell and animal models reported by Raun and colleagues, it evoked growth hormone release with a potency comparable to earlier secretagogues such as GHRP-6, but with a notable difference in specificity.

Ghrelin itself does more than prompt growth hormone release. In various research systems it influences appetite signaling, gastric activity, and energy balance through the same receptor. A synthetic agonist like this one lets researchers isolate the growth hormone arm of that biology from ghrelin's other roles, because the peptide was engineered around the secretagogue function rather than the full spread of ghrelin signaling. That separation is part of why selective secretagogues became valued bench reagents.

Selectivity was the headline finding of the 1998 work. Several first-generation secretagogues also nudged the release of prolactin, adrenocorticotropic hormone, and cortisol. In the reported models, Ipamorelin left those outputs largely unchanged at concentrations that still produced a strong growth hormone response. That is what the phrase selective secretagogue points to: a comparatively narrow signal rather than a blunt push on the whole pituitary. For a research tool, the narrowness makes the compound useful for pulling GHS-R1a-driven effects apart from the broader stress-axis response.

Later modeling work by Gobburu and colleagues examined how the peptide behaves over time in a research population, fitting its pharmacokinetic and pharmacodynamic profile. That kind of analysis links the concentration curve of the peptide to the size and timing of the growth hormone pulse it evokes, and it remains a reference point for anyone building models of GHS-R1a agonists. The short circulating presence typical of these small peptides means the growth hormone response reads as a pulse rather than a sustained plateau.

Why researchers reach for it

In the laboratory, Ipamorelin functions mainly as a reference or probe compound. Its clean selectivity profile makes it a convenient positive control when studying GHS-R1a signaling, and it appears in comparative work alongside ghrelin, GHRP-2, GHRP-6, and hexarelin. Researchers also contrast it with growth hormone-releasing hormone analogues such as the CJC-1295 and sermorelin families, since those act on a different receptor and drive the same downstream output by a separate route. None of this work is a claim about outcomes in living people; it is bench and preclinical characterization of receptor pharmacology.

Handling and storage

As with most research peptides, the lyophilized powder is generally kept frozen and shielded from light and moisture. Once reconstituted in a suitable sterile solvent, the working solution is usually stored cold and used within a limited window, since small peptides in solution are prone to gradual breakdown. Repeated freezing and thawing of a stock is avoided because each cycle chips away at peptide integrity. These are ordinary bench precautions and apply only to controlled laboratory settings.

For laboratory research use only. Not for human or veterinary use.

Common questions

Is Ipamorelin itself a growth hormone?

No. It is a small synthetic peptide that binds the growth hormone secretagogue receptor and prompts the pituitary to release its own stored growth hormone. It is not growth hormone itself, and it is not a growth hormone-releasing hormone analogue; that second class acts on a separate receptor to reach a similar output.

What makes it described as selective?

In the published preclinical models, it raised growth hormone output while leaving prolactin, adrenocorticotropic hormone, and cortisol comparatively unaffected. Older secretagogues tended to move several of those hormones at once. That narrower signal is the basis for the selective label, and it is the property that makes the peptide useful for isolating one receptor pathway.

Why can it not be written as a simple sequence?

Because it contains residues that fall outside the twenty standard amino acids. Aib is a non-standard, non-coded residue, and two of the aromatic positions use D-configured amino acids rather than the usual L-forms. One-letter sequence codes assume standard L-amino acids, so the full residue notation Aib-His-D-2-Nal-D-Phe-Lys-NH2 is used instead of a compact letter string.

References

1. Raun K, Hansen BS, Johansen NL, et al. Ipamorelin, the first selective growth hormone secretagogue. European Journal of Endocrinology. 1998;139(5):552-561.
2. Gobburu JV, Agerso H, Jusko WJ, Ynddal L. Pharmacokinetic-pharmacodynamic modeling of ipamorelin, a growth hormone releasing peptide, in human volunteers. Pharmaceutical Research. 1999;16(9):1412-1416.